Mapping of salinity distribution in an unconfined aquifer overlying rock salt bodies with an electromagnetic survey

Transcription

1 Mapping of salinity distribution in an unconfined aquifer overlying rock salt bodies with an electromagnetic survey Peangta Satarugsa, Kriengsak Srisuk, Sakorn Seangchomphu and Winit Youngmee Department of Geotechnology, Faculty of Technology Khon Kaen University, Thailand, Abstract Groundwater is one of the natural resources with the potential for domestic, agricultural and industrial consumption. However, in the Khorat and Sakon Nakhon basins, Northeastern Thailand, the use of groundwater for such purposes has been restrictive. This is because of the salinization of aquifers generated by rock salt present below the aquifers. Thus, this study was aimed to evaluate the capability of an electromagnetic survey for subsurface characterization in the Khorat and Sakon Nakhon basins. We conducted electromagnetic sounding and profiling in the two areas with known subsurface geology and hydrological processes. We have found that the electromagnetic survey can provide the mapping of an interface between fresh water and saline water of an unconfined aquifer. It provides also a rough diagnosis of near surface rock salt body. Introduction The Khorat and Sakon Nakhon basins in Northeastern Thailand (Figure 1) cover an area of 46,000 km 2 which is approximately 28% of the entire Northeastern region (Satarugsa, et al., 2005). Rapid increasing levels of salinity in soil and water are among the growing geo-environmental problems in these Khorat and Sakon Nakhon basins. High concentrations of salt in soil cause reduction in agricultural produce while high concentrations of salt in water cause brackish to saline surface water and groundwater. White granular salt is exposed on the ground in several places especially during summer time period. A number of communities lacking water supply have been reported to gradually increase. The geo-environment problem of soil and water salinity in Khorat and Sakon Nakhon basins is caused by natural dissolution of rock salt bodies that contain within the basins. However human activities such as over pumping of groundwater, inappropriate salt production activity, deforestation have accelerated further the natural dissolution of the rock salt. Results from boreholes and regional seismic data have shown depths to the rock salt vary largely between places which could be a few meters or thousands meters apart (Satarugsa, et al., 2005). Management of the increasing salinity levels in soil and water requires thorough understanding of subsurface geology and hydrological processes. A three dimension subsurface geology especially the depth to the top of the rock salt bodies need to be characterized and localized together with groundwater flows and pathways through subsurface. The groundwater flows and pathways could reveal vertical and lateral mobility of salt while they could provide also information on salt accumulation and salt migration. Distributions of fresh-saline groundwater with depth could also be defined by the groundwater flow pathways. The subsurface geology and hydrological process knowledge thus could play a role in management of problems associated with increasing soil and water salinity in the Khorat and Sakon Nakhon basins. 1

2 Figure 1: The Khorat and Sakon Nakhon basins in the Northeastern Thailand which contains a large volume (18 million million tons) of rock salt bodies. The depths of rock salt bodies range from a few meters to thousands meters below the ground surface. At the present, detailed subsurface geology in the Khorat and Sakon Nakhon basins has been confined in small areas, considered to be experimental areas. Geophysical methods, seismic, resistivity and gravity have been evaluated and it has indeed been suggested to have a value in mapping of rock salt bodies (Satarugsa, et al., 2000; Satarugsa and Srisuk, 2000). Thus, in this paper we present a case study with an electromagnetic survey for the purposes of mapping salinity distribution in an unconfined aquifer. A scientific objective of the study was to determine the capability of electromagnetic survey in delineating an interface between fresh and saline groundwater in shallow aquifers. Geology of the Study Area Surface geology of the study is composed of unconsolidated soil, categorized as Quaternary alluvial (Qa) and Quaternary terrace (Qt) soils and weathered Maha Sarakham Claystone and Rock Salt Formation (KTms). Typical beneath unconsolidated soil is weathered claystone, claystone and rock salt. The presence of salt diaper causes the thicknesses of weathered claystone and claystone to vary distinctively from a few meters to thousands meters. As rock salt is a soluble mineral; it can be dissolved, mobilized and accumulated along pathways where groundwater flow and pathways run through. These salt properties have produced naturally saline soil and saline water in the areas. Method of Study We conducted experiments in two known subsurface areas. The first study area was mapped as unconsolidated soil (Qt) in geologic map with ten piezometers installed at various depths with water quality from each piezometer analyzed. The second study area was mapped as Maha Sarakham Claystone and Rocks Salt Formation in geologic map with subsurface information of boreholes, seismic 2

3 reflection and resistivity data. The first study area was chosen to represent thick unconfined aquifer and deep rock salt body model. The second study area was to represent thin unconfined aquifer and shallow rock salt model. We acquired electrical conductivity data with Geonics EM34 instrument of 10, 20, and 40 m intercoil spacing, with station interval of 40 m, and with horizontal dipole. Bulk apparent conductivities detected from varying intercoils were considered as a sounding with depth. We acquired 6 profiles with length of ~4,000-18,000 meters in the first area and 4 profiles with length of ~3,500-5,500 meters in the second area. In addition, we collected available results of Electrical Conductivity (EC), Total Dissolved Solids (TDS) and chloride ion that were analyzed for surface water and groundwater samples in study areas and nearby for correlative findings and supportive evidence. Results and Interpretations Results from the two study areas have indicated the apparent conductivities to rise with depths. These apparent conductivities data have further revealed that a lateral near surface geology is heterogeneous whereas a vertical subsurface geology is relatively homogeneous. Figure 2 shows the representative electromagnetic sounding profiles of thick unconsolidated soil with deep rock salt body. Figure 3 shows the electromagnetic sounding profiles which represent thin unconsolidated soil with shallow rock salt body. Apparent conductivities from three sounding measurements with 10, 20 and 40 m intercoil spacing in Figures 2 and 3 reveal the variation to be uniform in the vertical, whereas they suggest distinctive variation in the lateral. Difference in apparent conductivities between 20 and 40 intercoil spacing in Figure 3 is much greater than those of Figure 2. Figure 2: (a) and (b) Representative profiles of horizontal dipole electromagnetism of 40 m station interval from three different intercoil spacings, acquired in the areas with Quaternary soil thickness of greater than meters, overlaying claystone and rock salt. Small differences in apparent conductivity are noted between 20 and 40 meter intercoil spacings. A plot of electrical conductivity vs. total dissolved solids [Figure 4 (a)] and a plot of total dissolved solids vs. chloride ion concentrations [Figure 4 (b)] both show a correlation between the pairs of measured parameters. High levels of total dissolved solids are also high in chloride levels (TDS:Cl = 1:0.3). It is noteworthy that a typical concentration of total dissolved solids of greater than 10,000 mg/l in the Khorat and Sakon Nakhon basins is found to be almost the same as saline water. 3

4 Figure 3: (a) and (b) Representative profiles of horizontal dipole electromagnetism of 40 m station interval from three different intercoil spacings, acquired in the areas with Quaternary soil thickness of about 1-5 meters, overlaying clatstone and rock salt. The Large differences in apparent conductivity are noted between 20 and 40 meter intercoil spacings. Figure 4: (a) A plot of electrical conductivity vs total dissolved solids, indicating a correlation coefficient value (R 2 ) of (b) A plot of total dissolved solids vs chloride ions, indicating a correlation coefficient (R 2 ) of Data are supported by Groundwater Research Center, Khon Kaen University) Figure 5 displays subsurface geological profiles and the interpretation of depth to saline ground water derived from apparent conductivity data. A two-layer case approximation (McNeill, 1980; Stewart, 1988) is assumed for calculating depths to the fresh and saline water interface [Figure 5 (b)]. An unconsolidated zone is combined with a saturated fresh water zone to generate a single layer with an average conductivity of 10 ms/m because the ground water level in the area is very thin [Figure 5 (c)]. A second layer is a saline saturated water zone with a conductivity of 100 ms/m. An equation used for a single-layer thickness Z, a depth below the surface to the interface, is Z=S(1-C 2 )/4C, and C=(σ a -σ 1 )/( σ 2 - σ 1 ) where; σ 1 is average conductivity of the first layer (10 ms/m); σ 2 is average conductivity of the second layer (100 ms/m); σ a is apparent of field conductivity and S is intercoil spacing. The data shown in Figure 5 (c) reveal an interface between fresh and saline water which could be identified by the electromagnetic survey together with calibration with electromagnetic-derived interface with TDS of the groundwater where the apparent conductivity is greater 100 ms/m, suggesting the TDS should be greater than 10,000 mg/l. 4

5 Figure 5: (a) A profile of horizontal dipole electromagnetism of 40 m station interval. (b) Fresh-saline water interface derived from 40 m intercoil spacing, twolayer approximation with average unsaturated and saturated fresh water of 10 ms/m and salt water of 100 ms/m. (c) Generalized geologic cross sectional illustration of the electromagnetic-derived fresh-saline water interface. Qt, Quaternary terrace soil. Peizometer wells are designated by PNSK1/50 where PNSK designates well name and location, 1, well number, 50, well depth. The first number below the peizometer location represents groundwater level, the second number represents total dissolved solids in mg/l. Figure 6 displays subsurface cross section where rock salt body is located near the ground surface. The apparent conductivity data in Figure 3 (a) were measured along the entire profile shown in Figure 6 (d). Results from Figures 3 and 6 reveal that in the area where the rock salt body is located near by the surface apparent conductivity from different sounding are sharply increased (Figures 2 vs 3). 5

6 Figure 6: (a) A profile of common offsets gathered at 60 meter indicating flank of near-surface salt body. (b) Seismic section with 1200% full coverage over the top of near-surface salt body. (c) A profile of horizontal dipole electromagnetism of 40 m station interval from two different intercoil spacing shown also Figure 3 (a). (d) Two-dimension resistivity earth model with the seismic data shown in (a) and (b), together with electromagnetic data shown in (c) plus core drilling boreholes. Discussions and Conclusions Electromagnetic survey with the assumption that the groundwater table is shallow and that the subsurface geology is relatively homogeneous can be adapted for mapping of fresh and saline water interface in the Khorat and Sakon Nakhon basins where their surface geology, described in a geologic map as Quaternary terrace soil 6

7 (Figure 5). Apparent conductivity values in soil of greater than 100 ms/m can be used as an indicator of saline water. Gradual increases in measured apparent conductivity values with depth, similar to the feature shown in Figure 2 can be used as an indicator of a deep rock salt body (Figures 2 and 5). Sharp increases in apparent conductivity with depth similar to the feature shown in Figure 3 can be used as an indicator of shallow rock salt body (Figures 3 and 6). Interpretation of apparent conductivity data must be done with cautions because these measurable data are notoriously known to be under the influences of a number of factors which include organic and inorganic compositions of soils and rocks, permeability, porosity, pore fluid of saturation, solid solution and ion content in fluids, and temperature (McNeill, 1990). However, in the study areas that the unconsolidated soil, weathered claystone and claystone show homogeneity in their mineral composition, fluid property is a single factor that could significantly influence both lateral and vertical conductivity data. Additionally, the concentrations of sodium and chloride ions in the fluid contribute markedly to variation in conductivity recorded (Figure 4). Based on these data, we conclude that electromagnetic survey can be applied for a rough estimate of near surface rock salt body and for effective mapping of salinity distribution in the unconfined aquifer in the study areas. Acknowledgements The study constitutes a part of two projects; the saline water and saline soil in Songkram River project and the rock salt and rock salt cavity project. The study was supported by research funds of Department of Geotechnology, Groundwater Research Center, and Khon Kaen University. References McNeil, J. D Interpretation procedures for the EM-34 and TN-8, Geonics Ltd., Missisauga, Ontario. McNeill, J. D Use of electromagnetic methods for groundwater studies. In: Geotechnical and environmental geophysics; Volume 1: Review and tutorial. In: Wards, S. H. (ed.). Society of Exploration Geophysicists Investigations in Geophysics, v.5, Satarugsa, P and Srisuk, K Applied geophysics in solving the hydrogeological problems in the Northeast, Thailand. Journal of the Geological Society of Thailand, no Satarugsa, P., Virasri, V., Navawitphaisith, S., and Chotrasri, O Evaluation of geophysical investigations for rapid mapping of the rock salt in Maha Sarakham Formation in Northern Thailand. In: Singharajwarapan, S., Rieb, S. L., and Wongpornchai, P. (eds.), Proceeding of the International Conference on Applied Geophysics, Geophysics Chiang Mai 2000, November 9-10, Chiang Mai, Thailand Satarugsa, P., Youngmee, W., and Meesawat, S New regional boundary of Maha Sarakham Formation in the Northeastern Thailand: results from 2D seismic mapping. In: Wannakao, L., Youngmee, W., Sirsuk, K., and Lertsirivorakul, R. (eds.), Proceedings on International Conference on Geology, Geotechnology and Mineral Resources of Indochina (GEOINDO 2005), November 2005: Khon Kaen,Thailand Stewart, M. T., Electromagnetic mapping of fresh-water lenses on small oceanic islands. Ground Water, v. 26, no.6,