Ci a ion: Sanju jo-Sánchez, J.;
Ba ien os Rod íguez, V.;
A ce Chamo o, C.; Al es, C.
Es ima ing he Radioac i e Hea
P oduc ion o a G ani ic Rock in he
Uni e si y o A Co uña (Galicia,
No hwes Spain) by Gamma- ay
Spec ome y. Appl. Sci. 2022,12,
11965. h ps://doi.o g/10.3390/
app122311965
Academic Edi o s: Da id
Be e mann, Jin Luo and
Joachim Rohn
Recei ed: 5 No embe 2022
Accep ed: 20 No embe 2022
Published: 23 No embe 2022
Publishe ’s No e: MDPI s ays neu al
wi h ega d o ju isdic ional claims in
published maps and ins i u ional a il-
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Copy igh : © 2022 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
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A ibu ion (CC BY) license (h ps://
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applied
sciences
A icle
Es ima ing he Radioac i e Hea P oduc ion o a G ani ic Rock
in he Uni e si y o A Co uña (Galicia, No hwes Spain)
by Gamma- ay Spec ome y
Jo ge Sanju jo-Sánchez 1,* , Vic o Ba ien os Rod íguez 1, Ca los A ce Chamo o 1and Ca los Al es 2
1Uni e si y Ins i u e o Geology, Uni e si y o A Co uña, Campus de El iña, 15017 A Co uña, Spain
2LandS/Lab2PT-Landscapes, He i age and Te i o y Labo a o y (FCT-UIDB/04509/2020),
Ea h Sciences Depa men , School o Sciences, Uni e si y o Minho, 4710-057 B aga, Po ugal
*Co espondence: jo [email p o ec ed]; Tel.: +34-981167000 (ex . 2695)
Abs ac :
Geo he mal ene gy is a o m o enewable ene gy wi h a long adi ion in Eu opean
coun ies, al hough i is sca cely used in Spain. One o he easons o his is he poo ly s udied
geo he mal po en ial o he Spanish e i o y. In ecen yea s, da a published on e es ial gamma
adia ion and he geochemis y o adioiso opes in ocks ha e sugges ed ha he adiogenic hea
p oduc ion (RHP) in some a eas o Spain is high. In his wo k, we assessed he RHP by analysing he U,
Th, and K con en s o he ocks unde lying he mos impo an campus o he Uni e si y o A Co uña
(no hwes Spain), using in si u handheld gamma- ay spec ome y (GRS) and X- ay luo escence
spec ome y (XRF). Ou esul s p o ide a good i o he adioiso ope con en s and unexpec edly
high RHP, compa ed wi h a e age da a obse ed in simila ocks (g anodio i e). These esul s e eal
ha GRS is a e y eliable ool o s udying he RHP o ock su aces, and ha geo he mal ene gy can
be used in he a ea (i.e., he s udied campus, bu also mos o he ci y o A Co uña, as i is buil on he
same unde lying ock) o cen al hea ing in buildings using g ound-sou ce hea pumps (GSHPs).
Keywo ds:
geo he mal ene gy; adiogenic hea p oduc ion; adioiso ope con en ; gamma- ay
spec ome y; g ani oid
1. In oduc ion
Al hough he e m geo he mal ene gy can be employed o desc ibe he in e nal ene gy
o he Ea h, in e ms o i s use as a sou ce o ene gy i e e s o he he mal ene gy s o ed
in he Ea h’s c us . The main sou ces o his geo he mal ene gy a e he esidual ene gy
a ailable om plane o ma ion and he ene gy con inuously gene a ed om p imo dial
adionuclide decay [
1
,
2
]. In p ac ice, geo he mal esou ces consis o he mal ene gy s o ed
in bo h ock and apped s eam o liquid wa e . Such sou ces p o ide ene gy wi h wo
possible uses: elec ici y gene a ion, and di ec hea use. The o me can be achie ed
unde a s eep geo he mal g adien when high empe a u es a e obse ed in some a eas o
he Ea h’s uppe c us . The la e is a ainable e en when he hea s o ed is conside ed
o be low, which occu s in mos o he Ea h’s c us due o he ela i ely low he mal
conduc i i y o ocks [
3
]. This second ype o geo he mal ene gy can be ha nessed wi h
low-en halpy echnologies such as g ound-sou ce hea pump (GSHP) and g oundwa e
hea pump (GWHP) sys ems [4].
Geo he mal ene gy is coun ed among enewable ene gies, wi h a long adi ion,
expe ience, and g ea po en ial o he u u e [
5
]. A di ec i e by he Eu opean Pa liamen
and he Eu opean Council es ablished ha his ype o ene gy should be conside ed
enewable in coun ies o he Eu opean Union [
6
]. Despi e his di ec i e, he de elopmen
and use o such ene gy is poo in coun ies such as Spain when compa ed wi h o he
coun ies o he EU, as well as when conside ing he es ima es o po en ial geo he mal
ene gy in his coun y [
7
–
9
]. Al hough i s po en ial de elopmen depends on he ene gy
Appl. Sci. 2022,12, 11965. h ps://doi.o g/10.3390/app122311965 h ps://www.mdpi.com/jou nal/applsci
Appl. Sci. 2022,12, 11965 2 o 15
policy, a good assessmen o he geo he mal po en ial is essen ial o p o ide p esen and
u u e p ojec ions o i s po en ial use. In his wo k, we ocus on a local case s udy in
no hwes Spain in an a emp o e lec he high po en ial o his ene gy in some a eas
o he coun y and he need o es ablishing bo h de elopmen and in es men plans o
i s explo a ion.
E en hough he Eu opean Geo he mal Ene gy Council [
10
] has s a ed ha i s ob-
jec i es un il he yea 2050 include he es ablishmen o a Eu opean geo he mal indus y
base (2020) o ob aining a subs an ial pa o he elec ici y supply om geo he mal ene gy
(2050), hese objec i es a e a om being ul illed in Spain. S udies o geo he mal po en ial
ca ied ou in Spain in he las 45 yea s ha e been ew and ha e o en been educed o
conse a i e es ima es ob ained om heo e ical o abula ed pa ame e s [
7
,
11
]. In mos o
he e i o y o Spain, based on he VDI 4640 (2001) s anda d [
12
], i has been es ima ed
ha he e is a e y low hea lux, wi h medium en halpy esou ces ( empe a u es be ween
100
◦
C and 150
◦
C) and e y low en halpy (T < 30
◦
C), wi h some excep ions—such as he
Cana y Islands [
7
]. This s anda d es ablishes es ima ed alues o assess he geo he mal
po en ial, wi hou conside ing how he a iabili y in he mine al composi ion o ocks
a ec s hei he mal conduc i i y and, abo e all, he a iable hea gene a ion in hem due
o hei con en s o p imo dial adioiso opes o u anium, ho ium, and po assium. The
mos ambi ious s udy ca ied ou by he Ins i u e o he Di e si ica ion and Sa ing o
Ene gy [7] acknowledges, in ac , ha i s own es ima es a e conse a i e. Cu en ly, he e
a e mo e p ecise s udies o es ima e he eal geo he mal po en ial, al hough wi h poo
spa ial esolu ion [
8
]. Fo he no hwes o Spain, i has been p oposed ha he geological
cha ac e is ics may be o pa icula in e es o he use o geo he mal ene gy h ough
GSHPs [13].
In ecen yea s, published s udies ha e p o ided da a on e es ial gamma adia-
ion [
14
], po en ial adon emissions [
15
], and e en da a on he geochemical composi ion
o adioiso opes on he su ace, in he i s Geochemical A las o Spain [
16
]. These new
da a, sepa a ely, ha e made i possible o obse e ha in no hwes Spain, bo h U and Th
iso opes a e especially abundan in ce ain ocks (such as g ani es), exceeding he a e age
alues o his ype o ock [
17
–
19
]. These adioiso opes a e esponsible o 85% o he hea
p oduc ion in ocks by adioiso opes [
17
], so hey a e he main cause o he geo he mal
g adien a he su ace o he c us being highe han he a e age g adien . Due o his
abundance o U and Th, e y high alues o e es ial gamma adia ion and po en ial
emission o adon in ocks a e obse ed in he au onomous communi y o Galicia [
14
,
15
]. In
his egion, g ani ic ocks o igina ing du ing he Va iscan O ogeny a e especially abundan ,
in which high concen a ions o U, Th, and K ha e been obse ed [
13
,
20
], exceeding he
a e age concen a ions expec ed in g ani es, as occu s in some Palaeozoic g ani es [
17
–
19
].
Despi e his, he geo he mal po en ial o his a ea has no ye been s udied in de ail.
1.1. Radiogenic Hea P oduc ion
Rocks con ain a iable amoun s o p imo dial adionuclides ha gene a e hea o a
ce ain ex en , due o he con e sion o he adioac i e decay mass o ene gy. These include
40K and he adioiso opes o he decay se ies o 238U, 235U, and 232Th.
The hea gene a ed by he na u al adioac i e decay in he Ea h’s c us cons i u es a
subs an ial po ion o he e es ial hea low. Some au ho s [
21
,
22
] ha e p oposed ha he
adiogenic hea p oduc ion o g ani e ocks (RHP in
µ
W
·
m
−3
) can be calcula ed by aking
in o accoun he hea gene a ion cons an (i.e., he amoun o hea eleased pe uni ime
and pe g am o U, Th, and K) and he u anium, ho ium, and po assium concen a ions
(CU,CTh, and CK, espec i ely) in a ock by using he ollowing exp ession:
RHP = 10 −5ρ(9.52 CU+ 2.56 CTh + 3.48 CK) (1)
whe e
ρ
is he densi y o he ock (kg m
−3
) and C
U
,C
Th
, and C
K
a e he concen a ions o
u anium (weigh ppm), ho ium (weigh ppm), and po assium (weigh %), espec i ely.
Appl. Sci. 2022,12, 11965 3 o 15
The hea p oduc ion uni (
µ
Wm
−3
) may be con e ed in o hea gene a ion uni s
(
1 HGU
= 10
−13
cal cm
−3
s
−1
) by he equi alence 1 HGU =
µ
W m
−3
es ima ed o he
Gansboden g ani e gneiss a he Guspisbach hea low si e in he Cen al Alps o Swi ze -
land [
23
]. The adioelemen concen a ion (U ) is calcula ed by U equi alen s: 1 ppm o U
in equilib ium (1 ppm eU) = 1 U ; 1 ppm o Th equilib ium (1 ppm eTh) = 0.5 U ; 1 w % o
K=2U [24,25].
The RHP is a scala pe ophysical p ope y independen o in si u empe a u e and
p essu e. Bo h
232
Th and
238
U a e he mos impo an adionuclides ha con ibu e o hea
p oduc ion (abou 85%), while
40
K con ibu es o a lesse ex en [
17
]. They also gi e ise o
daugh e adionuclides p esen in he Ea h’s c us ocks, wi h he U and Th decay chains
being he mos geologically signi ican in such hea p oduc ion.
The e a e se e al me hods ha allow de e mina ion o he adioiso ope concen a ions
in ocks o assessing he a e o hea p oduc ion. Some labo a o y analy ical echniques
indi ec ly assess he ac i i y concen a ions in ock samples, namely, induc i ely cou-
pled plasma a omic emission spec ome y and mass spec ome y, ins umen al neu on
ac i a ion analyses, and X- ay luo escence spec ome y. Low backg ound gamma- ay
spec ome y di ec ly es ima es he ac i i y concen a ions o such adionuclides [
22
,
26
].
The la e echnique is he only one ha gi es di ec indica ion o any possible disequilib-
ium in he U and Th decay chains. Howe e , disequilib ium is in equen in ocks [27].
I is also possible o assess he concen a ions o adioiso opes by handheld, ca -
bo ne, and ai bo ne gamma- ay spec ome y. These echniques a e used o es ima ing he
concen a ions o adioelemen s in he su ace ocks, and bo ehole gamma- ay spec ome y
is used o acqui e a con inuous spec um o he U, Th, and K concen a ions o subsu ace
ocks [
28
]. This kind o ield me hod allows a quick, albei imp ecise, assessmen o he
adioiso ope con en s o ocks, making i possible o assess he hea p oduc ion om hese
da a [
29
]. Labo a o y analyses equi e ieldwo k, sampling, and analyses ha a e cos ly and
ime-consuming. Howe e , po able gamma- ay spec ome y (GRS) can be a quick and
cheap p ocedu e. When GRS measu emen s a e ca ied on ock ou c ops, a 2
π
geome y
(plain a ea) is needed o a oid o e - o unde es ima ion o he adioiso opes o K, U, and
Th [29].
1.2. Aim o he S udy
In his wo k, ou main goal was o assess he po en ial adiogenic hea p oduc ion
o he unde g ound ock unde lying he main campus o he Uni e si y o A Co uña (A
Co uña, no hwes Spain). To ha end, we analysed he K, Th, and U con en s o ock
ou c ops on he campus. We also es ima ed he K, Th, and U con en s o he sampled ock
ou c ops by GRS, o compa ison pu poses. We buil adionuclide and hea p oduc ion
maps o he campus based on hese da a, o con i m whe he GRS is sui able o es ima ion
o he RHP, wi h he p ospec o being used in he u u e o he use o geo he mal ene gy
on he campus and explo ing he RHP o he a ea wi h GRS.
2. S udy A ea
The au onomous communi y o Galicia (no hwes Spain) is mos ly on P ecamb ian
and Palaeozoic basic–ul abasic me amo phic and g ani oid ocks [
20
]. Th ee ypes o
g ani ic ocks can be ound: calc-alkaline syncinema ic, pe aluminous syncinema ic and
la e-collisional cinema ic, and calc-alkaline la e-collisional [
20
]. In he i s g oup, bo h
g anodio i e and monzog ani es a e he mos equen , wi h high alues o K [
30
]. Pe a-
luminous g ani oids— he mos abundan — a y widely in composi ion and ex u e [
31
],
while a ange o monzog ani es p edomina e among la e-collisional g ani es [
32
]. One o
he mos ex ended pe aluminous g ani oid is he g anodio i e o A Co uña. The ock is
p esen in mos o he a ea o he ci y o A Co uña (abou 80%), which has 250,000 inhabi-
an s (Figu e 1), wi h a ound 90% o he popula ion (a ound 225,000) li ing in he a ea o
such ock.
Appl. Sci. 2022,12, 11965 4 o 15
Appl. Sci. 2022, 12, x FOR PEER REVIEW 4 o 15
la e-collisional cinema ic, and calc-alkaline la e-collisional [20]. In he i s g oup, bo h
g anodio i e and monzog ani es a e he mos equen , wi h high alues o K [30]. Pe -
aluminous g ani oids— he mos abundan — a y widely in composi ion and ex u e [31],
while a ange o monzog ani es p edomina e among la e-collisional g ani es [32]. One o
he mos ex ended pe aluminous g ani oid is he g anodio i e o A Co uña. The ock is
p esen in mos o he a ea o he ci y o A Co uña (abou 80%), which has 250,000 in-
habi an s (Figu e 1), wi h a ound 90% o he popula ion (a ound 225,000) li ing in he
a ea o such ock.
Figu e 1. Loca ion o he El iña-A Zapa ei a campus o he Uni e si y o A Co uña (no hwes
Spain), wi h a geological map o he s udy a ea (modi ied om [33]). Sa elli e image om Google
Ea h. Sampling si es a e ma ked and explained in Table 1.
The main campus o he Uni e si y o A Co uña (El iña-A Zapa ei a) is loca ed on
his ock. Such ock shows wo acies: ea ly-s age and la e-s age; bo h a e e y simila in
colou (g ey), ex u e (coa se-g ained), and chemical and mine alogical composi ion,
wi h qua z, plagioclase, mic ocline, and bio i e as he main mine als [20]. Mo e musco-
i e is obse ed in he la e s age. The ock is usually wea he ed, showing an och e colou
Figu e 1.
Loca ion o he El iña-A Zapa ei a campus o he Uni e si y o A Co uña (no hwes Spain),
wi h a geological map o he s udy a ea (modi ied om [
33
]). Sa elli e image om Google Ea h.
Sampling si es a e ma ked and explained in Table 1.
The main campus o he Uni e si y o A Co uña (El iña-A Zapa ei a) is loca ed on
his ock. Such ock shows wo acies: ea ly-s age and la e-s age; bo h a e e y simila in
colou (g ey), ex u e (coa se-g ained), and chemical and mine alogical composi ion, wi h
qua z, plagioclase, mic ocline, and bio i e as he main mine als [
20
]. Mo e musco i e is
obse ed in he la e s age. The ock is usually wea he ed, showing an och e colou and loss
o cohesion. Acco ding o he geological map o he ci y (and campus) o A Co uña, he
con ac be ween bo h acies is loca ed in he El iña campus a ea, al hough a p esen i is
below he buildings and u ban s uc u es o he campus and below he sedimen s in some
pa s. In his s udy, we sampled bo h acies.
Appl. Sci. 2022,12, 11965 5 o 15
Table 1.
Posi ion and coo dina es o he s udied ou c ops and samples (WGS84). WD:
wea he ing deg ee.
Sample La i ude Longi ude WD Loca ion
P1 43◦19031.1900 S 8◦24031.600 W I Back o he P o esso s0building in Facul y o Philology
P2 43◦19037.7800 S 8◦24027.4800 W I Slope be ween Facul ies o Philology and Sciences
P3 43◦19043.9800 S 8◦24026.400 W I Beside Casa del F ancés
P4 43◦19034.8100 S 8◦24035.3900 W I Beside Facul y o Sciences
P5 43◦2008.7100 S 8◦2500.2300 W I Beside UDC kinde ga en
P6 43◦2001.7200 S 8◦24056.700 W I Ca pa k o Facul ies o Law and Educa ion
P7 43◦19058.7100 S 8◦24030.5900 W II Back o CICA building
P8 43◦19033.2500 S 8◦24033.0900 W II Sou hwes o Facul y o Sciences
P9 43◦19041.1700 S 8◦24050.8300 W II Su oundings o Cas o de El iña a chaeological si e
P10 43◦19058.7100 S 8◦24034.5900 W III Ca pa k o he Schools o Ci il Enginee ing and In o ma ics
P11 43◦2001.2300 S 8◦2503.5900 W II Pa h in on o Facul y o Law
P12 43◦2007.9900 S 8◦24054.1400 W II Close o Spo s Cen e
P13 43◦19050.1900 S 8◦24019.0500 W I Monumen o El iña’s Ba le
P14 43◦19035.5500 S 8◦24044.6400 W III Slope in e campus oad
P15 43◦19042.900 S 8◦24037.0500 W III Canedo 15, El iña
P16 43◦19052.2300 S 8◦24041.2200 W V Slope Cen al Resea ch Se ice
The El iña-A Zapa ei a campus (Figu e 1) has a o al su ace a ea o 63 ha and 27.3 ha
o buil a ea dis ibu ed o e 20 buildings: 12 o eaching and 8 o spo s, esea ch, and
adminis a i e use. Mos o hese buildings we e buil be ween 1995 and 2006 and a e used
o 13–14 h a day. The as majo i y a e hea ed by gas oil, excep o i e ha a e hea ed by
elec ic adia o s and one wi h a hea pump [
34
]. The o al a e age annual consump ion
o hese buildings is app oxima ely 14,300 MWh [
34
]. The a e age annual consump ion o
diesel o hea ing is 9500 MWh/yea , which is equi alen o app oxima ely 725,880 li es o
diesel. This uel gene a es CO2emissions o 260 ons pe yea on a e age.
3. Ma e ials and Me hods
3.1. Sampling and Da a Acquisi ion
Selec ion o loca ions on he campus o sampling was pe o med a e a p elimina y
explo a ion was ca ied ou ia Google Ea h and conside ing he geological map. As he
con ac be ween he wo g anodio i e acies occu s in he campus a ea ( he ea ly-s age acies
on he wes and he la e-s age acies on he eas o he campus), we conside ed bo h acies in
he sampling p ocess. A e he p eselec ion o a eas o in e es , a walking ield explo a ion
was ca ied ou , looking o ock ou c ops. Mos such ou c ops we e loca ed in he wes
and sou h pa s o he a ea. A o al o 16 ou c ops o bo h acies we e chosen o he s udy
(Table 1and Figu e 1). In such ou c ops, gamma- ay spec ome y measu emen s we e
pe o med, and ock samples we e aken o geochemical analyses on he same ou c ops
whe e he gamma- ay measu emen s we e ca ied ou .
3.2. In Si u Gamma-Ray Spec ome y
Gamma- ay spec a we e acqui ed in si u in he ield wi h a po able gamma- ay
spec ome e (GF Ins umen s Gamma Su eyo Va io), equipped wi h a BGO p obe
(Bi
4
Ge
3
O
12
) wi h 2018 channels, along wi h a 51 mm
×
51 mm de ec o (103 cm
3
) wi h
p obe dimensions o 70 mm and a leng h o 290 mm (VB6), and a shielded pho omul i-
plie . This equipmen enables measu emen o ene gies up o 3 MeV. Measu emen s we e
aken by di ec p obe con ac a ock ou c ops wi h a iable geome y. We ollowed he
ecommenda ions gi en by he In e na ional A omic Ene gy Agency (IAEA), as desc ibed
by E di-K ausz e al. [
28
] o s a ic measu emen s (i.e., eco ding a spec um in a gi en
poin du ing a speci ic amoun o ime). A pe iod o 180 s is ecommended by he man-
u ac u e o op imise ime and p ecision. S a is ical e o unce ain ies—es ima ed o be
6% o K, 30% o U, and 16% o Th—we e conside ed, acco ding o he manu ac u e ’s
speci ica ions o low gamma- ay emissions. h p://www.g ins umen s.cz/index.php?

Appl. Sci. 2022,12, 11965 6 o 15
menu=gi&smenu=ig &con =su eyo _V_&ea = s (accessed on 11 Oc obe 2022). These
e o s we e due o backg ound adia ion ha mos ly o igina ed om h ee main sou ces:
a mosphe ic adon and i s daugh e s, cosmic ays, and ins umen backg ound adia ion.
Mo eo e , he geome y o measu ed su ace mus be plana (2
π
geome y), as possible
su ace oughness o he measu ed e ain and any nea by opog aphic ea u es can bo h
cause possible inaccu acies in he measu emen s. A second measu emen was ca ied
ou o each s udied ou c op using a lead collima o specially designed by he de ice’s
manu ac u e o educe he backg ound gamma adia ion om su oundings.
F om hese spec a, we can ex ac es ima ed con en s o K (po assium, mass pe -
cen age), eU (u anium equi alen s, in mass pa s pe million (ppm)), and eTh ( ho ium
equi alen s, also in ppm). While po assium con en s we e es ima ed om he peak o 40K
a 1.461 MeV (ene gy ange 1.366 keV–1.564 keV), he e we e se e al adioac i e iso opes
in he u anium and ho ium decay se ies. Thus, we used “equi alen s” o u anium and
ho ium, assuming secula equilib ium. Es ima ions o eU and eTh we e made om peaks
o
214
Bi (bismu h) a he 1.764 MeV peak (ene gy ange 1.57 keV–1.959 keV) and
208
Tl
( hallium) a he 2.615 MeV peak (ene gy ange 2.42 keV–2.81 keV), espec i ely.
3.3. Geochemical Analyses
Fo compa ison wi h da a acqui ed wi h he gamma- ay spec ome e , he ock samples
we e analysed ia X- ay luo escence spec ome y (XRF) o assess he con en s o bo h
majo and mino elemen s in he ock samples. The samples we e c ushed o a g ain size
below 63
µ
m, and XRF measu emen s we e pe o med using a B uke -Nonius S4 Pionee
wa eleng h-dispe si e luo escence spec ome e unde helium pu ging a he Uni e si y
o A Co uña.
To assess ace elemen s, induc i ely coupled plasma mass spec ome y (ICP-MS)
was used. ICP-MS analyses we e ca ied ou using a The mo Scien i ic
™
ELEMENT XR
™
ICP-MS de ice. Fo sample p epa a ion, he powde ed samples we e mixed wi h an equal
amoun o li hium e abo a e lux, placed in a ca bon c ucible, and used a 1000
◦
C in
a u nace o 30 min. A e cooling o he mel , he esul an usion bead was b ie ly
g ound and dissol ed in 100 mL o 4% HNO
3
/2% HCl
3
solu ion, which was hen analysed
by ICP-MS.
The densi y o he samples was es ima ed using an Accupyc 1340 Mic ome i ics gas
pycnome e , which used 99.995% pu e helium o de e mine he eal densi y o samples, by
measu ing he p essu e change o helium in a calib a ed olume.
3.4. Geog aphic In o ma ion Sys em
Spa ial da a p ocessing was ca ied ou using he QGIS geog aphic in o ma ion sys em
( .2.18.25). Using his so wa e, an in e pola ion o he da a was ca ied ou o geochemical
analyses and GRS da a, using he in e se dis ance weigh ing (IDW) me hod o he alues
ob ained o he concen a ions o U, Th, and K, as well as he alues ob ained o RHP. Bo h
ypes o da a we e p ojec ed on o he MDT05-LiDAR digi al e ain models co esponding
o shee s H0021 and H0045 [35], based on he UTM coo dina es o each sampling poin .
4. Resul s
4.1. Geochemical Analyses
The XRF esul s (Table 2) indica ed ha he samples’ esul s we e in he usual ange
be ween g anodio i es and g ani es sensu s ic o [
36
], since hal o he samples had a highe
SiO
2
con en han expec ed o g anodio i es. The SiO
2
con en s anged be ween 74.36%
(P7) and 62.9% (P15), wi h an a e age o 69.04%, al hough in one sample i was 53.7% (P16).
This Si en ichmen was obse ed in bo h acies and seemed mo e common owa ds he
sou h o he s udy a ea. Simila ly, he concen a ions o Al
2
O
3
anged be ween a maximum
o 17.1% (P12) and a minimum o 13.12% (P2), being e en highe in sample P16 (21.1%),
wi h an a e age o 15.53%. In sample P16, he concen a ion o Fe
2
O
3
was a ypically high,
while ha o Na
2
O was a ypically low. This was due o i s high wea he ing, making i a
Appl. Sci. 2022,12, 11965 7 o 15
sap oli e (i.e., g ade V in he wea he ing classi ica ions, [
37
]). The emainde o he samples
we e be ween g ades I and III o wea he ing. Gi en he impo ance o K
2
O in he calcula ion
o he a e o hea po en ial, he concen a ion o K (%) was es ima ed s oichiome ically
om hese da a (Table 3). This enabled compa ison o he K concen a ion es ima ed by
XRF wi h ha es ima ed by GRS.
Table 2. Oxides p o ided by XRF esul s (in %) and es ima ed densi y (in kg m−3).
Sample SiO2Al2O3Fe2O3CaO MgO Na2O K2O TiO2MnO P2O5S O BaO Z O2LOI To al Densi y (kg m−3)
P1 70.04 14.96 2.66 1.12 0.45 3.32 4.54 0.21 0.05 0.07 0.03 0.06 <0.001 2.35 99.88 2.646
P2 71.4 13.12 3.64 1.59 0.85 2.9 3.8 0.36 0.07 0.22 0.03 0.07 <0.001 1.84 99.9 2.663
P3 69.88 14.15 3.03 1.32 0.81 2.76 5.16 0.35 0.06 0.17 0.03 0.09 <0.001 2.1 99.91 2.644
P4 72.73 13.96 1.99 0.36 0.39 2.81 4.34 0.25 0.02 0.08 0.01 0.04 <0.001 2.93 99.92 2.656
P5 71.57 14.46 2.12 0.94 0.47 3.26 4.88 0.24 0.03 0.23 0.02 0.05 <0.001 1.66 99.93 2.656
P6 72.42 14.62 1.36 0.52 0.37 2.8 5.1 0.23 0.02 0.18 0.02 0.05 <0.001 2.24 99.93 2.666
P7 74.36 14.37 1.22 0.2 0.15 3.26 4.28 0.05 0.03 0.08 0.01 0.02 <0.001 1.93 99.96 2.674
P8 71.1 14.86 2.21 0.37 0.38 3.23 5.2 0.23 0.03 0.09 0.01 0.05 <0.001 2.16 99.92 2.65
P9 71.67 14.32 2.18 0.45 0.49 2.36 5.14 0.26 0.02 0.17 0.02 0.05 <0.001 2.78 99.91 2.638
P10 67.29 15.7 3.3 1.69 0.85 3.59 5.34 0.35 0.08 0.17 0.05 0.09 <0.001 1.42 99.92 2.679
P11 68.7 16.3 1.2 1 0.44 4.9 5.4 0.19 0.024 0.22 0.009 <0.008 0.013 1.5 99.948 2.656
P12 67.4 17.1 1.4 0.81 0.68 4.1 6.1 0.24 0.019 0.29 0.011 <0.008 0.015 1.8 100.025 2.654
P13 72.3 13.2 2.4 1.6 0.99 3.7 4.1 0.35 0.053 0.22 0.026 0.052 0.026 0.9 99.957 2.688
P14 67.2 17.1 1.4 0.75 0.47 4.8 5.3 0.23 0.027 0.25 0.015 <0.008 0.014 1.9 99.575 2.672
P15 62.9 19.1 1.7 1.3 0.56 4.4 7.4 0.22 0.039 0.1 0.032 0.12 0.018 1.8 99.777 2.635
P16 53.7 21.1 8.1 0.43 1.3 1.2 5.2 0.71 0.032 <0.005 0.014 0.05 0.071 7.6 99.578 2.619
Mean 69.04 15.53 2.49 0.90 0.60 3.34 5.08 0.28 0.04 0.17 0.02 0.06 - 2.31 99.88 2.66
Sd 4.97 2.17 1.67 0.49 0.29 0.94 0.85 0.14 0.02 0.07 0.01 0.03 - 1.50 0.13 0.02
Table 3. Resul s o GRS measu emen s and geochemical analyses.
Scheme
GRS GRS wi h Collima o XRF + ICPMS
U (ppm) Th (ppm) K (%) U (ppm) Th (ppm) K (%) U (ppm) Th (ppm) K (%)
P1 16.7 46.6 4.63 20.3 45.4 4.38 24.9 36.5 3.77
P2 12.2 44.9 4.24 14.7 45.4 4.26 20.7 52.1 3.15
P3 18.9 45.4 4.27 22.5 43.6 4.17 15 42.4 4.28
P4 7.2 17.6 4.65 9.4 17.1 4.38 16.25 25.5 3.60
P5 10.3 19.8 4.23 13 18.3 3.95 12.15 24.4 4.05
P6 11.9 24.3 4.7 16.3 22.1 4.28 16.5 22.9 4.23
P7 14.8 50.5 5.16 17.4 53.7 5.16 2.35 4.68 3.55
P8 11.5 23.2 4.52 14.2 21.8 4.26 12.35 22 4.32
P9 9.8 25.4 5.08 11.3 24.5 5.43 14.4 26.4 4.27
P10 14.5 46.4 4.48 17.9 48.7 4.05 10.35 61.2 4.43
P11 9.1 20.7 4.78 11.9 23.3 4.72 9.83 19.15 3.23
P12 8.5 23.2 4.21 11.1 23.6 3.65 17.5 21.7 3.64
P13 11.5 57.1 6.23 13.4 54.1 6.4 14.45 49.6 2.45
P14 18.2 31.9 6.77 23 30 6.68 22 23.2 3.17
P15 33.8 106.8 12.76 9.9 26.7 2.54 15 33.6 4.42
P16 17.1 60.2 6.71 23 67.3 6.35 24 73.1 3.11
Mean 14.13 40.25 5.46 15.48 33.65 3.73 15.58 35.35 4.67
Sd 6.34 22.78 2.12 5.72 17.80 0.59 4.66 15.59 1.10
The densi y measu emen s (Table 2) show ha i anged be ween 2.619 kg m
−3
and
2.688 kg m
−3
, wi h an a e age alue o 2.660 kg m
−3
. The lowes alue co esponded o
he mo e wea he ed sample (P16).
The ICP-MS esul s p o ide he concen a ions o U (ppm) and Th (ppm) in he
samples s udied. Table 3shows hese da a and includes an es ima e o he K concen a ion
ob ained by XRF, which was used o es ima e he geo he mal po en ial. The alues ob ained
o U anged be ween 2.45 ppm and 24.9 ppm, wi h an a e age o 15.6 ppm. The lowes
alue, obse ed in sample P7, was a ypical when compa ed o he es o he samples, since
he closes was 9.83 ppm (P11). These da a allow us o compa e he concen a ion o U
Appl. Sci. 2022,12, 11965 8 o 15
wi h ha o Th h ough he Th/U a io (Table 4). In he samples s udied, he a io anged
be ween 0.25 and 0.74 om he geochemical analyses (Table 4), wi h he a e age Th/U
a io being 0.49. In he case o K, he ange was smalle —be ween 2.45% and 4.43%, wi h
he a e age being 4.67%, and wi hou ou lie s (Table 3).
Table 4.
Es ima ed a ios o Th/U and K/U o he geochemical analyses and GRS measu emen s. U,
Th, and K a ios ob ained by geochemical analyses and he wo di e en GRS measu emen s a e also
p o ided. Ou lie s a e ma ked in ed.
Sample GRS GRS wi h
Collima o
XRF +
ICP-MS GRS/GRS wi h Collima o GRS/XRF + ICP-MS GRS wi h Collima o /XRF
+ ICP-MS
Th/U K/U Th/U K/U Th/U K/U U/U Th/Th K/K U/U Th/Th K/K U/U Th/Th K/K
P1 0.68 3.61 0.36 4.63 0.45 6.61 0.82 1.03 1.24 0.67 1.28 1.23 0.82 1.24 1.16
P2 0.40 2.88 0.27 3.45 0.32 6.56 0.83 0.99 0.87 0.59 0.86 1.34 0.71 0.87 1.35
P3 0.35 4.43 0.42 5.40 0.52 3.50 0.84 1.04 1.03 1.26 1.07 1.00 1.50 1.03 0.97
P4 0.64 1.55 0.41 2.15 0.55 4.51 0.77 1.03 0.67 0.44 0.69 1.29 0.58 0.67 1.22
P5 0.50 2.43 0.52 3.29 0.71 3.00 0.79 1.08 0.75 0.85 0.81 1.04 1.07 0.75 0.98
P6 0.72 2.53 0.49 3.81 0.74 3.90 0.73 1.10 0.97 0.72 1.06 1.11 0.99 0.97 1.01
P7 0.50 2.87 0.29 3.37 0.32 0.66 0.85 0.94 11.47 6.30 10.79 1.45 7.40 11.47 1.45
P8 0.56 2.54 0.50 3.33 0.65 2.86 0.81 1.06 0.99 0.93 1.05 1.05 1.15 0.99 0.99
P9 0.55 1.93 0.39 2.08 0.46 3.38 0.87 1.04 0.93 0.68 0.96 1.19 0.78 0.93 1.27
P10 0.17 3.24 0.31 4.42 0.37 2.34 0.81 0.95 0.80 1.40 0.76 1.01 1.73 0.80 0.91
P11 0.51 1.90 0.44 2.52 0.51 3.05 0.76 0.89 1.22 0.93 1.08 1.48 1.21 1.22 1.46
P12 0.81 2.02 0.37 3.04 0.47 4.80 0.77 0.98 1.09 0.49 1.07 1.16 0.63 1.09 1.00
P13 0.29 1.85 0.20 2.09 0.25 5.90 0.86 1.06 1.09 0.80 1.15 2.54 0.93 1.09 2.61
P14 0.95 2.69 0.57 3.44 0.77 6.95 0.79 1.06 1.29 0.83 1.38 2.14 1.05 1.29 2.11
P15 0.45 2.65 0.32 3.90 0.37 3.39 3.41 4.00 3.18 2.25 3.18 2.89 3.41 3.18 2.89
P16 0.33 2.55 0.28 3.62 0.34 7.72 0.74 0.89 0.92 0.71 0.82 2.16 0.96 0.92 2.04
Mean 0.53 2.60 0.38 3.41 0.49 4.56 0.80 1.01 1.04 0.81 1.00 1.41 1.01 0.99 1.37
Sd 0.20 0.73 0.10 0.93 0.16 1.75 0.04 0.07 0.06 0.27 0.20 0.48 0.32 0.19 0.50
R 0.88 0.85 0.62 0.47 0.48 −0.38 0.98 0.99 0.97 0.50 0.90 −0.62 0.54 0.93 −0.64
The GRS allowed us o es ima e he concen a ions o U, Th, and K in he ou c ops
s udied and om which he samples analysed in he labo a o y we e aken. The esul s o
he measu emen s aken by he GRS, wi h and wi hou he collima o , a e shown in Table 3
and Figu e 2. These da a can be compa ed wi h hose o K concen a ion ob ained by XRF
and hose o U and Th concen a ions ob ained by ICP-MS. Compa a i ely, he es ima es o
he concen a ion o U by he GRS wi h and wi hou a collima o we e simila (Figu e 2A),
wi h a sligh a e age o e es ima ion o 20% being obse ed in he measu emen s made
wi h he collima o (Tables 3and 4), al hough one o he samples (P15) p o ided a e y
conside able unde es ima ion, which is why i was excluded om he compa ison. Fo Th,
he collima o measu emen s p o ided a e y good i , wi h a a io o 1.01 (Figu e 2D and
Tables 3and 4). A ela ionship simila o ha o Th was ob ained o K, wi h a a io o 1.04
(Figu e 2G and Tables 3and 4).
Appl. Sci. 2022,12, 11965 9 o 15
Appl. Sci. 2022, 12, x FOR PEER REVIEW 10 o 17
Figu e 2. Es ima ed con en s o he di e en s udied elemen s wi h geochemical analyses and GRS:
(A,D,G) es ima ed con en s ob ained by GRS wi hou s. wi h collima o ; (B,E,H) es ima ed con-
en s ob ained by GRS wi hou collima o s. geochemical analyses (XRF + ICP-MS); (C,F,I) es i-
ma ed con en s ob ained by GRS wi h collima o s. geochemical analyses (XRF + ICP-MS).
Compa ing he es ima es o U ob ained by GRS and ICP-MS, i can be seen ha he
measu emen s made wi h and wi hou he collima o unde es ima e he concen a ions
ob ained ia ICP-MS by 20% ( a io = 0.81) (Figu e 2B,C and Tables 3 and 4) i we exclude
he abno mally low di e ences in samples P7 and P15 (including hese wo samples, i is
o e es ima ed by 50%). In he case o Th (Figu e 2E,F and Tables 3 and 4), samples P7 and
P15 also p o ide a ypical di e ences o measu emen s wi h and wi hou he collima o ,
and he a io o bo h wi hou conside ing hese samples is close o 1.00 (wi hou he col-
lima o ) and 0.99 (wi h he collima o ). Fo K, g ea e a iabili y is obse ed in he esul s
(Figu e 2H,I and Tables 3 and 4), and he a e age a io be ween he measu emen s wi h
he collima o and XRF is 1.37, while he a e age a io wi hou he collima o is e y
simila (1.41), wi hou conside ing he sample P15. The e o e, GRS o e es ima es he K
concen a ion by app oxima ely 40%. In ac , a nega i e co ela ion can be obse ed be-
ween he K es ima es made by GRS and he geochemical analyses (Figu e 2H,I and Table
4).
4.2. Geo he mal Po en ial
Fo he whole se o samples, he esul s o calcula ing he adiogenic hea p oduc-
ion (RHP) indica e a alue o 6.54 ± 2.25 μW m−3 (Table 5). Sample P7 is he one wi h he
lowes RHP (1.20 μW m−3), being is clea ly di e en om he o he samples. The ange o
Figu e 2.
Es ima ed con en s o he di e en s udied elemen s wi h geochemical analyses and GRS:
(
A
,
D
,
G
) es ima ed con en s ob ained by GRS wi hou s. wi h collima o ; (
B
,
E
,
H
) es ima ed con en s
ob ained by GRS wi hou collima o s. geochemical analyses (XRF + ICP-MS); (
C
,
F
,
I
) es ima ed
con en s ob ained by GRS wi h collima o s. geochemical analyses (XRF + ICP-MS).
Compa ing he es ima es o U ob ained by GRS and ICP-MS, i can be seen ha he
measu emen s made wi h and wi hou he collima o unde es ima e he concen a ions
ob ained ia ICP-MS by 20% ( a io = 0.81) (Figu e 2B,C and Tables 3and 4) i we exclude
he abno mally low di e ences in samples P7 and P15 (including hese wo samples, i
is o e es ima ed by 50%). In he case o Th (Figu e 2E,F and Tables 3and 4), samples
P7 and P15 also p o ide a ypical di e ences o measu emen s wi h and wi hou he
collima o , and he a io o bo h wi hou conside ing hese samples is close o 1.00 (wi hou
he collima o ) and 0.99 (wi h he collima o ). Fo K, g ea e a iabili y is obse ed in he
esul s (Figu e 2H,I and Tables 3and 4), and he a e age a io be ween he measu emen s
wi h he collima o and XRF is 1.37, while he a e age a io wi hou he collima o is e y
simila (1.41), wi hou conside ing he sample P15. The e o e, GRS o e es ima es he
K concen a ion by app oxima ely 40%. In ac , a nega i e co ela ion can be obse ed
be ween he K es ima es made by GRS and he geochemical analyses (Figu e 2H,I and
Table 4).
4.2. Geo he mal Po en ial
Fo he whole se o samples, he esul s o calcula ing he adiogenic hea p oduc ion
(RHP) indica e a alue o 6.54
±
2.25
µ
W m
−3
(Table 5). Sample P7 is he one wi h he
lowes RHP (1.20
µ
W m
−3
), being is clea ly di e en om he o he samples. The ange o