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.

Interest in halocarbons has been sparked by experimental evidence suggesting that these compounds are potential carcinogens and teratogens. The U.S. EPA started a nationwide program in 1970 to identify and quantify trace organic compounds in public water supplies and sewage effluents. To date no detailed large-scale areal ground-water surveys delineating concentrations of these halocarbons have been reported in the literature at the part per trillion level and below. The objectives of this dissertation are twofold: (1) identification, quantification, and detailed areal mapping of several halocarbon species in ground water near the Santa Cruz River northwest of Tucson, Arizona; and (2) development of halocarbon techniques for age dating recently recharged (0-40 years old) ground water. The halocarbon compounds considered are: trichlorofluoromethane (CC1₃F), dichlorodifluoromethane (CC1₂F₂), carbon tetrachloride (CC1₄), chloroform (CHC1₃), trichloroethylene (C₂HC1₃), methyl chloroform (CH₃CC1₃), and tetrachloroethylene (C₂C1₄). Objective (1) includes discussions of the areal halocarbon distributions detected in the ground water, their sources, and a qualitative comparison of the distributions to ground-water quality and land use patterns adjacent to the Santa Cruz River. Objective (2) utilizes the exponential atmospheric concentration buildup of CCl₂F₂, CC1₃F, and CCl₄, and the ratios of CCl₂F₂ to CCl₃F and CCl₄ to CCl₃F. Water samples from wells and the Santa Cruz River were collected in glass syringes and brought to the laboratory for analysis. An electron-capture gas chromatograph coupled to a gas stripping/concentration unit was used to quantify the halocarbons. Four high concentration areas were delineated, each probably associated with a different source: the CWUA area which was irrigated with sewage effluent during the 1960's; Rillito Creek (near its confluence with the Santa Cruz) which is a major ground-water recharge source; the Ina Road treatment plant/landfill/oxidation ponds area; and the Roger Road treatment plant "sewer farm" which is irrigated with sewage effluent. The CCl₂F₂ to CCl₃F ratio distribution indicates that most ground water in the study area is at least partially mixed with recharge less than 30 years old. The ground water adjacent to the Rillito has an apparent age of less than 10 years, in agreement with the CCl₃F distribution. Ground water in the Cortaro area has an apparent age of about 25 years, correlating with the start of irrigation in the area.

Organic contamination is associated with inorganic contamination in a plume resulting from acidic mine drainage water in an area of copper mining, Globe, Arizona. The level of dissolved organic carbon (DOC) is low, but is eight times the 0.5 mgC/l background. The source is probably organic reagents used in mineral processing. DOC fractionation using XAD-8 resin sorption chromatography gave a hydrophobic acid fraction (humic substances) of 1.0 mgC/l, and the hydrophobic neutral fraction was also anomalous, being as high as 49% of DOC. The fractionation data matched that for a waste-solution lake, believed to be a source of the aquifer contamination. Loss of DOC is occurring downgradient in the aquifer, based on comparison of DOC/chloride ratios. Loss occurs as the pH increases to over 4 or 5. Sorption onto metal precipitates being formed, particularly of the hydrophilic fraction, may be occurring.

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