NAFIZ, DAVID L., and LAWRENCE E. SPANGLER, U.S. Geological Survey, Salt Lake City, UT

ABSTRACT: Using Geochemical Techniques to Identify Salinity Sources in the Freshwater Navajo Aquifer, Aneth Oil Field, Utah

The salinity of water in the Triassic and Jurassic Navajo Sandstone in southeastern Utah has increased locally since 1952. The Navajo aquifer, within the Navajo Sandstone, is a major source of water for domestic use and livestock in the area. From 1989 to 1991, concentration of dissolved solids in one well increased by

as much as 5500 mg/L. The source or sources of the saline water and the reasons for the local increases are not known; however, mixing with either oil-field brines (OFB) or non-oil-field brines (NOFB) from the Pennsylvanian Paradox Formation could possibly cause the increased salinity. One of the objectives of an ongoing study is to use end-member mixing models and step-wise discriminant analysis to determine the possible source or sources of saline water causing the observed increases in salinity in the Navajo aquifer.

Two end-member mixing models, using the bromide/chloride x 10,000 (Br/Cl x 104) weight ratios and chloride concentrations, were constructed to identify saline sources. Three groups of end-member waters were defined from existing data: (1) fresh water from the Navajo aquifer, defined by chloride concentrations less than or equal to 50 mg/L; (2) OFB waters from the Paradox Formation, within and adjacent to the study area; and (3) NOFB from the Paradox Formation, northeast of the study area. OFB end-members had BR/Cl x 104 weight ratios much larger than 10 and NOFB end-members had weight ratios less than 10. The ratios larger than 10 in OFB waters probably reflect decomposition of bromide-enriched organic matter associated with oil reserves. Comparison of Br/Cl x 104 weight ratios from 2 water samples from the Navajo aquifer with the constructed mixing curves suggests that NOFB are the major source of saline water that mixes with fresh water in the Navajo aquifer.

Discriminant analysis was used on the major-ion/chloride ratios to identify saline-water sources that could have mixed with Navajo aquifer water. The discriminant function using the weight ratios of magnesium/chloride correctly identified 85.7% of the known end-member waters and had a Mahalanobis' distance (D2) of 165.4. Applying the discriminant function to water samples from 35 wells completed in the Navajo aquifer, 27 were classified as having mixed with NOFB, 8 were classified as having not mixed with saline water, and 0 were classified as having mixed with OFB. Results of the discriminant analysis were in agreement with classifications determined by the Br/Cl x 104 weight ratios.

AAPG Search and Discovery Article #90993©1993 AAPG Rocky Mountain Section Meeting, Salt Lake City, Utah, September 12-15, 1993.