Chlorinated hydrocarbons plumes in a residential area. Site investigation to assess indoor vapor intrusion and human health risks

This case study deals with a severe groundwater contamination by chlorinated ethenes and ethanes beneath the urbanized area of via Caretti, immediately to the east of the ancient historical center of the city of Ferrara (Emilia-Romagna region, northern Italy). Based on the concentrations as measured in the groundwater and conventional risk modeling, the contamination potentially poses a serious threat to on-site receptors (inhabitants) by indoor and outdoor vapor inhalation. The sources of contamination are two unlined waste dumps located inside abandoned clay quarries and filled, during the 1960’s-70’s, by a mixture of urban solid wastes and chlorinated pitches originated from the chloromethanes distillation process of nearby petrochemical plants. The area is located in the eastern and lowest sector of the Padana plain, a few kilometers southward of the Po river. The underground stratigraphy consists of alternating alluvial sand and silty-clayey layers that form a multi-aquifer system with alternance of three sandy permeable layers, down to about 50 m depth below ground level (b.g.l.), with low permeability aquitards. Contaminants occurrence in the aquifers, soil gas and indoor air were determined by a multi-step investigation approach based upon state of the art techniques. Contaminant plumes of mainly vinyl chloride (VC) occur in the two shallowest aquifers and have migrated up to a maximum distance of 500 m from the source below the houses of via Caretti. VC concentrations in the first aquifer attain a peak of about 1x103 μg/L (with a maximum value at the source of more than 2x104 μg/L); concentrations of about one order of magnitude higher were detected in the upper confined aquifer with an absolute peak of more than 1x105 μg/L. In addition high concentrations of perchloroethene (PCE), trichloroethene (TCE), dichloroethene DCE (1,1 and cis-1,2), 1,1,2,2-TeCA (tetrachloroethane) and chloromethanes were detected in both aquifers. Indoor air measurements were performed by means of sorption tubes by combining active and passive techniques, in two different sampling campaigns in the ground floor and the first floor of the buildings. Seasonal measurements of groundwater concentrations were also performed at four piezometers located in close proximity to the houses. Two different analytical models for risk assessment (Tier II) were applied with available site-specific data to predict indoor air concentrations and estimate risks for adult and children as receptors. In contrast to such severe concentrations in groundwater and to results of Tier II human health risk assessment based on analytical models, low and spotty evidence of contaminated vapors was found either in soil or in indoor gas and no direct relationship with subsoil conditions resulted. The occurrence of fine grained sediments (clays and silts) over the shallowest aquifer appears to block vapor intrusion and protect the residents from direct exposure by inhalation. Alternatively, biodegradation processes occurring at the interface between the saturated and the (oxygen rich) unsaturated zone may be capable of reducing the fluxes of contaminant that could potentially reach receptors. Detailed process and modeling studies are needed to elucidate these phenomena and improve site investigation tools and risk models.