Simulation of Groundwater Flow Through
A Tertiary Fanglomerate Aquifer

Situation

Groundwater resources in a tertiary fanglomerate aquifer were potentially affected by remedial groundwater pumping. An assessment of the extent to which drinking water supplies could be developed and sustained from the aquifer was needed.

The fanglomerate has a relatively complex shape, is faulted and heterogeneous, and has a thickness ranging from less than 100 to greater than 4,000 feet. Playa and lake bed deposits are believed to occur locally within the fanglomerate. Abrupt lateral changes in permeability that occur within the fanglomerate have been reported to result from local depositional variation and from faulting.

The fanglomerate is in hydraulic contact with a shallow alluvial aquifer that fills in-cut channels. The alluvial aquifer has a permeability at least three orders of magnitude higher than that of the fanglomerate, and acts to drain water from the fanglomerate. The fanglomerate overlies bedrock consisting of igneous, metamorphic, and sedimentary rock that locally serves as an aquifer. The permeability of the bedrock is generally believed to be at least one order of magnitude lower than the fanglomerate.

Hydro Geo Chem’s Approach

We developed a three-dimensional numerical groundwater flow model of a fanglomerate aquifer in Central Arizona as part of an ongoing groundwater remedial effort at an Arizona mine site.
The current version of the numerical model has been designed using available data to test our conceptual understanding of flow through the aquifer and of the hydraulic interaction between the fanglomerate, the overlying shallow alluvial aquifer, and the underlying bedrock. This model is also being used to direct further remedial efforts at the site.

Results

As the model is refined based on the collection of additional data, it will be used for a number of purposes including: 1) estimating the sustainable long-term groundwater pumping levels in the fanglomerate aquifer, and
2) predicting the hydraulic interaction between the alluvial and fanglomerate aquifers under present conditions and conditions of anticipated remedial pumping in the alluvial aquifer.

The figure shows the simulated hydraulic head at the present time within the shallow portion of the fanglomerate.