STUDENT ABSTRACTS

Chromium is one of the most common and troublesome soil and groundwater contaminants. One of the processes that controls the behavior of chromium in contaminated soil is the formation of chromium-containing solids. One type of solids that can form are solid solutions between the sulfate mineral jarosite (KFe₃(SO₄)₂(OH)₆) and its chromate analog (KFe₃(CrO₄)₂(OH)₆). The solid solutions (KFe₃(Cr_xS_1-xO₄)₂(OH)₆) have intermediate chromate to sulfate ratios. This project is part of a larger study trying to understand the role of these mixed solids in contaminated soils.

Seven solids with intermediate compositions from long-term dissolution experiments were analyzed to see if they had been altered after remaining in solution for an extended period of time. Powder X-ray diffraction scans of the solids revealed a continuous peak shift, suggesting a continuous solid solution and no change of the solids during the dissolution experiments. No evidence for the formation of secondary solids was found. These results contribute to overall findings that the formation of solid solutions between jarosite and KFe₃(CrO₄)₂(OH)₆ in contaminated areas reduces the mobility of chromium, thus interfering with the clean-up of groundwater and soils.

Highly acidic chrome-plating solutions are a common source of chromium contamination in soils and groundwater. Groundwaters at sites where chrome-plating solutions have been released into the environment are characterized by very high chromate concentrations and low pH. They also often exhibit high Fe(III) concentrations due to dissolution of ferric oxyhydroxide soil minerals at low pH and high Fe(III) concentrations in the plating solutions. Under these low pH and high Cr(VI) and Fe(III) conditions, the aqueous concentrations and mobility of chromate may be controlled by the precipitation of Fe(III)/Cr(VI) solid phases. Identifying the solid phases that can form under these conditions and determining their solubility will help in the assessment of the risks to human health and the environment associated with releases of chrome-plating solutions and in the design of clean-up methods.

The solid FeOHCrO₄ was synthesized from a solution of 0.75 M Na₂Cr₂O₇-2H₂O, 1.5 M Na₂CrO₄-4H₂O, and 1.5 M Fe(NO₃)₃-9H₂O which was kept at 110^oC for 48 hours. The precipitate was characterized by chemical digestion, XRD, SEM/EDX, FTIR, and Thermogravimetric Analysis (TGA). We then set up a series of 16 dissolution experiments in which we placed small amounts of FeOHCrO₄ in solutions which had been adjusted to a range of acidic pH values using CrO₃. The dissolution experiments were sampled at regular intervals until concentrations of Fe(III) and Cr(VI) and the pH remained stable. Based on the final Fe(III) and Cr(VI) concentration and final pH, equilibrium ion activities for Fe³⁺ and CrO₄^2- were calculated using the geochemical speciation program MINTEQA2. For the reaction FeOHCrO₄ (s) + H⁺ ó Fe³⁺ + CrO₄^2- + H₂O the log K_SP is –7.1±0.2.

Throughout the dissolution experiments, the solids were examined by XRD to make sure that the original solid was still present and to evaluate whether secondary phases formed. At pH values above 2, FeOHCrO₄ becomes unstable and an amorphous Fe(III)/Cr(VI) hydroxide precipitates. Chemical digestion, SEM/EDX, and TGA of the amorphous solid yield a composition of Fe₄(OH)₁₀CrO₄. Based on the final solution compositions, the log K_SP for the reaction Fe₄(OH)₁₀CrO₄(s) + 10H⁺ ó 4Fe³⁺ + CrO₄^2- + 10H₂O is 0.7±0.8.

The White Wolf fault, responsible for the magnitude 7.7 Tehachapi earthquake of 1952, crosses agricultural land in the southern San Joaquin Valley, California. Despite the importance of groundwater in this area for irrigation purposes, the influence of the fault on groundwater hydrology is poorly understood. The purpose of this study was to investigate the long-standing belief that the fault acts as barrier to groundwater flow.

To determine the effects of the White Wolf fault zone on groundwater movement, historical records were examined and a short-term constant rate pumping test was conducted across the accepted location of the fault. The historical records date back to the 1950s and include both groundwater elevation and groundwater quality data. Groundwater elevation data were contoured for spring and fall records during a period of drought (1991 through 1994) and a wet period (1983 through 1988). Bicarbonate, calcium, carbonate, sodium, sulfate and total dissolved solids values recorded between 1950 and 1999 for wells on both sides of the fault zone were averaged over ten year periods and contoured.

The records of groundwater elevation for spring 1992 and 1993 and fall 1993 show a steep gradient in the vicinity of the assumed surface trace of the fault. Groundwater elevations for spring 1985 and 1986 do not show this gradient although it is suggested by the data for fall 1985. The groundwater quality contour maps show no apparent trends along the fault zone. The transmissivity of the aquifer affected by the pumping test was determined to be approximately 570 ft²/day (53 m²/day) (4,300 gpd/ft), with a hydraulic conductivity of 0.38 ft/day (0.12m/day). This is the first direct measurement of aquifer properties published for the area.

Groundwater elevation contour maps suggest that the fault zone acts as a partial barrier to groundwater flow when the aquifer system is stressed, as in a period of drought or during fall irrigation when groundwater pumping is at its maximum. Groundwater quality contour maps do not appear to reflect a groundwater barrier along the fault zone, and therefore suggest that there is movement of groundwater across the fault zone.

The California Well Sample Repository is the only public “library” of core material (cylinders of rock recovered from oil wells) in California. The Repository’s web site contains basic information about its inventory in spreadsheet form. Spatial information from the California Division of Oil and Gas was linked to the Repository’s database to create a map of cored wells on file at the repository. The resulting product allows geologists to query the repository database geospatially using a GIS (ArcView). This project provides valuable information to geologists studying areas with potential untapped oil reserves to help increase our domestic petroleum supply.

Recent strontium isotope data suggest that the San Joaquin Formation at Elk Hills, California is older than previously assumed. Four samples were taken from two cores of the San Joaquin Formation in the Dry Gas Zone at Elk Hills. One sample contained an oyster shell from the First Mya sand at a depth of 935 feet. The other three samples were thin-shelled pelecypods (Mya sp.) taken from the Third Mya sand at depths of 1717, 1875, and 2149 feet. Carbon and oxygen isotope analyses indicated very little post-depositional alteration of the samples however, thin section analysis indicated some diagenetic alteration in the Mya shells.

⁸⁷Sr/⁸⁶Sr ages of the Mya shells from the Third Mya Sand range from 5.1-13.9 Ma. This wide range of ages and the relatively old dates from these samples suggest that the thinner-walled Mya shells may be more susceptible to alteration. The ⁸⁷Sr/⁸⁶Sr age for the oyster shell from the First Mya sand is approximately 3.75 Ma with an uncertainty of +/- 1.55 Ma using the strontium sea water curve from Farrell et al. (1995). This sample showed no alteration in thin section, possibly due to the relative stability of the oyster shell material. The date for the First Mya sand suggests that the Tulare-San Joaquin Formation boundary in the subsurface of the southern San Joaquin Basin is older than early Pleistocene as previously estimated and may, in fact, be early Pliocene. This older date compares well with ash dates of 6.5 Ma from the correlative, to slightly older, Kern River Formation to the east (Miller et al., 1998). The older strontium age also compares well with ash dates of 5.0 Ma from a tuff at the base of the San Joaquin Formation farther north near Coalinga (Loomis, 1992).

Obsidian samples from known geographic locations will be analyzed for trace elements using Laser Ablation Inductively Coupled Plasma Mass Spectroscopy, (LA-ICP-MS) and compared with the trace-element analyses of archeological artifacts in order to determine the geographic origin of the artifacts and to establish a baseline for further study and comparisons.

The Kern Water Bank stores surplus water underground for withdrawal during droughts. The objective of this study was to determine the distribution and possible sources of arsenic in sediments from two wells within the water bank, one with water with high arsenic (63 ppb), the other with low concentrations (1.9 ppb). Although arsenic levels in Kern Water Bank groundwaters are generally very low (<5ppb), the proposed drinking water standard for arsenic (10ppb) has led to an interest in arsenic distribution and sources.

100 grab samples from the well were analyzed for total arsenic and other trace elements by microwave digestion and ICP/MS, total organic and inorganic carbon, grain size, soil pH, and magnetic susceptibility. We also conducted sequential extractions with 50 sediment samples to determine how the arsenic is bound to the sediments.

Total arsenic in the sediments ranges from 0.5 to 15 ppm, with the two wells showing similar total arsenic levels despite dramatically different concentrations in the water. High total arsenic levels correlate with small grain size, high organic carbon, and high magnetic susceptibility. Sequential extractions show that much of the total arsenic is exchangeable and easily soluble with a MgCl extraction. Exchangeable arsenic is higher by a factor of 2-3 in the high well containing relatively high arsenic in its waters.

The Coso Volcanic Field (CVF) has been a major source of toolstone for prehistoric populations for at least the past 12,000 years. Hundreds of small prehistoric quarries exist within the CVF, the most notable being the “Colossal Quarry” first described by Harrington (1951) on the south-facing side of Sugarloaf Mountain, a large Tertiary rhyolite dome. Geochemical sourcing of CVF obsidian for archaeological studies has been an important tool in assessing prehistoric trading patterns in eastern California, and the CVF source was first characterized using X-ray fluorescence in the later 1970s. Subsequent work by Hughes (1988) demonstrated the occurrence of four distinct geochemical subsources of obsidian within the CVF (Joshua Ridge, West Sugarloaf, Sugarloaf Mountain, West Cactus Peak).

We examined obsidian from Joshua Ridge, West Sugarloaf, Sugarloaf Mountain, and West Cactus Peak by Laser Ablation ICP/MS. This technique allows for quick and non-destructive trace element analysis of obsidian and other solids making it a promising tool in for archaeological investigations. Obsidian samples were analyzed for a suite of 25 trace elements including rare earth elements. The samples exhibited a generally similar trace element composition but differences in Rb, Ba, Zr, Sr, Ce, Dy, Eu, and Sm were large enough to allow distinction of the different source areas. An attempt was also made to distinguish obsidian from four different quarries in on Sugarloaf Mountain. However, the composition of obsidian from these quarries was too similar to allow unequivocal identification.

References:
R.E. Hughes Geoarchaeology: An International Journal 3, 253 (1988)
M.R. Harrington The Masterkey 25, 14 (1951).