Seasonal and longitudinal water quality dynamics in three effluent-dependent rivers in Arizona

Authors: Hamdhani Hamdhani, Drew Eppehimer, David Quanrud, Michael Bogan

Year: 2019

Abstract

In the past 100 years, many rivers in the southwest USA have dried up, but some reaches in these rivers have remained perennial due to the continuous input of effluent discharged by wastewater treatment facilities. The re-establishment of perennial reaches has brought back habitat for numerous aquatic and riparian taxa. However, because these reaches are artificially supported by effluent, flow regimes and water quality gradients may differ from former natural conditions and water quality below effluent outfalls may be a limiting factor in supporting native fish populations. The objective of this study was to quantify spatial and temporal changes in basic physical and chemical water quality parameters below the outfalls of three effluent-dependent rivers in Arizona: Santa Cruz (Tucson), Salt (Phoenix) and Rio de Flag (Flagstaff). Six reaches were studied; three reaches were short (~3-5 km), one was intermediate (~9 km), and two were long (>24 km). In 2018, we measured dissolved oxygen, temperature, pH, specific conductance, and nutrients at 26 river sites. In the long reaches, dissolved oxygen increased with distance from effluent outfalls, whereas temperature and ammonia concentration decreased; most other water quality variables did not exhibit clear longitudinal trends. No trends were apparent in the short reaches studied. In all reaches, most variables exhibited at least some seasonal variation; for example, in summer, dissolved oxygen and pH tended to decrease, whereas temperature increased. In all seasons, water quality conditions in most reaches were within ranges tolerated by native desert fishes. These findings help us understand the potential for managing effluent to augment perennial river habitats in arid regions.