THE FALSE POSITIVE CONUNDRUM: IDENTIFYING FALSE POSITIVES OF CONTAMINATION FROM LANDFILLS IN SEMI-ARID TO ARID WESTERN WATERSHEDS

Many landfills in the semi-arid to arid American West have been engineered on basin fill clayey rocks and soils and without modern underlying protective barriers. Recently, the Wyoming Department of Environmental Quality (WDEQ) published multiple cases of groundwater contamination under its landfills. Has contamination actually occurred? What meaningful aquifers might be contaminated? What remediation would be an appropriate response and societal cost given natural attenuation?

We reviewed the published groundwater geochemical database for Wyoming’s landfills and found multiple false positives: naturally poor groundwater that exceeds regulatory standards because of ion exchange, mineral dissolution, and evaporation. At one landfill, isotopic measurement of dissolved inorganic carbon and water indicates that water immediately under landfills placed on basin fill does not constitute part of a regional water resource, but occurs in isolated perched water zones containing waters thousands of years old.

Our further geochemical characterization of groundwater chemistry in the Wind River Basin will provide a geochemical model and regulatory approach to define where leachate contamination actually occurs in the water poor West, where natural chemistry leads to false positives of contamination, and where remediation would be an appropriate societal cost.