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The hydrologic performance of a grass swale, a common stormwater control measure often utilized to drain roads, may potentially be improved using simple retrofits. Two rock check dams were retrofitted into an existing grass swale located in Knightdale, North Carolina, USA. The swale was monitored before and after check dam installation, and the addition of check dams improved runoff volume reduction (17%), peak flow mitigation, and hydraulic retention time in the swale, particularly for small (< 19 mm) and moderate (19–38 mm) rainfall events. The check dams were effective filters of gross solids, which eventually led to clogging and caused extended inundation and subsequent loss of swale vegetation. Because check dams are relatively inexpensive and simple vis-à-vis other stormwater control measure enhancements, their use for stormwater treatment is encouraged, provided they are adequately maintained. [ABSTRACT FROM AUTHOR]

The objective of this research project was to compare two stormwater management strategies within a nutrient-sensitive watershed: impervious cover limits versus pollutant-load regulations. A case study was conducted in the nutrient-sensitive Falls Lake watershed in North Carolina, USA, where a commercial fitness complex was constructed in a zone previously restricted to low-density housing. The Falls Lake watershed has a stormwater regulation that limits total nitrogen and total phosphorus export loads to 2.47 kg/ha/yr and 0.37 kg/ha/yr, respectively. Hydrology and water quality were monitored pre- and post-development to quantify changes to stormwater volumes, pollutant concentrations, and annual export loading rates. On-site stormwater control measures (SCMs) reduced nutrient export loading rates below the regulatory standard. However, increased stormwater volumes and nutrient export loading rates were observed from pervious surfaces that were disturbed during construction (total nitrogen increased from 2.06 to 4.24 kg/ha/yr, total phosphorus increased from 0.41 to 0.73 kg/ha/yr). Results from this case study suggest that (1) impervious cover limits do not adequately account for a parcel's nutrient export loads and (2) SCMs that reduce volume and treat pollutants can reduce nutrient export loads below regulatory levels in the Falls Lake watershed. [ABSTRACT FROM AUTHOR]

Across the globe, water quality standards have been implemented to protect receiving waters from stormwater pollution, motivating regulators (and consequently designers) to develop tools to predict the performance of stormwater control measures such as constructed stormwater wetlands (CSWs). The goal of this study was to determine how well the relaxed tanks-in-series (P-k-C*) model described the performance of CSWs in North Carolina. Storm events monitored at 10 CSWs in North Carolina were used for calibrating the model, and statistical evaluations concluded the model could adequately predict the performance for all pollutants except organic nitrogen. Nash-Sutcliff calibration/validation values were determined to be 0.72/0.78, 0.78/0.74, 0.91/0.87, 0.72/0.62, 0.88/0.73, and 0.91/0.63 for total nitrogen, total ammoniacal nitrogen, oxidized nitrogen, organic nitrogen, total phosphorus, and total suspended solids, respectively. Sensitivity analysis revealed only one calibration parameter with strong sensitivity, the Arrhenius coefficient (temperature dependent model coefficient). With this model, CSWs can be optimized to treat watershed-specific influent concentrations to meet effluent targets. In general, the current design technique used in North Carolina and many other locations (a first flush volume detention method) oversizes CSWs for water quality vis-à-vis the method herein, suggesting improved designs for water quality may be possible through scientifically-informed methods. [ABSTRACT FROM AUTHOR]

Permeable pavements allow stormwater to pass through the pavement surface, filtering out sediment and debris; over time, regular preventative maintenance will be needed to maintain the pavement surface infiltration rate (IR). IR testing is commonly used to determine maintenance needs and frequencies. ASTM standard methods may be used to measure permeable pavement IR; however, these tests can take hours to complete and require infiltrometers not readily available to maintenance contractors. A simple infiltration test (SIT) has been devised which (1) is conducted using easily acquired materials, (2) has a larger surface area (i.e., more representative of average pavement conditions), and (3) requires, on average, 72% less time to conduct than the ASTM test. ASTM and SIT methods were compared by conducting a total of 873 IR tests at the same locations on 12 permeable pavements in North Carolina, Ohio, and Sweden. Results showed that (1) a segmented linear relationship related SIT and ASTM-measured IRs; (2) the SIT and ASTM tests predicted approximately the same IR up to 250 mm/min; (3) the larger surface area of the SIT reduced the variability in measurements (average 40% less) compared to the ASTM method. The SIT took one-quarter the time to run, on average, making this newly-devised tool more efficient when assessing IR than ASTM methods, potentially saving maintenance personnel time and money. [ABSTRACT FROM AUTHOR]

Thermally-enriched stormwater runoff can negatively impact coldwater streams and their associated ecosystem services. The introduction of local and federal guidelines specifically targeting urban runoff temperature in the United States and Canada promulgate the use of practices that reduce thermal pollutant load from a catchment. Several stormwater control measures (SCM) have been shown to buffer thermal impacts to waterways by passing water through cooler subsurface media or by reducing overall discharge volume. Permeable pavement exposes stormwater to these mechanisms, which makes it a promising practice for thermal mitigation. A newly constructed permeable interlocking concrete paver (PICP) parking lot was monitored for 1 year in the mountain region of North Carolina to quantify runoff reduction, temperature buffering, and thermal load export. The effects on hydrology caused by internal water storage (IWS) in the pavement profile were also investigated by dividing the lot into three cells with varying aggregate depths and drainage configurations. Results showed substantial stormwater volume reductions from all permeable pavement configurations, with the most pronounced reductions (99.6 and 100%) experienced in the shallow and deep IWS cells, respectively. Thermal loads from the conventionally drained cell, deep IWS cell, and shallow IWS cell were consequently reduced proportionally to rates of runoff reduction. Median stormwater effluent temperature from the conventionally drained cell (78% runoff reduction) exceeded the critical trout threshold temperature of 21°C; however, outflow only occurred during eight storm events (out of the 54 recorded) and exceeded 21°C for 10.5 h of the stream temperature monitoring period (4.7 h more than ambient stream conditions). Effluent from the two cells with IWS never exceeded trout threshold temperatures because of retention and exfiltration of virtually all incoming stormwater. Temperature differentials between the top and bottom of the PICP profiles suggest that buffering of extreme runoff temperatures can occur. The subsoil interface never reached freezing temperatures during the unseasonably cold winter months, suggesting that frost heaving of permeable pavement systems is not expected in similar climes. These results merit the use of PICP and perhaps other forms of permeable pavement, particularly with the incorporation of IWS, where thermal impact to streams is of concern. [ABSTRACT FROM AUTHOR]

Forty-three bioretention cells (BRCs) throughout North Carolina were assessed for maintenance needs, soil media composition, and as-built surface storage volume to determine whether BRCs are typically constructed per their intended design specifications. Visual inspections identified hydric soils and redoximorphic features (indicators of prolonged anaerobic conditions) and quantified the occurrence of issues requiring maintenance. Particle-size distribution and permeability of soil media were measured at multiple locations within each cell, and topographic surveys were performed to calculate as-built surface storage volumes. Field drawdown tests were used to verify permeability measurements and assess overall BRC function. Of the studied cells, 22% contained redoximorphic features in the upper 30 cm, and 53% were in need of maintenance due to one or more factors. Although 71% of assessed cells failed to meet soil media particle-size distribution specifications, 98% of BRCs contained media with adequate or high permeability. Over half (65%) of cells were undersized vis-à-vis intended design, with 53% exhibiting moderately or severely undersized surface storage capacity. Discrepancies in average ponding depth and incorporation of inspection/maintenance programs significantly influenced the accuracy of as-built BRC storage volume; therefore, it is important for inspectors to verify that the bed elevations of BRCs have been accurately graded. Despite being typically undersized, it was concluded that most observed bioretention cells are meeting the hydrologic goals of North Carolina standards. [ABSTRACT FROM AUTHOR]

The thousands of kilometers of highways in North Carolina have the potential to generate large amounts of storm-water runoff. Thus, investigation of storm-water control measures (SCMs) for these somewhat unique linear catchments, where space is limited for SCM implementation, was needed. This study examined the quantity and quality of highway runoff at four sites over a 48-km stretch of Interstate 40 in the coastal plain of North Carolina. The highway had a 4-cm overlay of permeable asphalt, known as permeable friction course (PFC), which influenced the export of sediment-bound pollutants and produced median effluent concentrations of total suspended solids (TSS) of 8 mg/L, 8 mg/L, 9 mg/L, and 17 mg/L at the four sites, well below concentrations observed from standard asphalt highway runoff. Two vegetative filter strips (VFSs), two traditional dry swales, and two wetland swales were also tested for pollutant removal efficacy at the four highway research sites. The filter strips generally produced higher pollutant concentrations than the edge of pavement due to a lack of vegetative cover, substantial soil compaction, and high slopes. Total nitrogen (TN) median effluent concentrations were significantly lower for wetland swales (1.02 mg/L and 1.03 mg/L) than for dry swales (1.50 mg/L and 1.63 mg/L). TN load export appeared lower from wetland swales when compared with dry swales, potentially due to the greater number of nutrient removal mechanisms that are present in wetland swales. Maintenance of highway SCMs was shown to be extremely important, as one of the dry swales produced an effluent TSS concentration of 70 mg/L due to a head cut in the thalweg of the channel. As noted in other scientific literature, vegetative SCMs did not further reduce pollutant concentrations released by the PFC because TSS and sediment-bound pollutant concentrations seemingly were at or near irreducible concentrations. This study showed that incorporating wetland elements in the design of swales and the use of PFC should be considered by roadway designers. [ABSTRACT FROM AUTHOR]

Storm water ocean outfalls discharging into recreational waters pose a human health threat because of increased potential exposure to bacteria and other pathogens. The dune infiltration system (DIS) was designed and implemented at two ocean outfall sites in response to concerns by the North Carolina Department of Transportation and the town of Kure Beach, North Carolina The systems were designed to divert storm water runoff from 1.9 ha (4.7 acre) and 3.2 ha (8.0 acre) watersheds into the beach dunes. Following construction, data were collected from 25 storms during March through October 2006. The systems captured a combined total of nearly 1,800 m3 (63,500 ft3), or 95% of the influent storm water runoff-a significant reduction of runoff volume and peak flow discharging directly onto the beach (p<0.0001). Fecal coliform and enterococci concentrations were measured in the inflowing storm water runoff and groundwater downslope of the systems. Both groundwater bacteria concentrations near the systems were significantly lower than the bacteria concentrations in the inflowing storm water (p<0.001). Furthermore, groundwater fecal coliform concentrations after implementing the DISs were statistically similar to preconstruction levels (p<0.05). The initial results are promising, and the system should be considered for more widespread use. However, further comprehensive research is recommended to more thoroughly understand the viability of the DIS as a storm water best management practice and the fate and transport of the bacteria within the dunes. [ABSTRACT FROM AUTHOR]

n urban watershed in Raleigh, NC, was evaluated for Escherichia coli ( E. coli), fecal coliform, enterococci, and total suspended solids (TSS) over 20 storm events. Sampling procedures allowed collection of multiple discrete samples per event, resulting in a relatively detailed description of microbe and TSS export for each storm. Data were evaluated to determine if a first flush effect was present for indicator bacteria and TSS in stormwater runoff. Analyses suggested there was a significant first flush effect for fecal coliform and TSS, although the first flush effect for fecal coliform was relatively weak. For E. coli and enterococci, no significant first flush effect was noted. Overall, the first flush effect was not always present for indicator bacteria and, if present, tended to be weak. The first flush effect for TSS was substantially stronger than that of any indicator bacteria. Further analysis showed poor correlation between first flush strength and antecedent climate variables, storm characteristics, and flow characteristics. However, seasonal differences for first flush strength were noted. Specifically, winter storms showed a stronger first flush effect for all indicator bacteria. The results of this study indicate that stormwater runoff presents a potential public health hazard due to elevated indicator bacteria levels for all portions of the storm event. Further, stormwater management practices cannot be expected to treat proportionally more indicator bacteria when sized for the water quality event. Instead, removal will simply be a function of a management practice's volume capture and microbe sequestration efficiency. [ABSTRACT FROM AUTHOR]

Abstract: Effluent organic nitrogen concentrations from seven constructed stormwater wetlands in North Carolina were examined to compare background organic nitrogen (ON) concentrations and the fraction of organic nitrogen relative to total nitrogen discharged. Seasonal influences on organic nitrogen concentrations were also examined. The median ON concentration from the stormwater wetlands was 0.78mgl−1, and despite differences in wetland design and influent ON characteristics, outlet ON concentrations from all but one wetland were not significantly different. ON export from all stormwater wetlands was significantly less than untreated runoff entering the wetlands (p =0.002). In addition, median organic:total nitrogen (ON:TN) ratios from stormwater wetlands (0.75) were significantly greater than from untreated urban runoff (0.66), comparing more closely to ON:TN ratios collected from a naturally occurring wetland and reported in the literature for natural landscapes. Seasonal differences in organic nitrogen concentrations were identified with significantly lower concentrations during the winter. Though stormwater wetlands will not (and perhaps should not be expected to) completely remove total nitrogen loads from runoff, these results suggest constructed wetlands can play a role in restoring the balance between organic and inorganic nitrogen forms closer to that of an undisturbed landscape. The presence of background organic nitrogen concentrations from stormwater wetlands similar to those from a naturally occurring wetland highlights the importance of choosing appropriate metrics (e.g., effluent concentrations) when assessing treatment performance. [Copyright &y& Elsevier]

Bioretention, a key structural practice of low impact development (LID), has been proved to decrease peak flow rates and volumes, promote infiltration and evapotranspiration, and improve water quality. Exactly how well bioretention mimics predevelopment (or 'natural') hydrology is an important research question. Do bioretention outflow rates mirror shallow groundwater interevent stream recharge flow associated with natural or nonurban watersheds? Streamflow from three small, nonurban watersheds, located in Piedmont, part of central North Carolina, was compared with bioretention outflow from four cells also in North Carolina's Piedmont region. Each benchmark watershed drained to a small stream, where flow rate was monitored for an extended period of time. After normalizing the flow rates and volumes by watershed size, data were combined to form two data sets: bioretention outflow and stream interevent flow. Results indicate that there is no statistical difference between flow rates in streams draining undeveloped watersheds and bioretention outflow rates for the first 24 h following the commencement of flow. Similarly, there is no statistical difference between the cumulative volumes released by the two systems during the 48 h following the start of flow. These results indicate that bioretention cells behave comparably to watersheds in natural or nonurban conditions, with respect to both flow rates and flow volumes, and suggest that bioretention outflows may mirror post-storm event shallow groundwater interevent stream recharge flow. Solely considering bioretention outflow as a conjugate to runoff may be a misinterpretation of a flowrate that actually resembles shallow interflow. [ABSTRACT FROM AUTHOR]

Storm-water best management practices (BMPs) are typically assessed using the performance metric of pollutant concentration removal efficiencies. However, debate exists whether this is the most appropriate metric to use. In this study, a storm-water wetland constructed and monitored in the coastal plain of North Carolina is evaluated for water quality and hydrologic performance using four different metrics: concentration reduction, load reduction, comparison to nearby ambient water quality monitoring stations, and comparison to other wetlands studied in North Carolina. The River Bend storm-water wetland was constructed in spring 2007 and was monitored from June 2007 through May 2008. Twenty-four hydrologic and 11 water quality events were captured and evaluated. The wetland reduced peak flows and runoff volumes by 80 and 54%, respectively. Reductions were significant. Concentrations for the following pollutants increased: total kjeldahl nitrogen (TKN), NH4-N, total nitrogen (TN), and total suspended solids (TSS); inflow and outflow concentrations did not change for total phosphorus (TP), while only NO2-3-N and orthophosphorus (OP) concentrations were lower at the outlet. Using a load reduction metric, results were strikingly different, showing positive load reductions of 35, 41, 42, 36, 47, 61, and 49% for these respective pollutants: TKN, NO2-3-N, NH4-N, TN, TP, OP, and TSS. When comparing the effluent concentrations from the wetland to ambient water quality in the Trent River, all effluent nitrogen species concentration were either similar or lower. TP and TSS concentrations leaving the wetland were higher than ambient water quality data. Finally, by comparing pollutant concentrations among different North Carolina wetlands, it is apparent the River Bend wetland received relatively 'clean' water and released water with pollutant concentrations comparable to all other studies examined. Major conclusions from this study include: (1) storm-water wetlands sited in sandier soils (such as those of the North Carolina coastal plain) should be considered a low impact development tool and (2) the selection of performance metric has a pronounced bearing on how a BMP's performance is perceived. Sole reliance on a concentration reduction metric is discouraged. [ABSTRACT FROM AUTHOR]

With increasing development in areas of trout sensitive waters, the effect of urban storm-water runoff temperature on the aquatic ecosystem has become a concern. A study was conducted in western North Carolina, along the southeastern extent of U.S. trout populations, to determine the effect of storm-water wetlands and wet ponds on the temperature of urban storm-water runoff. Measurements included temperature at the inlets, outlets, and at several depths within the best management practices (BMPs). Parking lot runoff temperatures were significantly higher than the 21°C temperature threshold for trout during peak summer months and water temperatures consistently increased from the inlet to the outlet in the storm-water wetland and wet pond, implicating these BMPs as sources of thermal pollution. Despite similar inflow temperatures, effluent temperatures from the wet pond were significantly warmer than those from the storm-water wetland for the period from June to September. Substantial cooling was observed as runoff was conveyed from the parking surface to the BMPs through buried pipes, which could be incorporated into BMP design to achieve thermal pollution mitigation goals. Temperatures at the bottom of the water columns were cooler than water leaving the current outlet structures, providing support for the installation of modified outlet structures in regions with cold water fisheries. [ABSTRACT FROM AUTHOR]

Abstract: Recent severe droughts, concerns over the environmental impact of stormwater runoff and increased water demands have generated interest in rainwater harvesting systems in humid, well developed regions, such as the southeastern United States. In order to evaluate the use of rainwater harvesting systems in this region, a monitoring study was conducted at three rainwater cisterns in North Carolina, a computer model was developed to simulate system performance, and simulations were conducted for 208l rain barrels and larger cisterns. Results of the monitoring study showed that the rainwater harvesting systems were underutilized, which was suspected to result from poor estimation of water usage and public perception of the harvested rainwater. The computer model simulated system performance by evaluating a water balance using historical rainfall data and anticipated usage. Simulation results showed that a rain barrel was frequently depleted when used to meet household irrigation demands and overflowed during most rainfall events. Simulations also illustrated the improved performance of large systems while providing an indication of diminishing returns for increased cistern capacity. [Copyright &y& Elsevier]

A 1 year-old parking lot in eastern North Carolina consisting of four types of side-by-side permeable pavement and standard asphalt was monitored from January 2007 to July 2007 for water quality differences among pavement types. The four permeable sections were pervious concrete (PC), two different types of permeable interlocking concrete pavement (PICP) with small-sized aggregate in the joints and having 12.9% (PICP1) and 8.5% (PICP2) open surface area, and concrete grid pavers (CGP) filled with sand. The site was located in poorly drained soils, and all permeable sections were underlain by a crushed stone base with a perforated pipe underdrain. Composite, flow-weighted samples of atmospheric deposition and asphalt runoff were compared to those of permeable pavement subsurface drainage for pH, TN, NO2,3–N, TKN, NH4–N, and ON concentrations and loads. All pavements buffered acidic rainfall pH (p<0.01). The pH of permeable pavement subsurface drainage was higher than that of asphalt runoff (p<0.01) with the PC cell having the highest pH values (p<0.01). Permeable pavement subsurface drainage had lower NH4–N(p<0.01) and TKN concentrations than asphalt runoff and atmospheric deposition. With the exception of the CGP cell, permeable pavements had higher NO2,3–N concentrations than asphalt (p<0.01), a probable result of nitrification occurring within the permeable pavement profile. The CGP cell had the lowest mean TN concentrations; however, results were not significantly different from those of asphalt runoff. The possible nitrogen removal exhibited by the CGP cell is similar to that observed in sand filter research, not surprising considering CGP contained a 10 cm (4 in.) sand bedding layer. Overall, different permeable pavement sections performed similarly to one another with respect to water quality, with the CGP cell appearing to best improve storm-water runoff nitrogen concentrations. [ABSTRACT FROM AUTHOR]

Storm-water experts agree that the currently used best management practice (BMP) percent removal methodology metric has many flaws, and some have suggested using a BMP effluent concentration metric. This case study examines establishing an effluent target concentration for BMPs that relates to the health of macroinvertebrates in the receiving water. In North Carolina, 193 ambient water quality monitoring stations were paired with benthic macroinvertebrate health ratings collected in very close proximity. Water quality for the sites ranged from excellent to poor and was divided into three distinct ecoregions: Mountain, Piedmont, and Coastal. Statistically significant relationships were found in one or more ecoregions for dissolved oxygen, fecal coliform, NH3, NO2-3-N, total Kjeldahl nitrogen, total nitrogen (TN), and total phosphorus (TP). BMPs can then be selected and designed to meet these target effluent concentrations. Based upon this research, a development, and therefore set of BMPs, in Piedmont North Carolina could be required to release TN and TP effluent concentrations of 0.99 mg/L and 0.11 mg/L, respectively. These concentrations are both associated with “good” benthos health. The new method was most effective in the Piedmont ecoregion, however with more data collection, the Mountain and Coastal ecoregions may also benefit. [ABSTRACT FROM AUTHOR]

The objectives of this study were to characterize asphalt parking lot runoff quality and determine factors influencing nutrient concentrations and loads. Event mean concentrations (EMCs) and loads were measured from eight asphalt parking lots in North Carolina using automated flow meters and rain gauges. The number of water quality samples collected varied from 11 to 26 per site. EMCs and loads were statistically analyzed for six nutrient forms: total nitrogen, total Kjeldahl nitrogen, ammonia-nitrogen, nitrate-nitrogen, total phosphorus, and ortho-phosphate. The mean EMCs (in mg/L) were 1.57, 1.19, 0.32, 0.36, 0.19 and 0.07, respectively. Nitrogen species’ concentrations were slightly lower than those from highway runoff found in the literature; whereas, phosphorus EMCs were similar to those in highway runoff. Current load prediction models, generally based on highway or roadway nutrient concentrations, are therefore expected to over-estimate nitrogen loads from asphalt parking lots. Spring and summer presented the highest EMCs and loads, respectively. Significant seasonal differences in concentration (p<0.05) were found mainly between spring and the other three seasons, while loads in summer differed from those of fall and winter. In an attempt to determine the factors affecting EMCs and loads, Pearson correlation tests and multiple linear regression analyses were performed. Strong correlations were found among the variables of each group of factors referred to as climate, physical characteristics and surrounding land use. Rainfall depth, catchment area, the percentage of asphalt and natural surrounding land use were good predictors of nutrient concentrations and loads. [ABSTRACT FROM AUTHOR]

As an increasingly adopted storm water best management practice to remedy hydrologic impairment from urban imperviousness, bioretention facilities need rigorous field performance research and monitoring to confirm performance and improve design and maintenance recommendations. This study investigated hydrologic performance at six bioretention cells in Maryland [College Park (CP), a 181 m2 cell, 50–80 cm media depth, monitored for 22 events, and Silver Spring (SS), a 102 m2 cell, 90 cm media depth, monitored for 60 events] and North Carolina [Greensboro (G1 and G2), each approximately 317 m2, 120 cm media depth, both monitored for 46 events, and Louisburg (L1=surface area of 162 m2, L2=surface area of 99 m2); each had 50–60 cm fill depths, monitored for 31 and 33 events, respectively] over 10–15 month periods. Outflow from each cell was recorded and inflow was either recorded or calculated from rainfall data. In Louisburg, L2 was lined with an impermeable membrane to eliminate exfiltration while L1 was unlined to allow both exfiltration and evapotranspiration. Results indicate that bioretention facilities can achieve substantial hydrologic benefits through delaying and reducing peak flows and decreasing runoff volume. A large cell media volume: drainage area ratio, and adjustments to the drainage configuration appear to improve the performance. Media layer depth may be the primary design parameter controlling hydrologic performance. Performance diminishes as rainfall depths increase and rainfall durations become longer. Annual water budget analysis suggests that approximately 20–50% of runoff entering the bioretention cells was lost to exfiltration and evapotranspiration. [ABSTRACT FROM AUTHOR]

A permeable pavement parking lot in eastern North Carolina consisting of four types of permeable pavement and standard asphalt was monitored from June 2006 to July 2007 for hydrologic differences in pavement surface runoff volumes, total outflow volumes, peak flow rates, and time to peak. The four permeable sections were pervious concrete (PC), two types of permeable interlocking concrete pavement (PICP) with small-sized aggregate in the joints and having 12.9% (PICP1) and 8.5% (PICP2) open surface area, and concrete grid pavers (CGP) filled with sand. The site was located in poorly drained soils, and all permeable sections were underlain by a crushed stone base layer with a perforated underdrain. All permeable pavements significantly reduced surface runoff volumes and peak flow rates from those of asphalt (p<0.01). Of the permeable pavements, CGP generated the greatest surface runoff volumes (p<0.01). The PICP1 and CGP cells generated significantly lower total outflow volumes than all other sections evaluated (p<0.01), and had the lowest peak flows and the longest time to peak. The response of the PICP1 cell was likely due to an increased base storage volume resulting from an elevated pipe underdrain whereas the CGP cell response was attributed to water retention in the sand fill layer. Overall, different permeable pavement sections performed similarly, but were substantially different from asphalt. Subtle differences in the performance of CGP were primarily due to the characteristics of the sand filled media compared to small aggregate typically used in PC and PICP joints and bedding. [ABSTRACT FROM AUTHOR]

Bioswales are a promising stormwater control measure (SCM) for roadway runoff management, but few studies have assessed performance on a field scale. A bioswale is a vegetated channel with underlying engineered media and a perforated underdrain to promote improved hydrologic and water quality treatment. A bioswale with a rip-rap lined forebay was constructed along state highway NC 211 in Bolivia, North Carolina, USA, and monitored for 12 months. Thirty-seven of the 39 monitored rain events exfiltrated into underlying soils, resulting in no appreciable overflow or underdrain volume. The bioswale completely exfiltrated a storm event of 86.1 mm. The one event to have underdrain-only flow was 4.8 mm. The largest and third-largest rainfall depth events (82.6 and 146 mm, respectively) had a large percentage (85%) of volume exfiltrated, but also had appreciable overflow and underdrain volumes exiting the bioswale, resulting in no peak flow mitigation. Overall, this bioswale design was able to capture and manage storms larger than the design storm (38 mm), showing the positive hydrologic performance that can be achieved by this bioswale. The high treatment capabilities were likely due to the high infiltration rate of the media and the underlying soil, longer forebay underlain with media, gravel detention layer with an underdrain, and shallow slope. [ABSTRACT FROM AUTHOR]

Bridge deck runoff sometimes directly discharges through deck drains to water bodies. As such, the runoff is usually not treated; however, recent pressures have led Departments of Transportation to install closed pipe drainage systems beneath bridges to deliver stormwater to a stormwater control measure (SCM). This can be costly both in terms of up-front and long-term maintenance capital. This study compared bridge runoff concentrations of nutrients, sediment, and heavy metals to effluent concentrations from six commonly used SCMs. Runoff quality samples from 15 bridges in North Carolina were collected and compared to those from 41 different SCMs across North Carolina. The SCMs examined in this study were permeable friction course (PFC) overlays, wet retention ponds (WP), bioretention cells (BRC), vegetated filter strips (VFS), constructed stormwater wetlands (CSW), and grassed swales (GS). Bridge deck runoff concentrations were not statistically different from SCM effluent concentrations for total nitrogen (TN). For total phosphorus (TP), all SCMs produced effluent concentrations lower than bridge runoff concentrations, although only PFC, BRC, and WP did so significantly. For total suspended solids (TSS), median effluent concentrations from the SCMs were significantly and substantially lower (a difference of more than ) than those from bridges. Comparison against water quality threshold concentrations developed for North Carolina suggested that BRC and WP were best for TN treatment and that PFC, WP, and BRC were appropriate for TP treatment. For TSS, all six SCMs were capable of improving the bridge runoff. Similar results were observed for copper, lead, and zinc; BRC, GS, and CSW were able to reduce total metals concentrations significantly. Dissolved metal concentrations appeared difficult to reduce with current SCM technology. These results suggest that for certain pollutants, treatment of bridge runoff may yield improvement. However, the appropriateness of installing SCMs to treat bridge deck runoff must account for the increased cost of closed pipe drainage systems beneath bridges and limited space in the right-of-way and weighed against the relative ease of retrofitting stormwater treatment infrastructure into other transportation corridors. [ABSTRACT FROM AUTHOR]

• Permeable pavement (PP) hydrologic performance varies widely. • Long-term model needed to understand how design affects the PP water balance. • DRAINMOD, an agricultural drainage model, was tested herein for PP modeling. • Model calibrated to field-collected hydrologic data from four PP systems. • Model effective for long-term modeling of PP hydrology, not single storm events. The hydrologic performance of permeable pavement varies widely due to underlying soil type, drainage configuration, contributing drainage area, surface infiltration rate, and aggregate depth. A long-term hydrologic model is needed to better understand the influence of these design variables on surface runoff, drainage, exfiltration, and evaporation from permeable pavement systems. Most permeable pavement models have not been calibrated with field monitored data, are usually unable to accurately model internal water storage (IWS) zones, and do not account for evaporation from the aggregate profile. Because permeable pavement employs drainage and exfiltration as primary hydrologic mechanisms, it was hypothesized that DRAINMOD, a model shown to accurately simulate bioretention hydrology, could be calibrated to predict the hydrologic response from permeable pavements. Hydrologic data were collected from two permeable pavement applications in North Carolina (Boone and Durham) and two permeable pavement applications in Ohio (Perkins Township and Willoughby Hills) to calibrate and test the model. The specific permeable pavements applications varied widely in aggregate depth, drainage configuration, underlying soil type, and contributing drainage area. Nash-Sutcliffe Efficiencies for drainage ranged from 0.77 to 0.95 during calibration and validation of all sites. Nash-Sutcliffe Efficiencies for exfiltration/evaporation ranged from 0.55 to 0.97 for Boone and Perkins Township, but prediction of exfiltration/evaporation volumes on an event basis was poor for the remaining two sites due to low exfiltration rates (less than 0.20 mm/hr). Despite this, the cumulative volume of exfiltration/evaporation was predicted to within 1–9 % of what was measured during monitoring. Modeled and monitored surface runoff from the Willoughby Hills application (8 % of the water balance) was equivalent. The model predicted the percentage of drainage, surface runoff, and exfiltration/evaporation to within 2 % of what was monitored/estimated at each site, suggesting long-term modeling of permeable pavement hydrology with DRAINMOD is viable. [ABSTRACT FROM AUTHOR]

Abstract: Nutrient removal by stormwater control measures (SCMs) is typically reported in terms of an average annual percent removal or effluent concentration. However, when the performance of these systems is considered in light of downstream receiving aquatic ecosystems, which display seasonal sensitivities to nutrient loadings, the use of an annual-based metric seems arbitrary. To investigate the potential temporal mismatch between nutrient-sensitive periods in receiving water bodies and average annual reporting periods adopted for SCM performance metrics, a case study is presented for four SCM types (constructed stormwater wetlands, bioretention, vegetated filter strips, and swales) draining to the Neuse River Estuary (NRE) in North Carolina, USA. Outbreaks of harmful algal blooms (HABs) in the NRE have been related to different nutrient forms at different times of the year, resulting in a “window” of importance for a given nutrient. These windows were utilized herein to define seasons of interest for various pollutants, and thus how SCM effluent data should be grouped to evaluate seasonal differences in performance. Effluent SCM nutrient concentrations were analyzed on an annual and HAB-sensitive seasonal basis. Although the use of annual performance metrics was deemed either appropriate or conservative for total phosphorus and nitrate–nitrite, effluent concentrations of total ammonia–nitrogen were significantly higher during HAB-sensitive seasons from both bioretention areas and stormwater wetlands. In the case of the NRE, these data suggest SCMs such as bioretention and stormwater wetlands may perform less effectively during periods of HAB sensitivity and that the contribution of SCMs to HAB control in sensitive water bodies may be overstated using presently accepted annual evaluation metrics. Though the seasonal analysis presented is specific to HAB formation in the Neuse River Estuary, evaluation of SCM effluent nutrient concentrations on the basis of nutrient-sensitive periods in receiving water bodies has broad application to evaluation of SCMs in any nutrient-sensitive watershed. The present study suggests that evaluation of SCM performance with respect to nutrient-sensitive periods in receiving aquatic ecosystems warrants further study. [Copyright &y& Elsevier]

Regenerative stormwater conveyance (RSC) is a recently developed stormwater control measure that marries the concepts of bioretention and stream restoration. RSC mitigates stormwater runoff by converting surface flow to subsurface seepage using a series of pools and riffles built over a sand media bed. Subsurface seepage flows through media and exits the RSC beneath the outlet weir. Previous studies on RSC pollutant mitigation have focused on surface flow discharges from the RSC. To date, no known research has been conducted on the potential pollutant contributions of RSC seepage, despite the fact that this water also enters receiving waters. This research used Multi-Point Sampling coupled with in-situ ultraviolet-visual spectroscopy to measure nitrogen in seepage during simulated storm events (n = 9) at a field-scale RSC in Raleigh, North Carolina. Calibrations between light absorbance and concentrations were acceptable (Nash-Sutcliffe coefficient > 0.65) for nitrate and total ammoniacal nitrogen (TAN) and very good (Nash-Sutcliffe coefficient > 0.90) for total Kjehdahl nitrogen (TKN). Early storm simulations revealed some initial nutrient flushing from the substrate, which subsided by the third simulation. Overall, subsurface seepage nitrate, TAN, and TKN concentrations were lower by 29%, 57%, and 4% relative to storm inflow concentrations, respectively. Computed subsurface nitrogen concentrations demonstrated temporal variability, highlighting dynamic transport and biogeochemical transformations in saturated and unsaturated conditions. Nitrogen concentrations were lower in seepage than in surface flow; however, due to the high volume of runoff converted to seepage, nitrogen loads discharged in seepage can be larger than those of surface flow. Further research is needed to examine subsurface pollutant reductions under varying hydrologic and seasonal conditions., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

Stormwater control measures (SCMs) such as constructed stormwater ponds and constructed stormwater wetlands (CSWs) are designed to regulate runoff hydrology and quality. However, these created ecosystems also provide a range of other benefits, or ecosystem services, which are often acknowledged but rarely quantified. In this study, additional ecosystem services, including carbon sequestration, biodiversity, and cultural services, were assessed and compared between 20 ponds and 20 CSWs in North Carolina, USA. Carbon sequestration was estimated through the carbon content of pond and wetland sediments across a gradient of system age. Biodiversity was quantified in terms of the richness and Shannon diversity index of vegetative and aquatic macroinvertebrate communities. Cultural services were qualitatively assessed based on the potential for recreational and educational opportunities at each site. Ponds and wetlands were found to support similar levels of macroinvertebrate diversity, though differences community composition arose between the two habitat types. CSWs demonstrated greater potential to provide carbon sequestration, vegetative diversity, and cultural ecosystem services. This assessment provides an initial framework upon which future assessments of ecosystem service provision by SCMs can build., (Copyright © 2011 Elsevier Ltd. All rights reserved.)