NATURAL RECHARGE TO THE NAVAJO SANDSTONE AQUIFER OF SOUTHWESTERN UTAH

The Navajo aquifer of southwestern Utah provides most of the culinary water for the region. Infiltration of precipitation is the main source of natural recharge to this aquifer. Because of unique data-collection opportunities made possible by the construction of a surface-water reservoir near St. George, Utah, spatial distribution of the rate of natural recharge could be studied in detail. The ratio of chloride in ground water compared to rain samples indicates that about 5 percent of precipitation recharges the aquifer at Sand Hollow. Unsaturated-zone solute data from 3,000 m of trenches and 12 borehole cores, however, indicate wide spatial variability in recharge rates.

Chloride, stable-isotope, and tritium data from unsaturated-zone pore water all indicate that recharge is focused through discrete pathways. Most recharge to the aquifer does not occur beneath the ephemeral wash, but below exposed sandstone outcrops as infiltration of runoff during rainfall. On the basis of the large mass of anthropogenic tritium and the small accumulation of solutes in unsaturated zone, it is estimated that as much as 20 percent of the average annual 22 cm of precipitation may recharge the aquifer at these locations. This is confirmed by the similar lack of evaporative shift in the stable isotopes of oxygen and hydrogen in unsaturated-zone pore waters from these sites, similar to the underlying ground water.

Recharge elsewhere within the basin appears to be controlled by the particle size of the soils overlying the sandstone. In areas with coarse soils, infiltration can move rapidly beyond the root zone into the underlying sandstone. Conversely, finer-grained soils support a denser plant community that can intercept most of this infiltrating moisture. Recharge rates are generally negligible, based on the small mass of anthropogenic tritium and the large accumulation of solutes in the unsaturated zone. Pore-water stable isotopes at these sites show evaporative enrichment, markedly different than the underlying ground water. However, low solute concentrations in fractures beneath these fine-grained soils indicates that even in low-recharge areas, occasional ponding along the bedrock contact has occurred during wet periods, allowing for fractures to episodically saturate and conduct water downward toward the water table.