Discipline: Technology and Engineering
Margaret E. Carolan - Virginia Tech
Co-Author(s): Negin Ashoori, YeoMyoung Cho, and Richard G. Luthy, Stanford University, Stanford, CA
Urban stormwater harvesting and treatment is imperative to augment water supply in drought-prone regions in the western United States, and constructed wetlands for stormwater treatment can decrease pollutant concentrations before recharging ground and/or surface water. Chemical contaminants of emerging concern in urban stormwater such as the xenobiotic anticorrosive agent benzotriazole (BT) impair surface water quality have the potential to uptake, assimilate, bioaccumulate, and metabolize in wetland vegetation. Subsurface treatment mesocosms were planted with Carex praegracilis and compared to unvegetated mesocosms in triplicate after receiving 7 gallons of synthetic stormwater spiked with 1 mg/L BT, 1 ug/L BT, and no BT in four weekly simulated storm events at an average flow rate of 0.2 gpm. After 5 gallons were flushed through each mesocosm, the systems were left saturated for one week during which daily 200 mL bulk water samples were harvested, as well as randomized plant tissue samples twice a week. Bulk water was analyzed for BT and plant tissue was analyzed for BT and BT metabolites with liquid chromatography-mass spectrometry (LC-MS/MS). For mesocosms spiked with 1 ug/L BT, 79.4-92.7% of influent BT was removed from synthetic stormwater during the first storm event. After 4 days, all mesocosms experienced a further 66.9-79.1% reduction. Following the last unspiked event, residual BT levels (11.1-14.6 ug/L) were established in each mesocosm. The control mesocosms exhibited a 2.7-17.9% higher BT residual than treated mesocosms.
A background level of BT was present in both leaf and root tissue samples from planted C. praegracilis, and spikes corresponded to dosing. The fluctuations in BT levels between the mesocosms could be explained by the phytotransformation of BT to benzotriazole alanine and glycosylated benzotriazole (GBT), both BT metabolites detected in heightened levels. Increases in GBT levels were demonstrated in both leaf and root tissue relative to a blank BT internal standard with LC-MS/MS. The rate of BT removal during the four week sampling period demonstrates that model constructed wetlands can naturally remove a significant amount of BT from synthetic stormwater. However, concerns about breakthrough of BT suggest that future studies should focus on the effect of retention time on effluent BT concentration. In addition, it is imperative to measure and account for BT metabolites when sampling for BT in plant tissue. Simply accounting for BT is insufficient because of fast phytotransformation following assimilation. Future work must increase sampling frequency of C. praegracilis, target metabolite production, and investigate the effect of metabolite synthesis on plant function.
Funder Acknowledgement(s): Re-Inventing the Nation's Water Infrastructure, NSF ERC 1028968
Faculty Advisor: Negin Ashoori, email@example.com
Role: Experimental planning, design, and construction, daily sampling and monitoring of experimental mesocosms, chemical analyses of drinking water quality samples, processing and preparation of water and plant tissue samples, liquid chromatography, mass spectrometry analysis of water and plant tissue samples