Development of a Preliminary Hydrogeology Conceptual Model for a Heterogeneous Alluvial Aquifer using Geological Characterization

M Gomo G Van Tonder

doi:10.4172/2329-6755.1000128

Abstract

Geological characterization of a heterogeneous alluvial channel aquifer was conducted using outcrop mapping and analysis of lithological logs from 15 boreholes drilled at the site. The study was aimed at developing a preliminary hydrogeology conceptual model. A total of 12 lithological samples were tested in the laboratory using falling head permeameter to determine the hydraulic conductivity of each unconsolidated lithology unit. Shale and calcrete outcrops were observed on the study site. Outcrop mapping on the site also indicated the presence of a contact plane between the overlying unconsolidated sediments and the shale of the underlying bedrock where a groundwater discharge zone has been created. Geology logs show that the alluvial channel aquifer comprises of three distinct layered unconsolidated sediment materials (calcrete, clay-silt, sand-gravel deposited formations) that overlie low permeable shale formation. Geological lithology revealed the spatial variation in the nature and type of unconsolidated sediments deposits between boreholes that can influence groundwater occurrence and flow in the aquifer system. The gravel-sand aquifer layer that consists of medium to large pebbles is conceptually the main hydraulically conductive unit with an average laboratory determined hydraulic conductivity of 8.5 m/day. Hydraulic gradient calculated using a combination of boreholes ranges from 0.013-0.022.

Highlights

A considerable part of today’s farmers in Southern Africa rely on groundwater abstraction from alluvial aquifers for irrigation purposes during the dry and winter planting season
Background geology: The background geology generally consists of soil, clay-silt and calcrete unconsolidated deposits overlying the shale bedrock with the exception of BH14 that does not intersect clay-silt materials (Figure 8)
Geological characterization shows that the alluvial aquifer is composed of calcrete, clay-silt and gravel-sand unconsolidated sediments overlying low permeable shale bedrock

Summary

Introduction

A considerable part of today’s farmers in Southern Africa rely on groundwater abstraction from alluvial aquifers for irrigation purposes during the dry and winter planting season. Ephemeral rivers are more common in Southern Africa due to extended periods (>9 months) without significant rainfall events [2]. During these dry periods farmers rely heavily on groundwater. In Southern Africa, the alluvial river channel aquifers comprises mostly of sedimentary deposits, composed of various lithological components such as clay, silt, calcrete and sand. The background geology generally consists of soil, clay-silt and calcrete unconsolidated deposits overlying the shale bedrock with the exception of BH14 that does not intersect clay-silt materials (Figure 8). The terrestrial land is characterized by thicker unconsolidated vadose zone sediments and deep water levels as compared to the shallow alluvial channel aquifer. The presence of thick unconsolidated sediments on the terrestrial background aquifer can be attributed low erosional effects when compared to the riparian zone

Methods

Results

Conclusion