Analytic Element Modeling of Steady Interface Flow in Multilayer Aquifers Using AnAqSim

This paper presents the analytic element modeling approach implemented in the software AnAqSim for simulating steady groundwater flow with a sharp fresh‐salt interface in multilayer (three‐dimensional) aquifer systems. Compared with numerical methods for variable‐density interface modeling, this approach allows quick model construction and can yield useful guidance about the three‐dimensional configuration of an interface even at a large scale. The approach employs subdomains and multiple layers as outlined by Fitts (2010) with the addition of discharge potentials for shallow interface flow (Strack 1989). The following simplifying assumptions are made: steady flow, a sharp interface between fresh‐ and salt water, static salt water, and no resistance to vertical flow and hydrostatic heads within each fresh water layer. A key component of this approach is a transition to a thin fixed minimum fresh water thickness mode when the fresh water thickness approaches zero. This allows the solution to converge and determine the steady interface position without a long transient simulation. The approach is checked against the widely used numerical codes SEAWAT and SWI/MODFLOW and a hypothetical application of the method to a coastal wellfield is presented.     

Modeling Dewatered Domains in Multilayer Analytic Element Models

This proposed technique allows sensible and numerically stable behavior in multilayer analytic element models when layers dewater. When saturated thickness approaches zero in an unconfined or fresh/salt interface domain, the domain transitions to a very thin confined domain with a minimum saturated thickness M. M is an adjustable input parameter, so you can make the horizontal flow in dewatered domains negligibly small by making the minimum saturated thickness very small. Vertical flows can pass through a dewatered domain, whether it is near the surface or at depth. For example, recharge may pass through a shallow dewatered layer to a deeper layer that is not dewatered. This approach is examined in detail in an example multilayer model of mine dewatering.     

Model Calibration Techniques for Use with the Analytic Element Method

The combination of the analytic element method and a nonlinear parameter estimation technique forges a computationally efficient, information-rich, and cost-effective solution to the inverse ground-water flow problem. The recommended model calibration method uses a nonlinear least-squares objective, as quantified by misfitting the measured and modeled heads, and a modified Levenberg-Marquardt algorithm. As detailed and demonstrated by a steady-state regional aquifer model of Bemidji, Minnesota, automated calibration techniques make ground-water modeling feasible for a wider variety of projects where tight budgets and a lack of tools may have previously made such modeling inappropriate.