Bioavailability of Dissolved Organic Carbon and Ambient Redox Processes in Groundwater associated with Agricultural Land Use
U.S. Geological Survey
Agricultural practices have the potential to mobilize dissolved organic carbon (DOC) from soil zones and transport it to underlying aquifers. The bioavailability of this potential DOC source, and thus its ability to affect ambient redox processes in aquifers affected by agricultural practices, is uncertain. One potential metric of DOC bioavailability is its observed reactivity with dissolved oxygen. If DOC is highly bioavailable, it would be expected that DOC and dissolved oxygen would be inversely related according to a hyperbolic function. That is, high concentrations of DOC would be expected to correlate with low concentrations of dissolved oxygen and vice versa. Conversely, if DOC is relatively nonreactive, little correlation between the two parameters would be expected. Thus, the statistical correlation of DOC-dissolved oxygen plots to a hyperbolic function may allow comparison of the relative bioavailability of DOC characteristic of different agricultural settings. This approach was applied to a variety of agricultural settings in the United States including the San Joaquin Valley of California, the Great Salt Lake Valley, the High Plains of Nebraska, the White River Valley of Indiana, the western Lake Michigan drainage, the Delmarva Peninsula of Maryland, and the Santee River Basin of South Carolina using a large data base compiled by the National Water-Quality Assessment (NAWQA) program of the U.S. Geological Survey. The results suggest that (1) DOC associated with western agricultural areas is generally less bioavailable than DOC associated with eastern agricultural areas; (2) the bioavailability of DOC decreases as the thickness of the unsaturated zone increases, and (3) relatively bioavailable DOC identified with the dissolved oxygen metric also correlates hyperbolically with nitrate concentrations.