Conductivity and flux measurements in porous aquifers

Knowledge of hydraulic conductivities as well as water and contaminant mass fluxes in porous aquifers is of paramount importance for many hydrogeological processes, such as irrigation, drainage, quantitative and qualitative water resources management, contaminant risk assessment, etc. No theoretically well-based method is currently available to measure horizontal and vertical conductivities in the saturated zone and methods for measuring field saturated hydraulic conductivities in the unsaturated zone are limited, time consuming and sometimes unreliable. Furthermore, no method is currently available to directly measure vertical water and contaminant fluxes in porous aquifers or at the groundwater/surface water interface. The proposed work is based on the development of an exact solution of the flow field for the case of water injection/extraction from an arbitrary number of different injection screens along an otherwise impermeable casing with or without the presence of different boundary conditions close-by. This solution allows for the computation of shape factors of the flow fields required to estimate hydraulic conductivities from injection heads and flow rates. Approximate formulae in current use for packer injection tests can be evaluated against the exact solution and effects of anisotropic conductivity can be taken into account. A two-screen drive point probe is proposed to perform measurements of horizontal and vertical conductivities at variable depths under the water table without previous installation of boreholes. The test consists of two stages: (1) injection through both screens and (2) recirculating water between the screens (injection/extraction). The same probe design is proposed for the passive vertical flux meter to directly and simultaneously measure local cumulative vertical water and contaminant fluxes. No more pumping takes place at the screens, which are now hydraulically connected to each other by a sorbent column containing resident tracers. The vertical ambient gradient drives a flow through the column that gradually eludes tracers at the same time as the sorbent retains contaminants from the water. By analyzing the sorbent column for remaining tracer and retained contaminant masses and knowing the properties of the flow field around the meter, the vertical ambient water and contaminant fluxes can be determined. A coned two-screen drive point probe is furthermore proposed to directly measure field saturated hydraulic conductivities both above and under the water table. The method is based on steady state coupled saturated/unsaturated flow from spherical injection cavities. The possibilities of a transient single screen method and of measuring anisotropic conductivities are evaluated. The proposed concepts will be elaborated in theory and validated by saturated/unsaturated sand barrel tests in the laboratory.