Timothy P. Rose

ISOTOPE HYDROLOGY OF VOLUMINOUS COLD SPRINGS IN FRACTURED ROCK FROM AN ACTIVE VOLCANIC REGION, NORTHEASTERN CALIFORNIA

The more than 1550 km2 (600 mi2) Hat Creek Basin in northeastern California is host to several first magnitude cold springs that emanate from Quaternary basaltic rocks with individual discharge rates ranging from 1.7 to 8.5 m3 s−1 (60–300 ft3 s−1). Stable isotope (δ18O, δD, δ13C) and 14C measurements of surface and groundwater samples were used to identify recharge areas, and to evaluate aquifer residence times and flow paths. Recharge locations were constrained from the regional decrement in meteoric water δ18O values as a function of elevation, determined to be −0.23‰ per 100 m for small springs and creek waters collected along the western Cascade slope of this region. In general, the large-volume springs are lower in (δ18O than surrounding meteoric waters, and are inferred to originate in high-elevation, high-precipitation regions up to 50 km away from their discharge points. Large spring 14C abundances range from 99 to 41 % modern carbon (pmc), and most show evidence of interaction with three distinct carbon isotope reservoirs. These reservoirs are tentatively identified as (1) soil CO2 gas equilibrated under open system conditions with groundwater in the recharge zone [δ13CDIC ≈ −18‰, 14C > 100 pmc], (2) dissolved carbon equilibrated with atmospheric CO2 gas [δ13CDIC ≈ +1‰, 14C > 100 pmc], and (3) dissolved carbon derived from volcanic CO2 gas emissions [δ13CDIC≈0‰, 14C=0 pmc]. Many regional waters show a decrease in 14C abundance with increasing δ13C values, a pattern indicative of interaction with dead carbon originating from volcanic CO2 gas. Several lines of evidence suggest that actual groundwater residence times are too short (⩽ 200 years) to apply radiocarbon dating corrections. In particular, water temperatures measured at springs show that deep groundwater circulation does not occur, which implies an insufficient aquifer volume to account for both the high discharge rates and long residence times suggested by 14C apparent ages. The large springs also exhibit rapid decreases in flow during periods of drought that suggests a high level of aquifer interconnectivity to the recharge area. The estimated amount of volcanic CO2 dissolved in surface and groundwater originating from the Lassen highlands is consistent with the conversion of approximately 10% of the geothermal CO2 flux into dissolved inorganic carbon.

The use of temperature and the isotopes of O, H, C, and noble gases to determine the pattern and spatial extent of groundwater flow

Isotopic tracer and temperature measurements at large volume cold springs in the central Oregon Cascades are used to understand the pattern of groundwater flow. Standard oxygen and hydrogen isotope interpretations are used to determine the mean recharge elevation for springs. Carbon and helium isotopes are used to measure the component of dissolved magmatic gas in the spring waters. Inferences from isotopic measurements are compared with temperature measurements made at the springs to determine whether groundwater circulates to shallow or deep depths in the subsurface. Integrating the measurements of tracers derived at the surface, tracers derived from the subsurface, and temperature measurements can thus be used to derive a three dimensional picture of groundwater flow.

Isotope hydrology of southern Nevada groundwater: Stable isotopes and radiocarbon

A new δ18O map of southern Nevada groundwater shows a systematic decrease in δ18O of ∼5‰ from 36° to 39°N latitude. The variation is consistent with higher-latitude recharge following continuous flow paths along north-south trending graben valleys and systematically increasing in δ18O due to mixing with lower-latitude, higher-δ18O recharge. The data do not suggest that large masses of groundwater with unusually low δ18O values were recharged during the last pluvial period as suggested by previous workers. Many δ18O-δD pairs in groundwater indicate variable amounts of evaporation relative to the global meteoric water line. The precipitation rate and type (rain versus snow) for a given geographic area controls the extent of evaporation. A “model” δ18O value was calculated from evaporated groundwaters by subtracting postcloud evaporation. The distribution of these model δ18O values suggest new regional groundwater flow paths previously undocumented. Dissolved inorganic carbon in groundwaters collected from deep regional flow systems typically has low 14C concentrations (≤12% modern carbon). In contrast, groundwaters collected from the carbonate rock of the Spring Mountains and alluvium of Forty Mile Canyon have higher 14C contents, indicating more recent recharge. Because of nonlinear mixing of 14C, groundwater in the regional flow system likely acquired most of its observable 14C from mixtures of young, locally recharged groundwater.

CO2 degassing in the Oregon Cascades

The carbon isotope content of dissolved inorganic carbon was measured for large cold springs in the central Oregon Cascades. Low [sup 14]C activities in some of the springs are interpreted to result from the dissolution of diffuse emissions of magmatic CO[sub 2], even though volcanic activity has not occurred in this area for more than 1300 yr. On the basis of dissolved magmatic carbon concentrations in the springs, the authors infer a diffuse magmatic CO[sub 2] degassing rate of 3.4 [times] 10[sup 5] kg/yr per kilometer of arc for the central Oregon Cascades. The CO[sub 2] flux calculated from estimates of the mean magmatic intrusion rate and experimentally determined values of CO[sub 2] content in melts is consistent with that determined from their measurements of the dissolved CO[sub 2] flux at springs.

Effect of reducing groundwater on the retardation of redox-sensitive radionuclides

Laboratory batch sorption experiments were used to investigate variations in the retardation behavior of redox-sensitive radionuclides. Water-rock compositions were designed to simulate subsurface conditions at the Nevada Test Site (NTS), where a suite of radionuclides were deposited as a result of underground nuclear testing. Experimental redox conditions were controlled by varying the oxygen content inside an enclosed glove box and by adding reductants into the testing solutions.Under atmospheric (oxidizing) conditions, radionuclide distribution coefficients varied with the mineralogic composition of the sorbent and the water chemistry. Under reducing conditions, distribution coefficients showed marked increases for 99Tc (from 1.22 at oxidizing to 378 mL/g at mildly reducing conditions) and 237Np (an increase from 4.6 to 930 mL/g) in devitrified tuff, but much smaller variations in alluvium, carbonate rock, and zeolitic tuff. This effect was particularly important for 99Tc, which tends to be mobile under oxidizing conditions. A review of the literature suggests that iodine sorption should decrease under reducing conditions when I- is the predominant species; this was not consistently observed in batch tests. Overall, sorption of U to alluvium, devitrified tuff, and zeolitic tuff under atmospheric conditions was less than in the glove-box tests. However, the mildly reducing conditions achieved here were not likely to result in substantial U(VI) reduction to U(IV). Sorption of Pu was not affected by the decreasing Eh conditions achieved in this study, as the predominant sorbed Pu species in all conditions was expected to be the low-solubility and strongly sorbing Pu(OH)4.Depending on the aquifer lithology, the occurrence of reducing conditions along a groundwater flowpath could potentially contribute to the retardation of redox-sensitive radionuclides 99Tc and 237Np, which are commonly identified as long-term dose contributors in the risk assessment in various radionuclide environmental contamination scenarios. The implications for increased sorption of 99Tc and 237Np to devitrified tuff under reducing conditions are significant as the fractured devitrified tuff serves as important water flow path at the NTS and the horizon for a proposed repository to store high-level nuclear waste at Yucca Mountain.

Oxygen isotope evidence for hydrothermal alteration within a Quaternary stratovolcano, Lassen Volcanic National Park, California

Brokeoff volcano, a Quaternary stratocone located in the Lassen volcanic center in northern California, has been deeply eroded, exposing a 10-km2 meteoric hydrothermal alteration zone at the core of the volcano. Portions of the former volcanic edifice are sufficiently well preserved that an unusual opportunity exists wherein the alteration pattern can be correlated with the position of the volcanic cone. The δ18O analyses of more than 100 whole rock samples, consisting primarily of andesitic lavas, vary from +9.8 to +0.6 per mil. The highest δ18O values occur in bleached, solfatarically altered rocks that have interacted with low-pH, fumarolic hot springs associated with the present-day hydrothermal system. Low δ18O values are found in propylitically altered rocks that underwent isotopic exchange with meteoric hydrothermal fluids at elevated temperatures, mostly during the stratovolcanic stage (650–400ka) of the hydrothermal system, but probably continuing today at depth. Electron microprobe analyses of secondary layer silicate minerals in strongly propylitized samples (δ18O < +5.0) revealed the presence of discrete chlorite, suggesting that temperatures up to 200 to 250°C were attained in the shallow levels of the system. Two zones of pervasive meteoric hydrothermal alteration, defined by concentric 18O contours that are probably interconnected at depth, are located within the original topographic edifice of the volcano. The most intensely altered rocks within these equant zones of alteration define NNW trends that coincide with stream valleys and with regional structural patterns. A comparison of the characteristics of the 18O-depleted zone at Brokeoff with those of more deeply eroded volcanic centers, such as the Comstock Lode mining district (Criss and Champion, 1991), permits the construction of composite 18O cross sections through a hypothetical intact stratovolcano. At both Brokeoff and Comstock, hydrothermal fluids were strongly focused into plumelike zones of intense 18O depletion. At Comstock, these low-18O plumes are associated with faults. Although major fault displacements are not observed at Brokeoff, the topographic and alteration patterns are consistent with the presence of a linear array of faults that acted as conduits for fluid flow up into the shallow levels of the volcano.

Formation of 238U16O and 238U18O observed by time-resolved emission spectroscopy subsequent to laser ablation

We have measured vibronic emission spectra of an oxide of uranium formed after laser ablation of the metal in gaseous oxygen. Specifically, we have measured the time-dependent relative intensity of a band located at approximately 593.6 nm in 16O2. This band grew in intensity relative to neighboring atomic features as a function time in an oxygen environment but was relatively invariant with time in argon. In addition, we have measured the spectral shift of this band in an 18O2 atmosphere. Based on this shift, and by comparison with earlier results obtained from free-jet expansion and laser excitation, we can confirm that the oxide in question is UO, consistent with recent reports based on laser ablation in 16O2 only.We have measured vibronic emission spectra of an oxide of uranium formed after laser ablation of the metal in gaseous oxygen. Specifically, we have measured the time-dependent relative intensity of a band located at approximately 593.6 nm in 16O2. This band grew in intensity relative to neighboring atomic features as a function time in an oxygen environment but was relatively invariant with time in argon. In addition, we have measured the spectral shift of this band in an 18O2 atmosphere. Based on this shift, and by comparison with earlier results obtained from free-jet expansion and laser excitation, we can confirm that the oxide in question is UO, consistent with recent reports based on laser ablation in 16O2 only.

Irradiative coloration of quartz and feldspars with application to preparing high-purity mineral separates

Visible discrimination of quartz and feldspars can be greatly facilitated by the irradiative coloration of mineral concentrates using γ-radiation from a ^(137)Cs source. Irradiated quartz samples typically become smoky colored. Sanidine phenocrysts generally acquire a yellow color while plagioclase and plutonic K-feldspars respond weakly or negligibly to irradiation. Manual separation of 0.25–3.0-mm mineral grains that were initially identical in appearance was readily accomplished following radiation treatment. Available data indicate that this process does not modify the chemical or isotopic composition of the sample.

Recharge and Flow in the Medicine Lake Volcano–Fall River Springs Groundwater Basin, California

Isotopic measurements of the 34 m3/s discharge from the Fall River Springs of northern California indicate recharge from 50 km upgradient in high elevation regions of Medicine Lake Volcano. Age determinations suggest less than 20-year travel time. Data demonstrate Klamath Basin further north cannot be a recharge source. Mass balance calculations support that annual precipitation on the volcano supplies observed spring discharge, requiring 50%–75% recharge rates. Radiocarbon and δ13C of dissolved inorganic carbon indicate 30%–40% is derived from magmatic CO2. Measured excess 3He is also consistent with the presence of magmatic gas derived from the Quaternary Age Medicine Lake Volcano.

Radionuclide Partitioning in an Underground Nuclear Test Cavity

In 2004, a borehole was drilled into the 1983 Chancellor underground nuclear test cavity to investigate the distribution of radionuclides within the cavity. Sidewall core samples were collected from a range of depths within the re-entry hole and two sidetrack holes. Upon completion of drilling, casing was installed and a submersible pump was used to collect groundwater samples. Test debris and groundwater samples were analyzed for a variety of radionuclides including the fission products {sup 99}Tc, {sup 125}Sb, {sup 129}I, {sup 137}Cs, and {sup 155}Eu, the activation products {sup 60}Co, {sup 152}Eu, and {sup 154}Eu, and the actinides U, Pu, and Am. In addition, the physical and bulk chemical properties of the test debris were characterized using Scanning Electron Microscopy (SEM) and Electron Microprobe measurements. Analytical results were used to evaluate the partitioning of radionuclides between the melt glass, rubble, and groundwater phases in the Chancellor test cavity. Three comparative approaches were used to calculate partitioning values, though each method could not be applied to every nuclide. These approaches are based on: (1) the average Area 19 inventory from Bowen et al. (2001); (2) melt glass, rubble, and groundwater mass estimates from Zhao et al. (2008); and (3) fission product mass yield data from England and Rider (1994). The U and Pu analyses of the test debris are classified and partitioning estimates for these elements were calculated directly from the classified Miller et al. (2002) inventory for the Chancellor test. The partitioning results from this study were compared to partitioning data that were previously published by the IAEA (1998). Predictions of radionuclide distributions from the two studies are in agreement for a majority of the nuclides under consideration. Substantial differences were noted in the partitioning values for {sup 99}Tc, {sup 125}Sb, {sup 129}I, and uranium. These differences are attributable to two factors: chemical volatility effects that occur during the initial plasma condensation, and groundwater remobilization that occurs over a much longer time frame. Fission product partitioning is very sensitive to the early cooling history of the test cavity because the decay of short-lived (t{sub 1/2} < 1 hour) fission-chain precursors occurs on the same time scale as melt glass condensation. Fission product chains that include both volatile and refractory elements, like the mass 99, 125, and 129 chains, can show large variations in partitioning behavior depending on the cooling history of the cavity. Uranium exhibits similar behavior, though the chemical processes are poorly understood. The water temperature within the Chancellor cavity remains elevated (75 C) more than two decades after the test. Under hydrothermal conditions, high solubility chemical species such as {sup 125}Sb and {sup 129}I are readily dissolved and transported in solution. SEM analyses of melt glass samples show clear evidence of glass dissolution and secondary hydrothermal mineral deposition. Remobilization of {sup 99}Tc is also expected during hydrothermal activity, but moderately reducing conditions within the Chancellor cavity appear to limit the transport of {sup 99}Tc. It is recommended that the results from this study should be used together with the IAEA data to update the range in partitioning values for contaminant transport models at the Nevada National Security Site (formerly known as the Nevada Test Site).