Estimating Pollutant Mass Accumulation on Highways during Dry Periods Lee-Hyung Kim 1 ; Kyung-Duk Zoh 2 ; Sang-man Jeong 3 ; Masoud Kayhanian 4 ; and Michael K. Stenstrom, F.ASCE 5 Abstract: For determining the accumulated pollutant mass on highways, two years of monitoring data were used from eight highway sites in southern California. Buildup over antecedent dry days was calculated from mass washed off from the following storm and retained pollutant mass. Mass accumulation rates were determined for total suspended solids TSS, chemical oxygen demand COD, oil and grease, total Kjeldahl nitrogen, and total phosphorus, and are reported in g / m 2 -day. A revised buildup model is proposed using an alternative modeling approach to describe buildup during dry days between storms. The result shows that, between 1 and 10 antecedent dry days, the pollutant mass buildup rates are determined to be 0.544 g / m 2 -day for TSS, 0.114 g/m 2 -day for COD, and 0.0113 g/m 2 -day for oil and grease. Buildup rates decline in subsequent periods rates decreased by 79% for TSS, 78% for COD, and 61% less for oil and grease in the following 10– 70 day period. DOI: 10.1061/ASCE0733-93722006132:9985 CE Database subject headings: Stormwater management; Pollutants; Highways; Runoff; California. Introduction Many of the waters of the United States are classified as impaired because of pollutant inputs from point and nonpoint sources. The Nationwide Urban Runoff Program NURP expanded the state of knowledge of urban runoff pollution by instituting data collection at many different sites Driscoll et al. 1990; USEPA 1994, 1995, 1996. The study showed that significant quantities of organics, nutrients, pesticides, herbicides, and heavy metals are contained in runoff and caused the USEPA, using the authority of Section 208 of the Clean Water Act, to require that regional urban plan- ning agencies develop ways to reduce pollution from nonpoint sources. Many plans developed to minimize nonpoint-source pollution use total maximum daily loads TMDLs as a control mechanism. A TMDL is the sum of the allowable loads of a single pollutant from all contributing point and nonpoint sources. The calculation must include a margin of safety to ensure that the receiving water can still be used for its designated purpose e.g., drinking water supply, contact recreation, etc.. The success in developing and implementing a TMDL depends largely on a better understanding of nonpoint sources, because most point sources e.g., domestic wastewater treatment plants have already been addressed. Be- tween 40 and 80% of the total annual organic pollutant loading that enters receiving waters from a typical city originates from nonpoint sources USEPA 1995, 1996. Other pollutants also have high fractions originating from nonpoint sources. The origins of pollutants from nonpoint sources are varied and range from illegal discharges to washoff of natural substances to atmospheric deposition. Buildup of pollutants from various depo- sition sources is of interest, and a better understanding may assist in best management practices BMP selection or justification. This paper analyzes the buildup of pollutants from eight high- way sites in southern California over two years of monitoring. Estimates are made for six water quality parameters, and a revised buildup model is proposed. Background The sources of urban runoff pollution can be categorized as fol- lows: wet and dry atmospheric deposition; street refuse deposition including litter, street dirt, vegetation, and organic residues; traffic emissions; erosion; and road deicing chemicals. Dry deposition includes dust particles that arise from unpaved roads, parking lots, construction and demolition sites, urban refuse litter or garbage, surrounding soils, and industrial activities. A significant portion of pollutant loadings from urban areas can be attributed to rain or snowfall. This is especially true for nitrogen, and precipitation is one major source of nitrogen Crittenden 1998. Yuzhou et al. 2002 measured wet and dry atmospheric nitro- gen deposition on the East Coast of the United States. The mean values were 0.611 and 3.37 mg N/m 2 -day for wet and dry depo- sition, respectively. Lang et al. 2002 estimated 0.186 and 1 Assistant Professor, Dept. of Civil and Environmental Engineering, Kongju National Univ., Kongju, Chungnamdo, 314-701, Korea. 2 Assistant Professor, Dept. of Environment and Health, Seoul National Univ., Seoul 110-799, Korea. 3 Professor, Dept. of Civil and Environmental Engineering, Kongju National Univ., Kongju, Chungnamdo, 314-701, Korea. 4 Center for Environmental and Water Resources Engineering, Dept. of Civil and Environmental Engineering, Univ. of California, Davis, CA 95616. 5 Professor, Dept. of Civil and Environmental Engineering, 5714 Boelter Hall, Univ. of California, Los Angeles, CA 90095-1593. Note. Discussion open until February 1, 2007. Separate discussions must be submitted for individual papers. To extend the closing date by one month, a written request must be filed with the ASCE Managing Editor. The manuscript for this paper was submitted for review and pos- sible publication on April 4, 2005; approved on February 13, 2006. This paper is part of the Journal of Environmental Engineering, Vol. 132, No. 9, September 1, 2006. ©ASCE, ISSN 0733-9372/2006/9-985–993/ $25.00. JOURNAL OF ENVIRONMENTAL ENGINEERING © ASCE / SEPTEMBER 2006 / 985