Collaborative Research: Redox Boundaries in Shallow Groundwater: Effects of Flow Paths, Sediment Chemistry and Kinetics

关于地下水地球化学演化的许多信息都是在相对较深的含水层系统中进行的。然而，许多影响营养循环、温室气体浓度和污染物降解的生物地球化学反应是发生在近地表地下水中的氧化还原反应。在各种情况下，氧化还原边界既可以是扩散的，也可以是非常尖锐的。这些氧化还原边界的性质受沉积物化学、氧化还原反应动力学(包括细菌的中介反应)、地下水流动路径和地下水混合，或这些因素的组合的影响。在大多数关于硝酸盐浓度或参与氧化还原反应的其他溶质的研究中，可能的物理和化学系统没有被同时或足够详细地检查，以确定对地下水氧化还原反应的主要影响。我们建议对具有不同物理特征和沉积物化学的地点的含水层和水的物理和化学特征进行广泛的现场监测。将使用嵌套式压力计和张力计来监测地下水流动路径，将使用多级取样器来确定水化学。地下水年龄将通过测量溶解在水中的氯氟烃(氟氯化碳)浓度来确定。将对氮和其他生物分馏同位素进行测量，以确定溶质浓度的降低是由于生物调节过程还是由于地下水混合稀释造成的。

Much of the information on the geochemical evolution of groundwater has been conducted in relatively deep aquifer systems. Many of the biogeochemical reactions that influence nutrient cycling, concentrations of greenhouse gases, and degradation of contaminants, however, are redox reactions that occur in near- surface groundwaters. Redox boundaries can be either diffuse and extremely sharp under various situations. The nature of these redox boundaries are influenced by sediment chemistry, the kinetics of the redox reaction (including mediation of the reactions by bacteria), groundwater flow paths and mixing of groundwater, or combinations of these factors. In most studies of nitrate concentrations or other solutes involved in redox reactions, the possible physical and chemical systems have not been examined simultaneously or in sufficient detail to identify major influences on groundwater oxidation-reduction reactions. We propose to use extensive field monitoring to physical and chemical characteristics of the aquifers and water at sites with different physical characteristics and sediment chemistry. Nested piezometers and tensiometers will be used to monitor groundwater flow paths, multi-level samplers will be used to determine water chemistry. Groundwater ages will be determined by measuring concentrations of chlorofluorocarbons (CFC's) dissolved in the water. Measurements of nitrogen and other biologically fractionated isotopes will be made to determine whether reductions in solute concentrations are due to biologically mediated processes or due to dilution via groundwater mixing.