Water Research 36 (2002) 3989–4000

Sulphur isotopes as tracers of the influence of potash mining ingroundwater salinisation in the Llobregat Basin (NE Spain)

N. Otero*, A. Soler

Departament de Cristal.lografia, Mineralogia i Dip "osits Minerals, Facultat de Geologia, Universitat de Barcelona, Mart!ı i Franqu"es, s/n,

08028, Barcelona, Spain

Received 9 August 2001; accepted 19 February 2002

Abstract

Conventional chemical data for spring and river waters are presented together with sulphur isotopic data for

dissolved sulphate to elucidate the source of water salinisation in the middle section of the Llobregat River. As dilution

processes do not affect sulphur isotopic composition, the analysis of d34S of dissolved sulphate in waters provides an

excellent tool for quantifying the environmental impact caused by the mining activity existing in the area.

The d34S of dissolved sulphate from mining effluents and saline springs unrelated to mining activity was analysed.

The results obtained range from+18% to +20% (VCDT) for mining effluents and from+10% to +14% (VCDT) for

natural saline springs. These values are in accordance with the pattern of sulphur isotopic composition of sulphates

from the evaporite materials of this area. This distinctive isotopic composition has allowed us to determine the origin of

salinity in those cases in which chemical features are not conclusive. In addition, two fertilisers widely used in the

studied area are chemically and isotopically characterised and their contribution to groundwater salinisation is assessed.

r 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Sulphur isotopes; Potash mining; Water salinisation; Fertilisers; Contamination

1. Introduction

In the middle section of the Llobregat Basin (North-

eastern Spain) several potash mining works exist in the

localities of Sallent, S !uria, Balsareny and Cardona

(Fig. 1). This activity produces large salt mining tailings,

which are stored around the mining sites, with no

waterproofing. Some aquifers near the mining zones are

highly salinised. The origin of the salinity is controver-

sial, as it can be related to natural water interaction with

saline formations, or it could be caused by contamina-

tion from mine tailing effluents. Fertilisers could also

contribute to groundwater salinisation, as agriculture is

an important economic activity in the Llobregat Basin.

Sulphur isotopes of dissolved sulphate can be used to

identify natural sources of sulphate in surface waters [1–

6] and in groundwater [7–9]. In some cases they can be

applied to distinguish between natural and anthropo-

genic sources [10–16] and to trace processes in the soil

[17,18].

In a previous paper [19], we reported differences

between sulphur isotopic composition of natural

and anthropogenic sources in the Llobregat River

waters large enough to consider d34S as a good tool

to discriminate their different origins. This tool,

however, is more useful when the studied area is small

since the inputs can be better constrained and more

limited. On the basis of our previous results, the aim

of the present paper is to determine the origin of

groundwater salinisation in a specific area of Llobregat

Basin with some saline springs, using the sulphur isotope

composition of dissolved sulphate as well as water

chemistry.

*Corresponding author. Tel.: +34-93-402-1345; fax: +34-

93-402-1340.

E-mail address: notero@geo.ub.es (N. Otero).

0043-1354/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved.

PII: S 0 0 4 3 - 1 3 5 4 ( 0 2 ) 0 0 1 2 5 - 2

2. Study area

The area studied is the middle section of the Llobregat

River. For more information on the river’s character-

istics see [19]. In this area, besides the main flow, there

are three tributaries, all of which have several evaporitic

outcrops in their catchments (Fig. 2). The Cardona

Diapir, which appears in the Cardener River basin, is

composed dominantly of halite, sylvite, carnallite and

gypsum of Late Eocene age. In the studied basin

different sulphate formations outcrop: gypsum of

Oligocene age [20], and both anhydrite and gypsum of

Late Eocene age [21]. These evaporite materials cause

high natural values of salinity (1–4%wt) in some of the

small tributaries of the rivers.

On the other hand, the salinity of the Llobregat River

has increased significantly since the early 1920s [22] due

to the extensive development of potash and salt mining.

Fig. 1. Llobregat River Basin, showing major tributaries and sample location.

N. Otero, A. Soler / Water Research 36 (2002) 3989–40003990

The potash mine tailings mainly consist of halite with

minor amounts of sulphates (gypsum, polihalite and

anhydrite) and are considered by law as a resource and

not as waste, as halite can be reused for the chlorine

industry. Because of this, there are large mine tailings,

some without waterproofing (Plate 1), stored in the

surroundings of the mining sites (Balsareny, Cardona,

Sallent and S !uria). The mining industry also dumps

scrap metals into the tailings, which cover an area of

98 ha and the estimated mass is 61Mt, with an annual

increase of about 2.7Mt [23].

Since 1989, a brine collector drives the brines from the

potash mining zones, and from several springs, draining

evaporitic materials, to the sea, discharging it 10 km

before reaching the coast. However, not all saline

springs are collected and some flow into the Llobregat

River. Furthermore, two of the main fertilisers used in

the agricultural activity of the region are ammonium

Fig. 2. Simplified geological map of the Llobregat Basin, see Fig. 1 for sample location.

N. Otero, A. Soler / Water Research 36 (2002) 3989–4000 3991

sulphate and the NPK 5.7.10 (5% of N, 7% of P2O5 and

10% of K2O) that supply nitrate, sulphate, potassium,

etc. to groundwater.

3. Methodology

In order to characterise the water chemistry and the

sulphur isotopic signature of dissolved sulphate, saline

samples (conductivity up to 15mS/cm) from springs and

river waters were collected. Five water samples corre-

spond to springs with a salinity of natural origin, 10 are

from waters flowing through potash mine tailings, and

10 from springs where the origin of salinity is uncertain.

The samples were collected between July 1997 and

November 2000. Sampling localities are shown in Fig. 1.

Samples located at sites 1, 2, 6, 7 and 10 are from springs

that have highly variable discharges, whereas samples

located at site 4, 5, 8 and 9 are from springs that have

relatively constant discharge. Samples from site 3 are

from a Llobregat tributary with irregular flow regimen.

The samples from mine tailing effluents sampled at sites

2, 3 and 4 are drained to the salt collector; however, at

the time of sampling site 2, the effluent flowed into the

Cardener River. The rest of the samples from mining

effluents (site 1), natural origin (sites 5 and 6) and

uncertain origin (sites 7, 8, 9 and 10) are not drained to

the salt collector.

Physicochemical parameters (pH, temperature and

dissolved O2) were measured in situ. Rainwater samples

were collected in a pluviometer between October 1997

and December 1998 (see location in Fig. 1). In order to

avoid sulphate reduction during sample collection, the

dissolved sulphate was precipitated as BaSO4 by adding

a solution with excess of BaCl2 � 2H2O in the pluvi-

ometer. To constrain the importance of the possible

supply of fertilisers to the global isotope composition of

dissolved sulphate, sulphur isotope composition of two

of the main fertilisers used in the area was also analysed.

Before analysis, sampleswere filteredwith aMillipores

filter of 0.45mm pore size (fertilisers were previously

dissolved in distilled water). Major anions (NO3�, SO4

�2

and Cl�) were analysed by HP liquid chromatography.

Concentrations of the major cations (Ca, Mg, Na, Sr

and K) were determined by ICP-OES, and the trace

elements (Mn, Ba, Zn, Cu, Cr, As, Se, Cd, V, Hg, Pb

and Co) were analysed by ICP-MS; due to the high

chloride content, most samples needed a dilution and

therefore, most trace elements were under the detection

limits. Alkalinity was measured by titration.

For sulphur isotope analysis the dissolved sulphate

was precipitated as BaSO4 by the addition of

BaCl2 � 2H2O. The sulphur isotopic composition was

determined with an elemental analyser (Carlo Erba

1108) coupled with a mass spectrometer (Finningan

Matt delta C). Notation is expressed in terms of d34S per

mil relative to the Vienna Canyon Diablo Troilite (V-

CDT) standard. The isotope ratios were calculated using

the NBS-127, IAEA-S1, IAEA-S2, IAEA-S3 and inter-

nal laboratory standards. Reproducibility of the samples

calculated from standards systematically interspersed in

the analytical batches is 70.2%. All chemical and

sulphur isotope analyses were carried out at the Serveis

Cient!ıfic T"ecnics (Universitat de Barcelona).

4. Results and discussion

4.1. Major and trace elements

Results of pH, conductivity, major anions and

sulphur isotopic composition of dissolved sulphate are

shown in Table 1; the results of major cations, trace

elements and estimated flow are shown in Table 2. All

spring and river water samples are considered as ‘‘Cl–

Na’’ waters (Fig. 3). According to their relative cation

abundance, two groups of samples may be distin-

guished: one with Na>Ca>K, and another with

Na>K>Ca. The Na/K ratio of the first group ranges

from 100 to 500 whereas in the second it is one order of

magnitude lower (2–20) (Table 2).

The Na versus K and Na versus Mg diagrams

(Figs. 4a and b) show three clusters of samples. The

first, A, has Na contents from 0.1% to 1.5%, K ranging

from 10 to 500 ppm and Mg from 200 to 1500 ppm. The

second, B, is characterised by B11% of Na, from 0.5%

to 1.5% of K and B0.1% of Mg. The third, C, has

B5% of Na and high contents of K (2–3%) and Mg

(B0.6%). All water samples having a salinity of natural

origin plot in group A. Four samples (8a, 9a, 9b and

10a) with salinity from uncertain sources, also plot

within group A, suggesting a natural origin as well. The

rest of the samples with salinity from uncertain sources

Plate 1. Mine tailings located at site 4.

N. Otero, A. Soler / Water Research 36 (2002) 3989–40003992

form group C. Group B includes water samples from the

Salat Stream (draining the Cardona diapir as well as

mine tailings, site 3) and samples from mine tailing

effluents (1a, 2a and 4b). Sample 4a from mine effluents

plots outside these groups. The mine effluents collected

at site 4a drain active mine tailings; the effluents

collected at sites 1, 2, 3 and 4b are from abandoned

mine tailings. As solubility of silvite and carnallite is

greater than solubility of halite, the active and the

abandoned mine tailings would have different K and Mg

contents.

On a Na/Mg versus Na/K diagram (Fig. 4c) samples

plot in two distinct trends: one with a wide Na/K range

and low Na/Mg ratio which are the natural springs; and

a second trend with a low Na/K ratio and high Na/Mg

variability which includes samples from mining efflu-

ents. The samples with salinity of uncertain origin

plot in the lower Na/K and Na/Mg ratio region,

and cannot clearly be associated either to a natural or

to a mining origin, although samples 9a and 9b seem to

fit in the trend defined by the salinity of the natural

springs.

K and Mg contents allow the distinction between

samples of natural origin and samples of mining

effluents because the potash unit is mainly made up of

sylvite (KCl) and carnallite (KMgCl3 � 6H2O). Never-

theless, K and Mg data are not conclusive indicators of

the origin of salinisation in uncertain cases since (a)

there are other sources for K and Mg in the basin such

as NPK fertilisers (nitrate–phosphate–potassium) and/

or industry pollutants; and (b) Na/K and Na/Mg ratios

from the mine tailings are time dependant as solubility

of silvite and carnallite is greater than solubility of

halite. Therefore, as suggested above, K and Mg

contents in active mine tailings are greater than in

abandoned mine tailings.

In the chloride versus sulphate diagram (Fig. 5a) the

natural and mining samples are well differentiated:

natural samples have o2% of Cl� and o0.5% of

SO4�2(group A), whereas mining samples contain B15%

of Cl� and variable amounts of SO4�2 (group B). As

previously suggested by Na–K–Mg contents (Fig. 4a

and b), the Cl�/SO4�2ratios in the samples 8a, 9a, 9b and

10a also point to a natural origin (group A). The rest of

Table 1

Anion concentration and sulphur isotopic composition of dissolved sulphate

Site Sample Location Date pH Cond Cl NO3 SO4 HCO3 CO3 d34S(month-yr) (mS) (201C) (mg/l) (mg/l) (mg/l) (mg/l) mg/l (%) VCDT

1 1a Mining lixiviates (Vilafruns) Apr-99 7.4 562,000 162,000 154 10,340 125 o1 20.1

2 2a Mining lixiviates (Cardona) Mar-98 7.4 584,000 176,300 90 9950 194 o1 17.9

3 3a Rierol Salat Mar-98 7.4 604,000 177,800 74 9980 215 o1 18.9

3b Rierol Salat Jul-97 n.d. n.d. 139,200 o200 9480 n.d. n.d. 19.4

3c Rierol Salat Apr-98 7.4 188,400 160,100 o100 12,700 153 17.9 18.5

3d Rierol Salat May-98 7.4 173,700 136,800 o100 11,700 217 o1 19.3

3e Rierol Salat Aug-98 7.3 158,700 189,300 o200 8030 204 o1 19.4

3f Rierol Salat Aug-98 7.4 158,000 175,300 o200 7300 213 o1 19.6

4 4a Mining lixiviates Mont Salat Nov-00 n.d. n.d. 213,000 o100 5310 399 o1 20.0

4b Mining lixiviates La Botjosa Nov-00 n.d. n.d. 189,200 o100 10,800 147 o1 18.3

5 5a Gorg Salat, surge Mar-98 7.3 15,050 3020 2 2190 298 o1 10.2

6 6a Ol !o Stream, surge 1 Mar-98 7.4 66,100 19,700 34 3420 352 o1 13.8

6b Ol !o Stream, surge 1 Apr-99 7.7 75,600 22,300 22 3780 200 o1 12.8

6c Ol !o Stream, surge 2 Apr-99 7.7 17,200 4080 11 1000 330 o1 11.8

6d Ol !o Stream, surge 3 Apr-99 7.6 86,800 23,500 15 4970 140 o1 13.5

7 7a Sallent, surge 1 Mar-98 6.8 388,000 109,200 42 5370 377 o1 18.5

7b Sallent, surge 2 Apr-99 7.6 535,000 137,300 220 6840 170 43.2 19.8

7c Sallent, surge 3 Apr-99 7.0 491,000 141,300 131 6900 421 o1 18.2

7d Sallent, surge 1 Apr-99 7.2 456,000 130,000 o200 4100 316 o1 18.9

7e Sallent, surge 1 Jun-98 7.2 252,000 75,300 o100 3560 383 o1 18.2

7f Sallent, surge 1 Dec-99 7.2 318,000 92,900 o100 5140 365 o1 19.1

8 8a Call !us, surge Dec-99 7.2 40,100 13,500 o20 1420 278 o1 8.6

9 9a Hortons Stream, surge Mar-98 7.7 18,190 4850 17 300 210 o1 13.7

9b Hortons Stream, surge Jun-98 7.8 14,040 5560 5 288 212 o1 13.7

10 10a S !uria, surge C-1410 Apr-99 7.8 39,600 11,300 7 481 212 o1 2.4

P P Pluviometer Dec-98 n.d. n.d. o2 1 2 n.d. n.d. 7.2

FK Fertiliser NPK (5.7.10) — n.d. n.d. 51,700 o100 402,800 n.d. n.d. 6.9

FA Ammonium sulphate — n.d. n.d. 30,500 4600 796,500 n.d. n.d. �1.1

n.d.=not determined. Fertiliser concentrations are in mg/kg.

N. Otero, A. Soler / Water Research 36 (2002) 3989–4000 3993

the samples with salinities of uncertain origin (7a–7f,

group C) have intermediate values (B10% of Cl� and

B0.5% of SO4�2) suggesting a mixing between samples

from natural springs (A) and mining sources (B).

No clear correlation between nitrate and sulphate

contents is observed (Fig. 5b), which suggests that

nitrate comes from a source unrelated to the mine

tailing effluents. As nitrates are not present in the

bedrock, they must be introduced in the waters by some

human activity and therefore they might be tracers of

anthropogenic pollution. In the studied area the main

source of nitrates are fertilisers. The nitrate content in

the analysed water samples is highly variable and

depends largely on the sampling period (sites 3, 6, 7

and 9 were sampled several times). This variability could

be explained by a non-constant supply of nitrate in the

waters, although denitrification processes in subsurface

waters cannot be ruled out.

Several water samples are characterised by high

contents of heavy metals (Fig. 5c), especially Mn, Zn,

Pb and Cu (up to 12 ppm of Mn, 9 ppm of Zn, 5 ppm of

Pb and 3 ppm of Cu). Although there is no correlation

between chloride and heavy metal concentrations, all of

the samples with metal contents up to 4 ppm have

chloride contents above 7.5% (Fig. 5c). On the other

Table 2

Major and trace cation contents and estimated flow

Site Sample Na K Mg Ca Sr Mn Zn Cu Pb Na/K Estimated flow

(mg/l) (mg/l) (mg/l) (mg/l) (mg/l) (mg/l) (mg/l) (mg/l) (mg/l)

1 1a 108,200 4850 725 1242 30 0.03 o0.01 o0.01 0.09 22 0.003 l/s

2 2a 111,900 12,470 1431 504 20 3.33 o0.01 2.70 o0.01 9 0.5 l/s

3 3a 118,000 11,590 886 789 19 0.86 9.49 o0.01 0.27 10 0.034m3/s

3b 112,900 7730 439 1012 31 0.87 0.10 0.14 o0.01 15 0.034m3/s

3c 113,550 11,090 794 754 19 0.72 0.19 0.19 o0.01 10 0.034m3/s

3d 110,450 11,480 852 683 16 0.92 o0.01 o0.01 0.20 10 0.034m3/s

3e 116,750 14,510 691 955 16 0.79 o0.01 o0.01 0.14 8 0.034m3/s

3f 111,950 13,530 669 956 16 0.75 o0.01 o0.01 0.14 8 0.034m3/s

4 4a 85,260 54,050 18,540 1177 9 12.64 0.81 o0.5 0.1 2 3 l/s

4b 133,400 4310 974 749 31 0.31 o0.5 o0.5 o0.05 31 0.005 l/s

5 5a 1780 14 204 886 17 0.02 0.13 0.01 o0.01 128 8 l/s

6 6a 13,170 54 287 932 30 0.14 1.35 o0.2 0.02 243 0.016 l/s

6b 13,810 62 237 1001 29 0.10 o0.01 o0.1 o0.01 223 0.003 l/s

6c 2880 12 108 360 11 o0.05 o0.05 o0.05 o0.05 237 0.003 l/s

6d 15,630 30 298 1406 30 o0.05 o0.01 o0.1 o0.01 517 0.016 l/s

7 7a 47,970 27,320 6570 1095 23 7.92 1.93 0.26 0.89 2 0.016 l/s

7b 66,520 26,530 5516 1923 36 2.49 o1 o1 1.51 3 0.003 l/s

7c 69,030 25,350 6161 1366 27 9.48 2.27 o1 3.69 3 0.004 l/s

7d 57,810 29,830 7444 1198 23 5.50 o1 o1 1.08 2 0.003 l/s

7e 42,870 21,280 5572 1013 21 6.12 o0.5 o1 0.41 2 0.002 l/s

7 7f 45,060 22,470 5154 1215 24 7.24 3.15 o1 4.98 2 0.016 l/s

8 8a 4640 548 1408 2089 71 0.04 0.64 o0.1 0.01 8 0.25 l/s

9 9a 2580 24 210 535 15 o0.001 0.02 o0.01 o0.001 107 0.008 l/s

9b 2680 22 225 551 15 o0.001 o0.05 o0.01 o0.001 123 0.006 l/s

10 10a 4500 310 896 1452 61 o0.005 o0.05 o0.05 o0.005 15 0.002 l/s

P 2 o0.5 0.4 3.1 0.01 0.004 0.03 o0.01 o0.001 — —

FK 3834 66,380 7180 120,090 201 n.d. n.d. n.d. n.d. 0.06 —

FA 388 o250 o10 266 2 n.d. n.d. n.d. n.d. — —

n.d.=not determined. Fe, Ni, Co, B, Cr, As, Se, Cd, V and Hg were under the detection limit (between 1 ppb and 5 ppm depending on

the dilution needed). Fertiliser concentrations are in mg/kg.

Fig. 3. Piper plot of major ions for samples of natural (K),

mining (m) and uncertain (’) origin.

N. Otero, A. Soler / Water Research 36 (2002) 3989–40003994

hand, mine tailing effluent samples show variable

amounts of heavy metals (P

[Mn, Zn, Pb, and Cu] from

0.1 to 13 ppm), the origin of which could be related to

the local presence of scrap metal observed in the mine

tailings, which could explain also the heterogeneity of

these metal contents. Consequently, the geochemical

signature of the potash tailing effluents is not only the

chloride and potassium contents but in some cases, the

presence of heavy metals as well. The samples from

uncertain origin collected at site 7 also show high values

in heavy metals (P

[Mn, Zn, Pb, and Cu] from 4 to

15 ppm) suggesting a mining origin for these samples.

However, using heavy metals as tracers of mining

effluent pollution is not a conclusive tool, as there are

several sources that can contribute heavy metals to

groundwater, e.g. fertilisers and sewage sludge usually

have high contents of Zn, Cu, and Pb ([24], among

others).

Using conventional chemistry, samples with salt

content of natural or mining origin can be well

differentiated. In some cases, where salinity has an

uncertain origin (samples 9a and 9b) they seem to fit

with the natural salinity values but in others (samples 8a

and 10a), despite their chemical similarity to the natural

values, they exhibit an unusually low Na/K ratio. The

remaining water samples with salinities of uncertain

origin (collected at site 7) are not clearly related to a

natural or a mining source; however, the high contents

in chloride, potassium and heavy metals in these samples

suggest an influence of mine effluents. In order to

confirm these results and to clarify the origin of salinity

in the studied area, a systematic isotope analyses of the

studied samples was carried out.

4.2. Sulphur isotopes

4.2.1. Sources and isotopic composition of sulphate

The natural sources of dissolved sulphate in the

Llobregat River are evaporites from the bedrock and

rainwater. The sulphur isotopic composition determined

in sulphate from rainwater has a mean value of +7%,

(Table 1). However, the input of sulphate from rain-

water is negligible as the mean annual precipitation in

the studied area is between 500 and 750mm and the

sulphate concentration measured is below 2ppm.

5a6c 6a

6b6d

3a

4b3d

3f 3e

4a

2a

1a3c

3b9a9b

8a10a

7c7e

7d 7b7a

7f

0

10000

20000

30000

40000

50000

60000

0 25000 50000 75000 100000 125000

A

B

C

5a6c

6a6b 6d4b

3 a t o f1a

4a

2a9a9b

8a10a

7d7a

7e7f

7b7c

0

5000

10000

15000

20000

0 25000 50000 75000 100000 125000

A

C

B

6d6b6a

6c5a

3d

4b

3b

3e3f

3c

3a 1a

2a

7 a to

f

9b9a10a8a

4a

0

100

200

300

0 100 200 300 400 500 600

Na/

Mg

(a) (b)

(c)

Na (ppm)Na (ppm)

Na/K

K (

ppm

)

Mg

(ppm

)

Fig. 4. (a) Plot of Na versus K, (b) plot of Na versus Mg, (c) plot of Na/K versus Na/Mg for (K) samples of natural origin, (m)

samples of mining origin and (’) samples of uncertain origin.

N. Otero, A. Soler / Water Research 36 (2002) 3989–4000 3995

The different outcropping evaporites have distinctive

sulphur isotopic composition. According to Cend !on

[25], the isotopic composition of sulphate from the

evaporite sequence of the Upper Eocene in the studied

area shows two main stages during the development of

the basin: marine and continental. The first stage

corresponds to the formation of the Marginal Sulphates,

the Basal Anhydrite and Lower Halite Units. The

sulphur isotopic composition of the Marginal Sulphates

is closely constrained around +21.670.4%. The Basal

Anhydrite and Lower Halite Units are characterised by

a wider range of d34S values—between +20% and

+23%—which overlap the Basal Anhydrite value. The

second stage corresponds to the formation of the Potash

and the Upper Halite Units. The d34S of the Potash Unit

ranges from 21.3% to 18.2% and decreases to +14%during the precipitation of the Upper Halite Unit. On

the other hand, the sulphur isotopic composition of

Oligocene sulphates from the Anoia Lacustrine Basin is

more d34S depleted, ranging from +11.4% to +12.2%,

due to the recycling of Triassic evaporites [26].

Consequently, the expected d34S natural values of

dissolved sulphate in the studied waters would be related

to the sulphur isotope composition of the drained

evaporite minerals. In places where the Potash Unit

outcrops, such as in the Cardona diapir, the d34S of

dissolved sulphate would range from +18% to +21%,

in accordance with the d34S of gypsum from the Potash

Unit. In all the others sites, these natural values would

be in accordance with the values of the Upper Halite

Unit (around +14%), except for site 5, where the

natural value is around +12% (Anoia Lacustrine

Basin).

The main anthropogenic sources of sulphate in the

studied waters are mining effluents and fertilisers. The

d34S of the dissolved sulphate from mine tailing effluents

(samples 1a, 2a, 4a and 4b) ranges from +18% to

+20% in accordance with the isotope values of the

potash ore. The sulphur isotopic composition of the

fertilisers analysed is �1.1% for ammonium sulphate,

and +7% for NPK 5.7.10.

4.2.2. Isotopic composition of dissolved sulphate

Sulphur isotopic composition of sulphate dissolved in

waters flowing through different rock units is shown in

Table 1. Sulphate in natural springs is characterised by

d34S values from +10% to +14%, consistent with the

isotope composition of the evaporite outcropping

6c 5a6a 6b 6d

2a3a

3d3c

4b

1a

3b

3e3f

4a

9a9b

10a 8a

7b7c

7e7f7a

7d

0

50000

100000

150000

200000

250000

0 2000 4000 6000 8000 10000 12000 14000

B

A

C

5a6c6a

6b 6d4a

3c3d4b3b3e3f

1a

2a3a

9b10a 8a 7e7d 7f

7b

7c

7a9a

0

50

100

150

200

250

0 2000 4000 6000 8000 10000 12000 14000

NO

3 (p

pm)

6a

10a6d

6b 6c

4a

3a

2a

3d 3b 3c 3f 3e4b1a

7c7f

7a

7d7e

8a

9a9b

5a

7b

0

2

4

6

8

10

12

14

16

0 50000 100000 150000 200000 250000

(a) (b)

(c)

Cl (

ppm

)

Σ Σ [M

etal

s] (

Mn+

Zn+

Pb+

Cu)

ppm

Cl (ppm)

SO4 (ppm) SO4 (ppm)

Fig. 5. (a) Plot of sulphate versus chloride concentration, (b) plot of sulphate versus nitrate concentration and (c) plot of chloride

concentration versus sum of metals (Zn, Pb, Cu and Mn) for samples of natural (K), mining (m) and uncertain (’) origin.

N. Otero, A. Soler / Water Research 36 (2002) 3989–40003996

materials. Thus, samples 6a–6d (site 6) have d34S values

around +13% in agreement with evaporites from the

Upper Halite Unit. Sample 5a (site 5), from waters

flowing through evaporites of the Anoia Lacustrine

System, has a d34S=+10.2% close to the isotope

composition of the gypsum formation of Oligocene age

(d34S=+11%).

Water samples collected at site 3 (3a–3f) flow through

mine tailings and evaporites from the Cardona diapir

(Lower Halite and Potash Units) and their d34SSO4

ranges from +18.5% to +19.6% (Plate 2). As sulphur

isotopic composition of bedrock and mine tailings in this

site is similar (d34S=+1970.5%), the sulphate con-

tribution to waters of mining tailings is, in this case,

impossible to evaluate.

The variation in d34S versus [SO4] of the analysed

samples is presented in Fig. 6a. Samples with the highest

sulphate concentration have the highest d34S values but

low isotopic variability, whereas samples with lower

sulphate concentration have a broader range of isotopic

compositions. Data delineate a distribution compatible

with a mixing process. Different mixing curves between

end-members, natural waters, mining effluents and

fertilisers have been calculated and plotted with the

water results (Fig. 6b). Assumed isotopic values and

sulphate concentrations for mining tailing effluents

(M) are d34S=+19.5% and SO4=12000 ppm; and

for natural springs (N) d34S=+13.5% and

SO4=5000 ppm. The concentration of the fertiliser

end-members is estimated following the values obtained

in small streams of the Llobregat River controlled by

fertilisers [19]. The maximum concentration of dissolved

sulphate in these streams is 500 ppm, so the assumed

values for ammonium sulphate fertiliser (FA) are

d34S=�1.1% and SO4=500 ppm; and for NPK fertili-

ser (FK) d34S=+7% and SO4=500ppm.

Water samples of uncertain origin collected at site 7

(7a–7f) are from springs that used to flow into the

Llobregat River, but due to a flow increase, from 0.003 l/s

(March 1998) to 0.1 l/s (January 2000), have recently

been taken up by the salt collector to avoid river water

salinisation. These springs are located in a Quaternary

fluvial terrace overlying Tertiary materials, where

significant mining tailings are deposited over the

Plate 2. Headwaters of the Salat River, draining the Cardona

Diapir as well as mine tailings.

Upper Halite Unit

5a

6c

6d6b

6a

4b3d

3c

3b3a

1a4a

2a3e

3f

10a

8a

7e7d

7b

7c7a

7f

9b9a

NP K

Am. Sulph.

-2

0

2

4

6

8

10

12

14

16

18

20

22

24

0 5000 10000 15000

SO4 (ppm )

Lower Haliteand Potash

Units

Oligocene Gypsum

δ34 S

(0 /

00)

0 4000 8000 12000

0

4

8

12

16

20

90

6090

30

90

6030

90

60

30

90

6030

60

Am. Sulph .

Natural Mining Uncertain Model FK-M Model FA-M Model N-M Model FK-N

NPK

Model FA-N

SO4 (ppm )

δ34 S

(0/ 0

0)

(a)

(b)

Fig. 6. (a) Plot of d34S versus SO4 concentration for samples of

natural (K), mining (m) and uncertain (’) origin. Dashed lines

show the d34S values for the main inputs. (b) Mixing models for

d34S versus SO4 concentration (w/w). The end members

considered are FA (fertiliser: ammonium sulphate), FK

(fertilisers: NPK), M (mining effluents), N (natural surges).

The calculated models are FA–M, FA–N, FK–M, FK–N and

N–M.

N. Otero, A. Soler / Water Research 36 (2002) 3989–4000 3997

Quaternary sediments. The d34S of dissolved sulphate

for these springs has a mean value of +18.8%. This

isotopic composition is in accordance with the values of

the potash unit implying that the waters are flowing

through the mine tailings. The d34S of these samples can

be achieved by mixing 35% sulphate from mine effluents

and 65% sulphate of natural origin (Fig. 6b, model N–

M). If the model considered is a mixing between mine

effluents and fertilisers (models FA–M or FK–M), the

proportion of sulphate from mine effluents is 25%, and

75% fertilisers. These results are especially important

since, unlike conventional chemistry, the sulphur iso-

topic signature of the waters is not sensitive to high

dilution of the original brines. As this result indicates,

even when an anthropogenic source is highly diluted,

and cannot be detected by classical chemistry, its

sulphur isotopic signature may be used as a fingerprint.

Sample 8a has a sulphur isotope value of +8%.

Although the Na/K ratio points to salinity related to

the mining activity (Fig. 4c), the Na, K, Cl� and

SO4�2concentrations are compatible with a natural

source. The d34S values of the outcropping evaporites

in the area are between +12% and +14%. Therefore,

this low value (+8%) could be caused by the addition of

isotopically light sulphate from another source, such as

fertilisers which may also contribute to the high contents

of K. This d34S value can be obtained in a mixture of

75% sulphate from ammonium sulphate fertiliser and

natural sulphate (FA–N model) or in a mixture of 95%

NPK fertilisers and natural sulphate (FK–N model)

(Fig. 6b).

Although a natural origin could be assumed for

sample 10a from the major ion contents, the Na/K ratio

also points to a mining origin. According to the nearby

evaporite rocks, the expected sulphur isotope value

should be 1371%. However, the measured d34S for that

sample is +2.4%, close to the ammonium sulphate

fertiliser value which has a sulphur isotopic composition

of�1.1%. The calculated mixing models (Fig. 6b, model

FA–N) indicate that this isotopic composition can be

achieved by an input of B95% of sulphate derived from

fertilisers to the waters.

Sampling site 9 is located in a small stream with mine

tailings nearby. The samples collected at this site (9a and

9b) have d34SSO4=+13.7%; this value is in agreement

with the isotopic composition of sulphates from the area

(Upper Halite Unit), suggesting a natural origin, which

is already supported by the high Na/K ratio of these

samples (Fig. 7). This value (around +13%) cannot be

explained by a mixture of mining effluents and fertiliser

because the Na/K ratio in this case would be an entire

order of magnitude lower.

The sulphur isotopic composition of dissolved sul-

phate in waters allows the recognition of the origin of

water salinisation in the studied area. The results

obtained indicate that the mine effluents are the source

of salinisation at site 7. The samples from site 9 have a

salinity of natural origin (dissolution of sulphates from

the bedrock) whereas fertilisers are suggested to be

major contributors to salinisation for sites 8 and 10.

5. Conclusions

This study shows the usefulness of combining

elemental and isotopic geochemistry to trace the origin

of water salinisation in the middle section of the

Llobregat River. The chemical composition of waters

allows for the distinction between natural and anthro-

pogenic sources of sulphate. Natural waters are char-

acterised by Na/K ratios higher than 100, whereas

waters containing salts from human activities (mining

effluents and fertilisers) have ratios lower than 20. The

mine tailing effluents are characterised by high Cl, Na,

K and Mg content, as well as the occurrence of heavy

metals. Groundwaters contaminated by mine effluents

have significant metal contents also, therefore, the

environmental impact of potash mine tailings is not

only water salinisation, but in some cases, the presence

of heavy metals as well.

According to isotope data three main sources of

dissolved sulphate in saline springs can be identified:

bedrock evaporites (d34S between +10 and +14%),

mining effluents (d34S between +18 and +20%) and

fertilisers (d34S=–1.1% for ammonium sulphate and

d34S=+7% for NPK). The difference in d34S between

the outcropping sulphates and sulphates from the ore

unit (potash unit) allows us to know the origin of the

5a

6d

6c6b

6a

10a

8a

9b9a

0

5

10

15

20

25

0 100 200 300 400 500 600

Na/K

3d 3e3f

3c3b

2a

4a 1a

3b

3a

7b

7a7c7e

7f7d

17.5

20.5

δ34S

(0/ 0

0)

Fig. 7. Plot of Na/K versus d34S for (K) samples of natural

origin, (m) samples of mining origin and (’) samples of

uncertain origin.

N. Otero, A. Soler / Water Research 36 (2002) 3989–40003998

sulphate contamination caused by the mining effluents,

except in the Cardona Diapir where the potash unit

outcrops. Accordingly, this study has shown that the

origin of salinisation in the springs located at site 7 is

contamination from mine tailing effluents, also a natural

origin has been determined for the springs from site 9.

Fertilisers used in this area are characterised by sulphur

isotopic values of dissolved sulphate lower than the

bedrock allowing us to identify their contribution to

groundwater salinisation. Elemental and isotope geo-

chemistry point to fertilisers as the major source of

sulphate in sites 8 and 10. Coupling isotopic composi-

tion analysis together with geochemistry allowed us also

to quantify the relative contribution of these sources to

groundwater salinisation.

Since the data reveal that fertilisers play an important

role as salinity source, a more detailed sampling and

isotopic characterisation of fertilisers is proposed in a

further project. Also, the analysis of the oxygen isotopic

composition of sulphate could be used in future studies

to assess the extent of the redox processes, which have

been considered negligible in the present study.

Acknowledgements

This study has been financed by the CICYT project

HID99-0498 of Spanish Government, and partially

by the SGR99-00062 from the Catalonian Government.

We would like to thank J. Capdevila for the pluvio-

meter sampling, Ag"encia Catalana de l’Aigua (Environ-

mental Department of Catalonian Government, espe-

cially Llu!ıs God!e and Josep Bertran) for the field

assistance, and the Serveis Cient!ıfics T"ecnics (University

of Barcelona). We also wish to thank S.H. Bottrell and

an anonymous reviewer for their constructive comments

on the paper.

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