Summary of the San Juan structural basin regional aquifer-system analysis, New Mexico, Colorado, Arizona, and Utah

Ground-water resources are the only source of water in most of
the San Juan structural basin and are mainly used for municipal,
industrial, domestic, and stock purposes. Industrial use increased
dramatically during the late 1970's and early 1980's because of
increased exploration and development of uranium and coal resources.

The San Juan structural basin is a northwest-trending,
asymmetric structural depression at the eastern edge of the Colorado
Plateau. The basin contains as much as 14,000 feet of sedimentary
rocks overlying a Precambrian basement complex. The sedimentary
rocks dip basinward from the basin margins toward the troughlike
structural center, or deepest part of the basin. Rocks of Triassic
age were selected as the lower boundary for the study. The basin is
well defined by structural boundaries in many places with structural
relief of as much as 20,000 feet reported. Faulting is prevalent in
parts of the basin with displacement of several thousand feet along
major faults.

The regional aquifers in the basin generally are coincident with
the geologic units that have been mapped. Data on the hydrologic
properties of the regional aquifers are minimal. Most data were
collected on those aquifers associated with uranium and coal
resource production. These data are summarized in table format in
the report. The regional flow system throughout most of the basin
has been affected by the production of oil or gas and subsequent
disposal of produced brine. To date more than 26,000 oil- or gas-
test holes have been drilled in the basin, the majority penetrating
no deeper than the bottom of the Cretaceous rocks.

The general water chemistry of the regional aquifers is based on
available data. The depositional environments are the major factor
controlling the quality of water in the units. The dominant ions are
generally sodium, bicarbonate, and sulfate. A detailed geochemical
study of three sandstone aquifers--Morrison, Dakota, and Gallup--was
undertaken in the northwestern part of the study area. Results of
this study indicate that water chemistry changed in individual wells
over short periods of time, not expected in a regional flow system.
The chemistry of the water is affected by mixing of recharge, ion
filtrate, or very dilute ancient water, and by leakage of saline
water.

The entire system of ground-water flow and its controlling
factors has been defined as the conceptual model. A steady-state,
three-dimensional ground-water flow model was constructed to
simulate modern predevelopment flow in the post-Jurassic rocks of
the regional flow system. In the ground-water flow model, 14
geologic units or combinations of geologic units were considered to
be regional aquifers, and 5 geologic units or combinations of
geologic units were considered to be regional confining units. The
model simulated flow in 12 layers (hydrostratigraphic units) and
used harmonic-mean vertical leakance to indirectly simulate aquifer
connection across 3 other hydrostratigraphic confining units in
addition to coupling the 12 units.