Assessment of retoxification factors variability in a heavy metal contaminated coastal aquifer in South Eastern Sardinia (Italy)

Mobilization of heavy metals accumulated in soil and sediments in coastal aquifers represent a potential risk of groundwater contamination when soil-water systems react to natural/anthropic variation of environmental conditions. In coastal aquifers, chemical status of groundwaters is influenced by seawater intrusion: freshwater-seawater mixing and/or salt solution induce groundwater salinity increase and variation of chemical equilibria. Such variations affect heavy metals mobility, by altering the ion-exchange equilibrium, by promoting soluble metal complex and decreasing chemical thermodynamic activities in solution. With salinity decrease, heavy metal concentration in groundwaters decreases: anyhow, heavy metal concentration is subject to cyclic changes according to the dynamics of salinization/seawater intrusion. In a coastal aquifer in SE Sardinia, affected by heavy metal contamination due to the past mining activity, the spatial and time variability of Cl, EC, Eh, pH and heavy metals was analyzed by geostatistical methods in order to assess system potential for retoxification. The results of the study suggest that, under salinization from various sources, heavy metals are released from the contaminated sediments, and Se and Sb result the heavy metals most sensitive to salinity changes. Results of the study imply that remediation strategies and actions for restoring heavy metals contaminated sites located on the coasts have to consider the effects of possible retoxification processes on groundwaters and seawater by release of heavy metals from soils and sediments.