WSP09541

• File Number: 8273.300
• Description: Colorado River Basin Salinity Control - Federal Agencies - USGS
• Basin: Colorado Mainstem
• Water Division: 5
• Date: 1/1/1996
• Author: USGS
• Title: Hydrology/ chemical quality/ and characterization of salinity in the Navajo aquifer in and near the Greater Aneth Oil Field - San Juan County - Utah
• Water Supply Pro - Doc Type: Report/Study

Table 5. Chemical analysis 01 selected major-, minor-, and trace-element constituents lrom blank samples processed on site in
and near the Greater Aneth Oil Field, San Juan County, Utah, November 1992 to September 1994
W [mglL, milligrnms per liIer; <, less than reported value; IJ.g1L, micrograms per liter)
CJl
-..J Chemical Number of Concentration
.,L:,. constituent Unit samples Median Low High
Calcium mgIL 6 0.05 <0.02 0.08
Chloride mgIL 6 .2 .1 .3
Magnesium mglL 6 <.01 <.01 .01
Potassium mgIL 6 <.1 <.1 <.1
Sodium mgIL 6 <.2 <.2 .3
Sulfate mglL 6 .35 <.1 .8
Barium IlgIL 6 <2 <2 <2
Boron IlgIL 6 <10 <10 <10
Bromide mgIL 6 <.01 <.01 <.01
Fluoride mgIL 6 <.1 <.1 .1
Iodide mgIL 6 dlOl <.001 .001
Iron IlgIL 6 <3.0 <3.0 12
Lithium IlgIL 6 <4 <4 <4
Manganese IlgIL 5 <I <I <I
Silica mgIL 6 <.1 <.1 <.1
Strontium IlgIL 6 2 I 4
Vanadium IlgIL 6 <I <I <I
Dissolved solids mglL 6 <I <I 2

S., R. 25 E., Sec. 17 (J.G. Crawford, unpub. data, n.d.,
Rocky Mountain oil-field waters, p. 64, table 35).

Howells (1990) mapped the depth to the base of
moderately saline ground water (3,000 to 10,000 mg/L)
in San Juan County, Utah. The base of moderately
saline ground water was defined as the top of the first
permeable interval containing water with a dissolved-
solids concentration greater than 10,000 mg/L. The
configuration of the base of moderately saline ground
water is affected by stratigraphy and geologic structure;
thus, Howells' map indicates that the altitude of the
base of moderately saline water ranges from as low as
1,500 ft in the southwestern part of the study area to as
high as 4,500 ft in the northeastern part of the area.
Consequently, water with dissolved-solids concentra-
tions greater than 10,000 mg/L may be less than 500 ft
below land surface in places in the Greater Aneth Oil
Field. Howells' map also indicates valleys or troughs in
the base of moderately saline water, implying inflow of
fresh or less saline water from adjacent mountain
recharge areas.

Stiff diagrams can be used to illustrate the rela-
tive concentrations of cations and anions in water from
wells, to understand changes in water chemistry along
ground-water flow paths, and to distinguish different
water types (Stiff, 1951). Stiff diagrams for selected
wells and springs in the study area are shown on plate
I. Results of chemical analysis of water from wells in
the Navajo aquifer indicate that the major cation is
sodium, regardless of the salinity concentration.
Sodium is also the major cation in water from wells and
springs in overlying aquifers in the study area (pI. I).
The high concentrations of sodium relative to calcium
are attributed to water-rock interactions along ground-
water flow paths from recharge to discharge areas,
because in upgradient areas, sodium also is predomi-
nant in freshwater thar contains low concentrations of
chloride. Increased levels of sodium may be the result
of precipitation of calcium carbonate, hydrolysis of
sodium feldspars, or ion-exchange reactions (Kimball,
1992, p. 98). Excess concentrations of sodium also
might result from mixing with sodium chloride type
water in downgradient areas.

35