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Robert L. Michel

Publications and source records attributed to Robert L. Michel.

16 recordsLinked to original sources
USGS2018

Tritium deposition in precipitation in the United States, 1953–2012

Tritium is a radioactive isotope of hydrogen (half-life is equal to 12.32 years). Since it is part of the water molecule, tritium can be used to track and date groundwater and surface water when the history of tritium in precipitation and recharge is known. To facilitate that effort, tritium concentrations in precipitation were reconstructed from measurements and correlations for 10 precipitation stations in the continental United States. Using these data, and other data sets and correlations, estimates of tritium concentrations in precipitation from 1953 through 2012 and total tritium deposition from 1953 through 1983 were derived for 2 degrees latitude by 5 degrees longitude quadrangles of the continental United States. For August 1953 through December 1987 correlations were derived from the data of the Ottawa, Canada, station; for estimates after 1987, the Vienna, Austria, station was used. For quadrangles where no precipitation station records are available from measurements or correlations, concentrations were interpolated. The International Atomic Energy Agency has proposed that a correlation with the Vienna, Austria, station, which has been in operation since 1960, be used instead of the Ottawa, Canada, station for the period after 1987. Linear correlations calculated for all stations with the Vienna data are reported. Correlations also have been calculated for each latitude-longitude quadrangle using the estimated concentrations and the measured Vienna data for 1960–87. Because the Vienna, Austria, station is on a different continent, and not subject to the same seasonal weather patterns that affect North American stations, a correlation was obtained for each month individually. This reflects the fact that the North American stations were strongly affected by the input of low-tritium moisture from the south during the summer. The correlated station values were then used with measured Vienna data to obtain estimates of tritium concentrations in precipitation for the period 1988–2012 at each precipitation station and latitude-longitude quadrangles. This approach has two major advantages: (1) it blends the Ottawa correlation with the Vienna correlation to cover the period 1953 to present, and (2) it yields correlation coefficients for each quadrangle that can be used in future years when more data become available from the Vienna, Austria, station. The data, the estimated tritium concentrations derived from the correlations, and the correlation coefficients are provided as spreadsheets and associated comma delimited files in a data release that accompanies this report.

Scientific Investigations Report↗
USGS2013

On the conversion of tritium units to mass fractions for hydrologic applications

We develop a general equation for converting laboratory-reported tritium levels, expressed either as concentrations (tritium isotope number fractions) or mass-based specific activities, to mass fractions in aqueous systems. Assuming that all tritium is in the form of monotritiated water simplifies the derivation and is shown to be reasonable for most environmental settings encountered in practice. The general equation is nonlinear. For tritium concentrations c less than 4.5×10 12 tritium units (TU) - i.e. specific tritium activities<5.3×10 11 Bq kg -1 - the mass fraction w of tritiated water is approximated to within 1 part per million by w ≈ c×2.22293×10 -18 , i.e. the conversion is linear for all practical purposes. Terrestrial abundances serve as a proxy for non-tritium isotopes in the absence of sample-specific data. Variation in the relative abundances of non-tritium isotopes in the terrestrial hydrosphere produces a minimum range for the mantissa of the conversion factor of [2.22287; 2.22300].

Isotopes in Environmental and Health Studies↗
USGS2010

Dating of shallow groundwater: Comparison of the transient tracers 3H/3He, chlorofluorocarbons, and 85Kr

This paper describes a direct comparison of apparent ages derived from 3 H/ 3 He, chlorofluorocarbons (CCl 3 F and CCl 2 F 2 ), and 85 Kr measurements in shallow groundwater. Wells chosen for this study are completed in the unconfined surficial aquifers in late Cenozoic Atlantic Coastal Plain sediments of the Delmarva Peninsula, on the east coast of the United States. Most of the apparent tracer ages agree within 2 years of each other for recharge dates between 1965 and 1990. Discrepancies in apparent tracer ages usually can be explained by hydrological processes such as mixing in a discharge area. Recharge rate calculations based on apparent tracer age gradients at multilevel well locations agree with previous recharge estimates. High recharge rates on the Delmarva Peninsula result in nearly complete dissolved-gas confinement in the groundwater. The remarkable agreement between the different tracer ages indicates negligible mixing of waters of different ages, insignificant dispersion, minimal gas loss to the atmosphere, and insignificant sorption-desorption processes at this location.

Water Resources Research↗
USGS2009

Radionuclides as tracers and timers in surface and groundwater

Environmental radionuclides—in combination with stable isotopes, geochemistry, and other hydrological techniques—provide a powerful tool, often indispensable, for studying the cycling of water in continental hydrological systems. The use of environmental radionuclides in surface water studies is reviewed in the chapter. The chapter also briefly discusses groundwater and geothermal water taking into consideration the fact that most applications in groundwater and geothermal water studies require the combined use of radioactive and stable isotopes. There are several sources of radionuclides in the environment, and the sources control the ways in which isotopes can be applied to hydrologic systems. Another group of radionuclides that can be utilized are those produced by cosmic-ray spallationin the atmosphere or near-surface lithosphere. Many of these nuclides, such as carbon-14 ( 14 C) and tritium ( 3 H), are also produced by nuclear weapons testing, and it is necessary to separate the two source functions when using them.

Book chapter↗
USGS2009

Transport of tritium contamination to the atmosphere in an arid environment

Soil–plant–atmosphere interactions strongly influence water movement in desert unsaturated zones, but little is known about how such interactions affect atmospheric release of subsurface water-borne contaminants. This 2-yr study, performed at the U.S. Geological Survey's Amargosa Desert Research Site in southern Nevada, quantified the magnitude and spatiotemporal variability of tritium (3H) transport from the shallow unsaturated zone to the atmosphere adjacent to a low-level radioactive waste (LLRW) facility. Tritium fluxes were calculated as the product of 3H concentrations in water vapor and respective evaporation and transpiration water-vapor fluxes. Quarterly measured 3H concentrations in soil water vapor and in leaf water of the dominant creosote-bush [ Larrea tridentat a (DC.) Coville] were spatially extrapolated and temporally interpolated to develop daily maps of contamination across the 0.76-km2 study area. Maximum plant and root-zone soil concentrations (4200 and 8700 Bq L−1, respectively) were measured 25 m from the LLRW facility boundary. Continuous evaporation was estimated using a Priestley–Taylor model and transpiration was computed as the difference between measured eddy-covariance evapotranspiration and estimated evaporation. The mean evaporation/transpiration ratio was 3:1. Tritium released from the study area ranged from 0.12 to 12 μg d−1 and totaled 1.5 mg (8.2 × 1010 Bq) over 2 yr. Tritium flux variability was driven spatially by proximity to 3H source areas and temporally by changes in 3H concentrations and in the partitioning between evaporation and transpiration. Evapotranspiration removed and limited penetration of precipitation beneath native vegetation and fostered upward movement and release of 3H from below the root zone.

Vadose Zone Journal↗
USGS2004

Movement and Age of Ground Water in the Western Part of the Mojave Desert, Southern California, USA

Tritium and carbon-14 data in water from wells in the Mojave River and the Morongo ground-water basins in the western part of the Mojave Desert show recent recharge focused in the floodplain aquifer along the Mojave River. Older ground water was present in parts of the regional aquifer that surround and underlie the floodplain aquifer. Movement of water between the floodplain and the regional aquifers occurs near on the upgradient side of faults as water from the regional aquifer discharges to the floodplain aquifer and on the downgradient side of the faults where water from the floodplain aquifer recharges the regional aquifer. On the basis of carbon-14 ages, corrected for mineralogic reactions with aquifer materials, water from some wells was recharged more than 20,000 years ago. Geochemical data show ground-water recharge has gradually decreased as the climate changed since that time.

Water-Resources Investigations Report↗
USGS2004

Plume-scale testing of a simplified method for detecting tritium contamination in plants and soil

Cost-effective methods are needed to detect contamination near radioactive-waste and other contaminated sites. Such methods should be capable of providing an early warning of contaminant releases and should be accurate and robust enough for assessing the long-term performance of waste-isolation facilities and remediation measures. Recently, a simplified method for detecting tritium contamination in plants and soil was developed (1). The method includes solar distillation of plant water from foliage, followed by filtration and adsorption of scintillation-interfering constituents on a graphite-based solid-phase-extraction column prior to direct-scintillation counting. The objectives of the in-progress study described here are to (i) test the simplified contamination-detection method for collection and analysis of plume-scale tritium data and (ii) gain insight into tritium migration pathways and processes.

Nevada↗
USGS2004

Monitoring radionuclide contamination in the unsaturated zone - Lessons learned at the Amargosa Desert Research Site, Nye County, Nevada

Contaminant-transport processes are being investigated at the U.S. Geological Survey’s Amargosa Desert Research Site (A DRS), adjacent to the Nation’s first commercial disposal facility for low-level radioactive waste. Gases containing tritium and radiocarbon are migrating through a 110-m thick unsaturated zone from unlined trenches that received waste from 1962 to 1992. Results relevant to long- term monitoring of radionuclides are summarized as follows. Contaminant plumes have unexpected histories and spatial configurations due to uncertainties in the: (1) geologic framework, (2) biochemical reactions involving waste components, (3) interactions between plume components and unsaturated-zone materials, (4) disposal practices, and (5) physical transport processes. Information on plume dynamics depends on ex-situ wet-chemical techniques because in-situ sensors for the radionuclides of interest do not exist. As at other radioactive-waste disposal facilities, radionuclides at the ADRS are mixed with varying amounts of volatile organic compounds (VOCs). Carbon-dioxide and VOC anomalies provide proxies for radioactive contamination. Contaminants in the unsaturated zone migrate along preferential pathways. Effective monitoring thus requires accurate geologic characterization. Direct- current electrical-resistivity imaging successfully mapped geologic units controlling preferential transport at the ADRS. Direct sampling of water from the unsaturated zone is complex and time consuming. Sampling plant water is an efficient alternative for mapping shallow tritium contamination.

Nevada↗
USGS2003

Hydrogeology and geochemistry of aquifers underlying the San Lorenzo and San Leandro areas of the East Bay Plain, Alameda County, California

The East Bay Plain, on the densely populated eastern shore of San Francisco Bay, contains an upper aquifer system to depths of 250 feet below land surface and an underlying lower aquifer system to depths of more than 650 feet. Injection and recovery of imported water has been proposed for deep aquifers at two sites within the lower aquifer system. Successful operation requires that the injected water be isolated from surface sources of poor-quality water during storage and recovery. Hydraulic, geochemical, and isotopic data were used to evaluate the isolation of deeper aquifers. Ground-water responses to tidal changes in the Bay suggest that thick clay layers present within these deposits effectively isolate the deeper aquifers in the northern part of the study area from overlying surficial deposits. These data also suggest that the areal extent of the shallow and deep aquifers beneath the Bay may be limited in the northern part of the study area. Despite its apparent hydraulic isolation, the lower aquifer system may be connected to the overlying upper aquifer system through the corroded and failed casings of abandoned wells. Water-level measurements in observation wells and downward flow measured in selected wells during nonpumped conditions suggest that water may flow through wells from the upper aquifer system into the lower aquifer system during nonpumped conditions. The chemistry of water from wells in the East Bay Plain ranges from fresh to saline; salinity is greater than seawater in shallow estuarine deposits near the Bay. Water from wells completed in the lower aquifer system has higher pH, higher sodium, chloride, and manganese concentrations, and lower calcium concentrations and alkalinity than does water from wells completed in the overlying upper aquifer system. Ground-water recharge temperatures derived from noble-gas data indicate that highly focused recharge processes from infiltration of winter streamflow and more diffuse recharge processes from infiltration of precipitation occur within the study area. However, recharge of imported water from leaking water-supply pipes, believed by previous investigators to be a large source of ground-water recharge, was not supported on the basis of oxygen-18 and deuterium data collected as part of this study. Based on tritium/helium-3 ages, most water in the upper aquifer system is relatively young and was recharged after 1952; however, water in the lower aquifer system is older and does not contain detectable tritium. Carbon-14 ages interpreted for water from wells in the lower aquifer system and underlying partly consolidated rocks range from 500 to more than 20,000 years before present. The greatest ages were in water from wells completed in the partly consolidated deposits that underlie the northern part of the study area. Ground water from wells in the lower aquifer system near the proposed Bayside injection/recovery site was recharged about 9,400 years before present and appears to be isolated from surface sources of recharge and ground-water contamination.

California↗
USGS2002

Use of natural 35 S to trace sulphate cycling in small lakes, Flattops Wilderness Area, Colorado, U.S.A.

Measurements of the cosmogenically-produced 35 S, a radioisotope of sulphur (t 1/2 = 87 days), are reported for the Ned Wilson Lake watershed in Colorado. The watershed contains two small lakes and a flowing spring presumed to be representative of local ground water. The watershed is located in the Flattops Wilderness Area and the waters in the system have low alkalinity, making them sensitive to increases in acid and sulphate deposition. Time series of 35 S measurements were made during the summers of 1995 and 1996 (July–September) at all three sites. The system is dominated by melting snow and an initial concentration of 16–20 mBq L -1 was estimated for snowmelt based on a series of snow samples collected in the Rocky Mountains. The two lakes had large initial 35 S concentrations in July, indicating that a large fraction of the lake water and sulphate was introduced by meltwater from that year's snowpack. In 1995 and 1996, 35 S concentrations decreased more rapidly than could be accounted for by decay, indicating that other processes were affecting 35 S concentrations. The most likely explanation is that exchange with sediments or the biota was removing 35 S from the lake and replacing it with older sulphate devoid of 35 S. In September of 1995 and 1996, 35 S concentrations increased, suggesting that atmospheric deposition is important in the sulphate flux of these lakes in late summer. Sulphur-35 concentrations in the spring water were highly variable but never higher than 3.6 mBq L -1 and averaged 2 mBq L -1 . Using a simple mixing model, it was estimated that 75% of the spring water was derived from precipitation of previous years.

Water, Air, and Soil Pollution: Focus↗
USGS2000

Timescales for migration of atmospherically derived sulphate through an alpine/subalpine watershed, Loch Vale, Colorado

Sulphur 35, a cosmogenically produced radioisotope with a short half‐life (87 days), was measured in snowpack during 1993–1997 and at four locations within the Loch Vale watershed during 1995–1997. The four sites include the two main drainages in the watershed, Andrews Creek and Icy Brook, a small south facing catchment flowing into Andrews Creek (Andrews Spring 1), and a similar north facing catchment flowing out of a scree field into Icy Brook (Spring 19). Concentrations ranged from a high of almost 50 mBq/L for a sample from Spring 19 in June 1996 to a concentration near the detection limit for a sample from Andrews Creek in April 1997. Sulphur 35 concentrations were normalized to sulphate (as mBq/mg SO 4 −2 ) and were decay‐corrected to a Julian day of 90 (April 1) for each year. Snowpack had the highest 35 S concentration with an average concentration of 53 mBq/mg SO 4 −2 . Concentrations in the streams were much lower, even when corrected for decay relative to JD 90. The large 35 S concentrations found in Spring 19 were the result of increases in concentration due to sublimation and/or evapotranspiration and were lower than snowpack when normalized to sulphate. Using 35 S concentrations found in snowpack as of JD 90 as a beginning concentration, the fraction of sulphate in streamflow that was derived from atmospheric deposition within the prior water year was estimated. For Icy Brook and Andrews Creek the fraction of the sulphate in streamflow derived from that year's snowpack and precipitation was low prior to the beginning of the main spring melt, reached a maximum during the period of maximum flow, and decreased as the summer progressed. A calculation of the seasonal flux indicated that about 40% of the sulphate that flowed out of the watershed was derived from atmospheric sulphate deposited during the previous year. This suggests that more than half of the sulphate deposited in the watershed by atmospheric processes during the previous year was removed during the following summer. Thus sulphate retention in alpine watersheds like Loch Vale is very limited, and changes in sulphate deposition should be quickly reflected in stream chemistry.

Colorado↗
USGS2000

Data from a thick unsaturated zone underlying Oro Grande and Sheep Creek washes in the western part of the Mojave Desert, near Victorville, San Bernardino County, California

This report presents data on the physical properties of unsaturated alluvial deposits and on the chemical and isotopic composition of soil water and soil gas collected at 12 monitoring sites in the western part of the Mojave Desert, near Victorville, California. Sites were installed using the ODEX air-hammer method. Seven sites were located in the active channels of Oro Grande and Sheep Creek Washes. The remaining five sites were located away from the active washes. Most sites were drilled to a depth of about 100 feet below land surface; two sites were drilled to the water table almost 650 feet below land surface. Drilling procedures, lithologic and geophysical data, and site construction and instrumentation are described. Core material was analyzed for water content, bulk density, water potential, particle size, and water retention. The chemical composition of leachate from almost 1,000 subsamples of cores and cuttings was determined. Water extracted from selected subsamples of cores was analyzed for tritium and the stable isotopes of oxygen and hydrogen. Water from suction-cup lysimeters and soil-gas samples also were analyzed for chemical and isotopic composition. In addition, data on the chemical and isotopic composition of bulk precipitation from five sites and on ground water from two water-table wells are reported.

California↗
USGS1999

Tritium and 14C concentrations in unsaturated-zone gases at test hole UZB-2, Amargosa Desert Research Site, 1994-98: A section in U.S. Geological Survey Toxic Substances Hydrology Program: Proceedings of the technical meeting, Charleston, South Carolina, March 8-12, 1999: Volume 3 (Part C) (WRI 99-4018C)>

Tritium concentrations have been determined yearly since April 1994 from water-vapor samples collected at test hole UZB-2. The hole was drilled about 100 m (meters) south of the southwest corner of a commercial burial site for low-level radioactive wastes in September 1993. UZB-2 is equipped with ten 2.5-cm (centimeters) diameter air ports permanently installed in the unsaturated zone between the depths of 5.5 and 108.8 m below land surface. Depth to ground water is about 110 m. Additional sampling ports were driven by hand to depths of 0.5, 1.0 and 1.5 m in May 1997. Initial samples of water vapor collected in April 1994 showed elevated tritium concentrations of more than 100 TU (tritium units) from all 10 air ports, with a maximum concentration of 762±10 TU from an air port at a depth of 24.1 m. Subsequent tritium concentrations increased in all air ports, although tritium concentrations at depths of less than 34.1 m have remained relatively constant since July 1995. The largest observed increase in tritium has been at a depth of 47.9 m. There, tritium concentration has increased from 198±5 TU in April 1994 to 2,570±30 TU in June 1998. Large increases also have been measured in samples collected from air ports at depths of 106.4 and 108.8 m, just above the water table. During September and October 1998, carbon dioxide samples were collected from all ten air ports in UZB-2 and at a depth of 1.5 m, and analyzed for radioactive carbon-14 ( 14 C). 14 C concentrations are highest in air ports at depths less than 6 m where they exceed 2,000 pmc (percent modern carbon). Concentrations decrease rapidly in air ports at depth and are about 20 pmc below 94.2 m. However, at 47.9 meters, the 14 C concentration is 205±1 pmc, which is 2 to 4 times higher than concentrations in air ports immediately above and below. This depth corresponds to the largest tritium increase in UZB-2. Concentrations of both tritium and 14 C are greater than what could be expected from atmospheric fallout. The distribution of tritium and 14 C likely represent a complex pattern of lateral and vertical transport through the unsaturated zone from buried wastes to UZB-2.

Nevada↗
USGS1998

Isotopes as indicators of environmental change

In addition to providing an understanding of processes within a catchment system, isotopic techniques have been instrumental in providing reconstructions of catchment climate and other environmental indicators at various time scales. Many recent changes are a direct consequence of anthropogenic activities. Isotopic analysis serves as a valuable tool for distinguishing between natural variations in long-term climatic patterns and anthropogenic effects, yielding improved understanding of natural feedback mechanisms and the development of realistic remediation strategies. This chapter discusses the examples of isotopic techniques that have been applied to understand several types of ongoing and recent environmental changes, and in paleo-environmental studies. It discusses isotope geochemistry, hydrology, and climatology to look at new ways of applying isotopic tracing techniques to provide information on environmental change. It also gives an overview on how isotopic indicators are being applied in investigations of environmental change in continental settings.

Book chapter↗
USGS1994

Use of long-term tritium records from the Colorado River to determine timescales for hydrologic processes associated with irrigation in the Imperial Valley, California

Tritium records were used to study hydrologic processes associated with irrigation and drainage in the Imperial Valley, a 2000-km 2 agricultural area in the southeastern California desert. Tritium was analyzed in surface water, ground water, soil-pore water and drain water, and the results were compared to the historical record of tritium in the Colorado River. The Colorado River record was reconstructed using a simple reservoir model and precipitation data in the Colorado River Basin for the period prior to 1965, and from continuous measurements in the river for 1965–1988. This historical record is especially useful in the arid Imperial Valley because recent agricultural development has been entirely dependent on irrigation water diverted from the Colorado River and local recharge is negligible. Results indicate that it takes about 5 a for irrigation drainage to move through the soil to a depth of 2–3 m. Drainwaters have a wide range in tritium concentrations because of varying degrees of influence from ground-water intrusion, and from rapid percolation of irrigation through preferred pathways. The net result is that drainwater from about 40 fields had a range in tritium concentration similar to that of the Colorado River over the last 9 a (1980–1988), a period during which tritium concentration was declining about 15% annually in the river.

California↗