An analysis of U.S. wastewater treatment plant effluent dilution ratio: Implications for water quality and aquaculture.

Wastewater discharge and surface flow data from 2007 to 2017 was used to calculate wastewater dilution factors (WWDF) for U.S. Geological Society hydrologic unit codes (HUC) in the contiguous U.S. HUC 10-year average WWDF values generally increased from the east coast (HUC 1–3: WWDF range 125–466) as you move west to the Mississippi River (HUC 7, 8, 10: 1435–1813) before further declining moving west (HUC 13–18: 7–908), particularly in the California (HUC 18: 9) and southwestern states (HUC 13–16: 7–351). Within HUCs, watersheds with higher population centers had lower WWDF values. This population effect on WWDF was greater in drier regions (e.g. Southwestern U.S.) or during drought. This is particularly pronounced in the regions of the Southwest and West where populations are growing in an already water limited region. Moderate WWDF improvement was observed and projected through 2022 in these regions. A few areas of the country where surface water is used for aquaculture overlap with areas of low (<2) WWDF, but it is not widespread for the period examined. With continued population growth and the intensification of climate change, the proportion of treated wastewater effluent in surface waters may grow and potentially influence users of that water, but over the 10-year period examined WWDF values were relatively stable or improving for most regions of the contiguous U.S.