Hydrogeology and simulation of ground-water flow in the aquifers underlying Belvidere, Illinois

The U.S. Geological Survey investigated the
ground-water-flow system and distribution of
contaminants in the vicinity of Belvidere, Illinois,
during 1992?2000. The study included the
compilation, collection, and analyses of
hydrogeologic and water-quality data and
simulation of the ground-water-flow system.
Hydrogeologic data include lithologic,
stratigraphic, geophysical, hydraulic-property,
water-level, ground-water withdrawal, and
streamflow data. Water-quality data include
analyses of water samples primarily for volatile
organic compounds (VOC?s) and selectively for
tritium and inorganic constituents. Data were
collected from about 250 wells and 21 surfacewater
sites. These data were used (1) to describe
the hydrogeologic framework of the ground-waterflow
system, preferential pathways and directions
of ground-water movement and contaminant
distribution, ground-water/surface-water relations,
and the water budget and (2) to develop and
calibrate the ground-water-flow model.
The glacial drift (sand and gravel with some
clay) and Galena-Platteville (fractured dolomite)
aquifers and the sandstone aquifers of the
Cambrian-Ordovician aquifer system compose the
ground-water-flow system underlying Belvidere
and vicinity. The Glenwood confining unit
separates the Galena-Platteville aquifer from the
underlying sandstone aquifers. The Galena-
Platteville aquifer and confining unit may be
absent in parts of the Troy Bedrock Valley, about
1.5 miles west of Belvidere.
Throughout the study area, the Kishwaukee
River and its tributaries seem to be gaining flow
from shallow ground-water discharge.
Potentiometric levels in the glacial drift and
Galena-Platteville aquifers range from about 900
feet above sea level in the upland areas to 740 feet
along the Kishwaukee River.
Estimated horizontal hydraulic conductivity
of the glacial drift aquifer ranges from about 0.13
to 280 feet per day. The Galena-Platteville aquifer
is a dual-porosity unit with the greatest percentage
of flow through fractures and bedding-plane
partings. Estimated horizontal hydraulic
conductivity ranges from about 0.005 to 2,500
feet per day. Estimated horizontal hydraulic
conductivity of the St. Peter aquifer (the uppermost
sandstone aquifer of the Cambrian-Ordovician
aquifer system ranges from about 4.7 to 17.5 feet
per day.
Volatile organic compounds have been
detected in all aquifers underlying Belvidere.
Trichloroethene and tetrachloroethene are the
principal VOC?s detected at concentrations above
regulatory levels, with the largest number of
detections and highest concentrations in the glacial
drift aquifer. VOC?s generally are not detected in
the glacial drift aquifer farther than 1,000 feet from
known or potential source areas (industrial or
disposal sites), because most source areas are near
the Kishwaukee River, where shallow ground
water discharges. Across most of the study area,
the Glenwood confining unit seems to restrict
downward movement of VOC?s into the
underlying St. Peter aquifer; in the immediate
vicinity of Belvidere, downward movement also
seems restricted by lateral movement toward the
municipal wells through permeable intervals in the
2 Hydrogeology and Simulation of Ground-Water Flow in the Aquifers Underlying Belvidere, Illinois
Galena-Platteville aquifer. Fractures and (or)
unused wells that may penetrate the confining unit
seem to provide local pathways for limited
movement of VOC?s to the sandstone aquifers. At
least two municipal wells seem to intercept the
bedding-plane partings at about 525 and 485 feet
above sea level. Water levels in the lower one-third
of the Galena-Platteville aquifer rapidly respond to
withdrawals at these wells.
The ground-water-flow system underlying
Belvidere was simulated to test the conceptual
model of the system. The three-dimensional,
steady-state model represents the glacial drift,
Galena-Platteville, and sandstone aquifers
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