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

ECHEGOYEN C.; LECOMTE, K.L.; CAMPODONICO V.A.; YACIUK, P.; JOBBÁGY, E.; HIEDER, G.; SEPULVEDA, L.; PASQUINI, A.I.; DE MICCO, G. ; BOHÉ, A.E.

BOLETIN GEOLOGICO Y MINERO

Instituto Geológico y Minero de España

Lugar: Madrid; Año: 2021 vol. 132 p. 99 - 106

0366-0176

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, here the water table reaches the surface and fills the deepest depressions. The aim of this work 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 groundwaters are of the HCO3 - - Ca+2 29 type, with variable pH and electrical conductivity values. In the northern portion of the lake, concentrations of 222 30 Rn in surface water were ~ 70 Bq m-3 31 , one order of magnitude greater than those of the southern sector, which registered values < 5 Bq m-3 . By means of a 222 32 Rn mass balance model it was possible to determine that 33 the groundwater discharge occurs in the northern sector, with an inflow rate discharge of about 185.3 ± 39.1 m3 d -1 34 , whereas in the southern sector, an outflow from the lake to surrounding aquifers can be 35 detected. The intense groundwater inflow into these lakes may explain their relatively low salinity under 36 a semiarid climate in which evaporation exceeds by far direct rainfall inputs. 37 Keywords: aquifer, environmental tracers, geochemistry, lake, radioactive isotopes.