Geochemistry of ground water in the Gallup, Dakota, and Morrison aquifers, San Juan Basin, New Mexico

Ground water was sampled from wells completed in the Gallup,
Dakota, and Morrison aquifers in the San Juan Basin, New Mexico,
to examine controls on solute concentrations. Samples were
collected from 38 wells primarily from the Morrison aquifer (25
wells) in the northwestern part of the basin. A series of samples
was collected along ground-water flow paths; dissolved
constituents varied horizontally and vertically.

The understanding of the flow system changed as a result of
the geochemical analyses. The conceptual model of the flow system in
the Morrison aquifer prior to the study reported here assumed the
Westwater Canyon Member of the Morrison aquifer as the only
significant regional aquifer; flow was assumed to be two
dimensional; and vertical leakage was assumed to be negligible.
The geochemical results indicate that the Westwater Canyon Member
is not the only major water-yielding zone and that the flow
system is three dimensional. The data presented in this report
suggest an upward component of flow into the Morrison aquifer. The
entire section above and below the Morrison aquifer appears to be
controlled by a three-dimensional flow regime where saline brine
leaks near the San Juan River discharge area.

Predominant ions in the Gallup aquifer were calcium
bicarbonate in recharge areas and sodium sulfate in discharge areas.
In the Dakota aquifer, predominant ions were sodium bicarbonate and
sodium sulfate. Water in the Morrison aquifer was predominantly sodium
bicarbonate in the recharge area, changing to sodium sulfate
downgradient.

Chemical and radioisotopic data indicate that water from
overlying and underlying units mixes with recharge water in the
Morrison aquifer. Recharge water contained a large ratio of
chlorine-36 to chlorine and a small ratio of bromide to chloride.
Approximately 10 miles downgradient, samples from four wells
completed in the Morrison aquifer were considerably different in
composition compared to recharge samples. Oxygen stable isotopes
decreased by 2.8 per mil and deuterium decreased 26 per mil,
relative to recharge. Carbon-14 radioisotope activities were not
detectable. Chloride-36 radioisotope ratios were small and
bromide to chloride concentration ratios were large. These results
suggest two potentially viable processes: ion filtration or trapping of
ancient dilute water recharged under a humid climate. For water
samples near the San Juan River, pH decreased to about 8.0,
chloride concentrations increased to more than 100 milligrams per
liter, and ratios of chlorine-36 to chlorine and bromide to
chloride were small. Leakage of deep basin brine into the fresher
water of the Morrison aquifer appears to control ion concentrations.