G. Bryant Hudson

Chemical evolution of shallow groundwater as recorded by springs, Sagehen basin; Nevada County, California

Abstract Springs in Sagehen basin, California, were used to document the effect of chemical weathering on the chemical evolution and composition of groundwater in a high elevation catchment. Geochemical tracer ages were determined with chlorofluorocarbons (CFCs) and tritium/ 3 He dating techniques. The spring water ages range from less than 5 years to almost 40 years. Mass balance calculations performed by NETPATH were combined with spring water ages to calculate chemical weathering rates observed throughout the basin, which range from 0.0116 to 0.0018 and from 0.0036 to 0.0006 mmol l −1 year −1 , for plagioclase and hornblende, respectively. Major cation concentrations, pH, and spring water conductivity were found to correlate positively ( R 2 =0.7) with spring water age. This suggests that shallow groundwater, as represented by the springs, is a chemically evolving system.

Old groundwater influence on stream hydrochemistry and catchment response times in a small Sierra Nevada catchment: Sagehen Creek, California

[1] The relationship between the chemical and isotopic composition of groundwater and residence times was used to understand the temporal variability in stream hydrochemistry in Sagehen basin, California. On the basis of the relationship between groundwater age and [Ca2+], the mean residence time of groundwater feeding Sagehen Creek during base flow is approximately 28 years. [Cl-]:[Ca2+] ratios in Sagehen Creek can be used to distinguish between two important processes: changes in the apparent age of groundwater discharging into the creek and dilution with snowmelt. The mean residence time of groundwater discharging into the creek is approximately 15 years during snowmelt periods. The results from this study have implications for hydrograph separation studies as groundwater is not a single, well-mixed chemical component but rather is a variable parameter that predictably depends on groundwater residence time. Most current models of catchment hydrochemistry do not account for chemical and isotopic variability found within the groundwater reservoir. In addition, this study provides valuable insight into the long-term hydrochemical response of a catchment to perturbations as catchment-flushing times are related to the mean residence time of water in a basin. Copyright 2005 by the American Geophysical Union.

Cosmetic Isotope Analyses Applied to River Longitudinal Profile Evolution: Problems and Interpretations

The use of cosmogenic isotopes to determine surface exposure ages has grown rapidly in recent years. The extent to which cosmogenic nuclides can distinguish between mechanistic hypotheses of landscape evolution is an important issue in geomorphology. We present a case study to determine whether surface exposure dating techniques can elucidate the role knickpoint propagation plays in longitudinal profile evolution. Cosmogenically produced 10Be, 26Al, 36Cl, 3He and 21Ne were measured in olivines collected from 5·2 Ma basalt flows on Kauai, Hawaii. Several obstacles had to be overcome prior to the measurement of In situ-produced radionuclides, including removal of meteoric 10Be from the olivine grains. Discrepancies between the radionuclide and noble gas data may suggest limits for exposure dating. Approximate surface exposure ages calculated from the nuclide concentrations indicate that large boulders may remain in the Hawaiian valley below the knickpoint for hundreds of thousands of years. The ages of samples collected above the knickpoint are consistent with estimates of erosion based on the preservation of palaeosurfaces. Although the exposure ages can neither confirm nor reject the nickpoint hypothesis, boulder ages downstream of the knickpoint are consistent with a wave of incision passing upvalley. The long residence time off the coarse material in the valley bottom further suggests that knickpoint propagation beneath a boulder pile is necessary for incision of the bedrock underlying the boulders to occur.

The Use of Noble Gas Isotopes for Monitoring Leakage of Geologically Stored CO 2

This chapter focuses on the usage of noble gas isotopes for monitoring leakage of geologically stored CO 2 . One of the primary concerns in CO 2 storage is monitoring the storage site on a long-term basis for possible leakage of CO 2 . Concentrations of CO 2 vary widely in the Earths crust, making detection of very small releases difficult. Small amounts of noble gas isotopes can be dissolved into the CO 2 being injected for storage and used as tracers to monitor CO 2 movement. Noble gases are chemically inert, environmentally safe, and are persistent and stable in the environment. The unique isotopic compositions that can be imparted to the CO 2 can be unambiguously identified during monitoring. Among the noble gases, xenon isotopes have commercial costs and availability suitable for use in large CO 2 storage operations. Required xenon volumes are low, simplifying handling and injection. Multiple batches of injected CO 2 at the same site could be imparted with different xenon isotopic compositions, making each of them identifiable with only a single xenon analysis. These characteristics are believed to make xenon a superior tracer to other option, SF 6 and 14 CO 2 .

Measurements of Helium in Electrolyzed Palladium

The result of a double-blind, cold fusion experiment are reported, in which six laboratories measured the helium content of five identically shaped 2-mm-diam {times} 10-cm-long palladium rods supplied by Fleischmann and Pons. Three rods were initially implanted with {sup 4}He. Before analysis, three of the rods had served as cathodes during electrolysis in cold fusion experiments: two in 0.1 M LiOD, and one in 0.1 M LiOH. The other two, one implanted and one not, served as references. The major observations reported on in this paper are as follows: All the materials, including the as-received palladium stock, contained easily measured quantities of {sup 4}He, well above amounts normally found in high-purity palladium. The {sup 4}He could be totally removed from at least two of the materials, including the as-received palladium stock, by surface etching the samples to a depth of {approximately}25 {mu}m. Helium implanted by alpha-particle bombardment remained in the electrodes through out the electrolysis. No {sup 3}He was measured above detection limits in any of the materials by any of the six laboratories.