Estimation of Aquifer Parameters in Basaltic Terrain and the Application of Wireless Sensor Networks; Chikaldhara Region, Amravati District, Maharashtra, India

The estimation of aquifer parameters, like transmissivity (T) and storativity (S), is vital for evaluating and managing groundwater resources. Hydrograph analysis and pumping tests are among the different methods available to estimate aquifer parameters, but pumping tests are the most commonly used. The study and data presented in this paper was conducted in the Chikaldhara Region (Amravati District) in the state of Maharashtra, in India. Large diameter wells, which are abundant in this area, are a cost-effective means to conducting pumping tests. The tests were performed at 20 locations using the local farmers’ well pumps. This region has a predominantly basaltic terrain. The data collected was interpreted using both analytical and numerical methods. In comparison to the field observations, results from the two analytical methods produced a percentage of error of up to 35% in the calculated and residual drawdowns. In contrast, the numerical method used produced less than a 4.5% error compared to the field observations. Hence, the study and results presented here show the numerical method to be more accurate than the analytical curve matching methods for estimating aquifer parameters. Results from the numerical method showed the ‘T’ value to vary from 16.21 to 41.98 m/day in basalts and from 16.54.6 to 108.02 m/day in laterites. The ‘S’ value produced from the numerical method varied from 0.6 x 10 to 9.8 x 10 in basalts and 0.10 x 10 to 7.8 x 10 in laterites. In the second half of the paper, the strategy of using sensors and wireless sensor networks is presented. Their use could greatly benefit the effort to provide more accurate and timely data for the development of aquifer models, used by officials, managers, and conservationists. 1.0 INTRODUCTION Systemic mapping of hydrogeological conditions in basaltic rock is of prime importance in understanding its water bearing characteristics – especially since many factors like weathering, structural set-up, and the nature of the rock type – affect its water bearing characteristics. To understand the physical state of groundwater within a geological framework, shallow and unconfined aquifers are mapped and the relationship between the lithology and aquifer characteristics is studied. Determining groundwater potentiality, movement, storage and other parameters in an area is only possible only when the characteristics of the rock formation are known (Krusemam and de Ridder 1970, Karanth 1987, Walton 1989). 1.1 Pumping Tests and Methods of Analysis A pumping test is the best available method to evaluate aquifer parameters. This test involves extracting water from a well at a controlled rate and observing the water level changes in the pumped well and/or in one or more observation wells, with respect to time (Theis 1935, Singhal and Gupta 1999). During the past few decades, researchers have proposed several different methods to analyze the pumping test data and estimate the aquifer parameters (Theis 1935, Cooper-Jacob 1946, Chow 1952, Hantush and Jacob 1955, Hantush 1960, 1966, Javandel and Witherspoon 1983, Thiem 1906, Pradeep Raj 2001). Among the main techniques are analytical/conventional methods and numerical methods. Analytical/conventional methods involve one of the following: 1) curve matching, 2) finding inflection points, or for special cases, 3) fitting straight lines to the pumping test data. In curve matching techniques, field results are generally plotted and matched against the results of analytical solutions. There are also several alternative techniques for estimating aquifer parameters from the pumping phase (like log-log plots and log-arithmetic plots), as well as techniques for estimating aquifer parameters from the recovery phase. Comparatively, the numerical method uses a single model to obtain a “best fit” between the field and modeled results for both the pumping and recovery phases (using different parameters). A trial and error technique is employed to obtain a best fit (Rushton 2003). The entire computation procedure and hydrological equations is typically written into a computer program(s). Each of the methods described above are based on basic assumptions relating to the basic type of well, such as well diameter, dug well, and bore well. Therefore, it is important to choose the right method of interpretation based on the field conditions (Krusemam & de Ridder 1970). 2.0 CURRENT STUDY, METHODS, AND RESULTS In the study presented here, pumping tests were conducted on large diameter wells in basaltic terrain within the Chikaldhara region of Maharashtra. An attempt was made to interpret the pumping data by using two conventional methods: the Papadopulos and Cooper method (1967) and the Mishra and Chachadi method (1985), and one numerical method, as proposed by Singh and Gupta (1991). The results of the analysis obtained by these methods are discussed in this paper. 2.1 Climate and Evaporation Rate The climate of the area studied is classified as semi-arid, according to Thornthvaite’s scheme of classification (1948). The temperature of the area is considerably hot. The difference between the maximum and minimum temperature is moderate. The mean annual rainfall is 850 mm. The evaporation rate in the area ranges from 3.2 to 7.6 mm per day. Evaporation is highest during the months of May, June, and July. The mean wind velocity ranges between 7 km and 16 km per hour. The wind velocity is highest during June and July. 2.2 Geological and Hydrogeological Setting Basalts are the main litho types found in the study area. The basalts exposed on the surface area along the southern flank of the Deccan traps belong to the upper cretaceous age. These basalts are devoid of primary openings but possess secondary openings in the form of fractures and joints. These features aid in the infiltration of surface water. Pores and fractures in laterites and fractures and joints in basalts act as reservoirs of groundwater. In the area studied, groundwater occurs in phreatic, unconfined conditions in the weathered basalts that outcrop at the surface. Groundwater is also present in the basalts under semi-confined to confined conditions. The groundwater in the study is extracted by either dug wells or bore wells. In areas where the water table is shallow (depths of 2.0 to 15.0 m), dug wells are more popular due to their lower cost and simplicity in construction and operation. The bore wells are drilled in basalts where the water level is deeper, with depths varying from 35.0 to 45.0 m. The dug wells are replenished by shallow aquifers while the bore wells are replenished from deep aquifers. 2.3 Methodology In the present study, three methods are adapted to estimate the aquifer parameters in a basaltic terrain. Out of the three methods, two are conventional/analytical curve matching techniques: the Papadopulos & Cooper Method (1967) and the Mishra & Chachadi Method (1985). The other technique is a numerical method, as proposed by Singh & Gupta (1991). As the conventional/analytical curve methods are well known, only the numerical method is discussed in detail in this paper. 2.4 The Numerical Method (Singh & Gupta, 1991) In the study presented here, the aquifer parameters were estimated using the numerical approach and methods proposed by Singh & Gupta (1991). This method considers both the pumping and recovery phases to interpret the data from the pumping tests. This method assumes that the following conditions are valid: 1) the water level in the well is static prior to the pumping test, 2) the pumping well fully penetrates the aquifer, 3) the flow towards the well is radially symmetrical (implying isotropy and homogeneity of the aquifer), and 4) the drawdown is significantly smaller compared to the total saturated thickness. Hence, transmissivity could be regarded as invariable during the pumping test, making the system linear and allowing the principles of super-position to be used. The method also permits the pumping durations to be separated into a number of equal time steps for analysis. The aquifer response is then calculated for each time step through the computation of the abstraction with the impulse response function. The extraction rate during each time step is assumed to be constant (even though it could have different rates during various time steps). The drawdown for each time step is calculated considering “guess values” of aquifer parameters. The computed drawdown is then compared with the observed drawdown. The aquifer parameters are progressively modified in an iterative manner until a satisfactory match is achieved between the observed and the calculated drawdown/recovery time. The best-fit drawdown/recovery time curve gives the representative aquifer parameters. The entire computational procedure for this method was written into an interactive, user friendly, computer program. The software application takes into account the combined effects of one or more field conditions, like variable abstraction rate, seepage face, and the effect of nearby hydrogeological boundaries. The software is used to compute the response of an aquifer for a given set of parameters – instead of using a large number of type curves. 2.5 Data Acquisition and Interpretation For this study, 20 pumping tests were conducted to characterize the aquifer parameters of dug wells in the Chikaldhara region: a region with basaltic terrain. Eight of the tests were conducted in laterites and 12 tests were conducted in basalts. The pumping phase of the tests had a short duration of 60 to 210 minutes; the recovery phase of the tests had a longer duration of 90 to 300 minutes. Details of the pumping tests are given in Table 1. The discharge rates varied from 12.74 to 307.13 m/day and the drawdown rates varied from 0.33 to 3.19 m. Except at well location #11, 70-100% of the recovery from drawdown was observed at each location. The data was analyzed and interpreted by the three methods mentioned in Section 2.3 – Methodology. The results are presented in Table 2. 3.0 REGIONAL GEOLOGICAL SETTINGS AND STRATIGRAPHY The following subsections present the geological settings and strategraphy of the study area. The values obtained during the pumping tests are also discussed.