John A. Izbicki Joseph M. Murtaugh Peter W. Swarzenski Thomas D. Bullen Jill N. Densmore 2014 <p><span>The unadjusted 72-h gross alpha activities in water from two wells completed in marine and alluvial deposits in a coastal southern California aquifer 40&nbsp;km north of San Diego were 15 and 25&nbsp;picoCuries per liter (pCi/L). Although activities were below the Maximum Contaminant Level (MCL) of 15&nbsp;pCi/L, when adjusted for uranium activity; there is concern that new wells in the area may exceed MCLs, or that future regulations may limit water use from the wells. Coupled well-bore flow and depth-dependent water-quality data collected from the wells in 2011 (with analyses for isotopes within the uranium, actinium, and thorium decay-chains) show gross alpha activity in marine deposits is associated with decay of naturally-occurring&nbsp;</span>²³⁸<span>U and its daughter&nbsp;</span>²³⁴<span>U. Radon activities in marine deposits were as high as 2230&nbsp;pCi/L. In contrast, gross alpha activities in overlying alluvium within the Piedra de Lumbre watershed, eroded from the nearby San Onofre Hills, were associated with decay of&nbsp;</span>²³²<span>Th, including its daughter&nbsp;</span>²²⁴<span>Ra. Radon activities in alluvium from Piedra de Lumbre of 450&nbsp;pCi/L were lower than in marine deposits. Chromium VI concentrations in marine deposits were less than the California MCL of 10&nbsp;&mu;g/L (effective July 1, 2014) but &delta;</span>⁵³<span>Cr compositions were near zero and within reported ranges for anthropogenic chromium. Alluvial deposits from the nearby Las Flores watershed, which drains a larger area having diverse geology, has low alpha activities and chromium as a result of geologic and geochemical conditions and may be more promising for future water-supply development.</span></p> application/pdf 10.1016/j.apgeochem.2014.09.016 en Pergamon Press Alpha-emitting isotopes and chromium in a coastal California aquifer article