Biogeochemistry at the zone of intermittent saturation: Field-based study of the shallow alluvial aquifer, Rio Grande, New Mexico

The Rio Grande in central New Mexico ( USA) flows through a semiarid alluvial valley; the river is regulated by levees, riverside drain ditches, irrigation structures, and upstream dams. As a large river in a semiarid region, the Rio Grande experiences large variability in flows and solute concentrations due to riparian evapotranspiration, aquifer recharge, and upstream contributions. In order to characterize biogeochemical processes in this setting, surface water and groundwater from the shallow alluvial aquifer between the river and a parallel drain ditch were sampled from 1 to 13 m depth, including high-resolution multilevel sampling near the fluctuating water table, at a representative site on the middle Rio Grande. The zone of intermittent saturation is a region, similar to 50 cm in vertical extent at this site, in which the water table shifts in response to changes in river level and riparian evapotranspiration. Sediment extractions of iron and manganese oxides indicate that these solids are more prevalent in the zone of intermittent saturation, where oxic-anoxic cycling occurs. River water chemistry varies with time, strongly influencing influent waters to the alluvial aquifer. This chemistry evolves significantly in the similar to 100 m from the river to four wells. River concentrations of dissolved oxygen ( mean 6.9 mg L-1) and nitrate ( mean 2.8 mg L-1) are reduced to near zero in the shallower wells ( 1-3 m depth), but are less reduced from the river in the deeper wells ( 5-13 m depth). Mn and Fe increase from near zero in the river to maximum concentrations from 1 to 3 m depth ( mean 1.0 mg L-1 and 2.1 mg L-1, respectively), with lesser increases at 5 and 13 m depth. Sulfate concentration decreases relative to chloride in many samples, especially from 1 to 3 m depth, most likely due to sulfate reduction. Overall, patterns in water and sediment chemistry indicate that waters in the region of the shifting water table are more evolved from the river via terminal electron-accepting processes than deeper waters, including aerobic respiration, denitrification, manganese oxide reduction, iron oxide reduction, and sulfate reduction. This implies that a greater extent of organic carbon metabolism occurs at shallower depths. The influence of sulfate reduction on organic carbon oxidation is facilitated by sulfate concentrations of river water that varied from 31 to 83 mg L-1 during the study. These results illustrate that sulfate reduction may constitute a significant portion of organic carbon metabolism in higher-sulfate shallow alluvial aquifers associated with freshwater rivers. Biogeochemical processes in the shallow alluvial aquifer depend on the river for solute inputs, and may in turn influence large-scale river chemistry.