Alpha-emitting isotopes and chromium in a coastal California aquifer

Applied Geochemistry
By: , and 

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Abstract

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 km north of San Diego were 15 and 25 picoCuries per liter (pCi/L). Although activities were below the Maximum Contaminant Level (MCL) of 15 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 238U and its daughter 234U. Radon activities in marine deposits were as high as 2230 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 232Th, including its daughter 224Ra. Radon activities in alluvium from Piedra de Lumbre of 450 pCi/L were lower than in marine deposits. Chromium VI concentrations in marine deposits were less than the California MCL of 10 μg/L (effective July 1, 2014) but δ53Cr 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.

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Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Alpha-emitting isotopes and chromium in a coastal California aquifer
Series title Applied Geochemistry
DOI 10.1016/j.apgeochem.2014.09.016
Volume 51
Year Published 2014
Language English
Publisher Pergamon Press
Publisher location Oxford, UK
Contributing office(s) California Water Science Center
Description 12 p.
First page 204
Last page 215
Country United States
State California
Other Geospatial Camp Pendleton
Online Only (Y/N) N
Additional Online Files (Y/N) N