Monitoring the impacts of rainfall characteristics on sediment loss from road construction sites.

Exposed soils associated with active construction sites provide opportunities for erosion and sediment transport during storm events, introducing risks associated with excess sediment to downstream infrastructure and aquatic biota. A better understanding of the drivers of sediment transport in construction site runoff is needed to improve the design and performance of erosion and sediment control measures (ESCMs). Eleven monitoring locations on 3 active road construction sites in central Ohio were established to characterize runoff quality from points of concentrated flow during storm events. Grab samples were analyzed for total suspended solids (TSS), turbidity, and particle size distribution (PSD). Median TSS concentrations and turbidity levels across all samples were 626 mg/L (range 25-28,600 mg/L) and 759 NTU (range 22-33,000 NTU), respectively. The median PSD corresponded to a silty clay loam, mirroring the soil texture of much of Ohio's subsoils. TSS concentrations and turbidity were significantly positively correlated with the rainfall intensity 10 min prior to sample collection, suggesting that higher flow rates created greater shear stress on bare soil which resulted in more erosion. Conversely, rainfall duration was negatively correlated with particle size, indicating that prolonged moisture from rainfall promoted the dispersion of soil aggregates which mobilized smaller particles. Multivariable linear regression models revealed that higher rainfall intensities corresponded to higher turbidity values, while higher TSS concentrations were associated with higher rainfall intensities, depths, and durations. Results from this study highlight the importance of reducing raindrop impact and subsequent shear stress applied by concentrated flows through the use of ESCMs to limit sediment export from construction sites.

The potential to manage releases of Bacillus anthracis using bioretention and a high flow media filter: Results of simulated runoff testing with tracer spores Bacillus globigii.

The threat of bioterrorism has spurred research on the decontamination and containment of different agents. Anthrax [causative agent Bacillus anthracis (Ba)] is a disease that can lead to severe infections within human and animals, particularly when inhaled. This research investigated the use of spore-contaminated simulated runoff events into stormwater control measures (SCMs), which are designed to retain and improve the quality of runoff and may have the potential to filter and contain the spores. In this study, the effectiveness of a bioretention cell (BRC) and high flow media filter (HFMF) in Huron, Ohio, were evaluated for removal of Bacillus globigii (Bg) spores (a harmless cognate of Ba). Three 4-8 mm simulated runoff events were created for each SCM using a fire hydrant and Bg spores were injected into the runoff upstream of the SCM inlets. The BRC significantly (p < 0.001) outperformed the HFMF in reducing Bg concentrations and loads, with an average load reduction of 1.9 log (∼99% reduction) compared to 0.4 (∼60% reduction), respectively. A probable critical design factor leading to these differences was the infiltration rate of the media and subsequent retention time within the filters, which was supported by similar disparities in suspended solids reductions. Differences in spore removal may also have been due to particle size distribution of the HFMF, which was more gravelly than the bioretention cell. At 3 and 6 months after the-simulated runoff tests, soil samples taken from both SCMs, yielding detectable Bg spores within the top 15 cm of media, with increased spore concentrations where ponding occurred for longer durations during the tests. This suggests that forebays and areas near inlets may be hotspots for spore cleanup in a real-world bioterrorism incident.

Measuring sediment loads and particle size distribution in road runoff: Implications for sediment removal by stormwater control measures.

Road runoff contributes an array of pollutants which degrade the quality of receiving waters. Sediment conveyed in runoff results in loss of habitat and loss of reservoir capacity, among other undesirable impacts. To select and design stormwater control measures (SCMs), the sediment particle size distribution (PSD) is needed to quantify the required hydraulic retention time for particle settling and to understand what other treatment processes (e.g., filtration) are needed to meet sediment removal targets. A two-year field monitoring study was undertaken across the state of Ohio, USA, to evaluate the PSD of sediment in runoff at twelve roads. The highest TSS concentrations were observed on interstate highways (highest annual average daily traffic [AADT]) and minor arterials (low AADT), suggesting factors beyond AADT, such as antecedent dry period, rainfall intensity, and windborne dust and particulates, contribute to the varied sediment characteristics in runoff. The median TSS load across all samples collected was 2.7 kg/ha per storm event, while annual TSS loads for the monitoring sites varied from 98 kg/(ha·yr) to 519 kg/(ha·yr), with a mean value of 271 kg/(ha·yr). Particle size distributions varied across the monitoring sites, with mean and median d50 of 48.6 µm and 52.5 µm, respectively. Interstate highways (highest AADT) had significantly finer PSDs than other functional classes, while roads in low density residential areas had coarser PSDs than other land uses. Observed differences in PSD across road characteristics may guide SCM selection; dry detention basins and wet ponds/wetlands were predicted to provide effective removal across a variety of PSDs, while TSS reductions provided by hydrodynamic separators and high-flow media filters (which effectively remove larger particles) may be maximized in areas with coarser PSDs (e.g., roads surrounded by low density residential areas studied herein).

Environmental regulations in the United States lead to improvements in untreated stormwater quality over four decades.

Identifying sources of pollutants in watersheds is critical to accurately predicting stormwater quality. Many existing software used to model stormwater quality rely on decades-old data sets which may not represent current runoff quality in the United States. Because of environmental regulations promulgated at the federal level over previous decades, there is a need to understand long-term trends (and potential shifts) in runoff quality to better parameterize models. Pollutant event mean concentrations (EMCs) from the National Stormwater Quality Database (NSQD) were combined with those from recent sources to understand if untreated stormwater quality has changed over the past 40 years. A significant decreasing monotonic trend (i.e., continually decreasing in a nonuniform fashion) was observed for total suspended solids (TSS), total phosphorus (TP), total Kjeldahl nitrogen (TKN), total copper (Cu), total lead (Pb), and total zinc (Zn) in the resultant database, suggesting that runoff quality has become less polluted with time. Median EMCs decreased from 99.2 to 42 mg/L, 0.34 to 0.26 mg/L, 1.27 to 1.03 mg/L, 40 to 6.8 µg/L, 110 to 3.7 µg/L, and 375 to 53.3 µg/L for TSS, TP, TN, Cu, Pb, and Zn, respectively, from the 1980s to the 2010s. These significant reductions often aligned temporally with advancements in clean manufacturing, amendments of the Clean Air Act, and other source control efforts which impact pollutant bioavailability and atmospheric deposition. Results suggest environmental regulations not specifically targeting stormwater management have had a positive impact on stormwater quality and that temporal fluctuations should be considered in modeling.

Curbing sediment: The effects of added surface roughness in the curb and gutter as a novel pretreatment for green infrastructure stormwater control measures.

Stormwater control measures (SCMs) are employed to reduce the multitude of deleterious impacts of urban runoff on receiving waters. Sediment accumulation in infiltration-based SCMs can clog these systems, resulting in lack of hydraulic function and reduced stormwater treatment efficacy. As such, pretreatment devices, such as forebays, filter strips, or catch basin sumps, are typically employed upstream of SCMs to remove sediment and prolong maintenance intervals. However, the tendency of SCMs to be retrofitted into space-constrained, ultra-urban areas makes including pretreatment technologies difficult. An alternative pretreatment device for green infrastructure SCMs was developed and tested in the laboratory; alterations were made to the standard curb and gutter, which is ubiquitous within urban environments, to increase the roughness of these surfaces. Roughness was added to the curb and/or gutter of mock road sections constructed of expanded polystyrene (EPS) foam using a computer numerical control (CNC) router. Twenty-one patterns with varying degrees of depth, shape, and spacing were implemented to trap sediment from simulated runoff; samples were collected upstream and downstream of the added roughness and analyzed for sediment removal and particle capture. Patterns which included added roughness in both the curb and gutter reduced total suspended solids (TSS) concentrations by up to 95% (median 85%) and reduced median d50 and d90 in runoff from 46.9 to 39.4 µm and 322 to 100 µm, respectively. Continued TSS removal was observed during repeated testing designed to simulate up to seven runoff events, indicating the potential for sustained sediment accumulation before the need for maintenance via regular street sweeping. With routine maintenance performed at appropriate intervals, these findings indicate that added roughness to curb and gutters could be utilized as a viable pretreatment technology for green infrastructure SCMs.

Abundance and composition of anthropogenic macrolitter and natural debris in road runoff in Ohio, USA.

Urban stormwater conveys dissolved pollutants, micropollutants, particulate matter, natural debris, and anthropogenic macrodebris to receiving waters. Though it is widely recognized that anthropogenic macrodebris mobilized by stormwater contributes to global pollution management issues (e.g., ocean garbage patches), these materials often are not the focus of stormwater sampling campaigns. Furthermore, macrodebris can cause clogging of sewer systems, exacerbating flooding and public health hazards. Due to their engineered structures draining directly connected impervious areas (e.g., catch basins, inlets, and pipes), roads present a unique opportunity to mitigate the conveyance of macrodebris in stormwater. To optimize control measures, data are needed to understand expected volume and mass of macrodebris in road runoff. To address this gap in knowledge, a field monitoring study was conducted in Ohio (USA) to quantify the mass, volume, and moisture content of macrodebris transported by road runoff. Designed to filter macrodebris (i.e., material with diameter greater than 5 mm) while maintaining drainage, purpose-built inserts were deployed in catch basins at eleven geographically diverse locations across the state. Macrodebris samples were collected from the inserts every 11.6 days (mean) over a two-year monitoring period. Volume and mass of total and categorical (i.e., vegetation, cigarettes, plastic, glass, metal, wood, fabric, gravel, and paper) debris were characterized. Mean total macrodebris volume and mass were 4.62 L and 0.49 kg per sampling window, corresponding to mean volumetric and mass loading rates of 8.56 L/ha/day and 0.79 kg/ha/day, respectively. Natural debris (e.g., vegetation) was the primary contributor to macrodebris (mean 80.3% (i.e., 3.94 L of the mean 4.66 L total sample volume) and 79.7% (i.e., 0.42 kg of the mean 0.53 kg total sample mass) of total volume and mass, respectively), and exhibited seasonal peaks in autumn due to leaf drop. Road functional class (i.e., interstate, principal arterial, and minor arterial routes), land use, and development density significantly impacted macrodebris generation, with increased total and categorical macrodebris along urbanized interstate highways near commercial and residential areas. Macrodebris moisture content was highly variable (ranging from 1.5 to 440%; mean 78.5%), indicating additional management (e.g., drying, solidification) may be required prior to landfilling. Results of this study inform macrodebris mitigation strategies and required maintenance frequencies for pre-treatment devices for other stormwater control measures treating road runoff, including catch basin inserts and hydrodynamic separators, among others.

Monitoring the effects of urban and forested land uses on runoff quality: Implications for improved stormwater management.

Urban stormwater is a substantial source of non-point source pollution. Despite considerable monitoring efforts, little is known about stormwater quality in certain geographic regions. These spatial gaps induce uncertainty when extrapolating data and reduce model calibration capabilities, thereby limiting pollutant load reduction strategies. In this study, stormwater quality was monitored from 15 watersheds to characterize pollutant event mean concentrations (EMCs) and loads as a function of urban and forested (i.e., surrogates for pre-development) land use and land covers (LULCs) and rainfall patterns from a geographic region where these data are sparse. Residential and heavy industrial, heavy industrial, and industrial and commercial LULCs, respectively, were the primary generators of nutrients, total suspended solids (TSS), and heavy metals. Increased rainfall intensities (average and peak) significantly increased the EMCs of all particulate bound pollutants. Pollutant loads increased with rainfall depth and, in general, did not follow the same LULC trends as EMCs, suggesting loads were influenced substantially by watershed hydrologic responses. Mean annual urban loads of total phosphorus, total nitrogen, TSS, and zinc (Zn) ranged from 0.4 (low density residential [LDR]) to 1.5 (heavy industrial), 3.2 (single family residential [SFR]) to 11.5 (heavy industrial), 122.6 (SFR) to 1219.9 (heavy industrial), and 0.1 (LDR) to 0.7 (commercial) kg/ha/yr, respectively. Annual urban loads of TSS were 3.5 to 34 and - 1.5 to 6.8-fold greater than annual loads from forested and agricultural watersheds, respectively. Mean annual loads of heavy metals from urban LULCs were substantially greater than loads produced by forested and agricultural watersheds (e.g., 8.6 to 92 and 6.8 to 73-fold greater, respectively, for Zn), while loads of nutrients were generally similar between urban and agricultural watersheds. Findings herein suggest non-point source pollution will continue to threaten surface water quality as land is developed; results can help guide the development of cost-efficient stormwater management strategies.

Occurrence, concentration, and distribution of 38 organic micropollutants in the filter material of 12 stormwater bioretention facilities.

The increased use of bioretention facilities as a low impact development measure for treating stormwater runoff underscores the need to further understand their long-term function. Eventually, bioretention filter media must be (partly) replaced and disposed of at the end of its functional lifespan. While there are several studies of metal accumulation and distributions in bioretention media, less is known about organic pollutant pathways and accumulation in these filters. The present study considers the occurrence and accumulation of 16 polycyclic aromatic hydrocarbons, 7 polychlorinated biphenyls, 13 phthalates, and two alkylphenols throughout 12 older bioretention facilities (7-13 years old) used for stormwater treatment in Michigan and Ohio, USA. These pollutant groups appear to behave similarly, with greater instances of detection and higher concentrations in the upper media layers which decrease with increased depth from the surface. The patterns of detection and concentration in the filter material may be explained by characteristics of the pollutants, such as molecular structures and solubility that affect the removal of the organic pollutants by the filter material. There is also a large variation in concentration magnitudes between the bioretention sites, most likely due to differences in pollutant sources, contributing catchment size and/or land uses.

Optimizing floating treatment wetland and retention pond design through random forest: A meta-analysis of influential variables.

Floating treatment wetlands (FTWs), artificial systems constructed from buoyant mats and planted with emergent macrophytes, represent a potential retrofit to enhance the dissolved nutrient removal performance of existing retention ponds. Treatment occurs as water flows through the dense network of roots suspended in the water column, providing opportunities for pollutants to be removed via filtration, sedimentation, plant uptake, and adsorption to biofilms in the root zone. Despite several recent review articles summarizing the growing body of research on FTWs, FTW design guidance and strategies to optimize their contributions to pollutant removal from stormwater are lacking, due in part to a lack of statistical analysis on FTW performance at the field scale. A meta-analysis of eight international FTW studies was performed to investigate the influence of retention pond, catchment, and FTW design characteristics on effluent concentrations of nutrients and total suspended solids (TSS). Random forest regression, a tree-based machine learning approach, was used to model complex interactions between a suite of predictor variables to identify design strategies for both retention ponds and FTWs to enhance treatment of nutrient and sediment. Results indicate that pond design features, especially loading ratio and pond depth (which should be limited to 200:1 and 1.75 m, respectively), are most influential to effluent water quality, while the benefits of FTWs were limited to improving mitigation of phosphorus species and TSS which was primarily influenced by FTW coverage and planting density. Findings from this work inform wet retention pond and FTW design, as well as guidance on scenarios where FTW implementation is most appropriate, to improve dissolved nutrient and sediment removal in urban runoff.

Effects of land use, climate, and imperviousness on urban stormwater quality: A meta-analysis.

Many natural and anthropogenic factors cause degradation of urban stormwater quality, resulting in negative consequences to receiving waters. In order to improve water quality models at a variety of scales, accurate estimates of pollutant (nutrients, total suspended solids, and heavy metal) concentrations are needed using potential explanatory variables. To this end, a meta-analysis was performed on aggregated stormwater quality data from the published literature from 360 urban catchments worldwide to understand how urban land use and land cover (LULC), climate (i.e., KÓ§ppen-Geiger zone), and imperviousness (1) affect runoff quality, and (2) whether they are able to predict stormwater pollutant concentrations. Runoff pollutant concentrations were more influenced by LULC and climate than imperviousness. Differences in LULC significantly affected the generation of metals and some nitrogen species. Road, city center, and commercial LULCs generally produced the most elevated pollutant concentrations. Changes in climate zones resulted in significant differences in concentrations of nutrients and metals. Continental and arid climate zones produced runoff with the highest pollutant concentrations. Rainfall patterns seemed to have a more important role in affecting runoff quality than seasonal temperature. Differences in imperviousness only significantly affected chromium and nickel concentrations, although increased imperviousness led to slightly (not significantly) elevated concentrations of nutrients, suspended solids, and other heavy metals. Multiple linear regression models were created to predict the quality of urban runoff. Predictive equations were significant (p < 0.05) for 67% of the pollutants analyzed (ammonia, total Kjeldahl nitrogen, total nitrogen, total phosphorus, cadmium, nickel, lead, and zinc) suggesting that LULC, climate, and imperviousness are useful predictors of stormwater quality when local field monitoring or modeling is not practical. This study provides useful relationships to better inform urban stormwater quality models and regulations such as total maximum daily loads.

Detection of SARS-CoV-2 in urban stormwater: An environmental reservoir and potential interface between human and animal sources.

While wastewater has been found to harbor SARS-CoV-2, the persistence of SARSCoV-2 in stormwater and potential transmission is poorly understood. It is plausible that the virus is detectable in stormwater samples where human-originated fecal contamination may have occurred from sources like sanitary sewer overflows, leaky wastewater pipes, and non-human animal waste. Because of these potential contamination pathways, it is possible that stormwater could serve as an environmental reservoir and transmission pathway for SARS-CoV-2. The objectives of this study are: 1) determine whether the presence of SARS-CoV-2 could be detected in stormwater via RT-ddPCR (reverse transcription-digital droplet PCR); 2) quantify human-specific fecal contamination using microbial source tracking; and 3) examine whether rainfall characteristics influence virus concentrations. To accomplish these objectives, we investigated whether SARS-CoV-2 could be detected from 10 storm sewer outfalls each draining a single, dominant land use in Columbus, Xenia, and Springboro, Ohio. Of the 25 samples collected in 2020, at minimum one SARS-CoV-2 target gene (N2 [US-CDC and CN-CDC], and E) was detected in 22 samples (88%). A single significant correlation (p = 0.001), between antecedent dry period and the USCDC N2 gene, was found between target gene concentrations and rainfall characteristics. Grouped by city, two significant relationships emerged showing cities had different levels of the SARS-CoV-2 E gene. Given the differences in scale, the county-level COVID-19 confirmed cases COVID-19 rates were not significantly correlated with stormwater outfall-scale SARS-CoV-2 gene concentrations. Countywide COVID-19 data did not accurately portray neighborhood-scale confirmed COVID-19 case rates. Potential hazards may arise when human fecal contamination is present in stormwater and facilitates future investigation on the threat of viral outbreaks via surfaces waters where fecal contamination may have occurred. Future studies should investigate whether humans are able to contract SARS-CoV-2 from surface waters and the factors that may affect viral longevity and transmission.

Assessing maintenance techniques and in-situ pavement conditions to restore hydraulic function of permeable interlocking concrete pavements.

Permeable pavements are increasingly implemented to mitigate the negative hydrologic outcomes associated with impervious surfaces. However, the hydraulic function of permeable pavements is hindered by clogging in their joint openings, and systematic maintenance is needed to ensure hydraulic functionality throughout the design lifespan of these systems. To quantify the effectiveness of various maintenance measures, surface infiltration rates (SIRs) were measured before and after five different maintenance techniques were applied to five permeable interlocking concrete pavements (PICPs) in central Ohio, USA. Three maintenance techniques, the Municipal Cleaning Vehicle (MCV), the Rejuvenater, and a pressure washer and the Rejuvenater performed in series, significantly improved median SIRs from 16 to 26, 5 to 106, and 11 to 37 mm/min, respectively. However, pressure washing alone resulted in no significant difference to PICP SIR (median SIRs increased from 8 to 20 mm/min). Regenerative air street sweeping significantly worsened SIRs when performed during wet weather (median SIRs decreased from 19 to 4 mm/min) but had no significant impact on SIRs during dry weather (median SIRs decreased from 21 to 18 mm/min). This work captured the maintenance effectiveness of two techniques for the first or second time, namely the Rejuvenater and MCV, to investigate their use as a suitable maintenance technique. Further, the maintenance techniques were tested on multiple PICPs, thus the effect of in-situ pavement conditions had on hydraulic improvement via maintenance could be addressed. Differences in general upkeep, traffic, and runoff routed to a PICP affected the depth of clogging below the pavement surface, which forestalled hydraulic improvement. Though shown to improve the SIR of PICP systems, results indicate that the maintenance techniques were not capable of restoring pavement hydraulics to initial conditions. These results demonstrate the need for regular, routine maintenance and topping up of joint aggregate before clogging migrates deeper into the pavement profile.

Conventional and amended bioretention soil media for targeted pollutant treatment: A critical review to guide the state of the practice.

Bioretention systems are widely used green infrastructure elements that utilize engineered bioretention soil media (BSM) for stormwater capture and treatment. Conventional bioretention soil media, which typically consists of sand, sandy loam, loamy sand or topsoil amended with compost, has limited capacity to remove and may leach some stormwater pollutants. Alternative engineered amendments, both organic and inorganic, have been tested to supplement BSM. Yet, municipalities and regulatory agencies have been slow to adopt these alternative amendments into their design specifications, partly because of a lack of clear guidance on how to select the right amendment to treat a target stormwater contaminant under highly variable climatic conditions. This article aims to provide that guidance by: (1) summarizing the current design BSM specifications adopted by jurisdictions worldwide, (2) comparing the performance of conventional and amended BSM, (3) highlighting advantages and limitations of BSM amendments, and (4) identifying challenges for implementing amendments in field conditions. The analysis not only informs the research community of the barriers faced by stormwater managers in implementing BSM amendments but also provides guidelines for their adoption by interested agencies to comply with existing regulations and meet design needs. This feedback loop could catalyze further innovation in the development of sustainable stormwater treatment technologies.

The seasonality of nutrients and sediment in residential stormwater runoff: Implications for nutrient-sensitive waters.

The discharge of excess nutrients to surface waters causes eutrophication, resulting in algal blooms, hypoxia, degraded water quality, reduced and contaminated fisheries, threats to potable water supplies, and decreases in tourism, cultural activities, and coastal economies. An understanding of the contribution of urban runoff to eutrophication is needed to inform management strategies. More broadly, the seasonality in nutrient concentrations and loads in urban runoff needs further analysis since algal blooms and hypoxia are seasonal in nature. This study quantifies the variation of nutrients and sediment in stormwater runoff across seasons from four urban residential sewersheds located in Columbus, Ohio, USA. An average of 62 runoff events at each sewershed were sampled using automated samplers during stormflow and analyzed for nutrients and total suspended solids (TSS). Spring total nitrogen concentrations had a significantly (p < 0.05) higher median concentration (2.19 mg/L) than fall (1.55 mg/L) and summer (1.50 mg/L). Total phosphorus concentrations were significantly higher in spring (0.22 mg/L) and fall (0.23 mg/L) than summer (0.15 mg/L). TSS concentrations were significantly higher in the spring (74.5 mg/L) and summer (56.5 mg/L) than the fall (34.0 mg/L). In contrast, seasonal loading differences for nutrients or sediment were rare because runoff volume varied in such a way as to offset significant concentration differences and significant seasonality in rainfall intensity. Annual pollutant loadings were similar in magnitude to other residential and even some agricultural runoff studies. Although nutrient loads are the key indicator for determining algal biomass, nutrient concentrations are important for real-time algal growth. Future research efforts should be focused not only on understanding how seasonal urban concentrations and loads impact coastal eutrophication, but also developing improved watershed management focused on critical periods. Improved designs for stormwater control measures need to account for seasonality in pollutant discharge.

Stormwater quality performance of permeable interlocking concrete pavement receiving run-on from an asphalt traffic lane in a cold climate.

Cities have turned to permeable pavements as one tool to mitigate the detrimental effects of urban runoff. Permeable pavements permit rainfall to infiltrate through a series of aggregate layers, where pollutants are filtered out before the water discharges via an underdrain or exfiltrates into native soils. This study reports on the water quality performance of a parking area retrofitted with permeable interlocking concrete pavement in Vermilion, OH, USA. The practice was constructed in 2015, received run-on from an asphalt traffic lane and was operational for 2 years before the onset of monitoring. During the 15-month monitoring period, the permeable pavement provided significant reductions of sediment and particulate nutrients, which were removed via filtration in the upper aggregate layers. Despite poorly draining underlying soils, runoff volumes were reduced by 26%, leading to significant load reductions for nearly all nutrient and heavy metals in the study. Seasonal variations in runoff and effluent composition were investigated, showing that restorative maintenance performed in spring and fall has the potential to further improve the treatment provided by the practice by removing entrained particulates from the upper aggregate layers and restoring the filtering capacity of the system. Correlation analyses revealed a first flush of particulate nitrogen species, as well as the potential occurrence of erosive flows within the aggregate subbase which resulted in elevated sediment concentrations during high intensity rain events. Results from this study demonstrate the effectiveness of permeable pavements several years after construction, even when design features to specifically improve treatment were not implemented and additional run-on is routed onto the pavement from adjacent impervious surfaces. Findings also highlight the importance of timely maintenance of these practices, which could further improve their performance by removing seasonally deposited pollutants throughout the year.

Hydrologic and water quality performance of permeable pavement with internal water storage over a clay soil in Durham, North Carolina.

Permeable pavement is an effective tool for improving stormwater hydrology and water quality when sited over soils with high infiltration rates, but its efficacy over less permeable soils is uncertain. This study examined permeable pavement performance when built over a low-conductivity, clay soil. Four parking stalls (50 m2 total area) were retrofitted with permeable interlocking concrete pavement (PICP) to treat 15.2 m2 of contributing impervious area (0.3:1 run-on ratio). Using an elevated underdrain, the site incorporated a 150-mm internal water storage (IWS) zone to increase exfiltration and promote anaerobic conditions for denitrification. From March 2014-April 2015, 22% of influent runoff volume was reduced via exfiltration and evaporation. Inter-event drawdown of the IWS zone created storage to capture and exfiltrate more than 70% of the runoff volume from precipitation events less than 8 mm, and peak flows were significantly reduced (median 84%). Relative to stormwater runoff from a nearby impermeable asphalt reference watershed, the permeable pavement produced significantly lower event mean concentrations (EMCs) of all pollutants except nitrate, which was significantly higher. Permeable pavement effluent and reference watershed runoff were 99%, 68%, and 96% different for total suspended solids (TSS), total nitrogen (TN), and total phosphorus (TP), respectively. Significantly lower permeable pavement effluent EMCs for copper (Cu, 79%), lead (Pb, 92%) and zinc (Zn, 88%) were also observed. The median effluent concentrations of TN (0.52 mg/L), TP (0.02 mg/L), and TSS (7 mg/L) were all very low relative to the literature. Sampling of nitrogen species in the IWS zone 12, 36, 60, and 84 h post-rainfall was done to better understand mechanisms of nitrogen removal in permeable pavement; results indicated denitrification may be occurring in the IWS zone. Effluent pollutant load from the permeable pavement was at minimum 85% less than from nearby untreated asphalt runoff for TP, TSS, Cu, Pb, and Zn, and was 73% less for TN. Permeable pavements built over low-permeability soils with internal water storage can considerably improve long-term hydrology and water quality.

Quantifying volume reduction and peak flow mitigation for three bioretention cells in clay soils in northeast Ohio.

Green infrastructure aims to restore watershed hydrologic function by more closely mimicking pre-development groundwater recharge and evapotranspiration (ET). Bioretention has become a popular stormwater control due to its ability to reduce runoff volume through these pathways. Three bioretention cells constructed in low permeability soils in northeast Ohio were monitored for non-winter quantification of inflow, drainage, ET, and exfiltration. The inclusion of an internal water storage (IWS) zone allowed the three cells to reduce runoff by 59%, 42%, and 36% over the monitoring period, in spite of the tight underlying soils. The exfiltration rate and the IWS zone thickness were the primary determinants of volume reduction performance. Post-construction measured drawdown rates were higher than pre-construction soil vertical hydraulic conductivity tests in all cases, due to lateral exfiltration from the IWS zones and ET, which are not typically accounted for in pre-construction soil testing. The minimum rainfall depths required to produce outflow for the three cells were 5.5, 7.4, and 13.8mm. During events with 1-year design rainfall intensities, peak flow reduction varied from 24 to 96%, with the best mitigation during events where peak rainfall rate occurred before the centroid of the rainfall volume, when adequate bowl storage was available to limit overflow.

Maintenance measures for preservation and recovery of permeable pavement surface infiltration rate--The effects of street sweeping, vacuum cleaning, high pressure washing, and milling.

The surface infiltration rates (SIR) of permeable pavements decline with time as sediment and debris clog pore spaces. Effective maintenance techniques are needed to ensure the hydraulic functionality and water quality benefits of this stormwater control. Eight different small-scale and full-scale maintenance techniques aimed at recovering pavement permeability were evaluated at ten different permeable pavement sites in the USA and Sweden. Maintenance techniques included manual removal of the upper 2 cm of fill material, mechanical street sweeping, regenerative-air street sweeping, vacuum street sweeping, hand-held vacuuming, high pressure washing, and milling of porous asphalt. The removal of the upper 2 cm of clogging material did not significantly improve the SIR of concrete grid paves (CGP) and permeable interlocking concrete pavers (PICP) due to the inclusion of fines in the joint and bedding stone during construction, suggesting routine maintenance cannot overcome improper construction. For porous asphalt maintenance, industrial hand-held vacuum cleaning, pressure washing, and milling were increasingly successful at recovering the SIR. Milling to a depth of 2.5 cm nearly restored the SIR for a 21-year old porous asphalt pavement to like-new conditions. For PICP, street sweepers employing suction were shown to be preferable to mechanical sweepers; additionally, maintenance efforts may become more intensive over time to maintain a threshold SIR, as maintenance was not 100% effective at removing clogging material.