Groundwater flow dynamic investigation without drilling boreholes
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The flow net map is a basic tool for groundwater flow dynamics investigation. In areas where there are no boreholes or piezometers are not available, constructing flow net map may be difficult. This work proposes a simple methodology to construct flow net map without drilling boreholes. The flow net map constructed using the proposed approach represents an expected flow net map, which can draw conceptual flow model of the site. The major benefit from constructing the expected flow net map is it gives guidance for locating new boreholes for site investigation, carrying out investigation of the groundwater flow directions and estimating recharge/discharge from the site boundary. An illustrative example for the proposed approach was presented to show how the data required to construct the expected flow net map can be collected. The constructed, expected flow net map using the proposed methodology was compared with actual flow net map constructed from measured water levels. Both maps give consistent hydrological information about the site. The suggested approach represents a simple and cheap way to carry out investigation of groundwater flow dynamics in areas where there are no boreholes are available.
KeywordsGroundwater Flow direction Flow net construction No boreholes
Constructing flow net map is a well-accepted practice in investigation of groundwater flow directions. A flow net is a 2D diagram of equipotential and flow lines (Braja 2013; Casagrande 1940; Driscoll 1986; Freeze and Witherspoon 1967). The flow net map can be used to estimate the quantity of recharge across site feeding front boundary, and contaminant load can be estimated from flow net map (Harr 1962; Domenico and Schwartz 1990). Flow net can show the topographic control of groundwater flow (Fetter 1988). To construct a flow net map, water levels are measured in a network of boreholes and their surfaces are interpolated between measuring points (Fels and Matson 1996). Water table surface is a representation of the surface of saturated zone, below which all the geological formation voids are fully filled with water (Heath 1988). Position of the water table is a result of natural processes controlling the rate at which water enters and leaves the saturated zone. If the rate of water enters the saturated zone (recharge) exceeds the rate of water leaving (discharge) the aquifer, the water table rises and vice versa. The water table surface is not static, nor flat, but reflects the climatic, vegetative and geomorphic conditions. The groundwater water table could be subdued replica of the land surface (King 1899; Domenico and Schwartz, 1990).
Groundwater aquifer investigation in an area requires drilling boreholes to know the water depth to determine water level and construct flow net map. Having constructed a flow net map, groundwater flow dynamics such as pressure head distribution throughout the aquifer, flow direction, hydraulic gradient and definition of discharge/recharge areas can be studied (Freeze and Cherry 1979; Fitts 2012). All above benefits from flow net map cannot be obtained if there are no boreholes at the study site. Resistivity measurements with Schlumberger array configurations (Zohdy et al. 1974) are considered one of the best configurations for water depth sounding to explore the groundwater aquifer occurrences. In this configuration, the center point of the electrode array remains fixed; however, the spacing between the electrodes is increased in a progressive way to obtain more information about the subsurface layers.
This work proposes a simple nondestructive methodology to construct water level and flow net maps in regions where there are no boreholes. The methodology was applied to a site to show how the required data for the methodology can be collected.
Materials and methods
- 1.GPS (Global Positioning System) (NUS, 1995): GPS was used to define the coordinates of four points selected in the site (Table 1; Fig. 2).Table 1
Estimated water depth from geo-electric survey and calculated water level
Longitude (decimal) from GPSa
Latitude (decimal) from GPSa
Height above sea level (m) from Google Earthb
Estimated depth to water (m) from geo-electric survey
Water level above sea level (m) without wellsc
Google Earth: Using the coordinates of the four points and Google Earth, the height of the selected points above sea level was measured (Table 1).
- 3.Geo-electric survey was applied to estimate water depth at the four selected points. Geo-electric technique with Schlumberger layout (Fig. 3) was applied. The method is based on the estimation of resistivity of the medium. An apparent resistivity is calculated from a resistance value and geometric factors that account for the electrode spacing configuration. Estimation is performed based on the measurement of voltage of electrical field induced by the distant grounded electrodes (current electrodes). The interpretation of the measurements can be performed based on the apparent resistivity values. The depth of investigation depends on the distance between the current electrodes. In order to obtain the apparent resistivity as the function of depth, the measurements for each position are performed with several different distances between current electrodes. The measurements were taken using 200 m distance between polars A and B to know the subsurface layers till 70 m depth (Fig. 4). Resistivity curves were plotted on logarithmic scale between resistivity value and distance AB/2. RESIST software was used to analyze the data. Estimated water depth values are indicated in Table 1.
Geo-electric cross section for the site in the west–east direction.
- Four boreholes were drilled at same locations of the four points, the water depth was measured, and the water level was calculated according to Eq. (1) and Fig. 5. The measured water depth and calculated water levels are indicated in Table 2.Table 2
Data used in constructing actual flow net map using measured water depth values
Longitude (decimal) from GPS
Latitude (decimal) from GPS
Height above sea level (m) from Google Earth
Measured water depth (m) in drilled boreholes
Water levels above sea level (m) in drilled boreholes above sea level (m)a
Measured water depth values (Table 2) at the selected points were collected from the boreholes drilled at same points by private sector. The water levels at drilled boreholes above sea level (Fig. 4) were calculated from Eq. (2).
Where casing height above ground surface = 10 cm
Results and discussion
Water depth estimation and subsurface characterization
Site topography characterization
Groundwater dynamics investigation
Construction of depth to water map
Expected and actual flow net map
In areas where there are no boreholes or information about groundwater level and direction, expected flow net map of the site can be constructed without drilling boreholes, using the suggested simple nondestructive methodology to study groundwater flow dynamics and contaminate migration. Comparing the flow net map using water levels estimated from the suggested approach to that constructed from measured water depth values gives same hydrological groundwater flow information concerning flow direction and recharge/discharge areas. If the errors between estimated water depth from resistivity survey and that measured in boreholes were constant at all measuring points , both actual and expected flow net map have the same hydrological conclusion.
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