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.

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.

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.