“Tectonic Influences on Groundwater Quality”

Presented by Dr. Sam Earman

Millersville University

Groundwater derives nearly all its dissolved solids via water-rock interaction, so we would normally expect groundwater in basins with similar lithologies to have similar chemical characteristics. The San Bernardino Valley in southern Arizona and northern Sonora, Mexico is bounded by mountain ranges of identical lithology to its neighboring basins, meaning the alluvial aquifers in each basin should be similar. However, the San Bernardino Valley has significantly different hydrogeologic characteristics than its neighboring basins, including water quality, chemical evolution, storage, and residence time. The differences result from the locally unique geology of the San Bernardino Valley, which is due to the presence of a magmatically active accommodation zone (a zone separating two regions of normal faults with opposite dips). The geological differences and the resultant hydrological differences between the San Bernardino Valley and its neighboring basins may serve as a model for the distinctive nature of chemical evolution of ground water in other basins with locally distinct tectonic histories.