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We will probably need to develop several different-scale explanatory frameworks. Most immediately, in order to design the topical studies we will need to develop a set of fairly fine-scale questions that can be applied to multiple Study Units. In addition, we will likely need to explore several different types of explanatory framework.
Several national efforts may help us describe general arsenic controls and occurrence across the US; and at the same time guide the direction of our regional frameworks.
There are also a number of national or large-area geologic datasets that may help provide broad explanatory frameworks.
Deriving this type of framework can take the form of starting with a broad aquifer classification such as the "Glacial deposits" principal aquifer, and refining the area into finer classifications using broad depth-categories along with analysis of existing water-quality data. This approach may be the simplest to "aggregate back up" to the scale of the 16 principal aquifers.
One example of this approach is Bob Rogers' RASA work, which divided the "Glacial deposits" and "New York-New England crystalline" principal aquifers into three regions based on ground-water geochemistry:
Testing geochemical process hypotheses can be an explanatory framework in itself. Typically such an hypothesis might investigate what combination of source materials and geochemical processes are driving arsenic concentration. (Alan Welch has created several PowerPoint illustrations of common geochemical processes (combinations of natural source and mechanism) that may mobilize arsenic into ground water.)
For example, many wells in the upper Midwest's glaciated quaternary sediments have high arsenic concentrations. Two main theories for release of arsenic from geologic sources in this region are: (1) Oxidation of arsenic-bearing sulfides (a good example is Fox Valley in Wisconsin), and (2) Dissolution or desorption of iron oxides. Distinguishing between these alternatives could include obtaining sulfur isotope data for sulfide minerals and dissolved sulfate. Additionally, the oxygen isotopic composition of sulfate and water also can be useful in determining the contribution of sulfate from sulfide minerals.
Release of arsenic from aquifer solids such as iron oxide in alkaline, oxic ground water is another possibility. Present ground water in contact with Na-rich exchange sites can promote dissolution of calcite, if present, by exchange of Na for Ca leading to alkaline pH values. Where iron oxide is present, arsenic can be desorbed. Evaluation of this hypothesis could include sequential extraction of arsenic from cores and determination of the arsenic content of iron oxides. This scenario seems to be the cause of high arsenic ground water in the central Oklahoma aquifer.
The presence of high dissolved iron concentrations is suggestive of anoxic conditions and the dissolution of iron oxide, another possible mechanism for the release of arsenic. Mineralogic examination of aquifer materials and sequential extraction would again provide useful constraints on potential geochemical models involving arsenic.
The Oklahoma Pilot Study work is an excellent example of this type of geochemically-driven synthesis. The Water-Supply Papers below very thoroughly describe the study's synthesis of geochemical process hypotheses with data-collection and modeling techniques to pull together a large-scale study. Jamie Schlottmann's on-line papers focus more on the arsenic-specific work.
Figure 1 and Table 1 of Alan's SEGH '98 paper ilustrate how a literature search can set up this kind of work.
[frameworks tied to events (e.g. glaciation), structures (e.g. faulting), and physiography or geography]
Geologic controls on arsenic in ground water need to be considered in conjunction with examination of geochemical aspects. For instance, various investigators have suggested that different glacial lobes have distinct water chemistries, presumably due to differences in the sources of the glacial materials. Incorporation and oxidation of arsenic-rich sulfide minerals may be a necessary condition leading to release arsenic to ground water in these deposits. Others have suggested that stagnant moraines tend to contain high-arsenic ground water. If true, weathering of the glacial material may be important, as seems to be the case in some ground water of Bangladesh.
A large-area example of this type of framework, based on physiographic provinces, is shown in Figure 4 of:
Glaciation-- at two scales:
- scale of individual lobes and moraines, and
- local
Buried Valley aquifer systems came up on a recent conference call, as a possible common question to investigate in several Study Units.
Lithochemical characterization is similar in technique to the aquifer-delineation approach, but starts with geologic maps and reclassifies based on lithologies and chemistries associated with the formations in the map.
While the general assumption is that widespread elevated arsenic concentrations are from natural sources, there are certainly anthropogenic sources as well. In several urbanizing areas, the coring project has found arsenic concentrations in reservoir bottom-sediments increasing over time.
With respect to ground water, we have evidence that in some Study Units land use may play a role in several ways:
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Last modified: 30Mar2001