St. Peters aquifer

Because residential wells are often open to both the St. Peter aquifer and the overlying confining unit, water levels do not provide an accurate measurement of hydraulic head in the aquifer. Therefore, the water levels measured in residential wells are termed static water levels . This is distinguished from the potentiometric surface , defined by water levels in wells open only to the confined St. Peter and deep sandstone aquifers. 

As previously discussed, DO, nitrate, and Fe(III) can each serve as an oxidant of sulfides. In the St. Peter aquifer in the study area, nitrate is generally present at concentrations less than 0.5 mg/1 as N (Saad, 1996) and the near-neutral pH and low DOC content of ground water precludes the possibility of significant concentrations of dissolved Fe(III) on a regional basis. On a local basis, some wells exhibit low pH, and in these wells Fe(III) may be an important oxidant. Given that DO is the most ubiquitous oxidant in the near-surface, it is likely the most significant oxidant of the SCH. 

Where the St. Peter aquifer is confined by the Sinnipee Group, hydraulic-head measurements indicate downward flow though the Sinnipee (Batten and Bradbury, 1996 Conlon, 1998). This flow may induce vertical leakage of DO from the upper unconfined aquifer through the Sinnipee and into the St. Peter aquifer (Fig. 9b). 

The source of the oxidant in the severely impacted wells is most likely the borehole. The strong correlation between high arsenic and the interface of the SCH with the static water level in the well, in both confined and unconfined areas if the St. Peter aquifer, indicates that this interaction exerts significant control on arsenic release. Under confined conditions, the air-water-sulfide interface only occurs within the well. Field data also show that in the confined portion of the aquifer, the DO concentration in borehole water is significantly higher than in formation water. The borehole appears to promote oxidation of sulfides in the SCH in wells where the static water level is approximately coincident with the SCH. 

The St. Peter Sandstone (Ordovician) aquifer of eastern Wisconsin (Brown, Outagamie, Winnebago Counties) is a locally important source of water for private supplies. Arsenic contamination was first identified at two locations in 1987, and subsequent investigations showed that 18 out of 76... 

Figure 3. Cross-section showing major hydrogeologic units in the Fox River valley (see Fig. la for location of section A-A j. The regional recharge area for the St. Peter and deep sandstone aquifers occurs between the erosional edge of the Sinnipee and the regional ground water divide (denoted as "B " in figure). Modified from Batten and Bradbury (1996).

The nitrogen content of humic substances from groundwater is similar to that of humic substances from surface water. Data from the St. Peters and Laramie-Fox Hills aquifers suggest that the nitrogen content of humic substances from groundwater may be somewhat lower than the average nitrogen... 

Humic substances from groundwater are considerably less colored per unit of carbon than humic substances from surface water their absorbance at 465 nm (a wavelength commonly used for color in Standard Methods, 1971) is 3-10 times less than the absorbance of humic substances from surface water. Table 6 compares absorbances of humic substances from groundwater with absorbances of humic substances from surface water. Absorbances of samples from the Red River, St. Peters, and Laramie-Fox Hills aquifers are considerably less than absorbances of humic substances from an average surface water. Only the Madison and the Biscayne samples are similar in color to humic substances from surface water. The Biscayne is... 

FIGURE 2. Infrared spectra of fulvic acid from the Madison (A), St. Peters (B), and Red River (C) aquifers, and of humic and fulvic acids from the Laramie-Fox Hills aquifer (D and E, respectively) (Thurman, 1979). 

Other evidence supporting the kerogen hypothesis is that kerogen is enriched in carbon and hydrogen and depleted in oxygen, as is the case for humic substances from the Red River, Madison, St. Peter, and Laramie-Fox Hills aquifers. The only conflicting evidence is the fractionation,... 

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