Innovative methods for the characterisation of fractured rock aquifers
Akoachere, Richard Ayuk II
MetadataShow full item record
Bulk flow is regional flow. The word region is used in two ways viz; i) A region may be a hydrogeologically and geographically distinct area. Ex: the Karoo basin. ii) A region maybe discontinuous but widespread encompassing related non adjacent aquifer systems such as surficial aquifers, coastal aquifers or as in our research project study case, some selected fractured rock aquifers in South Africa. In case (ii) regions, topical investigations are optimized for regional applications. In such investigations, focus is on processes rather than properties of specific aquifers (Groundwater science). Characterization tends towards common processes (drivers of the various processes) rather than geographical locations and particularities. Two new methods have been developed to determine inclined and horizontal fracture apertures b, in fractured rock aquifers. These methods are; i) The SLUG-TRACER (ST) TEST; ii) The NAPL ENTRY PRESSURE (NEP) TEST/ NAPL INJECTION PRESSURE (NIP) TEST. Mathematical formulations were developed from laboratory experimentation using transparent Perspex parallel plate physical models and 27 apertures of 0.008 mm to 6 mm, created by using aluminum foil and thickness gauges between 20 mm thick clamped Perspex plates. The ST test uses a slug in which is added NaCI as tracer (500mg-5000 mg/I) and an EC meter is used to detect breakthrough in the observation boreholes. The NEP test uses a NAPL (Sunflower oil) hydraulic head and transducers to get the entry pressure. Using these mathematical formulae, fracture apertures are then determined for horizontal and inclined apertures. The NIP test uses the entry pressure recorded by transducers, of a NAPL (Sunflower oil) by injection and its volume to determine the fracture aperture for horizontal and inclined fractures. Results from smooth and rough (Buffed to 10x20 microns) fracture surfaces gave accurate results for 96-98 % aperture determinations of twenty six (26) apertures from 0, 04 mm to 6.3 mm. The Phreatic Hydraulic Conductivity (PHC) apparatus was developed to measure the hydraulic head gradient of samples. The PHC apparatus was made of a solid body divided into three chambers, mounted on a ten liter capacity water reservoir, with a pump. Three types of samples can be used; Consolidated (in-situ), loose/friable (insitu), and unconsolidated samples (Drill/auger cuttings, Mine tailings/ash etc.). The apparatus was used to determine hydraulic conductivities of samples ranging from coarse gravel to very fine clayey dam tailings. The values ranged from 2.81 E-03 to 4.32E+03 (m/d). The results were reproducible and compared well with those of other methods. The PHC apparatus' advantages are: Can be used in the field and laboratory (compact); Simple to use and needs limited maintenance (Three components); Economical, needing small volumes of water (ten liters); Light (6kg) and compact (0.16 rrr'): Rapid results (Complete determination for a sample within tens of minutes); This apparatus is particularly suited to determine the hydraulic conductivity of clastic formations for non-confining flow under atmospheric conditions. Laboratory experiments on the small (cm) scale aimed at determining the effect of variable thickness of formations on the hydraulic conductivity, determine the effect of composition, layering ,spatial disposition and develop a tool for predicting bulk hydraulic conductivity in phreatic aquifers were carried out. From these, the partial hydraulic conductivity formulation was developed empirically, to determine the bulk hydraulic conductivity of the samples, irrespective of the spatial disposition. With geologic insight, the bulk hydraulic conductivities were determined using the partial hydraulic conductivity theory. When the thicknesses of the layered sequences varied, the laws of composition broke down. The Trigger-tube is an apparatus developed for mixing solutes and tracers for injection tests in boreholes. It is a simple cap-trigger tube segment and the technique mixes solutes in boreholes in two minutes. Solutes are introduced into the well and the trigger is released. The tube is withdrawn and the solute mixes instantaneously to give a homogeneous mixture of solute with the borehole groundwater. Field tests using this method and apparatus for point dilution tests gave a Darcy velocity of 4.06 m/day, Seepage velocity of 122.89 m/day and effective porosity of 0.33. Natural gradient tests gave a Darcy velocity of 4.06 m/day and natural velocity of 123 m/day using NaCI for the same fracture at 21m in borehole U05. This apparatus takes comparatively a shorter time to carry out SWIW tests than using the pump mixing method. Field tests gave 13 minutes for the trigger-tube method and 25 minutes for the pump mixing method for a point dilution test using NaCI. This apparatus can be used for any test that needs the introduction of a homogenous mixture in single well tests. The thermal dilution test is a test developed to determine the position, number and groundwater (Darcy) velocity of fractures found in a single borehole drilled into a fractured rock aquifer using temperature as a tracer. Using a trigger-tube apparatus, cold at 2 degrees Celsius is introduced into a single well. The rate at which warmer groundwater flows into the well is measured as the change in temperature and used to determine flow zones, the position of fractures, their depths and the Darcy velocity of the various lithologies and fractures with flow present, from top to bottom of the borehole the method was used in a single well test on borehole U05 to determine fractures at 14m, 15m, 16.8m, 18m, 19.4m, 21m, 22.4m, 24.2m,26m and 27.5m below the surface. These fractures had Darcy velocities ranging from 1.54m/day t04.17m/day, with the largest fracture contributing to flow in the borehole being that at 21 m. This was confirmed by acoustic scan and borehole camera images of the borehole. This method is very useful to determine the hydraulic properties of fractures and formations under natural conditions (Without pumping) using a single well.