Partitioning of Mercury into Several Size Fractions in Highway Runoff Marcia Ferreira, Ph.D., P.E. 1 ; Mi-Hyun Park, Ph.D. 2 ; Jenny A. Jay, Ph.D. 3 ; Peter G. Green, Ph.D. 4 ; and Michael K. Stenstrom, Ph.D., P.E., F.ASCE 5 Abstract: Due to their large impervious coverage, highways are ideal sites for the transport of pollutants to either treatment units or direct discharge to waterways. Mercury, a highly toxic and ubiquitous environmental contaminant, is rarely studied in stormwater runoff. Removal of particulate-bound mercury in a dry detention basin was analyzed for the five size fractions: <0.45 μm, 0.45–8 μm, 8–20 μm, 20–100 μm, and >100 μm. The highest concentration of mercury for both influent and effluent is observed in the 8–20 μm size fraction. Treatment units are often characterized by suspended solids removal efficiency and particle-bound pollutants correlated to suspended solids removal; how- ever, different suspended solids removal efficiency may occur in specific particle size fractions. Therefore, the benefits of treatment units may be different than originally anticipated, especially if the particle-bound concentrations are greater in fractions more efficiently removed. For sedimentation-based treatment units, the removal is dependent on size fraction, and when one size fraction is captured more efficiently, the removal rate calculated using the entire sample might indicate particle-bound pollutant removal quite different than actually occurring. DOI: 10.1061/(ASCE)EE.1943-7870.0001838. © 2020 American Society of Civil Engineers. Author keywords: Stormwater control measures; Highway runoff; Mercury fractionation; Particle size distribution; Size fractionation; Stormwater runoff. Introduction Mercury is a rarely studied contaminant in stormwater runoff. In fact, out of approximately 500 studies reported to the International Stormwater Best Management Practice [BMP Database (Jones and Strecker 2020)], only two studies reported mercury concentration in dry detention basins. Furthermore, neither of them makes a distinc- tion between concentrations in dissolved and particulate phases, which is necessary when dealing with sedimentation-based treat- ment units, such as dry detention basins. The absence of mercury studies in stormwater is surprising, since mercury in its inorganic form is present in every ecosystem due to long-range atmospheric transport (Ullrich et al. 2001). The sources of atmospheric mercury range from volcanic eruptions to combustion processes, such as burning of coal and wildfires (Mason et al. 1995; Schroeder and Munthe 1998; Biswas et al. 2008; Burke et al. 2010; Pirrone et al. 2010). Even though volcanic eruptions are responsible for about 20%– 40% of all the mercury (Hg) released into the atmosphere by natural emissions (Pyle and Mather 2003), the contribution from natural sources to the global atmospheric Hg pool is decreasing. Until the 1970s, chlor-alkali plants were the largest anthropogenic source of atmospheric Hg, but their impact has been reduced due to stricter regulations (Schroeder and Munthe 1998). Currently, burning of fossil fuels, artisanal gold mining, nonferrous metals manufactur- ing, cement production, waste disposal, and caustic soda produc- tion are the major sources of atmospheric Hg (Pirrone et al. 2010). Locations with a Mediterranean climate, such as southern California, are prone to wildfires due to their long dry summers and the occurrences of high-speed warm dry winds during the Fall (Mensing et al. 1999). In the year 2006, the state of California saw 4,805 fires burn 222,896 acres (State of California 2020), which is below the 5-year average (2002–2006) for the number of fires (average number of fires is 5,674 fires) but above the average for the burned area at 688 km 2 (170,126 acres). The past seasons (2017 and 2018) saw the largest wildfire in California history. These wildfires release contaminants into the atmosphere, which can either travel across the globe or settle nearby via ash depo- sition. Rothenberg et al. (2010) have shown that for two relatively close lakes (40 km apart), the accumulation of Hg in the lakes’ sediment was greater on the lake affected more frequently with wildfires. Thus, besides long-range atmospheric transport, an im- portant source of mercury to watersheds might be wildfires (Burke et al. 2010; Rothenberg et al. 2010). Mercury is often found in stormwater runoff from urban water- sheds (Lawson and Mason 2001; Eckley and Branfireun 2008; McKee and Gilbreath 2015), and land uses with no obvious local mercury sources such as highways may have elevated mercury con- centrations because it is a global pollutant. Typical concentration of mercury in urban runoff was found to be 200 ng=L, and the range 1 Associate Engineer, Water Resource Management, Metropolitan Water District of Southern California, 700 Alameda St., Los Angeles, CA 90012. Email: mferreira@mwdh2o.com 2 Dept. of Civil and Environmental Engineering, Univ. of California, Los Angeles, 420 Westwood Plaza, Los Angeles, CA 90095 (corresponding author). Email: parkmh@ucla.edu 3 Professor, Dept. of Civil and Environmental Engineering, Univ. of California, Los Angeles, 420 Westwood Plaza, Los Angeles, CA 90095. Email: jennyjay@ucla.edu; jjay@seas.ucla.edu 4 Professional Research Engineer, Dept. of Civil and Environmental Engineering, Univ. of California, Davis, One Shields Ave., Davis, CA 95616. Email: pggreen@ucdavis.edu 5 Distinguished Professor, Dept. of Civil and Environmental Engineer- ing, Univ. of California, Los Angeles, 420 Westwood Plaza, Los Angeles, CA 90095. ORCID: https://orcid.org/0000-0001-6157-0718. Email: stenstro@seas.ucla.edu Note. This manuscript was submitted on February 21, 2020; approved on September 10, 2020; published online on November 20, 2020. Discus- sion period open until April 20, 2021; separate discussions must be sub- mitted for individual papers. This paper is part of the Journal of Environmental Engineering, © ASCE, ISSN 0733-9372. © ASCE 04020146-1 J. Environ. Eng. J. Environ. Eng., 2021, 147(2): 04020146 Downloaded from ascelibrary.org by UCLA Digital Coll Svcs on 11/20/20. Copyright ASCE. For personal use only; all rights reserved.