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

Abstract

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

“…When the freshwater saturated thickness is M or less, Φ( h ) changes to a confined domain equation with a constant that makes Φ( h ) continuous at this transition. An example of dewatered layers in multilayer fresh/salt interface analytic element models is given by Fitts et al ().…”

“…A stabilizing approach to this issue has been implemented in AnAqSim (analytic aquifer simulator) for unconfined, convertible confined/unconfined, and fresh/salt interface domains (Fitts 2017). This approach is briefly mentioned without details in a paper about multilayer AEM fresh/salt interface modeling (Fitts et al 2015). The present paper examines the approach in detail, focusing on unconfined domains.…”

Simulation of Drying‐Rewetting Processes in Numerical Groundwater Models Using a New Picard Iteration‐Based Method

“…Current methods for preventing or addressing problems with the drying and rewetting of grid cells in groundwater models can be summarized into two categories. The first has no physical mechanism and is more of a modeling technique, such as increasing the tolerance of the model convergence and lowering the bottom of key areas (Painter et al., 2008), adjusting the vertical discretization of the model (Keating & Zyvoloski, 2009; Markstrom et al., 2008), assigning minimal saturation thicknesses to dry cells (Doherty, 2001; Lin et al., 2010), converting the nearly dry unconfined domain into a confined domain to simulate (Fitts, 2018; Fitts et al., 2015), etc., which avoid the drying of the simulation domain. In some cases, this method can produce an acceptable simulation effect, but such nonphysical adjustments also raise concerns about the accuracy and reliability of the simulation results (Hunt & Feinstein, 2012; Painter et al., 2008).…”

Hydrogeological Uncertainty Estimation With the Analytic Element Method

“…While initially developed for two‐dimensional, steady‐state settings, AEM has since been extended to support three‐dimensional model domains (Haitjema, 1985; Janković & Barnes, 1999), smooth inhomogeneities (Craig, 2009), and transient dynamics (Furman & Neuman, 2003). AEM code has been distributed in modeling frameworks such as TIMML (Bakker, 2006; Bakker & Strack, 2003), Visual AEM (Craig et al., 2009), TTim (Bakker, 2013), or AnAqSim (Fitts et al., 2015).…”