Persistent Trace Organic Contaminants Are Transformed Rapidly under Sulfate- and Fe(III)-Reducing Conditions in a Nature-Based Subsurface Water Treatment System.

Subsurface treatment systems, such as constructed wetlands, riverbank filtration systems, and managed aquifer recharge systems, offer a low-cost means of removing trace organic contaminants from treated municipal wastewater. To assess the processes through which trace organic contaminants are removed in subsurface treatment systems, pharmaceuticals and several major metabolites were measured in porewater, sediment, and plants within a horizontal levee (i.e., a subsurface flow wetland that receives treated municipal wastewater). Concentrations of trace organic contaminants in each wetland compartment rapidly declined along the flow path. Mass balance calculations, analysis of transformation products, microcosm experiments, and one-dimensional transport modeling demonstrated that more than 60% of the contaminant removal could be attributed to transformation. Monitoring of the system with and without nitrate in the wetland inflow indicated that relatively biodegradable trace organic contaminants, such as acyclovir and metoprolol, were rapidly transformed under both operating conditions. Trace organic contaminants that are normally persistent in biological treatment systems (e.g., sulfamethoxazole and carbamazepine) were removed only when Fe(III)- and sulfate-reducing conditions were observed. Minor structural modifications to trace organic contaminants (e.g., hydroxylation) altered the pathways and extents of trace organic contaminant transformation under different redox conditions. These findings indicate that subsurface treatment systems can be designed to remove both labile and persistent trace organic contaminants via transformation if they are designed and operated in a manner that results in sulfate-and Fe(III)-reducing conditions.

Fate of Dissolved Nitrogen in a Horizontal Levee: Seasonal Fluctuations in Nitrate Removal Processes.

Horizontal levees are a nature-based approach for removing nitrogen from municipal wastewater effluent while simultaneously providing additional benefits, such as flood control. To assess nitrogen removal mechanisms and the efficacy of a horizontal levee, we monitored an experimental system receiving nitrified municipal wastewater effluent for 2 years. Based on mass balances and microbial gene abundance data, we determined that much of the applied nitrogen was most likely removed by heterotrophic denitrifiers that consumed labile organic carbon from decaying plants and added wood chips. Fe(III) and sulfate reduction driven by decay of labile organic carbon also produced Fe(II) sulfide minerals. During winter months, when heterotrophic activity was lower, strong correlations between sulfate release and nitrogen removal suggested that autotrophic denitrifiers oxidized Fe(II) sulfides using nitrate as an electron acceptor. These trends were seasonal, with Fe(II) sulfide minerals formed during summer fueling denitrification during the subsequent winter. Overall, around 30% of gaseous nitrogen losses in the winter were attributable to autotrophic denitrifiers. To predict long-term nitrogen removal, we developed an electron-transfer model that accounted for the production and consumption of electron donors. The model indicated that the labile organic carbon released from wood chips may be capable of supporting nitrogen removal from wastewater effluent for several decades with sulfide minerals, decaying vegetation, and root exudates likely sustaining nitrogen removal over a longer timescale.

Use of stable nitrogen isotopes to track plant uptake of nitrogen in a nature-based treatment system.

In nature-based treatment systems, such as constructed wetlands, plant uptake of nutrients can be a significant removal pathway. Current methods for quantifying plant uptake of nitrogen in constructed wetlands, which often involve harvesting biomass and assuming that all nitrogen stored in plants was derived from wastewater, are inappropriate in pilot- and full-scale systems where other sources of nitrogen are available. To improve our understanding of nitrogen cycling in constructed wetlands, we developed a new method to quantify plant uptake of nitrogen by using stable isotopes and a mixing model to distinguish between nitrogen sources. We applied this new method to a pilot-scale horizontal levee system (i.e., a subsurface constructed wetland) over a two-year monitoring period, during which 14% of nitrogen in plants was wastewater-derived on average and the remaining plant nitrogen was obtained from the soil. Analysis of nitrogen isotopes indicated substantial spatial variability in the wetland: 82% of nitrogen in plants within the first 2 m of the slope came from wastewater while less than 12% of plant nitrogen in the remainder of the wetland originated from wastewater. By combining these source contributions with remote-sensing derived total biomass measurements, we calculated that 150 kg N (95% CI = 50 kg N, 330 kg N) was taken up and retained by plants during the two-year monitoring period, which corresponded to approximately 8% of nitrogen removed in the wetland. Nitrogen uptake followed seasonal trends, increased as plants matured, and varied based on design parameters (e.g., plant types), suggesting that design decisions can impact this removal pathway. This new method can help inform efforts to understand nitrogen cycling and optimize the design of nature-based nutrient control systems.

The horizontal levee: a multi-benefit nature-based treatment system that improves water quality and protects coastal levees from the effects of sea level rise.

Municipal wastewater treatment plants in coastal areas are facing numerous challenges, including the need to provide a cost-effective approach for removing nutrients and trace organic contaminants from wastewater, as well as adapting to the effects of climate change. The horizontal levee is a multi-benefit response to these issues that consists of a sloped subsurface treatment wetland built between a coastal levee and tidal marshes. The wetland attenuates storm surges and can provide space for wetland transgression to higher elevations as sea levels rise, while simultaneously removing contaminants from treated wastewater effluent. To assess the ability of the horizontal levee to improve water quality and to identify optimal operating conditions, a 0.7-ha experimental system was studied over a two-year period. The removal of nitrate and trace organic contaminants was particularly sensitive to hydrology; rapid and near complete removal (>97%) of these contaminants was observed in water flowing through the subsurface, whereas surface flows did not exhibit measurable contaminant removal. Removal of F+ coliphage also appeared to be sensitive to hydrology, with up to 99% removal of these indicator viruses in subsurface flow. For phosphate, removal was not as sensitive to hydrology, but significant removal (>83%) was still observed when overland flow was eliminated. Although removal of contaminants did not appear to be sensitive to other design considerations, parameters such as soil texture and planting regimes affected the maximum subsurface flows, which in turn controlled contaminant mass loadings. Rapid subsurface removal of contaminants suggests that water quality benefits of these systems are limited by physical constraints (i.e., the ability of the system to maintain subsurface flow) and not chemical or biological conditions in the subsurface.

Sulfur Cycle in a Wetland Microcosm: Extended 34S-Stable Isotope Analysis and Mass Balance.

The sulfur cycle is an important part of constructed wetland biogeochemistry because it is intimately intertwined with the carbon, nitrogen, and iron cycles. However, to date, no quantitative investigation has been conducted on the sulfur cycle in constructed wetlands because of the complexity of wetland systems and the deficiencies in experimental methodology. In this study, 34S-stable isotope analysis was extended in terms of the calculation for the enrichment factor and the kinetic analysis for bacterial sulfate reduction. With this extended method, we attempted for the first time to assess the true rate of bacterial sulfate reduction when sulfide oxidation co-occurs. The joint application of the extended 34S-stable isotope and mass balance analyses made it possible to quantitatively investigate the primary sulfur transformation in a wetland microcosm. Accordingly, a sulfur cycle model for constructed wetlands was quantified and validated. Approximately 75% of the input sulfur was discharged. The remainder was mainly removed through deposition as acid volatile sulfide, pyrite, and elemental sulfur. Plant uptake was negligible. These findings improve our understanding of the physical, chemical, and biological transformations of sulfur among plants, sediments, and microorganisms, and their interactions with carbon, nitrogen, and iron cycles, in constructed wetlands and similar systems.