CHARACTERISING DNAPL FRACTURE FLOW IN THE FIELD USING A SURROGATE DNAPL

A field experiment was done to determine the nature of the DNAPL flow in South African aquifers using a surrogate DNAPL. FC-77, a Fluorinert chemical with similar physical properties to PCE was used. The experiment was done at the Campus Test Site at the UFS. The mode 1 fracture is the most significant fracture on the site and all boreholes with high yields intersect a bedding-plane fracture a depth of around 21 m. Six new percussion boreholes and two core boreholes were drilled around the previous boreholes. The latter were drilled until the bedding plane fracture was intersected leaving only a short interval below this. The site was characterised using several methods, including down-the-hole geophysics and tracer testing. To execute the DNAPL injection more accurately, several water and brine injections were undertaken in the laboratory and field to optimise the experimental procedure. Field equipment was custom-designed using inert, robust material and allowing instantaneous fluid release. Pressure changes were measured by transducers. Laboratory testing of different liquids into a fixed aperture parallel plate apparatus confirmed the critical factors for fracture mass transport of dissolved, density driven and DNAPL flow.

The field test showed DNAPL injection into fractured rock elicits a different response to injection of high density brine, emphasizing the importance of NAPL flow. The field injection showed that DNAPLs can migrate in directions opposite to groundwater flow and that local variations in fracture strike and dip are far more important than the regional fracture dip or flow directions. The pressure response in the aquifer surrounding the injection was not radial and is a function of hydraulic conductivity and fracture connectivity. It also showed that even under controlled injection, determination of DNAPL mass distribution at field scale is difficult, and large amounts of DNAPL residual rest in local depressions within the fracture. The injection was recorded by down-hole video, with the non-reactive FC-77 contrasted with dye tracer in the aqueous phase. This experiment confirmed that accurate fracture identification and characterization are the most important considerations in understanding DNAPL flow in the majority of local aquifers.