Urea fluctuations in stream baseflow across land cover gradients and seasons in a coastal plain river system

Authors: TZILKOWSKI, SARAH - Pennsylvania State University; Buda, Anthony; BOYER, ELIZABETH - Pennsylvania State University; Bryant, Ray; Kleinman, Peter; Kennedy, Casey; ALLEN, ARTHUR - University Of Maryland Eastern Shore (UMES); Folmar, Gordon; MAY, ERIC - University Of Maryland Eastern Shore (UMES)

Submitted to: Journal of the American Water Resources Association
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/27/2018
Publication Date: 12/24/2018
Citation: Tzilkowski, S.S., Buda, A.R., Boyer, E.W., Bryant, R.B., Kleinman, P.J., Kennedy, C.D., Allen, A.L., Folmar, G.J., May, E.B. 2018. Urea fluctuations in stream baseflow across land cover gradients and seasons in a coastal plain river system. Journal of the American Water Resources Association. 55(1):228-246. https://doi.org/10.1111/1752-1688.12716.
DOI: https://doi.org/10.1111/1752-1688.12716

Interpretive Summary: Urea is an important form of organic nitrogen whose increased use as a commercial fertilizer has heightened concerns about its transport in watershed runoff and its potential to degrade water quality. In this study, we examined how land cover type and season of the year affect urea concentrations in nontidal and tidal streams that flow directly into the Chesapeake Bay. Results showed that urea concentrations in stream water peaked in the summertime, long after fertilizers were applied in early spring, and that wetlands exerted a greater influence on urea levels than agriculture. Findings demonstrate the importance of wetlands as a key determinant of urea concentrations in streams draining a mixture of land cover types.

Technical Abstract: Urea-N is an important component of the bioavailable dissolved organic nitrogen (DON) pool that ostensibly induces many distinctive signs of eutrophication, including harmful algal blooms. In this study, we assessed urea-N fluctuations in baseflow across land cover gradients and seasons in the upper Manokin River watershed, a 98-km**2 basin located within the poultry producing region of the Lower Delmarva Peninsula. From March 2010 to June 2011, we sampled eleven streams (4 tidal and 7 nontidal), two wastewater treatment plants, an agricultural ditch, and groundwater underlying a cropped field on a monthly basis. At each location, we measured urea-N, DON, dissolved organic carbon (DOC), total dissolved nitrogen (TDN), NO3-N, and NH4-N concentrations. In general, urea-N constituted between 1.1 and 5.7% of TDN, with the highest urea-N concentrations in drainage ditches (0.054 mg L**- 1) and wetland-dominated streams (0.035 to 0.045 mg L**-1). While urea-N did not vary seasonally in tidal rivers, nontidal streams experienced pronounced urea-N peaks in summer (0.038 mg L**-1) that occurred long after the main period of cropland urea fertilization in spring. In light of this evidence, it was notable that the proportion of wetlands explained 78% of the variance in baseflow urea-N levels across the Manokin River basin. In wetland-dominated catchments, we found urea-N was positively related to water temperature and negatively related to DOC:DON ratios, indicating urea-N generation was driven more by in-stream and wetland-driven processes than by anthropogenic urea-N inputs. Study findings demonstrate important controls of wetlands on baseflow urea-N concentrations in mixed land-use basins.