DEEP GROUNDWATER QUALITY AND PROTECTION FROM OIL AND GAS DEVELOPMENT (Invited Presentation)

Fresh and brackish deep groundwaters may be an increasingly valuable resource, as society faces water shortages caused by growing demands, droughts, and climate change. Because of potential uses, it is important to characterize deep groundwater quality and protect this resource from contamination by oil and gas and other subsurface-based activities. In this presentation, I will discuss database analysis of total dissolved solids (TDS), toxic element (Na, Cl, and B) and 17 trace element concentrations in the southwestern U.S., depths of oil and gas wells in California, and densities of oil and gas wells across the U.S.

Fresh (<1,000 mg/L or <3,000 mg/L TDS) and brackish (<10,000 mg/L TDS) groundwater is easily found in seven out of the 17 southwestern basins analyzed. These seven basins include the Central Valley of California, Las Vegas – Raton, Wasatch Uplift in Utah, and the Great Basin. The Great Basin, encompassing Nevada and parts of Utah, Arizona, and California, has the largest percentage of fresh and brackish deep (>150 m) groundwater, with 88%, 96%, and 98% of TDS concentrations less than 1,000 mg/L, 3,000 mg/L, and 10,000 mg/L respectively. Moreover, the Great Basin and the Central Valley contain large quantities of deep groundwaters with sufficiently low toxic and trace element concentrations for irrigation use without treatment.

A comparison of oil and gas well depths and TDS distributions in the Central Valley show that many wells are drilled to depths where usable fresh and brackish groundwaters exist. The depths of oil and gas wells in California has a median of 900 m and an average of 1200 m, but can be much shallower. California also has the highest density of oil and gas wells in the southwestern U.S., reaching 60 wells per km² in southern Central Valley where critically overdrafted groundwater basins are located. These findings highlight the overlap between managing deep groundwater aquifers and developing oil and gas resources.