Mitigating agrichemicals from an artificial runoff event using a managed riverine wetland

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Abstract

We examined the mitigation efficiency of a managed riverine wetland amended with a mixture of suspended sediment, two nutrients (nitrogen and phosphorus), and three pesticides (atrazine, metolachlor, and permethrin) during a simulated agricultural runoff event. Hydrologic management of the 500 m-long, 25 m-wide riverine wetland was done by adding weirs at both ends. The agrichemical mixture was amended to the wetland at the upstream weir simulating a four-hour, ~ 1 cm rainfall event from a 16 ha agricultural field. Water samples (1 L) were collected every 30 min within the first 4 h, then every 4 h until 48 h, and again on days 5, 7, 14, 21, and 28 post-amendment at distances of 0 m, 10 m, 40 m, 300 m and 500 m from the amendment point within the wetland for suspended solids, nutrient, and pesticide analyses. Peak sediment, nutrient, and pesticide concentrations occurred within 3 h of amendment at 0 m, 10 m, 40 m, and 300 m downstream and showed rapid attenuation of agrichemicals from the water column with 79–98%, 42–98%, and 63–98% decrease in concentrations of sediments, nutrients, and pesticides, respectively, within 48 h. By day 28, all amendments were near or below pre-amendment concentrations. Water samples at 500 m showed no changes in sediment or nutrient concentrations; pesticide concentrations peaked within 48 h but at ≤ 11% of upstream peak concentrations and had dissipated by day 28. Managed riverine wetlands  1 ha and with hydraulic residence times of days to weeks can efficiently trap agricultural runoff during moderate (1 cm) late-spring and early-summer rainfall events, mitigating impacts to receiving rivers.

Highlights

► Mitigation efficiency of a managed wetland dosed with agrichemicals was assessed. ► Peak sediment, nutrient, and pesticide loads occurred within 3 h of dosing. ► Agrichemicals were rapidly attenuated by 42–98% within 48 h. ► By day 28, all agrichemicals were near or below pre-dosing concentrations. ► Managed wetlands can efficiently trap agricultural runoff after rainfall events.

Introduction

Agricultural regions wherein major rivers with broad, low-gradient floodplains exist often contain numerous natural backwater aquatic habitats, such as wetlands, conducive to anthropogenic manipulation (Mitsch et al., 2005, Shields et al., 2005, Shields and Pearce, 2010, Lizotte et al., 2009). Such freshwater wetlands, with minimal cost, can potentially be hydrologically managed to maximize their natural filtering capabilities to mitigate storm runoff from adjacent agricultural fields (Mitsch et al., 2005, Lizotte et al., 2009, Shields and Pearce, 2010). Costs to stakeholders such as farmers, land managers, land owners, and regulatory agencies would be less than the cost of full construction, implementation, and management of a constructed wetland of comparable size (Shields et al., 2005, Kadlec, 2006). Also, because natural backwater wetlands already provide pre-existing hydrology, hydrophytes, and hydrosoils, these conditions would not need any “conditioning period” as for constructed wetlands (Mitsch and Gosselink, 2007). Despite these advantages, little information exists regarding the ability of anthropogenically manipulated natural backwater wetlands in mitigating contaminants from agricultural runoff under controlled conditions.

Riverine backwater wetlands within river floodplains have important economic and ecological functions such as acting as filters and processors of a variety of agricultural contaminants including suspended sediment, nutrients and pesticides entering from adjacent agricultural fields (Reddy and DeLaune, 2008). The hydrology of such wetlands can be controlled to increase the efficacy of their natural filtering capabilities (Mitsch et al., 2002, Lizotte et al., 2009). Nutrient mitigation from agricultural sources has been a primary focus for several decades due to the increase in eutrophication of receiving lakes, rivers, streams and estuaries worldwide (Wetzel, 1992, Scanlon et al., 2007) and wetlands have long been known to be highly efficient at removing nutrients under favorable conditions (Mitsch and Gosselink, 2007). For these reasons, there is an increasing need to expand our knowledge of nutrient mitigation capabilities to efficiently maximize available wetland resources via hydraulic modification of riverine floodplain wetlands, when applicable. The purpose of this study was to assess the trapping efficiency of a modified riverine backwater wetland amended with a mixture of suspended sediment, two nutrients [nitrogen (N) and phosphorus (P)], and three pesticides (atrazine, metolachlor, and permethrin) during a simulated agricultural runoff event. Previous study by Lizotte et al. (2009) within the same wetland system assessed the trapping of pesticides only. The study was limited in scope both spatially (only two sites: inflow and furthest downstream weir) and pollutant mixture complexity. The current study expands the previous work of Lizotte et al. (2009) by incorporating a broader spatial assessment, and more complex, and realistic, pollutant mixture, to better address questions of agricultural pollutant trapping and attenuation efficiency of the managed riverine wetland study site.

Section snippets

Area description

A reach of the Coldwater River ~ 20 km downstream from Arkabutla Lake Dam in Tunica County, Mississippi, was selected because of the presence of > 20 severed riverine backwater meander bends and other floodplain water bodies (Fig. 1). A severed riverine compound meander bend backwater (~ 2.5 km long × 40 m wide) was selected for this study. The study site, inside the mainstem flood control levee, is the result of a 0.4 km cutoff constructed in 1941–42. Land-use both inside and outside the bend are in

Methods

On June 24, 2009, 611 m3 of water was released from the upstream lake cell portion of the study site into the modified wetland cell portion over about 4 h (Fig. 2), simulating agricultural runoff during an ~ 1-cm rainfall event from a 16-ha cultivated field. Simulated agricultural runoff comprised of local source suspended sediment (adjacent field soil), nutrients as P (42% P2O5) and N (34% NH4NO3), and pesticides as atrazine, S-metolachlor and permethrin was amended once simulating a “first

Results

The simulated hydrograph was quite similar to the targeted model, with peak discharge of 85 L s 1 about 1 h after flow initiation (Fig. 2). No outflow from the wetland occurred during simulated event, and although a total of approximately 149 mm of rainfall was recorded by the nearest rain gage during the monitoring period (reported at Sarah, Mississippi), no outflow occurred during the period following the event until day 22 (Fig. 3). Local thunderstorms triggered outflows from the wetland to the

Discussion

The current study provides valuable information on the use and efficacy of natural wetlands modified to enhance their natural filtering capabilities when inundated with a complex mixture of sediment, nutrients, and pesticides typically occurring in agricultural runoff. As a result, such studies as the current one are important in understanding the viability of using and managing available existing adjacent riverine floodplain wetlands within agricultural watersheds that can be modified to

Conclusions

Overall results of our study indicate that hydraulic management of a natural riverine backwater wetland can effectively trap a variety of contaminants commonly occurring in agricultural runoff during small to moderate rainfall events, mitigating potential ecological effects downstream within the main river channel. Controlled hydrology can be used to increase the efficiency of natural wetland filtering capabilities. The hydrologically modified riverine backwater wetland in the present study can

Acknowledgments

The authors thank the numerous technicians and support personnel who provided assistance with logistics, sample collection, and analysis. We also thank the several anonymous reviewers who provided helpful comments. Mention of equipment, computer programs, or a chemical does not constitute an endorsement for use by the US Department of Agriculture nor does it imply pesticide registration under FIFRA as amended. The US Department of Agriculture is an equal opportunity employer.

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