USGS Groundwater Information: Hydrogeophysics Branch
Figure 1. The EC-Logger handheld fluid conductivity meter being deployed in Bangladesh. Image credit: U.S. Geological Survey/photo by Alton Anderson.
EC-Logger is a low-cost, portable, down-well fluid conductivity logging tool that can be operated by one person. The tool was developed for identifying leaks in well casing in regions where down-well geophysical equipment typically is unavailable or prohibitively expensive.
Borehole geophysical logging methods can be used to characterize and monitor aquifers. However, the availability of geophysical logging systems in developing countries is limited by the high cost of the tools and by the need for specially trained logging personnel, vehicles for transport, and generators to power the equipment and associated computers. EC-Logger was developed for use in fluid-replacement logging well-casing leakage tests to address these common limitations of conventional logging systems in the developing world (fig. 1).
Leaking casing in a water-supply well is of concern if the well casing passes through a contaminated aquifer to reach deeper, uncontaminated drinking water. For example, shallow aquifers in Bangladesh are often severely contaminated by arsenic; deep groundwater wells have been installed throughout Bangladesh to provide drinking water that meets arsenic safety standards. If water leaks into the well from the contaminated zone, the water in the well may be unsafe to drink. Under certain hydraulic conditions, it also may be possible for the contaminated water to flow into the well through leaky casing, flow through the well, and then flow out into the deeper aquifer, contaminating a previously safe drinking-water source.
The fluid-replacement test identifies leaking wells through the comparison of fluid conductivity logs collected before and after spiking the water column with a sodium chloride tracer. Spiking the well with sodium chloride increases the electrical conductivity of the water column in the well above ambient values. Groundwater flowing into the well would have lower electrical conductivity than the spiked water column. Leaks are identified as zones in the casing where the fluid conductivity data indicate electrical fluid conductivity values decreasing towards background values with time.
This handheld fluid-conductivity logging tool is
Figure 2. Close up of EC-Logger tool being lowered into well. The four metal clamps function as electrodes. The white disc attached at the top is salt used to increase the fluid conductivity in the well during tests to determine if the well casing was leaking. Image credit: U.S. Geological Survey/photo by Alton Anderson.
The EC-Logger (fig. 2) is designed as a four-electrode conductivity probe to be used in 2- to 10-inch diameter groundwater wells.
Electrodes: Four pieces of stainless steel hose clamps serve as electrodes. The clamps are fastened to a weighted PVC pipe and attached to a four-wire multiconductor cable.
Current and Potential Array Geometry: Electrodes in this meter are arranged in a Wenner configuration. An alternating current (AC) is injected into the outer electrodes, and the potential is read from the two inner electrodes using a digital AC volt meter.
Measurement Value: Voltage is related to fluid conductivity through a calibration process. Calibration curves must be generated for each tool, as the curves may vary based on the unique construction of the individual tool. The current path of the electrode is affected by the diameter of the well, so a calibration curve must be generated for the different well diameters in which the tool will be used. Resolution of the meter is typically on the order of ±2 milliSiemens per centimeter.
Measurement Depth: A measuring tape attached to the weighted probe is used to determine the depth of insertion of the probe in a borehole.
We apply fluid-replacement logging in arsenic-contaminated regions of Bangladesh using a low-cost, down-well fluid conductivity logging tool to detect leaks in the cased section of wells. The fluid-conductivity tool is designed for the developing world: it is lightweight and easily transportable, operable by one person, and can be built for minimal cost. The fluid-replacement test identifies leaking casing by comparison of fluid conductivity logs collected before and after spiking the wellbore with a sodium chloride tracer. Here, we present results of fluid-replacement logging tests from both leaking and non-leaking casing from wells in Araihazar and Munshiganj, Bangladesh, and demonstrate that the low-cost tool produces measurements comparable to those obtained with a standard geophysical logging tool. Finally, we suggest well testing procedures and approaches for preventing casing leaks in Bangladesh and other developing countries.
Final copy as submitted to Groundwater for publication as: Stahl, M.O., Ong, J.B., Harvey, C.F., Johnson, C.D., Badruzzaman, A.B.M., Tarek, M.H., van Geen, A., Anderson, J.A., and Lane, J.W., 2014, Detecting well casing leaks in Bangladesh using a salt spiking method: Groundwater, vol. 52, pp. 195-200. doi: 10.1111/gwat.12200
Support is available for questions about EC-Logger construction and about how the tool is intended to be used. Only limited assistance can be provided for specific applications of the EC-Logger tool.
Users are encouraged to submit suggestions for improvements or corrections to the tool or to the materials provided on this site. We also are interested in hearing about your experiences using EC-Logger.
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