Use of fly ash layer as a barrier to prevent contamination of rainwater by contact with Hg-contaminated debris

Academic Edi o : Nelson Belzile
Recei ed: 5 Feb ua y 2025
Re ised: 24 Ma ch 2025
Accep ed: 25 Ma ch 2025
Published: 1 Ap il 2025
Ci a ion: Rod íguez, R.; Bascomp a,
M.; Ga cía-O diales, E.; Ayala, J. Use o
Fly Ash Laye as a Ba ie o P e en
Con amina ion o Rainwa e by
Con ac wi h Hg-Con amina ed
Deb is. En i onmen s 2025,12, 107.
h ps://doi.o g/10.3390/
en i onmen s12040107
Copy igh : © 2025 by he au ho s.
Licensee MDPI, Basel, Swi ze land.
This a icle is an open access a icle
dis ibu ed unde he e ms and
condi ions o he C ea i e Commons
A ibu ion (CC BY) license
(h ps://c ea i ecommons.o g/
licenses/by/4.0/).
A icle
Use o Fly Ash Laye as a Ba ie o P e en Con amina ion o
Rainwa e by Con ac wi h Hg-Con amina ed Deb is
Ra ael Rod íguez 1,*, Ma c Bascomp a 2, E én Ga cía-O diales 1and Julia Ayala 3
1Depa men o Mining Exploi a ion and P ospec ing, Poly echnical School o Mie es, Uni e si y o O iedo,
Gonzalo Gu ié ez Qui ós s/n, 33600 Mie es, As u ias, Spain; ga [email p o ec ed]
2
Depa men o Mining, Indus ial and ICT Enginee ing, Poly echnic Uni e si y o Ca alonia (UPC), A . Bases
de Man esa, 61-73, 08242 Man esa, Ca alonia, Spain; ma [email p o ec ed]
3
Depa men o Ma e ials Science and Me allu gical Enginee ing, Poly echnical School o Mie es, Uni e si y o
O iedo, Gonzalo Gu ié ez Qui ós s/n, 33600 Mie es, As u ias, Spain; [email p o ec ed]
*Co espondence: [email p o ec ed]
Abs ac : Highly con amina ed was e om an old me cu y mine acili y was co e ed wi h
ly ash om a coal-bu ning powe plan ha was analyzing he ainwa e in il a ion in a
ull-scale es in which he in luencing a iables we e moni o ed o a yea . A su icien ly
low hyd aulic conduc i i y and su icien ly high po osi y o he ash, and he ela ionship
be ween e apo anspi a ion and p ecipi a ion we e he mos impo an ac o s con olling
ainwa e in il a ion h ough he ly ash laye o p oduce con amina ed leacha e. A ly ash
laye wi h a hickness be ween 10 and 50 cm, depending on clima ic condi ions, wo ks as
a ba ie o pa ially o o ally p e en , depending on he scena io conside ed, ainwa e
con amina ion. O e all, he solu ion p oposed in his s udy esul s in economic sa ings in
all he cases conside ed, because ea men s o elimina ing PTEs om was e a e usually
expensi e. On he o he hand, he e ec is pe manen o e ime, as i is based on a physical
ba ie e ec , while he con amina ion educ ion is independen o he ini ial concen a ion
and he con amina ion educ ion is o any PTE (Hg, Pb, Zn, e c.).
Keywo ds: ly ash; physical emedia ion; abandoned mines; pollu ion; wa e con amina ion
1. In oduc ion
Indus ial p ocesses can gene a e signi ican quan i ies o was e, some o which pose
long- e m en i onmen al isks [
1
]. Among hese byp oduc s, ly ash—a esidue om coal
combus ion in he mal powe plan s— ep esen s a majo challenge due o i s olume and
chemical composi ion. Globally, coal- i ed powe plan s p oduce eno mous quan i ies o
coal combus ion esidues annually [
2
], wi h ly ash cons i u ing a signi ican p opo ion [
3
].
While a po ion o his ma e ial is u ilized in applica ions such as aw ma e ials o he
cons uc ion and indus ial sec o s [
4
–
7
], subs an ial amoun s a e disposed o in land ills
due o a low economic alue o limi ed ma ke demand [
8
,
9
], c ea ing isks o wa e and
soil con amina ion [
10
]. Some o he cu en applica ions include he usage o ly ash o
conc e e p oduc ion [
11
], was e s abiliza ion [
12
], oad basemen ma e ial [
13
], cemen
clinke s [
14
], and geopolyme conc e e applica ions [
15
]. Howe e , he e is s ill a lack o
new uses o ma ch he cu en su plus.
On he o he hand, po en ially oxic elemen s (PTEs) such as me cu y and a senic can
be ound in abandoned indus ial and mining si es. Many o hese si es, which ceased ope -
a ion be o e he ad en o s ingen en i onmen al egula ions, became pe sis en sou ces
o con amina ion [
16
,
17
]. Hea y me als like me cu y, due o hei oxicological signi icance
En i onmen s 2025,12, 107 h ps://doi.o g/10.3390/en i onmen s12040107
En i onmen s 2025,12, 107 2 o 30
and bio-accumula i e na u e [
18
], a e o pa icula conce n. Me cu y con amina ion a ec s
wa e bodies, en e ing aqua ic ecosys ems and, ul ima ely, he ood chain, posing se e e
isks o human heal h and biodi e si y [19,20].
T adi ional emedia ion s a egies o me cu y and o he hea y me als include soil
washing, s abiliza ion, and chemical immobiliza ion [
21
]. Howe e , hese me hods a e o -
en associa ed wi h high cos s, complex logis ics, and addi ional en i onmen al dis up ions.
Fly ash has eme ged as a p omising cos -e ec i e app oach wi h adequa e physicochemical
p ope ies and low hyd aulic conduc i i y [
22
–
24
]. I s use as a ba ie ma e ial in capping
and encapsula ion sys ems has shown po en ial o mi iga ing leacha e gene a ion, p e-
en ing con amina ion o g oundwa e and su ace wa e , and helping in he physical and
chemical s abiliza ion o he con amina ed a ea [
25
]. The applica ion o ly ash in en i on-
men al emedia ion aligns wi h he p inciples o sus ainabili y and ci cula economy [
26
].
U ilizing ly ash no only add esses was e managemen challenges bu also educes he
demand o i gin ma e ials [27,28].
This s udy builds on ea lie wo k by Rod íguez e al. [
29
], which in es iga ed he use o
ly ash o a oid gaseous me cu y emission. The cu en esea ch ocuses on e alua ing he
po en ial o ly ash as a su ace capping ma e ial o p e en ainwa e con amina ion in deb is.
Fi s , he si e in which he s udy was ca ied ou is b ie ly desc ibed. Then, labo a o y and
ield-scale es s a e desc ibed. A model o explain he in il a ion o he ainwa e h ough
he ash is de eloped and pa ame e s in luencing he p ocess (po osi y and pe meabili y o
ashes and e apo anspi a ion o p ecipi a ion a e) a e iden i ied. Then, di e en en i onmen al
scena ios a e explo ed o de e mine he long- e m pe o mance o he solu ion p oposed.
2. Si e Desc ip ion and Rainwa e Con amina ion
2.1. Abandoned Me cu y Mining Facili ies
This s udy was pe o med a he La So e aña mining si e composed o an unde g ound
mine and a smel ing acili y (Figu e 1). The mine aliza ion exploi ed in he La So e aña
mine is an epigene ic- ype o e deposi ha o igina ed om low- empe a u e hyd o he mal
solu ions. The pa agenesis o he o e deposi is cons i u ed by cinnaba , o pimen , ealga , py i e
(usually wi h high concen a ions o As), a senopy i e, ma casi e, and pa a ealga , in a gangue
o qua z and calci e. Na i e Hg has been obse ed o be associa ed wi h o ganic ma e - ich
limes ones [
24
]. Me cu y o e was ex ac ed using a oom and pilla me hod and, subsequen ly,
p ocessed and eco e ed a he su ace ins alla ions om he 19 h cen u y up o he 1970s.
A e wa d, he complex was abandoned wi hou mo phological es o a ion o any p e en i e
measu e o a oid me cu y con amina ion. The me allu gical plan buildings we e demolished
in 1989 and deb is om he demoli ion, which has a high me cu y con en [
29
], emained on
si e (Figu e 1). I is a he e ogeneous was e, a mix u e o demoli ion deb is om he buildings
and conc e e s uc u es ha suppo ed he oas ing u naces, while he o he was e is ound in
chimneys and duc s (dus , soo , e c.).
En i onmen s2025,12,xFORPEERREVIEW3o 32



Figu e1.Facili iesinea ly1970sanddemolisheds uc u eswi hinsi udeb isin2017.
2.2.P ecipi a ionandE apo anspi a ionin heS udyA ea
Figu e2showsamapo Spainwi h heloca iono As u ias.Thea e ageannual
ain allinAs u iasisabou 1200mm/yea [30].In hecen ala ea,whe e hepa icula 
cases udyisloca ed, hea e ageannual ain allissomewha lowe ,800–1000mm/yea ,
whilei  eaches1400–1800mm/yea insomemoun ainousa easo  heCan ab ianMoun-
ains(Figu e2).Consequen ly,e apo anspi a ionislowcompa ed oo he  egions,wi h
anannuala e ageo abou 700mm/yea [31].In hecen ala eaissligh lyhighe ,700–
800mm/yea ,while, u he sou h owa ds heCan ab ianMoun ains,i dec eases o600–
700mm/yea (Figu e3).Ascanbeseen, hep ecipi a ionPande apo anspi a ionET
0

a equi ebalancedin hea eas udied,wi h heET
0
/P a iobeingclose ouni y.

Figu e2.A e ageannualp ecipi a ioninSpainandAs u iasmodi ied omChaza ae al.[30].

Figu e3.A e ageannuale apo anspi a ioninSpainandAs u ias,modi ied omSanchoe al.
[31].
Figu e 1. Facili ies in ea ly 1970s and demolished s uc u es wi h in si u deb is in 2017.
En i onmen s 2025,12, 107 3 o 30
2.2. P ecipi a ion and E apo anspi a ion in he S udy A ea
Figu e 2shows a map o Spain wi h he loca ion o As u ias. The a e age annual ain all
in As u ias is abou 1200 mm/yea [
30
]. In he cen al a ea, whe e he pa icula case s udy is
loca ed, he a e age annual ain all is somewha lowe , 800–1000 mm/yea , while i eaches
1400–1800 mm/yea in some moun ainous a eas o he Can ab ian Moun ains (Figu e 2).
Consequen ly, e apo anspi a ion is low compa ed o o he egions, wi h an annual a e age o
abou 700 mm/yea [
31
]. In he cen al a ea is sligh ly highe , 700–800 mm/yea , while, u he
sou h owa ds he Can ab ian Moun ains, i dec eases o 600–700 mm/yea (Figu e 3). As can
be seen, he p ecipi a ion P and e apo anspi a ion ET
0
a e qui e balanced in he a ea s udied,
wi h he ET0/P a io being close o uni y.
En i onmen s2025,12,xFORPEERREVIEW3o 32



Figu e1.Facili iesinea ly1970sanddemolisheds uc u eswi hinsi udeb isin2017.
2.2.P ecipi a ionandE apo anspi a ionin heS udyA ea
Figu e2showsamapo Spainwi h heloca iono As u ias.Thea e ageannual
ain allinAs u iasisabou 1200mm/yea [30].In hecen ala ea,whe e hepa icula 
cases udyisloca ed, hea e ageannual ain allissomewha lowe ,800–1000mm/yea ,
whilei  eaches1400–1800mm/yea insomemoun ainousa easo  heCan ab ianMoun-
ains(Figu e2).Consequen ly,e apo anspi a ionislowcompa ed oo he  egions,wi h
anannuala e ageo abou 700mm/yea [31].In hecen ala eaissligh lyhighe ,700–
800mm/yea ,while, u he sou h owa ds heCan ab ianMoun ains,i dec eases o600–
700mm/yea (Figu e3).Ascanbeseen, hep ecipi a ionPande apo anspi a ionET
0

a equi ebalancedin hea eas udied,wi h heET
0
/P a iobeingclose ouni y.

Figu e2.A e ageannualp ecipi a ioninSpainandAs u iasmodi ied omChaza ae al.[30].

Figu e3.A e ageannuale apo anspi a ioninSpainandAs u ias,modi ied omSanchoe al.
[31].
Figu e 2. A e age annual p ecipi a ion in Spain and As u ias modi ied om Chaza a e al. [30].
En i onmen s2025,12,xFORPEERREVIEW3o 32



Figu e1.Facili iesinea ly1970sanddemolisheds uc u eswi hinsi udeb isin2017.
2.2.P ecipi a ionandE apo anspi a ionin heS udyA ea
Figu e2showsamapo Spainwi h heloca iono As u ias.Thea e ageannual
ain allinAs u iasisabou 1200mm/yea [30].In hecen ala ea,whe e hepa icula 
cases udyisloca ed, hea e ageannual ain allissomewha lowe ,800–1000mm/yea ,
whilei  eaches1400–1800mm/yea insomemoun ainousa easo  heCan ab ianMoun-
ains(Figu e2).Consequen ly,e apo anspi a ionislowcompa ed oo he  egions,wi h
anannuala e ageo abou 700mm/yea [31].In hecen ala eaissligh lyhighe ,700–
800mm/yea ,while, u he sou h owa ds heCan ab ianMoun ains,i dec eases o600–
700mm/yea (Figu e3).Ascanbeseen, hep ecipi a ionPande apo anspi a ionET
0

a equi ebalancedin hea eas udied,wi h heET
0
/P a iobeingclose ouni y.

Figu e2.A e ageannualp ecipi a ioninSpainandAs u iasmodi ied omChaza ae al.[30].

Figu e3.A e ageannuale apo anspi a ioninSpainandAs u ias,modi ied omSanchoe al.
[31].
Figu e 3. A e age annual e apo anspi a ion in Spain and As u ias, modi ied om Sancho e al. [
31
].
2.3. Rainwa e Con amina ion
One o he mo e se ious p oblems in si es highly con amina ed wi h me cu y and
a senic is he con amina ion o ainwa e p oducing leacha es wi h a high concen a ion o
me cu y and a senic. As has been said be o e, in he s udied a ea, he e was he e ogeneous
was e o med on one side by he was e p oduced in he demoli ion o s uc u es, and on he
o he side, by he was e emaining in oas ing u naces, chimneys, and duc s like dus , soo ,
e c. In he ollowing, we will use was e o deb is indis inc ly o denomina e his mix u e.
Be o e building he ea men cell, his was e occupied h ee di e en zones o abou
200 m
2
each, as i is shown in Figu e 4. In zones 1 and 2, bo h on he lowe pla o m, he e
was was e ela ed o s uc u es ha suppo ed he oas e , while in zone 3, on he uppe
pla o m, he e was was e ela ed o se e al duc s o he chimneys.
En i onmen s 2025,12, 107 4 o 30
En i onmen s2025,12,xFORPEERREVIEW4o 32


2.3.Rainwa e Con amina ion
Oneo  hemo ese iousp oblemsinsi eshighlycon amina edwi hme cu yand
a senicis hecon amina iono  ainwa e p oducingleacha eswi hahighconcen a ion
o me cu yanda senic.Ashasbeensaidbe o e,in hes udieda ea, he ewashe e oge-
neouswas e o medononesideby hewas ep oducedin hedemoli iono s uc u es,
andon heo he side,by hewas e emainingin oas ing u naces,chimneys,andduc s
likedus ,soo ,e c.In he ollowing,wewillusewas eo deb isindis inc ly odenomina e
hismix u e.Be o ebuilding he ea men cell, hiswas eoccupied h eediffe en zones
o abou 200m
2
each,asi isshowninFigu e4.Inzones1and2,bo hon helowe pla -
o m, he ewaswas e ela ed os uc u es ha suppo ed he oas e ,whileinzone3,on
heuppe pla o m, he ewaswas e ela ed ose e alduc s o hechimneys.
A o alo 11solidsampleswe e aken:4inzone1,4inzone2,and3inzone3.The
dis ibu ionwasapp oxima elyuni o m,andsamplingwasca iedou by akingsamples
o e e y ypeo was e.Tobuild he ea men cell, hewas eo zone1and,pa ially, he
was eo zone3we emo ed ozone2.Thismix u eo was ewasla e pu in he ea men 
cellandco e edwi hashes.Consequen ly, he11solidsamplescouldbeconside ed ep-
esen a i eo  hehe e ogeneouswas eo zone2,whichwas inally ea ed.
On heo he hand,11leacha esampleswe e akeninzone2du inga ainype iod
a e collapsing hewallandmixing hedeb is.
Figu e5shows heloca iono  he11solid( ed)and11leacha e(blue)sampling
poin s.I mus beconside ed ha solidandleacha esampleswe eno  akena  hesame
ime.Ne e heless, he imeelapsedbe weensolidsamplingandleacha esamplingwas
onlya ewmon hs,andnosigni ican changesin hesolidma e ialsoccu ed.

Figu e4.Diffe en  ypeso was eanddeb iso me allu gicalplan demoli ionin hes udya ea.

Figu e5.Loca iono  he11solidsamplingpoin s( ed)andleacha esamplingpoin s(blue).
Figu e 4. Di e en ypes o was e and deb is o me allu gical plan demoli ion in he s udy a ea.
A o al o 11 solid samples we e aken: 4 in zone 1, 4 in zone 2, and 3 in zone 3. The
dis ibu ion was app oxima ely uni o m, and sampling was ca ied ou by aking samples
o e e y ype o was e. To build he ea men cell, he was e o zone 1 and, pa ially, he
was e o zone 3 we e mo ed o zone 2. This mix u e o was e was la e pu in he ea men
cell and co e ed wi h ashes. Consequen ly, he 11 solid samples could be conside ed
ep esen a i e o he he e ogeneous was e o zone 2, which was inally ea ed.
On he o he hand, 11 leacha e samples we e aken in zone 2 du ing a ainy pe iod
a e collapsing he wall and mixing he deb is.
Figu e 5shows he loca ion o he 11 solid ( ed) and 11 leacha e (blue) sampling poin s.
I mus be conside ed ha solid and leacha e samples we e no aken a he same ime.
Ne e heless, he ime elapsed be ween solid sampling and leacha e sampling was only a
ew mon hs, and no signi ican changes in he solid ma e ials occu ed.
En i onmen s2025,12,xFORPEERREVIEW4o 32


2.3.Rainwa e Con amina ion
Oneo  hemo ese iousp oblemsinsi eshighlycon amina edwi hme cu yand
a senicis hecon amina iono  ainwa e p oducingleacha eswi hahighconcen a ion
o me cu yanda senic.Ashasbeensaidbe o e,in hes udieda ea, he ewashe e oge-
neouswas e o medononesideby hewas ep oducedin hedemoli iono s uc u es,
andon heo he side,by hewas e emainingin oas ing u naces,chimneys,andduc s
likedus ,soo ,e c.In he ollowing,wewillusewas eo deb isindis inc ly odenomina e
hismix u e.Be o ebuilding he ea men cell, hiswas eoccupied h eediffe en zones
o abou 200m
2
each,asi isshowninFigu e4.Inzones1and2,bo hon helowe pla -
o m, he ewaswas e ela ed os uc u es ha suppo ed he oas e ,whileinzone3,on
heuppe pla o m, he ewaswas e ela ed ose e alduc s o hechimneys.
A o alo 11solidsampleswe e aken:4inzone1,4inzone2,and3inzone3.The
dis ibu ionwasapp oxima elyuni o m,andsamplingwasca iedou by akingsamples
o e e y ypeo was e.Tobuild he ea men cell, hewas eo zone1and,pa ially, he
was eo zone3we emo ed ozone2.Thismix u eo was ewasla e pu in he ea men 
cellandco e edwi hashes.Consequen ly, he11solidsamplescouldbeconside ed ep-
esen a i eo  hehe e ogeneouswas eo zone2,whichwas inally ea ed.
On heo he hand,11leacha esampleswe e akeninzone2du inga ainype iod
a e collapsing hewallandmixing hedeb is.
Figu e5shows heloca iono  he11solid( ed)and11leacha e(blue)sampling
poin s.I mus beconside ed ha solidandleacha esampleswe eno  akena  hesame
ime.Ne e heless, he imeelapsedbe weensolidsamplingandleacha esamplingwas
onlya ewmon hs,andnosigni ican changesin hesolidma e ialsoccu ed.

Figu e4.Diffe en  ypeso was eanddeb iso me allu gicalplan demoli ionin hes udya ea.

Figu e5.Loca iono  he11solidsamplingpoin s( ed)andleacha esamplingpoin s(blue).
Figu e 5. Loca ion o he 11 solid sampling poin s ( ed) and leacha e sampling poin s (blue).
The sampling p ocedu e was he same as ollowed in p e ious s udies, including
om Ayala and Fe nández [
32
]. Samples o was e o abou 0.5 kg we e aken in he s udy
a ea. The was e was d ied a oom empe a u e in o de o minimize he loss o ola ile
con aminan s. Then, i was c ushed using a jaw c ushe and subsequen ly sie ed h ough a
2 mm ape u e sc een be o e use. A ac ion o he solids was milled o a pa icle size o
<100 µm o de e mine he chemical composi ion and pH.
Liquid samples o 10 cm
3
we e aken om uno wa e o wa e accumula ed in
puddles wi hin he deb is a ea. The wa e samples we e in oduced in ubes adding
Hg- ee acid and s o ed in a e ige a o un il he analysis.
X- ay luo escence (Phillips PW2404) was used o de e mine he majo elemen s,
while he quan i ica ion o he ace elemen s was pe o med by mass spec ome y wi h
induc i ely coupled plasma (ICP-MS Agilen 7700, Agilen Technologies, San a Cla a, CA,
USA) p io o dissolu ion wi h aqua egia using an An on Paa 3000 mic owa e sys em,
Os ilde n-Scha nhausen, Ge many.
En i onmen s 2025,12, 107 5 o 30
The pH alue o he samples was measu ed using an Eu ech pH 2700 m, Eu hec,
Syngapo e) in a 1:2.5 (w/ ) sample/wa e mix u e a e equilib a ing o 30 min. In addi ion,
he con en o dissol ed Ca, Mg, and Fe in he supe na an liquid om he byp oduc s was
de e mined by a omic abso p ion spec oscopy (Pe kin Elme AAnalys 200, Shel on, CT, USA).
The mine al composi ion was analyzed by X- ay di ac ion (PANaly ical X
′
Pe P o MPD
wi h CuK
α
adia ion, Mal e n Panaly ical, Mal e n, UK). The di ac ome e was ope a ed a
45 kV and 40 mA, o e he ange o 2θ om 5◦ o 90◦, wi h a de ec o speed o 1◦/min.
The o al a senic p esen in he was e can be ound in di e en o ms: some a e soluble
in wa e (a sena e and weakly adso bed species), soluble in Na
2
HPO
4
(s ongly adso bed
on o mine al su aces), soluble in NH
4
F (associa ed wi h Al oxyhyd oxides), soluble in
Na
4
P
2
O
7
(As bound o o ganic ma e ), soluble in ammonium oxala e/oxalic acid (As
associa ed wi h amo phous Fe oxyhyd oxides), soluble in bica bona e and asco bic acid
(associa ed wi h poo ly c ys alline Fe hyd oxide), and acid diges ion in a mic owa e o en
wi h HCl and HNO3(As cop ecipi a es wi h e ac o y mine als) [32].
Table 1p o ides he analysis esul s o he 11 samples and he ep esen a i e s a is ical
alues o he concen a ion o a senic and me cu y in he soil and in he leacha e p oduced.
Ex emely high concen a ions o As and Hg a e ela ed o s upp and soo om oas ing
u naces and lue dus in chimneys. The bigges As concen a ion in sample 8 could be
conside ed un ealis ic due o an e o du ing sampling o labo a o y analysis. Ne e heless,
e en i sample 8 is no conside ed in he calcula ion, he a e age concen a ion o As and
Hg a e 89.41 and 16.39 g/kg, which can also be conside ed ex emely high.
Table 1. Concen a ion o As and Hg in solid ma e ials and leacha es.
Solid Sample As (g/kg) Hg (g/kg) Leacha e
Sample
As
(mg/L)
Hg
(µg/L)
1 84.28 14.64 1 94.44 40.69
2 254.07 41.50 2 83.87 31.49
3 46.18 6.35 3 203.33 14.94
4 54.80 34.69 4 160.98 14.34
5 44.42 7.49 5 137.82 10.14
6 44.12 22.19 6 188.63 8.47
7 29.64 19.05 7 246.11 8.03
8 603.34 25.29 8 820.38 12.88
9 210.00 16.00 9 732.89 12.24
10 48.00 0.53 10 571.92 16.05
11 98.59 1.45 11 332.06 12.51
N 11 11 N 11 11
Min 29.64 0.53 Min 83.87 8.03
Max 603.34 41.50 Max 820.38 40.69
A e age 136.13 17.20 A e age 324.77 16.53
SD 172.07 13.17 SD 261.85 10.20
An analysis o a ep esen a i e sample o his was e was ca ied ou in [
32
] in which
he physical–chemical cha ac e is ics we e de e mined (Table 2). In he p esen s udy, we
ocused only on me cu y and a senic.

En i onmen s 2025,12, 107 6 o 30
Table 2. Chemical composi ion o was e a e Ayala and Fe nández [32].
SiO2(w %) 61.7 Hg (mg/kg) 34,691
Fe2O3(w %) 7.1 As (mg/kg) 54,801
MgO (w %) - Zn (mg/kg) 0.03
K2O (w %) 0.8 Cu (mg/kg) 420
Al2O3(w %) 7.1 C (mg/kg) 920
CaO (w %) 3.9 Pb (mg/kg) 3400
SO3(w %) 7.2 Ni (mg/kg) 0.02
TiO2(w %) 0.55 Cd (mg/kg) 0.01
MnO (w %) 0.02 pH 5.1
A cha ac e is ic o his deb is is i s abili y o p oduce con amina ed leacha e. When
a wa e low comes in o con ac wi h hem, some o ha a senic and me cu y passes in o
he wa e , p oducing he leacha e. This has been obse ed du ing ainy pe iods. Fo
example, a e 2 weeks o hea y ain whe e 115.4 mm accumula ed o e 14 days, wi h an
a e age empe a u e o 8.4
◦
C du ing he pe iod, a wa e sample was aken om a ain
puddle o med on he deb is. The a senic and me cu y concen a ions in he wa e eached
203.33 mg/L and 14.94
µ
g/L, espec i ely. Subsequen ly, he ain all dec eased, and a
week la e (wi h a mo e mode a e p ecipi a ion o only 15.0 mm), he a senic and me cu y
concen a ions in he puddle a ied in e sely. While a senic concen a ion inc eased o
246.11 mg/L (possibly due o wa e e apo a ion), he me cu y concen a ion dec eased o
8.03
µ
g/L ( o example, due o p ecipi a ion), om which is in e ed ha he ainy/d y
pe iods in luence.
This con amina ion is p ac ically ins an aneous; he wa e does no need o be in
con ac wi h he deb is o a long ime, and i can each much highe con amina ion le els
a e pe iods wi hou ain. Due o d ough condi ions, a es was conduc ed in July, whe e
he deb is had o be wa e ed wi h simula ed ainwa e . A ha ime, he deb is was
comple ely d y. O e he las wo weeks, he a e age empe a u e had been 20.3
◦
C, and
he accumula ed ain all had only eached 3.8 mm, wi h he peculia i y ha i did no
ain on he p e ious 5 days. The wa e was in con ac wi h he deb is o only a ew
minu es, ye he a senic concen a ion in he wa e eached 94,436 and 83,867
µ
g/L, and he
me cu y concen a ion eached 40.69 and 31.49
µ
g/L. These alues a e in acco dance wi h
some es s ca ied ou in he labo a o y [
32
] in which he leacha e ob ained acco ding o
s anda d es [
33
] has a pH o 5.24, 196.8 mV edox po en ial, and 2.2 mS/cm conduc i i y.
The analysis pe o med by mass spec ome y wi h ICP-MS shows a high con en in As
(59,056
µ
g/L) and much smalle amoun s o Hg (0.98
µ
g/L), o o he PTEs such as Ni, Zn,
Cu, Cd, and Pb. Abou 1% o he a senic p esen in he was e was solubilized du ing he
es . Al hough he esidue om me cu y p oduc ion has high concen a ions o As and Hg,
54.80 g/Kg and 34.69 g/Kg, espec i ely, hei mobili y is small.
The a e age alues o Hg concen a ion in bo h soils and leacha es a e in ag eemen
wi h he esul s ound by Vaselli e al. [34] o a la ge Hg mine si e in cen al I aly.
3. Ma e ials and Me hods
3.1. Wa e Balance
The solu ion p oposed, capping was e wi h ashes, is based on he hyd aulic p ope ies
o ly ashes. I can be analyzed h ough he wa e balance in he soil ep esen ed by he
ollowing o mula:
P=E+ETR +R+I (1)
En i onmen s 2025,12, 107 7 o 30
P is he p ecipi a ion, he ainwa e o a gi en pe iod. A pa o his ainwa e does
no pene a e in soil and becomes in he uno E. Ano he pa pene a es he soil, bu
i e u ns o he a mosphe e as apo h ough he ac ual e apo anspi a ion ETR. The
emaining wa e is s o ed in he uppe zone o he soil as echa ge R. When his zone is
sa u a ed, he wa e begins o in il a e I.
I he ashes ha e su icien ly high echa ge capaci y and su icien ly low pe meabili y
o acili a e e apo anspi a ion, hen he wa e in il a ion will be educed and, conse-
quen ly, i will no be con amina ed. To analyze his p ocess, a wa e balance model, which
is desc ibed in de ail in Appendix A, has been de eloped.
3.2. Tes s’ Desc ip ion
3.2.1. Labo a o y Tes s
The ashes used in his s udy come om a con en ional coal-bu ning powe plan ,
which uses ha d coal, mo e conc e ely, subbi uminous coal. Physical and chemical p op-
e ies we e de e mined om labo a o y es s. The chemical composi ion o ashes was
de e mined and p esen ed in ano he wo k p e iously [
33
]. Table 3summa izes he esul s.
Table 3. Chemical composi ion o ly ash used in he s udy a e Ayala and Fe nández [32].
SiO2(w %) 56.5 Hg (mg/kg) 2
Fe2O3(w %) 9.5 As (mg/kg) 59
MgO (w %) 0.9 Zn (mg/kg) 90
K2O (w %) 2.61 Cu (mg/kg) 57
Al2O3(w %) 23.9 C (mg/kg) 83.6
CaO (w %) 3.4 Pb (mg/kg) 16
SO3(w %) 2.04 Ni (mg/kg) 65.4
TiO2(w %) 0.85 Cd (mg/kg) 1.84
MnO (w %) - pH 10.9
Howe e , in his case, he ashes a e s udied as soil capable o e aining mois u e om
ain all, a use ha is di e en om he o he uses gi en abo e. Labo a o y es s ha e been
ca ied ou o de e mine hose cha ac e is ics ha in luence he ashes’ capaci y o abso b
wa e and s o e i up o a limi (po osi y echa ge) and o ansmi wa e a a cons an low
a e (hyd aulic conduc i i y). Table 4shows he esul s.
Table 4. Physical p ope ies o he ashes used in he s udy.
Pa ame e Value S anda d
Bulk densi y (g/cm3)0.96 [35]
Ac ual pa icle densi y (g/cm3)2.38 [36]
Mois u e con en o he ash in s ockpile (% by weigh ) 1.24 [37]
Po osi y (% by olume) 58.8 [35]
Hyd aulic conduc i i y (m/s) 2.83 ×10−6[38]
I should be no ed ha he de e mina ion o he po osi y is made by aising he
empe a u e o 106
◦
C. Howe e , unde no mal condi ions o ash deposi ion as supplied
by he he mal powe plan , he ash has a e y low mois u e con en , in he o de o
1.24% (by weigh ). The e o e, assuming no mal condi ions in which he s ockpiled ashes
a e p o ec ed om ain, he humidi y will always be e y low, and we can ake his
alue as a e e ence. An amoun o 1.24% by weigh is app oxima ely equi alen o
En i onmen s 2025,12, 107 8 o 30
3.0% by olume. Tha means ha he maximum alue o po osi y o be conside ed unde
no mal condi ions will no be 58.8% (by olume), bu app oxima ely 55.8% (by olume),
since he e is 3% mois u e ha is always p esen in he ash.
The alue o bo h he physical and chemical p ope ies is wi hin he ange o his
ype o coal acco ding o p e ious s udies ha compa ed ashes om di e en powe plan s
in Spain [39,40].
Gi en he ease o de e mining he p ope ies o in e es , scale expe imen s we e also
ca ied ou , which we e e y use ul and p o ided g ea in o ma ion since hey could be
ca ied ou bo h in he labo a o y and in he ield.
In he labo a o y expe imen s, anspa en plas ic es ubes wi h a diame e o 5 cm
we e used. Deposi ing 200 g o d y ash in i (wi h 1–2% humidi y, as supplied by he
he mal powe plan ). This amoun o ash eaches a heigh o abou 10 cm, which, as will
be jus i ied la e , is he heigh aken as a e e ence. Wa e was hen added, obse ing how i
pene a ed he ash and slowly descended, as shown in Figu e 6A. The p ocess was epea ed
un il i was sa u a ed wi h wa e . A his poin , wa e began o low om he bo om and
he cons an low a e h ough he es ube was measu ed, as in Figu e 6B.
En i onmen s2025,12,xFORPEERREVIEW8o 32


condi ionswillno be58.8%(by olume),bu app oxima ely55.8%(by olume),since
he eis3%mois u e ha isalwaysp esen in heash.
Table4.Physicalp ope ieso  heashesusedin hes udy.
Pa ame e ValueS anda d
Bulkdensi y(g/cm
3
)0.96[35]
Ac ualpa icledensi y(g/cm
3
)2.38[36]
Mois u econ en o  heashins ockpile(%byweigh )1.24[37]
Po osi y(%by olume)58.8[35]
Hyd aulicconduc i i y(m/s)2.83×10
−6
[38]
The alueo bo h hephysicalandchemicalp ope iesiswi hin he ange o  his
ypeo coalacco ding op e iouss udies ha compa edashes omdiffe en powe 
plan sinSpain[39,40].
Gi en heeaseo de e mining hep ope ieso in e es ,scaleexpe imen swe ealso
ca iedou ,whichwe e e yuse ulandp o idedg ea in o ma ionsince heycouldbe
ca iedou bo hin helabo a o yandin he ield.
In helabo a o yexpe imen s, anspa en plas ic es  ubeswi hadiame e o 5cm
we eused.Deposi ing200go d yashini (wi h1–2%humidi y,assuppliedby he
he malpowe plan ).Thisamoun o ash eachesaheigh o abou 10cm,which,aswill
bejus i iedla e ,is heheigh  akenasa e e ence.Wa e was henadded,obse inghow
i pene a ed heashandslowlydescended,asshowninFigu e6A.Thep ocesswas e-
pea edun ili wassa u a edwi hwa e .A  hispoin ,wa e began o low om hebo -
omand hecons an  low a e h ough he es  ubewasmeasu ed,asinFigu e6B.

Figu e6.Wa e pene a ingbe o esa u a ion(A)andwa e in il a iona e sa u a ion(B).
3.2.2.Full-ScaleTes sin heAbandonedMineFacili ies
Inaddi ion o helabo a o y es s,i wasnecessa y oca you a ull-scale es  o
check hebeha io o  lyashin he ealcondi ionsinwhichi isapplied.
The ubblewas emo ed obuild heimpe meablecelland hen eloca edinsidei 
(Figu e7A).Thecellwasapoolo 600m
2
limi edby hehighwalls ha p e iouslyexis ed
andnewsmallconc e ewallso 1.20mhigh(Figu e7B).Thewo kwasca iedou unde 
e y iskycondi ions,because heHgconcen a ionin heai was e yhigh,andasa e y
p o ocol o wo kingin hisa eahad obede elopedbyRod ígueze al.[29].
Flyashesa econside edno  obedange ouswas eandslagsa econside edanine 
ma e ial, ollowingSpanishlegisla ion,and hei useiscommoninci ilwo ks o  oad
illingo  oadslopes.Bo hby-p oduc swe echa ac e ized oa oid hein oduc iono a
newconce n o  helocalen i onmen : heconcen a iono HgandAsin heseby-p od-
uc swasnegligibleincompa isonwi h hedemoli ionwas e[32].On heo he hand,
companies ha supplyby-p oduc sha eexpe ienceinmanagingla geland illsinwhich
Figu e 6. Wa e pene a ing be o e sa u a ion (A) and wa e in il a ion a e sa u a ion (B).
3.2.2. Full-Scale Tes s in he Abandoned Mine Facili ies
In addi ion o he labo a o y es s, i was necessa y o ca y ou a ull-scale es o
check he beha io o ly ash in he eal condi ions in which i is applied.
The ubble was emo ed o build he impe meable cell and hen eloca ed inside i
(Figu e 7A). The cell was a pool o 600 m
2
limi ed by he high walls ha p e iously exis ed
and new small conc e e walls o 1.20 m high (Figu e 7B). The wo k was ca ied ou unde
e y isky condi ions, because he Hg concen a ion in he ai was e y high, and a sa e y
p o ocol o wo king in his a ea had o be de eloped by Rod íguez e al. [29].
En i onmen s2025,12,xFORPEERREVIEW9o 32


heseby-p oduc sa edeposi ed;consequen ly, hei po en ialeffec son heen i onmen ,
o  e ylowin ensi y,a ewellknown.
Asdesc ibedbyRod ígueze al.[29], heashcoa ingwaseffec i eins oppingme -
cu yemissionsin o heai .E enin hesamewo k,i was epo ed ha i p e en edwa e 
con amina ion.Ne e heless, he ac o s ha in luencewa e in il a ion(ashpe meabili y
andpo osi yandsomeclima iccondi ions)a es udiedindep hin hep esen wo k.

Figu e7.Co e ingdemoli iondeb is(A)andloca iono samplingpoin sin heashcapping(B).
Fieldda acollec ionwasconduc ed o  h eemon hs, om4Ap il2024 o4July
2024,whichcanbeconside edano mal ain all egime(95%o  hea e ageo  helas  en
yea s,a e AEMET).Se e al a iableswe emeasu eddi ec lyin he ield(Figu e8).P e-
cipi a ionwasmeasu edbydi ec  eadingina aingauge.Sola  adia ionwasdeduced
omul a iole  adia iondi ec lymeasu edwi hapo ablePho o- adiome e HD2102.1
wi hasenso LP471-PA omDel aOhmmanu ac u e ,CassellediSel azzano,I aly.Am-
bien  empe a u ewasmeasu edwi hacon en ional he mome e o by empe a u esen-
so sSensi TG8P 1000/3850,Os a a,CzechRepublic,coupled oaCome da alogge ,
Come Sys ems,Rozno podRadhos em,CzechRepublic.Wi h hese a iables,e apo-
anspi a ioncanbeempi icallyde e mined(usingTu c’slawo simila ),and, oge he 
wi h hep ecipi a ion, he heo e icalwa e balancein heashescanbeempi icallyes ab-
lished.

Figu e8.Moni o ing ainwa e , empe a u e,andul a iole  adia ion.
On heo he hand,sampleso  heashcoa ingwe epe iodically aken o mois u e
de e mina ion(Figu e9).In hiscase,i wasimpo an  o  heashes obecomesa u a ed
inwa e wi h heno malle elo p ecipi a iondu ing hesemon hs.The e o e,asmall
a eawasp epa edby educingi s hickness oonly100mm hick(Figu e7B).A o alo 
27ashsampleswe e aken.Samplesconsis edo 33mLo ashandbe ween3and5sam-
pleswe e akena diffe en dep hsbe ween0and100mm.Ashsampleswe ed ieda 
oom empe a u eand hen hewe nesswasde e minedasdesc ibedabo e.
Figu e 7. Co e ing demoli ion deb is (A) and loca ion o sampling poin s in he ash capping (B).
Fly ashes a e conside ed no o be dange ous was e and slags a e conside ed an ine
ma e ial, ollowing Spanish legisla ion, and hei use is common in ci il wo ks o oad
illing o oad slopes. Bo h by-p oduc s we e cha ac e ized o a oid he in oduc ion o
a new conce n o he local en i onmen : he concen a ion o Hg and As in hese by-
p oduc s was negligible in compa ison wi h he demoli ion was e [
32
]. On he o he hand,
En i onmen s 2025,12, 107 9 o 30
companies ha supply by-p oduc s ha e expe ience in managing la ge land ills in which
hese by-p oduc s a e deposi ed; consequen ly, hei po en ial e ec s on he en i onmen ,
o e y low in ensi y, a e well known.
As desc ibed by Rod íguez e al. [
29
], he ash coa ing was e ec i e in s opping me cu y
emissions in o he ai . E en in he same wo k, i was epo ed ha i p e en ed wa e
con amina ion. Ne e heless, he ac o s ha in luence wa e in il a ion (ash pe meabili y
and po osi y and some clima ic condi ions) a e s udied in dep h in he p esen wo k.
Field da a collec ion was conduc ed o h ee mon hs, om 4 Ap il 2024 o 4 July 2024,
which can be conside ed a no mal ain all egime (95% o he a e age o he las en yea s,
a e AEMET). Se e al a iables we e measu ed di ec ly in he ield (Figu e 8). P ecipi a-
ion was measu ed by di ec eading in a ain gauge. Sola adia ion was deduced om
ul a iole adia ion di ec ly measu ed wi h a po able Pho o- adiome e HD2102.1 wi h a
senso LP 471-PA om Del aOhm manu ac u e , Casselle di Sel azzano, I aly. Ambien
empe a u e was measu ed wi h a con en ional he mome e o by empe a u e senso s
Sensi TG8 P 1000/3850, Os a a, Czech Republic, coupled o a Come da alogge , Come
Sys ems, Rozno pod Radhos em, Czech Republic. Wi h hese a iables, e apo anspi a-
ion can be empi ically de e mined (using Tu c’s law o simila ), and, oge he wi h he
p ecipi a ion, he heo e ical wa e balance in he ashes can be empi ically es ablished.
En i onmen s2025,12,xFORPEERREVIEW9o 32


heseby-p oduc sa edeposi ed;consequen ly, hei po en ialeffec son heen i onmen ,
o  e ylowin ensi y,a ewellknown.
Asdesc ibedbyRod ígueze al.[29], heashcoa ingwaseffec i eins oppingme -
cu yemissionsin o heai .E enin hesamewo k,i was epo ed ha i p e en edwa e 
con amina ion.Ne e heless, he ac o s ha in luencewa e in il a ion(ashpe meabili y
andpo osi yandsomeclima iccondi ions)a es udiedindep hin hep esen wo k.

Figu e7.Co e ingdemoli iondeb is(A)andloca iono samplingpoin sin heashcapping(B).
Fieldda acollec ionwasconduc ed o  h eemon hs, om4Ap il2024 o4July
2024,whichcanbeconside edano mal ain all egime(95%o  hea e ageo  helas  en
yea s,a e AEMET).Se e al a iableswe emeasu eddi ec lyin he ield(Figu e8).P e-
cipi a ionwasmeasu edbydi ec  eadingina aingauge.Sola  adia ionwasdeduced
omul a iole  adia iondi ec lymeasu edwi hapo ablePho o- adiome e HD2102.1
wi hasenso LP471-PA omDel aOhmmanu ac u e ,CassellediSel azzano,I aly.Am-
bien  empe a u ewasmeasu edwi hacon en ional he mome e o by empe a u esen-
so sSensi TG8P 1000/3850,Os a a,CzechRepublic,coupled oaCome da alogge ,
Come Sys ems,Rozno podRadhos em,CzechRepublic.Wi h hese a iables,e apo-
anspi a ioncanbeempi icallyde e mined(usingTu c’slawo simila ),and, oge he 
wi h hep ecipi a ion, he heo e icalwa e balancein heashescanbeempi icallyes ab-
lished.

Figu e8.Moni o ing ainwa e , empe a u e,andul a iole  adia ion.
On heo he hand,sampleso  heashcoa ingwe epe iodically aken o mois u e
de e mina ion(Figu e9).In hiscase,i wasimpo an  o  heashes obecomesa u a ed
inwa e wi h heno malle elo p ecipi a iondu ing hesemon hs.The e o e,asmall
a eawasp epa edby educingi s hickness oonly100mm hick(Figu e7B).A o alo 
27ashsampleswe e aken.Samplesconsis edo 33mLo ashandbe ween3and5sam-
pleswe e akena diffe en dep hsbe ween0and100mm.Ashsampleswe ed ieda 
oom empe a u eand hen hewe nesswasde e minedasdesc ibedabo e.
Figu e 8. Moni o ing ainwa e , empe a u e, and ul a iole adia ion.
On he o he hand, samples o he ash coa ing we e pe iodically aken o mois u e
de e mina ion (Figu e 9). In his case, i was impo an o he ashes o become sa u a ed
in wa e wi h he no mal le el o p ecipi a ion du ing hese mon hs. The e o e, a small
a ea was p epa ed by educing i s hickness o only 100 mm hick (Figu e 7B). A o al o
27 ash samples we e aken. Samples consis ed o 33 mL o ash and be ween 3 and 5 samples
we e aken a di e en dep hs be ween 0 and 100 mm. Ash samples we e d ied a oom
empe a u e and hen he we ness was de e mined as desc ibed abo e.
En i onmen s2025,12,xFORPEERREVIEW10o 32



Figu e9.Ashsampling o ashwe nessde e mina ion.
Theashwe nessallowsus oes ablish heac ualwa e balance ha  ookplacein ha 
soilandcanbecompa edwi h hep edic ionmade omo he  a iables.
Themechanismbywhich heac ualin il a ionoccu s,once heashesa esa u a ed
inwa e ,is ela ed o hehyd aulicconduc i i yo  heashes.Tos udy hisp ocessin
mo ede ailand obeable ocompa ei wi hwha wasobse edin helabo a o y,se e al
specimensequal o heonein helabo a o ywe ebu iedin heashes(Figu e10).Hal o 
hespecimenswe e illedwi hcomple elyd yasheswhile heo he hal we e illedwi h
we ashes om hesi ei sel .

Figu e10.Tes  ubeswi hashes ode e mine hein il a ion.
Since heissueo wa e balanceis e yimpo an  o ag icul u e,allcoun iesha e
hei me eo ologicalagencies ha p o ide e y eliableda aon he a iableso in e es .
Fo  his eason, o akeall hein o ma iona ailable om heseagenciesis ecommended.
In hiscase,daybyday,AEMETp o ides he ollowing a iables egis e edin hes a ion
o Rozón(Lena),a ewkilome e saway om hes udya ea:
 Summa yo p ecipi a ion, empe a u e,e c.,hou byhou .
 Summa yo p ecipi a ions, empe a u es,e c.,o  hep e iousdays.
Inaddi ion o his,e e y endays,i issuesabulle inon hewa e balance ha al-
eadygi esa alueo  heaccumula ede apo anspi a iondu ing hose endays.Theaim
is oadjus wi h heda acollec edinsi uap edic ionmodel ha ,using heAEMETda a,
allowsus oes ablish hewa e balancein heashes ha co e  hedeb is.
In hisway, hiswell-knownme hodologycanbeuseda anysi e.I canbeused
di ec lywhe eda a omame eo ologicalagencya ea ailable,whe eas hep ocedu e
p e iouslyexplainedshouldbe ollowedi  he ea enoinsi umeasu emen s.


Figu e 9. Ash sampling o ash we ness de e mina ion.
The ash we ness allows us o es ablish he ac ual wa e balance ha ook place in ha
soil and can be compa ed wi h he p edic ion made om o he a iables.
En i onmen s 2025,12, 107 16 o 30
En i onmen s2025,12,xFORPEERREVIEW17o 32



Figu e20.Clima ologicalda ainzone6:mon hlya e age(A)andcumula i e alues(B).
Thecha ac e is ico  hiszoneis ha 100%o in il a ionisa oided,bu ag ea e 
laye  hicknessis equi ed(Figu e21A).Figu e21Bshows hee olu iono  echa gewi h
a50cm hicklaye .O he zonessimila  ozone6a ezones5,7,and8.In hem, heET
0
/P
a ioiso  heo de o 0.8–1.2.

Figu e21.Wa e in il a ion(A)andwa e  echa ge(B)inzone6.
Finally, helas  ypicalcaseiszone2,whe ein ensi eleadandzincmining ook
place.Theclima eis ypicalo humidSpain, empe a e,wi hou a ulyd yseasonand
wi hamildsumme (C bacco ding oKöppenclassi ica ion).P ecipi a ionexceedse ap-
o anspi a ionp ac icallyallyea (Figu e22A).Accumula edp ecipi a iondu ing he
yea ,a ound1200mm,ismuchhighe  hane apo anspi a ion,a ound700mm,and
e apo anspi a ionin hed ype iodisno enough ocompensa e o  hehumidi ydue
o ain all(Figu e22B).Unde  hesecondi ions, heashlaye mus be a he  hinso ha 
i canexhaus i s echa geby heac iono e apo anspi a ion.Figu e23A ep esen s his
ac bymaking hecu eno cu  heho izon alaxis.Howe e ,i mus besaid ha in he
condi ionso zone2,alaye o only20cm(Figu e23B)allowsless han30%o  he ain-
wa e  oin il a e,p e en ing70%o i  ombecomingcon amina ed.Zones1,2,3,and4
a esimila ,wi h heET
0
/P a io<0.8.
Figu e 20. Clima ological da a in zone 6: mon hly a e age (A) and cumula i e alues (B).
The cha ac e is ic o his zone is ha 100% o in il a ion is a oided, bu a g ea e
laye hickness is equi ed (Figu e 21A). Figu e 21B shows he e olu ion o echa ge wi h a
50 cm hick laye . O he zones simila o zone 6 a e zones 5, 7, and 8. In hem, he ET
0
/P
a io is o he o de o 0.8–1.2.
En i onmen s2025,12,xFORPEERREVIEW17o 32



Figu e20.Clima ologicalda ainzone6:mon hlya e age(A)andcumula i e alues(B).
Thecha ac e is ico  hiszoneis ha 100%o in il a ionisa oided,bu ag ea e 
laye  hicknessis equi ed(Figu e21A).Figu e21Bshows hee olu iono  echa gewi h
a50cm hicklaye .O he zonessimila  ozone6a ezones5,7,and8.In hem, heET
0
/P
a ioiso  heo de o 0.8–1.2.

Figu e21.Wa e in il a ion(A)andwa e  echa ge(B)inzone6.
Finally, helas  ypicalcaseiszone2,whe ein ensi eleadandzincmining ook
place.Theclima eis ypicalo humidSpain, empe a e,wi hou a ulyd yseasonand
wi hamildsumme (C bacco ding oKöppenclassi ica ion).P ecipi a ionexceedse ap-
o anspi a ionp ac icallyallyea (Figu e22A).Accumula edp ecipi a iondu ing he
yea ,a ound1200mm,ismuchhighe  hane apo anspi a ion,a ound700mm,and
e apo anspi a ionin hed ype iodisno enough ocompensa e o  hehumidi ydue
o ain all(Figu e22B).Unde  hesecondi ions, heashlaye mus be a he  hinso ha 
i canexhaus i s echa geby heac iono e apo anspi a ion.Figu e23A ep esen s his
ac bymaking hecu eno cu  heho izon alaxis.Howe e ,i mus besaid ha in he
condi ionso zone2,alaye o only20cm(Figu e23B)allowsless han30%o  he ain-
wa e  oin il a e,p e en ing70%o i  ombecomingcon amina ed.Zones1,2,3,and4
a esimila ,wi h heET
0
/P a io<0.8.
Figu e 21. Wa e in il a ion (A) and wa e echa ge (B) in zone 6.
Finally, he las ypical case is zone 2, whe e in ensi e lead and zinc mining ook place.
The clima e is ypical o humid Spain, empe a e, wi hou a uly d y season and wi h a
mild summe (C b acco ding o Köppen classi ica ion). P ecipi a ion exceeds e apo anspi-
a ion p ac ically all yea (Figu e 22A). Accumula ed p ecipi a ion du ing he yea , a ound
1200 mm, is much highe han e apo anspi a ion, a ound 700 mm, and e apo anspi a ion
in he d y pe iod is no enough o compensa e o he humidi y due o ain all (Figu e 22B).
Unde hese condi ions, he ash laye mus be a he hin so ha i can exhaus i s echa ge
by he ac ion o e apo anspi a ion. Figu e 23A ep esen s his ac by making he cu e
no cu he ho izon al axis. Howe e , i mus be said ha in he condi ions o zone 2, a
laye o only 20 cm (Figu e 23B) allows less han 30% o he ainwa e o in il a e, p e-
en ing 70% o i om becoming con amina ed. Zones 1, 2, 3, and 4 a e simila , wi h he
ET0/P a io < 0.8.

En i onmen s 2025,12, 107 17 o 30
En i onmen s2025,12,xFORPEERREVIEW18o 32



Figu e22.Clima ologicalda ainzone2:mon hlya e age(A)andcumula i e alues(B).

Figu e23.Wa e in il a ion(A)andwa e  echa ge(B)inzone2.
4.2.3.C i e ion oDe e mine heAshLaye Thickness o MinimumIn il a ion
Thep e iousanalysisallows hees ablishmen o ac i e ion ha canbeusedasa
i s app oxima ion oe alua e hesui abili yandlimi so usinganashlaye  oco e an
a eacon amina edwi hPTEs op e en  hecon amina iono  ainwa e .
Thein insicp ope ieso ash,pe meabili yandpo osi y,dono  ullyde e mine he
esul , equi ing heclima ological a iable obeconside ed.AscanbeseeninFigu e24,
heET
0
/P a iocanbeusedasa ep esen a i e a iableo  heclima e,whe eET
0
is he
e apo anspi a ionandPis hep ecipi a ionaccumula edin heyea .Figu e24Acanbe
used ode e mine,app oxima ely, he hicknesso  heashlaye h
u
(cm)necessa y o each
minimumin il a ion.Addi ionally,Figu e24Bgi es he alueo  heminimumin il a-
ionexpec edI(%).
Figu e 22. Clima ological da a in zone 2: mon hly a e age (A) and cumula i e alues (B).
En i onmen s2025,12,xFORPEERREVIEW18o 32



Figu e22.Clima ologicalda ainzone2:mon hlya e age(A)andcumula i e alues(B).

Figu e23.Wa e in il a ion(A)andwa e  echa ge(B)inzone2.
4.2.3.C i e ion oDe e mine heAshLaye Thickness o MinimumIn il a ion
Thep e iousanalysisallows hees ablishmen o ac i e ion ha canbeusedasa
i s app oxima ion oe alua e hesui abili yandlimi so usinganashlaye  oco e an
a eacon amina edwi hPTEs op e en  hecon amina iono  ainwa e .
Thein insicp ope ieso ash,pe meabili yandpo osi y,dono  ullyde e mine he
esul , equi ing heclima ological a iable obeconside ed.AscanbeseeninFigu e24,
heET
0
/P a iocanbeusedasa ep esen a i e a iableo  heclima e,whe eET
0
is he
e apo anspi a ionandPis hep ecipi a ionaccumula edin heyea .Figu e24Acanbe
used ode e mine,app oxima ely, he hicknesso  heashlaye h
u
(cm)necessa y o each
minimumin il a ion.Addi ionally,Figu e24Bgi es he alueo  heminimumin il a-
ionexpec edI(%).
Figu e 23. Wa e in il a ion (A) and wa e echa ge (B) in zone 2.
4.2.3. C i e ion o De e mine he Ash Laye Thickness o Minimum In il a ion
The p e ious analysis allows he es ablishmen o a c i e ion ha can be used as a i s
app oxima ion o e alua e he sui abili y and limi s o using an ash laye o co e an a ea
con amina ed wi h PTEs o p e en he con amina ion o ainwa e .
The in insic p ope ies o ash, pe meabili y and po osi y, do no ully de e mine he
esul , equi ing he clima ological a iable o be conside ed. As can be seen in Figu e 24,
he ET
0
/P a io can be used as a ep esen a i e a iable o he clima e, whe e ET
0
is he
e apo anspi a ion and P is he p ecipi a ion accumula ed in he yea . Figu e 24A can be
used o de e mine, app oxima ely, he hickness o he ash laye h
u
(cm) necessa y o each
minimum in il a ion. Addi ionally, Figu e 24B gi es he alue o he minimum in il a ion
expec ed I (%).
Based on hese esul s, he ollowing can be s a ed:
•
Fo ET
0
/P
≥
1, ash can be used as a ba ie laye , achie ing ze o in il a ion o
wa e p oo ing i he ash laye is hick enough; o alues o ET
0
/P
≈
1, hicknesses
o h = 50–60 cm will be necessa y, which a e d as ically educed o h = 10–15 cm
when ET
0
/P
≥
2. This ensu es ha wa e will no pass h ough he ash laye because
i has su icien maximum echa ge o abso b excess wa e du ing ainy seasons.
Theo e ically, he ash laye could eplace a laye o clay o a laye o clay plus a
HDPE shee .
•
Fo ET
0
/P < 1, he ash laye can no longe be conside ed impe meable. Wi h alues
o ET
0
/P
≈
0.7, a laye o 40–50 cm can be used o s op mo e han 80% o ainwa e .
Howe e , a HDPE wa e p oo ing shee may be equi ed. Wi h ET
0
/P
≈
0.5 alues,
En i onmen s 2025,12, 107 18 o 30
he wo s condi ions, i does no make sense o he ash laye o be e y hick because
i becomes o ally sa u a ed and does no ha e ime o d y; howe e , he ash laye can
s ill be use ul since a hickness o only 10–15 cm allows less han 50% o he ainwa e
o pass h ough.
En i onmen s2025,12,xFORPEERREVIEW19o 32



Figu e24.Es ima iono  henecessa ylaye  hickness(A)andin il a ion(B)basedonET
0
/P.
Basedon hese esul s, he ollowingcanbes a ed:
 Fo ET
0
/P≥1,ashcanbeusedasaba ie laye ,achie ingze oin il a iono wa e -
p oo ingi  heashlaye is hickenough; o  alueso ET
0
/P≈1, hicknesseso h=50–
60cmwillbenecessa y,whicha ed as ically educed oh=10–15cmwhenET
0
/P≥
2.Thisensu es ha wa e willno pass h ough heashlaye becausei hassufficien 
maximum echa ge oabso bexcesswa e du ing ainyseasons.Theo e ically, he
ashlaye could eplacealaye o clayo alaye o clayplusaHDPEshee .
 Fo ET
0
/P<1, heashlaye cannolonge beconside edimpe meable.Wi h alueso 
ET
0
/P≈0.7,alaye o 40–50cmcanbeused os opmo e han80%o  ainwa e .
Howe e ,aHDPEwa e p oo ingshee maybe equi ed.Wi hET
0
/P≈0.5 alues, he
wo s condi ions,i doesno makesense o  heashlaye  obe e y hickbecausei 
becomes o allysa u a edanddoesno ha e ime od y;howe e , heashlaye can
s illbeuse ulsincea hicknesso only10–15cmallowsless han50%o  he ainwa e 
opass h ough.
The i s cu ecanbe ep esen edbyaWeibull unc ion:
ℎ

ℎ

𝑋𝑋

𝑋
󰆒
𝑋

𝑒𝑥𝑝1 𝑋𝑋

𝑋
󰆒
𝑋

(2)
whe eh
u
is heashlaye  hickness o minimumin il a ionandX=ET
0
/P.
X
min
=(ET
0
/P)
min
is he alueand he alue o  heh
u
isnull; o X<X
min
, heashisno 
use ulandi shouldno beused.X’=(ET
0
/P)’is heabsciseo  hemaximumuse ul hick-
nessh
umax
.Inou caseX
min
=0.4,X’=0.85,andh
umax
=60cm.
On heo he hand, hesecondcu eisanega i eexponen ial unc ion:
𝐼𝐼

𝑒𝑥𝑝󰇩𝑙𝑛󰇧𝐼′
𝐼

󰇨 𝑋𝑋

𝑋
󰆒
𝑋

󰇪(3)
whe eI(%)is heminimumin il a ionwhenh
u
isusedandX,X’,andX
min
ha e hesame
meaning.I
max
is hemaximumin il a ion o X=X
min
andI’is hein il a ion o X=X’.
Themaximumpossiblein il a ionisI
max
=

100%,and,empi ically,wesee ha alow
enough alue o  hein il a ion o X’=0.85isI’=5%.
Al hough heefficiencyislowe  o lowET
0
/P,i isonlyin ela i e e ms; he olume
o wa e  ha doesno ha e obe ea edisbigge inwe clima es.In hes udieda eas,P
andET
0
/Pa eno comple elyindependen and he eisa ela ionship(Figu e25A):
𝑃𝑃

󰇛𝑃

𝑃

󰇜 exp 󰇟𝑘 󰇛𝑋𝑋

󰇜󰇠(4)
Figu e 24. Es ima ion o he necessa y laye hickness (A) and in il a ion (B) based on ET0/P.
The i s cu e can be ep esen ed by a Weibull unc ion:
hu=humax X−Xmin
X′−Xmin
exp1−X−Xmin
X′−Xmin  (2)
whe e huis he ash laye hickness o minimum in il a ion and X = ET0/P.
X
min
= (ET
0
/P)
min
is he alue and he alue o he h
u
is null; o X < X
min
, he ash is
no use ul and i should no be used. X
′
= (ET
0
/P)’ is he abscise o he maximum use ul
hickness humax. In ou case Xmin = 0.4, X′= 0.85, and humax = 60 cm.
On he o he hand, he second cu e is a nega i e exponen ial unc ion:
I=Imax exp−lnI′
Imax X−Xmin
X′−Xmin (3)
whe e I (%) is he minimum in il a ion when h
u
is used and X, X
′
, and X
min
ha e he
same meaning. I
max
is he maximum in il a ion o X = X
min
and I
′
is he in il a ion o
X = X
′
. The maximum possible in il a ion is I
max
= 100%, and, empi ically, we see ha a
low enough alue o he in il a ion o X′= 0.85 is I′= 5%.
Al hough he e iciency is lowe o low ET
0
/P, i is only in ela i e e ms; he olume
o wa e ha does no ha e o be ea ed is bigge in we clima es. In he s udied a eas,
P and ET0/P a e no comple ely independen and he e is a ela ionship (Figu e 25A):
P=Pmin +(Pmax −Pmin)exp[−k(X−Xmin)] (4)
whe e P
max
is he maximum yea ly p ecipi a ion in mm/yea ; P
min
is he minimum
yea ly p ecipi a ion in mm/yea and k is an empi ical coe icien . Acco ding o he da a,
Xmin = 0.4, Pmax = 1400 mm/yea , Pmin = 400 mm/yea , and k = 1.5.
En i onmen s 2025,12, 107 19 o 30
En i onmen s2025,12,xFORPEERREVIEW20o 32


whe eP
max
is hemaximumyea lyp ecipi a ioninmm/yea ;P
min
is heminimumyea ly
p ecipi a ioninmm/yea andkisanempi icalcoefficien .Acco ding o heda a,X
min
=

0.4,P
max
=1400mm/yea ,P
min
=400mm/yea ,andk=1.5.
The olumeo  ainwa e  ha isno con amina ed,inm
3
pe m
2
o co e ingandyea ,
willbeas ollows:
𝑉 𝑃
1000 1𝐼
100
(5)
Bysubs i u ingEqua ions(3)and(4)in(5),anewcu ecanbed awn(Figu e25B).
I ise iden now ha inabsolu e e ms,i isbe e  oa oid hecon amina iono  ainwa e 
inwe  egions hanina idones(70%o 1200mm/yea isbigge  han100%o 500
mm/yea ).

Figu e25.Yea lyp ecipi a ion(A)andcon amina ed ainwa e  olume(B)asa unc iono ET
0
/P.
5.DiscussionandConclusions
Cappingcon amina edwas e odiminish hequan i yo leacha ep oducedisa ech-
niquewi h he ollowingad an ages:
 Theeffec ispe manen o e  imeasi isbasedonaphysicalba ie effec .
 Thecon amina ion educ ionisindependen o  heini ialconcen a ion.
 Thecon amina ion educ ionis o anyPTE(Hg,Pb,Zn,e c.).
Thesead an agesshouldinspi eapa adigmshi because,inmanycases, hiscould
beasolu ion ohelp emedia econ amina eda eas.Thes anda dsaccep edup onow
sough  oelimina e hepossibili yo wa e in il a ionbyusing o allyimpe meableba -
ie elemen s,ei he HDPEshee ingand/o claylaye s,basing hisimpe meabili yonhy-
d aulicconduc i i iesaslowas10
−11
m/s.Howe e , his equi es heuseo ma e ialswi h
asigni ican ca bon oo p in ( heshee and/o clay)o  heexploi a iono a aluablena -
u al esou ce(clay ha mee s hes anda ds)wi h heco espondingen i onmen alim-
pac .On heo he hand, hema e ialsa eexpensi e,and,becauseo  heex en o  he
dumps, heeconomiccos isamajo de e en  ounde aking his ypeo p ojec .Thus,
henewpa adigmcanbesumma izedin he ollowingpoin s:
 No  ying oachie eabsolu ewa e p oo ingbasedonlyona e ylowhyd aulic
conduc i i y.
 Usingby-p oduc ssuchas lyash,wi hsufficien lylowhyd aulicconduc i i yand
sufficien lyhighpo osi y,wi hsufficien  echa gecapaci y o acili a ee apo anspi-
a ionand educewa e in il a ionas a aspossible.
Figu e 25. Yea ly p ecipi a ion (A) and con amina ed ainwa e olume (B) as a unc ion o ET0/P.
The olume o ainwa e ha is no con amina ed, in m
3
pe m
2
o co e ing and yea ,
will be as ollows:
V=P
1000 1−I
100(5)
By subs i u ing Equa ions (3) and (4) in (5), a new cu e can be d awn (Figu e 25B). I
is e iden now ha in absolu e e ms, i is be e o a oid he con amina ion o ainwa e in
we egions han in a id ones (70% o 1200 mm/yea is bigge han 100% o 500 mm/yea ).
5. Discussion and Conclusions
Capping con amina ed was e o diminish he quan i y o leacha e p oduced is a
echnique wi h he ollowing ad an ages:
•The e ec is pe manen o e ime as i is based on a physical ba ie e ec .
•The con amina ion educ ion is independen o he ini ial concen a ion.
•The con amina ion educ ion is o any PTE (Hg, Pb, Zn, e c.).
These ad an ages should inspi e a pa adigm shi because, in many cases, his could
be a solu ion o help emedia e con amina ed a eas. The s anda ds accep ed up o now
sough o elimina e he possibili y o wa e in il a ion by using o ally impe meable ba ie
elemen s, ei he HDPE shee ing and/o clay laye s, basing his impe meabili y on hyd aulic
conduc i i ies as low as 10
−11
m/s. Howe e , his equi es he use o ma e ials wi h a
signi ican ca bon oo p in ( he shee and/o clay) o he exploi a ion o a aluable na u al
esou ce (clay ha mee s he s anda ds) wi h he co esponding en i onmen al impac .
On he o he hand, he ma e ials a e expensi e, and, because o he ex en o he dumps,
he economic cos is a majo de e en o unde aking his ype o p ojec . Thus, he new
pa adigm can be summa ized in he ollowing poin s:
•
No ying o achie e absolu e wa e p oo ing based only on a e y low hyd aulic
conduc i i y.
•
Using by-p oduc s such as ly ash, wi h su icien ly low hyd aulic conduc i i y and su -
icien ly high po osi y, wi h su icien echa ge capaci y o acili a e e apo anspi a ion
and educe wa e in il a ion as a as possible.
•
P e en he ainwa e om being con amina ed is always a good esul and, e en i
100% wa e p oo ing is no achie ed, i can be a success ul esul in many cases.
•
Reducing he olume o con amina ed wa e esul s in economic sa ings because
ea men s o elimina ing PTEs o he con amina ed wa e a e usually expensi e.
En i onmen s 2025,12, 107 20 o 30
•
The leacha e con amina ion educ ion, due o he dilu ion in he wa e o he i e s,
g ea ly educes he ex en o con amina ion so ha he maximum concen a ion le els
a e close o he sou ce o con amina ion.
The p oposed solu ion is complemen a y o he exis ing ones and can be e y use ul
in cases such as leacha es wi h concen a ions sligh ly highe han he legal limi s. In his
sense, i is necessa y o look o an ash hickness ha complies wi h he ollowing:
•
The echa ge is su icien ly la ge o abso b he maximum di e ence be ween p ecipi a-
ion and e apo anspi a ion.
•In il a ion is heo e ically null o as li le as possible.
•
The echa ge balances e apo anspi a ion in he d y season, ensu ing ha he plan ed
species always ha e mois u e.
Gi en he pe meabili y and po osi y cha ac e is ics o he ashes and he balanced
ela ionship ET
0
/P, in he case s udy om he no h o Spain, his is achie ed wi h a
hickness o he ash laye o abou 45 cm, which p o ides a echa ge o abou 250 mm.
Au ho Con ibu ions: Concep ualiza ion, R.R. and M.B.; me hodology, R.R., M.B. and J.A.; alida-
ion, R.R., E.G.-O. and J.A.; in es iga ion, R.R., M.B., E.G.-O. and J.A.; da a cu a ion, R.R.; w i ing—
o iginal d a p epa a ion, R.R., M.B., E.G.-O. and J.A.; w i ing— e iew and edi ing, R.R. and M.B.;
supe ision, R.R.; p ojec adminis a ion, R.R.; unding acquisi ion, R.R. All au ho s ha e ead and
ag eed o he published e sion o he manusc ip .
Funding: The au ho s would like o hank he p og am LIFE o he Eu opean Commission o he
unding ecei ed o he p ojec SUBp oduc s4LIFE ( e e ence LIFE16 ENV/ES/000481).
Da a A ailabili y S a emen : The da a p esen ed in his s udy a e a ailable wi hin he manusc ip .
Acknowledgmen s: Au ho s would like o acknowledge he collabo a ion o he ins i u ions and
p i a e companies ha pa icipa ed in he p ojec SUBp oduc s4LIFE: Bios e a consul o ía Medioam-
bien al (BIOSFERA), Esco ias y De i ados (EDERSA), Global Se ice (GSe ice), Hid oeléc ica del
Can áb ico (EDP), Ins i u o As u iano de P e ención de Riesgos Labo ales (IAPRL), Recupe ación
y Reno ación (R&R), and Uni e sidad de O iedo (UNIOVI). Finally, he collabo a ion o sponso s
A celo Mi al, Ingenie ía de Mon ajes No e S.A. (IMSA), As u belga de Minas, Lena Council, and
he Ins i u o Nacional de Silicosis (INS) is also g ea ly app ecia ed.
Con lic s o In e es : The au ho s decla e no con lic s o in e es .
Appendix A. Model o he Wa e Balance
Appendix A.1. In il a ion and E apo a ion Models
Appendix A.1.1. In il a ion and E apo a ion Analy ical Models
When wa e om p ecipi a ion is in il a ed in o he soil, a pa is los due o he
e apo a ion p ocess; he balance ep esen s he quan i y o wa e e ained in he soil [
41
,
42
].
The e a e some analy ical models o ep esen bo h p ocesses. Fo example, he Philip
model, he Kos iako model, and he Ho on model we e used o analyze he in il a ion
p ocess [
43
], while he Powe unc ion, he Rose model, and he Black model we e applied
in he analysis o he e apo a ion [
44
,
45
]. Ne e heless, in p oblems like he p esen s udy,
empi ical models a e be e because hey a e easie o use and accu a e enough o p edic
he esul s.
Appendix A.1.2. Empi ical E apo anspi a ion Models
Many empi ical models a e a ailable o es ima e e apo anspi a ion. A e iew wi h
he desc ip ion o hese classical models can be ound in [
46
]. The Spanish Me eo ological
Agency (AEMET) uses he Penman–Mon ei h exp ession o es ima e e apo anspi a ion.
En i onmen s 2025,12, 107 21 o 30
Howe e , i is a ela i ely complex o mula wi h many pa ame e s. When only empe a-
u es a e a ailable, i is be e o use he Ha g ea es–Samani model, as sugges ed by Allen
e al. [
47
]. Howe e , his s udy mainly applied he Tu c model due o i s simplici y and
sui abili y, as i is shown in he ollowing sec ion.
Appendix A.2. Wa e Balance
Appendix A.2.1. Hypo hesis and Basic Fo mulae
Figu e A1 ep esen s he componen s o a adi ional wa e balance. Howe e , se -
e al o hese componen s (d ainage, deep pe cola ion, and capilla y ise) a e null in he
es conduc ed.
En i onmen s2025,12,xFORPEERREVIEW22o 32


AppendixA.1.2.Empi icalE apo anspi a ionModels
Manyempi icalmodelsa ea ailable oes ima ee apo anspi a ion.A e iewwi h
hedesc ip iono  heseclassicalmodelscanbe oundin[46].TheSpanishMe eo ological
Agency(AEMET)uses hePenman–Mon ei hexp ession oes ima ee apo anspi a ion.
Howe e ,i isa ela i elycomplex o mulawi hmanypa ame e s.Whenonly empe a-
u esa ea ailable,i isbe e  ouse heHa g ea es–Samanimodel,assugges edbyAllen
e al.[47].Howe e , hiss udymainlyapplied heTu cmodeldue oi ssimplici yand
sui abili y,asi isshownin he ollowingsec ion.
AppendixA.2.Wa e Balance
AppendixA.2.1.Hypo hesisandBasicFo mulae
Figu eA1 ep esen s hecomponen so a adi ionalwa e balance.Howe e ,se -
e alo  hesecomponen s(d ainage,deeppe cola ion,andcapilla y ise)a enullin he
es conduc ed.

Figu eA1.Wa e balancecomponen s.
The ollowinga e hemainhypo heses:
 The es ispe o medina ea men cellwi hawa e p oo ingshee on hewallsand
loo ; he e o e,wa e canonlyen e  hecell om he opsu ace.Thecapilla y ise
anddeeppe cola ioncomponen sa enull.
 Wa e canonlyexi  hecellei he bye apo a ingo  h oughad ain ha exis sin he
slaglaye jus below heash; he ea enoplan sand hen he eisno anspi a ion,
andnod ainagewasobse ed.
 Theonlywa e supply omou side hecellis ainwa e ; he eisnoi iga ion.
 The ea men cellisho izon al,so he eisnoho izon alwa e  low;wa e canonly
ascend/descendo e apo a e.
 I is es edonanashlaye o  hicknessh; o i  obecomesa u a edwi h ain all,a
smalla eao ashlaye isp epa edby educingi s hickness oh=10cm.
 The ainwa e e apo a eso passesin o heashes,inc easing hei humidi y.Aslong
asamaximumdeg eeo humidi yisno  eached, hewa e no e apo a ed emains
e ainedin heashlaye .
 Thewa e begins oin il a ein o helowe laye o slagassoonas heashesa e
sa u a edwi hwa e ,ac ingasad ainagelaye .
 The ea enoplan s,so he eisnoelimina iono wa e by anspi a ion;i isonlyby
di ec e apo a ion.
 Capilla i ydoesno en e di ec lyin o hecalcula ions.
 I isassumed ha  hebeha io desc ibedby hemodelis alid o anashlaye up o
h≈50–60cm.
Al hough hewa e balanceispe o med o longpe iods—weekso mon hs—a
day-by-daywa e balanceismadein hiss udy.Fo anygi enday(i),i iss a ed om
Figu e A1. Wa e balance componen s.
The ollowing a e he main hypo heses:
•
The es is pe o med in a ea men cell wi h a wa e p oo ing shee on he walls and
loo ; he e o e, wa e can only en e he cell om he op su ace. The capilla y ise
and deep pe cola ion componen s a e null.
•
Wa e can only exi he cell ei he by e apo a ing o h ough a d ain ha exis s in he
slag laye jus below he ash; he e a e no plan s and hen he e is no anspi a ion,
and no d ainage was obse ed.
•The only wa e supply om ou side he cell is ainwa e ; he e is no i iga ion.
•
The ea men cell is ho izon al, so he e is no ho izon al wa e low; wa e can only
ascend/descend o e apo a e.
•
I is es ed on an ash laye o hickness h; o i o become sa u a ed wi h ain all, a
small a ea o ash laye is p epa ed by educing i s hickness o h = 10 cm.
•
The ainwa e e apo a es o passes in o he ashes, inc easing hei humidi y. As long
as a maximum deg ee o humidi y is no eached, he wa e no e apo a ed emains
e ained in he ash laye .
•
The wa e begins o in il a e in o he lowe laye o slag as soon as he ashes a e
sa u a ed wi h wa e , ac ing as a d ainage laye .
•
The e a e no plan s, so he e is no elimina ion o wa e by anspi a ion; i is only by
di ec e apo a ion.
•Capilla i y does no en e di ec ly in o he calcula ions.
•I is assumed ha he beha io desc ibed by he model is alid o an ash laye up o
h≈50–60 cm.
Al hough he wa e balance is pe o med o long pe iods—weeks o mon hs—a
day-by-day wa e balance is made in his s udy. Fo any gi en day (i), i is s a ed om an
ini ial s a e in which he wa e echa ge in he soil is assumed o be known a he beginning
o he day, equal o he end o he p e ious day R
i−1
. The ini ial wa e echa ge on he i s
day R
0
mus be known o s a he calcula ion. I he balance is s a ed a he beginning o
he calenda wa e yea , Sep embe , he wa e echa ge is conside ed o be ze o R
0
= 0 mm.

En i onmen s 2025,12, 107 22 o 30
Wi h he echa ge a he beginning o day R
i-1
and he p ecipi a ion P
i
and po en-
ial e apo anspi a ion ETP
i
da a o ha day, he soil wa e echa ge a he end o day
R
i
is calcula ed. Fi e di e en scena ios, complemen a y o each o he , a e assumed. The
o mulas employed in each scena io a e de ailed below:
(1)
In he i s scena io, p ecipi a ion o he analyzed day is less han po en ial e apo an-
spi a ion (P
i≤
ETP
i
), bu he e is su icien wa e echa ge o all ha e apo anspi-
a ion o occu (P
i
+ R
i-1 ≥
ETP
i
). Ac ual e apo anspi a ion, ETR
i
, will be equal o
po en ial e apo anspi a ion, ETP
i
, since he e is enough wa e . Recha ge is inc eased
by p ecipi a ion, bu dec eased by e apo anspi a ion (Equa ions (A1)–(A4)).
Pi≤ETPi& Pi+Ri−1≥ETPi(A1)
ETRi=ETPi(A2)
∆Ri=Pi−ETRi(A3)
Ri=Ri−1+∆Ri(A4)
(2)
Scena io ypical o e y d y pe iods. P ecipi a ion is less han po en ial e apo an-
spi a ion (P
i≤
ETP
i
) and he e is no enough echa ge o p oduce all ha pos-
sible e apo anspi a ion (P
i
+ R
i-1
< ETP
i
). The ac ual e apo anspi a ion ETR
i
will equal ain all and wha e e wa e emains in he echa ge. Recha ge is in-
c eased by p ecipi a ion, bu echa ge is deple ed due o e apo a ion being so in ense
(Equa ions (A5)–(A8)).
Pi≤ETPi& Pi+Ri−1<ETPi(A5)
ETRi=Pi+Ri−1(A6)
∆Ri=Pi−ETRi(A7)
Ri=Ri−1+∆Ri=0 (A8)
(3)
The hi d scena io is ypical o humid pe iods, in which p ecipi a ion is highe han
e apo anspi a ion (P
i
> ETP
i
), leading o inc eased echa ge, bu wi hou eaching
he s a e o soil sa u a ion and wi hou eaching he maximum mois u e ha he soil
can s o e (R
i≤
R
max
). The pa o ain all ha does no e apo a e inc eases soil wa e
echa ge (Equa ions (A9)–(A12)) is as ollows:
Pi>ETPi& Ri≤Rmax (A9)
ETRi=ETPi(A10)
∆Ri=Pi−ETRi(A11)
Ri=Ri−1+∆Ri(A12)
(4)
The ou h scena io occu s in ainy pe iods (Pi > ETPi). A e se e al days o ain all,
he maximum echa ge (R
i
> R
max
) is eached and wa e begins o pe cola e o in il a e
o he lowe laye s (I
i
> 0). Howe e , he p ecipi a ion in ensi y does no each a
minimum alue (Pi ≤PN) o uno o occu (Equa ions (A13)–(A18)).
Pi>ETPi& Ri>Rmax & Pi≤PN(A13)
ETRi=ETPi(A14)
En i onmen s 2025,12, 107 23 o 30
∆Ri=Pi−ETRi(A15)
Ri=Ri−1+∆Ri(A16)
Ii=Ri−Rmax (A17)
Ri=Rmax (A18)
(5)
The i h scena io occu s in imes o con inuous ain all (P
i
> ETP
i
), wi h days o
e y hea y ain all, abo e a minimum alue (P
i
> P
N
), which causes uno (E
i
> 0),
because he 1 h ainwa e low (I
1
) is g ea e han he maximum possible in il a ion
h ough he wa e -sa u a ed ash (I1> Imax) (Equa ions (A19)–(A26)).
Pi>ETPi& Ri>Rmax & Pi>PN(A19)
ETRi=ETPi(A20)
∆Ri=Pi−ETRi(A21)
Ri=Ri−1+∆Ri(A22)
I1=Pi
2424
1n
(A23)
E=I1−Imax (A24)
Ii=(Ri−Rmax)−E (A25)
Ri=Rmax (A26)
Appendix A.2.2. P ecipi a ion and E apo anspi a ion
The mos impo an a iable o es ablish he wa e balance is p ecipi a ion P (mm/day).
These da a can be easily ob ained in he ield using a ain gauge; howe e , go e nmen s
usually p o ide i because o i s impo ance o ag icul u e. In he case o Spain, he
Spanish Me eo ological Agency (AEMET) p o ides daily p ecipi a ion da a a di e en
moni o ing s a ions loca ed in di e en p o inces. These speci ic da a a e a ailable om
he in o ma ion o he nea es clima ological s a ion.
As u ias is a e y moun ainous egion, and ain all can a y g ea ly om one loca ion
o ano he . The e o e, i is con enien o ha e ield da a and compa e i wi h he AEMET
da a. Figu e A2A shows he p ecipi a ion da a ob ained in si u e sus he p ecipi a ion
da a ob ained a he nea es AEMET s a ion be ween 21 Ap il 2024 and 31 May 2024. The
da a measu ed in si u a e only 10% highe han hose eco ded by AEMET, a a ia ion ha
can be conside ed wi hin a no mal dispe sion.
E apo anspi a ion (ET) is he combina ion o wo main p ocesses d i en by a mo-
sphe ic demand: e apo a ion om he soil (E) and anspi a ion h ough he s oma a o
plan s (T). I is an impo an componen o he wa e balance, especially in semi-a id a eas.
Conce ning po en ial e apo anspi a ion, he classical dual me hod will be ol-
lowed [
47
,
48
]. The e apo anspi a ion o a gi en c op ET
c
can be calcula ed as he p oduc
o a cons an K
c
by a e e ence e apo anspi a ion ET
0
. I u he s a es he con ibu-
ions due o plan anspi a ion, coe icien K
cb
, and ha o pu e soil wa e e apo a ion,
Ke(Equa ion (A27)).
ETc=KcET0=(Kcb +Ke)ET0(A27)
En i onmen s 2025,12, 107 24 o 30
A simila ela ionship o he de e mina ion o ETP is p oposed o his s udy. Since
he e a e no plan s, he con ibu ion o anspi a ion is ze o and K
cb
= 0, ob aining
he ollowing:
ETP =Ke·ET0(A28)
In hyd ogeology, i is conside ed ha ETP = ET
0
, so he K
e
pa ame e mus be close
o uni y.
Due o i s impo ance o ag icul u e, he Spanish Me eo ological Agency (AEMET)
also p o ides he ET
0
da a accumula ed in he las 10 days, ET
0–10
. As in he case s udy, he
balance is de e mined day by day, ope a ing wi h Equa ion (A29).
ETPi=KeET0–10
10 (A29)
F om he measu emen o sola adia ion, i has been ound ha adia ion is much
highe on sunny days han on ainy days. Gi en he di ec ela ionship be ween adia ion
and e apo anspi a ion, i is p oposed o use a di e en coe icien on ainy and non- ainy
days. Assuming ha in As u ias i ains 50% o he days o he yea , he coe icien s
can be used ollowing Equa ions (A30) and (A31), whose weigh ed a e age is Ke = 1.0,
co esponding o he s a ing hypo hesis.
Pi>0→Ke=0.1 (A30)
Pi=0→Ke=1.7 (A31)
I he da a a e no a ailable om a s a e agency, he e a e se e al p o en me hods
o he de e mina ion o ET
0
om o he measu able da a such as empe a u e, ela i e
humidi y, sola adia ion, e c. Among he bes -known me hods a e Tho n hwai e [
49
],
Tu c [50], and Ha g ea es-Samani [51].
In he case s udy, he exp ession om Tu c [
50
] has been used. I is based on some easily
a ailable clima ic da a such as adia ion, ai empe a u e, and ela i e humidi y. The e o e,
i is easy o apply whene e a ull se o clima ic da a is no a ailable. The equa ion o
daily po en ial e apo anspi a ion calcula ion is gi en by Equa ions (A32)–(A34):
ET0=aC T
T+15(RG+b)(A32)
RH ≥50% ⇒C=1 (A33)
RH <50% ⇒C=1+50 −RH
70 (A34)
whe e ET
0
is in mm day
−1
, T is he mean daily ai empe a u e (
◦
C), R
G
is he global
adia ion (MJ m
−2
day
−1
), a and b a e empi ical cons an s wi h a = 0.31 (m
2
MJ
−1
mm
−1
),
and b = 2.094 (MJ m−2day−1).
Acco ding o Coly e al. [
52
], i is some imes con enien o adjus he pa ame e s so
ha he es ima e made wi h Tu c [
50
] is close o he ET
0
alue. Using he alue a = 0.280
in Tu c’s o mula and he mean global adia ion R
G
alues measu ed in si u, ET
0
alues
can be p edic ed and compa ed wi h hose gi en by AEMET. Figu e A2B shows he ET
0
alues es ima ed wi h Tu c’s o mula ( o a = 0.267) and hose gi en by AEMET be ween
1 Sep embe 2023 and 31 Augus 2024. The co ela ion coe icien is 2= 0.98.
En i onmen s 2025,12, 107 25 o 30
En i onmen s2025,12,xFORPEERREVIEW26o 32


alueses ima edwi hTu c’s o mula( o a=0.267)and hosegi enbyAEMETbe ween
1Sep embe 2023and31Augus 2024.Theco ela ioncoefficien is
2
=

0.98.

Figu eA2.Compa isonbe weenp ecipi a ion(A)ande apo anspi a ion(B) ominsi umeasu e-
men sand he aluegi enbyAEMET.
Theanalysiso wa e  echa gein heasheshasbeenpe o meddailysinceAEMET
gi esbasicme eo ologicalin o ma ione e yday.Howe e ,AEMETgi es heaccumu-
la ede apo anspi a ionda ae e y10days,ET
0-10
.Thismeans ha i in hewa e balance
model, hedailymean o  hepe iodET
0i
=ET
0-10
/10isused, hep edic iono mois u ein
heashesdoesno adjus  o he aluesob aineddaybyday,asshowninFigu eA3A.This
isbecause heET
0-10
 alueisacumula i e alue ha doesno conside  hedailyoscilla-
ions.Toachie eabe e  i ,insi usola  adia ionmeasu emen swe e akenande apo-
anspi a ionwases ima edusingTu c’s o mula,asshowninFigu eA3B.The indings
ha e apo anspi a ionishighondayswheni doesno  ainand heskyisclea ,whilei 
is e ylowoncloudydayswheni  ains.
Asmen ionedabo e, hisadjus men isachie edbyusingacoefficien o less han
uni yon ainydays(K
e
=0.1)andahighe coefficien onclea days(K
e
=

1.7),so hemean
conside ing he a ioo  ainy/non- ainydaysisapp oxima elyK
e
=

1.Wi h heseassump-
ions,wes a  om hemois u ein heashmeasu edon he i s dayanduse hemodel
oes ima e he a ia ioninashonsuccessi edays.

Figu eA3.ET
0
 aluegi enbyAEMETe e y10days(A)andET
0
 a ia ion omday oday(B).

Figu e A2. Compa ison be ween p ecipi a ion (A) and e apo anspi a ion (B) om in si u measu e-
men s and he alue gi en by AEMET.
The analysis o wa e echa ge in he ashes has been pe o med daily since AEMET
gi es basic me eo ological in o ma ion e e y day. Howe e , AEMET gi es he accumula ed
e apo anspi a ion da a e e y 10 days, ET
0–10
. This means ha i in he wa e balance
model, he daily mean o he pe iod ET
0i
= ET
0–10
/10 is used, he p edic ion o mois u e
in he ashes does no adjus o he alues ob ained day by day, as shown in Figu e A3A.
This is because he ET
0–10
alue is a cumula i e alue ha does no conside he daily
oscilla ions. To achie e a be e i , in si u sola adia ion measu emen s we e aken and
e apo anspi a ion was es ima ed using Tu c’s o mula, as shown in Figu e A3B. The
indings ha e apo anspi a ion is high on days when i does no ain and he sky is clea ,
while i is e y low on cloudy days when i ains.
As men ioned abo e, his adjus men is achie ed by using a coe icien o less han
uni y on ainy days (K
e
= 0.1) and a highe coe icien on clea days (K
e
= 1.7), so he
mean conside ing he a io o ainy/non- ainy days is app oxima ely K
e
= 1. Wi h hese
assump ions, we s a om he mois u e in he ash measu ed on he i s day and use he
model o es ima e he a ia ion in ash on successi e days.
En i onmen s2025,12,xFORPEERREVIEW26o 32


alueses ima edwi hTu c’s o mula( o a=0.267)and hosegi enbyAEMETbe ween
1Sep embe 2023and31Augus 2024.Theco ela ioncoefficien is
2
=

0.98.

Figu eA2.Compa isonbe weenp ecipi a ion(A)ande apo anspi a ion(B) ominsi umeasu e-
men sand he aluegi enbyAEMET.
Theanalysiso wa e  echa gein heasheshasbeenpe o meddailysinceAEMET
gi esbasicme eo ologicalin o ma ione e yday.Howe e ,AEMETgi es heaccumu-
la ede apo anspi a ionda ae e y10days,ET
0-10
.Thismeans ha i in hewa e balance
model, hedailymean o  hepe iodET
0i
=ET
0-10
/10isused, hep edic iono mois u ein
heashesdoesno adjus  o he aluesob aineddaybyday,asshowninFigu eA3A.This
isbecause heET
0-10
 alueisacumula i e alue ha doesno conside  hedailyoscilla-
ions.Toachie eabe e  i ,insi usola  adia ionmeasu emen swe e akenande apo-
anspi a ionwases ima edusingTu c’s o mula,asshowninFigu eA3B.The indings
ha e apo anspi a ionishighondayswheni doesno  ainand heskyisclea ,whilei 
is e ylowoncloudydayswheni  ains.
Asmen ionedabo e, hisadjus men isachie edbyusingacoefficien o less han
uni yon ainydays(K
e
=0.1)andahighe coefficien onclea days(K
e
=

1.7),so hemean
conside ing he a ioo  ainy/non- ainydaysisapp oxima elyK
e
=

1.Wi h heseassump-
ions,wes a  om hemois u ein heashmeasu edon he i s dayanduse hemodel
oes ima e he a ia ioninashonsuccessi edays.

Figu eA3.ET
0
 aluegi enbyAEMETe e y10days(A)andET
0
 a ia ion omday oday(B).

Figu e A3. ET0 alue gi en by AEMET e e y 10 days (A) and ET0 a ia ion om day o day (B).
Appendix A.2.3. Maximum Wa e Recha ge
Recha ge is he amoun o wa e ha soil accumula es. Once he pe cen age o wa e
by olume and he hickness o he soil laye a e known, he olume o wa e accumula ed
can be de e mined in li e s o wa e pe m
2
o soil su ace o , usually, in millime e s (mm).
An impo an pa ame e in his s udy is he maximum echa ge ha he ashes admi .
I he p opo ion o wa e in he ashes exceeds hei maximum echa ge, he ashes will
no be able o e ain mo e wa e and, he e o e, i ha wa e does no e apo a e, i will