Heavy metal and volatile organic chemical removal and treatment in on-site wastewater systems

Abstract

In Wisconsin, motor vehicle waste fluids (MVWF) enter catch basins along with rinse waters and are discharged to drainfields (soil absorption systems) after mixing with domestic wastewater in a septic tank (systems installed prior to 1992). The purpose of this study was to determine if removal/treatment of heavy metals and volatile organic chemicals (VOCs) found in spent oils, greases, and solvents occurs in drainfields that receive MVWFs. Soil samples were collected beneath and soil gas samples were collected above three gravel beds in drainfields installed in loamy sand or silt loam soils. Cadmium, chromium, and lead concentrations in soil 15 cm and greater beneath beds in loamy sand and silt loam soils were similar to background concentrations. Heavy metals in drainfields would most likely be found in the clogging layer at the infiltrative surface of gravel and soil. The VOCs 1,3,5-trimethylbenzene and m- and p-xylenes were found beneath beds in loamy sand soils; concentrations of detected VOCs ranged from 20–270 mg kg−1. Volatile organic chemicals were not detected beneath the bed in silt loam soils. Drainfields in loamy sand soil appear to provide less treatment of VOCs compared to drainfields in silt loam soils. Volatile organic chemicals were found in soil gas above drainfields in both soil types. Thus, some VOCs diffuse from the drainfield to the soil surface.