Transport and loss of agriculturally-derived nitrogen through the vadose zone and shallow aquifer in Sussex county, Delaware
Date
2014
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Publisher
University of Delaware
Abstract
Southern Delaware's Inland Bays - Rehoboth, Indian River, and Little Assawoman - are subjected to high nitrogen loads, contributing to the Bays' eutrophic conditions. These nitrogen loads are in large part derived from agricultural land use. Southern Delaware's surficial sediments are generally sandy and highly permeable, meaning that leaching of agricultural fertilizers - specifically nitrogen as nitrate - beyond the root zone of crops and into the underlying shallow aquifer is a regional problem. Groundwater discharge accounts for approximately 80% of all freshwater flowing into the Bays. Understanding the transport and loss of nitrate through the aquifer is therefore important in constraining and managing overall nitrogen fluxes to the Bays. For my thesis, I collected groundwater during multiple field samplings to measure nitrogen (as nitrate and ammonium) and evaluate potential nitrogen loss within the vadose zone (down to 1.65 m), shallow aquifer (down to 6 m), and deep aquifer (down to 18 m) beneath an agricultural field adjacent to the Indian River estuary - the primary tributary to Indian River Bay. Shallow groundwater was assumed to contain nitrogen derived from the agricultural field directly above, while deeper, older groundwater was assumed to contain nitrogen derived from similar agricultural sources elsewhere in the watershed. Dissolved oxygen, nitrogen gas, and isotopic signatures of nitrogen and oxygen in nitrate were analyzed as a means of assessing denitrification as a potential pathway of nitrogen loss. Overall, my data showed no evidence of nitrogen loss via denitrification in the vadose zone and shallow aquifer. However, some apparent loss of nitrogen occurred within the deep aquifer. This apparent loss was likely related to more favorable denitrifying conditions, potentially occurring in anoxic pockets associated with subsurface heterogeneity along the groundwater flow path. The data suggest that at this site, all fertilizer-derived nitrate leached beyond the root zone of crops ultimately makes its way through the shallow aquifer and into Indian River via groundwater discharge. This emphasizes the need for more controlled fertilizer management on agricultural land lying adjacent to surface water bodies, especially within a region highly susceptible to groundwater contamination.