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.

The State of Arizona adopted strict groundwater conservation policies under the Arizona Groundwater Act of 1980. The Act mandates direct controls on groundwater use and provides incentives to seek alternative water supplies to offset the groundwater overdraft and restrict the expansion of municipal well fields. The City of Tucson, to reduce its groundwater dependency, is contracting for Central Arizona Project (CAP) water. CAP water comes from the Colorado River and carries with it nearly a ton of salts per acre-foot. Conservation programs being investigated by the city include reclaimed wastewater reuse for municipal use and artificial recharge. Salinity, a conservative contaminant, will rise as the water carrying it evaporates away during use. Some saline incidental recharge from wastewater irrigated acreage in the municipal well field is picked-up by groundwater pumpage in what is described as the municipal water-salinity cycle. The rate of salinity pick-up is compounded in the cycle. Those responsible for achieving groundwater conservation under the mandates of the Act do not include the costs generated by salinity damages and suffered by municipal water-consumers into their conservation plans. Salinity damages costs are generated by the direct use of CAP water and the use of degraded groundwater supplies. The study results show that under assumptions of limited groundwater dilution volumes the annual rate of salinity pick-up can range from about 1.4 percent to nearly 2.0 percent. An annual average pick-up rate of 2.0 percent could degrade Tucson's groundwater supplies from the present average salinity of 300 mg/1 to 1000 mg/1 in 61 years. Thirteen scenarios were evaluated and the present value of incremental costs of both salinity damage and expenditures associated with conservation were calculated. While estimates of salinity damage costs are many times lower than the conservation program expenditures, increased salinity in groundwater will lower the future capital value of the resource in the future if ignored.

The geochemistry, of groundwater collected from deep wells in the western section of the lower Santa Cruz basin of southeastern Arizona, was studied in order to determine the extent to which geochemica] analysis can be used to assess fluid flow and major and trace element migration patterns along hydrologic flowpaths in desert basins. Interaction between groundwater and enclosing sediments, and mixing between chemically distinct basin groundwater is found to exert a significant control on the chemical patterns that have evolved in the system. Activity-activity diagrams of the Na-Si-O-H system show that groundwater throughout the basin clusters near the three phase boundary between fluid, kaolinite, and montmorillonite and trends along the boundary to higher log (aNa⁺/aH⁺) values. A reaction model generated with computer program PHREEQE, that combines silicate weathering, kaolinite-Na-beidellite equilibrium, calcite equilibrium, and solution mixing, can simulate trends in groundwater composition along flowpaths in the basin. Trace metals introduced into the basin by the weathering of a buried porphyry copper deposit become spatially separated upon migration. Metal concentrations are found to be correlated to major cation concentrations. Cu is associated with high Na concentrations and a high ratio of Carbonate:Ca, whereas Zn is associated with high Ca concentration and a low ratio of Carbonate:Ca. Behavior of Cu and Zn during low-temperature transport can be controlled by the effects of mineral alteration on groundwater composition. Computer analysis of early basin diagenesis shows that changes in major solute composition that accompany weathering, constrained by equilibrium with clays and calcite, can produce the metal segregation pattern observed in the basin. Because the aquifer is strongly influenced by silicate and carbonate mineral equilibrium, the introduction of Central Arizona Project recharge, which is not in equilibrium with alluvial minerals, into the basin, requires a response by mineral reactions that attempt to restore the system to a state of equilibrium with kaolinite, montmorillonite, and calcite. A reaction model is developed to predict the consequent effects of outside recharge on groundwater quality.

A geochemical approach to the evaluation of the chemistry of natural recharge processes in the Tucson basin was used to identify the major minerals controlling the evolution of ground water chemistry and to assess the viability of recharging imported Central Arizona Project water supplies. Well cuttings analyses and water quality samples from over 65 wells in the basin were used as input to the geochemical computer model PATH4 (Helgeson, 1970) and the sequence of aqueous species and mineral production in a recharge reference volume examined. The study reveals that natural processes in the basin lead to the increase in dissolved solids content in ground water over time and the production of secondary minerals such as calcite, calcium montmorillonite, kaolinite and poorly crystallized alumino-silicate phases. Secondary minerals grow into aquifer pore spaces and may, over time, be responsible for the reduction in aquifer porosity and the specific capacity of wells. The recharge of imported Central Arizona Project water will lead to an increase in the dissolved solids content of ground water and may, in certain areas of the basin, lead to the enhanced production of secondary minerals. The use of CAP water as a recharge source must be guided by the geochemical factors which influence the nature and scope of reactions between CAP water and the Tucson aquifer matrix. The study demonstrates the need for and identifies water quality and aquifer matrix criteria for the assessment of sources of recharge water and recharge facility sites. The use of geochemistry as a tool for quantitatively assessing ground water quality is demonstrated.

My deepest respect and appreciation are extended to Dr. L. Gray Wilson for providing the opportunity to conduct this research, for his advice and assistance during the course of the research, and for his review of and suggestions for improving this manuscript. I would also like to thank Michael Osborn for his assistance. This research was funded by the City of Tucson. The assistance and direction provided by Mt. Bruce Johnson and Mt. Joe Babcock at Tucson Water are gratefully acknowledged. My thanks are also extended to Dr. Daniel D. Evans, Dr. L.G. Wilson, and Dr. Stanley N. Davis for serving on my thesis committee. Bruce Smith's assistance is gratefully acknowledged. Bruce spent two long days with 110-degree temperatures In a parking lot describing the lithology of sediment samples collected during the drilling phase of this research. He also determined the moisture content and particle size distribution on samples collected from the borehole. The assistance of Ralph Marra and Steve Brooks is also gratefully acknowledged. Ralph waded through city and county bureaucracies to determine zoning at each dry-well site. Steve assisted with collecting dry-well settling chamber sediment samples.