Previous studies have shown that adsorption onto iron oxides and other phases present at sediment-grain surfaces plays an important role in controlling the concentrations and, therefore, mobility of As in sediments.^54,55,56,57 The potential role of adsorption in controlling As concentrations at the Cape Cod site was first examined by computing compositions at adsorptive equilibrium with hydrous ferric oxide and either As(III) or As(V) in the presence and absence of phosphate (Fig. 4). These computations used the generalized diffuse layer surface complexation model and database for adsorption onto hydrous ferric oxide of Dzombak and Morel.⁵⁸ The total concentration of adsorption sites was set equal to 4.77 mM (millimoles per liter), the estimated total concentration of adsorption sites in the aquifer.⁵⁹ For the purposes of these computations, the total As and total phosphate concentrations were set equal to 0.6 µM and 3.1 mM, respectively, which yield a concentration of As(V) equal to 0.06 µM and dissolved phosphate concentration equal to 50 µM at pH 6.1. These are typical values for the suboxic zone of the sewage plume (cf., Fig. 2). Parameter values used in the computations are summarized in Appendix 2.

Figure 4.

Results of the computations illustrate that, in the absence of phosphate, As(III) and As(V) adsorb extensively onto hydrous ferric oxide so that concentrations of both species are maintained at extremely low levels (well below the analytical detection limit) throughout the pH range 5–7 (Fig. 4). In the presence of phosphate, detectable concentrations of As(III) and As(V) are observed throughout the pH range 5–7 (Fig. 4). Adsorbed phosphate comprises greater than 90% of total phosphate throughout the pH range 5–7 and dissolved phosphate concentrations are not affected significantly by the adsorption of either As species at the low As concentrations used in the computations. The trends illustrated in Fig. 4 are consistent with those observed experimentally for the influence of phosphate on adsorption of As(III)^60,61 and As(V) (e.g., Refs. 61,62,63,64,65) on amorphous and crystalline hydrous ferric oxides.

In addition to phosphate, groundwater in the sewage-contaminated zone had sulfate and dissolved silica at concentrations of 100–200 µM and bicarbonate at concentrations of 50–250 µM. Experimental studies have shown that the effect of phosphate greatly exceeds the effects of such low concentrations of these other solutes on As(III) and As(V) adsorption onto hydrous ferric oxide in the pH range 5–7.^60,66,67,68

Adsorption properties of Cape Cod aquifer sediments are controlled by coatings on the surfaces of quartz and other minerals that make up the sediments.¹⁵ Iron is an important constituent of these coatings but quantitative differences between the adsorption properties of the coatings and those of hydrous ferric oxide have been noted.^52,69,70 The coatings range in thickness from approximately 10 nanometers (nm) to 30 µm and are comprised of Al, Si, and Fe.^15,71 High-resolution transmission electron microscopy (HRTEM) of the material coating quartz grains revealed nanometer-size hematite in contact with nanometer-size particles comprised of Al and/or Si.^15,72 HRTEM studies of coatings on quartz grains in Atlantic coastal plain sediments also revealed isolated, nanometer-sized Fe oxide crystallites (goethite in that case) encapsulated in aluminosilicate material, which resembled allophane or halloysite.¹⁶ Aluminum- and Si-rich material in coatings on quartz grains from the Cape Cod site has not been identified but kaolinite, illite, and other clay minerals, not detectable in assays of bulk sediments, have been isolated from feldspar grains.¹⁴

Hydrous Al oxides and clay minerals have a much lower affinity for adsorption of As(III) than do amorphous and crystalline hydrous Fe oxides.^73,74,75 In addition, As(V) adsorbs less extensively on kaolinite, illite, and other clay minerals than onto hydrous Fe or Al oxides at equal site concentrations.^74,76 The presence of phosphate at concentrations higher than As causes As(III) to adsorb very little onto hydrous Al oxide, kaolinite, illite, and other clay minerals and greatly decreases the extent of adsorption of As(V).^73,74,76

These trends in adsorption of As(III) and As(V) onto hydrous Fe and Al oxides and silicates indicate that, if adsorbed As(III) were present at significant concentrations in the sediments, it would have desorbed in response to increasing concentrations of phosphate during the tracer test. The observed increase in As(V) concentrations with increasing phosphate concentrations is consistent with the hypothesis that there is naturally occurring As present as As(V) adsorbed to constituents of the coatings on mineral grains in the sediments and that it is released as a result of competition with phosphate for adsorption sites. This mechanism also probably accounts for the cooccurrence of As(V) and phosphate in the suboxic zone of the sewage plume (Fig. 2).

Arsenic concentrations in the anoxic, Fe(II)-containing zone of the sewage plume were higher than those in the suboxic zone owing to the presence of As(III) in addition to As(V) (Table I). Reductive dissolution of Fe oxides in the coatings would promote the release of As(V) adsorbed to or occluded in Fe oxides and would lead to the greater importance of hydrous Al oxides and silicates in controlling adsorption of As species. Reduction of As(V) to As(III) in the anoxic zone of the sewage plume has been observed but whether the reduction was carried out by As(V)-respiring microorganisms^77,78 or one or more abiotic reactions has not yet been determined.⁷⁹ As discussed above, hydrous Al oxides and silicates adsorb As(III) very little, especially in the presence of high concentrations of phosphate, and have a lower affinity for adsorption of As(V) than Fe oxides. Therefore As(III) is likely to be more mobile and present at higher concentrations than As(V) under the chemical conditions in the anoxic zone of the aquifer. Greater mobility of As(III) than As(V) in anoxic, Fe(II)-containing groundwater is consistent with results of previously conducted field experiments at the Cape Cod site^79,80 as well as results from other studies.¹ Thus it was the combination of sewage-derived phosphate, reduction of As(V) to As(III), and reductive dissolution of iron oxides associated with the sediments that caused concentrations of dissolved As in the anoxic zone to exceed the 0.13 µM drinking water standard.

During the phosphate injection, As(V) concentrations increased steadily as phosphate concentrations increased (Fig. 3). This demonstrates that at least some of the As(V) adsorbed within the coatings on sediment-grain surfaces was desorbed rapidly. This is interesting in light of findings of experimental studies on Fe oxide and clay minerals that the fraction of As(V) taken up by the solid that can be rapidly desorbed decreases with increasing contact time.^74,81,82,83 Thus these results suggest that significant concentrations of As(V) can be desorbed from these sediments over short periods of time in response to changing chemical conditions.

The decrease in As(V) concentrations observed prior to the complete breakthrough of phosphate (Fig. 3) cannot be interpreted unambiguously. It likely results, at least in part, from a chromatographic effect, sometimes called the "snow plow" effect,^59,84 whereby increased mobility of As(V) within the cloud of elevated dissolved and adsorbed phosphate concentrations as compared to the surrounding aquifer causes the accumulation of As(V) near the advancing front of elevated phosphate concentrations. However, slow rates of adsorption and desorption reactions may have played a role in producing a similarly shaped As(V) breakthrough curve obtained in a column experiment where As(V) adsorbed to sand-sized quartz and feldspar grains was displaced by elevated concentrations of phosphate.^85,86 The possible contribution of rate-limited adsorption/desorption reactions in our experiment is suggested by the observed increase in As concentration 1.4 h after completing the injection (Fig. 3). Further increases in As concentrations observed 14 to 15 h after the end of the injection at the breakthrough curve sampling port (Fig. 3) and the injection port (Table II) could have resulted from continued progress toward adsorptive equilibrium between phosphate and As(V) but may be a result of horizontal transport of different solute concentrations from upgradient in the injected cloud. Studies of adsorption of As(V) onto hydrous iron oxide have shown that slow rates of As(V) adsorption and desorption result from slow rates of diffusion through aggregates.⁸⁷ The coatings on Cape Cod sediment-grains consist of intimately aggregated networks of nanometer-size particles,^15,72 a structure that is compatible with a possible limitation to the rate of adsorption by diffusion. Experimental studies have shown that ion exchange reactions responsible for lithium adsorption on Cape Cod sediments are limited by intragranular diffusion.¹⁴

Reductive extractions of the sediments yielded a narrow range of As concentrations (0.9–1.1 nmol/g, Table III). This narrow range of values is consistent with the hypothesis that the As leached by the reductive extractions was primarily associated with coatings on sediment grains, which are ubiquitous and relatively uniformly distributed.^15,29 At the average solid/liquid ratio in the aquifer of 4145 g/l,⁵⁹ an As concentration in the coatings of 1 nmol/g corresponds to a concentration of 4.1 µM. Concentrations of As(V) observed in the suboxic zone and released from the sediments in the uncontaminated zone during the tracer test were 0.03–0.08 µM, which corresponds to 0.7%–2% of the total As in the coatings. Some of the As leached by the reductive extractions may be occluded in Fe oxides or in some other form that is not readily desorbable.⁸³ Nevertheless, the results of these calculations suggest that the As(V) concentrations observed during the tracer test and in the suboxic zone of the sewage plume represent a small percentage of the As associated with the coatings.

Oxidative extractions, which are assumed to leach essentially all of the As from the sediments, yielded a wider range of As concentrations on the sediments (2.0–5.2 nmol/g, Table III). The wider range of total As concentrations on the sediments is consistent with the hypothesis that As not associated with coatings on sediment grains is in the interior of mineral grains and that the range of values reflects heterogeneity in the distribution of these grains in the sediments. Studies of As in drinking water supplies elsewhere in New England^88,89 concluded that, in aquifers where elevated As concentrations were observed, As was ultimately derived from pyrite and other sulfide minerals, which, in contrast to silicate minerals, typically have significant concentrations of As.¹ Thus the dominant As-containing minerals are likely to be pyrite and other sulfide minerals, and, possibly, minerals derived from chemical weathering of these minerals. Sulfide minerals have not been identified in mineralogical studies of the sediments³⁰ but their abundance is likely to be low. Based on the maximum As concentration determined in the oxidative extractions of the sediments and assuming that the As content of sulfide minerals present in the sediments exceeds 1.3 µmol/g (100 ppm), the lower limit for pyrite,¹ sulfide minerals should be present at less than 0.4% by weight of the sediment. Sulfide minerals would be difficult to identify at such low concentrations.

The abundance of As in these sediments is lower than the estimated average abundance of As in crustal rocks of approximately 27 nmol/g (2 ppm), but similar to the range of As concentrations determined in granitic rocks from outcrops at various places in New England of 0.5–9 nmol/g (0.04–0.7 ppm).⁸⁹ Thus the As concentrations in these rocks may be representative of As concentrations of source rocks for the aquifer sediments.

Arsenic associated with coatings on sediment grains likely resulted from adsorption and other reactions between constituents of the coatings and As released as a result of chemical weathering of pyrite and other sulfide minerals under the oxic, mildly acidic conditions characteristic of uncontaminated groundwater in the aquifer. Results of experimental studies suggest that arsenic associated with pyrite is readily released in contact with oxic water and is oxidized to As(III) and As(V) at a rate similar to the rate at which Fe(II) is oxidized to hydrous ferric oxide.⁹⁰ In his examination of the sediments from the site, Barber³⁰ identified grains comprised of amorphous and crystalline iron oxyhydroxides, likely products of chemical weathering of pyrite exposed to oxic, mildly acidic groundwater.^13,90,91 A field experiment carried out in the uncontaminated zone showed that As(III) was oxidized to As(V) over short transport distances.⁸⁰ Thus chemical weathering of pyrite would lead to the production of As(V), which is extensively adsorbed by the hydrous Fe and Al oxides and silicates that comprise the coatings on sediment grains. Arsenic leached by the reductive extractions constitutes 18%–49% of the total As in the sediments (Table III), which suggests that a significant fraction of the As originally associated with pyrite and other As-containing minerals at the time the sediments were deposited has been transferred to the coatings on sediment grains.