Tracing Sewage Effluent Recharge - Tucson, Arizona

Abstract

Dry washes or river beds are often used by southwestern communities to dispose of treated sewage effluent. Because many of these communities rely on ground water as a water supply, there is concern that this disposal practice may contaminate local aquifers. This has led to implementation of monitoring and tracing programs to quantify effluent and ground-water interactions and to development of efficient, easily used predictive models. The treated sewage effluent from the City of Tucson treatment plant has historically been used for irrigation and/or discharged to the normally dry Santa Cruz River. Numerous sampling programs have been undertaken to quantify the chemical quality, temperature, and microbiological activity of the ground water in the area near the Santa Cruz. Ground-water regions with high chloride and nitrate concentrations tend to be associated with areas irrigated with sewage effluent. Quality degradation due to channel recharge is not as evident because the effluent recharge is restricted by fine materials plugging the channel deposits. Recharging water tends to mound near the contact between the Recent and Fort Lowell formations spreading laterally more rapidly than downward. A new tracer, trichlorofluoromethane (trade name Freon 11, C13CF) with applications similar to environmental tritium is being evaluated. C13CF enters the hydro-logic cycle when it is partitioned between the gas and liquid phases during raindrop formation. C13CF in water samples is separated and quantitatively measured by a gas chromatograph with pulsed electron-capture detector. Preliminary Cl3CF analyses of ground water along the Santa Cruz do not correlate with nitrate values because mixing and increasing atmospheric Cl3CF concentrations were not accounted for. However, the presence of CI3CF in the ground water indicates recent recharge. Predictive modeling will be implemented using CI3CF and a finite-state mixing model.