Remote detection and preliminary hazard evaluation of volatile organic contaminants in groundwater

Abstract

Chapter I presents an investigation of a TCE-contaminated aquifer beneath Tucson International Airport, Arizona. Shallow soil gas (< 2m deep) was collected and analyzed for halocarbon contaminants to determine the relationship with vadose zone and groundwater contamination directly below. The TCE plume in shallow soil gas was mapped with 46 probes and spanned 3 orders of magnitude in concentration (< 0.001 to 2.0 μg/L). TCE concentrations in water from 5 monitoring wells around the Airport property ranged from 4 to 2800 ppb and had a correlation coefficient (r) of 0.90 with TCE concentrations in shallow soil gas. Vertical borings were completed to the water table at four locations in order to obtain profiles of soil gas contamination, air porosity, and water saturation. Vertical concentration gradients and effective diffusion coefficients were used to calculate flux rates for volatile organic contaminants (VOC's). Flux values ranged from 0.094 to 680 pg/day per m² for the four borings, and an annual mass transfer of TCE from groundwater to the atmosphere was estimated at 4 kg over the 0.5 km² site. Contaminant concentrations in shallow soil gas provided an accurate indication (r = 0.94) of vadose zone flux on a site-specific basis. The success in delineating the areal extent of subsurface contamination via soil gas sampling is not diminished by geologic or hydrologic anomalies if they are not laterally extensive compared to the plume. Chapter II introduces an environmental rates model which provides an estimate of the subsurface fate of volatile organic chemicals. Six hydrochemical indices are estimated (dissolved flow retardation, immiscible flow retardation, air-water partitioning, soil gas diffusion, hydrolysis, biodegradation) from compound-specific properties and site-specific characteristics. In addition, the carcinogenic potential and metabolic intermediates (two biochemical indices) of 75 volatile organic chemicals are estimated from the compounds' chemical structure. The model output is designed to assist water resource and environmental decision-makers in assessing the severity of existing problems and in avoiding potential problems.