Constructed wetlands as a component of the agricultural landscape: Mitigation of herbicides in simulated runoff from upland drainage areas
Research highlights
► We evaluate herbicide moving through a constructed wetland during a simulated runoff event. ► Atrazine and fluometuron in water decrease as water flows from cell to cell. ► Constructed wetlands improve water quality by pollutant sequestration or processing.
Introduction
Wetlands are an integral part of many landscapes, often serving as transition zones between upland areas and water bodies. As such, wetlands are a sink for a variety of pollutants, given proximity to anthropogenic activities such as industry or agriculture. The integrity of a wetland, therefore, may depend upon its ability to process contaminants as they move through the system. Although the physical, biological, and chemical processes are complex, interrelated, and not completely understood, factors contributing to effective processing include the size of the wetland, system hydrology, vegetative characteristics and density, proximity to contaminant sources, and the nature of the contaminants (Mitsch and Gosselink, 2000).
Constructing artificial wetlands in vulnerable areas where wetlands do not naturally exist may provide protection from pollutants moving to water bodies from adjacent areas, and they may be more efficient in processing contaminants than natural wetlands because constructed wetlands can be tailored to meet specific needs. Significant research effort has focused on the development and assessment of constructed wetlands for treatment of waste from urban, mining, and industrial activities (e.g., Bastian and Hammer, 1993, Kadlec and Knight, 1996). In areas affected by agriculture, the application of conservation management practices such as constructed wetlands has received attention for attenuating non-point contamination of surface waters by sediment, livestock wastewater, and agrochemicals in runoff (Rodgers and Dunn, 1992, Hammer et al., 1993, Higgins et al., 1993, Kadlec and Knight, 1996, Cooper et al., 1998). As part of an integrated agricultural management system, constructed wetlands can be used in conjunction with other edge-of-field measures, e.g., vegetative buffer strips (Dabney et al., 2006).
Surface water contamination from non-point sources is a concern. Herbicides applied to the soil surface in row crop production are susceptible to loss in surface runoff, and downstream conservation measures such as vegetated constructed wetlands may help to minimize contamination of adjacent water bodies. Atrazine and fluometuron are commonly used soil-applied herbicides and are candidates for evaluations of the efficacy of constructed wetlands in remediating runoff from row crop areas. Factors that may influence the dissipation of herbicides in wetland systems include vegetation and associated decomposed litter; high soil organic carbon, anaerobic environment, and active populations of anaerobic and aerobic microbes (Stoeckel et al., 1997).
The capability for wetlands to remove atrazine from water has been reported from studies ranging from in vitro assays, through mesocosms, to functioning natural and constructed wetlands. Atrazine mitigation may be biologically mediated. Enhanced mineralization of atrazine has been documented in numerous, diverse terrestrial systems (Krutz et al., 2010) and within a wetland after bio-augmentation of the sediment with soil from an enhanced-mineralizing atrazine spill site (Runes et al., 2001). Atrazine-degrading organisms have been isolated from other wetlands with a history of atrazine exposure (Runes et al., 2003). The molecular basis for enhanced degradation in wetland sediment was provided by in vitro mineralization assays; PCR detection of the gene trzD and PCR/Southern blot detection of the gene atzA (Anderson et al., 2002). A biological role for atrazine degradation may also be inferred from atrazine dissipation after the addition of sucrose to anaerobic sediment microcosms (Kao et al., 2001). Working with other wetland sediments, Weaver et al. (2004) detected little mineralization and noted that the rapid loss of atrazine (half-life 23 d) was the result of pH-mediated hydrolytic dechlorination of atrazine and enhanced binding to soil. Others have observed atrazine partitioning to sediments (Detenbeck et al., 1996) as well as enhanced sorption and irreversible binding of atrazine and metabolites in wetland soils as compared to agricultural soils (Mersie and Seybold, 1996). Atrazine degraded under strongly reduced conditions in wetland soils in a microcosm study (half-life 38 d) (Seybold et al., 2001), but degradation was slower (half-life 86 d) in the aqueous phase above the soil. In contrast, atrazine was not mineralized under either aerobic or anaerobic conditions in a wetland soil microcosm (Larsen et al., 2001). Other factors that may improve retention of atrazine include longer residence times in the wetland area (Alvord and Kadlec, 1995) and increased wetland buffer size (Moore et al., 2000).
In a microcosm study, fluometuron was metabolized under saturated conditions (half-life 25–27 d), but under flooded conditions, the half-life was 175 d (Weaver et al., 2004). In a laboratory study using soils collected from wetland sites, there was more rapid degradation of fluometuron in forested riparian soil as compared to field soil (Locke et al., 2002). Shankle et al. (2004) observed more fluometuron sorption in wetland soils compared to field soils, and this was attributed to increased organic matter. Similarly, Rose et al. (2006) found that organic matter and vegetation influenced fluometuron retention in an in situ wetland study (Rose et al., 2006).
Long-term monitoring of Beasley Lake, an oxbow lake in the Mississippi Delta (Locke, 2004), measured detectable concentrations of both atrazine and fluometuron, presumably from surrounding agricultural fields (Zablotowicz et al., 2006a). A wetland was constructed and established adjacent to Beasley Lake to evaluate its potential as a management practice to inhibit contamination of the lake from pesticide in runoff (Moore et al., 2007, Moore et al., 2009). One year after the constructed wetland was established, Weaver et al. (2004) reported changes in the vegetation and hydrology as well as shifts in the soil microbial community structure. The purpose of the study reported here is to describe the environmental fate of atrazine and fluometuron in a constructed wetland during a simulated rainfall event in order to evaluate the effectiveness of constructed wetlands in processing and sequestering pesticides and protecting downstream sites. A companion study evaluated the fate of insecticides introduced to the wetland system (Moore et al., 2007, Moore et al., 2009).
Section snippets
Description of the study site
A constructed wetland (180 m long × 30 m wide [average]) consisting of a sediment trap (average 0.45 m depth) and two treatment cells was developed beginning in Spring, 2002 in the Beasley Lake watershed, Sunflower County, Mississippi (Fig. 1, Fig. 2). The treatment cells included: (a) one long, shallow (average 0.3 m depth) non-excavated cell; and (b) one deeper (average 1 m depth), excavated cell. Cells were separated by earthen berms and connected by culverts. Wetland establishment was completed in
Results and discussion
Water flow was monitored during the 3-h simulated run when water was pumped into the constructed wetland from Beasley Lake (Fig. 3a). After pumping was terminated, water flow in the wetland gradually declined for approximately 6 h (Fig. 3a) before stopping. Bromide was used to monitor flow through the various cell compartments (Fig. 3b). The main pulse for Br− concentration in the first (non-excavated) cell ranged from 0.8 to 6 h after dosage, with the peak occurring at almost 3 h (Fig. 3b). The
Acknowledgments
The authors are grateful for contributions from Frank Gwin, Matt Moore, and Paul Rodrigue.
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