|Abstract:||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 monitoring stations - three on the South Platte River and two on tributary streams, beginning in October 2001 and continuing through October 11, 2005. Composite samples of stormwater were analyzed at the U.S. Geological Survey National Water Quality Laboratory during water years 2003-2005 and the Metro Wastewater Reclamation District Laboratory during water year 2002 for water-quality properties such as pH, specific conductance, hardness, and residue on evaporation at 105 degrees Celsius; and for constituents such as major ions (calcium, chloride, fluoride, magnesium, potassium, sodium, and sulfate) in 2005, 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. Samples analyzed for bacteriological indicators such as Escherichia coli and fecal coliform collected during selected 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 those analyzed in the composite samples but that did not include determinations for total phases of metals.
Streamflow characteristics associated with 176 composite stormwater samples indicate that most samples were collected from hydrographs classified as falling or event hydrographs and that only a few samples were collected from rising hydrographs. Results from laboratory analyses of the composite samples indicate spatial patterns in which concentrations for some constituents increase with contributing drainage area in the South Platte River and Sand Creek, but no well-defined relation with the amount of urban land cover was identified using data available from the U.S. Geological Survey National Land Cover data.
Results from 22 discrete samples collected during two storms and used to obtain composited results with various weighting methods indicate that correlation coefficients between time-weighted and volume-weighted concentrations were generally at least 0.65, indicating a strong direct correlation between the two weighting methods for the stations involved in this study. In addition, the central tendency for relative percent differences between the time- and volume-weighting methods typically has an absolute value of about 10 or less, indicating good agreement for these weighting methods for data collected as part of this study.
Comparison of stormwater results to numeric standards for streams developed by the Colorado Department of Public Health and Environment on the basis of use classifications indicates that, for water-quality properties and constituents other than bacteriological indicators, there were very few exceptions to numeric standards. Bacteriological indicators, however, such as Escherichia coli and fecal coliform consistently exceeded numeric standards in all bacteriological samples.
An evaluation of laboratory results from composite samples on the basis of annual means indicates the presence of some simple upward and downward temporal trends in concentrations. In general, for annual means of results for all stations, hardness, ammonia plus organic nitrogen, total phosphorus, most dissolved metals (lead, manganese, and zinc), and all total metals (copper, lead, manganese, and zinc) all indicate annual means that decrease each year, or downward trends. Some trends were indicated only at individual stations in the network rather than at all stations. Ammonia as nitrogen at Union, Denver, and Henderson, orthophosphate at Sand Creek, and nitrite plus nitrate at Denver and Henderson all indicate decreasing annual means, or downward tr