Use of radon-222 to assess the groundwater inflow in a phreatic lake of a dune field (San Luis, Argentine)

Abstract

In the southeastern portion of San Luis Province (33°53’10’’-34°19’00’’ S and 65°42’00’’-65°20’00’’ W), the sand dune landscape hosts a lacustrine system with more than 200 water bodies, where the water table reaches the surface and fills the deepest depressions. The aim of this study is to analyze surface-groundwater interactions using the radioactive isotope radon-222 (²²²Rn) in a lake known as “Los Pocitos”. During September 2017, in situ ²²²Rn determinations were performed in the air; in the water/sediment interface and in surface waters at 6 sampling stations, as well as in 4 groundwater samples collected in the dune and the lake mudflat using the RAD-7 equipment (Durridge Co.). Surface waters are of the HCO₃ - - Na⁺ - K⁺ type, with mean pH and electrical conductivity in Lake Los Pocitos of 8.7 and 1232 μS cm^-1, respectively, whereas the groundwater is of the HCO₃ - - Ca⁺² type, with variable pH and electrical conductivity values. In the northern portion of the lake, concentrations of ²²²Rn in surface water were ~ 70 Bq m^-3, one order of magnitude greater than those of the southern sector, which registered values < 5 Bq m^-3. By means of a ²²²Rn mass balance model it was possible to determine that the groundwater discharge occurs in the northern sector, with an inflow discharge rate of about 185.3 ± 39.1 m³ d^-1, whereas in the southern sector, an outflow from the lake to the surrounding aquifers can be detected. The intense groundwater inflow into these lakes may explain their relatively low salinity under a semi-arid climate in which evaporation by far exceeds direct rainfall input.