USGS Data Series 172 - USGS Publications Warehouse

In cooperation with the U.S. Army Corps of Engineers, Kansas City District

Two-Dimensional Direct-Current Resistivity Survey to Supplement Borehole Data in Ground-Water Models of the Former Blaine Naval Ammunition Depot, Hastings, Nebraska, September 2003

Data Series 172

U.S. Department of the Interior U.S. Geological Survey

Cover. (Left top) Collection of positional data at monitoring well. (Left bottom) Collection of borehole geophysical log. (Right top) Speed bump used to protect cables where profile extends over roads. (Right bottom) A set of multicore cables with 10-meter electrode spacing.

Two-Dimensional Direct-Current Resistivity Survey to Supplement Borehole Data in Ground-Water Models of the Former Blaine Naval Ammunition Depot, Hastings, Nebraska, September 2003 By Wade H. Kress, Lyndsay B. Ball, Andrew P. Teeple, and Michael J. Turco

In cooperation with the U.S. Army Corps of Engineers, Kansas City District

Data Series 172

U.S. Department of the Interior U.S. Geological Survey

U.S. Department of the Interior Gale A. Norton, Secretary U.S. Geological Survey P. Patrick Leahy, Acting Director

U.S. Geological Survey, Reston, Virginia: 2006 For sale by U.S. Geological Survey, Information Services Box 25286, Denver Federal Center Denver, CO 80225

For more information about the USGS and its products: Telephone: 1-888-ASK-USGS World Wide Web: http://www.usgs.gov/ Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although this report is in the public domain, permission must be secured from the individual copyright owners to reproduce any copyrighted materials contained within this report.

Suggested citation: Kress, W.H., Ball, L.B., Teeple, A.P., and Turco, M.J., 2006, Two-dimensional direct-current resistivity survey to supplement borehole data in ground-water models of the former Blaine Naval Ammunition Depot, Hastings, Nebraska, September 2003: U.S. Geological Survey Data Series 172, 31 p.

iii

Contents Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Purpose and Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Hydrogeology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Methodology and Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Borehole Geophysical Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Direct-Current Resistivity Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Inverse Modeling of Resistivity Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Two-Dimensional Direct-Current Resistivity Survey Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 References Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Appendix 1—Geologist Logs and Borehole Geophysical Resistivity Logs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Appendix 2—Inversion Results of Two-Dimensional Direct-Current Resistivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Figures 1. Map showing approximate boundary of the former Blaine Naval Ammunition Depot site . . . . . . . . 2 2. Schematic diagram showing rock stratigraphy, hydrogeologic units, monitoring well construction, ground-water-model layers, and electrical units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Maps showing two-dimensional direct-current resistivity profiles and monitoring well locations of the former NAD site at: (a) Study area 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 (b) Study area 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 4. Photographs showing collection of (a) borehole geophysical log at monitoring well 113BB3, and (b) positional data at monitoring well 113BB3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 5. Photographs showing setup of two-dimensional direct-current resistivity equipment, (a) multicore cable connected to electrode, (b) resistivity meter connected to multicore cables (orange cables connected in back of unit), (c) one of 11 sets of multicore cables with 10-meter electrode spacing, and (d) speed bump used to protect cables where profile extends over roads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 6. Diagrams showing geologist log superimposed on inversion results (true resistivity) of two-dimensional direct-current resistivity profile for: (a) Section plot 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 (b) Section plot 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 (c) Section plot 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 (d) Section plot 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 (e) Section plot 5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 (f) Section plot 6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

iv

7. Diagrams showing geologist log and borehole geophysical resistivity log for monitoring well: (a) MW9BB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (b) MW19BB4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (c) MW20C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (d) MW105BB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (e) MW113BB3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (f) MW155B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (g) MW170B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (h) MW175B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8. Diagrams showing inversion results of two-dimensional direct-current resistivity for: (a) Profile 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (b) Profile 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (c) Profile 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (d) Profile 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (e) Profile 5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (f) Profile 6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

19 20 21 22 23 24 25 26 29 29 30 30 31 31

Conversion Factors and Datums Inch/Pound to SI Multiply

By

To obtain

Length foot

0.3048

meter

mile

1.609

kilometer

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 (NAVD 88). Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83). Elevation, as used in this report, refers to distance above the vertical datum.

Two-Dimensional Direct-Current Resistivity Survey to Supplement Borehole Data in Ground-Water Models of the Former Blaine Naval Ammunition Depot, Hastings, Nebraska, September 2003 By Wade H. Kress, Lyndsay B. Ball, Andrew P. Teeple, and Michael J. Turco

Abstract The former Blaine Naval Ammunition Depot located immediately southeast of Hastings, Nebraska, was an ammunition facility during World War II and the Korean Conflict. Waste-management practices during operation and decommissioning of the former Depot resulted in soil and ground-water contamination. Ground-water models have been used by the U.S. Army Corps of Engineers to provide information on the fate and transport of contaminants on the former Depot site. During September 2003, the U.S. Geological Survey, in cooperation with the U.S. Army Corps of Engineers, Kansas City District, conducted a pilot study to collect two-dimensional direct-current resistivity data on the site along six profiles near existing monitoring wells. The inversion results of field data from five of the six two-dimensional direct-current resistivity profiles display distinct electrical stratigraphy consistent with three resistivity units (low resistivity, high resistivity, and low resistivity). These three resistivity units correlate with rockstratigraphic or hydrogeologic units described prior to this study. To interpret the resistivity profiles, additional data extending through the lower confining unit into the underlying Niobrara Formation could be used with the existing data to construct forward models for data analysis and interpretation.

Introduction The Blaine Naval Ammunition Depot (referred to hereinafter as former NAD site) located immediately southeast of Hastings, Nebr. (fig. 1), was an active facility for loading, assembling, and packing ammunition during World War II and the Korean Conflict. Waste-management practices during the operation of the former NAD site and during the decommissioning process resulted in contamination of soil and ground water (U.S. Environmental Protection Agency, 2004). The former

NAD site later became part of the Hastings Ground Water Contamination Site, which was added to the U.S. Environmental Protection Agency’s National Priorities List in 1986. In 1987, the U.S. Army Corps of Engineers (USACE) began the Remedial Investigation/Feasibility Study (RI/FS) of the former NAD site (Shaw Environmental, Inc., 2004). The RI/FS characterized the nature and extent of contamination at the former NAD site, developed and evaluated remedial action alternatives that addressed potential risk, and complied with regulatory requirements. Studies generated from the RI/FS have been used by the USACE to produce ground-water-flow models that provide information on the fate and transport of contaminants at the former NAD site (IT Corporation, 2002). The reliability of any ground-water-flow model is dependent, in part, on the quality and quantity of data available in a study area. Borehole logs, including geologist descriptions and geophysical data, often are used in conjunction with regional bedrock topology and outcrop observation to define layers in ground-water-flow models that reflect the scale and geometry of the aquifer under study (Merry and others, 2003). In heterogeneous aquifers, lack of subsurface data is a substantial limitation to model results and one of the common sources of model error (Konikow and Bredehoeft, 1992). Additional information on the aquifer system can be obtained by installing wells or test holes, collecting groundwater samples for chemical analysis, examining borehole logs, or conducting aquifer tests, but drilling additional wells and performing aquifer tests can be time-consuming and expensive. An alternative approach is to combine non-intrusive surface geophysical methods with a drilling and ground-water sampling program to provide additional aquifer-system information at a lower cost. Surface geophysical data can be used to interpret elevations of geologic or hydrogeologic units between wells or boreholes, vertical and horizontal distributions of hydrologic properties in the aquifer system, and vertical and horizontal locations of contaminant plumes (Stanton and others, 2003). By conducting a two-dimensional direct-current (2D–DC)

Two-Dimensional Direct-Current Resistivity Survey, Former Blaine Naval Ammunition Depot, September 2003

2

98°20’00”W

98°15’00”W

98°10’00”W

98°05’00”W

Hastings

Figure 3a

40°30’00”N

Former Blaine Naval Ammunition Depot

CLAY COUNTY

ADAMS COUNTY

Figure 3b

NEBRASKA Hastings

40°25’00”N

Location map

Base from U.S. Geological Survey digital orthophotography, 1999 Universal Transverse Mercator projection, Zone 14

0

0

Figure 1. Approximate boundary of the former Blaine Naval Ammunition Depot site.

1

1

2

2

3

3

4

4

5 KILOMETERS

5 MILES


40°35’00”N

98°25’00”W

Introduction

Clay and silt

Pleistocene alluvial aquifer (semiconfined)

Lower confining unit

Layer 2 Variable thickness Total thickness – 4

Upper confining unit

Layer 1

"C" wells

Pleistocene sand and gravel

Water table Pleistocene alluvial aquifer (unconfined)

"BB5" wells

Clay and silt

Unsaturated zone

"BB4" wells

Pleistocene sand and gravel

Ground-watermodel layers

Electrical unit 1

"BB3" wells

Surface soils and loess

Well construction

Hydrogeologic description

"B" wells

Rock-stratigraphic description

3

Electrical unit 2

Layer 3 Layer 4 Layer 5 Layer 6

Electrical unit 3

Figure 2. Schematic diagram showing rock stratigraphy, hydrogeologic units, monitoring well construction, ground-water-model layers, and electrical units (modified from IT Corporation, 2002).

resistivity surface geophysical survey in an area where boreholes are present, resistivity data can be compared to geologic, hydrogeologic, or water-quality data (from boreholes) to determine if there is a relation between these units and units of resistivity. Once a relation is established, 2D–DC resistivity surveys can be used to extend the depth, thickness, and information obtained from boreholes or can be used as an exploratory tool in areas where few or no borehole data are available. During September 2003, the U.S. Geological Survey (USGS), in cooperation with the USACE, Kansas City District, conducted a pilot study to collect 2D–DC resistivity data on the former NAD site along six profiles near existing monitoring wells.

Purpose and Scope The purpose of this report is to describe the methods and results of a 2D–DC resistivity surface geophysical survey conducted in September 2003 on the former NAD site. Borehole resistivity data collected in selected boreholes and inversion results of six profiles of 2D–DC resistivity data are documented. The inversion results and the borehole geologic and geophysical data that were collected or used in the study were input into a three-dimensional (3D) database of the former NAD site.

Hydrogeology The High Plains aquifer lies beneath most of the former NAD site and comprises the Pleistocene alluvial deposits and

the Miocene Ogallala Formation (Keech and Dreeszen, 1968). The surface geophysical investigation of the former NAD site was conducted in two areas where the Ogallala Formation is not present. The Niobrara Formation of Cretaceous-age limestone and chalk lies at the base of the High Plains aquifer (IT Corporation, 2002). The Niobrara Formation is overlain by unconsolidated Pleistocene deposits. The lower part of the unconsolidated Pleistocene deposits, consisting of silt and clay, is considered to be the local lower confining unit at the former NAD site. Thickness of the lower confining unit ranges from 0 to 150 feet. The upper part of the unconsolidated Pleistocene deposits is a sequence of sand and gravel that ranges from 200 to 300 feet in thickness and forms the Pleistocene alluvial aquifer, the primary source of drinking water in the Hastings area. Some areas of the Pleistocene alluvial aquifer on the former NAD site contain layers of clay and silt. In some places these clay and silt layers are as thick as 11 feet and serve as a local confining unit within the Pleistocene alluvial aquifer. About 50 feet of loess deposits overlie the Pleistocene sand and gravel (Shaw Environmental Inc., 2004). For the purposes of the ground-water-flow model, the subsurface beneath the former NAD site in the ground-water feasibility study report (IT Corporation, 2002) is divided as follows (fig. 2): unsaturated zone, Pleistocene alluvial aquifer (unconfined part) (model layer 1), upper confining unit (model layer 2), Pleistocene alluvial aquifer (semiconfined part) (model layers 3–6), and lower confining unit. The unsaturated zone is composed of surface soils, loess, sand, and gravel. The

4


thickness of the unsaturated zone ranges from about 95 to 115 feet. The unconfined part of the Pleistocene alluvial aquifer consists of sand, and to a lesser extent, sand and gravel and clayey or silty sand. Saturated thickness of the unconfined part of the aquifer varies but typically is about 15 feet. The (local) upper confining unit consists of silty clay, clayey silt, and clayey sand. The upper confining unit can be as thick as 11 feet; however, the thickness and extent vary throughout the former NAD site. The semiconfined part of the Pleistocene alluvial aquifer primarily consists of sand and gravel, with thin, discontinuous silty clay and clayey sand layers interbedded in the unit. The semiconfined part of the alluvial aquifer ranges from 140 to 160 feet thick. The (local) lower confining unit consists of silty clay and clayey silt overlying bedrock, but its thickness at the site is unknown.

Methodology and Approach Using surface geophysical methods to measure the physical properties of the subsurface, such as electrical conductivity or resistivity, dielectric permittivity, magnetic permeability, density, or acoustic velocity, provides a relatively quick and inexpensive means to characterize the subsurface (Powers and others, 1999). The results (measurements) can be influenced by chemical and physical properties of soils, rocks, and pore fluids. Interpretations from these measurements can be used to image the distribution of physical properties in the subsurface (American Society for Testing and Materials, 1999). Electrical surface geophysical methods can be used to detect changes in the electrical properties of the subsurface. The electrical properties of soils and rocks are determined by water content, mineralogical clay content, salt content, porosity, and presence of metallic minerals. However, the resistivity of the water typically has a greater effect on bulk resistivity than the soil or rock type. Variations in electrical properties of soils and rocks, either vertically or horizontally, produce variations in the electrical signature measured by surface geophysical tools. Changes in the received signal can be related to changes in the composition, extent, and physical properties of the soils and rocks within the subsurface, to the extent that differences in lithology or rock type are accompanied by differences in resistivity (U.S. Army Corps of Engineers, 1995). However, to effectively detect these differences there must be a contrast in the property measured; for example, the target to be detected or the geologic feature to be defined must have properties substantially different from background conditions (American Society for Testing and Materials, 1999). For electrical surface geophysical methods to successfully detect or define a geologic unit, the geologic unit of interest must have properties substantially different from the geologic unit immediately above or below it. Typically, clay and shale units are less resistive than sands and gravels, which in some cases can produce an electrical contrast that could be detectable with electrical surface geophysical methods (U.S. Army Corps of Engineers, 1995).

The 2D–DC resistivity method was used to collect field measurements of apparent resistivity along six profiles (fig. 3) in two study areas (fig. 1) within the former NAD site. Apparent resistivity is the resistivity of a homogeneous isotropic earth (subsurface) that will give the same resistance value for the same electrode arrangement. To estimate the distribution of resistivity for a heterogeneous anisotropic subsurface, the apparent resistivity data were processed using an inverse modeling software program (Loke, 2002). The results from this program were used to generate 2D–DC profiles of the subsurface distribution of resistivity. Locations for six 2D–DC resistivity profiles (fig. 3) were selected by USACE, Kansas City District, to collect data in different areas throughout the former NAD site where borehole data were available. Geologist descriptions of rock units, collected by USACE during the RI/FS, were supplied for 22 monitoring wells (fig. 3) (B.J. Roberts, U.S. Army Corps of Engineers, written commun., 2003). Electromagnetic inductionresistivity borehole geophysical logs (borehole resistivity logs) were collected by the USGS during September 2003 in eight of the 22 monitoring wells. In most wells, borehole data did not extend to the lower confining unit.

Borehole Geophysical Data Monitoring wells used in this report were named by the USACE according to the placement of the screened interval of the well within its respective aquifer. BB and C designate wells (fig. 3) screened in the upper and lower parts, respectively, of the semiconfined aquifer. B designates wells (fig. 3) screened in the unconfined aquifer (IT Corporation, 2002). Borehole resistivity logs were collected by the USGS at eight monitoring wells with a System VI logging system (fig. 4a) using a 9512 three-coil slim-hole induction tool supplied by Century Geophysical Corporation (2004). Borehole resistivity logs were collected along profile 1 at monitoring wells MW170B and MW175B, along profile 2 at monitoring wells MW19BB4 and MW20C, along profile 3 at monitoring wells MW105BB and MW155B, and along profile 4 at monitoring wells MW9BB and MW113BB3 (fig. 3a). Positional data (horizontal and vertical) for the monitoring wells (fig. 4b) and electrodes used in the surface geophysical survey were collected using an Ashtech Z-extreme real-time kinematic global positioning system (Thales Navigation Inc., 2004). All geologist descriptions and borehole geophysical logs of the boreholes used in this report were incorporated into Oasis montaj version 6.2 (Geosoft, Inc., 2004) and are presented in appendix 1.

Direct-Current Resistivity Data Resistivity surveys are made by transmitting current into the subsurface through two current electrodes and measuring the resulting voltage between two potential electrodes. The

Methodology and Approach

40°34'0"N

98°18'30"W

98°18'0"W

98°17'30"W

MW167B

MW115B

MW170B MW122B

MW175B MW166B

40°33'30"N

LINE 1

MW9BB

MW113BB3

LINE 4 MW101BB3

MW155B 40°33'0"N

MW156BB4 MW19BB4

MW157B

MW105BB

MW20C

LINE 3 LINE 2

0

Base from U.S. Farm Service Agency digital othophotography, 2003 Universal Transverse Mercator projection, Zone 14

Explanation See figure 1 for location of study area 1.

Figure 3a.

0

500

1,000 250

1,500

2,000 FEET 500 METERS

DC resistivity transects

MW20C

Monitoring well

Two-dimensional direct-current resistivity profiles and monitoring well locations of the former NAD site at study area 1.

5

6

98°14'0"W

98°13'30"W

98°13'0"W

98°12'30"W

MW312B

40°30'0"N

MW314B MW315B MW324B

LINE 5 MW319B MW310BB MW321BB

40°29'30"N

LINE 6

0

Base from U.S. Farm Service Agency digital orthophotography, 2003 Universal Transverse Mercator projection, Zone 14

See figure 1 for location of study area 2.

Explanation DC resistivity transects

MW21BB

Figure 3b.

0

Monitoring well

Two-dimensional direct-current resistivity profiles and monitoring well locations of the former NAD site at study area 2.

500

1,000 250

1,500

2,000 FEET 500 METERS


40°30'30"N

98°14'30"W

Methodology and Approach (a)

7

(b)

Figure 4. Collection of (a) borehole geophysical log at monitoring well 113BB3 and (b) positional data at monitoring well 113BB3.

resistance, R, then is computed by dividing the measured voltage by the transmitted current, as described by Ohm’s law: R = V/I,

(1)

where I is the current applied through the current electrodes, and V is the potential difference or voltage measured by the potential electrodes. The apparent resistivity of the subsurface is calculated by multiplying each resistance by a geometric factor determined by the geometry and the spacing of the electrode array (Zohdy, 1974). Deeper apparent resistivity measurements can be obtained by increasing the electrode spacing. The DC resistivity method is described in detail by Grant and West (1965) and Zohdy (1974). Resistivity data can be collected using different techniques. Traditionally, individual resistivity measurements are made by keeping the central point of the array at the same location and increasing the electrode spacing to obtain measurements at increasing depths, a technique known as resistivity sounding. The central point of the array then is moved and the process repeated until the desired area is covered. An alternative to resistivity sounding is resistivity profiling, in which a large number of electrodes are connected to a multiconductor cable and controlled by an automatically switching resistivity meter. The resistivity meter uses an initial set of four electrodes (two current electrodes and two potential electrodes) to make a measurement, switches to another four electrodes, and then continues until all electrodes have been used in a sequence of different spacings to create a 2D section of apparent resistivity. Using the Wenner-Schlumberger array (Zohdy, 1974), 2D–DC resistivity data at the former NAD site were collected using an IRIS Syscal R1 Plus switching unit (IRIS Instruments, 2004). Using four electrodes at a time, the unit switches among a combination of 11 sets of multicore cables (fig. 5c) with six

electrodes each at 10-meter spacing to collect multiple points from a single layout. After the initial section of data was collected, the first two cables of 12 electrodes were moved ahead of the survey profile. A partial section of data then was collected using the previous 54 electrodes (electrodes 13–66) and the 12 electrodes (electrodes 67–78) that were just moved. This process, known as the roll-along technique, was continued until all data along the desired profile length were collected. Plastic speed bumps were used to protect the cables when profiles extended over roads (fig. 5d). The data from the roll-alongs were combined into a single apparent resistivity dataset during processing.

Inverse Modeling of Resistivity Data Apparent resistivity, as calculated from the field measurements, is the electrical resistivity of an equivalent electrically homogeneous isotropic subsurface and is used to represent the average resistivity of the heterogeneous subsurface (Loke, 2000). To estimate the “true” subsurface resistivity, an inversion program develops a 2D model consisting of rectangular blocks of individual resistivities. The inversion program then determines the calculated system response of that model, the calculated apparent resistivity, on the basis of the collected field data properties. The root mean square (RMS) difference between the measured and calculated apparent resistivities is used to determine the accuracy of the model. The inversion program then attempts to reduce the RMS difference by altering the model resistivity values, and the apparent resistivity is recalculated; this alteration is known as an iteration. When the RMS difference between the calculated and measured apparent resistivity no longer improves substantially between iterations (more than 1 percent between iterations), a solution is reached. This final model represents a non-unique estimate of the true 2D

8


(a)

(c)

(b)

(d)

Figure 5. Setup of two-dimensional direct-current resistivity equipment, (a) multicore cable connected to electrode, (b) resistivity meter connected to multicore cables (orange cables connected in back of unit), (c) one of 11 sets of multicore cables with 10-meter electrode spacing, and (d) speed bump used to protect cables where profile extends over roads.

distribution of subsurface resistivity. This inversion process is described in detail by Loke (2003). DC resistivity data were inverted using the finite-element method with the least-squares approximation using RES2DINV version 3.54w (Geotomo Software, 2005)

Two-Dimensional Direct-Current Resistivity Survey Results Resistivity data were processed and inverted, then input into Oasis montaj version 6.2 (Geosoft, Inc., 2004). The inversion results of the resistivity data are displayed as gridded, 2D profiles. The same vertical and horizontal scales were used between all profiles, with distance from the origin of the profile on the x-axis and elevation above NAVD 88 on the y-axis. Resistivity data were gridded using the bi-directional line gridding method with a 0.25-foot cell size in Oasis montaj version 6.2 (Geosoft, Inc., 2004) and plotted as image maps ranging from 0 to 648 ohm-meters (appendix 2). To provide an alternative and complimentary presentation of the profiles, available geologist logs from monitoring wells were plotted against the inversion results of the 2D–DC resistivity data and referred to as section plots (fig. 6). The electrical stratigraphy in five of the six section plots (fig. 6b–f) displays three distinct electrical units—(from land surface downward) unit 1, a low-resistivity zone (less than 216 ohm-meters); unit 2, a high-resistivity unit (greater than 216

ohm-meters); and unit 3, a low-resistivity unit (less than 216 ohm-meters). The inversion results of the 2D–DC resistivity data in section plot 1 show discontinuity in unit 2, which is separated by two vertical low-resistivity (less than 216 ohmmeters) features along the profile. Section plots 2–6 all show a continuous unit 2. To illustrate the three-unit electrical stratigraphy, the 216 ohm-meter line has been displayed in the inversion results for section plots 1–6. Electrical unit 1 correlates with the surface soils and loess deposits of the upper part of the unsaturated zone (fig. 2). Electrical unit 2, which is more resistive than unit 1, correlates with the unconsolidated sand and gravel deposits that compose the lower part of the unsaturated zone and the Pleistocene alluvial aquifer. Unit 3, which is less resistive, correlates with the clay and silt of the lower part of the unconsolidated Pleistocene deposits that compose the upper part of the lower confining unit. Geologist logs from MW20C located on profile 2 (fig. 6b) extend to the lower confining unit. Although the full range of depth of the profile does not extend to the top of the lower confining unit identified in the geologist logs, the southernmost elevation of electrical unit 2 in profile 2 and the elevation of the top of the lower confining unit identified in MW20C are very similar. Because, in many cases, geologist logs from the existing monitoring wells did not extend to the total depth of the 2D–DC resistivity profiles, it would be difficult to develop interpretations identifying the top of the lower confining unit along each profile with the existing data. To interpret the 2D–DC resistivity profiles, additional data such as geologist

W 16 7B

-204 ft

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M

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M

M

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17 5B

Two-Dimensional Direct-Current Resistivity Survey Results

+204 ft

DISTANCE (feet) 1000

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Section Plot 1

RESISTIVITY (ohm-meter) 0

338

648

NAD83 / UTM zone 14N SCALE 1 : 5830

DESCRIPTION TOPSOIL

NOT RECORDED

CLAY

CLAY AND SAND

SILT

CLAY AND SILT

SAND

SAND AND GRAVEL

Figure 6a. Geologist log (B.J. Roberts, U.S. Army Corps of Engineers, written commun., 2003) superimposed on inversion results (true resistivity) of two-dimensional direct-current resistivity profile for section plot 1.

9


-279 ft

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DISTANCE (feet) 250

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DESCRIPTION


338

Section Plot 2 648


CLAY

CLAY AND SAND

SAND

CLAY AND SILT

SAND AND SILT

SAND AND GRAVEL

Figure 6b. Geologist log (B.J. Roberts, U.S. Army Corps of Engineers, written commun., 2003) superimposed on inversion results (true resistivity) of two-dimensional direct-current resistivity profile for section plot 2.


M

M

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4

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DESCRIPTION


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648

Section Plot 3 NAD83 / UTM zone 14N SCALE 1 : 7190

CLAY

CLAY AND SAND

SILT

CLAY AND SILT

SAND

SAND AND SILT

Figure 6c. Geologist log (B.J. Roberts, U.S. Army Corps of Engineers, written commun., 2003) superimposed on inversion results (true resistivity) of two-dimensional direct-current resistivity profile for section plot 3.

11


M

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3

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DISTANCE (feet) 750

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DESCRIPTION

0


Section Plot 4 648

CLAY

CLAY AND SAND

NAD83 / UTM zone 14N

SAND

CLAY AND SILT

SCALE 1 : 6010

SILT

SAND AND GRAVEL

SAND AND SILT

Figure 6d. Geologist log (B.J. Roberts, U.S. Army Corps of Engineers, written commun., 2003) superimposed on inversion results (true resistivity) of two-dimensional direct-current resistivity profile for section plot 4.


W 31 2B

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+263.5 ft

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1450 DESCRIPTION


338

648

Section Plot 5 NAD83 / UTM zone 14N SCALE 1 : 5190

CLAY

CLAY AND SILT

SAND

GRAVEL

SILT

Figure 6e. Geologist log (B.J. Roberts, U.S. Army Corps of Engineers, written commun., 2003) superimposed on inversion results (true resistivity) of two-dimensional direct-current resistivity profile for section plot 5.

13


-146.5 ft

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W 3 M 21B W 31 0B B M W 31 9B

14

+146.5 ft

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DESCRIPTION

Section Plot 6 648


CLAY

CLAY AND SILT

SAND

SAND AND GRAVEL

SAND AND SILT

Figure 6f. Geologist log (B.J. Roberts, U.S. Army Corps of Engineers, written commun., 2003) superimposed on inversion results (true resistivity) of two-dimensional direct-current resistivity profile for section plot 6.

Summary and borehole resistivity logs or other surface geophysical data extending beyond the lower confining unit into the Niobrara Formation could be used with the existing data to construct forward models for data analysis and interpretation similar to the method identified in Kress and Teeple (2005). The results of this pilot study have been compiled into Oasis montaj, which serves as a 3D database that allows the user to graphically display and compare the results of geologic and geophysical data collected at the former NAD site. The database for this pilot study was designed to allow input of additional geologic or geophysical data that could be used to determine the effectiveness of using 2D–DC resistivity data to extend hydrogeologic contacts between existing or new monitoring wells.

Summary The former Blaine Naval Ammunition Depot (former NAD site) located immediately southeast of Hastings, Nebr., was an ammunition facility during World War II and the Korean Conflict. Waste-management practices during the operation and decommissioning process of the former NAD site resulted in soil and ground-water contamination. Ground-water models have been used by the U.S. Army Corps of Engineers (USACE) to provide information on the fate and transport of contaminants on the former NAD site. The reliability of any ground-water-flow model depends, in part, on the quality and quantity of data available. In heterogeneous aquifers, lack of subsurface data substantially limits model results and is one of the common sources of model error. Additional information on the aquifer system can be obtained by installing wells or test holes, collecting ground-water samples for chemical analysis, examining borehole logs, or conducting aquifer tests, but drilling additional wells and performing aquifer tests can be time-consuming and expensive. An alternative approach is to combine non-intrusive surface geophysical methods with a drilling and ground-water sampling program to provide additional aquifer-system information at a lower cost. During September 2003, the U.S. Geological Survey, in cooperation with USACE, Kansas City District, conducted a pilot study to collect two-dimensional direct-current (2D–DC) resistivity data on the former NAD site along six profiles near existing monitoring wells. The inversion results of field data from five of the six 2D–DC resistivity profiles display distinct electrical stratigraphy consistent with three resistivity units (low resistivity, high resistivity, and low resistivity). These three resistivity units correlate with surface soils and loess deposits of the upper part of the unsaturated zone (unit 1), the unconsolidated sand and gravel deposits that compose the lower part of the unsaturated zone and the Pleistocene alluvial aquifer (unit 2), and the clay and silt of the lower part of the unconsolidated Pleistocene deposits that compose the upper part of the lower confining unit.

15

To interpret the 2D–DC resistivity profiles, additional data such as geologist and borehole resistivity logs or other surface geophysical data extending through the lower confining unit into the underlying Niobrara Formation could be used with the existing data to construct forward models for data analysis and interpretation.

References Cited American Society for Testing and Materials, 1999, Standard guide for selecting surface geophysical methods, in 1999 Annual book of ASTM standards: West Conshohocked, Pa., v. 04.08, p. 6,429–6,499. Century Geophysical Corporation, 2004, 9512 logging tool: accessed April 23, 2004, at http://www.century-geo.com/ 9512-index.html Geosoft, Inc., 2004, Oasis montaj version 6.2: Toronto, Ontario, Canada, accessed October 2005 at http://www.geosoft.com/ Geotomo Software, 2005, RES2DINV version 3.54w: accessed October 2005 at http://www.geoelectrical.com Grant, F.S., and West, G.F., 1965, Interpretation theory in applied geophysics, part 3—Electrical conduction and electromagnetic induction methods: New York, McGraw-Hill, p. 384–572. IT Corporation, 2002, Ground water feasibility study report, operable unit no. 14, former Blaine Naval Ammunition Depot, Hastings, Nebraska: IT Corporation, chap. 1, p. 2–27, fig. 1–2. IRIS Instruments, 2004, Syscal R1 Switch Plus: accessed September 21, 2004, at http://www.iris-instruments.com/ Pdf%20file/R1Plus_72_Gb.pdf Keech, C.H., and Dreeszen, V.H., 1968, Availability of ground water in Adams County, Nebraska: U.S. Geological Survey Hydrologic Investigations Atlas HA–287, 7 maps. Konikow, L F. and Bredehoeft, J.D., 1992, Ground water models cannot be validated: Advances in Water Resources, v. 15, no. 1, p. 75–83. Kress, W. H., and Teeple, A.P., 2005, Two-dimensional resistivity investigation of the North Cavalcade Street site, Houston, Texas, August 2003: U.S. Geological Survey Scientific Investigations Report 2005–5205, 27 p. Loke, M.H., 2000, Electrical imaging surveys for environmental and engineering studies—A practical guide to 2–D and 3–D surveys: 62 p. Loke, M.H., 2002, Tutorial—2–D and 3–D electrical imaging surveys: 28 p., accessed December 1, 2003, at http://www.geoelectrical.com Loke, M.H., 2003, RES2DINV version 3.52w—Rapid 2D resistivity and IP inversion using the least-squares method: Geoelectrical Imaging 2–D and 3–D, Geotomo Software, 125 p., accessed August 10, 2003, at http://www.geoelectrical.com Merry, A.G., Martin, P.J., and Sussman, J., 2003, Importance of local geologic structure in a regional groundwater flow and

16


contaminant transport model for Lansing, Michigan, in MODFLOW and more 2003—Understanding through Modeling, Golden, Colorado, September 17–19, 2003, Proceedings: International Ground Water Modeling Center, v. II, p. 655–660. Powers, C.J., Wilson, Joanna, Haeni, F.P., and Johnson, C.D., 1999, Surface-geophysical investigation of the University of Connecticut landfill, Storrs, Connecticut: U.S. Geological Survey Water-Resources Investigations Report 99–4211, 34 p. Shaw Environmental, Inc., 2004, Kansas City Total Environment Restoration Contract-Task Order—TO 001 Former Blaine NAD, Hastings NE: accessed September 21, 2004, at http://www.terc-itcorp.com/kcterc/task.html Stanton, G.P., Kress, Wade, Hobza, C.M., and Czarneci, J.B., 2003, Possible extent and depth of salt contamination in ground water using geophysical techniques, Red River Aluminum site, Stamps, Arkansas, April 2003: U.S. Geolog-

ical Survey Water-Resources Investigations Report 03–4292, 28 p. Thales Navigation Inc., 2004, Land surveying solutions— Z-Extreme: accessed September 21, 2004, at http://products.thalesnavigation.com/en/products/ product.asp?PRODID=38 U.S. Army Corps of Engineers, 1995, Geophysical exploration for engineering and environmental investigations: Engineering Manual 1110–1–1802, chap. 4, 57 p. U.S. Environmental Protection Agency, 2004, National Priority List sites in the Midwest, Nebraska—Hastings Ground Water Contamination: accessed September 21, 2004, at http://www.epa.gov/region7/cleanup/npl_files/index.htm Zohdy, A.A.R, Eaton, G.P., and Mabey, D.R., 1974, Application of surface geophysics to ground-water investigations: U.S. Geological Survey Techniques of Water-Resources Investigations book 2, chap. D1, 116 p.

Appendix 1—Geologist Logs1 and Borehole Geophysical Resistivity Logs

Appendix 1

1

B.J. Roberts, U.S. Army Corps of Engineers, written commun., 2003.

Blank Page

Appendix 1

19

MW9BB Easting (feet) 1833466.4

Northing (feet) 14730776.4

Elevation (feet above NAVD 88) 1891.6

Depth (feet below land surface) 179.0 RL 1890 1880

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1870 1860

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1850 1840 1830 1820

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1810 1800

100

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120

1770 1760

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1730 1720

100 50 0 DESCRIPTION CLAY

SAND AND SILT

SILT

CLAY AND SILT

SAND RESISTIVITY (ohm-meters)

Figure 7a.

Geologist log and borehole geophysical resistivity log for monitoring well MW9BB.


DEPTH (feet below land surface)

60

20


MW19BB4 Easting (feet) 1835299.3




20

1860 1850

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CLAY AND SAND

SAND RESISTIVITY (ohm-meters)

Figure 7b.

Geologist log and borehole geophysical resistivity log for monitoring well MW19BB4.



60

Appendix 1

21

MW20C Easting (feet) 1835409.4



Depth (feet below land surface) 257.5 RL

1860 20

1840 40

1820 60

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1740 140

1720 160

1700 180

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1660 220

1640 240

1620 150 100 50 0 DESCRIPTION CLAY

SAND AND SILT

SILT

CLAY AND SILT

SAND

SAND AND GRAVEL

RESISTIVITY (ohm-meters)

Figure 7c.

Geologist log and borehole geophysical resistivity log for monitoring well MW20C.



1780 100

22


MW105BB Easting (feet) 1836963.3




20 1850 1840 40

1820 60 1810 1800 80 1790 1780 100 1770 1760 120 1750 1740 140 1730 500

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300

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100

0

DESCRIPTION CLAY

SAND AND SILT

SAND

CLAY AND SILT

CLAY AND SAND RESISTIVITY (ohm-meters)

Figure 7d.

Geologist log and borehole geophysical resistivity log for monitoring well MW105BB.



1830

Appendix 1

23

MW113BB3 Easting (feet) 1832744.2




20

1860 1850 1840 1830

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Figure 7e.

Geologist log and borehole geophysical resistivity log for monitoring well MW113BB3.

2000

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DESCRIPTION CLAY



40

24


MW155B Easting (feet) 1837708.3



Depth (feet below land surface) 115.0

1870 1860 20 1850

1830 1820 60 1810 1800 80 1790 1780 100 1770 1760 150 100 50 0 DESCRIPTION CLAY

CLAY AND SAND

SAND

CLAY AND SILT


Figure 7f. Geologist log and borehole geophysical resistivity log for monitoring well MW155B.



1840 40

Appendix 1

25





1880 1870

20

1860 1850 1840 1830

60

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DESCRIPTION TOPSOIL

NOT RECORDED

CLAY

SAND AND SILT

SAND

SAND AND GRAVEL


Figure 7g.

Geologist log and borehole geophysical resistivity log for monitoring well MW170B.



40

26






1880 1870 20 1860

1840 1830 60 1820 1810 80 1800 1790 100 1780 1770 120 1760 600

500

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100

0

DESCRIPTION CLAY

CLAY AND SAND

SAND

CLAY AND SILT RESISTIVITY (ohm-meters)

Figure 7h.

Geologist log and borehole geophysical resistivity log for monitoring well MW175B.



1850 40

Appendix 2—Inversion Results of Two-Dimensional Direct-Current Resistivity

Appendix 2

Blank Page


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PROFILE 1

Vertical Exaggeration: 1

Inversion results of two-dimensional direct-current resistivity for profile 1.


Figure 8a.

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29

Figure 8b.

PROFILE 2

Appendix 2

Scale 1:12000

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Scale 1:12000 250

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PROFILE 3




Figure 8c.

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Scale 1:12000 0

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Figure 8d.


PROFILE 4



30

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Scale 1:12000 0

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PROFILE 5




Figure 8e.

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31

Figure 8f. Inversion results of two-dimensional direct-current resistivity for profile 6.

PROFILE 6

Appendix 2

Scale 1:12000