Vertical profiles showing groundwater chemistry (prior to conducting the tracer test) at the top of the sewage-contaminated zone are shown in Fig. 2. Just below the water table there is a zone of uncontaminated groundwater (above approximately 13 m to sea level in Fig. 2), which has high concentrations of dissolved oxygen, pH values near 5.8, and low concentrations of dissolved salts, indicated by the low specific conductance of approximately 20 µS/cm. Steep vertical gradients in groundwater chemistry mark the transition zone between uncontaminated groundwater and the upper part of the sewage plume (between approximately 11 and 13 m to sea level in Fig. 2). Across this transition zone there is a decrease in dissolved oxygen concentrations, a net increase in pH, an increase in specific conductance, and increases in concentrations of sewage contaminants like phosphorus. The upper part of the sewage plume, referred to as the suboxic zone³⁷ (below approximately 11 m to sea level Fig. 2), is characterized by low but measurable concentrations of dissolved oxygen (approximately 5 µM), pH values near 6.2, and elevated concentrations of phosphorus and dissolved salts (the latter indicated by specific conductance values above 100 µS/cm). Concentrations of dissolved Mn below an altitude of 11 m were 2 to 3 µM (data not shown), typical of values in the suboxic zone at this distance (approximately 300 m) downgradient of the sewage-disposal beds.^37,38 Previous comparisons between total dissolved phosphorus concentrations (determined by ICPAES) and dissolved reactive phosphate concentrations (determined colorimetrically) have shown that phosphate accounts for all of the dissolved phosphorus, within analytical error.^42,43

Figure 2.

The groundwater-chemical features shown in Fig. 2 are typical of those observed within 500 m of the former sewage disposal beds, a region that has been sampled frequently over the past 20 yr (e.g., Refs. 25, 26, 32, 37, 38, 44,45,46). The vertical locations of the dissolved oxygen, pH, and phosphorus gradients are virtually identical to those observed in 1993 at the same location (cf., Fig. 2 in Ref. 47).

Concentrations of dissolved As increased across the upper boundary of the sewage plume (Fig. 2). Speciation determinations showed that, within analytical errors, all of the dissolved As was present as As(V). Concentrations of As(V) in the upper part of the sewage plume were 0.06 to 0.07 µM, which is below the MCL for As in drinking water in the United States of 0.13 µM.⁴⁸

Arsenic concentrations were also determined in samples from the anoxic zone of the sewage plume (Table I). None of the MLS ports in or adjacent to 23A13 extend deep enough to reach the anoxic zone. Therefore data are presented for samples obtained from MLS F625 and F343, which are located approximately 30 and 50 m, respectively, upgradient of 23A13 (Fig. 1). At F625, which was sampled approximately 2 weeks after 23A13, Fe(II) concentrations of 69–236 µM, phosphorus concentrations of 18–93 µM, and pH values of 6.34–6.78 were observed. Total dissolved As concentrations were 0.04–0.20 µM (Table I). Arsenic(III) concentrations were 0.02–0.14 µM (Table I). At most depths, concentrations of As(V), determined by difference, were similar to those observed in the suboxic zone of the sewage plume. The total dissolved As exceeded the MCL for As in drinking water in the United States at every depth where the Fe(II) concentration exceeded 100 µM.

The MLS at F625 had not been sampled previously. In order to compare the groundwater chemical conditions in the anoxic zone determined in June 2003 with historical data we also present data from the anoxic zone at a nearby MLS, F343 (Fig. 1), which was sampled in June 2002 (Table I). Arsenic concentrations and other groundwater quality parameters from the anoxic zone at F343 are similar to those at F625. The Fe(II) concentrations of 214–330 µM, phosphorus concentrations of 24–90 µM, and pH values of 6.48–6.58 were within the range of values determined previously in the anoxic zone at this location.^32,34,37,44 Dissolved sulfide concentrations were less than 3 µM (the analytical detection limit).

Gschwend and Reynolds³⁹ observed ferrous phosphate colloids approximately 0.1 µm in diameter and Fe and phosphate concentrations supersaturated with respect to vivianite [Fe₃(PO₄)₂·8H₂O] in a nearby well sampled in 1985. Chemical compositions in Table I indicate supersaturation with respect to vivianite (thermodynamic data from Al-Borno and Tomson⁵⁰). If present, ferrous phosphate colloids 0.1 µm in diameter would have passed through the 0.45 µm filters and contributed to the observed Fe and P concentrations. However, in contrast to the light scattering results reported by Gschwend and Reynolds,⁴⁹ turbidity values for these samples were low (Table I) and similar to those of samples from the uncontaminated and suboxic zones. Furthermore, previous sampling at this MLS showed similar Fe and P concentrations but no difference between Fe and P concentrations in unfiltered samples and those filtered through 0.1, 0.45, and 8 µm filters.³² Why precipitation of vivianite would be inhibited in the anoxic zone of the sewage plume is unknown, but these observations suggest that it is unlikely that ferrous phosphate colloids contributed significantly to the Fe, P, and, by extension, As concentrations reported in Table I.

Groundwater to which phosphate had been added was pumped into a port in the uncontaminated zone and the port 0.3 m above it was sampled periodically (altitudes of these ports are indicated in Fig. 2). Chloride was also added to increase the specific conductance, which was used as a conservative, nonreactive tracer. Previous experience has shown that, during the injection (which lasted 7.7 h), the tracer cloud (injected groundwater with added tracers) displaces the ambient groundwater with minimal mixing as it expands away from the injection port. After the injection, solutes are transported horizontally at the ambient groundwater flow rate (e.g., Ref. 37). Assuming that the tracer cloud expanded spherically at the pumping rate (1 l per min) and the aquifer into which it was injected had an effective porosity of 0.39,¹⁹ the leading edge of the tracer cloud was expected to arrive at the sampling port 0.7 h after the beginning of the injection. In reasonable agreement with this, specific conductance reached half of the value in the injectate between 0.5 and 1 h after the beginning of the injection (Fig. 3). Phosphate concentrations reached half of the injectate concentration between 1 and 2 h after beginning the injection, indicating that adsorption of phosphate onto the sediments^35,44,51 caused phosphate to move more slowly than chloride and other ions contributing to the specific conductance. Values of pH increased but did not achieve 6.2, the pH of the injectate, by the end of the injection. This is consistent with the results of previous tracer tests, which have shown that groundwater pH values evolve slowly as a result of ion exchange and adsorption reactions driven by differences in chemical composition between the injected solution and ambient groundwater.⁵²

Figure 3.

Arsenic concentrations increased as phosphate concentrations increased over the first 2 h of the injection (Fig. 3). After reaching a maximum of 0.07 µM, As concentrations decreased through the end of the injection. All of the As was present as As(V) within analytical errors. Sampling conducted 1.5 h after completing the injection showed that As concentrations increased to approximately 0.06 µM but phosphate concentrations and pH values remained constant. Sampling conducted 14.5 h after completing the injection showed an As(V) concentration of 0.078 µM, phosphorus concentration of 435 µM, and a pH of 5.91.

The injection port was sampled 1.4 and 14.4 h after completing the injection (Table II). Phosphate concentrations, specific conductance, and pH values 1.4 h after completing the injection were the same as those in the injectate. Arsenic(V) was present at a lower concentration at the injection port than at the port 0.3 m above it (0.0215 µM, Table II, as compared to 0.06 µM, Fig. 3). Thirteen hours later, the As(V) concentrations had increased to 0.059 µM and the phosphate concentration had decreased to 530 µM. Specific conductance was not determined but likely remained equal to that in the injectate. As observed elsewhere in the uncontaminated and suboxic zones, As(III) concentrations were below the detection limit.

Chemical extractions of the sediments were used to estimate the quantity of As associated with coatings on sediment grains and the total quantity of As in the sediments. Reductive extractions, using hydroxylamine hydrochloride at 50 °C for 4 days, were used to estimate the quantity of As associated with the coatings. Previous work with Cape Cod sediments has shown that this method removes visible coatings containing Fe, Al, Mn, and silicon (Si)^15,29 as well as other elements, like zinc and phosphorus, likely to be adsorbed onto constituents of the coatings.^42,44 However, neither Fe nor Al is removed quantitatively from sediment-grain surfaces and it is likely that the extraction procedure leaches some Al and Fe associated with silicate minerals.^15,29 Oxidative extractions, using hot concentrated nitric acid and 30% hydrogen peroxide (EPA method 3050B) were used to estimate the total quantity of As in the sediments. Previous studies have shown that this method extracts As from Fe oxides, metal sulfides, and As sulfides.^53,54

Reductive extractions leached 0.9–1.1 nanomoles As per gram of sediment dry weight (nmol/g) from the <2 mm size-fraction of sediments collected adjacent to the tracer test site and from the <1 mm size-fraction of a composite sample comprised of sediments from the uncontaminated zone (Table III). Preparation of this composite sample has been described elsewhere.⁴⁷ Quantities of As extracted from duplicate samples from the same core varied by less than 10% (Table III). Quantities of extractable Fe (17–23 µmol/g), Al (14–20 µmol/g), and Mn (0.16–0.23 µmol/g) (Appendix 1) were in good agreement with values determined previously on sediments from this site.¹⁵ Gravel-size (>2 mm) material had significant quantities of reductively extractable As (0.9 to 1.5 nmol/g). Gravel-size material was comprised of individual quartz grains and rock fragments, most of which were granitic or gneissic in composition. The individual quartz grains, most of which had surface coatings ranging in color from white to yellow-brown, were separated from the rock fragments by hand and extracted separately. The quartz grains also had significant quantities of extractable As (0.7–1.8 nmol/g).

The oxidative extractions leached 2.0 to 4.0 nmol/g of As from the <2 mm size fractions of sediments adjacent to 23A13 and 5.1 nmol/g from the <1 mm size fraction of the composite sample (Table III). Greater variability was observed in the quantity of As leached by the oxidative extractions than by the reductive extractions. The quantity of As leached from the gravel-sized material was less than that leached from the <2 mm size-fraction from the same core. A separate gravel-sized sample of the whole-except-quartz fraction from core R23AWC02 at 13.4 m to sea level was ground to <0.125 mm and then extracted using the same procedure. The quantity of As leached from the ground sample was 0.56 nmol/g, as compared to 0.33 nmol/g for the sample that had not been ground. This difference is smaller than the difference in As extracted from the two samples of gravel-size sediment. Thus, whether this difference results from greater accessibility of As-containing minerals to the extracting solution owing to grinding or from variability in the As content of different samples of gravel-size sediment is unknown.