Impact of industrial and urban waste on the heavy metal content of salt marsh soils in the southwest of the province of Pontevedra (Galicia, Spain)

The impact of waste disposal on marsh soils in Lagares (Vigo, Spain) was assessed in topsoil samples collected at ten randomly selected sampling points in the salt marsh at 4-month intervals for two years. The study area is very close to the city of Vigo (population 300,000) and also to an industrial complex the raw materials and end products of which can contaminate the surrounding areas via atmospheric pollution and effluents; in fact, the river forming the salt marsh receives many discharges. Polluted soil samples were characterized in physico-chemical terms and their heavy metal contents determined for comparison with control, unpolluted samples. The influence of discharges was clearly apparent since, under normal environmental conditions, the soils in the area have low contents in heavy metals. On the other hand, the soils affected by discharges contained substantial amounts of total and DTPA-extractable Cd, Cu, Pb, and Zn. Based on the relationship between the oxidation–reduction potential and the total and DTPA-extractable Cd, Cu, Pb, and Zn contents, strongly reductive conditions raise the total contents by effect of the metals remaining in the soil as precipitated sulphides. Such contents, however, decrease as oxidative conditions gain prevalence. The DTPA extractable metal contents increase with increasing Eh by effect of the metals being released as ions to the soil solution under oxidative conditions. The contents in heavy metals concentrating in the polluted soils were found to be several times greater than those in the control soils (viz. 7 vs 16 for Cd, 9 vs 14 for Cu, 8 vs 32 for Pb, and 6 vs 27 for Zn). This can be expected to influence the amounts of available heavy metals, and hence the environmental quality of the area, in the near future. Geoaccumulation index (Class > 3) indicates that the Lagares is highly polluted for heavy metals studied. Enrichment factors confirm that the salt marsh is highly polluted, mainly by Cd in all the studied soils, principally due to the anthropogenic activity.