Estimating Aquifer Parameters from Time Series EC and Pressure Data Collected During River Flow Events

Numerous analytical models have been developed for predicting pressure responses in aquifers resulting from water level variations in rivers. These models have also been used for estimating aquifer parameters from observed responses in bores adjacent to the river. Where the river and aquifer have distinct chemistries, then this same process can also produce a water quality (solute) response in the adjacent aquifer. However, there are few observations of such responses in bores following river flow events, and limited theoretical development in the use of solute data to estimate aquifer parameters.

We present a new analytical solution that relates travel distance and time for movement of a solute into a confined aquifer to the increase in river stage (H), and aquifer parameters hydraulic conductivity, storativity (S), porosity (θ), and aquifer thickness (b). Combining solute travel time with an existing solution for pressure travel time yields a ratio that is solely a function of H, S, θ, and b.

A 2D numerical model of an aquifer slice perpendicular to a river was constructed in FEFLOW. Simulations of a confined system indicate good agreement with solute and pressure travel times predicted by the analytical solutions. The applicability of the relationships to unconfined systems is evaluated through simulations in an equivalent model that explicitly includes the unsaturated zone. Sensitivity analysis indicates that the predicted relationships generally hold, providing that unsaturated zone storage is appropriately represented.

The utility of the method for estimating aquifer thickness and storage is tested using extended time series data from an observation bore. The field example indicates that co-measurement of pressure and EC and application of the analytical relationships can reasonably estimate aquifer thickness using standard ratios for storage parameters, or conversely, if aquifer thickness is known, can provide an estimate of the storage parameters.