Bioretention and green roofs have become the centerpieces of Low Impact Development (LID) initiatives throughout North America. The well-publicized benefits of these two types of stormwater management practices focus on runoff reduction, stormwater quality treatment, and landscape aesthetics. Promotional literature provided by various regulatory agencies and environmental organizations typically highlights the processes and mechanisms within bioretention and green roofs that provide desirable outcomes. Normally, one finds mention of evapotranspiration, filtering, and adsorption characteristics of the vegetation and soil mix as beneficial mechanisms for the purposes of runoff reduction, particulate and hydrocarbon removal, and dissolved pollutant capture, respectively.
While it is certainly true that the soil matrix within bioretention and green roofs acts as an effective filter for removal of suspended solids and particulate-bound pollutants of concern, the effectiveness for removal of pollutants such as dissolved metals and dissolved phosphorus is much less certain. In fact, several recent high-profile monitoring studies by credible researchers and agencies in the United States and Canada (Washington State University, University of Waterloo, North Carolina State University, Wisconsin Department of Natural Resources and others) have demonstrated considerable leaching of these dissolved pollutants from bioretention and green roofs, particularly in systems where compost comprises a substantial portion of the soil mix. The term “leaching” is applied to circumstances in which the treatment system itself is the source of pollutants that enter the effluent stream.
The natural degradation of compost, leaf litter, vegetation, and other organic materials within bioretention and green roofs will typically release dissolved phosphorus and may also release dissolved metal and nitrogen species. Since it is common for municipal stormwater guidance manuals to specify a compost content of up to 40% of the soil mix, the release of dissolved phosphorus can be a substantial and sustained source of nutrients that are detrimental to the quality of downstream receiving waters. Excessive dissolved phosphorus promotes algae blooms, eutrophication, and degradation of aquatic habitat.
The “dirty little secret” of bioretention and green roof leaching has created substantial alarm in the stormwater regulatory and design community, especially since these practices have been so widely promoted as preferred LID treatment measures. In response, multiple studies have been conducted on bioretention and green roof systems that incorporate phosphorus-adsorptive engineered media as an amendment to the soil mix, or as a downstream polishing filter. These monitoring studies have demonstrated exceptional capture of dissolved phosphorus when soil is amended with as little as 5% phosphorus-adsorptive media. A key advantage of the soil amendment approach is that the engineered media blends easily into the sand portion of the soil mix. This feature demonstrates utility for amended sand filters as well, since sand typically has very low capacity for dissolved phosphorus.
With the success of phosphorus-adsorptive engineered media in bioretention and green roof applications, the actual treatment performance of these practices can now more closely approach the high expectations.