Effect of Diurnal Flow Variability on Water Quality Dynamics in the Effluent-Dependent Santa Cruz River

Authors: Drew Eppehimer, David Quanrud, Michael Bogan

Year: 2018

Abstract

The Santa Cruz River in Arizona is one of several river systems in Southwestern USA that have experienced loss of perennial flow due to groundwater pumping and drought. Along with the Gila and Salt Rivers, the Santa Cruz currently has perennial reaches that are supported by effluent discharge from wastewater reclamation facilities. The return of perennial reaches has brought back habitat for numerous aquatic and riparian taxa. Reliable flow rates are one of the crucial components that support healthy aquatic habitat. However, in the lower Santa Cruz, flow during a given 24-hour period fluctuates by more than a factor of two, dependent on timing of water use by Tucson-area residents, creating high and low flow “tides” each day. The effect of this daily flow variation on water quality conditions in this effluent-driven system is unknown. The objective of this study is to quantify physical and chemical water quality factors in the Santa Cruz River (1) temporally–during high and low flow periods and (2) spatially–along the river downstream from the effluent outfalls. Since September 2017 we have collected water quality data (DO, pH, conductivity, nutrient levels) monthly from six locations of the Santa Cruz River spaced ~2 miles apart during both high and low flow periods. Ammonia, which is potentially toxic to aquatic biota, was significantly higher during low flow than high flow conditions on average across the year. River water quality tended to improve with increasing distance from effluent outfalls. The findings of this study suggest we should sample effluent-driven systems during low flow periods in order to identify worst case water quality conditions. We hope that these results will also help us understand how effluent can be better used to restore perennial river habitats in arid climates, including habitat for native desert fishes.