Uncontaminated ground water

Table 1. Composition of ground water (August-October, 1997) contaminated by leachate from the Area 4 landfill, and uncontaminated ground water in bedrock and alluvial deposits adjacent to the Area 4 landfill. (Concentrations in milligrams per liter unless otherwise noted)...

After Area 4 was covered in 1998, natural dewatering of the landfill was estimated to take about two years. Pre-landfill ground-water-flow eonditions were expected to become reestablished and eontaminants removed as uncontaminated ground water from upgradient flowed through the aquifer. The primary objective of this study was to assess the rate of natural remediation of the aquifer with respect to As. A second objective was to determine if aquifer solids could be the primary souree of As in the landfill plume beneath and downgradient from Area 4. 

The dissolved organic carbon concentration in uncontaminated ground water eluent for the Fe reduction core experiment was 120 mg/L C, and 0 mg/L C for the remediation experiments. 

The composition of pore waters from contaminated cores 1 and 2 were used to initialize the model (Table 2). Concentrations represent leachate collected from the initial half pore volume of each core. Eluent specified in the transport simulations had the composition of uncontaminated ground water in Table 2. Reactions proposed to describe concentration changes for selected constituents within the cores are based on comparisons between eluent and leachate chemistry and analysis of selected constituents in the core samples. Equilibrium constants and kinetic rates for the reactions were adjusted to give the best fit to leachate concentrations from core 1. The same reactions, equilibrium constants, and kinetic rates were then tested by modeling the concentrations of constituents in leachate from core 2. This geochemical model will be used in the future to simulate evolution of contaminated ground water associated with the Area 4 landfill at the aquifer scale. 

Figure 5. Experimental and modeled O2 concentrations in leachate from core I (a) and core 2 (b). Dissolved oxygen in uncontaminated ground water eluent was 6 mg/L.

Figure 6. Dissolved organic carbon concentrations in leachate from cores I and 2. Initial DOC concentration in pore water of core I was 45 mg/L C, and in pore water from core 2 was 31 mg/L C. Dissolved organic carbon in uncontaminated ground water eluent was below detection.

The concentration of Fe in pore water from core 1 was 50 mg/L, and the concentration in pore water from core 2 was 32 mg/L (Table 2). Chemical analyses measured only Fe(II) Fe(III) was below the detection limit. The concentration of Fe(II) in leachate from the contaminated cores rapidly decreased as Fe-free uncontaminated ground water displaced the contaminated pore water (Figs. 8a-8b). Within a few pore volumes, Fe(II) concentrations were less than 5 mg/L. For the remainder of the experiments, Fe(II) concentrations deaeased at a much slower rate. The low concentrations (<5 mg/L) measured in the first 25 pore volumes of leachate coincided with higher flow velocities and measurable O2 concentrations. Based on the mass of Fe(II) in leachate from core 1, the concentration of reactive organic carbon necessary to reduce Fe(III) to Fe(II) was about 2% of the total organic carbon. 

Figure 1. Freundlich plot of Mo(VI) sorption parameters for sewage-contaminated ground water and uncontaminated ground water. The size of plotted experimental data points in all Figures encompasses the analytical error.

Figure 7. Simulation of Mo(VI) experimental data from uncontaminated ground water, using the equilibrium sorption, rate-controlled sorption, mixed side-pore diffusion, and profile side-pore diffusion models.

Type 2 systems are somewhat open in that entry of new components is possible but not frequent or likely, and components are tentatively defined. An example is the output from a drinking water treatment plant tha uses an uncontaminated ground water source and chlorinates it to produce a more or less constant variety of halogenated methanes. 

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