H41I-01 08:00h
Identifying Groundwater Discharge in the Merced River Basin, California Using Radon-222
Groundwater flow in fractured granite of the Sierra Nevada is poorly characterized, in particular, contributions of mountain block recharge are not known. Using a combination of water quality and isotopic analyses, groundwater inputs to the Upper Merced River were characterized. Between November 2003 and July 2004, monthly water quality samples were taken from Happy Isles to the inlet of Lake McClure, a 75 km reach. These samples demonstrated the expected dilution due to snowmelt in the spring. In the fall, the spatial profile matched the geology with anion concentrations increasing downstream of the transition from the Sierra Nevada batholith to the country rock, suggesting significant groundwater inputs. From July 19 to 21, 2004, radon-222 and other noble gases (He, Ne, Ar, Kr and Xe abundances and 3He/4He ratio) were measured along a 37 km reach of the Merced River, extending from the top of Yosemite Valley to the confluence of the South Fork of the Merced River. All radon samples were extracted into mineral oil immediately in the field and counted using liquid scintillation; noble gas samples were collected in copper tubes. Radon-222 activity varied from about 1 to 100 pCi/L (at collection time) indicating significant, spatially variable groundwater discharge into the Merced River. Two one-mile reaches of the Merced River were sampled for 222Rn on a fine scale. Large fracture sets in these two locations and previous temperature measurements suggested that groundwater discharge was higher relative to other locations along the river. Radon-222 activity was low upstream and downstream of large fractures observed in the bedrock; whereas, 222Rn activity was high at large fracture zones. Degassing is rapid downstream of fractures where no groundwater discharge is observed. For a representative groundwater end-member, radon-222 activity measured in Fern Spring, Yosemite Valley was about 1200 pCi/L. Excess 4He from U and Th decay is observed in samples with elevated 222Rn, however the ratio of 4He to 222Rn is significantly different between the two fracture sets.
H41I-02 08:15h
Radium Isotopes as an Indicator of Microbial Activity in a Deep Aquifer at Memphis, Tennessee, USA
Groundwater samples were collected from three well fields (Shaw, Morton, and Sheahan) at Memphis in southwestern Tennessee, to study the behavior of Ra isotopes ($^{226}$Ra, $^{228}$Ra, $^{224}$Ra, and $^{223}$Ra) in a semi-confined deep aquifer in the area. Elevated activities were observed for each of these radioisotopes ($^{226}$Ra: 0.4-0.8 dpm/L; $^{228}$Ra: 0.8-1.6 dpm/L; $^{224}$Ra: 1.4-2.4 dpm/L; $^{223}$Ra: 0.07-0.1 dpm/L) at the Shaw site where groundwaters are recharged to form the Memphis aquifer, while relatively low activities ($^{226}$Ra: 0.02-0.07 dpm/L; $^{228}$Ra: 0.03-0.1 dpm/L; $^{224}$Ra: 0.04-0.13 dpm/L; $^{223}$Ra: $<$0.003 dpm/L) were found at the Morton site where the deep Memphis aquifer is effectively isolated from the unconfined aquifer at shallow depths. At the Sheahan site, the Ra isotope activities in the aquifer all reach maximum values at a depth of $\sim$100 m below the ground surface. The high activities are found to be associated with high dissolved organic carbon concentrations, suggesting that leakage via plumes of contaminated groundwater from the shallow unconfined aquifer into the deep Memphis aquifer may have taken place. As Mn oxides are a strong scavenger of Ra isotopes in low-salinity groundwaters, our results imply that the elevated Ra isotope activities in these ground waters may be caused by a reduction/dissolution of Mn oxide coatings on the aquifer solids in association with enhanced microbial activities. This implication points to the potential of using the Ra isotopes as a proxy for assessing deep aquifer microbial activities and their influence on the chemistry of groundwater.
H41I-03 08:30h
Quantifying Radionuclides Migration From U-series in an Unconfined Aquifer
With the increase of contaminant flux of radionuclides in the environment, there is a need to understand and model the processes that control the distribution of uranium and its daughter products during transport within aquifers. We have used U-series disequilibria as an analogue for the transport of anthropogenic nuclides in a small watershed of a chalk aquifer in Eastern France. We have measured naturally occurring U and Th isotopes in rock and water phases in order to determine transport mechanisms at water/rock interface and quantify parameters controlling the migration of radionuclides. The studied aquifer has a double porosity and is characterised by a saturated matrix up to 40m above the water-table and variably saturated fractures. Measurements of U and Th isotopic composition in the water phase were performed on a MC-ICPMS which enable in particular high precision measurements of (230Th/232Th) activity ratio. Water samples were obtained from wells both in low and high water-table conditions. Chalk samples were collected at the same wells and were analysed also for U-series and Sr isotopes. We observed seasonal precipitation of uranium ((234U/238U) activity ratio above 1) in the zone of water-table fluctuation linked with the oversaturation of water with respect to calcite during low water-table period. (230Th/232Th) activity ratios in water are lower than in the substratum meaning that the time of water infiltration is not long enough to allow an equilibration of groundwater with rock. Water acquires its Th isotopic composition when interacting with another phase of the aquifer (soil) and joins the water-table mainly through fractures. The complete data set obtained on solid and liquid phases of the aquifer allow us mass balance calculations and to model the behaviour of uranium and thorium during transport. The 1-D model we have developed considers a transient behaviour of radionuclides in the aquifer and for the first time allows the calculation of the evolution of rock composition with time. We have then calculated a global uranium weathering rate within the aquifer of 2.5 10-6 y-1.
H41I-04 08:45h
Tracing and Apportioning Sources of Uranium to the Hanford Reach of the Columbia River Using Uranium Isotopes
The U.S. Department of Energy's Hanford Site is situated along an unimpounded portion of the Columbia River, the highest discharge volume river west of the continental divide. Decades of nuclear related activities have left significant local contamination (e.g. nitrate, U, tritium, Cr$^{6+}$, $^{99}$Tc) in the vadose zone and groundwater within the site. Some of this contamination has reached the Columbia River, and there remains the potential for further contaminant migration to the river. We collected and analyzed samples of Columbia River water for U and Sr isotopes in coordination with the ongoing sampling and monitoring of the river. The U and Sr isotopic data allow us to evaluate sources of U (e.g. natural background, Hanford related, agricultural runoff) and their relative contributions to the river's U budget. The data also provide constraints on the flux of contaminant U from the Hanford Site to the river. We analyzed two sample traverses across the Columbia, one near the Vernita Bridge, upstream from Hanford Site contamination, and a second about 5 km downstream of the Hanford Site. An island divides the downstream traverse into western (main channel) and eastern portions. Filtered (0.45 micron) water samples were analyzed for U isotopic composition (including $^{236}$U, one marker of spent U fuel) and U concentration, as well as $^{87}$Sr/$^{86}$Sr and Sr concentration. The samples from the upstream traverse had no detectible $^{236}$U ($^{236}$U/$^{238}$U $<$ 2x10$^{-8}$), one marker of spent U fuel, natural $^{238}$U/$^{235}$U, uniform ($^{234}$U/$^{238}$U, $^{87}$Sr/$^{86}$Sr, U and Sr concentrations. In contrast, the downstream traverse showed variation in all of these parameters. Concentrations of U are 0.5 ppb to 1.2 ppb and are all well below the EPA MCL of 30 ppb for drinking water. In the western channel, measured $^{236}$U/$^{238}$U is 3.4x10$^{-5}$ to $<$ 2x10$^{-8}$, with a co-variation in $^{238}$U/$^{235}$U toward enriched ratios. This correlation is consistent with the U isotopic compositions of a groundwater sample from the environs of a former fuel fabrication complex and samples of groundwater seeps at the adjacent river shore. These compositions indicate a component of 2nd cycle enriched U fuels. For the river water sample with the highest $^{236}$U/$^{238}$U, about 30% of the total U comes from Hanford contamination. For the eastern portion of the traverse, no detectable $^{236}$U was found, however $^{234}$U/$^{238}$U and $^{87}$Sr/$^{86}$Sr are significantly different from the Vernita Bridge traverse, likely reflecting contributions from agricultural runoff.
H41I-05 09:00h
Determining Sources and Transport of Nuclear Contamination in Hudson River Sediments with Plutonium, Neptunium, and Cesium isotope ratios
Different sources of radioactive contamination contain characteristic and identifiable isotopic signatures, which can be used to study sediment transport. We focus on Pu-239, Pu-240, Np-237 and Cs-137, which are strongly bound to fine grained sediments. The Hudson River drainage basin has received contamination from at least three separate sources: 1) global fallout from atmospheric testing of nuclear weapons, which contributed Pu, Np and Cs; 2) contamination resulting from reactor releases at the Indian Point Nuclear Power Plant (IPNPP) located on the Hudson River Estuary $\sim$70km north of New York Harbor, where records document releases of Cs-137; 3) contamination resulting from activities at the Knolls Atomic Power Laboratory (KAPL) located on the Mohawk River, where incomplete records document releases of Cs-137 but no mention is made of Pu or Np. Here we report measurements of Pu isotopes, Np-237 and Cs-137 for a series of sediment cores collected from various locations within the drainage basin: 1) Mohawk River downstream of KAPL, 2) Hudson River upstream of its confluence with the Mohawk River, and 3) lower Hudson River at a location in close proximity to IPNPP. In addition, we present data from selected samples from two other lower Hudson River locations: One site located $\sim$30km downstream of IPNPP and another $\sim$30km upstream of IPNPP. By comparing the isotopic ratios Pu-240/Pu-239, Np-237/Pu-239, and Cs-137/Pu-239, measured in fluvial sediments to mean global fallout values, it is possible to identify and resolve different sources of non-fallout contamination. To date, isotopic data for sediments indicate non-fallout sources of Pu-239, Pu-240, and Cs-137; Np-237, however, appears to originate from global fallout only. Mohawk River sediments downstream of KAPL exhibit enrichments in Pu-239, Pu-240, and Cs-137 that are 7 to 20 times higher than levels expected from global fallout as indicated from Np-237. The elevated levels, non-fallout isotopic signatures, and core location are all consistent with KAPL being a source of Pu and Cs isotopes. Sediments from upper Hudson River and a section of the lower Hudson Estuary both contain Cs-137 levels that are more than twice that expected from global fallout. While elevated Cs-137 in selected lower Hudson samples is consistent with reported reactor releases from IPNPP, there is no known source of non-fallout Cs in the upper Hudson. We have been able to estimate end-member isotopic compositions of radionuclides originating from KAPL as well as detect its presence and estimate its contribution to contaminant inventories far downstream in tidal Hudson sediments. By comparing KAPL-derived Pu-239 inventories measured in the Mohawk and Lower Hudson Rivers, we estimate a dilution factor of $\sim$140. While there is isotopic evidence of KAPL derived radionuclides in all the lower Hudson sediments that we have analyzed, elevated levels of Cs-137 (not attributable to KAPL)were only observed in sediments collected in the vicinity of IPNPP and those collected 30km downstream of the plant's location. We attribute the elevated Cs-137 levels in these Lower Hudson sediments to contamination originating from IPNPP. The lack of elevated levels of Cs-137 in sediments collected 30km upstream of the plant's location plus a dilution factor for Upper Hudson sediments that is larger than that estimated for Mohawk River sediments alone, allows us to conclude that Cs-137 enrichment observed in the Upper Hudson is not likely to be a significant source of Cs-137 contamination to lower Hudson River sediments.
H41I-06 09:15h
Tracing the Origin of Radioactivity in Groundwater from the Negev, Israel
In normal groundwater conditions natural radionuclides are typically retained on the aquifer matrix and their activity in the groundwater is low. Radium is exceptional since the ratio between adsorbed and dissolved radium depends the ionic strength of the solution. Under high salinity radium is rapidly desorbed and accumulates in the liquid phase. Here we report the results of a geochemical study that investigates the origin of radioactivity in brackish to saline groundwater from the Negev and Arava Valley, Israel. We use the Ra isotope quartet (226Ra-half life 1600 y, 228Ra - 5.6 y, 224Ra - 3.6 d, 223Ra - 11.4 d) to discriminate between radioactivity derived from a thorium source (high 228Ra/226Ra and 224Ra/223Ra ratios) found in groundwater flowing in the Nubian Sandstone aquifer and an uranium source (low 228Ra/226Ra and 224Ra/223Ra ratios) in groundwater flowing in carbonate (Upper Cretaceous) aquifer. We show that the activity of 226Ra in groundwater from the carbonate aquifer is positively correlated with that of the salinity. In the Nubian Sandstone aquifer, however, no such correlation was found. Instead, we observed an inverse correlation between 228Ra activity and sulfate and a positive correlation with barium contents. Given the high H2S content of the ground water, we hypothesized that sulfate reduction process triggers radium leaching to the water, probably due to barite dissolution and anoxic conditions in the aquifer. These findings indicate that high radioactivity can also be found even in low-saline groundwater and that the isotopic ratios of radium are sensitive tracers for the water-rock interactions and thus reconstructing the flow paths in different aquifer matrix (i.e., carbonate versus sandstone).
H41I-07 09:30h
One Million Year Old Groundwater in the Sahara Revealed by Krypton-81 and Chlorine-36
Measurements of 81Kr/Kr in deep groundwater from the Nubian Aquifer (Egypt) were performed by a new laser-based atom-counting method. 81Kr ages range from about 2x10e5 to about 1x10e6 yr, correlate with 36Cl/Cl ratios, and are consistent with lateral flow of groundwater from a recharge area near the Uweinat Uplift in SW Egypt. Low delta-2H values of the 81Kr-dated groundwater reveal a recurrent Atlantic moisture source during Pleistocene pluvial periods. These results indicate that the 81Kr method for dating old groundwater is robust and such measurements can now be applied to a wide range of hydrologic problems.