VARIATIONS IN WATER COMPOSITION IN AQUIFERS OF THE HOPI BUTTES (TSEZHIN BII) AREA OF THE NAVAJO NATION, NORTHEASTERN ARIZONA

Water from springs and wells in the Tsezhin Bii area varies in composition over the 1000 mi² study area. Much of this variation is within rather than among aquifers. Understanding these variations will aid water development plans of the Navajo Nation. The intra-aquifer variability reflects interconnection of aquifers, lateral changes within rock units, and the impact of surface features. Sampling for this study was done with permission of the Navajo Nation.

Water from modern alluvium along intermittent channels is Ca-Na or Na HCO3-type, with conductivities of 0.7 to 2.2 mS, pH values 7.7 to 8.9 and locally high arsenic and uranium contents. d¹⁸O and dD values are consistent with variably evaporated recent recharge. Groundwater near eroding channels contains 100 to 400 mg/L of sulfate. Sulfate <20 mg/L is measured in water from an aggrading alluvial valley that also contains greater concentrations of Ba (1.3 mg/L), Fe (0.3 mg/L), and Mn (0.2 mg/L). The latter composition is attributed to reducing conditions induced by preserved organic matter. The Tertiary Bidahochi Formation sandstones, diatremes, and lava flows produce Na-HCO3 type water with low to moderate conductivities (0.2 to 0.8 mS/cm) and pH of 7.0 to 9.4. Dissolved arsenic contents are >20 mg/L in water with pH > 8.6 and is inferred to result from desorption of arsenic from iron oxides. High pH values are due to coupled cation exchange and carbonate mineral dissolution, and minor silicate hydroloysis. The stable isotope composition of water from the Bidahochi is variable; springs and shallow wells resemble water from modern alluvium and deeper wells are similar to water in Mesozoic sedimentary rocks.

Mesozoic sedimentary rocks of the D and N aquifers produce water with conductivities of 0.4 to 24 mS, and are mainly Na-HCO3 water with pH of 8 to 9.8. N aquifer equivalents produce water with high pH and arsenic contents >20 mg/L; in contrast, high pH water in D aquifer units contain <10 mg/L As. d¹⁸O and dD values of water from the N aquifer are similar to water in the confined part of the N aquifer north of the study area that was recharged > 10,000 years B.P.