Analysis of pumping well near a stream

Abstract

A pumping test was conducted to evaluate aquifer characteristics, the percentage of tube well discharge derived from the stream, and the effective distance to the stream simulated as a fully penetrating plane source of constant head. The formulae are applicable although a natural stream is seldom fully penetrating, because the distance from the stream to an equivalent fully penetrating but imaginary plane source can be calculated. When pumping begins, the tube well obtains most of its water from the immediate vicinity, but as the cone of influence expands, it receives a part of its water from the stream until a time is reached when it draws the entire amount of water from the stream. This will of course depend upon the aquifer characteristics, the tube well discharge, distance of the tube well from stream and the infiltration regimen of the stream. Different methods were used to find the distance of stream equivalent to a fully penetrating plane source, the determination of coefficient of transmissibility and coefficient of storage, and, lastly, the amount of water that the tube well intercepts from the stream. The distance of the stream as a fully penetrating plane source by Rorabaugh’s modification of the Theis formula ranges from l'70 feet to ioo feet and the value of coefficient of transmissibility is 0.77 cubic ft./sec./ ft. The equivalent distance by Stallman’s modification of the Theis formula is l90 feet and coefficient of transmissibility 0.6g. The equivalent distance by the inflection point method of Hantush ranges from 1500 feet to 2100 feet, the coefficient of transmissibility from 0.73 to 0.79 cubic ft./sec. and coefficient of storage is 0.05. The coefficient of transmissibility by considering it as a leaky aquifer is 0.715 cubic ft./sec. The coefficient of storage by the volume method is 0.065 and the contribution of the stream to the tube well discharge is 86%. The contribution of the stream to the tube well is 93% by the image theory. The results derived, based on mathematical theory, were also checked by preparing contour maps of drawdown for different times and calculating the volume dewatered by the pumping well at different times and plotting it against total pumpage and thus estimating the coefficient of storage. }or the determination of effective distance, only distant pipes from the tube well were used to avoid effects due to partial penetration and non-radial flow. The data for later time were used assuming that canal leakage fully affected the pipe under analysis and it was not a time variable. As determination of T and S was based upon the inflection point on a time drawdown curve for the analysis, only pipes near the canals were chosen, where the recharge was predominant, and so the determination of inflection point and maximum drawdown were easier. To find the maximum drawdown, the drawdown data of pipes near the canal were replotted on uniform scale against l/t and the maximum drawdown was determined from the tangent to the curve.