Time trends of 1,1,1-trichloroethane, trichloroethylene, and perchloroethylene in confined and unconfined aquifers of a groundwater system in northern Italy.

The concentrations of 1,1,1-trichloroethane, trichloroethylene, and perchloroethylene were recorded in a groundwater system of Northern Italy over the period 1985-1997. In the unconfined recharging aquifer these chemicals showed a remarkable overall decline which was accompanied by a five-fold reduction in their consumption (from approx. 250 to approx. 50 tons year-1) over the same period. The time trends for the confined aquifers indicated a steady decline for 1,1,1-trichloroethane which was accompanied by a constant concentration of trichloroethylene and an increasing presence of perchloroethylene. It is suggested that the confined aquifers are recording a contamination which took place in the unconfined recharging aquifer before monitoring period (1982) started. At present, in most of the study area 1,1,1-trichloroethane contamination is below the detection limit (0.1 microgram/L). For trichloroethylene and perchloroethylene, the average unconfined aquifer contamination accounts for approx. 1 +/- 1 and approx. 4 +/- 3 micrograms/L, respectively, while in the confined aquifers the average concentrations are approx. 8 +/- 3 and approx. 35 +/- 5 micrograms/L, respectively.