Submitted to: Southwest Hydrology
Publication Type: Popular Publication
Publication Acceptance Date: 6/1/2004
Publication Date: 12/1/2004
Citation: Duran, N.L., Spencer, L.K. 2004. Pathogen distribution in an effluent-dominated stream. Southwest Entomology. 31(6): 22,23,31.
Interpretive Summary: Rapid urbanization and growing populations in the semiarid Southwest have led to increased discharge of reclaimed wastewater into stream channels. These water additions benefit the environment by contributing to the development of riparian habitats and increasing groundwater recharge, but this practice may also introduce poor quality water, potentially containing pathogenic microbes, to the environment. This study examined the fate and transport of several pathogens commonly found in wastewater using samples collected along a 13-mile transect downstream from a treatment plant on the Santa Cruz River in Tucson, Arizona. It was found that concentrations of E. coli increased with distance from the wastewater plant discharge point, contrary to what might be expected from bacteria die-off. Although the wastewater was chlorinated prior to discharge, it is possible that this commonly used disinfection process did not kill but only injured the E. coli population, which then re-established itself downstream. Several species of pathogens were also detected in shallow monitoring wells near the discharge point, but the environmental fate and transport of these microbes and their potential effects on drinking water are largely unknown. Although human pathogens have traditionally not been thought to flourish in soil environments, this study underscores the pressing need for further research to assess the impacts of reclaimed effluent on surface and groundwater quality.
Technical Abstract: Due to limited annual rainfall, streams of the arid west differ from the wet, flowing streams of perennial ecosystems in that they are dry most of the year and many are effluent-dependent. Effluent discharge from wastewater treatment plants is a growing source of water that can provide a constant stream flow in ephemeral streams. Although these streams cannot be used for recreational purposes, they allow riparian ecosystems to develop and contribute to groundwater recharge. The latter is particularly important to help replenish declining groundwater levels as a result of over-pumping of groundwater for urban and agricultural use. However, since microbial and chemical contaminants can persist in the treated effluent, it is necessary to evaluate the effects of effluent-dependent streams on groundwater quality. For this reason, surface and groundwater samples were collected from the effluent-dependent Santa Cruz River located near the city of Tucson, Arizona. Samples were then subjected to chemical and microbial quality analyses. Preliminary results demonstrated that, at the point of discharge, coliform numbers varied considerably over a twenty-four hour period emphasizing the need for average daily load measurements. Contrary to what might be expected, microbial numbers (E. coli, total coliforms, and heterotrophic plate counts) were greater downstream from the discharge. Although the groundwater samples had much lower bacterial counts than the surface water, Cryptosporidium and Giardia oocysts were identified in one groundwater sample as well as in the effluent. These preliminary results indicate that the impact of effluent-dependent streams on the underlying groundwater needs further assessment and highlights the need for proper monitoring of microbial survival, fate, and transport.