Vermont Water Resources and Lake Studies Center

Subsurface gravel wetlands (SGW) are water treatment practices that utilize a saturated bed of gravel and (sometimes) wetland vegetation to filter incoming water and remove pollutants through a combination of physical filtration, adsorption, biological uptake, and microbial transformation. Their modeled capacity to remove fine particles and nutrients of concern indicates that they have one of the highest capacities for phosphorus (P) removal of any passive stormwater treatment practice. With the promise of impressive P removal, Vermont municipalities planning and managing for impending phosphorus control requirements have focused state stormwater permit applications on these practices. While the State (and permittees) seek to allocate accurate P removal performance projections to gravel wetlands, they are limited in their ability to do so by a lack of performance data from field installations. Further, there are emerging concerns with the impact of chloride (Cl-) on natural waters and roadside soils. Due to questions surrounding the performance of SGW, researchers investigated the field performance of two permitted and installed subsurface gravel wetlands of similar age and drainage area characteristics over two growth seasons and sampling and conducted laboratory experiments to measure phosphorus capture in comparison to state standards and measure road salt chloride impacts on the performance of the SGW.