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

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)

From McNeill (1983)

Factors Affecting Bulk Terrain Conductivity:

• Porosity and permeability

• Moisture content

• Concentration of dissolved electrolytes

• Phase state of porewater

• Total Dissolved Solids (TDS)

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

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

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 fine-grained 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

• Strong positive linear correlation between total dissolved solids and SpC of monitoring wells

1000 2000 3000 4000 5000 6000 70000

1000

2000

3000

4000

5000

6000

7000

8000

f(x) = 1.19053068127263 x − 407.770594368917R² = 0.977646058410572

SpC (µmhos/cm)

To

tal D

iss

olv

ed

So

lids

(m

g/L

)

Results

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 conductivityf is the instrument operating frequency

With an average terrain conductivity of 36.6 mS/m and 6.4 kHz operating frequency

de ≈ 21 ft

Terrain Conductivity:

Range of 17-58 mS/m across study area

Fluid Specific Conductance (SpC):

Range of 1380-5410 µmhos/cm among monitoring wells

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

10 100 1000100

1000

10000

100000

f(x) = 43.5138177750653 x^1.15896673367569R² = 0.759553980482265

Spindler & Olyphant DataBrooks et al. DataMinnehaha Site Data

Apparent Conductivity (mS/m)

Sp

C (

μm

ho

s/c

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

• Positive correlation between terrain conductivity and hydraulic conductivity

• Correlation is in agreement with the physical parameters allowing electricity flow defined by Archie’s equation

10.0 100.0

1.0E-09

1.0E-08

1.0E-07

1.0E-06

1.0E-05

1.0E-04

1.0E-03

1.0E-02f(x) = 1.35019201288796E-22 x^11.7718902034291R² = 0.568707411625274

Apparent Conductivity (mS/m)H

yd

rau

lic C

on

du

cti

vit

y (

cm

/se

c)

Terrain Conductivity and Depth to Water

• 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

15.0 20.0 25.0 30.0 35.0 40.0 45.0 50.0 55.0 60.00.00

2.00

4.00

6.00

8.00

10.00

12.00

14.00

16.00

18.00

R² = 0.0723195982515204

Apparent Conductivity (mS/m)

De

pth

to

Wa

ter

(ft)

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

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

Correlation Matrix: n = 22 degrees of freedom = 18

ParameterEstimate

Standard Error t-ratio

Constant 36.30 (bo) 4.177 8.690 ***

SpC 0.32E-2 (b1) 0.001 3.347 ***

DTW 0.48 (b2) 0.337 1.425

ln(Ko) 1.39 (b3) 0.346 4.030 ***

Standard errors and t-ratios

*** 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

ReferencesAtekwana, E.A., E.A. Atekwana, R.S. Rowe, D.D. Werkema Jr., and F.D. Legall. 2004. The relationship of total dissolved solids measurements to bulk

electrical conductivity in an aquifer contaminated with hydrocarbon. p. 281-294. In: Journal of Applied Geophysics, 56.Archie, G.E. 1942. The electrical resistivity log as an aid in determining some reservoir characteristics. p. 54-62. In: Transactions of the American

Institute of Mining and Metallurgical and Petroleum Technology, 146. Benson, A.K., K.L. Payne, and M.A. Stubben. 1997. Mapping groundwater contamination using dc resistivity and VLF geophysical methods-a case

study. p. 80-86. In: Geophysics, 62, No. 1. Börner, F., M. Gruhne, and J. Schön. 1993. Contamination indications derived from electrical properties in the low frequency range. p. 83-98. In:

Geophysical Prospecting, 41.Bouwer, H. and R.C. Rice. 1976. A slug test for determining hydraulic conductivity of unconfined aquifers with completely or partially penetrating wells.

p. 423-428. In: Water Resources Research, 12, No. 3. Brooks, G.A., G.A. Olyphant, and D. Harper. 1991. Application of electromagnetic techniques in survey of contaminated groundwater at an abandoned

mine complex in Southwestern Indiana, U.S.A. p. 39-47. In: Environmental Geology and Water Sciences, 18, No. 1.Burger, H. R., A.F. Sheehan, and C.H. Jones. 2006. Introduction to Applied Geophysics: Exploring the Shallow Subsurface. New York, NY: W. W.

Norton & Company. Ebraheem, A.M., M.W. Hamburger, E.R. Bayless, and N.C. Krothe. 1990. A study of acid mine drainage using earth resistivity measurements. p. 361-

368. In: Ground Water, 28, No. 3.Karlik, G. and M.A. Kaya. 2001. Investigation of groundwater contamination using electric and electromagnetic methods at an open waste-disposal site:

A case study from Isparta, Turkey. p. 725-731. In: Environmental Geology, 40. Kearney, P. and M. Brooks. 1991. An Introduction to Geophysical Exploration. p. 227. Cambridge, MA: Blackwell Scientific Publications. Mazac, O., W.E. Kelly, and I. Landa. 1987. Surface geoelectrics for groundwater pollution and protection studies. p. 277-294. In: Journal of Hydrology,

93, No. 3. aMcNeill, J.D. 1980. Electrical conductivity of soils and rocks. In: Geonics Limited, Technical Note TN-5. (Mississauga, Ontario Canada, October,

1980).bMcNeill, J.D. 1980. Electromagnetic terrain conductivity measurements at low induction numbers. In: Geonics Limited, Technical Note TN-6.

(Mississauga, Ontario Canada, October, 1980).McNeill, J.D. 1983. Electromagnetic measurement of rock conductivity. p. 137-142. In: Potash ’83: Proceedings of the first international Potash

technology conference, Saskatoon, Saskatchewan. Merkel, R.H. 1972. The use of resistivity techniques to delineate acid mine drainage in ground water. p. 38-42. In: Ground Water, 10, No. 5. Schüring, J., M. Kölling, and H.D. Schulz. 1997. The potential formation of acid mine drainage in pyrite-bearing hard-coal tailings under water-saturated

conditions: an experimental approach. p. 59-65. In: Environmental Geology, 31.Spindler, K.M. and G.A. Olyphant. 2004. Geophysical investigations at an abandoned mine site subjected to reclamation using a fixated scrubber sludge

cap. p. 243-251. In: Environmental & Engineering Geoscience, 10, No. 3.Stollar, R.L. and P. Roux. 1975. Earth resistivity surveys – a method for defining ground-water contamination. p. 145-150. In: Ground Water, 13, No. 2.Urish, D.W. 1983. The practical application of surface electrical resistivity to detection of ground-water pollution. p. 144-152. In: Ground Water, 21,

No. 2. Waddle, R.C. and G.A. Olyphant. 2010. Groundwater flow modeling of an abandoned mine lands site scheduled for reclamation. Figure 4. In:

Proceedings of the American Society of Mining and Reclamation, this volume.