Death Valley Regional Flow System Research Articles

Subsurface microbial communities as a tool for characterizing regional-scale groundwater flow

Subsurface microbial community distribution patterns are influenced by biogeochemical and groundwater fluxes and may inform hydraulic connections along groundwater-flow paths. This study examined the regional-scale microbial community of the Death Valley Regional Flow System and evaluated whether subsurface communities can be used to identify groundwater-flow paths between recharge and discharge areas. Samples were collected from 36 sites in three groundwater basins: Pahute Mesa–Oasis Valley (PMOV), Ash Meadows (AM), and Alkali Flat–Furnace Creek Ranch (AFFCR). Microbial diversity within and between communities varied by location, and communities were separated into two overall groups that affiliated with the AM and PMOV/AFFCR basins. Network analysis revealed patterns between clusters of common microbes that represented groundwaters with similar geochemical conditions and largely corroborated hydraulic connections between recharge and discharge areas. Null model analyses identified deterministic and stochastic ecological processes contributing to microbial community assemblages. Most communities were more different than expected and governed by dispersal limitation, geochemical differences, or undominating processes. However, certain communities from sites located within or near the Nevada National Security Site were more similar than expected and dominated by homogeneous dispersal or selection. Overall, the (dis)similarities between the microbial communities of DVRFS recharge and discharge areas supported previously documented hydraulic connections between: (1) Spring Mountains and Ash Meadows; (2) Frenchman and Yucca Flat and Amargosa Desert; and (3) Amargosa Desert and Death Valley. However, only a portion of the flow path between Pahute Mesa and Oasis Valley could be supported by microbial community analyses, likely due to well-associated artifacts in samples from the two Oasis Valley sites. This study demonstrates the utility of combining microbial data with hydrologic, geologic, and water-chemistry information to comprehensively characterize groundwater systems, highlighting both strengths and limitations of this approach.

Hydrogeology of desert springs in the Panamint Range, California, USA: Geologic controls on the geochemical kinetics, flowpaths, and mean residence times of springs

AbstractOver 180 springs emerge in the Panamint Range near Death Valley National Park, CA, yet, these springs have received very little hydrogeological attention despite their cultural, historical, and ecological importance. Here, we address the following questions: (1) which rock units support groundwater flow to springs in the Panamint Range, (2) what are the geochemical kinetics of these aquifers, and (3) and what are the residence times of these springs? All springs are at least partly supported by recharge in and flow through dolomitic units, namely, the Noonday Dolomite, Kingston Peak Formation, and Johnnie Formation. Thus, the geochemical composition of springs can largely be explained by dedolomitization: the dissolution of dolomite and gypsum with concurrent precipitation of calcite. However, interactions with hydrothermal deposits have likely influenced the geochemical composition of Thorndike Spring, Uppermost Spring, Hanaupah Canyon springs, and Trail Canyon springs. Faults are important controls on spring emergence. Seventeen of twenty‐one sampled springs emerge at faults (13 emerge at low‐angle detachment faults). On the eastern side of the Panamint Range, springs emerge where low‐angle faults intersect nearly vertical Late Proterozoic, Cambrian, and Ordovician sedimentary units. These geologic units are not present on the western side of the Panamint Range. Instead, springs on the west side emerge where low‐angle faults intersect Cenozoic breccias and fanglomerates. Mean residence times of springs range from 33 (±30) to 1,829 (±613) years. A total of 11 springs have relatively short mean residence times less than 500 years, whereas seven springs have mean residence times greater than 1,000 years. We infer that the Panamint Range springs are extremely vulnerable to climate change due to their dependence on local recharge, disconnection from regional groundwater flow (Death Valley Regional Flow System ‐ DVRFS), and relatively short mean residence times as compared with springs that are supported by the DVRFS (e.g., springs in Ash Meadows National Wildlife Refuge). In fact, four springs were not flowing during this campaign, yet they were flowing in the 1990s and 2000s.

Groundwater geochemistry and flow in the Spring Mountains, NV: Implications for the Death Valley Regional Flow System

Groundwater geochemistry and 87Sr/86Sr from spring waters in the Spring Mountains, NV were used to understand how groundwater recharging in the Spring Mountains influences regional groundwater flow in the Death Valley Regional Flow System (DVRFS). Previous researchers hypothesized that groundwater from the Spring Mountains contributes to the DVRFS, an interbasin groundwater flow system (IGF). However, the Spring Mountains are geologically heterogeneous, therefore, a uniform contribution of groundwater flow across the range is unlikely. Here, we use spring water geochemistry, spring water and host rock 87Sr/86Sr, and observations of the surrounding geology to determine what regions of the Spring Mountains are likely to contribute more flow to the DVRFS. Based on geochemical and isotopic evidence, Spring Mountains groundwater can be divided into three flow regions with distinct geochemical signals reflecting silicate or carbonate water-rock interactions and location relative to major thrust faults. The Keystone Thrust flow region (KST) has a 87Sr/86Sr of 0.710–0.711, is recharged by both rain and snow, and weathers both Mesozoic sandstones and Permian and Cambrian carbonate. Central Spring Mountains (CSM) groundwater has a 87Sr/86Sr of ~0.708, is primarily recharged by snow, and weathers Permian and Cambrian limestones and dolostones. The Montgomery Hills (MH) flow region has a 87Sr/86Sr 0.711–0.733, is recharged by both rain and snow, and weathers both Permian and Cambrian carbonates and Precambrian siliciclastics. Groundwater mixing between flow groups occurs along major thrust faults in the study area, as evidenced by 87Sr/86Sr signatures that reflect one flow group and δ18O and δ2H and geochemical signatures that reflect another flow group. The combination of geochemical and isotopic results suggest CSM groundwater is the most likely to reach Death Valley because CSM groundwater composition reflects the geochemical and isotopic signatures of the highly permeable rocks that connect the Spring Mountains to Death Valley. Flow leaving the KST and MH also influences the DVRFS after mixing with CSM groundwater along major thrust faults.

Four Draft Single-Cell Genome Sequences of Novel, Nearly Identical Kiritimatiellaeota Strains Isolated from the Continental Deep Subsurface.

The recently proposed bacterial phylum Kiritimatiellaeota represents a globally distributed monophyletic clade distinct from other members of the Planctomycetes, Verrucomicrobia, and Chlamydiae (PVC) superphylum. Here, we present four phylogenetically distinct single-cell genome sequences from within the Kiritimatiellaeota lineage sampled from deep continental subsurface aquifer fluids of the Death Valley Regional Flow System in the United States.

Evaluating Groundwater Interbasin Flow Using Multiple Models and Multiple Types of Data.

The groundwater interbasin flow, Qy , from the north of Yucca Flat into Yucca Flat simulated using the Death Valley Regional Flow System (DVRFS) model greatly exceeds assessments obtained using other approaches. This study aimed to understand the reasons for the overestimation and to examine whether the Qy estimate can be reduced. The two problems were tackled from the angle of model uncertainty by considering six models revised from the DVRFS model with different recharge components and hydrogeological frameworks. The two problems were also tackled from the angle of parametric uncertainty for each model by first conducting Morris sensitivity analysis to identify important parameters and then conducting Monte Carlo simulations for the important parameters. The uncertainty analysis is general and suitable for tackling similar problems; the Morris sensitivity analysis has been utilized to date in only a limited number of regional groundwater modeling. The simulated Qy values were evaluated by using three kinds of calibration data (i.e., hydraulic head observations, discharge estimates, and constant-head boundary flow estimates). The evaluation results indicate that, within the current DVRFS modeling framework, the Qy estimate can only be reduced to about half of the original estimate without severely deteriorating the goodness-of-fit to the calibration data. The evaluation results also indicate that it is necessary to develop a new hydrogeological framework to produce new flow patterns in the DVRFS model. The issues of hydrogeology and boundary flow are being addressed in a new version of the DVRFS model planned for release by the U.S. Geological Survey.

Knowledge, transparency, and refutability in groundwater models, an example from the Death Valley regional groundwater flow system

This work demonstrates how available knowledge can be used to build more transparent and refutable computer models of groundwater systems. The Death Valley regional groundwater flow system, which surrounds a proposed site for a high level nuclear waste repository of the United States of America, and the Nevada National Security Site (NNSS), where nuclear weapons were tested, is used to explore model adequacy, identify parameters important to (and informed by) observations, and identify existing old and potential new observations important to predictions. Model development is pursued using a set of fundamental questions addressed with carefully designed metrics. Critical methods include using a hydrogeologic model, managing model nonlinearity by designing models that are robust while maintaining realism, using error-based weighting to combine disparate types of data, and identifying important and unimportant parameters and observations and optimizing parameter values with computationally frugal schemes. The frugal schemes employed in this study require relatively few (10–1000s), parallelizable model runs. This is beneficial because models able to approximate the complex site geology defensibly tend to have high computational cost. The issue of model defensibility is particularly important given the contentious political issues involved.

Comment on “An unconfined groundwater model of the Death Valley Regional Flow System and a comparison to its confined predecessor” by R.W.H. Carroll, G.M. Pohll and R.L. Hershey [Journal of Hydrology 373/3–4, pp. 316–328

Summary Carroll et al. (2009) state that the United States Geological Survey (USGS) Death Valley Regional Flow System (DVRFS) model, which is based on MODFLOW, is “conceptually inaccurate in that it models an unconfined aquifer as a confined system and does not simulate unconfined drawdown in transient pumping simulations.” Carroll et al. (2009) claim that “more realistic estimates of water availability” can be produced by a SURFACT-based model of the DVRFS that simulates unconfined groundwater flow and limits withdrawals from wells to avoid excessive drawdown. Differences in results from the original MODFLOW-based model and the SURFACT-based model stem primarily from application by Carroll et al. (2009) of head limits that can also be applied using the existing MODLOW model and not from any substantial difference in the accuracy with which the unconfined aquifer is represented in the two models. In a hypothetical 50-year predictive simulation presented by Carroll et al. (2009) , large differences between the models are shown when simulating pumping from the lower clastic confining unit, where the transmissivity is nearly two orders of magnitude less than in an alluvial aquifer. Yet even for this extreme example, drawdowns and pumping rates from the MODFLOW and SURFACT models are similar when the head-limit capabilities of the MODFLOW MNW Package are applied. These similarities persist despite possible discrepancies between assigned hydraulic properties. The resulting comparison between the MODFLOW and SURFACT models of the DVRFS suggests that approximating the unconfined system in the DVRFS as a constant-saturated-thickness system (called a “confined system” by Carroll et al., 2009 ) performs very well.

A Model‐Averaging Method for Assessing Groundwater Conceptual Model Uncertainty

This study evaluates alternative groundwater models with different recharge and geologic components at the northern Yucca Flat area of the Death Valley Regional Flow System (DVRFS), USA. Recharge over the DVRFS has been estimated using five methods, and five geological interpretations are available at the northern Yucca Flat area. Combining the recharge and geological components together with additional modeling components that represent other hydrogeological conditions yields a total of 25 groundwater flow models. As all the models are plausible given available data and information, evaluating model uncertainty becomes inevitable. On the other hand, hydraulic parameters (e.g., hydraulic conductivity) are uncertain in each model, giving rise to parametric uncertainty. Propagation of the uncertainty in the models and model parameters through groundwater modeling causes predictive uncertainty in model predictions (e.g., hydraulic head and flow). Parametric uncertainty within each model is assessed using Monte Carlo simulation, and model uncertainty is evaluated using the model averaging method. Two model-averaging techniques (on the basis of information criteria and GLUE) are discussed. This study shows that contribution of model uncertainty to predictive uncertainty is significantly larger than that of parametric uncertainty. For the recharge and geological components, uncertainty in the geological interpretations has more significant effect on model predictions than uncertainty in the recharge estimates. In addition, weighted residuals vary more for the different geological models than for different recharge models. Most of the calibrated observations are not important for discriminating between the alternative models, because their weighted residuals vary only slightly from one model to another.

An unconfined groundwater model of the Death Valley Regional Flow System and a comparison to its confined predecessor

Summary The MODFLOW version of the United States Geological Survey (USGS) Death Valley Regional Flow System (DVRFS) in California and Nevada is conceptually inaccurate in that it models an unconfined aquifer as a confined system and does not accurately simulate unconfined drawdown in transient pumping simulations. The transfer of geologic and hydrologic information from the confined MODFLOW DVRFS model to an unconfined MODFLOW–SURFACT (SURFACT) version was accomplished by maintaining cell structure between models and computing effective cell properties to translate the HUF2 package used in MODFLOW to the BCF4 package used by SURFACT. The confined version of the DVRFS was compared to the unconfined SURFACT version by examining head contour maps and the ability of the SURFACT model to match the 4900 observations of hydraulic head/drawdown, 49 observations of groundwater discharge, and 15 estimates of groundwater fluxes into/out of the model domain. Resultant weighted root mean squared error (ωRMSE) for the unconfined SURFACT model was lower than the USGS confined model. Despite a lower ωRMSE, unconfined conditions simulated with SURFACT did produce greater heads in mountainous regions compared to the confined MODFLOW with differences most pronounced in regions where cell thickness is large, horizontal conductivity small and recharge large. Difference in computed heads reflects computation schemes employed by both models to estimate interblock conductance. Specifically, interblock conductance for the unconfined SURFACT model is dependent on the relative saturation of a modeled cell while MODFLOW’s confined system is not. Despite head differences, SURFACT simulates comparable flux estimates to MODFLOW (e.g. observed ET, groundwater spring flow, and groundwater flux across model boundaries), while significantly improving transient well drawdown estimates. SURFACT is also capable of producing more realistic estimates of water availability from proposed groundwater development and resultant potential impacts to the region.

Expert elicitation of recharge model probabilities for the Death Valley regional flow system

Summary This study uses expert elicitation to evaluate and select five alternative recharge models developed for the Death Valley regional flow system (DVRFS), covering southeast Nevada and the Death Valley area of California, USA. The five models were developed based on three independent techniques: an empirical approach, an approach based on unsaturated-zone studies and an approach based on saturated-zone studies. It is uncertain which recharge model (or models) should be used as input for groundwater models simulating flow and contaminant transport within the DVRFS. An expert elicitation was used to evaluate and select the recharge models and to determine prior model probabilities used for assessing model uncertainty. The probabilities were aggregated using simple averaging and iterative methods, with the latter method also considering between-expert variability. The most favorable model, on average, is the most complicated model that comprehensively incorporates processes controlling net infiltration and potential recharge. The simplest model, and the most widely used, received the second highest prior probability. The aggregated prior probabilities are close to the neutral choice that treats the five models as equally likely. Thus, there is no support for selecting a single model and discarding others, based on prior information and expert judgment. This reflects the inherent uncertainty in the recharge models. If a set of prior probability from a single expert is of more interest, we suggest selecting the set of the minimum Shannon’s entropy. The minimum entropy implies the smallest amount of uncertainty and the largest amount of information used to evaluate the models. However, when enough data are available, we prefer to use a cross-validation method to select the best set of prior model probabilities that gives the best predictive performance.

A method for evaluating the importance of system state observations to model predictions, with application to the Death Valley regional groundwater flow system

We develop a new observation‐prediction (OPR) statistic for evaluating the importance of system state observations to model predictions. The OPR statistic measures the change in prediction uncertainty produced when an observation is added to or removed from an existing monitoring network, and it can be used to guide refinement and enhancement of the network. Prediction uncertainty is approximated using a first‐order second‐moment method. We apply the OPR statistic to a model of the Death Valley regional groundwater flow system (DVRFS) to evaluate the importance of existing and potential hydraulic head observations to predicted advective transport paths in the saturated zone underlying Yucca Mountain and underground testing areas on the Nevada Test Site. Important existing observations tend to be far from the predicted paths, and many unimportant observations are in areas of high observation density. These results can be used to select locations at which increased observation accuracy would be beneficial and locations that could be removed from the network. Important potential observations are mostly in areas of high hydraulic gradient far from the paths. Results for both existing and potential observations are related to the flow system dynamics and coarse parameter zonation in the DVRFS model. If system properties in different locations are as similar as the zonation assumes, then the OPR results illustrate a data collection opportunity whereby observations in distant, high‐gradient areas can provide information about properties in flatter‐gradient areas near the paths. If this similarity is suspect, then the analysis produces a different type of data collection opportunity involving testing of model assumptions critical to the OPR results.

A chlorine-36 study of regional groundwater flow and vertical transport in southern Nevada

Chlorine-36 data for groundwater from the Death Valley regional flow system is interpreted in the context of existing conceptual models for regional groundwater flow in southern Nevada. Chlorine-36 end member compositions are defined for both recharge and chemically evolved groundwater components. The geochemical evolution of 36Cl is strongly controlled by water–rock interaction with Paleozoic carbonate rocks that comprise the regional aquifer system, resulting in chemically evolved groundwater that is characteristically low in 36Cl/Cl and high in Cl. Groundwater from alluvial and volcanic aquifers that overlie the regional carbonate aquifer are generally characterized by high 36Cl/Cl and low Cl signatures, and are chemically distinct from water in the regional carbonate aquifer. This difference provides a means of examining vertical transport and groundwater mixing processes. In combination with other geochemical and hydrogeologic data, the end members defined here provide constraints on aquifer residence times and mixing ratios.

Methods for using groundwater model predictions to guide hydrogeologic data collection, with application to the Death Valley regional groundwater flow system

Calibrated models of groundwater systems can provide substantial information for guiding data collection. This work considers using such models to guide hydrogeologic data collection for improving model predictions by identifying model parameters that are most important to the predictions. Identification of these important parameters can help guide collection of field data about parameter values and associated flow system features and can lead to improved predictions. Methods for identifying parameters important to predictions include prediction scaled sensitivities (PSS), which account for uncertainty on individual parameters as well as prediction sensitivity to parameters, and a new “value of improved information” (VOII) method presented here, which includes the effects of parameter correlation in addition to individual parameter uncertainty and prediction sensitivity. In this work, the PSS and VOII methods are demonstrated and evaluated using a model of the Death Valley regional groundwater flow system. The predictions of interest are advective transport paths originating at sites of past underground nuclear testing. Results show that for two paths evaluated the most important parameters include a subset of five or six of the 23 defined model parameters. Some of the parameters identified as most important are associated with flow system attributes that do not lie in the immediate vicinity of the paths. Results also indicate that the PSS and VOII methods can identify different important parameters. Because the methods emphasize somewhat different criteria for parameter importance, it is suggested that parameters identified by both methods be carefully considered in subsequent data collection efforts aimed at improving model predictions.

234U/ 238U evidence for local recharge and patterns of ground-water flow in the vicinity of Yucca Mountain, Nevada, USA

Uranium concentrations and 234U/ 238U ratios in saturated-zone and perched ground water were used to investigate hydrologic flow and downgradient dilution and dispersion in the vicinity of Yucca Mountain, a potential high-level radioactive waste disposal site. The U data were obtained by thermal ionization mass spectrometry on more than 280 samples from the Death Valley regional flow system. Large variations in both U concentrations (commonly 0.6–10 μg l −1) and 234U/ 238U activity ratios (commonly 1.5–6) are present on both local and regional scales; however, ground water with 234U/ 238U activity ratios from 7 up to 8.06 is restricted largely to samples from Yucca Mountain. Data from ground water in the Tertiary volcanic and Quaternary alluvial aquifers at and adjacent to Yucca Mountain plot in 3 distinct fields of reciprocal U concentration versus 234U/ 238U activity ratio correlated to different geographic areas. Ground water to the west of Yucca Mountain has large U concentrations and moderate 234U/ 238U whereas ground water to the east in the Fortymile flow system has similar 234U/ 238U, but distinctly smaller U concentrations. Ground water beneath the central part of Yucca Mountain has intermediate U concentrations but distinctive 234U/ 238U activity ratios of about 7–8. Perched water from the lower part of the unsaturated zone at Yucca Mountain has similarly large values of 234U/ 238U. These U data imply that the Tertiary volcanic aquifer beneath the central part of Yucca Mountain is isolated from north-south regional flow. The similarity of 234U/ 238U in both saturated- and unsaturated-zone ground water at Yucca Mountain further indicates that saturated-zone ground water beneath Yucca Mountain is dominated by local recharge rather than regional flow. The distinctive 234U/ 238U signatures also provide a natural tracer of downgradient flow. Elevated 234U/ 238U in ground water from two water-supply wells east of Yucca Mountain are interpreted as the result of induced flow from 40 a of ground-water withdrawal. Elevated 234U/ 238U in a borehole south of Yucca Mountain is interpreted as evidence that natural downgradient flow is more likely to follow southerly paths in the structurally anisotropic Tertiary volcanic aquifer where it becomes diluted by regional flow in the Fortymile system.