Four different tracer methods were used to estimate groundwater flow velocity at a multiple-well site in the saturated alluvium south of Yucca Mountain, Nevada: (1) two single-well tracer tests with ...different rest or “shut-in” periods, (2) a cross-hole tracer test with an extended flow interruption, (3) a comparison of two tracer decay curves in an injection borehole with and without pumping of a downgradient well, and (4) a natural-gradient tracer test. Such tracer methods are potentially very useful for estimating groundwater velocities when hydraulic gradients are flat (and hence uncertain) and also when water level and hydraulic conductivity data are sparse, both of which were the case at this test location. The purpose of the study was to evaluate the first three methods for their ability to provide reasonable estimates of relatively low groundwater flow velocities in such low-hydraulic-gradient environments. The natural-gradient method is generally considered to be the most robust and direct method, so it was used to provide a “ground truth” velocity estimate. However, this method usually requires several wells, so it is often not practical in systems with large depths to groundwater and correspondingly high well installation costs. The fact that a successful natural gradient test was conducted at the test location offered a unique opportunity to compare the flow velocity estimates obtained by the more easily deployed and lower risk methods with the ground-truth natural-gradient method. The groundwater flow velocity estimates from the four methods agreed very well with each other, suggesting that the first three methods all provided reasonably good estimates of groundwater flow velocity at the site. The advantages and disadvantages of the different methods, as well as some of the uncertainties associated with them are discussed.
•Four different tracer methods were used to estimate groundwater flow velocity.•The tests were conducted in the same location in a saturated porous medium aquifer.•The flow velocity estimates from the different methods were in good agreement.•The relative merits and uncertainties of the different methods are discussed.
•Nonreactive, thermally-degrading, and cation-exchanging tracers were deployed.•The thermally-degrading tracer placed constraints on magnitude of thermal drawdown.•The cation-exchanging tracer placed ...constraints on timing of thermal drawdown.•Uncertainties associated with the thermal drawdown predictions are discussed.•Repeated use of thermally-degrading tracers could reduce predictive uncertainty.
A tracer test was conducted in a geothermal reservoir using a cation-exchanging tracer to interrogate flow pathway surface area to volume ratios and a thermally-degrading tracer to interrogate average flow pathway temperatures. The test interpretations are used to predict when thermal drawdown will occur at a production well due to injecting cooler spent geothermal water into an injection well. The cation-exchanging tracer provides constraints on the timing of thermal drawdown, and the thermally-degrading tracer provides constraints on the magnitude of thermal drawdown. Uncertainties associated with the method, including assumptions made about flow pathway geometry, are discussed.
We consider the late‐time tailing in a tracer test performed with a push‐drift methodology (i.e., quasi‐radial injection followed by drift under natural gradient). Numerical simulations of such tests ...are performed on 1000 multi‐Gaussian 2‐D log‐hydraulic conductivity field realizations of varying heterogeneity, each under eight distinct mean flow directions. The ensemble pdfs of solute return times are found to exhibit power law tails for each considered variance of the log‐hydraulic conductivity field,
σlnK2. The tail exponent is found to relate straightforwardly to
σlnK2 and, within the parameter space we explored, to be independent of push‐phase pumping rate, pumping duration, and local‐scale dispersivity. We conjecture that individual push‐drift tracer tests in wells with screened intervals much greater than the vertical correlation length of the aquifer will exhibit quasi‐ergodicity and that their tail exponent may be used to infer
σlnK2. We calibrate a predictive relationship of this sort from our Monte Carlo study, and apply it to data from a push‐drift test performed at a site of approximately known heterogeneity—closely matching the existing best estimate of heterogeneity.
Key Points
Monte Carlo study in heterogeneous simulated aquifers finds push‐drift tests exhibit power law late‐time breakthrough behavior
The power law exponent is found to relate straightforwardly to the variance of the log‐hydraulic conductivity
An empirical relation, independent of push‐phase injection rate and volume, is derived to estimate hydraulic conductivity variance
In situ recovery (ISR) uranium (U) mining mobilizes U in its oxidized hexavalent form (U(VI)) by oxidative dissolution of U from the roll-front U deposits. Postmining natural attenuation of residual ...U(VI) at ISR mines is a potential remediation strategy. Detection and monitoring of naturally occurring reducing subsurface environments are important for successful implementation of this remediation scheme. We used the isotopic tracers 238U/235U (δ238U), 234U/238U activity ratio, and 34S/32S (δ34S), and geochemical measurements of U ore and groundwater collected from 32 wells located within, upgradient, and downgradient of a roll-front U deposit to detect U(VI) reduction and U mobility at an ISR mining site at Rosita, TX, USA. The δ238U in Rosita groundwater varies from +0.61‰ to −2.49‰, with a trend toward lower δ238U in downgradient wells. The concurrent decrease in U(VI) concentration and δ238U with an ε of 0.48‰ ± 0.08‰ is indicative of naturally occurring reducing environments conducive to U(VI) reduction. Additionally, characteristic 234U/238U activity ratio and δ34S values may also be used to trace the mobility of the ore zone groundwater after mining has ended. These results support the use of U isotope-based detection of natural attenuation of U(VI) at Rosita and other similar ISR mining sites.
In-situ recovery is a common method of mining U ore. This technique involves injecting an oxidant to oxidize U deposited as insoluble U(IV) to soluble U(VI) and a complexing agent to form mobile ...U(VI) compounds, which are extracted from mining waters. Unfortunately, U concentrations often remain elevated in groundwater following this mining process. One promising technique to decrease U concentrations to acceptable levels is in-situ remediation where a reductant is injected into the mined area to reduce U(VI) back to insoluble U(IV). Here we assess the efficacy of the chemical reductant dithionite to reduce U(VI) concentrations. While dithionite can reduce environmental Cr(VI), dithionite has previously never been tested for reducing U(VI) in the environment. We determine how effectively dithionite remediates U(VI) contamination using aqueous U concentration and δ238U data. U concentrations and δ238U decrease during the dithionite experiment, demonstrating that dithionite induces U(VI) reduction. As dithionite is removed by pumping, U concentrations and δ238U increase towards background levels, but U(VI) reduction continues for an extended period. Measurements demonstrate that re-oxidation and remobilization does not appear to occur due to the reducing condition of groundwater.
•Dithionite induces reduction of U(VI) in a previously mined area.•Dithionite may be utilized to remediate U contamination in other settings.•U(VI) and δ238U provide complementary information on extent of U(VI) reduction.•Chemical reductant dithionite impacts the geochemistry of sulfur and iron species.
Diffusion cell and diffusion wafer experiments were conducted to compare methods for estimating effective matrix diffusion coefficients in rock core samples from Pahute Mesa at the Nevada Nuclear ...Security Site (NNSS). A diffusion wafer method, in which a solute diffuses out of a rock matrix that is pre-saturated with water containing the solute, is presented as a simpler alternative to the traditional through-diffusion (diffusion cell) method. Both methods yielded estimates of effective matrix diffusion coefficients that were within the range of values previously reported for NNSS volcanic rocks. The difference between the estimates of the two methods ranged from 14 to 30%, and there was no systematic high or low bias of one method relative to the other. From a transport modeling perspective, these differences are relatively minor when one considers that other variables (e.g., fracture apertures, fracture spacings) influence matrix diffusion to a greater degree and tend to have greater uncertainty than effective matrix diffusion coefficients. For the same relative random errors in concentration measurements, the diffusion cell method yields effective matrix diffusion coefficient estimates that have less uncertainty than the wafer method. However, the wafer method is easier and less costly to implement and yields estimates more quickly, thus allowing a greater number of samples to be analyzed for the same cost and time. Given the relatively good agreement between the methods, and the lack of any apparent bias between the methods, the diffusion wafer method appears to offer advantages over the diffusion cell method if better statistical representation of a given set of rock samples is desired.
•A new lab method is tested for determination of matrix diffusion coefficients.•This method compares well with a previously used method.•There is no systematic bias in estimates between this method and a previous method.•This new method is experimentally more simplistic.
Accurate prediction of the subsurface transport of iodine species is important for the assessment of long-term nuclear waste repository performance, as well as monitoring compliance with the ...Comprehensive Nuclear-Test-Ban Treaty, given that radioiodine decays into radioxenon. However, the transport of iodine through intact geologic media is not well understood, compromising our ability to assess risk associated with radioiodine migration. The current study's goal is to quantify the matrix diffusion of iodine species through saturated volcanic rock, with particular attention paid to the redox environment and potential speciation changes. Diffusion experiments were run for iodide through lithophysae-rich lava, lithophysae-poor lava, and welded tuff, whereas iodate diffusion was studied through welded tuff. Iodine transport was compared with a conservative tracer, HDO, and effective diffusion coefficients were calculated. Likely due to a combination of size and anion exclusion effects, iodine species diffused more slowly than the conservative tracer through all rock types tested. Furthermore, oxidation of iodide to iodate was observed in the lithophysae-poor lava, affecting transport. Results provide much needed data for subsurface transport models that predict radioiodine migration from underground sources, and indicate the pressing need for geochemical and redox interactions to be incorporated into these models.
•Matrix diffusion of iodide and iodate was quantified through volcanic rock cores.•Anion and size exclusion retarded iodine species relative to a conservative tracer•Oxidation of iodide was observed in the lithophysae-poor lava system.•Rock hydraulic conductivity was altered in systems with iodate present.
This research assesses the ability of a GC SCM to simulate uranium transport under variable geochemical conditions typically encountered at uranium in-situ recovery (ISR) sites. Sediment was taken ...from a monitoring well at the SRH site at depths 192 and 193 m below ground and characterized by XRD, XRF, TOC, and BET. Duplicate column studies on the different sediment depths, were flushed with synthesized restoration waters at two different alkalinities (160 mg/l CaCO3 and 360 mg/l CaCO3) to study the effect of alkalinity on uranium mobility. Uranium breakthrough occurred 25% - 30% earlier in columns with 360 mg/l CaCO3 over columns fed with 160 mg/l CaCO3 influent water. A parameter estimation program (PEST) was coupled to PHREEQC to derive site densities from experimental data. Significant parameter fittings were produced for all models, demonstrating that the GC SCM approach can model the impact of carbonate on uranium in flow systems. Derived site densities for the two sediment depths were between 141 and 178 μmol-sites/kg-soil, demonstrating similar sorption capacities despite heterogeneity in sediment mineralogy. Model sensitivity to alkalinity and pH was shown to be moderate compared to fitted site densities, when calcite saturation was allowed to equilibrate. Calcite kinetics emerged as a potential source of error when fitting parameters in flow conditions. Fitted results were compared to data from previous batch and column studies completed on sediments from the Smith-Ranch Highland (SRH) site, to assess variability in derived parameters. Parameters from batch experiments were lower by a factor of 1.1 to 3.4 compared to column studies completed on the same sediments. The difference was attributed to errors in solid-solution ratios and the impact of calcite dissolution in batch experiments. Column studies conducted at two different laboratories showed almost an order of magnitude difference in fitted site densities suggesting that experimental methodology may play a bigger role in column sorption behavior than actual sediment heterogeneity. Our results demonstrate the necessity for ISR sites to remove residual pCO2 and equilibrate restoration water with background geochemistry to reduce uranium mobility. In addition, the observed variability between fitted parameters on the same sediments highlights the need to provide standardized guidelines and methodology for regulators and industry when the GC SCM approach is used for ISR risk assessments.
•Comparisons between experimental and GC SCM models used to simulate uranium transport for ISR-U sites.•Calcite kinetics were source of error for parameters fitting.•Experimental methodology more responsible for uranium sorption behavior than sediment heterogeneity•Removal of residual pCO2 and restoration water geochemical equilibration important to reduce uranium mobility
A series of batch experiments were performed to assess the uranium sorption capacity of four mineralogically distinct lithologies from the Negev Desert, Israel, to evaluate the suitability of a ...potential site for subsurface radioactive waste disposal. The rock specimens consisted of an organic-rich phosphorite, a bituminous marl, a chalk, and a sandstone. The sorption data for each lithology were fitted using a general composite surface complexation model (GC SCM) implemented in PHREEQC. Sorption data were also fitted by a non-mechanistic Langmuir sorption isotherm, which can be used as an alternative to the GC SCM to provide a more computationally efficient method for uranium sorption. This is because all the rocks tested have high pH/alkalinity/calcium buffering capacities that restrict groundwater chemistry variations, so that the use of a GC SCM is not advantageous. The mineralogy of the rocks points to several dominant sorption phases for uranyl (UO22+), including apatite, organic carbon, clays, and iron-bearing phases. The surface complexation parameters based on literature values for the minerals identified overestimate the uranium sorption capacities, so that for our application, an empirical approach that makes direct use of the experimental data to estimate mineral-specific sorption parameters appears to be more practical for predicting uranium sorption.
The scale dependence of the matrix diffusion coefficient (Dm) for fractured media has been observed at variable scales from column experiments to field tracer tests. In this paper, we derive an ...effective Dm for multimodal heterogeneous fractured rocks using characteristic distributions of matrix properties and volume averaging of the mass transfer coefficient. The effective field‐scale Dm is dependent on the statistics (geometric mean, variance, and integral scale) of laboratory‐scale ln(Dm) and on the domain size. The effective Dm increases with the integral scales and is larger than the geometric mean of ln(Dm). Monte Carlo simulations with 1000 realizations of heterogeneous Dm fields were conducted to assess the accuracy of the derived effective Dm.
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