Costs of Water Quality Goals under Climate Change in Urbanizing Watersheds: Difficult Run, Virginia

Reducing nutrient loadings in urban areas is important for water quality improvement programs in many watersheds. Urban nutrient loadings are expected to increase and become more variable under climate change (CC). In this study, a water quality simulation model (SWMM), land cover data layers, and mathematical programming models were used to compare costs of abating nutrient loads under CC in the Difficult Run Watershed located in Fairfax County, Virginia. Predicted costs of abating mean total nitrogen (TN), total phosphorus (TP), and total suspended sediment (TSS) loadings under current climate conditions were compared with those for CC under certainty with a Cost Minimization Model and under uncertainty with a Safety First Model. Total nitrogen loadings abatement had the highest cost followed by TP and TSS abatement in that order. Costs of controlling TP and TSS increased with CC, whereas there was little change in TN control costs. Introducing uncertainty of loadings caused control costs to increase substantially for all three pollutants. The preferred pollutant control strategy was urban stream restoration. Policy makers seeking to meet water quality goals over a multiyear horizon should consider frontloading supplemental best management practices (BMPs) to offset the changes in nutrient loadings predicted for CC. (C) 2017 American Society of Civil Engineers.