of Groundwater Flow Through
A Tertiary Fanglomerate Aquifer
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 Chems Approach
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.
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
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.
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