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Maricopa County Urban Stormwater Quality

Project Number: 96719GJ
Project Chief: Ken Fossum
Cooperating Agencies: Cities of Phoenix and Glendale, AZ

Downtown Phoenix.

PROBLEM

Since 1991, the U.S. Geological Survey (USGS); in cooperation with the Flood Control District of Maricopa County (FCDMC), the city of Phoenix, and the city of Glendale; has been collecting, analyzing, and interpreting urban stormwater information from selected basins throughout the metropolitan Phoenix area (Lopes and others, 1995; Fossum and Davis, 1996). Water-resource managers and policy makers have used this information to determine pollutant loads discharged from urban basins to receiving waters and will be using this information in the future to develop and test management strategies designed to mitigate pollutant loads. The basins monitored during the first phase of the investigation are generally categorized by land use type. This approach has aided in determining the effect of land use type on the variability of pollutant loads discharged from basins during storms.

After extensive data had been collected on the chemistry of urban runoff in numerous basins throughout Maricopa County, the data were statistically aggregated in order to estimate conditions in similarly characterized unmonitored basins. This has been done through development of a series of pollutant-load regression equations similar to ones derived for the National Urban Runoff Program (NURP, Driver and Tasker, 1990). When the NURP equations were applied to urban basins in Maricopa County, the standard error of estimate for particular constituents was substantial, which indicated that adjustments were needed using stormwater data for these areas. As part of a previous study being completed under Intergovernmental Agreement number 98036 with the FCDMC, adjustment of the equations resulted in a significant decrease in the standard error associated with the equations, which indicated that the development of a larger set of local data through additional collection and analysis of stormwater runoff may reduce the standard error further.

OBJECTIVES

Objectives of this study are to (1) collect rainfall, runoff, and water-quality data to characterize stormwater quality from representative urban land uses at seven urban monitoring stations and (2) update pollutant-load regression equations to be used to transfer results of this investigation to similar unmonitored urban basins.

APPROACH

The USGS will collect stormwater samples during representative storm events at seven urban basins in the metropolitan Phoenix area and five urban basins in the metropolitan Glendale area. These samples will be analyzed to determine the basin characteristics and factors that contribute to the variability of urban runoff quality for selected land use types.

Four flow-weighted composite samples will be collected and analyzed annually for each of the urban sites. Two samples will be collected during the winter months and two samples will be collected during the summer months to characterize seasonal changes in pollutant loads. Automatic samplers already installed at most of the sites will collect the composite samples. Grab samples for bacteria, volatile organic compounds, total petroleum hydrocarbons, oil and grease, and synthetic organic chemicals, will be collected at the site by sampling personnel as soon as possible after the beginning of a storm event.

RELEVANCE AND BENEFITS

The collection, compilation, and analysis of urban stormwater information will result in increased knowledge of the chemistry of urban stormwater runoff in a major metropolitan area that is in an arid region. Because of the many municipal entities involved in characterizing the chemistry of stormwater runoff in Maricopa County, the information gathered and relations developed by the USGS will provide a substantial database and resource that the communities can use for planning and operation purposes. The information collected during this study is needed to develop better sampling criteria for future stormwater permitting in arid regions. The USGS is collecting similar stormwater chemistry information for major metropolitan areas throughout the United States; this study will contribute information to national databases that can be used to advance the understanding of regional variations in hydrologic conditions.

In most urban settings, the receiving waters for stormwater runoff are tributaries that eventually flow into major river systems. This region is unique in that most runoff enters into dry riverbeds and eventually is transpired by plants or percolates into the ground-water system. Very little is known about the chemicals entering the ground-water system and the fate of these chemicals as they move from surface-water systems into the unsaturated zone or the ground-water system. Data from this project combined with data from earlier work will provide a needed database for future studies of these processes.