Geohydrological characteristics of the Otjiwarongo marble aquifer, Namibia

Abstract

Abstract by author:

Abstraction over the years has increased from 0. 6 Mm3/a in 1970/71 to more than 1. 30 Mm3/a in 1988/89, thereby exceeding the level of recharge of 1, 2 Mm3/a as indicated during a previous investigation. The need for a geohydrological re-evaluation was further enhanced with the failure of the Omatjenne Dam, considered to be a significant source of local recharge to the Marble Aquifer. The general absence of deeply incised drainage channels in the study area restricts the exploitation of water resources other than groundwater

The aims of this study are to give a detailed account of the geohydrological characteristics of the aquifer and to introduce appropriate recharge and groundwater flow models that can be implemented and updated to facilitate optimal utilisation of the local groundwater resources

Marble (Karibib Formation) which belongs to the geosynclinal Damara Supergroup, has through the process of karstification established itself as the most productive aquifer in the area. Marble outcrops occur in a northeastly trending isoclinal synform. The isoclinal attitude of the folded limbs and the narrow strip-like outcrops are major determining factors when evaluating recharge potential, boundary conditions, aquifer geometry, topography and the vertical depth extension of favourable aquifer conditions

Geological and geohydrological evidence suggest that the present groundwater system developed as a result of the observed geo-structural framework. The dominant aquifer parameters are closely related to, and governed by the geometric configuration of the marble outcrops. The spatial distribution of hydraulic parameters (transmissivity and specific yield) in the Marble Aquifer represent characteristic heterogeneous and anisotropic conditions. The Marble Aquifer is considered to be a fractured watertable aquifer

The groundwater potential of the Marble Aquifer is closely related to the recharge potential of the area. The groundwater level fluctuation (GLF) and saturated volume fluctuation (SVF) methods, which are variations of the well-known groundwater balance method, were applied to facilitate estimates of recharge to the aquifer system. Recharge factors of between 3 percent and 1 percent are proposed and the average annual recharge, which equates to the safe yield of the Marble Aquifer, is estimated at 4. 3 Mm3/a for the area studied

A satisfactory simulation of groundwater levels in the present abstraction area, the so-called Omatjenne Compartment, was achieved by an in tandem utilisation of the inverse model AQUA-INV, and the two-dimensional finite element model AQUAMOD. Model calibration identified certain aspects in the present information framework that need to be improved upon before utilising the model effectively as a management or decision-making tool. In this regard, further field observations to define adequately aquifer boundary conditions and rainfall-recharge relationships, would enhance the model's prediction capabilities. However, the successes and shortcomings of the existing model should not be seen in isolation, but rather as a platform for similar endeavours to describe and simulate groundwater flow in the other compartments comprising the Marble Aquifer