No abstract

Intertidal aquifers host a reactive zone comprised of Fe mineral‐coated sands where fresh and saline groundwaters mix. This zone may significantly influence the export of C, N, P, Fe, and other metals in submarine groundwater discharge (SGD). Toward determining the roles of microbes in Fe and S mineralization, and the interplay of microbiology with geochemistry and physical hydrology, we conducted a biogeochemical study of pore waters at Cape Shores, Delaware. Here, fresh groundwater provides Fe(II), which precipitates as FeIIIOOH predominantly through microbial Fe(II) oxidation. Candidate division OP3 was the dominant microbial group associated with Fe(II)‐ and Fe(III)‐rich regions of the aquifer, suggesting that this uncharacterized phylum may be involved in Fe(II) oxidation. Saline water brings O2, sulfate, and organic C into the intertidal mixing zone. Microbial reduction of sulfate produces sulfide that is transported to the Fe‐mineralized zone leading to the transformation of FeOOH to Fe(II) sulfides. Microbial populations are structured by the availability of chemical species supplied along groundwater flow paths. Seasonal changes in the relative supply of fresh and saline groundwater affect solute fluxes, and therefore, microbial controls on the location and composition of the Fe‐mineralized zone. Ultimately, the composition, extent, and dynamics of the Fe‐mineralized zone will affect the sequestration, affinity, and residence time of solutes bound for export to coastal oceans through SGD.

We describe relationships between pH, specific conductance, calcium, magnesium, chloride, sulfate, nitrogen, and phosphorus and land‐use patterns in the Mullica River basin, a major New Jersey Pinelands watershed, and determine the thresholds at which significant changes in water quality occur. Nonpoint sources are the main contributors of pollutants to surface waters in the basin. Using multiple regression and water‐quality data for 25 stream sites, we determine the percentage of variation in the water‐quality data explained by urban land and upland agriculture and evaluate whether the proximity of these land uses influences water‐quality/land‐use relationships. We use a second, independently collected water‐quality dataset to validate the statistical models. The multiple‐regression results indicate that water‐quality degradation in the study area is associated with basin‐wide upland land uses, which are generally good predictors of water‐quality conditions, and that both urban land and upland agriculture must be included in models to more fully describe the relationship between watershed disturbance and water quality. Including the proximity of land uses did not improve the relationship between land use and water quality. Ten‐percent altered‐land cover in a basin represents the threshold at which a significant deviation from reference‐site water‐quality conditions occurs in the Mullica River basin.