Turnitin: Prosiding DFC 2018 - Effect of diurnal flow variability on water quality dynamics in the effluent-dominated Santa Cruz River

Abstract

The Santa Cruz River in Arizona is one of several river systems that have experienced groundwater drops and loss of perennial flow due to groundwater pumping and drought. Along with the Gila and Salt Rivers, the Santa Cruz River currently has artificially perennial reaches that are supported by effluent discharge from wastewater treatment plants. The return of perennial reaches has brought back habitat for numerous of aquatic and riparian taxa. Reliable flow rates are one of the crucial components that support healthy aquatic habitat. However, flow fluctuates dramatically in the Santa Cruz each day, depending on quantity of water used by Tucson residents, creating high and low flow “tides” each day. The effect of this daily flow variation on water quality conditions in this effluent-dominated system is unknown. The objective of this study is to quantify physical and chemical water quality factors in the Santa Cruz River (1) during high and low flow periods and (2) along gradients downstream from the effluent outfall. Since September 2017 we have been collecting water quality data (e.g. DO, pH, conductivity, nutrient levels) monthly from six reaches 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. Gradient water quality measurements below wastewater treatment plant discharge indicated that water quality parameters tended to get better further downstream. The findings of this study suggest we should sample effluent-dominated systems during low flow periods to detect worst case water quality conditions. We hope that these data will also help us understand how effluent can be used to restore perennial river habitats in arid climates, including habitat for native desert fishes.