AN INTEGRATED APPROACH FOR EVALUATION OF HYDRAULIC PROPERTIES OF ALLUVIAL AQUIFERS

Abstract

In order to evolve pragmatic and scientific planning for the management ofgroundwater resources, one needs toquantify the relevant hydrogeologic characteristics ofthe aquifer system(s) and adopt appropriate methodology for data interpretation and analysis. The present work is an attempt to accomplish this objective by devising fresh approaches using data on grain size parameters, pumping test data and electrical resistivity soundings. The applicability ofthese approaches are tested in an area which is approximately 38,000 square kilometer in areal extent located between Latitude 26° 45' 00" and 31° 00' 00" North and Longitudes 77° 11' 00" and 80° 22' 00" East, a part ofGangetic alluvial plain. 1. Grain Sifce Analysis For Estimating Hydraulic Conductivity Anumber of relationships exist between hydraulic conductivity and those parameters that describe the grain-size distribution of depositional medium. One common aspect ofthe reported grain size studies has been the determination ofvaried types ofrelationship between hydraulic conductivity and some statistical grain size parameters, such as the geometric mean, standard deviation (dispersion), or effective diameter. However, only afew such correlation give reliable estimates in case ofnon-uniform sediments owing to the difficulty ofincluding all possible variables in porous media. Further, most ofthese correlations are area-specific as they involve evaluation ofsome constants derived for specific conditions and can be seldom applied to porous media in general. The application of flow model equation(s) in estimating permeability also becomes less fruitful as these necessitate estimation of the value of parameters such as porosity and composite shape factor. Due to these reason(s), the available formulae do not yield consistent results in comparison to realistic hydraulic conductivity values. Itwas considered meaningful to obtain a viable expression for evaluating permeability as a function of both the size distribution (sediment characteristic) and fluid characteristics. The developed expression is of the form as given below: g d+2 / v2 K = - d.3g where,K is coefficient of permeability, g is acceleration due to gravity,v is kinematicviscosity of water at the given temperature, d+ is the transitional diameter, and C accounts for porosity and standard deviation ofthe sediment sample. It was found during calibration from data of a part of the area that C can be expressed as C = o + n, where, a is standard deviation and n is a transitional exponent. A fairly good correlation coefficient has been obtained between observed and calculated permeabilities, for the sediment samples considered for calibiration. The validation of the expression has been done on data pertaining to other parts of the area. Thus, a new method has been proposed for the evaluation of hydraulic conductivity from grain size data of alluvial sediments. &. Puniping Test Data Analysis For Estimation of Aquifer Properties Pumping Test-data analysis, by curve matching is one of the most widely used method for estimation of hydrological parameters of aquifers. However, the approach suffers from many limitations such as, inherent subjective bias in the graphical procedure of curve matching, and the number of unknownparametersinan aquifer test, limits the accuracy of the graphical solution. Available literature on pumping test data analysis reveals that computer programs are mostly available for analyzing pumping test data for a specific type of aquifer only. Anattempt hasbeen made to develop a composite computerprogramto interpret the pumping test data for estimating hydraulic properties of the aquifers undervarying conditions, bymaking appropriate computational modifications in the known methods. The developed computer program can analyze pumping test data of wells tapping a particular type of aquifer by Theis, Hantush-Jacob, Boulton's and Neuman's full and partial penetration approaches. Theprogram wastested ontwo pumping test sitesinthe alluvial aquifers of the study area. The generated output revealed that, the parameters so obtained gave a much better reproduction of drawdowns as compared to the graphical estimates, under the given hydrogeological conditions ofthe aquifer. The program helps to obviate the subjectivityinvolved Page vi j in graphical matching, to estimate a good number of unknown parameters, and to determine or verify the aquifer type. 3. Resistivity Studies in Tlie Evaluation of Aquifer Properties The common methods for evaluating aquifer characteristics involve drilling ofobservation wells, conducting pump tests and analysis ofpumping test data which is in totality time consuming and expensive. This suggest that some alternative inexpensive method be tried before theuse of pump test which should be adopted sparingly. Surface geo-electrical measurements in which large volumes ofearth materials are covered offer an attractive approach for the estimation of aquifer characteristics. Inspite ofbottlenecks regarding its nonuniqueness, the surface resistivity method, when generalized for different geological conditions holds considerable promise. In this context an attempt has been made to establish more meaningful relationships between hydraulic properties and surface geoelectrical parameters ofthe aquifer. The applicability ofgeoelectrical method ofexploration has been examined in parts ofthe study area for evaluating aquifer properties (like Transmissivity and hydraulic conductivity) ofalluvial anisotropic aquifers. Following types of empirical relations are established, for estimating the hydraulic parameters from the field data. Relation between Aquifer Formation Factor (FF) and Horizontal Hydraulic Conductivity (K). K = 8.953 (FF)0852 0 Relation between Aquifer Hydraulic Conductivity (K) and Modified Aquifer Resistivity (p1). K = -0.744 + 0.463 (p') D Relation for estimating Aquifer Transmissivity (T) from Modified Transverse Resistance (R). 0 244 T= 1.081 R The direct relation between the modified resistivity and the aquifer hydraulic conductivity Page vii I is more meaningful in the study area although the aquifers have a wide range of hydraulic anisotropy. It can be concluded that in an alluvial area where Darcy flow is deemed to be valid, we can estimate hydraulic Conductivity and transmissivity ofanisotropic aquifers with reasonable accuracy, at aquifer level, by using relations between hydraulic properties and resistivity parameters. These development significantly improve the characterization of aquifer parameters and as such management ofgroundwaterresource can be done morepurposefully.