Field Evidence of Denitrification in Shallow Groundwater Flow Systems

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Abstract
A detailed hydrogeological study in a small agricultural watershed near Lake Erie in southwestern Ontario has provided evidence that denitrification is widespread in shallow groundwater in this area. In the groundwater zone of the unconfined sandy aquifer that underlies the study area, NO3- concentrations within two metres of the water table are commonly between 5 and 50 mg/1 (expressed as N). At greater depth, however, NO3- concentrations are generally less than 0.02 mg/1. The aquifer varies in thickness from 3 to 10 m. The transition zone between the high nitrate zone and the low nitrate zone is very thin. In the high nitrate zone, dissolved oxygen occurs at concentrations above 2 mg/1 , methane is absent, and Eh values obtained by the platinum electrode method are greater than 300 mV. In the low nitrate zone, dissolved oxygen occurs at levels less than 2 mg/1, methane is present, and Eh values are generally between 50 and 200 mV. The hydrogeologic conditions are such that it is very improbable that these trends are due to different groundwater sources. The data support the hypothesis that denitrification is the cause of the lack of NO3- in the deeper part of the aquifer. Nitrate and dissolved oxygen data from shallow groundwater zones in four other agricultural watersheds in southwestern Ontario support this conclusion.

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