Nitrate removal in a first-order stream: reconciling laboratory and field measurements

A combination of laboratory and field experiments were carried out to evaluate nitrate(NO₃^t-) removal during stream transport in a first-order agricultural drainage stream. Intact stream sediment cores overlain with stream and NO₃^–-amended stream water indicated NO₃^– losses averaging 93 — 353 mg m^–2 day^–1, with NO₃^– concentration exerting a primary control on loss rate. Isotopic data indicated enrichment of NO₃^– - ¹⁵N over time as NO₃^– concentrations decreased, indicating a denitrification loss. Field experiments were designed to evaluate dilution of streamwater with low-NO₃^– groundwater in addition to other NO₃^– removal processes during transport. A series of bromide tracer and NO₃^– - addition experiments were carried out in the field; groundwater dilution dominated the downstream NO₃^– concentration trends, accounting for all observed decreases in NO₃^– concentration. Isotopic data did not point to denitrification downstream as a major NO₃^– removal process. This apparent disparity between simulated laboratory and in-situ stream removal rates appears to be a function of the hydrological processes controlling exchanges between stream bottom sediments and the overlying water. These results suggest that caution must be exercised in extrapolating potentials for NO₃^– removal measured in laboratory experiments to the field, as these rates could be overestimated in some watersheds.