Progress 08/15/01 to 08/14/05
Outputs
The goals of this study were to map the distribution of sedimentary units, improve the understanding of the relationship between the mapped units and groundwater quality/production through the analysis of sediment grab samples from wells, and to develop integrative models of depositional environment and geochemical evolution as aids to predict spatial and time-dependent variations in groundwater quality and production. Sedimentary units were mapped throughout the Kern Water Bank (KWB) to a depth of ~1,000 feet below ground surface (fbgs) after inferring these units from short-normal electric logs. The sedimentary units were then tied into groundwater production and quality through physical and chemical analyses of grab samples from wells, including grain-size, magnetic susceptibility, total organic carbon and the concentrations of major and minor elements. Also, grab samples were inspected and/or analyzed via petrographic microscope, scanning electron microscope, and
electron microprobe and the mineralogy of samples was determined using x-ray diffractometry. Sediments are progressively coarser-grained toward shallower depths and the coarsest-grained sediments occur in higher abundances toward the apex of the Kern River Alluvial Fan. Both patterns are expected distributions for an alluvial fan building outward over time into the basin toward the toe of the fan and from its apex. Sediments in the middle depth range are slightly, but significantly, finer-grained. One particular locus of fine-grained sediments is at the base of a coherent package of sediments characterized by LsCus2, an integrative unit defined by a Large-Scale Coarsening-Upward pattern. This unit is found in a depth zone between 600 and 300 ft below ground surface in sediments that are likely to be several hundreds of thousands of years old. The map pattern of LsCus2 coincided with the locations of the only wells in the KWB that have relatively high groundwater arsenic
concentrations. Analyses of well samples from the LsCus2 yielded relatively high concentrations of arsenic loosely bound to mineral surfaces. Low magnetic susceptibility and high total organic carbon of the same samples led to the hypothesis that the LsCus2 unit was deposited by a prograding delta and the geochemical conditions during this deposition were relatively reducing. This hypothesis predicted the presence of pyrite in these samples, a prediction that was supported by subsequent SEM/EDX analyses. SEM analyses also showed dissolution features in the pyrite and wavelength-dispersive microprobe analyses demonstrated that this pyrite contained significant amounts (up to a few percent) of arsenic. The new observations led to the enhanced hypothesis that high arsenic concentrations in this semi-arid to arid alluvial environment are associated with reducing geochemical reactions in sediments deposited in a lacustrine rather than an alluvial fan environment. Arsenic was bound in
pyrite after sulfate reduction and subsequently released when the geochemical conditions changed toward oxidation. Ongoing work is focused on isolating the source of the arsenic and the regional geological context of the prograding delta.
Impacts
Our study has improved the understanding of the spatial distribution of groundwater arsenic in arid to semi-arid alluvial sediments. It is also leading to a better understanding of the origin of the host sediments and the geochemical pathways of arsenic during water/rock interactions in such systems. The study has led to three M.S. projects, one of which is completed and another of which is almost completed. Furthermore, 11 undergraduates played a significant role in the research including running analyses on state-of-the-art analytical equipment (e.g., the ICP-MS). Several of the undergraduates have been included as coauthors on the numerous presentations given on this research at both local (e.g., the Kern Fan Element Monitoring Committee) and national (e.g., the Geological Society of America and the US Department of Agriculture) meetings. The funding from this project led to successful grant proposals to another agency that brought an additional ~$250,000 into the
project that enhanced our ability to further project goals and to include students in the research.
Publications
- Boockoff, L., 2005. Arsenic in Groundwater from Water Bank Well 30S/25E-23H01, San Joaquin Valley, California, M.S. Thesis, Department of Geology, California State University, Bakersfield, CA.
- Boockoff, L., D. Baron, R. Horton, R. Negrini, J. Parker, 2005. Arsenic in sediments and groundwater from a well in the southern San Joaquin Valley, California, Geol. Soc. Am. Abstr. w/ Prog., v. 37, p. 376.
- Horton, R. A., Jr.,, Durham, N, 2004. Mineralogy of Quaternary sediments of the Kern River alluvial fan, Kern Water Bank, California: California: American Association of Petroleum Geologists Pacific Section Meeting Official Program, p. A12.
- Huff, J., 2005, in progress. Quaternary Stratigraphy of the Kern Water Bank Area, Southern San Joaquin Valley, Kern County, California, M.S. Thesis, Department of Geology, California State University, Bakersfield, CA.
- Huff, J., R. Negrini, J. Gillespie, P. Philley, K. Blake, J. Parker, 2004. Mapping the lithofacies of the Kern Water Bank, Prog. Abstr. Annual Meeting Pac. Sect. AAPG, Bakersfield, CA, p. A13.
- Negrini, R., D. Baron, J. Gillespie, R. Horton, K. Blake, J. Huff, C. Meyer, E. Powers, A. Draucker, S. Draucker, N. Durham, G. Hilton, L. Mondrian, S. O Rear, P. Philley, C. Register, 2005. A Middle Pleistocene Lacustrine Delta Lobe in the Kern River Alluvial Fan and its Close Association with Groundwater Arsenic Concentrations: One Outcome of USDA-CREES Grant #2001-01170, USDA-CSREES National Water Quality Conference, La Jolla, CA, February.
- Negrini, R., D. Baron, J. Gillespie, R. Horton, K. Blake, J. Huff, C. Meyer, E. Powers, A. Draucker, S. Draucker, N. Durham, G. Hilton, L. Mondrian, S. O Rear, P. Philley, J. Parker, D. Bean, T. Haslebacher, 2005. Kern Water Bank Project: A progress report, Kern Fan Element Monitoring Committee Montly Meeting, Kern County Water Agency, Bakersfield, CA, February.
- Negrini, R., J. Gillespie, P. Philley, D. Baron, R. Horton, and J. Huff. 2004, Large-Scale Coarsening-Upward Sequences in the Stratigraphy of the Kern Water Bank: Prograding Fan-Deltas into a more Extensive Ancestral Buena Vista Lake?, Prog. Abstr. Annual Meeting Pac. Sect. AAPG, Bakersfield, CA, p. A20.
- Negrini, R., D. Baron, J. Gillespie, R. Horton, J. Huff, K. Blake, A. Draucker, S. Draucker, C. Register, P. Philley, J. Parker, T. Haslebacher, 2003. CSUB Kern Water Bank Project, San Joaquin Geological Society monthly meeting, Bakersfield, CA, April.
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Progress 10/01/02 to 09/30/03
Outputs
1. The construction of the Geographix database was completed. It consists principally of information from 372 wells, 162 of which have electric logs. The typical well has a total depth of several hundred feet. 2. Our 3-D mapping program of the Kern Water Bank is 75% completed. Results include the mapping of a 100-400 ft thick prograding delta sequence defined by a coarsening-upward e-log signature. The area where the sequence is thickest coincides with the area containing the highest arsenic concentrations observed in production wells. If this correspondence is causal in nature, then a reducing, lacustrine geochemical environment may be responsible for differential leaching of arsenic from sediment grains to groundwater. Completed mapping will also include sand and clay units as thin as 25 feet. This will be completed by June, 2004. Results from this task will provide constraints for groundwater flow models. 3. An analysis of grab samples from 2 KWB wells is nearly
complete. One well has high groundwater arsenic concentrations (63 ppb), the other has low concentrations (1.9 ppb). Analyses include ostracode paleontology (complete) and palynology (complete). Despite the processing of more than 100 samples, few fossil remains were found and none were diagnostic of depositional environment. However, the palynological study produced associated sedimentological observations which indicated the presence of a soil horizon at 550-570 fbgs based on soil clay and carbonate structures. Thin section petrography of all sands is underway and will be completed by March. This analysis will include identification of constituent minerals/rock fragments, plots of major constituents on ternary diagrams, and identification of mineral dissolution/precipitation. Initial results found 50-100 micron-sized pyrite grains, an indicator mineral for reducing conditions. Volcanic glass has also been identified, particularly in one zone at a depth of 850 fbgs. Original grab
samples from this horizon may provide enough glass for geochemical fingerprinting. If so, then we will fingerprint the ash layer to see if it is one of two well known ash layers found in Corcoran Clay layer elsewhere in San Joaquin Valley thereby testing our lacustrine delta model and possibly placing the KWB stratigraphy into a regional framework. Sedigraph granulometry of the <180 micron fraction is complete. These results will be used to test electric log signatures as indicators of sand/clay content. ICP-MS analyses of sediment after five-step sequence extraction procedure will be complete by March. This analysis may lead to identification of geochemical process responsible for extraction of arsenic into groundwater. Early results indicate that the sediment grains in both wells contain more than enough arsenic to supply observed concentrations in groundwater.
Impacts
Based on results to date, we are developing a depositional model wherein the KWB contains a lacustrine prograding delta sequence which may be strongly associated with high concentrations of arsenic in the groundwater. This relationship is potentially due to reducing environments associated with a lacustrine environment rather than the oxidizing alluvial fan environment traditionally associated with Kern Water Bank sediments. We will be testing this model in the coming months primarily using the imminent results from the sequential extraction/ICP-MS analysis. These results have the potential to implicate the dissolution or precipitation of minerals associated with reducing vs. oxidizing environments. This ongoing work potentially impacts groundwater banking in arid environments through constraining the origin of arsenic in groundwater. Our approach was driven first by developing depositional models (i.e., prograding lacustrine delta) with geological interpretation
software traditionally used by the petroleum industry. This methodology may prove to be a desirable model for water bank studies elsewhere on the Kern River alluvial fan and even elsewhere in arid regions.
Publications
- Huff, J., Gillespie, J., and Negrini, R., 2003. Log Correlations and Environments of Deposition for the Kern Water Bank, Kern County, CA, San Joaquin Geological Society May Meeting, Bakersfield, CA.
- Philley, P., and Negrini, R., 2003. Magnetic Susceptibility Method to Supplement Well Correlations in the Kern Water Bank, San Joaquin Geological Society May Meeting, Bakersfield, CA.
- Mahan, A., and Gillespie, J., 2003. Water Level Elevation Maps of the Kern Water Bank from 1994-1999, San Joaquin Geological Society May Meeting, Bakersfield, CA.
- Negrini, R., 2003. The CSUB Kern Water Bank Project, presentation at the San Joaquin Geological Society March Meeting, Bakersfield, CA
- Negrini, R., 2003. The CSUB/KWBA Kern Water Bank Project, presentation at the Groundwater Resources Association April Meeting, Fresno, CA
- Bennett K., Blake K., and Baron D., 2003. Creating a database to model water quality and improve geological interpretations at the Kern Water Bank, CA. San Joaquin Geological Society May Meeting, Bakersfield, CA.
- Register C.L., Draucker A.C., O'Rear S., Baron D., and Negrini R., 2003. Distribution and sources of arsenic in sediments from two wells in the Kern Water Bank, California. San Joaquin Geological Society May Meeting, Bakersfield, CA.
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