Hydrogeochemical evolution of ground water in an intensively pumped alluvial aquifer.

摘要

Ground water from 21 of 118 wells from the alluvial aquifer in the Bayou Bartholomew watershed in eastern Arkansas exceeded the maximum contaminant level (MCL) of 10 mug/L for arsenic. Ground water in the watershed is intensively pumped for agricultural usage. It is hypothesized that the intensive ground-water pumping causes significant ground-water level fluctuation that could affect the geochemical evolution and mobilization of heavy metals and, especially arsenic in the alluvial aquifer.; To test the hypothesis and conceptualize the arsenic mobilization mechanisms, field work and laboratory column experiments were conducted. Ground-water level and quality were measured at three monitoring sites (a shallow and a deep well at each site) and 21 irrigation wells. Disaggregated sediments from the boreholes of three monitoring wells were packed in 6-in inner diameter, 2-ft length acrylic columns. The sediment layers correspond to the sediment profile of lithology and thickness in the field. Field collected ground water was passed through a pre-treatment column, also packed with sediments collected in the field, to produce a reducing environment similar to field conditions. The regenerated ground water was used as input for three other columns: (1) a column exposed to air representing oxic water-level fluctuation, (2) a column isolated from air representing anoxic water-level fluctuation, and (3) a column isolated from air with continuous flow. Oxidation-reduction potential (ORP), pH, conductivity, dissolved oxygen (DO), and temperature were measured in situ in the columns, and water was collected periodically for chemical analyses. Ground water samples were collected from the monitoring and irrigation wells during the recharge season (April 2007) and growing season (July 2007), and analyzed for major and trace ions. The ground-water quality was generally similar to water samples from the laboratory column experiments (e.g. arsenic: 5--88 mug/L and 10.3--354 mug/L, iron: 0.016--38 mg/L and 0.029--50.5 mg/L for field and laboratory analyses, respectively).; Statistical and graphical analyses, and geochemical modeling with PHREEQC indicated that surface complexation of arsenic with iron oxyhydroxides, reductive dissolution and ion exchange were the main geochemical processes controlling arsenic concentrations and transport in the two columns representing water-level fluctuation, whereas mineral dissolution and ion exchange were the main geochemical processes operating in the continuous flow column. Reductive dissolution was very effective for controlling arsenic concentrations in the oxic water-level fluctuation column. In the anoxic water-level fluctuation system, reductive dissolution was less important than for the oxic water-level fluctuation column. In the continuous flow column, reductive dissolution and surface complexation were almost negligible to control arsenic concentrations. A small amount of competitive sorption by bicarbonate, silica and phosphate was observed in all columns.; These distinct conditions in the laboratory (oxic and anoxic fluctuation, and continuous flow) are aggregated in the natural environment. Based on the laboratory column experiments and ground-water chemistry from the field samples, the following mechanisms were determined for arsenic mobilization: (1) In an oxidizing environment (during the growing season when the water level declined and oscillated because of draw down by pumping for irrigation) arsenic was sorbed onto the iron oxyhydroxides in the sediment, (2) in a reducing environment (when the water table recovered during the winter and spring), arsenic was released into the ground water by the reductive dissolution, resulting in the increase of the arsenic concentrations, and (3) infiltration of phosphate, bicarbonate, and nitrate increased the common ion effects, including competitive sorption and interference with sorption. The infiltrated organic matter from the surface to the aquif