Stormwater carries dissolved and particle associated solid-phase contaminants from impervious surfaces to receiving water bodies, degrading their quality. Pollutants of concern include heavy metals, trace organics, sediments, and nutrients. “Green-infrastructure” infiltration systems of various designs are increasingly being used to provide passive treatment of these waters. Inclusion of plants in these systems provides erosion control, increased infiltration rates, and possible habitats for pollinators. However, plants and their associated microbes vary greatly in their provision of additional degradation, removal, and sequestration of stormwater pollutants. In this study, native and regional plants were investigated for their stormwater treatment capacity. The plants were: tufted hair grass, California oatgrass, spreading rush, hardstem bulrush, slough sedge, dense sedge, Oregon grape, rose checkermallow, salal, and yarrow.
During a 10-week greenhouse study, these plants were investigated for their abilities to treat Cu2+, Zn2+ and polyfluorinated substances (PFASs) from surface runoff collected from a 2-acre public works utility yard. Copper and zinc were naturally present, as were trace concentrations of PAHs and PCBs. Polyfluorinated substances (PFBA, PFDS, PFBS, PFPeA, PFHx, PFOA, PFHpA, PFPeS, PFHxS, PFOS, FHxSA, FOSA, PFNA, PFDA) were spiked into the influent at concentrations ranging from 400 – 1,500 ng/L.
Each plant (n=6) was obtained from a local nursery and transplanted into a one-gallon HDPE bucket with a spigot (also HDPE) drilled into the bottom. Buckets were elevated above 250 mL glass bottles allowing for the collection of the effluent from each planted system. Plants were grown in a 50:50 volumetric mix of sieved sand and local soil from an existing stormwater bioswale, with a gravel drainage layer. These systems were irrigated with tap water for 11 months prior to the start of the experiment. Once per week, plants were watered with stormwater spiked with PFASs. In the influent and effluent, total and dissolved Cu2+ and Zn2+ were measured with ICP-MS, organics were measured by LC-MS (PFASs) and GC-MS (PCBs, PAHs). Following extraction/microwave digestion, concentrations of organics and metals were similarly measured in the soil, roots, and shoots at the conclusion of the experiment. Enrichment of microbiota in the plant rhizosphere and the bulk soil was evaluated by amplicon sequencing and profiling of 16S, 18S, and ITS ribosomal DNA (bacteria, archaea, and fungi).
Preliminary data suggest that some plants were much more effective at treating PFASs, particularly the shorter chain, 4 carbon compounds perfluorobutanoic acid (PFBA) and perfluorobutanesulfonic acid (PFBS) which are most difficult to remove by adsorptive processes. The grasses, as well as yarrow and checkermallow, achieved significantly higher bioconcentration factors (BCF) of PFBA and PFBS (ratio of plant concentration to soil concentration). Yarrow achieved a BCF > 100 for PFBS. Yarrow, checkermallow, and salal were most successful in translocating PFBA and PFBS (ratio of shoot concentration to root concentration), with yarrow able to accumulate 40 times more mass of PFBS in its shoots than its roots.