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.     

Zinc Pollution Potential of Consumer Battery Litter

Consumer batteries in urban litter can be significant sources of storm-water pollution. Recent studies have reported annual urban battery litter rates as high as 215 batteries per hectare of pavement and 0.4 batteries per meter of street curb. On average, 75% of these batteries are leaking or have already discharged their internal reactants. However, many battery sizes, brands, and power chemistries are littered and this diversity makes it difficult to quantify their cumulative pollution potential. The amount of zinc released from AA size alkaline and zinc chloride/zinc carbon (ZnCl/ZnC) batteries which account for approximately 90% of urban battery litter is examined. Results are presented for batch rupture release extractions of 52 alkaline battery products yielding zinc releases of 20–40 mg/L and 57 ZnCl/ZnC products yielding zinc releases of 400–1,400 mg/L. Results of continuous flow column extractions are also presented to gauge releases following initial battery rupture. Disassembly analyses are used to bound the total zinc release potential of common battery products. Results indicate that ZnCl/ZnC batteries release more zinc when they are first ruptured, but if deterioration is complete, alkaline batteries can release approximately 25% more zinc. Therefore, the relative importance of these two classes of batteries depends on site-specific factors such as the proportion of each in litter, battery deterioration rates, and the length of time that battery litter remains unremediated by maintenance.     

Corrosion Deterioration in Consumer Battery Litter

Recent studies have shown that the annual rates of consumer battery litter on urban pavements can be as high as 215 batteries per hectare of pavement and 0.4 batteries per meter of curb. As littered batteries deteriorate, they release components (Ag, Ba, Cd, Cr, Cu, Hg, Li, Mn, Na, Ni, Pb, Ti, and Zn) that can be significant sources of storm-water contamination. Results of ambient environmental and laboratory-accelerated corrosion studies are presented to quantify the mechanisms that yield chemical deterioration of littered batteries. The analysis concentrates on AA size alkaline and zinc chloride/zinc carbon (ZnCl/ZnC) cells since these are the most commonly littered batteries. Results indicate that littered batteries exposed to ambient well-drained environmental conditions for more than two months will develop surface corrosion over most of their surface but fewer than 10% will be ruptured by corrosion within the first 6 months. This agrees well with field observations. Exposure under poorly drained conditions yields more rapid deterioration but it requires exposure to more aggressive conditions such as those produced by road salts to reproduce the high degree of deterioration observed in some battery litter.     

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.     

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.     

Storm-Water Filter Media Pollutant Retention under Aerobic versus Anaerobic Conditions

Storm-water runoff entering filters is usually aerobic and therefore the removal processes in the filter normally occur under oxidizing and aerobic conditions. However, storm-water filters differ from water and wastewater treatment filters because there are quiescent times when no influent enters the filter and the pore water stagnates. During this stagnation period, anaerobic conditions on a macro- or microscale could develop. This note presents the results of experiments conducted to determine if four potential filter media (sand, activated carbon, peat moss, and compost) could retain previously trapped pollutants when anaerobic conditions develop during interevent periods. The results indicated that permanent retention of heavy metals may occur even in an anaerobic environment (for the media and metals investigated). However, retention of some nutrients may not occur under these conditions, particularly for the organic media. This is an area of concern when the design of filters and bioretention devices includes an internal water storage zone where, between events, anaerobic conditions for nitrate removal are encouraged.     

Thermal Reduction by an Underground Storm-Water Detention System

Increases in stream temperatures by heated storm-water runoff from impervious surfaces are a serious environmental problem. An underground detention with slow-release facility is a versatile storm-water best management practice (BMP) for buffering high flows. Temperature reductions in underground storm-water storage BMPs, however, have not been quantified. A field study on an underground detention BMP located in Maryland was undertaken to characterize its effect on storm-water runoff temperatures. In colder months, when the runoff temperature ranged from 5 to 15°C, small or no temperature change was observed. Runoff produced during summer storm events, however, with event mean temperatures over 20°C, exhibited mean temperature reductions of 1.6°C through the BMP. While statistically significant, the reductions were not sufficient to cool the summer runoff discharges below the Maryland Class III temperature standard (20°C) 100% of the time. The results indicate that underground facilities can moderate high runoff temperatures, but that more efficient designs are needed for heat transfer.     

Incentive Index Developed to Evaluate Storm-Water Low-Impact Designs

In practice, the challenge of storm-water low-impact-development (LID) design is often related to how to quantify the effectiveness of a LID layout. In this study, the watershed imperviousness was chosen as a basis to evaluate the performances of various LID designs. Often, LID designs apply cascading planes to drain the runoff flow from the upstream impervious area to the downstream pervious area. In this study, the conventional area-weighting method is revised with a pavement-area-reduction factor (PARF) to produce the effective imperviousness. PARF is employed as an incentive index to quantify the on-site runoff volume reduction and cost savings from downsized sewers. Two sets of PARF are derived: conveyance-based and storage-based LID designs. The conveyance-based LID approach is to drain runoff flows on various porous surfaces while the storage-based LID approach is to temporarily store runoff flows in an on-site basin. For a specified LID layout, the PARF provides a consistent basis to translate the infiltration and storage effects into the reduction on the area-weighted imperviousness. The nondimensional governing equation derived in this paper indicates that the PARF depends on the ratio of the soil infiltration rate to rainfall intensity, the ratio of receiving pervious area to upstream impervious area, and the on-site storm-water storage capacity. The PARF serves as a basis for the engineers, planners, and/or developers to select a LID design and also for regulatory agencies to assess meritorious credits for cost savings.     

Solids Removal in Storm-Water Filters Modeled Using a Power Equation

Storm-water filters and biofilters are limited by physical clogging. Several models have been proposed to predict the flow rate through the media throughout its lifespan. Urbonas (1999) modeled flow rate through a downflow sand filter as a function of the suspended solids loading on the media surface using a power equation. This paper confirms this equation in describing the flow rate through mixed-media downflow filters at a laboratory scale but with unique coefficients for different media. Confounding effects of influent solids concentration and filter surface diameter on the relationship between flow rate and suspended solids loading were seen, indicating that other factors are important in describing that relationship. Maintenance issues also were investigated, with the results showing that removal of about 5–10% of the surface of the media had little long-term impact on flow rate recovery.     

Process Modeling of Storm-Water Flow in a Bioretention Cell

A two-dimensional variable saturated flow model was developed to simulate subsurface flow in bioretention facilities employing the Richards’ equation. Variable hydrologic performances of bioretention are evaluated using the underdrain outflow hydrographs, outflow volumes for 10 storms with various duration and depth, and flow duration curves for 25 different storms. The effects of some important design parameters and elements are tested, including media type, surrounding soils, initial water content, ratio of drainage area to bioretention surface area, and ratio of cell length to width. Model results indicate that the outflow volume via underdrain is less than the inflow; the flow peak is significantly reduced and delayed. Underdrain outflow volume from loamy sand media (with larger Ks) is larger than that from sandy clay loam media. The saturated hydraulic conductivity, storage capacity, and exfiltration into surrounding soils contribute to the hydrologic performance of a bioretention cell. Initial media storage capacity is affected by the hydraulic properties of media soils, initial water content, and bioretention surface area. The exfiltration volume is determined by the surrounding soil type and exfiltration area, dominated by flow through the bottom of the media.     

Storm-Water Management Using Street Sweeping

Although street sweeping is commonly regarded as a cost-effective storm-water best management practice, there is little quantitative evidence that street sweeping directly improves runoff water quality. In this paper, several previous street sweeping studies were reevaluated using statistical power analysis. Two-group, independent-sample one-sided t-test power analyses were performed using log-transformed event mean concentrations (EMCs) of total suspended solids, suspended sediment concentration or chemical oxygen demand. The effect size between the two groups was estimated using the sweepers’ pickup efficiency, which showed that the failure to detect the difference between mean EMCs of the two sample groups (i.e., unswept and swept groups) is likely due to limited sample numbers. Too few samples, which also resulted in a high coefficient of variation, were analyzed to detect the likely difference between swept and unswept observations. In addition, the temporal gap between street sweeping and subsequent storm events was not controlled to improve statistical power.     

Probabilistic Approach to Estimation of Urban Storm-Water Total Maximum Daily Loads: Unregulated Catchment

This work examines the basic processes and functions behind urban storm-water pollution delivery into surface waters and develops a set of tools that allow the estimation of pollutant load dynamics on receiving waters. In particular, the group of expressions developed in this paper allows the calculation of runoff parameters (volume, discharge rate and pollutant load) on an event average basis for an unregulated catchment. Using Monte Carlo simulation techniques, the runoff pollutant concentration probability distribution (as event averages) are obtained. Merging these runoff statistics with the stream parameters allows the receiving water pollutant concentration characteristics to be obtained as well as the probability of exceeding threshold pollutant concentrations in the mixing zone of a stream. The simulation can be performed with allowance for different levels of complexity with respect to catchment hydrologic representation and pollutant load functions. As a result, the magnitude of influence of urban runoff on a surface water body can be determined, pollutants of concern can be identified, and certain remedial measures recommended.     

Integrated Storm-Water Management for Watershed Sustainability

One aspect of integrated watershed management evaluates the impact of development on the local hydrologic cycle and, in particular, drinking water, wastewater, and storm-water infrastructure. Sustainable storm-water management focuses on selecting storm-water controls based on an understanding of the problems in local receiving waters that result from runoff discharges. For example, long-term problems associated with accumulations of pollutants in water bodies include sedimentation in conveyance systems and receiving waters, nuisance algal growths, inedible fish, undrinkable water, and shifts to less sensitive aquatic organisms. Short-term problems associated with high pollutant concentrations or frequent high flows (event-related) include swimming beach closures, water quality violations, property damage from increased flooding, and habitat destruction. A wide variety of individual storm-water controls usually must be combined to form a comprehensive wet weather management strategy. Unfortunately, combinations of controls are difficult to analyze. This will require new modeling techniques that can effectively evaluate a wide variety of control practices and land uses, while at the same time ensure that the flood-control objectives also are met. The results of these new models and novel techniques used for storm-water control then can be incorporated into an evaluation of the urban water cycle for a specific service area to determine whether storm-water controls can provide additional benefits such as reduction of potable water use and reduction of sanitary sewer overflow events.     

Characterizing Physicochemical Quality of Storm-Water Runoff from an Urban Area in Calgary, Alberta

Understanding storm-water runoff quality is required to develop effective urban storm-water runoff management for regions of semiarid climate. In this study, the quality of storm-water runoff from a semiarid, urban residential catchment, draining through separated storm-water sewers was investigated in 2006 and 2007. Water temperature, conductivity, pH, dissolved oxygen, and turbidity were continuously measured during 16 storm events. Storm-water runoff quality was characterized in terms of event mean values (EMVs), loads, and first flush (FF) loads and their relationships with rainfall characteristics. Discharge of total suspended solids (TSSs) is in general governed by the flow magnitude in storms and no significant relationships exist between the FF loads of TSS and rainfall intensity. The discharge of dissolved solids is independent of the flow magnitude. Strong FF effect for dissolved solids and weak FF effect for TSS were observed. This semiarid region provided no relationship between the EMVs of both TSS and conductivity and the antecedent dry period. This raises doubts on storm-water runoff being more heavily loaded with pollutants after a longer dry period in semiarid regions.     

Groundwater Mounding at a Storm-Water Infiltration BMP

This research presents an initial study of the impacts of storm-water infiltration on a shallow unconfined aquifer at a bioinfiltration best management practice (BMP) on the campus of Villanova University. The study site is a vegetated infiltration basin with a 0.52?ha drainage area consisting of parking areas and recreational fields and features approximately 35% directly connected impervious area. The research utilized continuous monitoring of precipitation, groundwater elevation, and groundwater temperature in conjunction with surface water hydrologic modeling to assess the duration, magnitude, and extent of groundwater mounding at a storm-water infiltration BMP. Results indicate that precipitation greater than 1.80?cm causes increased mounding at wells adjacent to the site. In addition, it was found that precipitation less than approximately 1.80?cm leads to larger increases in groundwater elevation at an upgradient control well located near the edge of a large grass field. The extent of groundwater mounding is observed to be localized to the BMP and does not extend a significant distance downgradient. In addition, the magnitude and duration of groundwater mounding is related to both infiltration rate and groundwater temperature, such that cooler temperatures correlate to increased mounding. This study demonstrates the utility of groundwater monitoring for the purpose of BMP hydraulic performance assessment, and recommends that additional research be conducted in the future and that groundwater monitoring be considered for site monitoring plans.     

On Parameter Estimation of Urban Storm-Water Runoff Model

An existing accumulation and wash-off model was applied and calibrated on a standard asphalt parking lot located in the northeastern United States. The field measured data consisted of rainfall, flow, and runoff samples taken from over 26 storm events monitored from 2004 to 2006. The contaminants under consideration include: total suspended solids, total petroleum hydrocarbons-diesel range hydrocarbons (TPH-D), dissolved inorganic nitrogen (DIN) (comprised of nitrate, nitrite, and ammonia), and zinc (Zn). The objective of the study was to provide probability distributions of model parameters for contaminants that have not been documented much (TPH-D, DIN, and Zn). The best fitting parameter values were found on a storm by storm basis. Subsequently, the range and variability of these parameters are provided for modeling purposes and other urban storm-water quality applications. A normal distribution was fitted to the optimized model parameter values to describe their distributions. A simulated annealing algorithm was used as the parameter optimization technique. Several examples are given to illustrate the methodology and the performance of the model. Finally, a Monte Carlo simulation was performed to assess the capability of the model to predict contaminant concentrations at the watershed’s outlet.     

Probabilistic Approach to Estimation of Urban Storm-Water TMDLs: Regulated Catchment

The present work is concerned with the development of a set of tools for the incorporation of various control measures—best management practices into an analytical probabilistic modeling approach for urban storm-water total maximum daily load (TMDL) estimation. Control measures are divided into two major groups—upstream and downstream, each requiring application of separate modeling principles elaborated in this paper. Applying Monte Carlo simulation to the developed set of expressions allows modeling the “end-of-pipe” parameters of urban storm-water discharges (runoff volume, discharge rate, and pollutant load) on an event average basis, as well as the stream parameters downstream of a storm-water discharge outlet. Model application is illustrated for a catchment regulated with an extended detention dry pond. Representative model results are presented, and a range of potential model applications is discussed. The capability to model the behavior of an urban storm-water system with the application of various control measures is the key precondition for the design of an optimal configuration of a water-protective strategy.     

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.     

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.     

Hydrologic Performance Monitoring of an Underdrained Low-Impact Development Storm-Water Management System

The use of low-impact development (LID) storm-water management facilities will grow with gains in understanding of their performance based on field tests. An innovative flow measurement system was designed and tested for monitoring of an interconnected system of underdrained LID components forming a site management strategy. Pressure transducers housed in stilling wells provided in-line flow measurements in pipes connecting the LID components. A combination of laboratory experiments, field measurements, and computer simulations was used to calibrate the flow measurement system to translate depth measurements to estimates of flow. The monitoring system is well suited for high-resolution temporal monitoring and provides important information for evaluating LID component performance. The measurement system is limited to open-channel flow, but calculations indicate that surcharge conditions are expected to occur at the case study site only under conditions more extreme than the 100-year storm event.