COMPARTMENTALIZATION OF GROUND WATER IN AN INTERMOUNTAIN BASIN: IMPLICATIONS on PERFORMANCE-BASED LANDFILL DESIGN AND MONITORING IN THE ARID AMERICAN WEST

Seven lines of independent evidence show groundwater found less than 100 feet deep under a landfill located in an arid Wyoming intermountain basin, occurs within disconnected and compartmentalized perched zones. Test holes intersected water within lenses of silty sandstone, encased by dry clayey siltstones. Ground penetrating radar and seismic reflection geophysical studies showed no continuous water table within at least the upper ~150 feet of rock. These data and a dry well drilled 200 feet deep show that any "regional water table" must be hundreds of feet deep. Tritium activities below detection, stable isotopic compositions of the ground waters that define an evaporative trend inconsistent with modern recharge waters, and carbon-14 ages of dissolved inorganic carbon show that water in the compartments recharged as precipitation thousands of years ago under drier conditions than found now. Finally, nearly constant water levels in monitoring wells after snowmelt show that modern precipitation does not meaningfully recharge the pockets of groundwater.

The governmental agencies that regulate the design, remediation, and monitoring of western municipal solid waste landfills need to explicitly recognize that arid zone hydrology differs from the hydrology in humid, water-rich areas from which many regulatory practices have been derived. At the very least, shallow geophysical methods and isotopic age dating of ground water should be coupled to the standard regulatory data obtained from monitoring wells to avoid "false positives", wherein a meaningful groundwater resource may be assumed present, but in reality, does not exist. Isotopic methods also can scientifically test if high concentrations of dissolved inorganic solutes downgradient of landfills show contamination from the landfill, or alternatively, the natural results of water rock chemical reactions within isolated perched water compartments.