Nepal is among the richest in terms of water resource availability and it is one of the most important natural resource of the country. Water resources are abundant throughout the country in the form of snow covers, rivers, springs, lakes, and groundwater. Despite the large availability, access of water is not so easy and equal for all population of the country. Rugged and difficult terrain, deep river valleys, sparsely distributed settlements and poorly developed infrastructures are the major factors that deprive the easy and equal access to water.
Groundwater is not found everywhere. Lithological layers consisting of coarse grains of sediments like sands, gravels form good reservoir or aquifer but layers containing of finer grains of sediments like silts, clay do not form good reservoir or aquifer. Such layers may contain water in them but cannot move from them easily due to lack of permeability. Similarly, there is less chance for groundwater to be trapped in the hard-compact rock terrain, but good chance exists in fractured, jointed rocks. Groundwater is the most convenient and important source of water in Terai region, Dun Valleys and some of Inter Mountain Basins like Kathmandu and Pokhara, there is need of accessing the very source in the hilly and mountainous regions as well. Since ancient time, most of the people in the mountainous area depend up on Kuwa, Pandhera, Nwalo etc. for Drinking water. This kind of water source is the form of groundwater which is the Spring Water.
Hence to increase the chance of success of groundwater exploitation (Drilling), a prior study employing suitable scientific method is to be conducted in the area under consideration.
For the assessment of groundwater potential (hydro-geological) condition of the area and to locate the probable area before drilling, Groundwater potential study along with Geophysical Investigation (Electrical Resistivity Survey) is desirable. So, the municipality has appointed Himalayan Earth Science Research Center and Solution Pvt. Ltd as a consultant for carrying out Groundwater potential study.
2. Field Visit Program
The field visit program for the geophysical survey (VES) for groundwater was conducted by the team comprising of Geophysicist, Geologist, Technical Assistants and Local Labors.
3. Location and Accessibility
The present study area is located at the ward no. 3 of the Nawalpur East District Provision No. 4, Nepal. Figure 1 shows the location map of the study area. The Geographical location of the central point of the study area for VES is 27°41’3.07″N, 84°12’9.15″E and 194 amsl. The site is easily accessible by the black topped road, feeder road to Mahendra Highway.
4. Scope and Objective
In ward no. 3 of Devchuli Municipality of Nawalpur East District, Provision No.4 Vertical Electrical Sounding was carried out for the purpose of extraction of groundwater for irrigation.
The main objective of the present geophysical exploration is to get hydrogeological information of the area so that one will be able to decide whether or not groundwater may be exploited for irrigation.
In order to meet the objectives, the scopes of works are as follows:
1）Review and conduct general geological survey of the area
2）Carryout Resistivity survey (VES) is the given area
3）Compute, analyze and interpret the field data with computer added software.
Make suggestion and recommendations on hydrogeological condition through geophysical, geological observations.
The methodology of the study consists of materials collection; literature collection and review, geological observation and geophysical (VES) survey, data computation, processing and interpretation; and finally report preparation.
The Vertical Electrical Soundings (VES) using Schlumberger Array was conducted with Geomative GD-10. Other accessories that were used during the study were District Map, GPS (Global Positioning System), Photographic Camera, Brunton Compass etc. Literatures and information related to topography, climate, geological condition of the area was collected and reviewed.
Resistivity Meter (Geomative GD-10) is used for vertical electrical resistivity sounding (VES) data collection. GPS is used to take spot location of the observation point (VES- Point) and other important features like dug wells, tube wells water body etc. Photograph were taken to characterize the topographical and morphological character of the area.
The general geology, hydrogeology of the area is taken from the geological map, hydrogeological map of the area prepared by Department of Mines and Geology. The description
of the geology of the area is done from the published and unpublished literatures and verified in the field by observation and field measurement.
Figure 2 Schematic Diagram of Vertical Geoelectrical Sounding
Figure 3 Schematic Diagram of Vertical Geoelectrical Sounding Data Acquisition
Data Acquisition for Vertical Electrical Sounding
In the present case, in the ward no. 3 of Devchuli Municipality Vertical Electrical Soundings was performed to cover the area using schlumberger resistivity method of electrode configuration. The survey point and VES profile is shown in Figure 4. In VES survey, the current electrodes were taken up to 300 meter distance on both sides of the sounding site, increasing the distance of current and potential electrodes after each successive observation. Thus, increasing the distance of the current and potential electrodes enables to obtain information from deeper part of the underground layers.
Figure 4 Google Map Showing Survey Area and Profile Lines
Figure 5 Data Acquisition in Field
Analysis and Interpretation
Geo-electrical sounding is widely used geophysical method for groundwater exploration, environmental geophysical exploration and other engineering application. The benefit of this method is to perform the survey quite fast and in the cost-effective manner. Detection of water table, variation of resistivity with depths (distinguishing layered earth), contaminants plume detection etc. are the objectives of VES survey. The interpretation of electrical resistivity sounding data is the process of deriving the values of true resistivity (ρ) and thickness (t) of various subsurface strata from the values of recorded resistance (R) or apparent resistivity (ρa) at electrode separations (a). There are a number of interpretation techniques for evaluating (ρ) and (t) of each of the stratum as proposed by many investigators. These can be grouped as analytical, numerical, empirical, graphical, computer (software) based etc. and several amongst each category.
In the present case Vertical Geoelectrical Sounding was made to cover the area using Schlumberger array of electrode configuration. The survey point is shown in Figure 4.
Topographically, this area consists of flat land. In qualitative interpretation, general conclusive remarks regarding to lithological variation are made in terms of apparent resistivity values. The data acquisition, data filtering and interpretation was carried using standard norms by Geophysicist and Hydrogeologist. The apparent resistivity values ρa were plotted against the electrode spacing (AB/2) on a log-log scale to obtain the VES sounding curves using the computer software IPI2WIN software which is developed for the purpose of data processing, analysis and interpretation. The field curves together with the best fit model curve of sounding site is drawn in double log-log sheet, photographs of the survey are presented in the Annex.
The resistivity map values shall be interpreted in terms of lithology and thickness in quantitative analysis. The resistivity of certain layer depends up on rock type, grain size, degree of void spaces and amount of water present, degree of weathering etc. Based on different studies carried out in different part of Nepal, the general correlation table is shown in Table 1.
Table 1 Correlation of Resistivity and Lithology
Table 2 Expected Lithology of the Survey Area from VES
Discussion of the Results
The study was mainly focused by geophysical methods and other assisting information for geophysical methods were also collected for the groundwater potential survey. Assisting information like geological, hydrogeological and lithological characteristics of the area and topography, geomorphology, drainage pattern and climate of the area shows the study area is recharged by the rainfall and supports for the shallow aquifer within the surveyed area. The VES result reveal the presence of open aquifer.
The VES profile shows the absence of lower confining strata up to the considerable depth of about 300 meter below due to which the aquifer is characterized as an open aquifer in the surveyed site. Water level in this type of aquifer has the seasonal variation.
Conclusion and Recommendation
It is revealed from above discussion that there is presence of open aquifer in the survey site. Groundwater extraction from this aquifer is possible but variation in water level might occur. Presence of fine, medium and coarse sand, pebbles and granules size particles indicate the water bearing strata in the survey areas.
It is recommended to drill the well for the purpose of the irrigation. Drilling up to the depth of 110 m is recommended in this location.