Estimating confined aquifer parameters using a simple derivative-based method

Abstract

The confined aquifer parameters, transmissivity and storage coefficient, are commonly determined using the pumping tests. Several methods have been developed to estimate confined aquifer parameters using pumping tests, but different methods suffer from different drawbacks. Those methods that use the truncated Theis well functionw(u), apply just early or late drawdowns, depending on the case, to estimate the aquifer parameters. Those methods, such as Theis (1935), that use non-truncated well functionw(u), can apply all drawdown data for aquifer parameter estimation but may still suffer from subjectivities such as personsal judgment in curve matching, time-consuming procedure and requiring values for Theis well function, w(u), and its argument u(u=r2S4Tt). The aim of this study is to present a new method to overcome the aforementioned drawbacks and subjectivities involved in available published methods. In this paper, a simple derivative-based method is presented to estimate confined aquifer parameters applicable for entire drawdowns during the pumping period. The time derivative of drawdowns relate non-linearly with pumping timet, and therefore, aquifer parameters are estimated using developed equations based on the least squares optimization approach. The method is applied to three sets of synthetic, published and field data and results show that the estimation accuracy is acceptable. The drawdown time interval measurement has a marginal effect on parameters estimation due to the analytical basis of derivative calculations. The method does not require construction of graphs, and numerical calculations may be performed on a calculator to determine the aquifer parameters on site. It does not require curve matching, initial guess of the parameters and values of w(u) and u.