The study was carried out in a shallow, unconfined aquifer at the U. S. Geological Survey research site on Cape Cod, MA¹⁷ (Fig. 1). The permeable, glacial outwash sediments consist of coarse-sand and gravel, with lesser fine-sand and silt.¹⁹ Mineralogical analyses have focused on the less-than-2-millimeter size-fractions (<2 mm), in which approximately 90% of the material is comprised of quartz;^14,15 the remainder consists of potassium and plagioclase feldspars, magnetite, hematite, goethite, glauconite, and lithic fragments.^15,28,30 Material greater than 2 mm in diameter comprises approximately 25% of the sediments and consists of quartz grains and lithic fragments, most of which are granitic in composition.³⁰ Locally, the hydraulic gradient direction has varied over approximately 15° during the past 15 yr when it has been measured frequently³¹ (Fig. 1). The groundwater flow direction follows closely the hydraulic gradient direction²² and the groundwater velocity has been steady at approximately 0.4 m per day.

Figure 1.

Land disposal of dilute, secondary sewage effluent at a sewage-treatment facility upgradient of the site over a 60-yr period resulted in a plume of sewage-contaminated groundwater approximately 6 km long, 1 km wide, and 50 m thick.^17,32 Effluent disposal ceased in December of 1995 and dissolved salt concentrations have steadily decreased since that time. However, the distribution of several characteristic redox-reactive species [dissolved oxygen, nitrate, ammonium, and dissolved Fe(II)], pH values, and distributions of strongly adsorbing sewage contaminants like phosphate and zinc have changed little in the region between the upper boundary and the core of the sewage plume.^27,33,34,35 Concentrations of dissolved organic carbon are 1 to 2 mg carbon per liter;³⁶ concentrations of particulate organic carbon on sediments downgradient of the disposal beds are less than 0.01%.^28,30 The boundary between the upper region of the sewage plume and uncontaminated groundwater above it is marked by steep vertical gradients in groundwater chemistry that are temporally and spatially persistent.^25,37,38

Samples were collected with peristaltic pumps.³² After purging, a 50 milliliter (ml) filtered [0.45 micrometer (µm) nominal pore size] sample was collected. Approximately 20 ml were transferred to a high-density polyethylene scintillation vial and acidified to pH 2 with trace-metal-grade nitric acid. The remaining 30 ml sample was preserved by adding ethylenediaminetetraacetic acid (EDTA), which sequesters Fe thereby preventing oxidation and precipitation of hydrous ferric oxide, to a concentration of approximately 600 µM and passed through a syringe packed with 500 mg of a strong anion exchanger (SAX, tetramethylammonium on styrene-divinylbenzene base) pretreated with methanol and water.³⁹ At a pH of 4 (resulting from addition of EDTA), As(V) oxyanions are retained by the SAX whereas the neutral As(III)-hydroxo species passes through.³⁹ Additional unfiltered samples were collected for determining field parameters, including turbidity, specific conductance,³² and pH.³⁴ Dissolved oxygen concentrations below 30 µM were determined in the field;³² dissolved oxygen concentrations greater than 30 µM were determined with a dissolved oxygen probe on samples collected in thoroughly purged biological-oxygen-demand bottles.³²

Concentrations of major, minor, and many trace metals and metalloids were determined on the nitric-acid preserved samples using inductively coupled plasma-atomic emission spectrometry (ICPAES) as described elsewhere.⁴⁰ Relative precision (2 times the standard deviation divided by the average concentration determined analytically) and accuracy (ratio of the average concentration determined analytically to the known concentration), determined by repeated analyses of multielement standards, for all elements reported here were 5% and 90%–110%, respectively, or better. Concentrations of total dissolved As were determined on nitric-acid-preserved samples using ICP-mass spectrometry (ICPMS). Concentrations of As(III) were determined on EDTA-preserved samples passed through SAX using ICPMS after processing to replace chloride with nitrate because high concentrations of chloride can cause an interference in the determination of As. Processing involved evaporating samples of known volume to dryness. The precipitate was reconstituted in 1 ml of concentrated nitric acid (trace metal grade) and once again evaporated to dryness, driving off chloride as hydrogen chloride vapors. This was repeated at least one more time, after which the sample was reconstituted in a known volume of 0.15 moles per liter (M) nitric acid (trace metal grade). Concentrations of As(V) were determined by difference. In some cases, As(V) concentrations were also determined by eluting the SAX column with 0.15 M nitric acid, processing the eluate as described above to diminish the chloride concentration, and analyzing using ICPMS. In all cases, concentrations of As(V) determined by difference were the same as those determined on the nitric acid eluates of the SAX columns within analytical error. The relative precision and accuracy of the As determinations by ICPMS were 3% and 103%, respectively, at a concentration of 0.033 µM; the limit of quantitation was determined to be 0.005 µM (0.4 µg/l). Analytical errors for the As(V) determinations in the SAX eluates were significantly higher than those for other samples largely because of uncertainties in the volume of sample passed through the SAX.

The tracer test involved pumping 445 l of groundwater from the uncontaminated zone with added sodium phosphate and sodium chloride (NaCl) into a single port of multilevel sampler (MLS), 23A13 (Fig. 1) at an altitude of 13.2 m. A detailed description of the construction and installation of MLS has been presented elsewhere.¹⁹ The added tracers brought the measured specific conductance to 441±6 microsiemens per centimeter (µS/cm), the phosphate concentration to 620±30 µM, and the pH to 6.22±0.03. The wells pumped to collect groundwater for the injectate and the pumping rate were chosen to have a minimal impact on the hydraulic gradient near the injection port. Samples were collected as described above.

Sediment samples were collected using a wire-line coring apparatus as described elsewhere⁴¹ and frozen within 4 h of collection. Sediment samples designated R23AWC2 and R23AWC3 were collected approximately 3.6 and 4.8 m west of 23A13, respectively. Subsequently, sediments were removed from the vinyl core liners, dried in a laminar flow hood, and dry-sieved to separate out gravel-sized material (greater than 2 mm in diameter).

Two types of chemical extractions of sediment samples were conducted. Sediment samples were extracted in 0.25 M hydroxylamine hydrochloride in 0.25 M hydrochloric acid at 50 °C for 96 h.¹⁵ For the <2 mm size fraction, approximately 5 g of sediment were extracted in 25 ml of solution. For the >2 mm size fraction, approximately 3 g of sediment were extracted in 20 ml of solution. After 96 h, samples were allowed to cool, filtered (0.45 µm), and evaporated to dryness. In order to diminish the chloride concentration prior to determining arsenic concentrations by ICPMS, samples were repeatedly dissolved in 1 to 2 ml of trace-metal grade concentrated nitric acid and evaporated to dryness. After 8 cycles, each sample was dissolved in 20 ml of 0.15 M nitric acid. Sediment samples were also extracted using hot, concentrated nitric acid and 30% hydrogen peroxide following Environmental Protection Agency (EPA) method 3050B. Approximately 2 g of the <2 mm size fraction and 3 g of the >2 mm size fraction were extracted using this method. After evaporating to dryness, samples were brought up in 10 ml 0.15 M nitric acid, heated for approximately 15 min to help dissolve precipitates formed during evaporation, and filtered (0.45 µm). Procedural blanks had dissolved concentrations of As, Al, Fe, and manganese (Mn) less than 0.1% of concentrations measured in samples.