Stormwater subsurface gravel wetland hydraulics, phosphorus retention, and chloride dynamics in cold climates

Subsurface gravel wetlands (SGW) are water treatment practices that use a saturated layer of gravel, sometimes below a vegetated soil layer, to filter urban stormwater and remove pollutants during horizontal flow. In recent years, the implementation of SGWs has proliferated among municipalities in the Northeast United States to meet phosphorus (P) control requirements. However, stormwater SGW performance is not well researched, creating knowledge gaps related to P removal performance and the effects of road salt on performance in cold climates. Here, we used field monitoring of two SGWs over two years and a complementary series of laboratory studies to examine SGW hydraulics, P retention, and chloride dynamics. Field results indicated reductions in peak flows and flow volumes, net P export for approximately half of the total storms monitored across a wide range of influent P loads, and chloride load reductions at one site. Lab results showed that both engineered soils and native soils are unlikely to restrict hydraulic conductivity to the degree desired. Furthermore, two out of three engineered soils tested, including the one used at the field sites, can release substantial P post-installation, while gravels have limited ability to sorb dissolved P. Neither soils nor gravels substantially influenced chloride concentrations in the lab experiments, and results illustrated two potential responses of wetland vegetation to chloride exposure in SGWs as well as chloride assimilation by vegetation. We recommend a testing protocol based on Mehlich-3 P saturation ratio to guide soil selection and reduce dissolved P leaching risk.