Publications—Scientific Investigation Report
By T.B. Reed and John B. Czarnecki
U.S. Geological Survey Scientific Investigations Report 2006-5097—ONLINE ONLY
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Extensive mining activities conducted at the Tar Creek
Superfund site, one of the largest Superfund sites in the United
States, pose substantial health and safety risks. Mining activities
removed a total of about 6,000,000 tons of lead and zinc by
1949. To evaluate the effect of this mining on the ground-water
flow, a MODFLOW 2000 digital model has been developed to
simulate ground-water flow in the carbonate formations of Mississippian
age underlying the Tar Creek Superfund site. The
model consists of three layers of variable thickness and a grid of
580 rows by 680 columns of cells 164 feet (50 meters) on a side.
Model flux boundary conditions are specified for rivers and
general head boundaries along the northern boundary of the
Boone Formation. Selected cells in layer 1 are simulated as
drain cells. Model calibration has been performed to minimize
the difference between simulated and observed water levels in
the Boone Formation. Hydraulic conductivity values specified
during calibration range from 1.3 to 35 feet per day for the
Boone Formation with the larger values occurring along the
axis of the Miami Syncline where horizontal anisotropy is specified
as 10 to 1. Hydraulic conductivity associated with the mine
void is set at 50,000 feet per day and a specific yield of 1.0 is
specified to represent that the mine void is filled completely
with water. Residuals (the difference between measured and
simulated ground-water altitudes) has a root-mean-squared
value of 8.53 feet and an absolute mean value of 7.29 feet for 17
observed values of water levels in the Boone Formation.
The utility of the model for simulating and evaluating the
possible consequences of remediation activities has been demonstrated.
The model was used to simulate the emplacement of
chat (mine waste consisting of fines and fragments of chert)
back into the mine. Scenarios using 1,800,000 and 6,500,000
tons of chat were run. Hydraulic conductivity was reduced from
50,000 feet per day to 35 feet per day in the model cells corresponding
to chat emplacement locations. A comparison of the
simulated baseline conditions and conditions after simulated
chat emplacement revealed little change in water levels, drainage
and stream flux, and ground-water flow velocity.
Using the calibrated flow model, particle tracks were simulated
using MODPATH to evaluate the simultaneous movement
of particles with water in the vicinity of four potential sites
at which various volumes of chat might be emplaced in the
underground mine workings as part of potential remediation
efforts at the site. Particle tracks were generated to follow the
rate and direction of water movement for a simulated period of
100 years. In general, chat emplacement had minimal effect on
the direction and rate of movement when compared to baseline
(current) flow conditions. Water-level differences between
baseline and chat-emplacement scenarios showed declines as
much as 2 to 3 feet in areas immediately downgradient from the
chat emplacement cells and little or no head change upgradient.
Chat emplacements had minimal effect on changes in surfacewater
flux with the largest simulated difference in one cell
between baseline and chat emplacement scenarios being about
3.5 gallons per minute.
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