Seasonal Flushing of Pollutant Concentrations and Loads in Urban Stormwater1

Abstract

Schiff, Kenneth C. and Liesl L. Tiefenthaler, 2011. Seasonal Flushing of Pollutant Concentrations and Loads in Urban Stormwater. Journal of the American Water Resources Association (JAWRA) 47(1):136‐142. DOI: 10.1111/j.1752‐1688.2010.00497.x Abstract: Despite broad observations of first flush within storms, the scientific understanding of seasonal flushing remains incomplete. Seasonal flushing occurs when initial storms of the season have greater concentrations or loads than storms later in the season. The goal of this study was to census stormwater concentrations and loads from an arid, urban watershed to quantify seasonal flushing. Samples were collected every 15 min during the 1997‐1998 wet season from the Santa Ana River and analyzed for total suspended solids. Initial storms of the season generated event mean concentrations 3‐10 times the event mean concentration of storms later in the season. Cumulative flow‐weighted mean concentrations were calculated as the season progressed. Early season storms discharged only 6% of the annual volume, but influenced flow‐weighted mean concentrations well past the midpoint of the wet season. Mass‐based estimates also indicated a disproportionate load in the early portion of the year; over 52% of the annual load was discharged in the first 30% of the annual volume from the highly urbanized lower watershed. Other stormwater pollutants, including six trace metals (Cd, Cr, Cu, Pb, Ni, Zn), were highly correlated with total suspended solids and also exhibited a significant seasonal flush.

Quantifying the effects of stream channels on storm water quality in a semi-arid urban environment

“…Chloride was less conservative earlier in the season (July 15th), likely due to a seasonal first flush that was captured in the synoptic sample collections, which occurred throughout the duration of the storm hydrograph. Others have observed similar first flush patterns (Lee et al, 2004;Schiff and Tiefenthaler, 2011).…”

“…However, processes unrelated to the characteristics of the stream channel, such as solute evapoconcentration (Henderson and Shuman, 2010;Meredith et al, 2009;Moravec et al, 2010), solute mobilization from the uplands (Lee et al, 2004;Schiff and Tiefenthaler, 2011) and water column biogeochemical processes like microbial respiration (Mallin et al, 2006) can alter stream water quality, but these factors are often overlooked in urban stream studies. Consequently, successful management of urban runoff requires understanding both water column processes and stream channel controls on urban runoff hydrochemistry.…”

Composition of nitrogen in urban residential stormwater runoff: Concentrations, loads, and source characterization of nitrate and organic nitrogen

“…Investigation of hydrologic trends and variability of N forms is useful to determine hydrologic variables that play important roles in N transport and to elucidate potential influences of rainfall patterns on N formsspecific transport mechanisms. Rainfall variables such as rainfall amount, duration, intensity, and antecedent dry periods have been used to determine the relationship between rainfall variables to nutrient and pollutant transport [17][18][19]. For example, a study by Schiff et al [20] on stormwater runoff from parking lots in California, US showed that 18 measured compounds were inversely correlated to rainfall duration where longer rain events decreased the concentrations of the constituents in parking lot runoff.…”

Design aspects and plant species affect pollutant removal in Southern California stormwater biofilters

“…Although columns dosed fortnightly removed significantly less TN, the difference was marginal (42% vs. 48% on average before the dry period). Although this result indicates that pollutant removal during less frequent rain events may diminish only slightly, longer antecedent dry periods often mean increased loads due to pollutant build-up on impervious surfaces, which can be substantial in Mediterranean climates like southern California (Schiff & Tiefenthaler 2010).…”