geosciences
A icle
Hyd ochemis y and 222Rn Concen a ions in
Sp ing Wa e s in he A id Zone El G ane o,
Chihuahua, Mexico
Ma usia Ren e ía-Villalobos 1,*, Alejand o Co a ubias-Muños 1, Al edo Pinedo-Ál a ez 1and
Guille mo Manjon-Collado 2
1Na u al Resou ces, Facul ad de Zoo ecnia y Ecologia, Uni e sidad Au ónoma de Chihuahua,
Pe i é ico F ancisco R. Almada km 1, 31415 Chihuahua, Mexico; [email p o ec ed] (A.C.-M.);
[email p o ec ed] (A.P.-Á.)
2Applied Nuclea Physics, ETSA, Uni e sidad de Se illa, A enida Reina Me cedes 2, 41012 Se illa, Spain;
[email p o ec ed]
*Co espondence: [email p o ec ed]; Tel.: +52-614-4340304
Academic Edi o s: Ca los Al es and Jesus Ma inez-F ias
Recei ed: 24 Oc obe 2016; Accep ed: 6 Ma ch 2017; Published: 11 Ma ch 2017
Abs ac :
Wa e in a id and semi-a id en i onmen s is cha ac e ized by he p esen a ion o complex
in e ac ions, whe e dissol ed chemical species in high concen a ions ha e nega i e e ec s on he
wa e quali y. Radon is p esen in a eas wi h a high u anium and adium con en , and i is he main
con ibu o o he annual e ec i e dose ecei ed by humans. Theobjec i e o his s udywas o e alua e
concen a ions o
222
Rn and he wa e quali y o sp ing wa e s. Wa e was classi ied as calcium sul a ed
and sodium sul a ed. Mos o he wa e samples wi h high adon concen a ions p esen ed highe
concen a ions o sul a es, luo ides, and o al dissol ed solids.
222
Rn concen a ions may be a ibu ed
o possible enhancemen o
226
Ra due o empe a u e and salini y o wa e , as well as e apo a ion
a e. In 100% o he sampled sp ing wa e s he
222
Rn le els exceeded he maximum accep able limi
which is p oposed by in e na ional ins i u ions. A idi y inc eases adiological isk ela ed o
222
Rn
dose because sp ing wa e s a e he main supply sou ce o local popula ions. The implemen a ion o
en i onmen al educa ion, s a egies, and echnologies o emo e he con aminan s om he wa e
a e essen ial in o de o educe he heal h isk o local inhabi an s.
Keywo ds: dissol ed chemical species; 222Rn; sp ings; a id zones; Chihuahua
1. In oduc ion
Su ace and g oundwa e na u ally con ain dissol ed chemical species, mainly as a esul o
leaching om ocks ha a e in con ac wi h he wa e . Howe e , he sum o he concen a ions o
Ca
+2
, Mg
+2
, K
+1
, Na
+1
, HCO
3−
, CO
3−2
, SO
4−2
, Cl
−
, F
−
, NO
3−
, and H
4
SiO
4
dissol ed in he wa e
can nega i ely a ec he wa e quali y o bo h use and human consump ion.
222
Rn is a adioiso ope ha belongs o he u anium amily. I is o med by he adioac i e
disin eg a ion o
226
Ra and is an alpha emi e [
1
]. Radon is an ine gas, which displaces ela i ely
eely h ough po es in he soil o agmen ed ocks. When hese po es a e sa u a ed wi h wa e , as
happens in soils and ocks si ua ed below he wa e able, adon is dissol ed in o he wa e in which i
is anspo ed. The e o e, adon p esen in g oundwa e can each he su ace wa e om whe e i
can emana e o he a mosphe e. The solubili y o adon in wa e depends mainly on i s concen a ion,
as well as wa e olume, empe a u e, and p essu e; hus, he con en o adon dec eases wi h an
inc ease in empe a u e [
2
,
3
] and inc eases o high p essu e [
4
]. Exposu e o adon and i s disin eg a ion
p oduc s has been associa ed wi h an inc ease in he isk o lung and s omach cance h ough inhala ion
Geosciences 2017,7, 12; doi:10.3390/geosciences7010012 www.mdpi.com/jou nal/geosciences
Geosciences 2017,7, 12 2 o 15
o inges ion, espec i ely [
5
]. The exposu e o adon by inhala ion is likely 4 imes highe han by
inges ion. The e o e, he isk o lung cance due o inhala ion is a much highe han he isk o
s omach cance by wa e consump ion. Fu he mo e, adon is conside ed o ep esen app oxima ely
50% o he annual e ec i e dose ecei ed by human beings, wi h an o igin in na u al sou ces [
6
].
Because o his, i is conside ed o i al impo ance o e alua e he le els o
222
Rn in he en i onmen .
P e ious esul s ha e shown ha he concen a ions o u anium, adium, and adon in su ace
and g ound wa e , e alua ed in se e al loca ions in he s a e o Chihuahua, a e mainly ele a ed in he
ci ies o Aldama and Chihuahua [
7
–
11
]. In addi ion, he wa e in he he mal sp ings o San Diego de
Alcalá, close o he in lux o he i e Chu ísca wi h he i e Conchos, has been obse ed o ha e a
high ac i i y concen a ion o dissol ed
226
Ra, wi h le els o 5.6 Bq/L [
7
]. Due o his, s udies on he
con en o dissol ed chemical species in na u al wa e bodies a e always impo an . The objec i e o
he p esen s udy was o e alua e concen a ions o
222
Rn and he wa e quali y o sp ing wa e s ha
a e he p incipal wa e sou ce o he local illages in he egion o El G ane o. These esul s can be
used o selec and es app op ia e ea men s o hese wa e s, be o e hey can be consumed o used
by human beings.
2. Ma e ials and Me hods
2.1. S udy A ea
The ese oi Luis L. León (G ane o dam) is loca ed in he municipali y o Aldama, a egion
cha ac e ized mos ly by a dese aspec , and p esen s a ocky moun ain ange o he no he n cen al
pla eau. Physiog aphically, i is loca ed in he ancien lake a ea o he Sie a Mad e O ien al [
12
],
a ound 28
◦
59
0
7
0 0
N and 105
◦
16
0
48
0 0
W, wi h an al i ude o 1080 m. The G ane o egion is composed o
elonga ed moun ain anges ha ollow a gene al N-NW o ien a ion and ha s and in isola ion om
he dese plains o his egion. The ou c opping ocks in he egion a e sedimen a y (mainly o ma ine
o igin), in usi e igneous, and olcanic, da ing om he Uppe Ju assic o he p esen [13].
On he o he hand, he semicon ined aqui e is placed in allu ial deposi and sequences o ocks
o low pe meabili y [
14
]. The exposed li hologic sequences a e composed o ca bona e and sandy-clay
ocks (Paleozoic ocks), calca eous-clayey sequence om Uppe Ju assic, e apo i ic and ca bona ed
ocks (C e aceous sequence), and con inen al ocks (composed by silica) om Paleogene-Neogene.
This aqui e is di ided in ou hyd ogeological uni s, acco ding o hei physical p ope ies. The sp ings
unde s udy a e placed in he hyd ogeological uni denomina ed UH3 [14]. I is cons i u ed by lowe
C e aceous limes ones ha ha e seconda y po osi y due o dissolu ion; owa ds moun ain anges
oo hills, he con ac o wa e wi h low pe meabili y ocks o igina es sp ings. The mos impo an
sp ing is loca ed in Cho e as Village. Figu e 1shows he loca ions o he sp ings ha we e sampled.
Likewise, he geological condi ions o El G ane o ese oi zone a e p esen ed in Figu e 2.
Geosciences 2017,7, 12 3 o 15
Geosciences 2017, 7, 12 3 o 16
Figu e 1. Sa elli e image o he s udy a ea, ese oi El G ane o. The sampling poin s indica e he loca ions
o he sp ings (P1, P2, P3, P4, P5, and P6).
Figu e 1.
Sa elli e image o he s udy a ea, ese oi El G ane o. The sampling poin s indica e he
loca ions o he sp ings (P1, P2, P3, P4, P5, and P6).
Geosciences 2017,7, 12 4 o 15
Geosciences 2017, 7, 12 4 o 16
Figu e 2. Geological map o G ane o ese oi zone.
2.2. Sampling
Sampling was ca ied ou in No embe 2014. Su ounding he El G ane o dam, he e a e six sou ces
o sp ing wa e which we e selec ed o sampling. The wa e om he sp ings eed he ese oi El
G ane o and a e he main wa e supply o he local inhabi an s. The physical-chemical p ope ies
empe a u e (T), o al dissol ed solids (TDS), oxida ion- educ ion po en ial (ORP), and pH o he sp ing
wa e s we e de e mined in si u. These pa ame e s we e measu ed using a mul ipa ame ic po able
equipmen (Oak on, Wa e p oo Mul i-pa ame e Tes s), p e iously calib a ed using con idence
solu ions o pH, ORP, and conduc i i y (OAKTON). To de e mine adon in he wa e , samples we e
collec ed in 20 mL scin illa ion ials [15]; a duplica e was collec ed o e e y sample. The sampling
loca ions we e ma ked wi h a global posi ioning sys em (GPS). In addi ion, a second wa e sample was
collec ed in 1 L polye hylene ecipien s wi h he capaci y o de e mine luo ides, ha dness, sul a es,
ni a es, calcium, magnesium, and sodium. These samples we e p ese ed on ice a a empe a u e o
app oxima ely 4 °C.
2.3. De e mina ion o Dissol ed Ions
Dissol ed chemical species we e de e mined using di e en no ma i e me hods s anda dized in he
Depa men o Wa e Quali y o he Resea ch Cen e o Ad anced Ma e ials (CIMAV).
E e y sample was analyzed by iplica e. Anions we e de e mined using he ollowing me hods: (a) o
sul a es he me hod p oposed by HACH was used, using a HACH spec opho ome e , model DR 2010
Figu e 2. Geological map o G ane o ese oi zone.
2.2. Sampling
Sampling was ca ied ou in No embe 2014. Su ounding he El G ane o dam, he e a e six
sou ces o sp ing wa e which we e selec ed o sampling. The wa e om he sp ings eed he ese oi
El G ane o and a e he main wa e supply o he local inhabi an s. The physical-chemical p ope ies
empe a u e (T), o al dissol ed solids (TDS), oxida ion- educ ion po en ial (ORP), and pH o he sp ing
wa e s we e de e mined in si u. These pa ame e s we e measu ed using a mul ipa ame ic po able
equipmen (Oak on, Wa e p oo Mul i-pa ame e Tes s), p e iously calib a ed using con idence
solu ions o pH, ORP, and conduc i i y (OAKTON). To de e mine adon in he wa e , samples we e
collec ed in 20 mL scin illa ion ials [
15
]; a duplica e was collec ed o e e y sample. The sampling
loca ions we e ma ked wi h a global posi ioning sys em (GPS). In addi ion, a second wa e sample was
collec ed in 1 L polye hylene ecipien s wi h he capaci y o de e mine luo ides, ha dness, sul a es,
ni a es, calcium, magnesium, and sodium. These samples we e p ese ed on ice a a empe a u e o
app oxima ely 4 ◦C.
2.3. De e mina ion o Dissol ed Ions
Dissol ed chemical species we e de e mined using di e en no ma i e me hods s anda dized in
he Depa men o Wa e Quali y o he Resea ch Cen e o Ad anced Ma e ials (CIMAV). E e y sample
was analyzed by iplica e. Anions we e de e mined using he ollowing me hods: (a) o sul a es he
me hod p oposed by HACH was used, using a HACH spec opho ome e , model DR 2010 (<1% o
measu emen unce ain y); and (b) luo ides we e analyzed using he ion-selec i e elec ode me hod
based on he ope a ion p ocedu e o he equipmen O ion 1260. Ca ions such as Ca, Mg, Na, K, and Si
we e de e mined ollowing Mexican egula ions [
16
] using an a omic abso p ion spec opho ome e
(Model A an a, Sigma, GBC Scien i ic Equipmen , Hampshi e, IL, USA). Because he ha dness o he
wa e is mainly in luenced by he ions Ca
+2
and Mg
+2
, exp essed in mg/L o CaCO
3
o meq/L, i was
calcula ed using Equa ion (1):
Geosciences 2017,7, 12 5 o 15
Ha dness (mg/L CaCO3) = (meq Ca/L + meq Mg/L) ×50 (1)
whe e 1 meq/L CaCO3is equi alen o 50 mg/L CaCO3.
2.4. De e mina ion o 222Rn
P epa a ion o he samples o de e mine he concen a ion o
222
Rn in su ace wa e was pe o med
using he ex e nal s anda d me hod ( ela i e analysis) o liquid scin illa ion. To ex ac adon, 20 mL
o he sample was aken and mixed wi h he liquid scin illan Be apla e HiSa e by shaking o 1 min.
The o ganic phase was placed in a glass ial wi h a capaci y o 20 mL. Once adon was ex ac ed,
he sample was s o ed o app oxima ely 3 h o p e en chemical “shu down”. The measu emen
o he samples was pe o med using he liquid scin illa ion de ec o TRIATHLER HIDEX OY, wi h
a measu emen ime o 50 min. Fo he ela i e analysis, 0.25 mL o a s anda d o
226
Ra (SRM-4965
ce i ied ma e ial by US NIST) was added o he ial and illed wi h Be apla e HiSa e liquid scin illa o
un il comple ing 20 mL. The measu emen e iciency was 60%.
The 222Rn ac i i y concen a ion (Bq/L) was de e mined using he ollowing equa ion:
CRn = CPSα/(a ×ε×V) (2)
whe e CPS
α
is a ne coun index in coun s pe second, egis e ed o he egion o de ec ion alpha,
ε
is
he de ec ion e iciency, a is he numbe o alpha disin eg a ions o
222
Rn and i s descendan s
218
Po
and
214
Po, and V is he wa e olume added o he ial in li e s. In addi ion, co ec ions we e applied
in unc ion o he decay ime o adon om he ime o sample collec ion o he ime o measu emen .
The ela i e e o o he 222Rn ac i i y concen a ion was less han 5%.
The annual e ec i e doses by inges ion om sp ing wa e samples was calcula ed using he
equa ion desc ibed in [5]:
Ewi (nS /y) = CRn ×Cw ×EDc (3)
whe e E
WI
is he e ec i e dose o
222
Rn inges ion, C
Rn
is he adon concen a ion in wa e (Bq/L), C
w
is he weigh ed es ima ed o wa e consump ion (60 L/y), and EDc is he E ec i e Dose Coe icien o
inges ion (3.5 nS /Bq).
2.5. S a is ical Analysis
A mul iple linea eg ession model was pe o med o de e mine he change o e e y ion wi h
espec o TDS, by using he PROC GLM, SAS
®
(SAS 9.1.3, 2006) (SAS Ins i u e: Ca y, NC, USA); unde
Equa ion (4). P e iously, an analysis o selec he ions ha con ibu e signi ican ly in he model was
pe o med, using he eg ession s epwise me hod.
Y = β0+β1X1+β2X2+β3X3+ . . . . . . . + βnXn+ε(4)
whe e Y is he TDS concen a ion,
β
is he ion coe icien , X is he ion concen a ions, and
ε
is he
andomness e o .
2.6. Gene a ion o he Spa ial Dis ibu ion o he In e se o he Dis ance (IDW)
The dis ibu ion map o he
222
Rn concen a ions in wa e was ob ained by Spa ial Analysis and
Geop ocessing-In e pola ion. This as e map was gene a ed using In e se Dis ance Weigh ed (IDW)
in e pola ion in A cGIS 9.0 (ESRI, Redlands, CA, USA).
3. Resul s
Wa e quali y is de e mined by i s physical and chemical pa ame e s, as well as he con en s
o ace elemen s. Mo eo e , wi h he e alua ion o hese cha ac e is ics, he wa e quali y can be
de e mined in e ms o i s di e en uses: consump ion, i iga ion, and suppo ing aqua ic li e. Table 1

Geosciences 2017,7, 12 6 o 15
shows he esul s o pa ame e s measu ed in si u such as pH, empe a u e (T), oxida ion- educ ion
po en ial (ORP), and o al dissol ed solids (TDS), as well as he geog aphic coo dina es o each
sampling poin .
Table 1.
Resul s o he physical-chemical pa ame e s measu e in si u, and geog aphic coo dina es o
each sampling poin .
Sample pH T (◦C) ORP (mV) TDS (mg/L) No h (N) Wes (O) Al i ude (MSL +)
P1 6.9 28.2 158 410 28◦5102.1100 105◦16011.4900 1168
P2 6.6 30.7 149 440 28◦5101.2300 105◦16013.8500 1168
P3 6.5 31.4 153 420 28◦50058.5200 105◦16020.1600 1160
P4 6.6 27.2 132 270 28◦54022.1000 105◦2009.8600 1029
P5 6.8 28.5 138 930 28◦5500.4500 105◦2004.6600 1043
P6 6.4 27.2 158 1180 28◦5407.2500 105◦16018.3600 1084
+MSL, me e s abo e sea le el. T, empe a u e, ORP, oxida ion- educ ion po en ial; TDS, o al dissol ed solids.
In he samples aken om he sp ing wa e s o El G ane o, he pH alues a ied om sligh ly
acidic o neu al. In addi ion, in he sp ing wa e he T was ound o a y be ween 27.2 and 31.4
◦
C,
whe eas he esul s om ORP p esen ed a sligh a ia ion om 132 o 158 mV, which indica es ha he
wa e ea u ed suboxic condi ions [
17
]. Fu he mo e, he alues o TDS p esen ed a high a iabili y,
wi h a concen a ion ange om 270 up o 1180 mg/L.
The esul s o he dissol ed ion analysis a e shown in Table 2. The dominan ca ions we e Ca,
Na, and Si. Based on he measu ed concen a ions o Ca and Mg, he alues o ha dness anged
om 104 o 293 mg CaCO
3
/L. The anions wi h he highes concen a ions we e he sul a es, anging
om 44 o 800 mg/L. Finally, he esul s o
222
Rn concen a ions and he calcula ed annual e ec i e
dose (calcula ed using Equa ion (3)) by inges ion a e showed in Table 3. The
222
Rn concen a ion in
he sp ing wa e s in he zone o El G ane o showed concen a ions anging om 52 o 229.3 Bq/L.
Likewise, he annual e ec i e dose caused by adon inges ion anged om 10.9 o 79.8 µS /y.
Table 2.
Concen a ion o he main anions and ca ions (mg/L) and hei s anda d de ia ion (SD).
Likewise, he maximum allowed limi s o some ions.
Sample SO4
−2F−Ca+2 Mg+2 Na+K+Si Ha dness *
NORM 400 1.5 - - 200 - - 500
WHO - 1.5 75 50 - - - 500
P1 45 (0.592) 1.05 (0.001) 37.7 (0.001) 2.3 (0.004) 47.4 (0.019) 6.3 (0.039) 17.3 (0.133) 104 (0.016)
P2 71 (0.295) 1.11 (0.002) 41.2 (0.001) 2.4 (0.010) 48.6 (0.006) 5.9 (0.036) 18.2 (0.362) 113 (0.042)
P3 80 (0.505) 1.12 (0.002) 38.6 (0.001) 2.5 (0.006) 47.7 (0.004) 6.4 (0.012) 16.6 (0.202) 107 (0.028)
P4 44 (0.092) 1.12 (0.001) 19.8 (0.002) 2.4 (0.016) 48.6 (0.006) 5.5 (0.056) 15.3 (0.054) 59 (0.061)
P5 420 (0.367) 2.38 (0.012) 48.5 (0.017) 14.0 (0.001) 79.3 (0.009) 2.3 (0.015) 35.2 (0.097) 179 (0.042)
P6 800 (0.182) 1.84 (0.002) 96.7 (0.012) 12.5 (0.001) 72.6 (0.016) 6.7 (0.081) 27.7 (0.014) 293 (0.029)
* Calcula ed based on he concen a ions o Ca and Mg; NORM [
18
] wa e o human consump ion. WHO [
19
]
wa e o human consump ion.
Table 3. 222
Rn concen a ions (i s unce ain y) and he annual e ec i e dose by inges ion based on
Equa ion (3). Likewise, he maximum allowed limi s o adon.
Sample 222Rn (Bq/L) Annual E ec i e Dose (µS /y)
EPA 11 -
UNSCEAR 40 -
EU 100 -
P1 67 (2.7) 14.07
P2 83 (4.1) 17.43
P3 52 (2.3) 10.92
P4 380 (11.4) 79.80
P5 161 (6.4) 33.81
P6 229 (6.9) 48.09
EPA [
20
] wa e o human consump ion; UNSCEAR [
21
] wa e o human consump ion; EU [
22
] wa e o
human consump ion.
Geosciences 2017,7, 12 7 o 15
4. Discussion
In gene al, he highes ca ion concen a ions we e o Ca, Na, and Si, and he lowes concen a ions
o Mg and K, whe eas in anions he sul a es we e dominan in he analyzed wa e . This beha io has
p e iously been obse ed in sp ing wa e s wi h simila T and pH condi ions o hose in he p esen
s udy [
23
,
24
]. The esul s show ha TDS in sampling poin s P5 and P6 we e highe compa ed o he
o he sampling poin s. The TDS we e composed o ino ganic sal s such as bica bona es, chlo ides,
and sul a es o Ca, Mg, K, and Na, as well as o small amoun s o o ganic ma e ial [
19
]. In 2006,
wa e om his aqui e was analyzed. The au ho s ob ained TDS concen a ions in he ange o 380
o 3286 mg/L [
14
], acco ding o hose alues ound in his esea ch. The highes alues o anions
and ca ions we e ound in hose sampling poin s, con i ming he esul s o he concen a ions o TDS
measu ed in si u. Mo eo e , some co ela ions we e ound be ween TDS and he de e mined anions
and ca ions. F om he sc eening made by eg ession s epwise me hodology, Si, Mg, Na, and F we e he
mos signi ican ions (p alue < 0.05) o con ibu e o TDS. The e o e, he ion coe icien s we e ound
by using Equa ion (4) and a e shown in Table 4.
Table 4. Ions con ibu ion o TDS concen a ions in he sp ing wa e s analyzed.
Ion Coe icien s Coe icien s Values pValue
-β0−5277.78 <0.0001
Si β1−42.53 <0.0001
Na+β2 177.77 <0.0001
Mg+2 β3 98.37 <0.0001
F−β4 105.01 <0.0001
In wa e , he p esence o magnesium and calcium sal s depends undamen ally on he
geological o ma ions. The landscape o El G ane o is mainly o med by limes one, muds one, and
gypsum [25,26]
. Likewise, he aqui e echa ge comes om he di ec in il a ion o p ecipi a ion and
su ace s eams [
14
]. Unde hese condi ions, when g oundwa e comes in o con ac wi h Ca and
Mg mine als, hey a e solubilized and p o ide concen a ions o hose non-alkaline elemen s o his
wa e . Thus, wi h he Ca and Mg concen a ions ound in hese sp ings, his wa e can be classi ied as
mode a ely ha d o ha d (>100 mg/L CaCO
3
). Likewise, Figu e 3shows some signi ican co ela ions
be ween dissol ed ions (p< 0.05). F om Table 4, TDS showed ha Mg con ibu es o i s concen a ions;
he e o e, because ha dness is de e mined by he concen a ion o dissol ed ions Ca and Mg, a linea
co ela ion be ween TDS and ha dness ( = 0.97) was obse ed (see Figu e 3a). Despi e Ca ion no
being a main con ibu o o TDS, i was also linea ly co ela ed wi h TDS ( = 0.90) (Figu e 3b). In wa e ,
he pH, T, and Eh de e mine he solubili y o chemical species. Thus, a a neu al pH ca bona es and
bica bona es a e usually he dissol ed species; when he pH inc eases hyd oxides a e he main anions
p esen in wa e [
27
], whe eas a a low pH chemical species such as chlo ides, luo ides, and acids
will be p edominan . The dissolu ion o elemen s p esen in mine als such as Mg, Ca, Al, Na, and K
in o he en i onmen depends mainly on he p esence o wa e wi h a ce ain acidi y [
28
]. The pH
measu ed in hese wa e samples was sligh ly acid (<7). Hence, he concen a ion o hese elemen s in
he wa e can be a ibu ed o his pa ame e and o he geological condi ions o he s udy a ea.
A id and semi-a id egions a e cha ac e ized by p ocesses ha ake place o e long ime pe iods,
due o low p ecipi a ion and high e apo a ion a es. The eby, he empe a u e (T) o wa e and
he d y en i onmen al condi ions play an impo an ole in he dissolu ion o chemical species.
In he s udy a ea, he en i onmen al T eaches alues o e 44
◦
C, wi h epo ed e apo a ion a es
o 200 mm/mon h [
13
], and wi h an annual a e age o 2635 mm measu ed in he las 40 yea s.
Consequen ly, sul a ed and ca bona e sal s p esen in sp ing wa e can be p ecipi a ed. In wa e ,
he CaCO
3
solubili y (solubili y p oduc cons an Ksp = 8.7
×
10
−9
) is lowe han MgCO
3
solubili y
(Ksp = 4.0
×
10
−5
) a 25
◦
C [
29
,
30
]. Addi ionally, CaCO
3
solubili y dec eases wi h he inc ease o T.
Thus, en i onmen al d yness and sp ing wa e T (>28
◦
C) induce mainly he p ecipi a ion o CaCO
3
Geosciences 2017,7, 12 8 o 15
sal s. In addi ion, a dis inc ion in he o al ha dness can be p oposed: empo al ha dness due o he
p esence o ca bona es and pe manen ha dness caused by sul a es, chlo ides, and luo ides. This is
co obo a ed by he s ong co ela ion showed be ween ha dness and sul a e con en ( = 0.98) in
hese wa e samples (see Figu e 3c). O he co ela ions we e ound o SO
4−2
wi h Ca ( = 0.93), Mg
(
= 0.88
), and Na ( = 0.85) (Figu e 3d– , espec i ely), indica ing he p esence o sul a ed sal s. In he
s udy a ea, hese sul a es (Ca, Mg, and Na) can be he esul o leaching om ocks ha con ain CaSO
4
(gypsum) and om sedimen s in he i e s o he egion ha con ain gypsum [
26
]. F om a geological
poin o iew, he ypical ocks in he municipali y o Aldama, speci ically in he zone o El G ane o,
a e limes ones (o sedimen a y ma ine o igin), hyoli es (ex usi e igneous ocks), and deposi s o
gypsum [
26
,
31
]. Likewise, he soil in his egion is mainly lep osol and calcisol, which ha e a high
con en o CaCO3as a esul o he a id clima e and he limes one ocks in he s udy a ea.
Geosciences 2017, 7, 12 9 o 16
Figu e 3. Co ela ions be ween ion con en s in sp ing wa e : (a) Ha dness s. TDS; (b) Ca s. TDS;
(c) Sul a es s. Ha dness; (d) Sul a es s. Ca; (e) Sul a es s. Mg; ( ) Sul a es s. Na; (g) F s. Si; (h) F s.
Na; and (i) F s. Mg.
On he o he hand, 222Rn is a gas and is pa o he disin eg a ion chain o he 238U; i s p esence in he
en i onmen is associa ed wi h he concen a ion o i s p ogeni o 226Ra. The in e ac ions
be ween wa e and ock di ec ly a ec he elemen al concen a ions in bo h sys ems. In ock sys ems,
secula adioac i e equilib ium is common, while su ace and g ound wa e s a e cha ac e ized by
signi ican disequilib ia. This adioac i e disequilib ium allows o he de e mina ion o he mobili y and
he dis ibu ion o he adioiso opes p esen in an aqui e . The concen a ions o 222Rn de e mined in he
p esen s udy each le els o 380 Bq/L, and a e no below 50 Bq/L. Some au ho s ha e ound ha adon
concen a ions in ock aqui e s a e cha ac e ized by hei a iabili y. Thus, 222Rn in g oundwa e a ies
om 1 o 50 Bq/L in aqui e s wi h sedimen a y ocks, om 100 o 50,000 Bq/L in aqui e s wi h c ys alline
ocks, and om 10 o 30,000 Bq/L in wells o allu ial deposi s [5,37–39]. The s udy a ea is cha ac e ized
by sedimen a y ocks such as lu i e-sands one and sands one-conglome a e. These ypes o ocks can
con ain high amoun s o 238U because o i s e en ion in he clay, lime, and o ganic ma e ha hey a e
composed o [40]. Addi ionally, in he geophysical in o ma ion o he subsoil, he ma e ial ha ills he
alleys con ains conglome a es and allu ial deposi s, which in some places al e na e la e ally wi h
hyoli es and acidic u s [14]. Rhyoli es end o ha e high con en s o ho ium and u anium in a ange
om 12 o 230 Bq/kg [41,42].
When 222Rn is eed om he 238U decay chain, i is solubilized in he wa e o aqui e s. In Figu e 4 he
dis ibu ion o he 222Rn concen a ions is shown using IDW.
Figu e 3.
Co ela ions be ween ion con en s in sp ing wa e : (
a
) Ha dness s. TDS; (
b
) Ca s. TDS;
(
c
) Sul a es s. Ha dness; (
d
) Sul a es s. Ca; (
e
) Sul a es s. Mg; (
) Sul a es s. Na; (
g
) F s. Si;
(h) F s. Na; and (i) F s. Mg.
Soils and di e en ypes o wa e na u ally con ain luo ine. Commonly, F
−
is ound as apa i e
in sedimen a y ocks and as luo i e in sedimen s [
32
]. Depending on he geog aphic zone, luo ine
shows concen a ions anging om 1.0 o mo e han 35 mg/L in g oundwa e [
33
], om 0.8 o
1.3 mg/L in sea wa e , and a ying om 1 o 25 mg/L in i e s [
34
]. The sp ing wa e analyzed in
he p esen s udy shows concen a ion o luo ides in ag eemen wi h hese concen a ion anges o
bo h g oundwa e and su ace wa e (Table 2). In addi ion, luo ine is a highly elec onega i e elemen
ha p esen s a s ong endency o be a ac ed by posi i ely cha ged ions such as Ca, Si, Al, and B [
35
].
The concen a ions o F
−
ha we e de e mined in hese sp ing wa e s showed a s ong co ela ion
wi h he con en s o Si ( = 0.99), Na ( = 0.98), and Mg ( = 0.97) (Figu e 3g–i, espec i ely). Di e en
au ho s ha e ound a simila co ela ion o F
−
wi h Si and Na in g oundwa e [
36
], which can be
explained by he o igin o his elemen in ock silica es. The e o e, luo ine p esen in silica es in he
ocks/sedimen s o he s udy a ea is being leached and pu in o solu ion in he wa e o hese sp ings.
On he o he hand,
222
Rn is a gas and is pa o he disin eg a ion chain o he
238
U; i s p esence in
he en i onmen is associa ed wi h he concen a ion o i s p ogeni o
226
Ra. The in e ac ions be ween
wa e and ock di ec ly a ec he elemen al concen a ions in bo h sys ems. In ock sys ems, secula
Geosciences 2017,7, 12 9 o 15
adioac i e equilib ium is common, while su ace and g ound wa e s a e cha ac e ized by signi ican
disequilib ia. This adioac i e disequilib ium allows o he de e mina ion o he mobili y and he
dis ibu ion o he adioiso opes p esen in an aqui e . The concen a ions o
222
Rn de e mined in he
p esen s udy each le els o 380 Bq/L, and a e no below 50 Bq/L. Some au ho s ha e ound ha
adon concen a ions in ock aqui e s a e cha ac e ized by hei a iabili y. Thus,
222
Rn in g oundwa e
a ies om 1 o 50 Bq/L in aqui e s wi h sedimen a y ocks, om 100 o 50,000 Bq/L in aqui e s wi h
c ys alline ocks, and om 10 o 30,000 Bq/L in wells o allu ial deposi s [
5
,
37
–
39
]. The s udy a ea is
cha ac e ized by sedimen a y ocks such as lu i e-sands one and sands one-conglome a e. These ypes
o ocks can con ain high amoun s o
238
U because o i s e en ion in he clay, lime, and o ganic ma e
ha hey a e composed o [
40
]. Addi ionally, in he geophysical in o ma ion o he subsoil, he ma e ial
ha ills he alleys con ains conglome a es and allu ial deposi s, which in some places al e na e
la e ally wi h hyoli es and acidic u s [
14
]. Rhyoli es end o ha e high con en s o ho ium and
u anium in a ange om 12 o 230 Bq/kg [41,42].
When
222
Rn is eed om he
238
U decay chain, i is solubilized in he wa e o aqui e s. In Figu e 4
he dis ibu ion o he 222Rn concen a ions is shown using IDW.
Geosciences 2017, 7, 12 10 o 16
Figu e 4. Spa ial dis ibu ion o
222
Rn in he sp ing wa e s o he egion El G ane o.
I is shown ha sampling poin s 4 and 6 p esen he highes adon concen a ions, which could be
o igina ing di ec ly om ocky ma e ial. Con e sely, he lowes adon concen a ions we e in sampling
poin s 1, 2, and 3, sp ings ha a e loca ed in he a ea wi h a highe amoun o conglome a e (sedimen a y
ocks) (see Figu e 2). Howe e , adon concen a ions in g oundwa e a e a ound 50 Bq/L in sa u a ed
soils wi h a po osi y o 20%, wi h
226
Ra concen a ions o 40 Bq/kg (wo ld a e age in he ea h c us ), and
a e unde condi ions o equilib ium [5]. In addi ion, in he sands one ock wi h U concen a ion o 0.5–1
mg/Kg (<12 Bq/Kg, o
238
U) and po osi y o 20%, he
222
Rn ac i i y ha could be p oduced by he ocks is
no mo e han 6000 dpm/L ( ens o Bq/L) [43], which is 10% o he adon e ec i ely ejec ed o he po e
space luid.
Ne e heless, he highes
222
Rn concen a ions in hese sp ings can be a ibu ed o an addi ional
sou ce.
226
Ra is con inuously p oduced om he
238
U decay chain, di ec ly om
230
Th disin eg a ion. This
las adioiso ope is p esen in mine al om sedimen s and ocks o aqui e . The
226
Ra mobili y is s ongly
in luenced by bo h high T and/o high sal con en s, as well as he edox s a e (lowe oxygen
concen a ions). In saline g oundwa e ,
226
Radium is mo e mobile due o ion exchange compe i ion wi h
o he majo ca ions p esen in wa e [39,44]. Wa e om he aqui e unde s udy is classi ied
p edominan ly as Calcium-sul a ed and Sodium-sul a ed [14]. He e, he co ela ion ound o sul a es
wi h Ca, Mg, and Na co obo a e ha classi ica ion. Thus, in his aqui e ,
226
Ra concen a ions may be
p esen in solu ion by wo ac o s: (1) he ela i ely high empe a u e o sp ing wa e s lowing h ough
ocks wi h bo h u anium and ho ium con en s; and (2) he high le el o salini y.
Mo eo e , in a id and semi-a id egions, he low p ecipi a ion and high e apo a ion induce he
p ecipi a ion o ce ain chemical species a he su ace le el. Hence, he main p ecipi a ed sal s may be
Figu e 4. Spa ial dis ibu ion o 222Rn in he sp ing wa e s o he egion El G ane o.
I is shown ha sampling poin s 4 and 6 p esen he highes adon concen a ions, which could
be o igina ing di ec ly om ocky ma e ial. Con e sely, he lowes adon concen a ions we e in
sampling poin s 1, 2, and 3, sp ings ha a e loca ed in he a ea wi h a highe amoun o conglome a e
(sedimen a y ocks) (see Figu e 2). Howe e , adon concen a ions in g oundwa e a e a ound 50 Bq/L
in sa u a ed soils wi h a po osi y o 20%, wi h
226
Ra concen a ions o 40 Bq/kg (wo ld a e age in he
ea h c us ), and a e unde condi ions o equilib ium [
5
]. In addi ion, in he sands one ock wi h U
concen a ion o 0.5–1 mg/Kg (<12 Bq/Kg, o
238
U) and po osi y o 20%, he
222
Rn ac i i y ha could
be p oduced by he ocks is no mo e han 6000 dpm/L ( ens o Bq/L) [
43
], which is 10% o he adon
e ec i ely ejec ed o he po e space luid.