Transcript b 1 - Indiana University

Mapping the Variability of Groundwater Quality in an
Abandoned Tailings Deposit Using Electromagnetic
Geophysical Techniques
D. Alex Gore and G.A. Olyphant
Indiana Department of
Natural Resources
Purpose of Study
• Geological heterogeneity makes characterizing Abandoned Mine Lands (AML)
difficult and expensive
• Electrical geophysical techniques have been used to characterize and map
groundwater quality at AML sites
-Merkel 1972, Stollar and Roux 1975, Ebraheem et al.1990, Brooks et al. 1991,
and Spindler and Olyphant 2004
• Electromagnetic (EM) techniques can be utilized for relatively inexpensive and quick
point measurements
-EM techniques have been used in shallow geophysical studies to characterize and
map groundwater quality
-McNeill 1980, Mazac et al. 1987, Brooks et al.1991, Börner et al. 1993, Karlik
and Kaya 2001, Atekwana et al. 2004, and Spindler and Olyphant 2004
Geonics Limited® EM34-3 Terrain Conductivity Unit
From McNeill (1983)
EM34-3 Instrument:
• Measures bulk conductivity (terrain conductivity) from
the ratio of the secondary magnetic field to the primary
magnetic field
• Reports values in milliSiemens per meter (mS/m)
EM34-3 Instrument Response
Relationship Between Bulk and Fluid
Conductivity (Archie 1942 and Atekwana et
al. 2004)
σb = a ϕm Swn σw
σb is the bulk electrical conductivity of the
Factors Affecting Bulk Terrain Conductivity:
• Porosity and permeability
• Moisture content
• Concentration of dissolved electrolytes
porous medium
a is a constant related to sediment type
ϕ is the porosity
m is the cementation factor
Sw is the water saturation
n is the saturation exponent
σw is the electrical conductivity of the pore fluid
• Phase state of porewater
• Total Dissolved Solids (TDS)
Study Approach
Mapping Variability of Groundwater Quality:
• Shallow geophysical technique - Electromagnetic conductance
-Instrument: Geonics Limited® EM34-3 Terrain Conductivity Unit
Evaluating the EM34-3 Instrument’s Ability to Respond to Variations in Groundwater
Quality:
• Compared instrument measurements to:
-Total Dissolved Solids (TDS) in groundwater
- Hydraulic conductivity
- Depth to Water (DTW)
Study Site: Minnehaha
• Abandoned surface coal mine located in
Sullivan County, southwestern Indiana
• Contains both coarse-grained and finegrained coal refuse materials
• Scheduled for on site reclamation
treatment by Indiana Department of
Natural Resources – Division of
Reclamation (IDNR-DOR)
Study Site: Minnehaha
Methods
Mapping Spatial Variation in Terrain Conductivity:
• Over 280 point conductivity measurements were taken using EM34-3 instrument with a
10 meter spacing
• Measurement locations were plotted using a GPS unit and ESRI ArcGIS® software
• Point measurements were interpolated using inverse distance weighting to create a continuous
terrain conductivity distribution
Evaluating the EM34-3 Instrument’s Ability to Respond to Variations in Groundwater
Quality:
• Terrain conductivity measurements were taken at each of the 27 monitoring well locations
• Terrain conductivity values were compared to:
- the Specific Conductance (SpC) of well water, to represent TDS
- Hydraulic conductivities determined from slug tests, to represent permeability
- Depth to Water (DTW), to represent instrument target depth
Total Dissolved Solids and SpC Correlation
Total Dissolved Solids (mg/L)
8000
7000
6000
y = 1.1905x - 407.77
R² = 0.9776
5000
4000
3000
2000
1000
0
0
1000
2000
3000
4000
5000
6000
7000
SpC (µmhos/cm)
• Strong positive linear correlation between total dissolved solids and SpC of monitoring
wells
Results
Terrain Conductivity:
Range of 17-58 mS/m across study area
Fluid Specific Conductance (SpC):
Range of 1380-5410 µmhos/cm among monitoring wells
EM34-3 Approximate Penetration Depth (Kearney and Brooks 1991):
de ≈ 100 (σ f )-1/2
de is the effective depth of penetration
σ is the bulk ground conductivity
f is the instrument operating frequency
With an average terrain conductivity of 36.6 mS/m and 6.4 kHz operating frequency
de ≈ 21 ft
Spatial Variation in Terrain Conductivity
Terrain Conductivity Over Active Mine Refuse Deposits
Interpolated Terrain Conductivity Distribution
Flow at Water Table
Terrain Conductivity and Fluid SpC Correlation
100000
SpC (μmhos/cm)
y = 43.514x1.159
R² = 0.7596
10000
Spindler &
Olyphant Data
Brooks et al.
Data
1000
Minnehaha
Site Data
100
1
10
100
1000
Apparent Conductivity (mS/m)
• Positive log-linear correlation between terrain conductivity and SpC
• Correlation is in agreement with studies conducted at AML sites having similar
hydrogeological settings to Minnehaha (Brooks et al. 1991 and Spindler and Olyphant
2004)
Terrain Conductivity and Hydraulic Conductivity
Apparent Conductivity (mS/m)
1.0
10.0
100.0
Hydraulic Conductivity (cm/sec)
1.0E+00
1.0E-01
1.0E-02
1.0E-03
1.0E-04
y = 1E-22x11.772
R² = 0.5687
1.0E-05
1.0E-06
1.0E-07
1.0E-08
1.0E-09
• Positive correlation between terrain conductivity and hydraulic conductivity
• Correlation is in agreement with the physical parameters allowing electricity flow
defined by Archie’s equation
Terrain Conductivity and Depth to Water
18.00
16.00
Depth to Water (ft)
14.00
12.00
R² = 0.0723
10.00
8.00
6.00
4.00
2.00
0.00
0.0
10.0
20.0
30.0
40.0
50.0
60.0
Apparent Conductivity (mS/m)
• No significant correlation between terrain conductivity and depth to water
• Lack of correlation is likely due to shallow water table and instrument response to
depth
EM34-3 Instrument Response to Depth
EM34 instrument relative response with depth where the y-axis is relative response
and the x-axis is skin depth (z), z = depth/intercoil spacing (McNeill, 1980_TN-6).
Statistical Analysis of Instrument Response
Statistical Model:
TC = bo + b1 SpC + b2 DTW + b3 ln(Ko) + e
TC = terrain conductivity measured using the EM34-3 instrument (mS/m)
bo = is a regression constant
b1 = regression coefficient for fluid specific conductance
SpC = fluid specific conductance (µS/cm)
b2 = regression coefficient for depth to water
DTW = depth to water table (ft)
b3 = regression coefficient for hydraulic conductivity
Ko = hydraulic conductivity (cm/sec)
e = random error term
Results of Statistical Analysis
Correlation Matrix: n = 22
degrees of freedom = 18
SpC
DTW
ln(Ko)
TC
SpC
1.000
-0.146
0.318
0.639
DTW
-0.146
1.000
-0.628
-0.267
ln(Ko)
0.318
-0.628
1.000
0.690
TC
0.639
-0.267
0.690
1.000
Standard errors and t-ratios
Parameter Standard
Estimate
Error
t-ratio
SpC
0.32E-2 (b1)
0.001
8.690 ***
3.347 ***
DTW
0.48 (b2)
0.337
1.425
ln(Ko)
1.39 (b3)
0.346
4.030 ***
Constant 36.30 (bo)
4.177
*** values are statistically different from 0
at the 99% confidence level
Conclusions
Electromagnetic Measurements
• Have positive correlation to fluid SpC and hydraulic conductivity
• Strongest correlation to hydraulic conductivity followed closely by fluid SpC
• Show no correlation to DTW because of shallow water table (<16ft)
Electromagnetic Investigation as an AML Reclamation Tool
• Should be used as an initial site characterization tool
-and to help in determining monitoring well locations
• Apparent conductivity (terrain conductivity) is not synonymous with the
concentration of contaminants at the study site
• Interpreting electromagnetic data requires special attention to variations in
permeability
Acknowledgements
• Dr. Gary Pavlis (Indiana University, Bloomington)
• Indiana Geological Survey, Center for Geospatial Data Analysis
-Shawn Naylor (director)
-Rob Waddle (data collection & processing)
-Jared Olyphant (data collection)
-Jeff Olyphant (data collection)
- Sally Letsinger (GIS processing)
- Jack Haddan (instrumentation)
- Dalton Hardisty (data collection)
• Research support was obtained through a contract with the Indiana Department of
Natural Resources - Division of Reclamation.
Indiana Department of
Natural Resources
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