Nitrate fate and transport through current and former depressional wetlands in an agricultural landscape, Choptank Watershed, Maryland, United States

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 depressional wetlands at mitigating nitrate (NO 3 ) transport from fertilized row crops, through groundwater, to local streams was examined in the watershed of the upper Choptank River, a tributary of Chesapeake Bay on the Atlantic Coastal Plain. Hydrologic, geochemical, and water quality data were collected from January of 2008 through December of 2009 from surface waters and networks of piezometers installed in and around current or former depressional wetlands of three major types along a gradient of anthropogenic alteration: (1) natural wetlands with native vegetation (i.e., forested); (2) prior-converted croplands, which are former wetlands located in cultivated fields; and (3) hydrologically restored wetlands, including one wetland restoration and one shallow water management area. These data were collected to estimate the orientation of groundwater flow paths and likely interactions of groundwater containing NO 3 from agricultural sources with reducing conditions associated with wetlands of different types. Natural wetlands were found to have longer periods of soil saturation and reducing conditions conducive to denitrification compared to the other wetland types studied. Because natural wetlands are typically located in groundwater recharge areas along watershed divides, nitrogen (N) from nearby agriculture was not intercepted. However, these wetlands likely improve water quality in adjacent streams via dilution. Soil and geochemical conditions conducive to denitrification were also present in restored wetlands and prior-converted croplands, and substantial losses of agricultural NO 3 were observed in groundwater flowing through these wetland sediments. However, delivery of NO 3 from agricultural areas through groundwater to these wetlands resulting in opportunities for denitrification were limited, particularly where reducing conditions did not extend throughout the entire thickness of the surficial aquifer allowing NO 3 to pass conservatively beneath a wetland along deeper groundwater flow paths. The complexity of N fate and transport associated with depressional wetlands complicates the understanding of their importance to water quality in adjacent streams. Although depressional wetlands often contribute low NO 3 water to local streams, their effectiveness as landscape sinks, for N from adjacent agriculture varies with natural conditions, such as the thickness of the aquifer and the extent of reducing conditions. Measurement of such natural geologic, hydrologic, and geochemical conditions are therefore fundamental to understanding N mitigation in individual wetlands.Key words: agriculture-Chesapeake Bay-denitrification-depressional wetlandsgroundwater-wetland conservation practices Local hydrologic and geochemical conditions control the movement of nitrate (NO 3 ) through and around depressional wetlands...

Wetland Restoration and Creation for Nitrogen Removal: Challenges to Developing a Watershed-Scale Approach in the Chesapeake Bay Coastal Plain

“…Further research examining factors affecting hydrologic transport of N to ditches, including the relative importance of lateral matrix flow and preferential flow in different hydrogeologic settings, would help us identify opportunities to capture and treat nitrate before it reaches the ditch. In cases where ditches intercept groundwater nitrate, wetland restoration adjacent to ditches may help maintain anoxic conditions beneath ditches (Denver et al, 2014), thereby encouraging denitrification in ditch soils and sediments. Controlled drainage structures installed in ditches manage drainage outflow and may enhance denitrification.…”

“…The complexity of N fate and transport complicates evaluating the effects of depressional wetlands on downstream water quality (Denver et al, 2014). In flat landscapes, groundwater flow paths do not always follow topographic gradients; seasonal reversals in the direction of groundwater flow can cause shallow groundwater to move away from the wetland to the agricultural upland (Denver et al, 2014) (Figure 3).…”

“…The complexity of N fate and transport complicates evaluating the effects of depressional wetlands on downstream water quality (Denver et al, 2014). In flat landscapes, groundwater flow paths do not always follow topographic gradients; seasonal reversals in the direction of groundwater flow can cause shallow groundwater to move away from the wetland to the agricultural upland (Denver et al, 2014) (Figure 3). Due to the multidimensionality of groundwater flow and variability in reducing conditions, limited geochemical and hydrologic measurements along a presumed hydrologic transect are often insufficient for determining the potential for nitrate interception and removal in wetlands (Denver et al, 2014).…”

“…Method of restoration can affect soil characteristics and the potential for nitrate removal in wetlands as well. For example, confining layers created by the addition of clay or compaction of soils during restoration may limit interaction between anoxic wetland sediments and nitrate in groundwater (Denver et al, 2014).…”

“…In addition to nitrate removal, wetlands may improve water quality in local streams through mixing and dilution (Denver et al, 2014). Forested wetlands upgradient from agriculture can be a source of low nitrate water to downstream waters, diluting nitrate concentrations and improving regional water quality (Denver et al, 2014).…”