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Quantifying Dissolved Nitrogen Flux Through a Coastal Watershed
Author(s) -
Weiskel Peter K.,
Howes Brian L.
Publication year - 1991
Publication title -
water resources research
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.863
H-Index - 217
eISSN - 1944-7973
pISSN - 0043-1397
DOI - 10.1029/91wr01910
Subject(s) - groundwater , flux (metallurgy) , environmental science , watershed , hydrology (agriculture) , aquifer , nitrate , nitrogen , soil science , tracer , nitrification , geology , chemistry , geotechnical engineering , physics , organic chemistry , machine learning , computer science , nuclear physics
Available nitrogen loading models, commonly used to estimate subsurface fluxes of dissolved nitrogen to coastal waters, have not been quantitatively or systematically compared; nor have they generally been field‐verified at regional scales. We employed three published loading models, a site‐specific model based upon water use data, and both Darcian and non‐Darcian field approaches to obtain estimates of steady state, dissolved nitrogen flux through a permeable Massachusetts watershed. The two field approaches, based on independent data, yielded similar results. Results of the published loading models agreed closely with each other, but exceeded the mean of the field approaches (130 ± 12 mol N m −1 aquifer width yr −1 ) by 60%, on average. The Water Use loading model agreed closely with the field results (within 4%), largely because it did not require estimates of occupancy rate, which was found to be the major source of error to the published models. The observed, median concentration of total dissolved nitrogen (TDN) in groundwater increased from 1.9 to 313 μM during transport through the subbasin, confirming loading model predictions that >99% of the TDN flux is anthropogenic. In contrast to the watershed inputs, downgradient TDN was dominantly nitrate (98%), indicating near‐complete nitrification during transport. Significant transverse horizontal and vertical variations were found in the groundwater TDN distribution at scales of meters and tenths of meters, respectively, consistent with a large number of discrete nitrogen sources at the ground surface, and low transverse macrodispersivities in the aquifer. Loading models, if properly verified by field measurements at the stream tube scale, hold promise for characterizing the effects of land use on subsurface nitrogen flux through coastal watersheds.