Urban stormwater quality, event-mean concentrations, and estimates of stormwater pollutant loads, Dallas-Fort Worth area, Texas, 1992-93

The quality of urban stormwater is characterized with respect to 188 properties and constituents. Event-mean concentrations and loads for three land uses (residential, industrial, commercial), and annual loads for 12 selected properties and constituents for 26 gaged basins in the DallasFort Worth study area are presented. During February 1992–June 1993, 182 water samples from the 26 gaged basins (each basin classified as primarily residential, industrial, or commercial) were collected and analyzed. Residential land-use basins had greater median concentrations of bacteria, nutrients, and total arsenic. Industrial land-use basins had greater median concentrations of suspended and dissolved solids, and total recoverable chromium, copper, nickel, and zinc. Diazinon was the most frequently detected pesticide in all three land-use basins. Diazinon was detected in 93 percent of samples from residential land-use basins, 70 percent from commercial land-use basins, and 33 percent from industrial land-use basins. Volatile organic compounds and base/neutral and acid extractable semivolatile organic compounds were detected more frequently in samples from industrial land-use basins than residential or commercial land-use basins.

Event-mean concentrations (EMCs) were computed for each land use for biochemical oxygen demand; chemical oxygen demand; suspended and dissolved solids; total nitrogen and ammonia plus organic nitrogen; total and dissolved phosphorus; total recoverable copper, lead, and zinc; and total diazinon. The EMCs of chemical oxygen demand; total nitrogen and ammonia plus organic nitrogen; total and dissolved phosphorus; and total diazinon were greatest in samples from residential land-use basins. The EMCs of biochemical oxygen demand; suspended and dissolved solids; and total copper, lead, and zinc were greatest in samples from industrial land-use basins.

Loads per square mile for the three land uses were estimated for the same properties and constituents from flow-weighted EMCs and runoff volume on the basis of seven sampled storms at each gaged site. Chemical oxygen demand and dissolved and suspended solids had the greatest mean loads per square mile. Mean loads per square mile were greatest for trace elements in industrial land-use basins and for total diazinon in residential land-use basins. Mean loads per square mile for total nitrogen in the three land-use basins were dissimilar.

Local regression equations were developed to estimate loads produced by individual storms. Mean annual loads were estimated by applying the storm-load equations for all runoff-producing storms in an average climatic year and summing individual storm loads to determine the annual load.