Metolachlor metabolite (MESA) reveals agricultural nitrate-N fate and transport in Choptank River watershed

Authors: McCarty, Gregory; Hapeman, Cathleen; Rice, Clifford; HIVELY, W - Us Geological Survey (USGS); McConnell, Laura; Sadeghi, Ali; LANG, MEGAN - Us Forest Service (FS); WHITALL, DAVID - National Oceanic & Atmospheric Administration (NOAA); Bialek Kalinski, Krystyna; Downey, Peter

Submitted to: Science of the Total Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/14/2013
Publication Date: 1/2/2014
Citation: McCarty, G.W., Hapeman, C.J., Rice, C., Hively, W.D., McConnell, L.L., Sadeghi, A.M., Lang, M.W., Whitall, D.R., Bialek Kalinski, K.M., Downey, P.M. 2014. Metolachlor metabolite (MESA) reveals agricultural nitrate-N fate and transport in Choptank River watershed. Science of the Total Environment. 473-474:473-482.

Interpretive Summary: A national assessment of water quality found that that of the water bodies surveyed in the U.S, 44% of streams/rivers, 64% of lakes/reservoirs and 30% of bays/estuaries were impaired as defined by the 1972 Clean Water Act with agriculture being a major source of impairment. Reducing impairment by agricultural pollution is a major focus of Total Maximum Daily Load (TMDLs) framework for regulating pollution sources within watersheds with varying degrees of success. Combined use of monitoring and modeling hold promise for improved estimates of non-point source loading to water bodies and reduced uncertainties in pollutant source predictions but often such approaches fall short of what is needed for adaptive implementation and targeting of conservation practices. In this study, we report on a novel environmental tracer that helps detect fate of agricultural nitrogen at watershed scales. This new approach for measurement will improve our ability to detect influence of conservation practices on water quantity in sensitive ecosystems such as the Chesapeake Bay.

Technical Abstract: Nitrogen from agricultural activities contributes to the hypoxic zones and severe declines in water quality in the Gulf of Mexico and the Chesapeake Bay. The Federal Clean Water Act requires nitrogen load reductions to restore the integrity of these important waterways. Tools are needed to track the transport and fate of agricultural nitrate-N in watersheds under different management practices. MESA (2-[(2-ethyl-6-methylphenyl) (2-methoxy-1-methylethyl)amino]-2-oxoethanesulfonic acid) is a stable, highly soluble metabolite of the extensively-used herbicide metolachlor. In a multi-year study examining nitrate-N and MESA, we found that MESA mimics nitrate-N transport in agricultural watersheds and provides a useful frame of reference for interpreting watershed nitrate-N processing. We further evaluated the utility of MESA to assess nitrate-N fate in the tidal portion of the Choptank River, an embayment of the Chesapeake Bay. MESA provided a more accurate reference of dilution effects than salinity and evidence that little nitrate-N was processed in the estuary.