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Fig. 1. Map, showing locations of sampling sites (23A13, F625, F343), sewage disposal beds, extent of historically sewage-contaminated groundwater (Ref. 32), the hydraulic gradient direction in June 2003 (middle arrow), and, for comparison, extremes in hydraulic gradient direction measured between 1987 and 2003. First citation in article

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Fig. 2. Groundwater chemistry at 23A13, sampled May 23, 2003 (prior to the phosphate injection). (a) Dissolved oxygen (DO) in micromoles per liter (µM), specific conductance (SpC) in microsiemens per centimeter (µS/cm), and concentrations of total dissolved arsenic (AsT), arsenic(III) [As(III)], and arsenic(V), [As(V)] in µM. (b) Phosphorus (P) in µM and pH. Bars show error limits for measured As(V) concentrations. Also shown are locations of the water table at the time of sampling, the injection and breakthrough-curve-sampling ports used for the tracer test, and the uncontaminated (UZ) and suboxic zones (SZ). First citation in article

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Fig. 3. Tracer test breakthrough curves. (a) Specific conductance (SpC) in microsiemens per centimeter (µS/cm) and pH. Expected arrival time of the tracers, calculated assuming spherical symmetry of the injected tracer cloud and an effective porosity of 0.39, and the end of the injection are indicated. (b) Concentrations of phosphorus, total dissolved arsenic (AsT), arsenic(III) [As(III)], and arsenic(V) [As(V)] in micromoles per liter (µM). Bars show error limits for measured As(V) concentrations. Sampling 14.5 h after completing the injection showed 0.078 µM As(V), 435 µM phosphorus, and a pH of 5.91. First citation in article

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Fig. 4. Dissolved concentrations of arsenic(III) [As(III)] or arsenic(V) [As(V)] and phosphate in adsorptive equilibrium with hydrous ferric oxide plotted against pH computed using the adsorption model of Dzombak and Morel (Ref. 58) at the total arsenic (dissolved plus adsorbed, AsTOT), total phosphate (PTOT), and adsorption site (Sites) concentrations indicated. Values used in the computations are presented in Appendix 2. First citation in article