Persistent Elevated Nitrate in a Riparian Zone Aquifer
2008
Robertson, William D. | Schiff, Sherry L.
Streamside vegetated buffer strips (riparian zones) are often assumed to be zones of ground water nitrate (NO₃ ⁻) attenuation. At a site in southwestern Ontario (Zorra site), detailed monitoring revealed that elevated NO₃ ⁻–N (4–93 mg L⁻¹) persisted throughout a 100-m-wide riparian floodplain. Typical of riparian zones, the site has a soil zone of recent river alluvium that is organic carbon (OC) rich (36 ± 16 g kg⁻¹). This material is underlain by an older glacial outwash aquifer with a much lower OC content (2.3 ± 2.5 g kg⁻¹). Examination of NO₃ ⁻, Cl⁻, SO₄ ²⁻, and dissolved organic carbon (DOC) concentrations; N/Cl ratios; and NO₃ ⁻ isotopic composition (δ¹⁵N and δ¹⁸O) provides evidence of four distinct NO₃ ⁻ source zones within the riparian environment. Denitrification occurs but is incomplete and is restricted to a narrow interval located within ∼0.5 m of the alluvium–aquifer contact and to one zone (poultry manure compost zone) where elevated DOC persists from the source. In older ground water close to the river discharge point, denitrification remains insufficient to substantially deplete NO₃ ⁻ Overall, denitrification related specifically to the riparian environment is limited at this site. The persistence of NO₃ ⁻ in the aquifer at this site is a consequence of its Pleistocene age and resulting low OC content, in contrast to recent fluvial sediments in modern agricultural terrain, which, even if permeable, usually have zones enriched in labile OC. Thus, sediment age and origin are additional factors that should be considered when assessing the potential for riparian zone denitrification.
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