QUALITY OF STORMWATER IN DENVER, CO
Stormwater in the Denver area was sampled by the U.S. Geological Survey, in cooperation with the Urban Drainage and Flood Control District, in a network of five stations, three on the South Platte River and two on tributary streams, beginning in October 1997 and continuing through September 2001. Composite samples of stormwater were analyzed at the Metro Wastewater Reclamation District Laboratory for physical properties such as specific conductance, calcium carbonate hardness, and residue after evaporation at 105 degrees Celsius; and for constituents such as organic carbon and nutrients, including ammonia, nitrite plus nitrate, ammonia plus organic nitrogen, phosphorus, and orthophosphate; and for metals, including total and dissolved phases of copper, lead, manganese, and zinc. Bacteriological samples for Escherichia coli and fecal coliform collected during some storms also were analyzed at the Metro Wastewater Reclamation Laboratory. Discrete samples collected during selected storms were analyzed at the U.S. Geological Survey National Water-Quality Laboratory for a suite of water-quality properties and constituents similar to composite samples that did not include determinations for total phases of metals.
Streamflow characteristics associated with 255 composite stormwater samples indicate that, for individual stations and storms, mean streamflow for portions of hydrographs representing rising, falling, or entire event streamflow conditions were generally within 35 percent of overall station means, and differences in mean streamflow for different portions of the storm hydrographs were not large. Results from chemical analyses of the composite samples indicate spatial patterns related to contributing drainage area in the South Platte River, but no well-defined relation with the amount of urban land cover identified using national land-cover data available from the U.S. Geological Survey National Land Cover data.
Results from 52 discrete samples collected during three storms indicate that correlation coefficients between time-weighted and volume-weighted concentrations were always at least 0.88, indicating a strong correlation between the two weighting methods for the stations involved in this study. In addition, the central tendency for relative percent differences between the various weighting methods typically has a value of about 1, indicating good agreement for the various weighting methods for data collected as part of this study.
Comparison of stormwater results from composite samples to historical results representing base flow indicates that concentrations for some water-quality properties and constituents such as specific conductance, hardness, ammonia, nitrite plus nitrate, ammonia plus organic nitrogen, phosphorus, and orthophosphate were elevated in base flow, whereas concentrations for some constituents such as dissolved copper, lead, and zinc were elevated in stormwater runoff. Comparison of stormwater results to numeric standards developed by the Colorado Department of Public Health and Environment on the basis of use classifications indicates that concentrations of some metals such as dissolved copper, manganese, and zinc did not meet standards in 10 to 25 percent of composite samples at some stations, and that bacteriological indicators such as Escherichia coli and fecal coliform did not meet standards in all bacteriological samples.
An evaluation of stormwater results by year on the basis of annual means indicates that, in general, there are few monotonic trends in concentrations, indicating generally uniform or stationary conditions, for most water-quality properties and constituents at individual stations. At some sampling stations, however, annual means for water-quality properties and constituents such as specific conductance, hardness, orthophosphate, and ammonia indicated a consistent increase in concentration, or upward monotonic trend, through the data-collection period. In addition, total organic carbon, ammonia plus organic nitrogen, and total phosphorus all indicated a consistent decrease, or downward monotonic trend, through the data-collection period, at some stations.
Two-sided 95-percent confidence tolerance intervals, generally with 99-percent coverage, were computed to be used as a basis for comparisons with future samples. Linear regressions and correlation coefficients also were computed to help evaluate intradistributional changes on the basis of future samples. Together, relatively weak correlation coefficients between water-quality properties and constituents with mean streamflow, and the lack of spatially consistent regressions, indicate a heterogeneous and mixed system.