MOVEMENT AND AGES OF GROUND WATER AND ISOTOPIC EVIDENCE OF LEAKAGE FROM THE D TO THE N AQUIFER, BLACK MESA AREA, NORTHEASTERN ARIZONA

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. The Navajo Nation and Hopi Tribe are concerned that pumping of water from the N aquifer will induce leakage from early Cretaceous and late Jurassic sandstones of the overlying D aquifer resulting, in the degradation of water quality in the N aquifer. Chemical composition of water collected from the D and N aquifers, integrated with characteristics of the stratigraphic section, is consistent with movement of water from the D aquifer to the N aquifer through semipermeable silt and mudstones. Ground-water composition in the D aquifer is variable and results from water-rock interactions along the flow paths. The ground water evolves from a calcium magnesium bicarbonate type water in the recharge area to a high pH, sodium bicarbonate type water in downgradient areas. Corrected carbon-14 ages for ground water in the D aquifer ranged from 10,000 years to more than 30,000 years. Delta oxygen-18 and delta hydrogen-2 data indicate that recharge occurred when the climate was cooler and more humid than at present. Ground water in the D and N aquifers evolve similarly along flow paths; however, water samples from the D aquifer had higher concentrations of dissolved solids than samples from the N aquifer. Isotopic analyses indicate that ground water in the N aquifer is similar in age to water in the D aquifer owing to similar climate conditions present when the water entered each of the aquifers. Delta strontium-87 data are consistent with water from the D aquifer leaking into the N aquifer in the southern part of Black Mesa. Delta strontium-87 values for the N and D aquifers are similar in this area; statistical means are -2.74 ‰ and -2.49 ‰, respectively. N aquifer delta strontium-87 values are more radiogenic than D aquifer values in the northern part of Black Mesa; statistical means are -0.14 ‰ and -2.49 ‰, respectively.