Journal of Sustainable Water in the Built Environment

Bioretention is a stormwater control measure commonly used to remove pollutants, including nitrogen (N) and phosphorus (P), from urban stormwater. This study evaluates the impacts of episodic high concentrations of sodium chloride (NaCl) on bioretention N, P, and heavy metals (Cu and Zn) removal performances. Bioretention mesocosm studies were conducted to examine N and P removal efficiencies following periodic 2,000-, 5,000-, and $\mathrm{10,000}\text{-}\mathrm{mg}/\mathrm{L}$ NaCl salt applications (every fifth stormwater event). Episodic washouts of particulate matter (PM), N, and P were observed for all three columns, generally occurring in the second stormwater event after the high-salt event. PM and associated particulate P and N releases were likely due to soil deflocculation, as noted by high concentrations of effluent total suspended solids (TSS) and turbidity. Solvation of organic matter in the presence of chloride and ion exchange with the sodium and chloride ions appear to have contributed to dissolved N and P washouts. Mass export of P was observed for the 2,000- and $\mathrm{5,000}\text{-}\mathrm{mg}/\mathrm{L}$ NaCl columns, and some removal of N and metals was noted. Based on mass balances for N, P, and heavy metals, it is recommended to limit the use of NaCl deicers to prevent long-term P export.

Low participation rates in existing US stormwater utility fee credit programs indicate that the benefits attributed to credits are not being realized, most notably, the financial incentive for private property owners to control stormwater on their sites. This is problematic if credits are to be used to achieve the level of private property participation in green infrastructure (GI) retrofits necessary to effect stream quality improvements in stormwater-impaired urban watersheds. This paper examines economic and policy issues related to the use of stormwater fee credits as a market-based incentive strategy for private commercial property owners to invest in GI retrofits. The City of Charlotte, North Carolina, stormwater user fee and credit program is assessed relative to the direct use and investment value of GI retrofits, and an economically efficient configuration is defined that will provide both the stormwater utility and private commercial property owners with adequate incentive for investment. The results indicate that a stormwater fee and credit combination for a stormwater utility based on the cost of capital and credits for GI retrofits, and credits to property owners equal to the cost of annual maintenance for GI retrofits, can provide adequate incentive for investment by both groups. The fee magnitude and credit structures of several other existing US municipal stormwater programs were evaluated relative to these results. This research can be used to address policy questions regarding the role of stormwater utility fee and credit programs in promoting sustainable urban water infrastructure goals in stormwater-impaired watersheds and in making beneficial connections between the seemingly disparate goals of water quantity and water quality management policies.

Stormwater best management practices (BMPs) are commonly emplaced to remove contaminants to reduce environmental impacts of runoff, in addition to their flood control function. The selection of BMPs for urban catchments is ideally targeted to local contaminants of concern; however, these decisions often fail to consider observed treatment performance. The International Stormwater BMP Database is a large repository of monitoring data that can inform performance estimates. A major goal of this work is to maximize the usefulness of the BMP database to inform data-driven decision making for BMP selection. However, the database currently lacks sufficient BMP design and construction data to parameterize complex models. In lieu of more complex models, this work uses water quality data from the BMP database to develop and evaluate easy-to-use linear models that, given influent concentration, predict effluent concentrations for a selection of BMPs and stormwater contaminants. Four linear models (percent removal, ordinary least squares regression, Theil–Sen robust line, and a linear decay model with an irreducible concentration parameter) are evaluated using percent bias (PBIAS), Nash–Sutcliffe efficiency (NSE), and root mean square error–standard deviation ratio (RMSE RSR) criteria. Results identify ordinary least squares as the best model and expose patterns in BMP categories, with acceptable model criteria for detention basins, grass swales, media filters, retention ponds, and wetland basins. The ordinary least squares model yielded acceptable model criteria for estimates of total suspended solids removal, as well as total phosphorus, nitrate plus nitrite, and total nitrogen. None of the models met “good” criteria thresholds ($\mathrm{PBIAS}<10\%$; $\mathrm{NSE}>0.6$; $\mathrm{RSR}<25\mathrm{th}\mathrm{percentile}$) for more than 40% of BMP-contaminant combinations, but the ordinary least squares model performed best overall. Ultimately, there is a need for more monitoring projects to report BMP design information to facilitate the use of these data in more complex treatment models.

Increasing imperviousness has driven regulation and design philosophies to offset consequent increases in runoff volumes and peak flows. Previous research has shown bioretention to reduce runoff volumes and peak flows. Since most research has focused on newly constructed systems, the long-term performance of bioretention has been questioned. Because bioretention is a biologically based practice, changes over time could impact hydrologic performance. This research examined and compared the hydrologic mitigation performance of three bioretention cells (BRCs) in central North Carolina with postconstruction ages ranging from 8 to 17 years old. Observed runoff volumes were significantly reduced at each of the three cells by 90%, 81%, and 64%. The volume discharge ratio for each cell was at or below low impact development (LID) target thresholds (0.33) for 63%, 67%, and 48% of observed storm events. Similar to volume reduction, all three BRCs significantly reduced peak flows. Peak discharge ratios at each site were less than the LID target threshold (0.33) for over 75% of observed storm events, and the interquartile range of peak discharge ratios was less than the LID target threshold for all observed storm events $<25.4\mathrm{mm}$. All three BRCs struggled to mitigate volumes and peak flows for large storm events ($>50\mathrm{mm}$). As the frequency and magnitude of larger events increases, guidance recommending additional surface storage should be considered. When compared to the hydrologic performance of “young” BRCs (less than 3 years old), “old” BRCs (at least 3 years old) perform at least as well with respect to peak flow mitigation while appearing to reduce runoff volumes better than newly constructed BRCs. That the three BRCs presented herein ranged from 8 to 17 years old during their respective monitoring periods while significantly reducing peak flows and runoff volumes (while meeting LID target thresholds) supports the prediction of long-term hydrologic mitigation of bioretention.

In addition to flooding and water quality management, blue-green infrastructure (BGI) provides multiple benefits to humans and ecosystems, including health and biodiversity. Various tools are available for assessing these benefits but few evaluate economic benefits. Two tools that monetize the benefits, the Benefits Estimation Tool (B£ST) (UK) and The Economics of Ecosystems and Biodiversity (TEEB) (Netherlands), have been used to estimate value for a case study in Luleå, Sweden. Three options for a newly developed area were assessed in comparison with two different baselines. The main economic benefits of the newly developed area were related to amenities, home values, health, and social cohesion rather than to stormwater. However, as a result of the proposed development, negative economic benefits (i.e., costs) were attributed to carbon sequestration and biodiversity when considering the value of the existing area due to a loss of green spaces and trees. B£ST gave higher negative impacts than TEEB. Direct comparison of each category used in each tool was not possible since these categories and the way in which the monetized values are determined in each case differ. While the overall approach used in both tools is applicable in Sweden, calculations and data used need to be adapted to local circumstances and valuation.