GEOCHEMICAL CHARACTERIZATION OF GROUNDWATER IN AN AREA OF CONTINUOUS PERMAFROST ADJACENT TO THE GREENLAND ICE SHEET, KANGERLUSSUAQ, WEST GREENLAND

In general, knowledge of groundwater chemistry in areas of continuous permafrost in Greenland relies on the study of springs and open pingos, while knowledge of deep groundwater chemistry is limited. Properly instrumented boreholes can provide additional insight into geochemical processes affecting groundwaters in cryogenic environments. As part of the Greenland Analogue Project (GAP), two deep boreholes were drilled and instrumented with sampling systems in the Kangerlussuaq Region of West Greenland. Borehole DH-GAP01 (221 m total length) was drilled at an angle and intercepts a talik beneath a lake less than 2 km from the Greenland ice sheet. Borehole DH-GAP04 (697 m total length) was drilled adjacent to the ice sheet and angled to allow sampling in the bedrock beneath the ice sheet. DH-GAP04 is equipped with three sampling sections installed below the base of the permafrost (350 m below ground surface).

Isotopic (d¹⁸O, d²H, ³H, d³⁴S-d¹⁸O_SO4, ⁸⁷Sr/⁸⁶Sr, d³⁷Cl and d⁸¹Br) and geochemical tools were used to compare the impact of mixing and water-rock interaction to cryogenic processes such as cryogenic concentration and solute exclusion during permafrost formation. Isotopic characterization of fracture minerals, crush and leach and out diffusion were also used to determine water-rock interaction and to obtain estimates of porewater composition to further define these end members.

Groundwater in the bedrock beneath the margin of the ice sheet has an isotopic signature (d¹⁸O/ d²H) similar to the enriched end of the range of isotopic values observed for regional meltwaters. Recharging glacial meltwaters interact with both bedrock and fracture minerals, evolving from dilute Ca, Na, K-HCO₃ type waters to brackish Ca-Na-SO₄ waters. Based on the d³⁴S/d¹⁸O and ⁸⁷Sr/⁸⁶Sr values of the groundwater and fracture fillings, the chemistry is dominated by dissolution of gypsum and other sulphate-bearing mineral phases found as fracture infillings. The extensive presence of gypsum, a highly soluble mineral, below 300 meters in the DH-GAP04 core suggests that the groundwater system below this depth has maintained a degree of stability during, and beyond, the Pleistocene.