Future Groundwater Resources at Risk (Proceedings of the Helsinki Conference, June 1994). IAHSPubl.no. 222, 1994. 231

Isotope studies (180/160, D/H and 87Sr/86Sr) of saline groundwater in Denmark

NIELS OLUF J0RGENSEN & PAUL MARTIN HOLM Geological Institute, Copenhagen University, 0ster Voldgade 10, DK-1350 Copenhagen, Denmark

Abstract Saline groundwaters are recognized to have three principal origins in Denmark: (a) Seawater infiltration into near-coastal aquifers, (b) saline formation water in aquifers of marine sedimentary origin and (c) intrusion of brines from deep saline formation waters and evaporitic deposits in the subsurface. Isotope hydrochemical investigations of the distribution patterns of hydrogen, oxygen and strontium isotopes in groundwater from three selected aquifers demonstrate that isotopic hydrochemical criteria can successfully unveil the sources of saline groundwater.

INTRODUCTION

The occurrence of saline groundwater is a well-known phenomenon in Denmark and is a serious problem in water resources management. Saline groundwater requires specific precautions to be taken by waterworks for which it is essential to know the source of the saline component. Saline groundwaters are recognized to have three principal origins in Denmark: (a) seawater infiltration into near-coastal aquifers, (b) saline formation water in aquifers of marine sedimentary origin and (c) intrusion of brines from deep saline formation waters and dissolution of evaporitic deposits in the subsurface by circulating meteoric water. However, it may be difficult to unveil the source of saline groundwater only by means of geological setting. The objective of the present study is to establish isotope hydrochemical criteria for the distinction between these sources for saline groundwater using the distribution patterns of hydrogen, oxygen and strontium isotopes.

ANALYTICAL METHODS

The CI contents in water samples were determined by a digital titrator using the mercuric nitrate method.

The oxygen stable isotope analyses of water samples were measured in a VG Sira 10 mass spectrometer with automatic inlet. The water samples for stable hydrogen isotope analyses were reduced to H2 by use of Zn in quartz ampules at 700°C. The analyses were carried out in a Finnigan MAT 250 triple collector mass spectrometer. The results for both isotopes are expressed in per mille deviation from the SMOW standard using the <5-scale. Reproducibility is better than 0.1 %o for both ratios.

Freeze-dried water samples were analysed for 87Sr/86Sr ratios using standard ion-exchange techniques and a VG Sector 54-30 mass spectrometer. The Sr concentration

232 Niels Oluf Jorgensen & Paul Martin Holm

in the water samples was determined in order to test the validity of the mixing hyperbola for the strontium isotopes. The Sr analyses were carried out with a Perkin-Elmer Atomic Absorption Spectrophotometer model 460.

The oxygen isotope analyses were determined at the Geophysical Institute, Copenhagen University. The determination of the Sr and CI contents and the hydrogen isotope and strontium isotope analyses were carried out in laboratory facilities at the Geological Institute, Copenhagen University.

ISOTOPES IN THE HYDROLOGICAL CYCLE

The relationship between <5D and ô180 in meteoric waters is linear and is represented by the meteoric water line with the equation:

ÔDsmow = 8ô18Osmow + 10 (Craig, 1961)

The oxygen and hydrogen isotope compositions of meteoric water in Denmark are 6180 = -8.00%o and ÔD = — 70%o, respectively. The ô180 and <5D of oceanic seawater are close to zero and only vary within narrow limits (Faure, 1986). The isotopic composition of brackish seawater in the inner Danish seas vary considerably and depend upon the mixing of meteoric water (i.e. precipitation and runoff from rivers) and oceanic seawater from the North Sea. However, groundwater influenced by intruding seawater will show an isotopic composition which correlates with the meteoric water-seawater mixing line.

The strontium isotopic composition of the precipitation in Denmark reflects the 87Sr/86Sr ratio in modern seawater, i.e. 0.7092. However, the strontium concentration in the precipitation is extremely low and, therefore, groundwater will have 87Sr/86Sr ratios which primarily reflect the isotopic composition of the strontium-bearing mineralogical components in the host rock as a result of the chemical equilibrium between formation and porewater. Groundwater in aquifers influenced by intruding sea­water or other Sr-containing fluids will show a Sr isotopic composition correlated to a mixing hyperbola between the end members of the groundwater and the infiltrating fluid.

CASE STUDIES

Three case studies of saline groundwater were carried out on locations selected with particular respect to the hydrogeological settings discussed above, i.e. a near-coastal limestone aquifer with intruding seawater, a near-coastal aquifer of Holocene marine sediments and a limestone aquifer in supposed contact with underlying Zechstein evaporites (Fig. 1).

Hanstholm

Hanstholm is located on the northwest coast of Jutland (Fig. 1). A number of water wells found on the coastal plain close to the seashore have been studied. The wells are approximately 40 m deep and were drilled into an aquifer of Danian limestone covered

Isotope studies of saline groundwater in Denmark 233

Fig. 1 Map of northern Denmark showing the locations of the three aquifers studied. 1. Hanstholm; 2. Erslev; 3. Skagen.

-6 -4 <5,80o/oo

Fig. 2 The ÔD vs. <5180 in groundwater samples from the Hanstholm aquifer and adjacent seawater from the Skagerak. The plot shows a close fit to the groundwater-seawater mixing line (r = 0.99).

10 15 20 CI mg/l

(Thousands) Fig. 3 The <5180 vs. CI contents in groundwater samples from the Hanstholm aquifer. The data shows a significant positive correlation (r = 0.99).

234 Niels Oluf Jorgensen & Paul Martin Holm

by up to 1 m of Recent marine sands and windblown quartz sands. The wells show variable influence by seawater infiltration and the groundwater exhibits CI concentrations in the range 50-17 100 mg l"1 CI depending on distance to the sea. The plot of ÔD vs. ô180 shows a significant correlation with the meteoric water-seawater mixing line (r = 0.99) (Fig. 2). The oxygen and hydrogen isotopes also show a significant positive correlation with the CI contents (r = 0.99) (Fig. 3).

However, the 87Sr/86Sr vs CI plot and the 87Sr/86Sr vs 1/Sr plot reveal the presence of two water types in the aquifer (Figs 4-5). The low-Cl water samples (i.e. < 100 mg 1_1 CI and < 1 mg r1 Sr) have 87Sr/86Sr ratios in the range from 0.7079 to 0.7083 which is appreciably above the ratio of the reservoir rock, the Danian limestone, i.e.87Sr/86Sr = 0.7076 (Jorgensen & Larsen, 1979) (Fig. 4). This is attributed to a mixing system between infiltrating precipitation (87Sr/86Sr = 0.7092) and modified groundwater containing Sr released by water-rock interaction. The 87Sr/86Sr ratios in the samples obtained from the marine influenced part of the aquifer (i.e. >200 mg r1 Cl and >2mgl_ 1 Sr) are significantly correlated with a flat mixing hyperbola with end members of modern marine water (0.7092) and low-Cl groundwater in the limestone aquifer (Fig. 4).

0 . 7 0 9 5 Î

0.7090

=£• 0.7085

0.7080

0.7075 20 10

CI mg/l (Thousands)

Fig. 4 The 87Sr/86Sr vs. CI contents in groundwater samples from the Hanstholm aquifer and adjacent seawater from the Skagerak. Cl-low groundwater samples: filled squares. Cl-rich groundwater samples: empty squares.

0.7095

0.7090-

0.7085

0.7080

0.7075 0.0 1.0 4.0 5.0 2.0 3.0

1/Srppm"1

Fig. 5 The 87Sr/86Sr vs. 1/Sr in groundwater from the Hanstholm aquifer. Note the existence of two water types: Cl-low groundwater samples (filled squares) and Cl-rich groundwater samples (empty squares). See text for further explanation.

Isotope studies of saline groundwater in Denmark 235

Erslev

Erslev is located on the island of Mors (Fig. 1) close to the centre of the Mors Dome, a salt diapir of Zechstein marine evaporites overlain by Upper Cretaceous chalk and Danian limestone. The cap rock is encountered at ca. 600 m below the surface and the interface between highly saline formation water and meteoric water is situated at approximately 100 m below the surface (Elsam & Elkraft, 1981). A single well, DGU nr. 37.928, approximately 80 m deep, was drilled into an aquifer of the carbonate sediments and has been sampled in order to examine the possible influence of any intrusion of saline brine from below.

The plot of <5D vs. <5180 reveals a constant isotopic composition and a relative close fit to the meteoric water line (Fig. 6). The CI concentration in the well ranges from 50-80 mg T1 CI in the uppermost 35 m to approximately 700 mg l"1 CI at the bottom of the well (Fig. 7). However, continuous pumping from the bottom of the well resulted in increasing CI contents with time (up to 1100 mg l"1 CI after 3 h with a capacity of 1 m3 h"1). However, the oxygen isotope composition is relatively constant with increasing CI contents indicating that there is no influence of modern seawater (Fig. 8).

The low-Cl water samples from the top of the well exhibit an 87Sr/86Sr ratio around 0.7079 (Fig. 9) which is in the same order as found for the low-Cl groundwater in the

o Q

-8.5 <5l8Oo/oo

Fig. 6 The 5180 vs. <5D in groundwater samples from the Erslev well DGU nr. 37.928. The data points are located slightly above the meteoric water line (MWL).

-80

-100 200 800 1000 400 600 CI mg/l

Fig. 7 CI vs. depth in Erslev well DGU nr. 37.928. Note the stratified water column in the well.

236 Niels Oluf Jargensen & Paul Martin Holm

o O

600 CI mg/l

800 1000 1200

Fig. 8 CI vs. <5I80 in Erslev well DGU nr. 37.928. The oxygen isotope compositions are relatively constant for both Cl-low water samples (empty squares) and the Cl-rich water samples (filled squares).

0.7095

0.7090

Sg 0.7085-

S 0.7080

0.7075-

0.7070

0.7072

0 1000 2000 3000 CI mg/l

Fig. 9 The 87Sr/86Sr ratios vs. CI contents in groundwater samples from the Erslev aquifer. The mixing hyperbola is indicated for groundwater and a subsurface brine which derives from dissolution of Zechstein evaporites.

Danian limestone aquifer of Hanstholm. However, the 87Sr/86Sr ratios in the samples with high CI contents are relatively constant and close to 0.7078, which is well below the groundwater-modern seawater mixing hyperbola as obtained from the Hanstholm data (Figs 4 and 9). Chemical analyses of the formation water from the Upper Cretaceous limestone above the cap rock in the Morse Dome show that the pore water is depleted with respect to bromide, suggesting that the brine derives from dissolution of evaporites (Laier & Jacobsen, 1992). Although it has not been possible to obtain any Sr isotope data from the brine itself, the 87Sr/86Sr ratio of the brine is most likely equal to that of Zechstein halite from the Morse Dome (87Sr/86Sr = 0.7072) which is in accordance with the strontium isotope composition of marine sediments from Upper Zechstein (Burke et al, 1982). Therefore, it is believed that the strontium isotope compositions of the groundwater from the Erslev well are most likely the result of a flat mixing hyperbola between low-Cl groundwater in equilibrium with Danian and Upper Cretaceous limestone and a residual brine which originated from formation waters of Zechstein evaporites in the subsurface.

Skagen

A large number of water wells and piezometers of Skagen Waterworks, North Jutland (Fig. 1), were examined in order to investigate the origin of the high CI content (40-

Isotope studies of saline groundwater in Denmark 237

1650 mg l"1 CI) in the groundwater. The aquifer consists of postglacial deposits of marine sand and silt. The plot of SD vs. ô180 shows good agreement with the meteoric water line (Fig. 10). However, the plot of oxygen isotopes versus the CI contents reveals

-6 -4 5I80 o/oo

Fig. 10 The <5I80 vs. 5D in groundwater samples from Skagen aquifer and adjacent seawater from Skagerak and Kattegat.

o o

-8.5

-9.0 300 600 900 1200 1500 1800

CI mg/l

Fig. 11 The 5180 vs. CI contents in groundwater samples from Skagen aquifer and adjacent seawater from Skagerak and Kattegat. The filled squares show analyses from a single water well in which the filters are influenced by intrusion of seawater. The empty squares show analyses of all other water wells investigated in the Skagen area.

0.7110

0.7080 1000 2000

CI mg/l 15 20 CI mg/l

(Thousands)

Fig. 12 The 87Sr/86Sr vs. CI contents in groundwater samples from the Skagen aquifer. Legend, filled triangles: seawater; empty squares: groundwater samples influenced by intrusion of seawater; filled squares: analyses of groundwater from other water wells in the Skagen area.

238 Niels Oluf Jergensen & Paul Martin Holm

two populations: (a) a single well in which the water type shows significant influence by seawater and (b) the majority of wells in which the water type is characterized by relatively constant oxygen isotopic composition but highly variable CI contents (Fig .11). Thus, the data suggest that the chlorinity of the water in the aquifer may have two sources: (a) intruded sea water in restricted parts of the aquifer and (b) a generally enhanced CI content which most likely derives from residual CI in the marine sediments themselves. It is noteworthy that the strontium isotopic data reveal relatively high 87Sr/86Sr ratios (up to 0.7105) particularly in low-Cl samples (Fig. 12). This indicates that the groundwater is influenced by Sr released from highly radiogenic Proterozoic K-feldspar and micas which are common accessory minerals in the marine sediments. The Cl-rich samples influenced by seawater show 87Sr/86Sr ratios in accordance with modern seawater (87Sr/86Sr = 0.7092).

CONCLUSIONS

The examination of the three selected aquifers demonstrates that isotope hydrochemical studies are a useful tool in identifying details in the mechanisms of salinization of groundwater and in differentiating sources of saline groundwater. (a) Seawater intrusion into near-coastal aquifers results in significant positive

correlation of the oxygen (and hydrogen) isotope data with the meteoric water-seawater mixing line. The strontium isotope data reveal a mixing hyperbola with the end members of modern marine composition, i.e. 0.7092, and the strontium isotopic composition of the reservoir rock.

(b) Groundwater in near-surface marine sedimentary aquifers which are influenced by remnant Cl-ions of marine origin generally reveal oxygen and hydrogen isotopic data which have a close fit to the meteoric water line. The strontium isotope ratios of the groundwater reflect the isotopic composition of the Sr released by water-rock interaction in the aquifer.

(c) Groundwater in aquifers influenced by intrusion of highly saline brines in the subsurface or by dissolution of evaporitic deposits by circulating meteoric water generally show oxygen and hydrogen isotopic compositions which are close to that of the meteoric water line. The strontium isotopic mixing hyperbola of the dissolved Sr in the groundwater reflects the Sr isotopic composition of the reservoir host-rock lithology and the subsurface brine.

REFERENCES

Burke, W. H., Denison, R. E., Hetherington, E. A., Koepnik, R. B., Nelson, H. F. & Otto, J. B. (1982) Variation of sea water 87Sr/86Sr throughout the Phanerozoictime. Geology 10, 516-519.

Craig, H. (1961) Isotopic variations in meteoric waters. Science, 133, 1702-1703. Elsam & Elkraft (1981) Disposal of high-level waste from nuclear power plants in Denmark. Salt dome investigations. In:

Geology, vol. II. Faure, G. (1986) Stable Isotope Geochemistry. 2nd Edition. Springer Verlag, Berlin. Jargensen,N. O. &Larsen, O. (1979) The strontium isotopic composition of Maastrichtian and Danian chalk. Bull. Geol.

Soc. Denmark!», 127-129. Laier,T. & JacobsenO. S. (1992) Saline groundwater in Denmark - Geochemical and isotopic studies. Presented at Nordic

Workshop on Mathematical Models for QuantitativeEvaluationof Isotope Contents in Groundwater Hydrology (Lund, Sweden, June 1992).