Location: Hydrology and Remote Sensing LaboratoryTitle: Nitrogen fate and transport through palustrine depressional wetlands along an alteration gradient in an agricultural landscape, upper Choptank Watersheds, Maryland, USA Author
Submitted to: Journal of Soil and Water Conservation
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/30/2012
Publication Date: 1/1/2014
Publication URL: http://handle.nal.usda.gov/10113/59939
Citation: Denver, J., Ator, S.W., Lang, M., McCarty, G.W., Clune, J., Fisher, T., Fox, R., Gustfson, A. 2014. Nitrogen fate and transport through palustrine depressional wetlands along an alteration gradient in an agricultural landscape, upper Choptank Watersheds, Maryland, USA. Journal of Soil and Water Conservation. DOI: 10.2489/jswc.69.1.1. Interpretive Summary: Nitrogen occurs widely in the form of nitrate in contaminated groundwater and groundwater is a source of agricultural nitrogen to many streams and tidal waters. The effectiveness of wetlands for mitigating agricultural nitrate depends primarily on local hydrology and movement of water through the landscape, which controls how much nitrate actually passes through the zone of wetland influence. Shallow groundwater flow systems around wetlands and other surface-water bodies are commonly transient and complex and reflect influences of local geology, variable precipitation. This study assesses ability of wetlands in an agricultural landscape to reduce export of agricultural nitrogen to streams. We found that the ability of wetlands to reduce nitrate export is a complex function of wetland setting in the landscape. The ability to better estimate the influence of these wetlands on water quality could be improved by the development of new approaches for mapping or estimating wetland hydrology.
Technical Abstract: Understanding local groundwater hydrology and geochemistry is critical for evaluating the effectiveness of wetlands at mitigating agricultural impacts on surface waters. The effectiveness of forested, prior-converted cropland (historic wetlands), and restored palustrine depressional wetlands at mitigating nitrate transport from fertilized row crops to local streams was examined in the watershed of the upper Choptank River, a tributary of Chesapeake Bay on the Atlantic Coastal Plain. Hydrogeologic, geochemical, and water-quality data were collected to estimate the orientation of groundwater flow paths and likely interactions with reducing conditions associated with wetland sediments. Forested wetlands, which represent natural conditions, have relatively long hydroperiods and reducing conditions conducive to denitrification, but are typically located in groundwater recharge areas along watershed divides and therefore intercept little nitrogen from nearby agriculture, although they likely improve water quality in adjacent streams via dilution. Restored wetlands and even prior-converted croplands often support soil and geochemical conditions conducive to denitrification, and substantial losses of agricultural nitrate were observed in groundwater flowing through these wetland sediments. However, delivery of nitrate from agricultural areas through groundwater to these wetlands and resulting opportunities for denitrification can be limited, particularly where reducing conditions do not extend throughout the entire thickness of the surficial aquifer and nitrate may pass conservatively beneath the wetland along deeper groundwater flow paths. The complexity of nitrogen fate and transport associated with palustrine depressional wetlands across the alteration gradient complicates estimation of their importance to water quality in adjacent streams. Although their individual contribution to nitrate mitigation varies with critical hydrogeologic and geochemical variables (e.g., aquifer thickness and the extent of reducing conditions) these wetlands are likely to be a significant landscape sink for nitrate and source of low nitrate water due to their abundance within the study site. The ability to better estimate the influence of these wetlands on water quality could be improved by the development of new approaches for mapping or estimating critical hydrogeologic and geochemical variables at sufficient resolution and accuracy.