Water-Resources Investigations Report 03-4189
Hydrogeology of the D Aquifer and Movement and Ages of Ground Water Determined from Geochemical and Isotopic Analyses, Black Mesa Area, Northeastern Arizona
Margot Truini and Steve A. Longsworth
The Navajo Nation and the Hopi Tribe in the Black Mesa area depend on ground water from sandstones of the N aquifer for domestic, agricultural, municipal, and industrial needs. They are concerned that pumping of water from the N aquifer will induce leakage from the overlying D aquifer, resulting in the degradation of water quality in the N aquifer. Water samples from the D aquifer contained higher concentrations of dissolved solids than samples from the N aquifer; however, ground waters in the D and N aquifers evolve similarly along their respective flow paths.
The ground-water composition in the D aquifer results from interaction with limestone and sandstone sediments. The ground water evolves from a calcium magnesium bicarbonate type in the recharge area to a sodium bicarbonate type in downgradient areas. 34S data indicate sulfate reduction occurs when ground water comes in contact with lignite seams in the Dakota Sandstone. Adjusted 14C ages for ground water in the D aquifer range from 4,000 to 33,000 years. δ18O and δ2H data indicate that most of the recharge occurred when the climate was cooler and more humid than at present. 3H data indicate that localized recharge has occurred in some areas in recent time.
Leakage between the D and N aquifers has been occurring for thousands of years. The area of highest leakage occurs in the southern areas of Black Mesa, where the N aquifer is thin, the predevelopment hydraulic gradient is small, and the vertical head differences between the D and N aquifers are small. Induced leakage from ground-water development in the last several decades could take centuries to detect geochemically because of the increased vertical difference between the potentiometric surfaces of the D and N aquifers, and possibly because of increases in the hydraulic gradient in the N aquifer that would increase flow rates, causing dilution.
87Sr/86Sr data are consistent with the leakage of ground water from the D aquifer into the N aquifer in the southern part of Black Mesa. 87Sr/86Sr values for the N and D aquifers are similar in this area; statistical means are -2.74 ‰ and -2.49 ‰, respectively, N aquifer 87Sr/86Sr values are more radiogenic than D aquifer values in the northern part of Black Mesa; statistical means are -0.14 ‰ and -2.49 ‰, respectively.
Geochemical evolution of ground water in the D aquifer
Estimated ground-water ages for the D aquifer
Recharge to the D aquifer
Ground-water leakage from the D aquifer to the N aquifer
Summary and conclusions
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