Geohydrology and simulation of ground-water flow near Los Alamos, north-central New Mexico

An existing model was modified in recognition of new
geohydrologic interpretations and adjusted to simulate
hydrographs in well fields in the Los Alamos area. Hydraulic-head
drawdowns at the Buckman well field resulting from two projected
ground-water-withdrawal alternatives were estimated with the
modified model.

The Chaquehui formation (informal usage) is the main new
feature of recent hydrologic interpretations for the Los Alamos
area. The Chaquehui occupies a 'channel' that was eroded or
faulted into the Tesuque Formation, and the Chaquehui is more
permeable than the Tesuque. The Chaquehui is a major producing
zone in the Pajarito Mesa well field and to a lesser extent in
the Guaje well field.

Model modification included splitting the four layers of the
McAda-Wasiolek model (McAda, D.P., and Wasiolek, Maryann, 1988,
Simulation of the regional geohydrology of the Tesuque aquifer
system near Santa Fe, New Mexico: U.S. Geological Survey Water-
Resources Investigations Report 87-4056, 71 p.) into eight layers
to better simulate vertical ground-water movement. Other model
modifications were limited as much as possible to the area of
interest near Los Alamos and consisted mainly of adjusting
hydraulic-conductivity values representing the Tesuque Formation,
Chaquehui formation (informal usage), and Puye Formation, and
adjusting simulated recharge along the Pajarito Fault Zone west
of Los Alamos. Adjustments were based mainly on simulation of
fluctuations in measured hydraulic heads near Los Alamos.

Two possible alternative plans for replacing Guaje well field
production were suggested by Los Alamos National Laboratory. In
the first plan (Guaje alternative), the Guaje field would be
renewed with four new wells replacing the existing production
wells in the Guaje field. In the second plan (Pajarito-Otowi
alternative), the Guaje well field would be retired and its
former production would be made up by additional withdrawals from the
Pajarito Mesa and Otowi well fields. A projection for each of
these alternatives was made through 2012 using the new eight-
layer model. In the Guaje field, projected hydraulic heads at the
end of 2012 were as much as 50 feet lower with the Guaje
alternative; in the Pajarito Mesa field, hydraulic heads were as
much as 12 feet higher with the Guaje alternative. At the western
end of the Los Alamos well field, projected hydraulic heads were
about 20 feet higher with the Guaje alternative; at the eastern
end of the Los Alamos field, the difference between alternatives
was much less. At the Buckman field, projected hydraulic heads
were about 2 feet higher with the Guaje alternative because the
Buckman field is closer to the Pajarito Mesa field than to the
Guaje field.

Ways of improving the understanding of the flow system
include developing a more accurate representation of the
structure and extent of the Tesuque Formation, Chaquehui
formation, and Puye Formation of the Santa Fe Group and obtaining
more detailed geologic and hydrologic data for the Chaquehui and
Puye. Data that describe water chemistry, hydraulic head, and
degree of saturation would be valuable for determining the
location and quantity of recharge on the Pajarito Plateau,
especially along the west side of the Plateau and in canyon
bottoms. Chloride concentrations in soil at the top of the
Bandelier Tuff could be used to verify the concept that
evapotranspiration accounts for nearly all precipitation over a
large area of the plateau.