The Effect of Grain-Coating Mineralogy on the Storage of Nitrate and Sulfate in the Unsaturated Zone, Atlantic Coastal Plain, USA.

Characteristics of unsaturated-zone sediments and the chemistry of shallow ground water underlying a small (~8 km²) watershed were studied to identify mechanisms responsible for anion storage within the Miocene Bridgeton Formation and weathered Coastal Plain deposits in southern New Jersey.  Lower unsaturated zone sediments and shallow ground-water samples were collected and concentrations of selected ions (including nitrate and sulfate) from 11 locations were determined.

Textural features (grain size, sorting) and sediment color of lower unsaturated zone was determined and the mineralogy of these sediment grains and the composition of coatings were analyzed by petrographic examination, SEM-EDAX, and quantitative whole rock XRD. The sediment grains, largely quartz and chert (80 to 94 wt%), are coated with a complex mixture of minerals (typically grain diameter <20 mm) with variably-charged ion exchange sites (kaolinite, halloysite, goethite, and possibly gibbsite and lepidocrocite). The mineral coatings are present as an open fabric resulting in a large surface area in contact with pore water.

Significant positive correlations between the amount of goethite in the grain coatings and the concentration of sediment-bound sulfate were observed, indicative of anion sorption. Other mineral-chemical relations indicate that negatively charged surfaces and competition with sulfate results in exclusion of nitrate from inner-sphere exchange sites. The observed nitrate storage may be a result of matrix forces within the grain coatings and outer-sphere complexation. Results of this study indicate that the mineralogy of grain coatings can have demonstrable affects on the storage of nitrate and sulfate in the unsaturated zone. Models that neglect the sorption of nitrate and sulfate in the unsaturated zone will not accurately predict the loading of these anions to surficial aquifers in this and similar hydrogeologic settings.