Assessing Different Methods to Characterize Hydraulic Properties of Fractures

Fractured rock aquifers are complex systems, which impedes the determination of their hydraulic properties. However, characterization of fracture properties is important when dealing with groundwater quality and quantity protection. Fractures have often shown to be good conduits for water and contaminants, leading possibly to fast contamination of these aquifers. No agreement exists on a preferred methodology for characterization of fractured aquifers.

We therefore used a combination of methods to determine the transmissive zones at two research sites drilled in the quartz arenite Table Mountain Group (TMG) Aquifer. This aquifer forms one of the major fractured rock aquifers in South Africa and is currently targeted for bulk water supply to the City of Cape Town. Methods used to characterize the aquifer included down-hole geophysics and water quality logs using a YSI multi parameter sonde to determine whether there are any significant changes in chemistry. A specific depth sampler was also used to collect chemical samples. Following the initial logging, Flowing Electrical Conductivity (FEC) logging was used to quantify the contribution of individual fractures. Constant discharge pump tests at three different rates followed at both research sites with water quality logging over time. Correlations between YSI, FEC, specific depth samples, hydraulic properties and time series water quality during pumping test are analyzed for correlation and contribution of different fractures to flow at specific pumping rates.

Results acquired from the different methods at these two sites are used to evaluate the benefit and usefulness of the various methods. The data also contribute to the management or protection plan for the specific aquifer due to an improved knowledge of the local pathways for the fluids and solutes.