WSP08953

• File Number: 8273.300
• Description: Colorado River Basin Salinity Control - Federal Agencies - USGS
• Basin: Colorado Mainstem
• Water Division: 5
• Date: 3/1/1995
• Author: USGS
• Title: Using Geochemical Data to Identify Sources of Salinity to the Freshwater Navajo Aquifer in Southeastern Utah - Aneth Technical Committee Fact Sheet
• Water Supply Pro - Doc Type: Publication

Salinity Increases in the Navajo Aquifer in Southeastern Utah

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It is apparent from the geochemical data presented
that the salinity sou,rcehas a bromide-to-chloride X
10,000 weight ratio Jess than 10 and an iodide-to-chlo-
ride X 10,000 weigh* ratio less than 0.1, both ratios
typical of non-organic sources. The unidentified salin-
ity source also is isowpically enriched with respect to
3180 and 3D values and probably has been evaporated
in a closed basin.
Although OFB and injection water are not the
salinity sources, it is possible that upward leakage of
an unidentified saline water source could occur from
natural causes or present-day and/or historical
human-induced effectis in the area. Examples of these
possible effects in~lude: (1) poorly plugged or
unplugged abandoneil wells and test holes; (2) leak-
age upward from aqQ.ifers containing very saline or
briny water through annuli between the well casings
and the borehole walls, especially in older oil wells
where only a hundred feet or less of surface casing
was cemented in pla~e and the seal is ineffective; (3)
leakage of saline water upward through natural frac-
tures, the movement induced by decreasing hydraulic
head in the Navajo a<l!uifer and related hydrogeologic
units because of withdrawal of water lor domestic,
stock, and other uses in the study area; (4) continuous
flow of artesian water from unplugged wells causing
decreasing hydraulic pead in the Navajo aquifer; (5)
interdunal facies wit~in the Navajo Sandstone which
contain evaporite dep6sit~; or (6) some combination of
the above.
As an example, a preliminary report prepared by
Uribe and Associates' (1992) showed selected aban-
doned exploratory oil iwells in the study area having
insufficient surface ca~ing to isolate the entire thick-
ness of underground Sources of drinking water. More
than 40 wells in the study area were identified as
having the potential to provide conduits for upward
water migration. Exploratory oil test holes that are
abandoned generally ~ndicate nonexistent or nonre-
coverable oil pools and could provide a human-
induced mechanism to transport non-OFB (in
contrast to OFB) intoifreshwater aquifers in an area
where OFB salinity so$-rces would be expected.
Additional study is needed to verify the non-OFB
salinity source in the ,study area and to identify the
most likely transport mechanism from the numerous
possibilities. Future study needs include: (1) utilizing
surface geophysical te~hniques to attempt to identify
high salinity source ~reas; (2) utilizing additional
organic and inorganic data from ground water in the
area in pattern-recognUion modeling to "fingerprint"
brine sources and sali~ity mechanisms; (3) collecting
and updating hydraulkliead data from throughout
the study area to claI1fy further the effect of water
withdrawal from the Navajo aquifer on possible
saline-water migration: pathways; (4) investigation of

possible effects of depositional systems (lor example,
interdunal saline facies) on local ground-water quali-
ty; and (5) collection and analysis of water samples
from other saline horizons in the vicinity of the
Greater Aneth Oil Field.

SUMMARY

Selected wells in the Navajo aquifer in southeast.:
ern Utah have shown large increases in dissolved-
solids concentration in recent years. Pre00us results
from the area have suggested that brines associated
with oil recovery and injection in the Greater Aneth
and satellite. oil fields were the most probable source
ofthe saline waters.
The average bromide-to-chloride ratio for the nine
OFB samples in and adjacent to the study area is
0.014, substantially enriched relative to the modern
ocean water ratio of 0.00344. This large ratio is
because of .the influence of bromide-enriched organic
material associated withOFB waters. NoncOFIl sam-
ples adjacent to the study area are depleted in pro-
mide relative to modern ocean water, possibly
denoting a limited amount of organic matter and rela-
tively small bromide concentrations in chloride miner-
als. As salinity in the Navajo aquifer increases,'
bromide-to-chloride ratios become depleted in bro-
mide relative to modern ocean water, suggestive of
mixing with a bromide-depleted salinity S/lurce, not
characteristic of OFB from the study area; A series
of end-member mixing models indicate that the
bromide-to-chloride weight ratio characteristic of OFB
in and adjacent to the study area could not 'be causing
the observed bromide depletion with increasing salini-
ty in the Navajo aquifer. According to the results of
the mixing model, a simulated mixture of one percent
OFB with 99 percent Navajo aquifer water would
more than double the bromide-to-chloride weight
ratio, instead of the decrease in the weight ratio.OFIl
samples in the study area also show enriched iodide-
to-chloride weight ratios relative to ratios typical of
halite solution brines, probably resulting from the
enrichment of iodide in organic material. Iodide-to-
chloride weight ratios compared with increasing chloe
ride concentrations in water samples from the study
area also indicate that OFB is not the source of salini-
ty in the Navajo aquifer.
Most of the 3180 and 3D values of samples from the
Navajo aquifer plot close to the North American mete-
oric water line; however, the more saline waters are
isotopically enriched and plot below the meteoric
water line, suggestive of mixing with an isotopically
enriched ,source. The mixing-line trend through the
isotopically enriched samples from the Navajo aquifer

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1133

WATER RESOURCES BULLETIN

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