Use of radon-222 to assess the groundwater inflow in a phreatic lake of a dune field.

CECILIA VANINA ECHEGOYEN; KARINA LETICIA LECOMTE; VERENA AGUSTINA CAMPODONICO; PABLO AGUSTÍN YACIUK; ESTEBAN GABRIEL JOBBÁGY; GUILLERMO HEIDER; LAURA DANIELA SEPÚLVEDA; ANDREA INÉS PASQUINI; GEORGINA DE MICCO; ANA ESTER BOHÉ

BOLETIN GEOLOGICO Y MINERO

Instituto Geológico y Minero de España

Lugar: Madrid; Año: 2019

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), 21 the sand dune landscape hosts a lacustrine system with more than 200 water bodies, where the water 22 table reaches the surface and fills the deepest depressions. The aim of this work is to analyze surface-23 groundwater interactions using the radioactive isotope radon-222 (222Rn) in a lake known as ?Los 24 Pocitos?. During September 2017, in situ 222Rn determinations were performed in the air; in the 25 water/sediment interface and in surface waters at 6 sampling stations, as well as in 4 groundwater samples 26 collected in the dune and the lake mudflat using the RAD-7 equipment (Durridge Co.). Surface waters 27 are of the HCO3- - Na+ - K+ type, with mean pH and electrical conductivity in Lake Los Pocitos of 8.7 28 and 1232 μS cm-1, respectively, whereas groundwaters are of the HCO3- - Ca+2 type, with variable pH 29 and electrical conductivity values. In the northern portion of the lake, concentrations of 222Rn in surface 30 water were ~ 70 Bq m-3, one order of magnitude greater than those of the southern sector, which 31 registered values < 5 Bq m-3. By means of a 222Rn mass balance model it was possible to determine that 32 the groundwater discharge occurs in the northern sector, with an inflow rate discharge of about 185.3 ± 3339.1 m3 d-1, whereas in the southern sector, an outflow from the lake to surrounding aquifers can be 34 detected. The intense groundwater inflow into these lakes may explain their relatively low salinity under 35 a semiarid climate in which evaporation exceeds by far direct rainfall inputs.