Effects of Stormwater Infiltration on Quality of Groundwater Beneath Retention and Detention Basins

Infiltration of storm water through detention and retention basins may increase the risk of groundwater contamination, especially in areas where the soil is sandy and the water table shallow, and contaminants may not have a chance to degrade or sorb onto soil particles before reaching the saturated zone. Groundwater from 16 monitoring wells installed in basins in southern New Jersey was compared to the quality of shallow groundwater from 30 wells in areas of new-urban land use. Basin groundwater contained much lower levels of dissolved oxygen, which affected concentrations of major ions. Patterns of volatile organic compound and pesticide occurrence in basin groundwater reflected the land use in the drainage areas served by the basins, and differed from patterns in background samples, exhibiting a greater occurrence of petroleum hydrocarbons and certain pesticides. Dilution effects and volatilization likely decrease the concentration and detection frequency of certain compounds commonly found in background groundwater. High recharge rates in storm water basins may cause loading factors to be substantial even when constituent concentrations in infiltrating storm water are relatively low.     

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.     

Experimental Assessment of Stormwater Infiltration Basin Evolution

Infiltration basins are frequently used for stormwater drainage. They can operate for periods over 20 years but long-term evolution is not well understood or controlled. The two main problems encountered are clogging, which compromises the hydraulic capacity of the basin, and possible contamination of underlying soil and groundwater. This paper focuses on studying long-term evolution of clogging and soil pollution of infiltration basins. Basins of different ages are compared. Also, clogging and soil pollutant concentrations are explored for four infiltration basins in Lyon, France. Ages of the sites range from 10 to 21 years old. Clogging is characterized by the hydraulic resistance. Soil samples were collected at different depths in each basin and analyzed for different pollution parameters (metals, hydrocarbons, pH, and particle size distribution). All four basins have good infiltration capacities. Their hydraulic resistance is low. Such uniformity is surprising because of the age difference between the basins. Pollutant concentrations decrease rapidly with depth whereas pH and grain size increase. Concentrations reach an acceptable value at a 30 cm depth, even after 21 years of operation. Multivariate data analysis does not show significant relation between age, hydraulic resistance, and pollution.     

Storm Flow from First-Flush Precipitation in Stormwater Design

Regulations for mitigating nonpoint source pollution from urban areas often include a requirement for treatment of a first-flush depth of stormwater. When the stormwater treatment technology requires specification of a design flow rate, the first-flush depth must be related to a first-flush flow rate. This paper describes a methodology to determine the relation between accumulated storm depth and corresponding flow to a treatment device for small, urban watersheds. The approach uses the rational method under the assumption that the time-of-concentration is small. Intensity–frequency relations, expressed in the form of return periods and associated intensities, were determined using a partial duration frequency analysis of regionalized precipitation data. The results of the method can be used to determine the flow rate to a treatment device that will meet a specified return period for a first-flush depth. The method is demonstrated using 13 years of 15-min data from seven Massachusetts precipitation stations.     

“First Flush,” Power Law and Particle Separation Diagrams for Urban Storm-Water Suspended Particulates

Commensurate with development of in situ storm-water control and treatment is the need to quantify the delivery and granulometry of the suspended particulate fraction in storm water. Consistent with this need, this study examined the so-called “first flush” phenomenon for suspended particles with a measured range from 2 to 75 μm (typically <50 μm), the appropriateness of a single- versus multiple-power-law model of particle-number density (PND), and the application of process selection diagrams for particle separation. In comparison to a defined concentration “first flush” during the early portion of the examined rainfall-runoff events, results indicate that a disproportionately high and in some cases a continuous suspended particle delivery phenomenon that followed the hydrology of the event occurred. Such results suggest that the entire event may require treatment, not solely the commonly designated “first flush” based on indices such as suspended solids. While a single-power law reasonably represented granulometric characteristics of suspended storm-water particulates, and in theory a continuously size-based power law is the most accurate representation; within the given suspended particle-size range a multiple-power law provided reasonable simplicity and accuracy for total PND, surface area, and particulate volume. Despite a wide range of hydrologic conditions for a series of nine rainfall-runoff events examined, process selection diagrams based on the number-volume mean size (lnv) and total PND led to a similar conclusion. Based on the combination of lnv and PND in the urban catchment sedimentation in a typical transportation land use drainage facility is capable of removing 90% of particulate matter by mass within a detention time of 120 min.     

Fractional Kinetic Model for First Flush of Stormwater Pollutants

By generalizing the urban ground as a fractal surface and revising the classical Fick’s formula as a law of dispersion with a fractional-order derivative, a fractional kinetic model is developed for simulation of the first flush phenomenon of urban stormwater pollutants. The model is comprised of (1) a fractional dispersion-advection equation (FADE); (2) the kinematic-wave overland flow equation; and (3) methods for numerical solution of the equations. A split-operator method is proposed for numerical solution of the FADE by means of a newly presented F.3 finite-difference scheme for fractional partial differential equations. The kinematic-wave overland flow equation is solved using the Lax–Wendroff explicit scheme. Under a constant rainstorm the hydrograph displays an initial rising limb followed by a constant flow discharge. The pollutograph exhibits a steep receding limb (the first flush), followed by a long stretched tail (heavy tail process). The agreement between simulated and measured dispersion characteristics is found to be good, demonstrating that the fractional kinetic model is capable of accurately predicting the characteristics of the first flush phenomenon.     

New Paradigm for Sizing Riparian Buffers to Reduce Risks of Polluted Storm Water: Practical Synthesis

Riparian buffers are commonly promoted to protect stream water quality. A common conceptual assumption is that buffers “intercept” and treat upland runoff. As a shift in paradigm, it is proposed instead that riparian buffers should be recognized as the parts of the landscape that most frequently generate storm runoff. Thus, water quality can be protected from contaminated storm runoff by disassociating riparian buffers from potentially polluting activities. This paper reviews and synthesizes some simple engineering approaches that can be used to delineate riparian buffers for rural watersheds based on risk of generating runoff. Although reference is made to specific future research that may improve the proposed methods for delineating riparian buffers, the approaches described here provide planners and engineers with a set of currently available scientifically defensible tools. It is recommended that planners and engineers use available rainfall and stream discharge data to parameterize the buffer-sizing equations and use variable-width buffers, based on a topographic index, to achieve a realistic representation of runoff generating areas.     

Modeling Storm Water Mass Emissions to the Southern California Bight

Storm water runoff is perceived as a major source of pollutants that results in adverse environmental effects, but large-scale assessments are rarely conducted. The problem is particularly pronounced in southern California where 17 million people have rapidly developed coastal watersheds. The goal of this study was to make regionwide estimates of mass emissions, assess the relative contribution from urbanized watersheds, and compare pollutant flux from different land uses. A geographic information system-based storm water runoff model was used to estimate pollutant mass emissions based on land use, rainfall, runoff volume, and local water-quality information. Local monitoring data were used to derive runoff coefficients; over 1,700 storm water sampling events were used to calibrate and validate annual loadings. An average rainfall year produced 1,073×109?L of runoff, 118,000 metric tons (MT) of suspended solids, 1,940 MT of nitrate-N, 108 MT of zinc, and 15 kg of diazinon. The majority of mass emissions were from urbanized watersheds except for suspended solids, total DDT, and chlorpyrifos. Agricultural areas had the greatest fluxes for pesticides, including total DDT and chlorpyrifos while open areas typically had the smallest.     

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.     

Comparison of Stormwater Solids Analytical Methods for Performance Evaluation of Manufactured Treatment Devices

As more manufactured stormwater treatment devices enter the market, stormwater managers are searching for effective and rapid methods for evaluating device performance. Many agencies require vendors to test full-scale versions of their devices under controlled conditions. The most common parameter used to document performance is suspended solids for several reasons: (1) many pollutants attach to solids; (2) a solids simulant is relatively easy to generate; and (3) solids are comparatively easy and inexpensive to quantify. However, a controversy still exists in the profession and some regulatory agencies as to whether total suspended solids (TSS) or suspended sediment concentration (SSC), or both, should be measured. This paper focuses on the comparability of the two methods/protocols used for sample solids analysis, including lessons learned during recent evaluations of two manufactured treatment devices. Analysis of 215 sample pairs (where both TSS and SSC were measured on aliquots of the same sample) showed that statistically the TSS measured using the wide-bore pipet method and SSC results were indistinguishable from one another and from the original simulant mixture.     

Influence of Hydrology on Rainfall-Runoff Metal Element Speciation

Examination of speciation for metal elements transported in urban rainfall-runoff events is critical when evaluating the potential fate, bioavailability, and effective control of such constituents. In many urban areas anthropogenic activities result in rainfall pH levels that are acidic and low in alkalinity. As a result, finely abraded metallic components and exposed metal infrastructure can be leached into rainfall-runoff. This study examines the influence of hydrology on storm water metal element speciation at the upper end of a Portland cement concrete small urban watershed. This study focused on Pb, Cd, Cu, and Zn; metal elements commonly found in urban and transportation land uses. For this site partitioning results demonstrated that Cd and Cu partitioned nearly equally between particulate and dissolved phases while Zn was generally particulate-bound and Pb was highly particulate-bound. Utilizing water quality analyses, measured ion balances and speciation modeling, results for Cd and Zn indicated that divalent ionic forms of these metals dominated the dissolved species for all events, while Pb was predominately associated with dissolved organic matter (DOM), and Cu was predominately associated with carbonate species or DOM. Of the three events examined, only the mass-limited events demonstrated a change in speciation during the passage of the hydrograph. Results from this study indicate that effective control of storm water metal elements at the upper end of the urban watershed requires unit operations and processes that account for the ionic, complexed and particulate-bound species and account for the hydrology at the upper end of the urban watershed.     

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.     

Infiltration Variability in Furrow Irrigation

In this paper, the contribution of different sources of variability to irrigation water depth variability was quantified using a combination of variance techniques. This method was applied using field measurements from irrigation events performed on a loamy soil with a low-infiltration rate. Infiltration variability was estimated with blocked furrow infiltrometers. The assumptions made for the application of the combination of variance techniques proved to be valid. The major variability source turned out to be the soil intake characteristics, whose variance accounted for 45–71% of the variance in infiltrated depth under first irrigation conditions. Opportunity time and wetted perimeter were less variable in subsequent irrigations and the soil intake characteristics variability accounted for a percentage of total variance beyond 76%, being at times beyond 95%. The combination of variance techniques can be used to complement standard evaluation methods in order to take into account the influence of different variability sources.     

Storm-Water Infiltration and Focused Recharge Modeling with Finite-Volume Two-Dimensional Richards Equation: Application to an Experimental Rain Garden

Rain gardens are infiltration systems that provide volume and water quality control, recharge enhancement, as well as landscape, ecological, and economic benefits. A model for application to rain gardens based on Richards equation coupled to a surface water balance was developed, using a two-dimensional finite-volume code. It allows for alternating upper boundary conditions, including ponding and overflow, and can simulate heterogeneous soil-layering or more complex geometries to estimate infiltration and recharge. The algorithm is conservative, and exhibits good performance compared to standard models for several test cases (less than 0.1% absolute mass balance error); simulations were also performed for an experimental rain garden and comparisons to collected data are presented. The model accurately simulated the matrix flow, soil water distribution, as well as deep percolation (potential recharge) for a natural rainfall event in the controlled experimental setup.     

Compaction’s Impacts on Urban Storm-Water Infiltration

Soil infiltration is a critical component of most urban runoff models. However, it has been well documented that, during urbanization, soils are greatly modified, especially in relation to soil density. Increased soil compaction results in soils that do not behave in a manner predicted by traditional infiltration models. Laboratory and field tests were conducted to investigate detailed infiltration behavior of disturbed urban soils for a variety of soil textures and levels of compaction. The results from traditional permeability tests on several soil groups showed that, as expected, the degree of compaction greatly affected the steady-state infiltration rate. The field tests highlighted the importance of compaction on the infiltration rate of sandy soils, with minimal effect seen from antecedent moisture conditions. For the clayey soils, however, both the compaction level and antecedent moisture conditions were important in determining the steady-state infiltration rate.     

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.     

Temperature Effects on the Infiltration Rate through an Infiltration Basin BMP

Infiltration Best Management Practices (BMPs) are becoming more readily acceptable as a means of reducing postdevelopment runoff volumes and peak flow rates to pre-construction levels, while simultaneously increasing recharge. However, the design, construction, and operation of infiltration basins to this point have not been standardized due to a lack of understanding of the infiltration processes that occur in these structures. Sizing infiltration BMPs to hold and store a predetermined volume of runoff, typically called the Water Quality Volume, has become a widely accepted practice. This method of sizing BMPs does not account for the infiltration that is occurring in the BMP during the storm event; which could result in significantly oversized BMPs. The objective of this study was to develop a methodology to simulate varying infiltration rates observed from a large scale rock infiltration basin BMP. The results should aid in improved design of such structures. This methodology is required to predict the performance of these sites using single event and continuous flow models. The study site is a Pervious Concrete Infiltration Basin BMP built in 2002 in a common area at Villanova University. The system consists of three infiltration beds filled with coarse aggregate, lined with geotextile filter fabric, overlain with pervious concrete and underlain by undisturbed silty sand. The BMP is extensively instrumented to facilitate water quantity and quality research. The infiltration performance of the site is the focus of the study. Recorded data indicates a wide variation of linear infiltration rates for smaller storm events. A model was developed using the Green–Ampt formula to characterize the infiltration occurring in the basin for small storm events characterized by an accumulated depth of water in the infiltration bed of less than 10?cm. The effectiveness and accuracy of the model were measured by comparing the model outputs with observed bed water elevation data recorded from instrumentation on site. Results show that for bed depths of <10?cm, hydraulic conductivity is the most sensitive parameter, and that the storm event measured infiltration rate is substantially less then the measured saturated hydraulic conductivity of the soil. The governing factor affecting hydraulic conductivity, and subsequently, infiltration rate is temperature; with higher rates occurring during warmer periods, affecting the infiltration rate by as much as 56%.     

Experimental Investigation of Surges in a Stormwater Storage Tunnel

Below-grade storage tunnels in stormwater systems are usually designed to operate in a free-surface flow regime. However, intense rain events may trigger flow regime transition to pressurized flow, which results in operational problems. To date, little guidance is available as to the considerations necessary to properly design a system undergoing flow regime transition. In this investigation, an experimental apparatus consisting of a 14.6-m-long, 94-mm-diameter acrylic pipe was used to observe the nature of flows in such conditions. It was noticed that the air near the pipe crown may pressurize and influence the flow dynamics. Qualitative observations regarding the interactions between the air and water phases during the filling events are included in this study. Generally the surge intensity was maximized when a hydraulic bore propagating towards the surge riser just filled the pipe cross section, and it increased with the pressure head behind the pressurization front. Furthermore, the results indicate that the effects of air phase pressurization should be properly included in numerical simulations if ventilation conditions are limited.     

Nutrient Loads Associated with Different Sediment Sizes in Urban Stormwater and Surface Pollutants

A significant amount of pollutants in urban stormwater runoff is transported as sediment-bound contaminants. It is important to have a clear understanding of the amount of pollutants attached to the different sediment sizes so that treatment facilities can be designed to effectively target the removal of the most polluted sediment sizes. This paper presents results from a study carried out to determine nutrient loads associated with different particle size ranges for dry surface pollutants and stormwater samples collected from an urban road surface. The results indicate that although more that half of the surface pollutant is coarser than 300 μm (microns), less than 15% of the total phosphorus (TP) and total nitrogen (TN) are attached to particle sizes greater than 300 μm. The pollutant loads in the different particle size ranges from different stormwater samples show a larger variability for TN than for TP. The dissolved component for TN ranges between 20 and 50% compared to 20–30% for TP. Practically all the particulate TP and TN in stormwater samples are attached to sediments between 11 and 150 μm. This suggests that to effectively remove TP and TN, pollutant treatment facilities must be able to remove particulates down to 11 μm.     

Integration of Low-Impact Development into the International Stormwater BMP Database

Low impact development (LID) strategies are being encouraged in many communities as an approach to reduce potential adverse impacts of development on receiving streams. Many questions exist regarding how well various LID strategies perform in different settings, just as similar questions have been raised regarding performance of traditional stormwater best management practices (BMPs). Whereas historical focus on BMP performance has been water quality concentrations or loads, characterization of volume reduction benefits for both conventional and LID practices is increasingly an objective of researchers and stormwater managers. More than a decade ago, Urban Water Resources Research Council (UWRRC) members worked to develop a set of standardized monitoring and reporting protocols for traditional BMPs and to establish a master database for the purpose of evaluating BMP performance and the factors affecting performance. This effort culminated in the International Stormwater BMP Database (www.bmpdatabase.org), which contains data for more than 360 BMPs and continues to operate as a clearinghouse for stormwater BMP data and performance analyses. During 2008–2009, the International Stormwater BMP Database project expanded to better integrate LID into the database and develop a set of metrics that can be used to characterize BMP performance with regard to surface runoff volume reduction. This paper provides a condensed overview and progress report on the LID-focused effort, including the following topics: (1)?monitoring guidance for LID at the overall site development level, (2)?an overview of recent changes to the International Stormwater BMP Database to better accommodate LID studies, (3)?a summary of LID studies currently included in the database, and (4)?a proposed approach for evaluating performance of LID studies with regard to reducing surface runoff volumes.