Saline waters in the coastal area of the national park of Doñana (SW of Spain) in absence of saline water intrusion.

Abstract

Wetlands in dune complexes in Doñana National Park are strongly influenced by groundwater. Hydrochemical studies have been performed to know the relationships between the small lakes existing at the inner side of the coastal dune belt and the aquifer, which is formed mainly by fine to medium silica sands. The aim of this work is to show the existence of brackish and saline waters in a coastal area with lakes in absence of marine water intrusion. These saline waters form in small lakes by evaporation, and affect the chemical and isotopic composition of the surrounding groundwaters. The mean electrical conductivity (EC) of unaffected groundwater (2 - 45 m depth) in the study area is around 0.25 mS/cm. It is of the Na-Cl type, and reflects mostly the local rain, including airborne marine salts incorporation into recharge water through dry deposition. The water of the small lakes present a wide range of EC, from 0.2 to 28 mS/cm, and are mainly of the Na-Cl type, but can be changed into Na-SO4 waters. In the surroundings of the lakes phreatic water (< 2 m depth) may acquire elevated EC values, from 0.1 to 7.5 mS/cm. The origin of this salinity increase is the mixing of groundwater and surface water affected by intense evaporation. The evolution of SO4 concentrations in lake water seems to be related with the chemical reactions taking place during dry and wet periods, mostly the reduction and precipitation of S compounds and their re-disolution afterwards. These processes modify the chemical and isotopic composition of the groundwater sampled near Dulce and Santa Olalla lakes, which show the mixing between surface and phreatic water. The observed salinity, chemical and isotopic composition distribution of the phreatic waters around these lakes suggests that fresh groundwater enters Santa Olalla lake along its N and W margins, while concentrated surface water infiltrates to the aquifer along its SE margin.