Pollutant Mass Flushing Characterization of Highway Stormwater Runoff from an Ultra-Urban Area

Water quality of highway stormwater runoff from an ultra-urban area was characterized by determining the event mean concentration (EMC) for several pollutants and by evaluating pollutant flushing. Thirty-two storm events were monitored between June 2002 and October 2003. Mean EMCs in mg/L were 0.035, 0.11, 0.22, 1.18, 420, 3.4, 0.14, 1.0, and 0.56 for Cd, Cu, Pb, Zn, total suspended solids (TSS), total Kjeldahl nitrogen (TKN), NO2–N, NO3–N, and TP. First flush as defined by flushing of 50% of the total pollutant mass load in the first 25% of the event runoff volume occurred in 33% of the storm events for NO2?, 27% for TP, 22% for NO3? and TKN, 21% for Cu, 17% for TSS, 14% for Zn, and 13% for Pb. Median values for the mass flushed in the first 25% of runoff volume were greater than the mass flushed in any 25% portion beyond the first for all pollutants. The mass in later 25% volume portions were greater than in the first 25% volume in at least 17% of the events for all pollutants, indicating that a significant amount of the pollutant load can be contained in later portions of the runoff volume. Nonetheless, management of the first 1.3?mm (1/2?in.) of runoff was able to capture 81–86% of the total pollutant mass.     

Documenting Stormwater Quality on Texas Highways and Adjacent Vegetated Roadsides

The primary objective of this study is the documentation of stormwater quality of vegetated roadsides of two Texas highways (State Highway 6 in College Station and Loop 360 in Austin), both of which had high average daily traffic. Three sites each in Austin and College Station were monitored using passive “first flush” stormwater samplers for 16 months. Results from this study indicate that significant removal of sediment and heavy metals occurred over the width of vegetated roadsides, but no apparent nutrient (nitrogen and phosphorus) removal was observed. The results also show that vegetation density has a direct effect on the performance of vegetated roadsides. When roadsides are densely covered with grasses above 90%, significant sediment removal is expected, often within the first 4?m of the edge of pavement. A stepwise regression analysis identifies the antecedent dry period (ADP) as the most significant predictor to pollutant concentration. The pollutant event mean concentration was found to decrease with increasing ADP for all pollutants at the College Station sites, but not the Austin ones.     

Particle Destabilization in Highway Runoff to Optimize Pollutant Removal

Sedimentation column studies and simulations using particle size distribution suggest that low removal efficiencies of smaller particles in highway runoff would be obtained using sedimentation if coagulation-flocculation is not performed. Coagulation-flocculation studies, using metal salts (alum and ferric chloride) and one organic polymer in three molecular weights, were evaluated over the 2004–2005 storm seasons. Only the first flush or approximately the first hour of runoff was coagulated. Efficiencies were quantified with particle size distribution measurements and turbidity. Results with low dosages of metal salts were ineffective and did not improve water quality. High dosages of metal salts using a sweep floc mechanism were effective in dramatically lowering runoff turbidity, but resulted in large quantities of sludge production and required pH control. A cationic organic polymer at low dosages (<10?mg/L) was effective in coagulating highway runoff and reducing particle charge. Extended mixing time was required to achieve low turbidities ( ～ 5 NTU). A combination of organic polymer, followed by small doses of alum (<10?mg/L), reduced mixing time and produced high quality effluent.     

Particle Size Distribution in Highway Runoff

Particles in highway runoff contain various sorbed pollutants, and many best management practices (BMPs) are selected for particle removal efficiency, which makes particle size distribution a crucial BMP design parameter. Particles between 2 and 1,000?μm in diameter were quantified for three rainfall events during the 2002–2003 rainy season at three highway sites in west Los Angeles. Rainfall, runoff flow rate, and a large suite of water quality parameters were also measured. An experimental protocol was developed for bottle cleaning, sample storage, and mixing that provided repeatable results. Particle aggregation occurred which required samples to be analyzed in less than 6?h; the concentration of small particles decreased with a corresponding increase in the concentration of larger particles in stored samples. The particle concentration decreased as the storm progressed and the number of large particles decreased more rapidly than the total number of particles. Particles demonstrated a strong first flush. On average, 40% of the particles were discharged in the first 20% of the runoff volume.     

Effects of Road Salts on Heavy Metal Mobility in Two Eastern Washington Soils

Heavy metals deposited on road surfaces and transferred to roadside environments by rainfall and snowmelt runoff can have serious impacts on receiving ecosystems. Infiltration is an effective best management practice for controlling metal contamination in runoff, although metals retention within infiltration facilities depends on a number of factors, including metal species, soil characteristics, and influent water quality concentrations. In cold climates, deicing compounds have been shown to mobilize heavy metals putting receiving waters at risk. This study ascertains the effects of two widely used road salts (NaCl and MgCl2) on heavy metal mobility in two eastern Washington soils. Infiltration experiments were conducted using a basic soil, exhibiting a soil pH of 8.3, taken from a highway infiltration pond site in Spokane Washington and an acidic soil, exhibiting a soil pH of 5.9, taken from an infiltration pond site in Richland Washington. Three concentrations of each salt were percolated through both soils using continuous flow soil columns. Leachate samples were collected and analyzed for dissolved metals, organic matter, and pH. Experiments were also performed without salt and used as controls. Results indicate that metal mobilization can occur by a number of mechanisms including cation exchange, chloride complex formation, and colloid dispersion (release of organic matter and/or clay that can complex metal species). Sodium chloride resulted in the largest release of copper and lead via indirect mobilization of organic matter. The magnesium salt had less of an effect on lead and copper but had a much greater effect on the mobilization of cadmium. Releases of metals during or immediately following salt application produced concentrations that ranged from 50% to 1000% greater than the concentrations released from the control experiments.     

Effects of a Permeable Friction Course on Highway Runoff

This project documents the impact of a porous asphalt overlay on the quality and quantity of highway storm-water runoff. A permeable friction course, also known as open graded friction course, is a layer of porous asphalt approximately 50?mm thick, which is often applied on top of conventional asphalt or concrete highways to enhance safety and reduce noise. Storm-water runoff from a four-lane divided highway in the Austin, Texas area was monitored at two sites before and after the installation of a PFC. Observed concentrations of total suspended solids and pollutants associated with particulate material were much lower in the runoff from the PFC than that derived from the conventional asphalt surface. Concentration reductions were observed for total suspended solids, total lead, total copper, and total zinc at both monitoring locations. In addition to the above-mentioned constituents, concentrations of chemical oxygen demand and total Kjeldahl nitrogen were also lower in the runoff from the PFC at a site collecting paired samples from both pavement types. Concentrations of dissolved constituents were not significantly different and concentrations of polycyclic aromatic hydrocarbons were below the detection limit for both pavement types. The runoff coefficient for the PFC appears to be higher than for conventional pavements.     

Oil and Grease Measurement in Highway Runoff—Sampling Time and Event Mean Concentrations

An event mean concentration (EMC), usually collected with an automatic, flow-weighted composite sampler, is often used to characterize stormwater pollutants. Automatic samplers are not recommended for collecting oil and grease (O&G) samples due to possible biases associated with interactions with tubing and pumps. To measure the EMC without sampler interferences, a series of grab samples (often over ten samples) must be collected along with the flow measurement to compute the EMC. This paper examines 22 O&G pollutographs from small, impervious highway sites, to determine when a single O&G grab sample most closely approximates a flow-weighted composite sample. Samples collected within the first hour of a storm event overestimated the O&G EMC by 20?mg/L or more, while samples collected toward the end of the event underestimated the EMC. The best time to collect a single grab sample ranged from 1 to 6 h after the beginning of runoff, and was related to site or storm-specific factors. Results obtained from this study also showed that strong correlations (R2 = 0.9) exist between O&G and other organic constituents, such as chemical oxygen demand (COD) and dissolved organic carbon (DOC). Correlations also exist between O&G EMC, antecedent dry days, and total rainfall. Depending upon site and regulatory specific factors, using COD or DOC EMCs in lieu O&G samples may be a better strategy.     

New Approach to Evaluate Pollutant Removal by Storm-Water Treatment Devices

A methodology was developed to monitor and evaluate the removal of solids and associated constituents by a nutrient separating baffle box (NSBB) storm-water treatment device treating runoff from a 4.3 ha (10.6 acre) residential watershed discharging into the Indian River Lagoon, Florida. The NSBB was monitored over a 359-day time period using autosamplers to quantify water column removal during runoff events, and by quantifying and analyzing solids that accumulated within the NSBB. Flow composited influent and effluent samples were collected to represent water column performance. Event mean concentration (EMC) reduction was moderate (mean: 17%) and variable (range: ?39 to 68%) for suspended solids, and negative for nitrogen, phosphorus, fecal coliforms chromium, and copper. The mass of solids that accumulated in bottom chambers and in a strainer screen was quantified and analyzed for nitrogen, phosphorus, heavy metals, and polycyclic aromatic hydrocarbons. A quantitative evaluative framework was devised to estimate the total pollutant mass removal by NSBB, which consisted of the summation of the separately calculated mass removals for water column, bottom chamber material, and strainer screen material. The water column accounted for only 4% of total solids that accumulated in the NSBB, which was equally divided between bottom chamber and strainer screen. Removal of nitrogen, phosphorus, and metals could be accounted for only by considering mass accumulations. Results suggest that overall assessment of pollutant removal by NSBB must be cognizant of the materials not captured by typical autosamplers: larger size sediment particles, large floating and suspended matter, and the pollutants associated with these materials. Using water column EMCs as the sole measure of performance significantly underestimated loading reduction of storm-water constituents by the NSBB. The monitoring and evaluative methodology applied to the NSBB may be applicable to load reduction evaluations for other storm-water treatment devices with a similar function.     

Quantifying Long-Term NPS Pollutant Flux in an Urbanizing Watershed

Long-term nonpoint source (NPS) pollutant flux is described within the rapidly developing Occoquan watershed west of Washington, D.C. Data consist of up to 24 years of observed rainfall, integrated pollutant discharge, and land use/land cover from four headwater basins of the Occoquan River. Three of the four study basins, ranging in size from 67 to 400?km2, are predominantly forest and mixed agriculture. The fourth basin, the 127?km2 Cub Run watershed, has urbanized rapidly over the past 20 years. Higher annual NPS sediment and nutrient fluxes in Cub Run after 1983 are linked to increased soil disturbance from urban construction and increased storm volumes resulting from increased mean impervious percent. Over the long-term, storm fluxes of NPS particulate P, soluble P, particulate N, and soluble N make up 92, 67, 89, and 50%, respectively, of the total fluxes of those constituents, with between 88 and 98% of mean annual total suspended solids fluxes delivered by storm flow. Higher NPS pollutant fluxes in Cub Run basin after 1983, and specifically during the growing season, indicate a seasonal impact of replacing vegetated cover with impervious surface.     

Impact of Annual Average Daily Traffic on Highway Runoff Pollutant Concentrations

The objective of this study was to evaluate correlations between annual average daily traffic (AADT) and storm water runoff pollutant concentrations generated from California Department of Transportation (Caltrans) highway sites. Analyses of data collected from the Caltrans four-year (1997–2001) highway runoff characterization program revealed that, in general, pollutant concentrations from urban highways were higher than those found from nonurban highways. For a limited number of pollutants, however, the concentrations from nonurban highways were found to be higher than the concentrations from urban highways. No direct linear correlation was found between highway runoff pollutant event mean concentrations and AADT. However, through multiple regression analyses, it was shown that AADT has an influence on most highway runoff constituent concentrations, in conjunction with factors associated with watershed characteristics and pollutant build-up and wash off. The other noticeable factors shown to influence the accumulation of pollutants on highways were antecedent dry period, drainage area, maximum rain intensity, and land use.     

Steady Groundwater Transport of Highway Deicing Agent Constituents from an Infiltration Basin

The highway deicing agent groundwater plume from an infiltration basin in the Plymouth-Carver Aquifer near State Route 25 in southeastern Massachusetts is modeled and measured in order to assess the impact of the basin on groundwater transport processes. The advective transport model superimposes the existing steady models of axisymmetric basin hydraulics of Zlotnik and Ledder in 1992 and the two-dimensional ambient flow of Gelhar and Wilson in 1974. The basin component incorporates a surface source of finite-radius into a Hankel transform model for unconfined aquifers, whereas the ambient component varies linearly in the horizontal and vertical directions. Contaminant streamlines describe the resulting groundwater plume, and highway deicing agent constituents provide useful tracers. A simple vertical dispersion model quantifies the spread of contaminants across the bottom of the plume. Deicing agent constituent data calibrate a 17?m basin radius, a 45?m plume width, a bottom streamline 6–10?m below the water table, and a vertical dispersivity of 34?cm. The latter value is comparable to the amplitude of vertical excursions caused by storm scale fluctuations of hydraulic head, as measured by Ostendorf et al. in 2007. Infiltration basins alter ambient advection by displacing streamlines downward and augment vertical mixing by imposing aperiodic vertical fluctuations on the ambient flow field.     

Water Quality Improvement through Reductions of Pollutant Loads Using Bioretention

As an increasingly adopted storm-water best management practice (BMP) to remedy hydrology and water quality impairment from urban development, bioretention facilities need rigorous investigation to quantify performance benefits and to allow design improvements. This study examines water quality improvements [total arsenic, total cadmium, chloride, total chromium, total and dissolved copper, E. coli, fecal coliform, lead, mercury, nitrogen species, oil and grease, phosphorus, total organic carbon (TOC), total suspended solids, and total zinc] via monitoring for a 15-month period at two bioretention cells in Maryland. Both bioretention cells effectively removed suspended solids, lead, and zinc from runoff through concentration reduction. Runoff volume reduction promotes pollutant mass removal and links BMP water quality benefits with hydrologic performance. From a load perspective (kg/ha?year), all but TOC at one cell showed pollutant reduction. Bioretention effluents exhibited good water quality for all significant pollutants except for nitrate, copper, and phosphorus in one cell, the latter two of which may be attributed to media organic matter dissolution. Copper dissolved/particulate analyses showed that significant changes in copper speciation behavior result from transport through the bioretention media.     

Diffusion Wave Model for Simulating Storm-Water Runoff on Highway Pavement Surfaces at Superelevation Transition

On a curved section of highway, the cross slope of the road is often designed to be superelevated to balance the centrifugal force and gravity applied on vehicles. The accumulation of storm-water runoff (sheet flow) near superelevation transitions may significantly increase due to the extended flow path and converging flow lines. A two-dimensional finite-volume-based diffusion wave model is developed to simulate the sheet flow on these geometrically complex surfaces. Both Dirichlet- and Neumann-type boundary conditions are developed for open boundaries based on kinematic wave theory. Results show that the distribution of sheet flow is closely related to the cross slope, longitudinal slope, rainfall intensity, and the width of the road. The analysis of sheet flow characteristics on superelevation transition areas suggests that the optimal longitudinal slope in the range of 0.3–0.4% minimizes the depth of storm-water runoff on the road surface.     

Investigation of Boundary Shear Stress and Pollutant Detachment from Impervious Surface during Simulated Urban Storm Runoff

Pollutant detachment rates have been determined for four chloride salts during simulated urban storm runoff. Under rainfall and/or overland flow conditions, chloride mass flux was measured and related to boundary shear stress of the test surface. Washoff coefficients, presumed to depend only on pollutant characteristics, were computed based on the slopes of dimensionless mass flux versus dimensionless time plots. Washoff coefficients were found to vary among and between the chloride compounds studied. In general, higher overland flow rates produced lower boundary shear and lower washoff coefficients. The combination of simulated rainfall and overland flow resulted in an increased boundary shear and an increased washoff coefficient. An empirical washoff coefficient based on a load characteristic curve derived from an exponential washoff relationship was also computed from the runoff data and compared with the previous washoff coefficient. A linear correlation between these two washoff coefficients was observed. The magnitude of the latter coefficient under simulated rainfall was consistent with reported values obtained from field data.     

Trace Metal Pollutant Load in Urban Runoff from a Southern California Watershed

In order to implement efficient and effective management strategies for coastal water quality in Southern California, it is important to consider the relative pollutant contributions from urban dry-weather flow (DWF) and wet-weather flow (WWF). This study uses both historical flow coupled with water quality monitoring data and computer modeling to characterize the annual DWF and WWF discharges from an urban catchment in Los Angeles, Calif. The DWF and WWF pollutant loading of the trace metals copper, lead, nickel, and chromium for 6 water years dating from 1991 to 1996 is predicted. The results indicate that DWF contributes a considerable amount of flow and pollutants. Approximately, 9–25% of the total annual Ballona Creek flow volume is DWF. The simulations indicate DWF accounts for 54, 19, 33, and 44% of the average annual load of total chromium, copper, lead, and nickel, respectively. In the dry season, the simulations indicate DWF accounts for 89, 59, 58, and 90% of the load of total chromium, copper, lead, and nickel, respectively. This research suggests DWF controls may be an important part of pollution mitigation plans for urban stormwater drainage systems in Southern California.     

Evaluation and Optimization of Bioretention Media for Treatment of Urban Storm Water Runoff

Bioretention is a relatively new urban storm water best management practice. The objective of this study is to provide insight on media characteristics that control bioretention water management behavior. Eighteen bioretention columns and six existing bioretention facilities were evaluated employing synthetic runoff. In columns, the runoff infiltration rate through different media mixtures ranged from 0.28 to 8.15?cm/min at a fixed 15 cm head. For pollutant removals, the results showed excellent removal for oil/grease (>96%). Total lead removal (from 66 to >98%) decreased when the total suspended solids level in the effluent increased (removed from 29 to >96%). The removal efficiency of total phosphorus ranged widely (4–99%), apparently due to preferential flow patterns, and both nitrate and ammonium were moderate to poorly removed, with removals ranging from 1 to 43% and from 2 to 49%, respectively. Two more on-site experiments were conducted during a rainfall event to compare with laboratory investigation. For bioretention design, two media design profiles are proposed; >96%?TSS, >96%?O/G, >98%?lead, >70%?TP, >9%?nitrate, and >20%?ammonium removals are expected with these designs     

Predictions of Resuspension of Highway Detention Pond Deposits in Interrain Event Periods due to Wind-Induced Currents and Waves

The paper presents a numerical study of resuspension of deposits from highway detention ponds based on a previous experimental study. The resuspension process is evaluated in dry weather periods with baseflow/infiltration flow through the ponds only. The resuspension is caused by the bed-shear stress induced by the return flow near the bed and waves both generated by the wind. Wind statistics for 30 years have been applied for prediction of the annual discharged bulk of suspended solids and associated pollutants; fluoranthene, benzo(b)fluoranthene, benzo(k)fluoranthene, benzo(a)pyrene, dibenzo(a,h)anthracene and indeno(1,2,3-cd)pyrene (PAHs) and the heavy metals of cadmium, chromium, copper, lead, nickel, and zinc. The current and wave-generated bed-shear stresses entail a discharged bulk of pollutants corresponding to approximately 10% of the annual accumulation of pollutants in the present pond due to the baseflow in the pond. The mean outlet concentration of suspended solids is well correlated with the wind speed. To reduce the resuspension of deposited materials, two mechanisms are prevailing; either by increase of the water depth of the pond to minimize the effect of the wind in the near-bed region or by reduction of the wind to some degree. The most efficient action for reducing the wind impact on the shallow waters is the establishment of shelterbelts as known from the agriculture. Just a 20% reduction of the yearly wind speeds will reduce the outlet mass with 70% and a 50% reduction with almost 100%. A 50% reduction of the wind speed is far from impossible to achieve with relatively small investments.     

Case Study of Steady Oxygen Concentration Gradients in a Groundwater Plume from a Highway Infiltration Basin

We measure and model the steady transport of specific conductivity and dissolved oxygen through a groundwater plume from a highway infiltration basin in southeastern Massachusetts. Specific conductivity is treated as a conservative surrogate for runoff contamination, and the data calibrate a 0.27-m vertical dispersivity α of the aquifer and the bottom streamline elevation of the plume, which falls to an 8-m depth below the water table. The dissolved oxygen degrades as a first order reactant in the plume to levels below 1 mg/L, with a decay constant λ of 0.12?day?1. The latter may be attributed in part to the historical use of an alternative de-icing agent calcium magnesium acetate on the highway, since acetate is a readily biodegradable substrate for microorganisms. The calibrated kinetics suggest that plume microbes and geochemistry degrade oxygen over two orders of magnitude faster than their ambient groundwater counterparts, which impose a linear decrease of dissolved oxygen concentration below the plume. Simulations suggest that the anoxic groundwater plume extends 1,600 m downgradient of the infiltration basin, a distance that will shorten by an order of magnitude if salt is used exclusively to de-ice the highway.     

Pollutant Removal and Peak Flow Mitigation by a Bioretention Cell in Urban Charlotte, N.C.

Bioretention is a stormwater treatment practice that has gained popularity due to its aesthetics, potential to reduce flooding, and early documented improvements to stormwater quality. A bioretention cell in an urban setting was examined in Charlotte, N.C. from 2004 to 2006. Flow-weighted, composite water quality samples were collected for 23 events and analyzed for TKN, NH4-N, NO2-3-N, TP, TSS, BOD-5, Cu, Zn, Fe, and Pb. Grab samples were collected from 19 storms for fecal coliform and 14 events for Escherichia coli (E. coli). There were significant reductions (p<0.05) in the concentrations of TN, TKN, NH4-N, BOD-5, fecal coliform, E. Coli, TSS, Cu, Zn, and Pb. Iron concentrations significantly increased (p<0.05). NO2-3-N concentrations were essentially unchanged. Efficiency ratios for TN, TKN, NH4-N, TP, and TSS were 0.32, 0.44, 0.73, 0.31, and 0.60, respectively. Fecal coliform and E. coli efficiency ratios were 0.69 and 0.71, respectively. Efficiency ratios for Zn, Cu, and Pb were 0.77, 0.54, and 0.31, respectively. Concentrations of Fe increased by 330%. The peak outflow of the bioretention cell for 16 storms with less than 42?mm of rainfall was at least 96.5% less than the peak inflow, with a mean peak flow reduction being 99%. These results indicated that in an urban environment, bioretention systems can reduce concentrations of most target pollutants, including pathogenic bacteria indicator species. Additionally, bioretention can effectively reduce peak runoff from small to midsize storm events.     

Modeling Flow and Pollutant Removal of Wet Detention Pond Treating Stormwater Runoff

A mathematical model of pollutant removal by wet ponds was developed based on the mass balance principle and the release–storage equation. The release–storage equation can be linear or nonlinear depending on the wet pond shape and the spillway crest features. If the exponent index of the storage relationship (d) is equal to the index of the outflow equation (b), the wet pond hydrological routing model is linear. Otherwise, the model is nonlinear. Substituting the release–storage equation into the continuity equation produces a nonlinear ordinary differential equation (ODE). A Runge–Kutta method was used to solve the resulting nonlinear ODE. When the ratio of indexes d/b = 2/3, the hydrologic wet pond model reduces to a special case that leads to an implicit analytical solution. The pollutant removal and flow routing models were tested with data obtained from an actual wet pond for treating highway runoff. The predicted flow discharges and pollutant concentrations compared well with the observed data.