Stormwater ponds and biofilters for large urban sites: Modeled arrangements that achieve the phosphorus reduction target for Boston's Charles River, USA

Urban rivers daily receive tons of phosphorus and other pollutants from stormwater generated by impervious surfaces. Constructed detention ponds and biofiltration cells (biofilters) are often effective for localized stormwater treatment, yet less is known about their effectiveness for large built areas. Our goals were to assess stormwater phosphorus-removal relative to total percent cover, number, size, and configuration of detention ponds and biofilters. Two approximately 200-ac. (80 ha) industrial and institutional sites near Boston's Charles River containing diverse smaller drainages, land uses, and runoff sources were analyzed. Using the model WinSLAMM, P-reduction percents were calculated and compared for detention ponds (1–40 per site; covering 5–15% of their drainage areas) and biofilters (two sizes, with and without underdrains; ∼900–4300 per site; 5–10% cover). The government's proposed TMDL target of 65% P-reduction was only achieved with designs that treated 100% of urban land with a pond or biofilter. The 65% target was met by a single pond covering 5% of the site and by several multi-pond or biofilter arrangements with coverage ranging from 5% to 10%. A stringent water quality goal of 75% P-reduction was also attained with certain consolidated and dispersed pond and biofilter designs. Configuration of treatment landscapes appeared to be more important than total treatment area. Results were generally similar for the large institutional and industrial sites. Stormwater P-reduction goals can be creatively met with diverse, realistic land allocations for ponds and biofilters, which also provide enhanced aesthetics, recreation opportunities, and other benefits beyond water quality.