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

Abstract

A combination of laboratory and field experiments were carried out to evaluate nitrate(NO-3) removal during stream transport in a first-order agricultural drainage stream. Intact stream sediment cores overlain with stream and NO-3-amended stream water indicated NO-3 losses averaging 93 – 353 mg m−2 day−1, with NO-3 concentration exerting a primary control on loss rate. Isotopic data indicated enrichment of NO-3 - δ15N over time as NO-3 concentrations decreased, indicating a denitrification loss. Field experiments were designed to evaluate dilution of streamwater with low-NO-3 groundwater in addition to other NO-3 removal processes during transport. A series of bromide tracer and NO-3 - addition experiments were carried out in the field; groundwater dilution dominated the downstream NO-3 concentration trends, accounting for all observed decreases in NO-3 concentration. Isotopic data did not point to denitrification downstream as a major NO-3 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 NO-3 removal measured in laboratory experiments to the field, as these rates could be overestimated in some watersheds.