Occurrence of natural and anthropogenic hexavalent chromium (Cr VI) in groundwater near a mapped plume, Hinkley, CA

Task 2

Sample collection and analyses of rock and alluvium

Physical, mineralogic, and chemical analysis will be done on selected archived core material and cuttings from wells drilled by PG&E and as part of previous work by the U.S. Geological Survey. Analyses also will be done on samples of alluvium and rock collected as part of this study by the U.S. Geological Survey within the study area and from other areas in the Mojave Desert known to have naturally-occurring concentrations of chromium. Rock and alluvium will be evaluated using a hand-held X-Ray Fluorescence (XRF) instrument prior to collection, description, and analyses.

Physical, mineralogic, and chemical data collected as part of this task will be used to determine natural geologic occurrence of chromium in rock and alluvium in the study area. Results will be compared to: 1) average chromium abundances in continental crust and in specific rock types, 2) chromium abundances in areas known to have high chromium in groundwater, and 3) chromium abundances within the study area to determine if there are differences in chromium abundance that are related to local geology.

Physical description of rock and alluvium

Core material and cuttings from selected wells drilled throughout the study area by PG&E and their consultants will be described and classified optically, using a petrographic microscope, with respect to texture (Folk, 1954), roundness (maturity) of sand grains (Folk, 1951), Quartz, Alkali feldspar, Plagioclase, Feldspathoid (QAPF) phaneritic mineralogy (Le Maitre, 2002), and the abundance and composition of lithic fragments. Archived core material and cuttings from USGS drilled wells along the Mojave River (Huff and others, 2002) will be used as reference material for alluvium from the Mojave River. Rock samples collected from outcrops and alluvium collected from streams draining upland areas adjacent to Hinkley Valley will be used as reference material for alluvium from those sources. Additional material from areas elsewhere in the Mojave Desert known to have high chromium concentrations also will be collected and described. XRF data, physical descriptions, and optical analysis will be used to select approximately 100 samples for preparation of thin sections. Thin sections will be examined optically to provide semi-quantitative estimates of differences in mineral occurrence and abundance in different areas. Results will be used to select material for additional analyses of mineralogy and sequential extraction for selected trace element compositions.

Physical descriptions (including field and office XRF measurements) of rock, alluvium, core material and cuttings, and analysis of thin sections will be done in the San Diego U.S. Geological Survey office in collaboration with Brett Cox and David Miller U.S. Geologic Survey, Geologic Discipline, Menlo Park, CA. Thin sections will be prepared by a contract laboratory.

Mineralogy of cores and cuttings

Mineralogy of 30 previously described core material and cuttings, rock and alluvium from representative source areas (including material from other areas in the Mojave Desert known to have high chromium concentrations) will be determined by X-Ray diffraction. Mineral identification from X-Ray diffraction data will be made using pattern-fit routines within the computer programs MDI Jade, and Rockjock for clay-mineral bearing rocks. Selected samples will be sorted according to density in bromoform (specific gravity 2.8 g/cm3) (Peacock and others, 2000). Minerals containing Cr and other selected trace elements are associated with denser minerals. Minerals having a density greater than 2.8 g/cm3 will be sorted into highly magnetic (C1), weakly magnetic (C2), and nonmagnetic (C3) fractions using a Franz unit (Peacock and Taylor, 1990; Taylor, 1990). Minerals in the nonmagnetic fraction will be identified optically, digested, and analyzed for chromium and other selected trace elements. Selected mineral grains from the bulk and C3 fractions will be examined using a Scanning Electron Microscope (SEM) to determine the morphology, physical integrity (with respect to the extent of weathering,) and chemical composition of mineral grains.

X-Ray diffraction analyses, optical description, and analyses of the C3 mineral fraction, and Scanning Electron Microscope analyses will be done by the U.S. Geological Survey, Geologic Discipline, in Denver, Colo. in collaboration with Jean Morrison and William Benzel.

Sequential extractions from cores and cuttings

Chromium and other selected trace elements (including iron, aluminum, manganese, arsenic, nickel, vanadium, and uranium) will be extracted from sorption sites, amorphous oxides, and crystalline oxides on the surfaces of mineral grains from 30 samples of alluvium collected during test drilling for existing monitoring wells and from samples collected from representative source areas. The sequential extraction procedure to be used is modified from Wentzel and others (2001). Each step within the procedure is intended to extract trace elements from operationally-defined sorption sites on the surfaces of the mineral grains. These sorption sites include: 1) "non-specifically sorbed" trace elements dissolved in pore water and associated with water-soluble material; 2) "Specifically-sorbed" trace elements potentially mobilized by changes in pH, or by exchange with more strongly sorbed oxyanions; 3) trace elements associated with poorly-crystalized (amorphous) iron, aluminum, and manganese oxides on the surfaces of mineral grains; and 4) trace elements associated with well-crystalized iron, aluminum, and manganese oxides on the surfaces of mineral grains. A fifth extraction step modified from Chao and Sanzolone (1989) will be done on a split of material obtained after the second extraction. This step is included to ensure all trace elements associated with well-crystalized iron, manganese, and aluminum oxides on mineral surfaces within the sample are measured. Results from the fifth step are expected to be comparable to strong-acid extraction data collected elsewhere in the Mojave Desert by Izbicki and others (2008). Chemical data from sequential extractions for chromium, or other trace elements, are commonly normalized to the occurrence of more abundant elements, such as iron, or to physical properties, such as particle-size or surface area, prior to evaluation of abundance.

Extractions will be done at the U.S. Geological Survey Laboratory in San Diego, Calif., with chemical analyses done at the U.S. Geological Survey National Water Quality Laboratory (NWQL) in Denver, Colo.

Unbiased procedures designed to relate results from more numerous, inexpensive descriptive data (XRF, physical and optical descriptions) of aquifer mineralogy, to results from fewer samples analyzed for more expensive mineralogy and extraction data will be developed by the project in consultation with USGS Geologic Discipline scientists and the TWG.

Cooperating Agency: Lahontan Regional Water Quality Control Board
Project Chief: John A. Izbicki
Phone: 619-225-6131
Email: jaizbick@usgs.gov

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