James L. Osiensky

Partitioning of REE between solution and particulate matter in natural waters: a filtration study

Rare earth element (REE) concentrations were determined in filtered and unfiltered fractions of one stream water and five shallow ground waters from springs and wells from the Palouse region of Idaho/Washington to characterize the contribution of suspended, colloidal, and solution fractions to the REE contents of each water sample. Similar characteristics were observed in the shallow ground waters as reported in the literature for rivers. In almost all cases the LREE were depleted substantially in filtered fractions relative to the unfiltered fractions, indicating substantial partitioning of LREE onto particulate matter. In some, but not all samples, the HREE were far less depleted, indicating greater mobility of the REE as dissolved species or small colloidal particles. Increased solubility of HREE relative to LREE in these neutral to slightly alkaline waters may be due to preferential complexation of the HREE with ligands such as carbonate, hydroxide, fluoride or organic anions. In one water studied, filtration through a finer pore-size filter resulted in markedly more pronounced Ce anomalies. Cerium anomalies are likely controlled by preferential sorption of Ce4+ onto Fe–Mn particle coatings.

Rare earth element geochemistry of groundwater in the Palouse Basin, northern Idaho–eastern Washington

Concentrations of rare earth elements (REE) were determined in groundwaters from the Palouse Basin, located in northern Idaho and eastern Washington. Samples were taken from wells and springs tapping waters in four different rock units: loess, Wanapum Basalt, Grande Ronde Basalt, and Moscow Mountain Granite. In general, unfiltered aliquots of the waters had higher concentrations of REE than those filtered through a 0.45 μm filter, sometimes by several orders of magnitude, indicating the presence of REE as suspended particles or large colloids. Overall REE concentrations in the waters decrease, and the unfiltered and filtered aliquots become more similar in concentration, with increasing depth within the stratigraphic section (i.e. loess → Wanapum → Grande Ronde). There are some distinctions in NASC (North American Shale Composite)-normalized REE patterns among the four groups of waters, but there is considerable variation among the waters within each group. When normalized to their host rocks, most filtered and many unfiltered aliquots of groundwaters from all four units show depletion of the LREE relative to the HREE. The results are consistent with a model in which meteoric water enters the basalt aquifers via the loess, picking up a heavy particulate and some dissolved REE load, followed by infiltration downward through the units. While moving downwards, the waters are subjected to natural filtration processes in which the particulate REE are largely removed.

Time series electrical potential field measurements for early detection of groundwater contamination

Abstract The adequacy of groundwater monitoring systems that consist of up‐gradient and downgradient monitoring wells is often questioned by regulatory agencies. Groundwater contaminants frequently migrate along preferred pathways formed by lateral and vertical heterogeneities within definable stratigraphic units. Many monitoring wells must be completed correctly within individual heterogeneities to provide for early detection of contaminant migration along these preferred flowpaths. The adequacy of a specific groundwater monitoring system is usually directly dependent upon the number of monitoring wells. Time series electrical potential field measurements in combination with groundwater monitoring wells can greatly improve the capability of groundwater monitoring systems to detect the presence of contaminants. An electrical geophysical method known as the mise‐a‐la‐masse method involves the measurement of electrical current flow through earth materials under investigation. Changes in the electrical poten...

Evaluation of drawdown curves derived from multiple well aquifer tests in heterogeneous environments

Aquifer coefficients derived from nonsteady-state, multiple well, aquifer tests in laterally heterogeneous environments often have uncertain meaning. Drawdown at observation wells reflects the removal of water from storage in the aquifer and transient refraction of ground water pathlines during the evolution of a non-symmetrical cone of depression. These effects are masked within observation well drawdown data such that “good” Theis (1935) type curve matches often result. Transmissivity and storativity values derived from independent drawdown curves plotted as drawdown versus time (t) or drawdown versus time/distance2 (t/r2) usually differ from observation well to observation well. These aquifer coefficients often are considered to represent some type of average of the materials between and/or about the pumping well and the observation wells. Simulations of two multiple well aquifer tests with simple, arbitrary distributions of block heterogeneities suggest that transmissivity (T) and storativity values derived from independent drawdown curves by the Theis (1935) method generally increase with distance from the pumping well. This apparent scale effect is related to the force-fitting of earlytime drawdown data to the steep portion of the Theis type curve without sufficient late-time drawdown data to constrain vertical shifting of the drawdown data relative to the type curve.Log-log plots of drawdown versus t/r2 for multiple well aquifer tests form families of curves that are characteristic of the distribution of observation wells and the degree of heterogeneity within the cone of depression. Separation between discrete drawdown curves within a family provides a qualitative measure of the degree of heterogeneity within the cone of depression. All of the drawdown curves within a family converge on a single curve at large values of t/r2. A composite analysis of all of the drawdown data within the family yields an estimate of the average T within the cone of depression. Analysis of discrete drawdown curves as integral members of the family of curves provides a means to constrain type curve matches and minimizes force-fitting if drawdown data are defined for large values of t/r2 for at least one well. The constrained type curve matches provide more reasonable estimates for T near individual observation wells than analysis of drawdown curves independently.

Distributed of downward flux in unsaturated heterogeneous hydrogeology environments

We used the finite-element computer program UNSAT2 to quantify the horizontal distribution of unsaturated downward flux in porous tuff as a function of hydrogeologic heterogeneities under two different recharge rates. The distribution of downward flux is important because it influences ground-water travel time at a potential geologic repository for high-level radioactive wastes. Hydraulic properties of the Topopah Spring Member of the Paintbrush Tuff Formation at Yucca Mountain, Nevada, were selected for use in the eight simulations that were conducted. All simulations were run essentially to steady state (referred to herein as quasi-steady state). The simulations show that (1) heterogeneities in an isotropic porous rock matrix will cause downward flux to be distributed nonuniformly, (2) zones in which saturated matrix hydraulic conductivity is less than the downward flux tend to develop positive pressures that may divert flow into fractures if they exist, (3) one-dimensional analysis of vertical flow in the unsaturated zone is insufficient to ensure that fracture flow does not occur even if the true magnitude of vertical flux is known, and (4) the true spatial distribution of hydraulic conductivity above the regional water table must be determined in order to obtain the true spatial distribution of downward flux. This analysis assumes constant spatial and temporal distribution of recharge for each simulation, but the differences among simulations verify the fact that changing recharge rates also play a significant role in the distribution of downward flux owing to the influence of recharge rate on the development of positive pore pressures in the otherwise unsaturated rock. The primary significance of this research is as follows. (1) We have introduced a new method of UNSAT2 time step control that facilitates convergence even in complex, large-scale partially saturated hydrogeologic environments. Previous studies have experienced convergence problems with UNSAT2 under complex hydrogeologic conditions. (2) Nonuniform downward flow should be anticipated in heterogeneous hydrogeologic environments at a heterogeneity scale of 50 m and a model scale of hundreds of meters, even if recharge is uniform spatially. (3) The impact of heterogeneities in the unsaturated zone on downward flux can now be quantified. (4) Hydrogeologic heterogeneities can create zones of positive pressure and consequent possible fracture flow in the unsaturated zone even when the recharge rate is less than the average saturated hydraulic conductivity, but most certainly when the recharge rate exceeds saturated hydraulic conductivity. (5) This type of study provides a basis for designing hydrogeologic property testing programs and recharge distribution testing programs that are compatible with the design of models that can characterize downward flux. (6) This work shows that field documentation of the absence of positive pressures above the water table constitutes the best evidence of exclusive matrix flow (absence of fracture flow) within Yucca Mountain. If positive pressures are encountered in the field, then fracture flow probably exists within Yucca Mountain.

Charged body potential monitoring of an electrolyte plume emanating from a dripping source.

Abstract Hole-surface charged body potential (CBP) measurements were taken over a 173-day period during a drip-injection, tracer experiment in partially saturated, fractured basalt. A continuous, enhanced conductivity, potassium chloride (KCl) solution was dripped into the fractured basalt and energized directly through a current electrode placed in the conductive solution. The constant concentration, KCl solution was introduced above a perched water table at an average rate of 10.07 L/day under a constant hydraulic head for 76 days. The KCl drip period was followed by a 34-day tap water drip period and a 62-day drainage period. Hole-surface CBP measurements were taken over time to delineate the evolution of the asymmetrical, vadose zone, plume. A 15 by 15 grid of land surface based, porous pot electrodes (copper sulfate), located symmetrically about the centrally located injection borehole, was used for the hole-surface CBP experiment. Ratios of electrical potentials measured at the land surface over time were contoured and profiled to delineate the evolution of the electrolyte plume.

Geostatistical based monitoring of soil water NO3- -N: a potential nonpoint source of ground water contamination.

Soil water NO3 −-N concentrations were monitored for an alfalfa-oat-bean rotation and an alfalfa–bean–bean rotation in the Idaho Snake River Plain as part of the USEPA Section 319 National Monitoring Program. This monitoring study was conducted to evaluate potential beneficial impacts of a USDA recommended crop rotation on subsurface NO3 −-N concentrations at a 4.9 hectare (ha) farm test field. Soil water monitoring was conducted in cooperation with the USDA Snake River Plain Water Quality Demonstration Project. Geostatistical and statistical analyses of NO3 −-N data collected from a network of soil water solution samplers (lysimeters) coupled with hydrogeological characterization of the field indicated that alfalfa followed by oats reduced soil water NO3 −-N concentrations at least temporarily compared to alfalfa followed by beans which is the traditional practice in the area. Soil water NO3 −-N sample data showed a unimodal distribution, through the first two months of the split field rotations, that changed to a distinct bimodal distribution three months into the rotations. Development of the bimodal distribution of soil water NO3 −-N appeared to correspond directly to the rotational split of the field. The median soil water NO3 −-N value calculated from the sample data was approximately 50 mg L−1 greater in the field half planted in beans as compared to the field half planted in oats. Geostatistical spatial mapping results using sequential Gaussian simulations (SGS) supported these findings. SGS results suggested that elevated concentrations of NO3 −-N in the soil water were related to both stratigraphic factors as well as the rotational split.

Geostatistical Ground Water Monitoring of a Point Source Plume Entering a Restored Riparian Zone

Abstract concentrations associated with a point-source ground water plume were monitored as the plume entered a restored riparian zone (RRZ) on the University of Idaho campus in Moscow, Idaho. Seasonal data collected for a network of piezometers installed in the RRZ were analyzed geostatistically and then modeled spatially using the conditional simulation method of sequential Gaussian simulation (SGS). SGS was utilized to predict the spatial distribution of ground water concentrations at unsampled locations in the RRZ and to quantify the uncertainty associated with the prediction. Maps prepared using SGS results illustrate the short-scale variability, or patchiness, expected of concentration distributions in a riparian zone. Manipulation of SGS output provided graphical and quantitative estimates of the likelihood of exceeding a specified concentration threshold at a given confidence level for any location within the RRZ. Geostatistical simulation tools for quantifying uncertainty also provide a potential risk assessment methodology for making remediation decisions and reducing remediation costs.

SIMULATION OF ELECTRICAL POTENTIAL DIFFERENCES NEAR A CONTAMINANT PLUME EXCITED BY A POINT SOURCE OF CURRENT

Finite-difference simulations of electrical excitation of conductive contaminant plumes indicated that approximate dimensions of a plume and the approximate location of its center of mass can be derived, under specified circumstances, from the resulting electrical potential fields. Direct electrical excitation of a contaminant plume by a point current source was simulated for homogenous and isotropic conditions as well as in the presence of conductive clay layers and lenses. When a very shallow water table was assumed, changes in the electrical potential field between baseline (preplume) conditions and conditions that included a developing plume graphically formed a difference dipole. Simulations suggested that electrical flow is channeled preferentially through the negative difference pole at the approximate location of the center of mass in a dispersive contaminant plume. Electrical flow was channeled directly through the negative difference pole at the terminal end of a conductive clay lens. Simulations showed that even in the presence of conductive clays, the approximate location of the center of mass of an evolving contaminant plume could be delineated. This illustrates the potential future value of this approach, assuming continued technological advances in the field.