Effects of the Green Valley Wastewater Treatment Facility Upon Groundwater Quality

Abstract

The Green Valley Wastewater Treatment Facility (GVWWTF) is about 20 miles south of Tucson, Arizona. Locally, the aquifer consists of interbedded sands, silts and gravels. Depth to water near the facility is about 160 feet with transmissivity ranging from 32,000 to 48,000 gpd /ft. Ground water quality has historically been poor near the facility due to deep percolation of irrigation return flow. With the retirement of farm land, however, ground water quality has improved considerably. The only problem with well water downgradient of GVWWTF has been with total coliform where several samples exceeded public drinking water standards. Current inflow of about 1.1 mgd will increase to 4.5 mgd by 2005. Flow net and mass balance analyses indicate effluent recharge by percolation beds will increase from 1060 AF /year to 4130 AF /year during this period. A mass balance model predicts TDS will increase from about 585 to 615 mg/l, nitrate-N will increase from 9.2 to 9.9 mg/l, chloride will increase from 50 to 75 mg/1, and sulfate will decrease by 25 mg/l to 115 mg/l. Further study of pond disinfection for prevention of microbiological contamination is suggested. Additionally, effluent reuse and an enhanced monitoring program including upgradient and downgradient wells are encouraged.