``Is Your Representative Outfall Really Representative?'' Factors That Can Affect Representative Discharge Sampling Data

In 1987 Congress amended the Clean Water Act requiring EPA to establish NPDES requirements for storm water discharges. As part of the NPDES permit requirements for Phase I municipal separate storm sewer systems (MS4), the operator of the system is required to perform field screening analysis of the major outfalls to detect illicit connections. The operator is allowed to designate "representative outfalls" to characterize discharges from drainage areas of similar land-uses. Volume-weighted water quality samples are collected at the representative outfalls during storm events and used to calculate event mean concentrations (EMCs). Data from this characterization may be used in a number of applications including: identification of illicit discharges, determination of load allocations on impaired stream segments in a TMDL process, selection of best management practices to reduce or eliminate pollutant discharge, and prioritization of infrastructure improvements. Each of these applications could result in significant resources to execute. Therefore, it is important to collect data from the representative discharges, which are accurate, and correctly characterize the drainage areas they represent. This paper presents a case study evaluation for one of Harris County Flood Control District representative outfall located in Houston, Texas. Houston has over 15 bayous that drain the relatively flat, coastal watershed. Houston receives 50-inches of rainfall on average, evenly distributed over the year. In addition, the region frequently experiences short-duration, high-intensity rainfall events that can cause brief flooding. There are four representative outfall land uses are used to characterize Harris County, Texas: single family residential, multi-family residential, commercial, and industrial. These designations do not fully represent the land uses in the County. A number of land use activities are not accounted for in these designations (i.e. transportation, recreational, institutional, and undeveloped areas.) This study is focused on identifying factors that may influence results collected during storm water sampling at representative outfalls. During this study, a procedure was developed to investigate the current status of a representative monitoring site. First, the drainage area of the representative outfall is delineated. This drainage area is used to identify land use and potential sources of pollution. Finally, identified sources were field verified, and GPS locations collected for each potential source.