Water quality indexes (WQI) are a useful way to summarize disparate water quality parameters into a single score to describe the ability of a waterbody to meet beneficial uses (e.g., recreation, aquatic life). They are frequently found in economic valuation studies and benefit-cost analyses for surface water regulations. However, the existing WQI used by the Environmental Protection Agency (EPA) features only six parameters that are not consistently monitored across states and do not provide context about potential uses of the waterbody. We develop a new WQI framework comprised of use scores for recreation, ecological, and fish consumption use categories. Individual use scores reflect the probability use-specific parameters meet nationally recommended criteria. Because the use scores represent a use and not a specific parameter, this new WQI can be tailored to different state monitoring programs and can provide context on potential waterbody impairments. The use scores can further be aggregated into a single WQI like the EPA WQI for benefit-cost analysis.
Leveraging the Assessment, Total Maximum Daily Load (TMDL) Tracking and Implementation System (ATTAINS) dataset, we identify water quality parameters frequently assessed by states for each use category on rivers, lakes, and estuaries. We then construct use-based use scores for three states using monitoring data from the Water Quality Portal (WQP). Our results demonstrate there is a wide set of parameters assessed for ecological use designations that vary by state and waterbody type, while parameters for fish consumption and recreation are more limited. Overall, we find our use-based use scores are more closely related to waterbody impairments than the EPA WQI, particularly in lakes and ponds. These findings suggest integrating state expertise in local ecology can improve methods utilized by economists and policy makers.