Stormwater biofilters are being implemented widely in urban environments to provide green space, alleviate flooding, and improve stormwater quality. However, biofilters with conventional media (sand, soil, and/or mulch or compost) do not reliably remove contaminants from stormwater. Research suggests addition of biochar to the biofilter media can improve the pollutant removal capacity of biofilters. In the current work, we present a systematic review of laboratory and mesocosm studies of biochar-augmented biofilters and an assessment of watershed-scale implementation of biofilters on local water quality. A full text review of 84 papers was conducted; of these, data were extracted from the 14 that met our inclusion criteria. log₁₀ removal of microbial pollutants and trace organic contaminants (TOrCs) by biochar-augmented media is generally greater than those of the controls containing just sand, soil, and/or compost. log₁₀ removal of nitrogen, phosphorous, total organic carbon, and total suspended solids in biochar-augmented biofilters is not clearly higher than those of control experiments. A supplemental analysis of four studies reporting longer-term breakthrough data revealed that TOrC removal effectiveness varies substantially among high temperature wood-based biochars, and that operational lifetimes of full-scale systems constrained by TOrC sorption capacity could range from five months to over seven years depending on the selected biochar. At the watershed-scale, biochar-augmented biofilters can provide enhanced treatment of runoff, resulting in the need for fewer treatment units or a smaller volume of watershed runoff treated to meet water quality criteria compared to their conventional counterparts. While their installation can reduce the load of pollutants to receiving waters, achieving concentration-based water quality targets may prove difficult even when pollutant removal capacity is high. This work highlights the importance of a systems approach to studying how biofilter installation affects water quality within a watershed. We identify several topical areas where further research is needed, especially as installation of biofilters and other stormwater control measures gain popularity in highly urbanized watersheds.

Integrated decision support tools are needed to investigate the tradeoffs of stormwater control measures (SCMs) and determine the optimal suite of SCMs based on the needs of watersheds. In this study, an urbanized watershed undergoing infill development (the Berkeley neighborhood located in Denver, CO, USA) was modeled using a modified version of the U.S. Environmental Protection Agency’s (EPA) System for Urban Stormwater Treatment and Analysis IntegratioN (SUSTAIN). The primary goal was to compare the relative performance between green and grey SCMs, use optimizations and a planning-level approach to assist in decision-making, and discuss how stakeholder and community preferences can shift which SCMs are optimal for the watershed. Green and grey SCMs have variable hydrologic performance based on design and function, and both offer benefits that may be important to decision makers. Our results showed that infiltration trenches and underground infiltration were optimal for reducing flow volumes while vegetated swales and underground detention were optimal for pollutant concentration reduction. Stakeholders value both of these benefits and so the optimal stormwater solution in the Berkeley neighborhood included a mix of green and grey SCMs. Determining the optimal SCMs while considering tradeoffs in costs and associated benefits was complex and multifaceted. Modeling results such as those presented here are critical for informing stakeholders’ decision-making process.

Hydrologic modeling is a crucial tool to understand the physical and hydrologic characteristics that contribute to water quality degradation in urban areas and the effectiveness of stormwater best management practices (BMPs) for meeting subsequent regulatory compliance. In this study, three highly urbanized watersheds, the Ballona Creek (BC), the Dominguez Channel (DC), and the Los Angeles River (LAR), located in Los Angeles, are modeled using the EPA’s System for Urban Stormwater Treatment and Analysis INtegration (SUSTAIN). The primary goal is to identify the key characteristics that contribute to water quality impairment, analyze the efficacy of utilizing specific BMPs to comply with regulatory compliance, and discuss how results can inform optimal stormwater management plans for each watershed. The BC watershed has the least degraded water quality and multiple BMP scenarios result in elimination of exceedances, while the DC watershed has the most degraded water quality and none of the modeled BMP scenarios meet water quality regulations. The LAR watershed is a large, diverse watershed and results show better water quality compliance than the DC watershed despite land use characteristics that would indicate otherwise. Watersheds that reach compliance with multiple BMP types have more available options when considering ancillary benefits. Modeling results such as those presented here are critical for informing stakeholders’ decision-making process and facilitating optimal BMP implementation for integrated water management.

As we move into an era of increased urbanization, stormwater practitioners are charged with creating multibenefit solutions through the installation of stormwater control measures (SCMs). Two drivers facilitate the accrual of benefits in SCMs: hydrologic or water quality process and vegetation. This study investigated the feasibility of incorporating benefits beyond water quality and quantity control into the SCM planning process. A critical review was used to determine which benefits would be assessed with hydrologic and water quality modeling or a complementary conceptual framework. Drawing on common themes in the literature, the conceptual framework of the 4 Cs (community, context, connectivity, and canopy) was created to assess vegetation-based benefits. To demonstrate the coupled benefit assessment, a case study was performed in a neighborhood of Denver, Colorado. Results from hydrologic and water quality modeling show that vegetated swales provide the most preferred solution. From the vegetation-based benefit perspective, we find that the modeled area of the vegetated swales is only 0.2% by area, which is likely too small to have a measurable benefit effect at the neighborhood scale. We show how the 4 Cs can be used to leverage existing greenness to maximize the potential vegetation-based benefit of swales and consider how existing sociodemographic and vegetation trends can be leveraged to make benefit-driven decisions.

Stormwater management is a multiobjective decision-making process. Integrating life-cycle costs (LCC) and a benefit analysis with hydrologic modeling can enhance planning-level analyses by expanding available information for decision makers. In this study, two urbanized watersheds located in Los Angeles, California, and Denver, Colorado, are modeled using an integrated decision support tool for green to grey stormwater infrastructure. The primary goals are to compare capital costs to LCC and to explore the preference of green versus grey solutions using varying management criteria and a hydrologic benefit analysis. Results show that optimizing on capital costs alone produces qualitatively different results than optimizing on LCC, which are substantially higher. Additionally, a shift in relative cost order between stormwater control measure (SCM) types, when comparing capital costs and LCC, may misguide decision makers on which SCMs minimize cost while achieving regulatory compliance. Watershed-specific management criteria relevant to Ballona Creek (Los Angeles) and Berkeley neighborhood (Denver) result in a preference for greener solutions given hydrologic process benefits. The optimal solutions for these basins are determined by evaluating costs to the community and environment, comparing the hydrologic performance of varying SCMs, and ensuring the priorities of the individual watershed are met. Using a framework that integrates stormwater modeling, LCC, and a benefit analysis advances decision-relevant information for urban planners and water resource managers.