Source identification for groundwater arsenic in the Verde Valley, Central Arizona, USA

摘要

The chemical composition of water samples collected from Montezuma Well andVerde Hot Springs in the Verde Valley of central Arizona, USA, is compared to thechemical composition of groundwater from the Verde Formation to identify theoriginal As source for Verde Valley groundwater. Verde Hot Springs water (As =719μg/1, Li = 1040 mg/1, B 9100μg/1) is very different chemically from Monte-zuma Well water (As = 100μg/1, Li = 180 mg/1, B = 760μg/1) and Verde Formationgroundwater (As = 30 μg/1, Li = 50 mg/1, B = 60 μg/1), being similar in compositionto geothermally influenced groundwater. As expected, Verde Formation ground-water shows the chemical characteristics of low-temperature groundwater. Strontiumisotope ratios (~(87/18)Sr) were measured by ICP–MS spectrometry. ~(87/86)Srisotope ratiosof 0.700 for Montezuma Well water indicate contact with Precambrian rocks, whileVerde Formation groundwater has ~(87/86)Srratios greater than 0.720, indicating thatthis water is from recent sediments. Lithium and boron concentrations were used toestimate the percentage of Montezuma Well water that can be attributed to geother-mally influenced groundwater at 10.5%. This is the first evidence that the As inMontezuma Well water and Verde Formation groundwater may have originated inPrecambrian rocks, 800-1000 m below the surface.