Origin of brackish groundwater in a sandstone aquifer on Bornholm, Denmark

TY - JOUR

T1 - Origin of brackish groundwater in a sandstone aquifer on Bornholm, Denmark

AU - Jørgensen, Niels Oluf

AU - Heinemeier, Jan

PY - 2008

Y1 - 2008

N2 - A multi-isotope approach in combination with hydrochemical data and borehole logging is applied to identify the source of brackish groundwater in a borehole in the well field of Neksø Municipal Waterworks in Bornholm, Denmark. The aquifer lithology consists of fractured Lower Cambrian sandstones resting on Proterozoic crystalline basement. The water body in the studied borehole is significantly stratified with respect to the hydrochemical and isotopic signatures and reveal a Na-Ca-Cl-HCO3 water type and TDS values >1,000 mg/L below a halocline at 40-55 m below measurement point (bmp). The occurrence of brackish groundwater is remarkable for this aquifer, which otherwise yields potable groundwater of good quality. The stable isotope (18O and 2H) compositions indicate a meteoric origin of the brackish groundwater, which rules out seawater intrusion into the aquifer. 14C activities show apparent 14C ages of the brackish groundwater in the range 2200-4300 yr (BP), whereas the freshwater samples above the halocline indicate modern age. Hydrochemical (Cl/Br and Sr) and isotopic studies (18O, 2H and 87Sr/86Sr) of the brackish groundwater point to a well-mixed and homogeneous water body reflecting long water-rock interaction and suggest a contribution of palaeowater from the fractured crystalline basement which has intruded into the Lower Cambrian sandstone aquifer.

AB - A multi-isotope approach in combination with hydrochemical data and borehole logging is applied to identify the source of brackish groundwater in a borehole in the well field of Neksø Municipal Waterworks in Bornholm, Denmark. The aquifer lithology consists of fractured Lower Cambrian sandstones resting on Proterozoic crystalline basement. The water body in the studied borehole is significantly stratified with respect to the hydrochemical and isotopic signatures and reveal a Na-Ca-Cl-HCO3 water type and TDS values >1,000 mg/L below a halocline at 40-55 m below measurement point (bmp). The occurrence of brackish groundwater is remarkable for this aquifer, which otherwise yields potable groundwater of good quality. The stable isotope (18O and 2H) compositions indicate a meteoric origin of the brackish groundwater, which rules out seawater intrusion into the aquifer. 14C activities show apparent 14C ages of the brackish groundwater in the range 2200-4300 yr (BP), whereas the freshwater samples above the halocline indicate modern age. Hydrochemical (Cl/Br and Sr) and isotopic studies (18O, 2H and 87Sr/86Sr) of the brackish groundwater point to a well-mixed and homogeneous water body reflecting long water-rock interaction and suggest a contribution of palaeowater from the fractured crystalline basement which has intruded into the Lower Cambrian sandstone aquifer.

KW - Faculty of Science

KW - strotium isotoper

KW - stabile isotoper

KW - Brackish groundwater

KW - strontium isotopes

KW - stable isotopes

KW - paleowater

U2 - 10.2166/nh.2008.043

DO - 10.2166/nh.2008.043

M3 - Journal article

SN - 1998-9563

VL - 39

SP - 209

EP - 222

JO - Hydrology Research

JF - Hydrology Research

IS - 3

ER -