Projects > Hydrology and Water Quality Associated With The Midwestern Reclamation Site
Midwestern Reclamation Site Project
Final Report from the Indiana Geological Survey to the Indiana Division of Reclamation
Geophysical Investigations
Purposes: Discussion of Geophysical Methods:
Electrical resistivity is a noninvasive geophysical method for exploring the
shallow subsurface. A current is induced into the ground and the resulting potential difference
(voltage) is measured at various spacings (called a-spacings). Typically, the wider the spacing,
the deeper the current penetrates. In this way the apparent resistivity of the subsurface can be
determined. Two different types of resistivity studies were performed at the Midwestern site:
vertical electrical soundings (VES) and fixed traverses. The
coarse pyritic refuse and acidic groundwater plume that underlies the central part of the site has
a very low resistivity when compared to natural earth materials and can be detected easily by this
method. Vertical electrical soundings were undertaken to identify the proper spacing of electrodes for use
in the fixed resistivity profiles, which will determine the lateral extent of the pyritic refuse
deposit. VESs were placed based on the type of material in the subsurface. Five soundings were
made in order to demonstrate the resistivity effects of various site materials. Locations of VES points VES soundings show that the pyritic refuse layer can be seen as a low-resistivity layer in the subsurface. By comparing the soundings between refuse and nonrefuse areas, it was determined that the optimal spacing for the fixed traverse was greater than 10 meters. At such spacings, the effect of the refuse can be seen at depth. Fixed traverses were used to determine the lateral extent of the buried refuse and (or) associated
contaminated plume in the central lowland area. Location of fixed resistivity profiles As the traverse progressed over the refuse deposit, the apparent resistivity of the subsurface
decreased from around 35 ohm-meters to around 17 ohm-meters. Instead of values simply decreasing
to a low and then increasing again, there was a slight increase before the sudden decrease. This
is indicative of a "vertical contact" between high and low resistivity materials. By knowing the
a-spacing and the step-size between measurements, the location of this contact (i.e., the edge of
the refuse) can be inferred. In line A-A', the low resistivity materials were approximately 140
meters wide. In line B-B', the low resistivity materials were approximately 100 meters wide.
Electromagnetic terrain conductivity was used to accurately delineate the lateral extent of the plume of acidic mine drainage in the subsurface. In EM methods, an electromagnetic field is emitted by a
transmitting antenna. This field interacts with materials in the subsurface and induces a
secondary EM field in the subsurface, which is detected by a receiving antenna. The strength
of this secondary field is directly related to the conductivity (inverse of resistivity) of
the subsurface at that point. EM methods can provide the same type of information as a
resistivity, but are typically much faster and more efficient. As shown in the accompanying graph, a strong correlation exists between conductivity
measurements and data obtained through geochemical
sampling. This correlation can be used to delineate the area
affected by acidic mine drainage in the subsurface. A three-dimensional view was created, showing the area of contaminated groundwater within the buried coarse-grained
refuse where total dissolved solids (TDS) exceeds 5,000 mg/L. This map was derived from the
conductivity survey, using the correlation discussed above. Blue dots indicate the locations of
monitoring wells; note that MW7, whose water samples consistently exhibit the highest
concentrations of contaminants, actually lies just outside the area of greatest contamination.
The EM survey indicates that approximately 40,700 square meters of the
central refuse area has groundwater with TDS that exceeds 5,000 mg/L. Ground-penetrating radar (GPR) was used to determine the lateral extent of the FSS
in the central lowland area.
Investigations were undertaken to:
Conclusions:
Electrical Resistivity
Electromagnetic Terrain Conductivity
Ground Penetrating Radar
Neutron Moisture Gauge
In a typical VES, current and voltage electrodes are moved progressively farther apart about a
central point. By increasing the spacing, the depth of penetration becomes progressively deeper
and a resistivity profile is constructed for that point. The raw data is then run through a
computer program, which produces an earth/resistivity model that serves as an interpretation of the data.
In the fixed traverses, the a-spacing was held constant at 16 meters as the line progressed
horizontally. The fixed traverses were used to detect lateral changes in the resistivity structure
of the subsurface, such as the presence of refuse and/or acidic water.
Vertical Electrical Soundings
Fixed Traverses
Three-dimensional view showing apparent conductivity in the central refuse area after reclamation.
The red peaks indicate the areas of highest conductivity, which are associated with the most
contaminated groundwater within the buried coarse-grained refuse.
Locations of GPR lines
In ground penetrating radar, an emitted electromagnetic pulse travels through the subsurface and
is partially reflected by materials of contrasting electrical impedance, such as layered geologic
materials. The timing of the returns from the reflectors provides information about the depth to,
and thickness of, geologic materials in the subsurface, or other features such as the water.
However, since returns are a function of travel-time rather than depth, it is important to remember
that GPR does not show an actual picture of the subsurface. Many of the returns have been
reflected off of several objects. These returns are called multiples and do not necessarily relate
to reflector in the subsurface.
In this investigation, a bistatic, 100 MHz pulseEkko system was used to record two profiles across the site. The lines were gathered with a one-meter antenna separation and a one-meter step size. Line C-C' (below) clearly shows the beginning and ending of the FSS layer. Although the pyritic refuse layer cannot be directly seen, it is most likely responsible for the multiples seen at long travel times. In this way, GPR can be used to map the lateral extent of the FSS and refuse deposits.
The FSS is approximately 1.5 meters thick, with the minimum thickness being approximately 1 meter. The overlying soil cap is approximately 2 meters thick. Along the GPR line C-C', the FSS extends for a distance of 150 meters. Along the resistivity line B-B' (below), which is in approximately the same location as C-C', the FSS layer completely caps the refuse and its associated contaminate plume.
A neutron moisture gauge was used to monitor changes in the storage of moisture in
the unsaturated zone of the central lowland and to evaluate the effectiveness of FSS in reducing
vertical recharge of the aquifer in the coarse-grained refuse.
A neutron moisture gauge emits fast-moving neutrons into surrounding materials. The neutrons are slowed through collisions with hydrogen atoms and are returned to the gauge. Thus, the ratio of neutrons returned to neutrons emitted is proportional to the amount of hydrogen in the soil. Because measurements are made repeatedly through the same material, changes in hydrogen are related to changes in water content of the material.
Red lines indicate the ranges of measurements through time at various depths. The blue bars represent standard deviations. The variation of moisture content within the FSS is much less than in the overlying materials, indicating that the water which infiltrates the soil cap does not percolate into the FSS. This indicates that the FSS is effectively functioning to prevent vertical recharge of the underlying aquifer in the pyritic refuse deposit.
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