Journal de géologie et géophysique
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Development of a Preliminary Hydrogeology Conceptual Model for a Heterogeneous Alluvial Aquifer using Geological Characterization

M Gomo and G van Tonder

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.