A Unit-Concentration Method to Quantify Source Contribution.

Solute migration is typically simulated to describe and estimate the fate and transport of contaminants in groundwater. The unit-concentration approach is investigated here as a method to enable solute transport simulations to expand the capabilities of groundwater flow modeling. The unit-concentration method uses a concentration value of one to identify sources of water to be assessed and a concentration of zero for all other water sources.

Investigation of transient freshwater storage in island aquifers.

Fresh groundwater in an island aquifer is an extremely important resource that is highly vulnerable to variations in natural weather cycles and climate change effects. On small islands, precipitation creates subsurface freshwater lenses that float on top of coalesced saltwater that has intruded from the surrounding seawater. The volume and shape of these highly vulnerable freshwater lenses depend on the size and shape of the island, underlying geology, and the rate and duration of groundwater recharge.

Axisymmetric Modeling Using MODFLOW-USG.

Axisymmetric groundwater models are used for simulating radially symmetric conditions. Groundwater simulators built specifically to model axisymmetric conditions are most commonly used for simulating aquifer tests. Although some numerical models capable of simulating flow and solute transport that are developed in the cartesian coordinate system framework offer flexibility to simulate axisymmetric conditions, most of the numerical groundwater models, such as the MODFLOW family of codes, are based on structured grids in which axisymmetric flows cannot be directly simulated.

Impact of Local Groundwater Flow Model Errors on Transport and a Practical Solution for the Issue.

A groundwater flow model is typically used to provide the flow field for conducting groundwater solute transport simulations. The advection term of the mass conserved formulation for groundwater transport assumes that the flow field is perfectly balanced and that all water flowing into a numerical grid cell is exactly balanced by outflows after accounting for sources/sinks or internal storage. However, in many complicated regional or site-scale models, there may be localized flow balance errors that may be difficult to eliminate through tighter flow convergence tolerances due to simulation time constraints or numerical limits on convergence tolerances.

Source screening module for contaminant transport analysis through vadose and saturated zones.

At complex sites there may be many potential sources of contaminants within the vadose zone. Screening-level analyses are useful to identify which potential source areas should be the focus of detailed investigation and analysis. A source screening module (SSM) has been developed to support preliminary evaluation of the threat posed by vadose zone waste sites on groundwater quality.

Approaches to the simulation of unconfined flow and perched groundwater flow in MODFLOW.

Various approaches have been proposed to manage the nonlinearities associated with the unconfined flow equation and to simulate perched groundwater conditions using the MODFLOW family of codes. The approaches comprise a variety of numerical techniques to prevent dry cells from becoming inactive and to achieve a stable solution focused on formulations of the unconfined, partially-saturated, groundwater flow equation. Keeping dry cells active avoids a discontinuous head solution which in turn improves the effectiveness of parameter estimation software that relies on continuous derivatives.