Maquoketa Formation. Sinnipee Group dolomite (about 300 feet thick) underlies the Maquoketa
Formation. Cambrian-Ordovician sedimentary units, estimated to be about 600 feet thick in this
area, underlie the Sinnipee dolomite (Eaton, et al, 1999).
Hydrogeologic Setting and Well Water Use
The hydrostratigraphy of Walworth County has been evaluated as a part of a larger study of
groundwater resources of southeastern Wisconsin (Eaton, et al, 1999). A simplified
hydrostratigraphic framework is illustrated in Figure 5. On a regional basis, the upper,
unconsolidated deposits and the Silurian Group dolomites are considered to be an unconfined
aquifer. The upper portion of the Silurian aquifer is highly fractured; wells completed in this unit
are fed predominantly by water moving through horizontal fracture networks within the Silurian
(verbal communication, K. Bradbury, WGNHS, 2000). The Maquoketa confining unit consists of
the Maquoketa and Sinnipee shale and dolomites. The Cambrian-Ordovician confined aquifer
system consists of a series of highly conductive sandstone formations (e.g. St. Peter, Wonewoc
and Mt Simon) interbedded with lower permeability aquitards (the Tunnel City and Eau Claire
units). The recharge area for the deep sandstone aquifer is to the west in Waukesha and Jefferson
counties, where the Maquoketa confining unit is not present.
Well water use in the study area is consistent with this regional characterization. Water yield
from shallow geologic materials is often sufficient for domestic use and to supply small public
water systems. Accordingly, many wells are screened in the thick sequence of sand, gravel and
clay, or are cased through these deposits and open to shallow bedrock (the Silurian dolomite). A
small number of domestic wells are open to the Maquoketa Formation and Sinnipee Group
dolomites, which provide sufficient water yield on a local basis. Larger public systems (such as
those that serve local resorts and hospitals) are cased through the Maquoketa, and occasionally
through the Sinnipee Group, and are open to the Cambrian-Ordovician (deep sandstone) aquifer
system. Table 5 lists the aquifer that wells sampled for this study are open to.
Static water levels in wells (recorded on WCRs) and the physiographic setting indicate that the
study area is an area of groundwater recharge to the upper, unconfined aquifer (Table 5). Water
levels are highest in wells located on the topographic high just east of the Wood School (about
880 feet M.S.L. at the Hartshorne and Brunk residences), and decrease towards the lakes (about
860 ft M.S.L. at the Johnson residence). There is a significant downward vertical gradient across
the Maquoketa confining unit, as demonstrated by water levels in near-by wells completed in the
Silurian dolomite and the deep sandstone aquifer: the Wood School well, open to shallow
bedrock, has a static water level of about 860 ft MSL, while the Sunset Hills Association well,
completed in the deep sandstone aquifer, has a static elevation of about 786 ft M.S.L.
Water levels in wells under pumping conditions were evaluated using information on the WCRs
(Table 5). On the basis of these data, it is highly unlikely that water levels in wells drop below
the base of the well casings under pumping conditions. This is significant because in the Fox
River Valley, at wells with high (greater than 100 [tg/L ) As concentrations, exposure of the
aquifer formation to air within well boreholes is thought to be the source of oxygen acting on
sulfide minerals in the Fox River Valley.
Occurrence of Arsenic in Well Water
Some of the sampled wells within each of the aquifers in the study area contained detectable
levels of arsenic, but only wells completed in the Silurian dolomite have concentrations that
exceed the current MCL of 50 jgg/L (Table 6). Within the subgroup of eight Silurian dolomite

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