HR: 10:35h
AN: H42C-02    [Abstracts]
TI: Use of Stable-isotope and Radioisotope Data to Assess Ground-water Leakage Between the D and N Aquifers in the Black Mesa Area, Arizona
AU: * Truini, M
EM: mtruini@usgs.gov
AF: U.S. Geological Survey, 2255 N. Gemini Drive, Flagstaff, AZ 86001 United States
AB: Ground-water leakage between the D aquifer and underlying N aquifer has been assessed, in part, by using isotopic data to calculate ground-water age, characterize general climatic conditions during ground-water recharge, delineate areas where ground-water mixing and leakage is occurring, and determine sources of sulfate. The hydrogeologic characteristics of the Carmel Formation, which is predominantly a confining unit between the D and N aquifers throughout the study area, and the relation of these characteristics to ground-water leakage are being described in an ongoing study. The Navajo Nation and the Hopi Tribe in the Black Mesa area depend on the N aquifer to supply water for domestic, agricultural, municipal, and industrial needs, and are concerned that pumping of ground water from the N aquifer could induce leakage from the D aquifer and degrade the water quality in the N aquifer. Adjusted 14C ages for ground water in the D aquifer range from 4,000 to 33,000 years. Values of 18O and D (hydrogen-2) indicate that most ground-water recharge occurred when the climate was cooler and more humid than at present. Detectable amounts of tritium in water from a few wells imply that localized recharge has occurred in some areas in recent time. Ratios of strontium-87 to strontium-86 in ground-water samples from the D and N aquifers in the southern part of Black Mesa support the hypothesis of leakage between the D aquifer into the N aquifer. Strontium isotope ratios in the N aquifer in the northern part of Black Mesa are larger than ratios in the southern part; statistical means are -0.14 % and -2.49%, respectively. Ratios in the N aquifer in the southern part of Black Mesa, however, are similar to ratios in the D aquifer in the same area; statistical means are -2.74 % and -2.49 %, respectively. The similarity of ground-water ages in the D aquifer to ages in the N aquifer suggests that leakage has been occurring for thousands of years. Elevated sulfur concentrations in the D aquifer occur from gypsum stringers and lignite seams in the rock units of the Dakota Sandstone. Sulfur-34 data indicate that sulfate reduction occurs when ground water comes in contact with lignite seams in the Dakota Sandstone, a rock unit of the D aquifer. Consequently, sulfur isotope values can be used to determine areas where ground water may be in contact with lignite seams and be used to identify the source of elevated sulfate concentrations in the water. Preliminary results from the ongoing study of the Carmel Formation indicate a relation between the thickness of the Carmel Formation and leakage indicated by 87Sr/86Sr data. Additional characterization of the thickness, lateral extent, and lithology of the Carmel Formation from the ongoing study will provide an improved understanding of the geographic extent of leakage in the Black Mesa area.
DE: 1831 Groundwater quality
DE: 1871 Surface water quality
DE: 1030 Geochemical cycles (0330)
DE: 1040 Isotopic composition/chemistry
DE: 1045 Low-temperature geochemistry
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting