Document Type

Report

Publication Date

January 1983

Abstract

Executive Summary: The basic purpose of this study was to conduct a finite-element computer model and give an dindependent appraisal of the reliability of the groundwater supply for the proposed Coastal Belt Water Project (CBWP_ Well Fields at Sarir and Tazerbo. There is no doubt that ample quantities of groundwater of acceptable quality occur at the sites selected for these well fields for the estimated 50-year life of the project and longer. Of major concern is the predicted drawdown of wells and total pumping lifts throughout the 50-year period. Excessive drawdowns could cause operating costs to become prohibitibely expensive. Average drawdown in wells not including well-field interference and hydraulic friction losses at the CBWP Well Field at Sarir is expected to be only about 30 m at the end of 50 years, but the static water level is relatively deep (56 m) which makes the total pumping lift fairly high when well-field interference and other losses are added. This optimistic estimate of drawdown is based upon computer analysis for almost 7 years of pumping history of the existing Sarir South agricultural well field nearby, where the subsurface geology is believed to be quite similar to that at the propsed CBWP Well Field at Sarir. The average dpumping lift, which includes many other factors besides aquifer drawdown, is estimated to be about 142 m. at the end of the 50-year pumping period at Sarir and should average about 136 m during the 50 years. Interference from pumpage at the existing agricultural well fields at Sarir may be responsible for about 10 m of this predicted pumping lift. Drawdown at Tazerbo is predicted to be about 90 m. at the end of 50 years, but the static water level is only about 9 m. there. Consequently its average pumping lift should be about 133 m at the end of the 50-year pumping period and should average about 118 m during the 50 years. no appreciable interference from nearby well fields is expected in the Tazerbo area based upon presently known conditions. Significatn reductions in pumping lifts at Tazerbo and in water-collection network costs at Sarir are possible by improved well field layout and well design. Suggested inprovements and estimated cost savings are presented in detail in Appendix A (especially see well-field design comparisons of Tables A-1 and A-4).

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