Water qualitiy and distribution of trace elements in the Doñana aquifer (SW Spain)

Abstract

The variations of groundwater quality in the unconfined zone of the Almonte-Marismas aquifer, upon which Donana National Park is located, are analysed. Most sampled points are multiple piezometers, allowing for the vertical distribution study of the hydrogeochemical characteristics in the aquifer. Temperature, pH, electrical conductivity, dissolved oxygen and redox potential were determined in the field. A large number of parameters, including major ions and a large amount of minor and trace elements, were analysed. In the southern zone, where aeolian sands crop out, water composition in the shallower part of the aquifer is of the sodium chloride type, with low pH (5.5–6) and mineralization (<200 μS/cm) values. As water circulates through the aquifer, bicarbonate and calcium concentrations increase slightly. In agricultural or urban zones, nitrates and sulphates present their highest contents in the upper part of the aquifer. In zones with low pH levels (around 6), concentration of many trace metals (Al, Co, Cr, Cu, Ni, Zn, etc.) also shows a distribution similar to that of sulphates and nitrates, which indicates its fertilizer-linked origin. In zones with neutral or alkaline pH, regardless of high nitrate content, concentration of the above mentioned metals is very low due to its immobilisation by surface adsorption processes. The distribution of Br contents also shows the effects of agricultural pollution.