Field Mapping Soil Conductivity to Delineate Dryland Saline Seeps with Four-electrode Technique 1

Continuing incidence of saline-seep areas in the northern Great Plains dryland soils has created a need for detecting and delineating encroaching saline seeps before plant growth is affected. We evaluated the four-electrode conductivity technique for field mapping surface and subsurface soil salinity under dryland conditions. Results indicated that the four-electrode conductivity technique can be used successfully to quickly field map surface and subsurface soil salinity boundaries of existing and potential saline-seep areas. This technique also depicted underground flow patterns of a shallow, saline ground water table. Maps of apparent bulk soil conductivity values (EC.,) were used to locate the position of the recharge area in relation to the discharge (seep) area. While maps of discrete depth interval conductivity values (ECX) provided more precise information, the time required may not warrant the additional required calculations unless a portable programable calculator is available. Mapping soil salinity with the four-electrode conductivity technique is easy, rapid, and relatively inexpensive. This technique provides information useful in making management decisions to prevent or alleviate a saline seep or other soil salinity problems. Additional Index Words: dryland salinity, four-electrode soil conductivity, shallow saline water tables. T is a need for determining the extent of encroaching >oil salinity conditions before salinity levels become high enough to affect plant growth and soil structure in dryland areas. New saline-seep areas in dry cropland areas of the northern Great Plains of the USA and Canada and similar areas in western Australia continue to develop yearly without warning. Therefore, early detection and delineation of the potential size of these salt-affected areas is important so that remedial measures can be initiated in time to prevent reduced crop production and further salinization of productive crop land (1, 3, 4, 5). The four-electrode soil conductivity technique has been used to confirm suspected saline-seep areas and to measure soil salinity in the field without recourse to soil sampling (6, 7). Our objective was to evaluate the usefulness of the technique for delineating the surface and subsurface boundaries of encroaching and developed saline seeps. This technique should be especially appropriate for this purpose since fourelectrode soil conductivity (ECa) increases in proportion to both soil salinity and water content (6). MATERIALS AND METHODS Root zone soil electrical conductivity (0to 120-cm soil depth) of a well-characterized saline-seep area (5) was mapped using four-electrode conductivity technique (Wenner configuration) described previously (6). 'Contribution from the Western Region, Agric. Res. Ser., USDA, in cooperation with the Mont. Agric. Exp. Stn. J. Ser. no. 627. Received 1-9 Jan. 1976. Approved 16 April 1976. Presented before Div. S-2, Soil Science Society of America, 13 Nov. 1974, Chicago, 111. Soil Scientists, USDA-ARS, P. O. Box 1109, Sidney, MT 59270, and U.S. Salinity Laboratory, Riverside, CA 92502, respectively. A 153 by 244 m area around the saline seep was gridded at 30.5-m intervals (Fig. 1). Surface topography and location of several observation wells are also shown in Fig. 1. Table 1 gives depth to the water table in several observation wells surrounding the seep for the spring 1972 (highest level) and at the time of this study (June 1974). The EC of the ground water in the seep area was about 7 mmhos/cm at the time of study. In June 1974, four-electrode soil resistance measurements were taken at each grid location with inter-electrode spacings, a, of 30, 60, 90, and 120 cm. Soil water content of the clay loam soil throughout most of the study area (excluding central part of seep) was near field capacity (Table 2). Within the central part of the saline-seep area (Fig. 1), water content in the 60to 120-cm depth was at or near saturation. Apparent bulk soil conductivity (ECa) values in mmhos/cm were calcalated as follows: ECa = 1000/271-atf [1] where a is the inter-electrode spacing (cm) and R is the measured soil resistance (ohms). Fig. 1—A map showing relative surface topography in meters, the area around the saline seep that was gridded and surveyed, and the location of several observation wells (triangles with numbers). 571 Published July, 1976