Potential Flow Paths Research Articles

Regional groundwater flow in structurally-complex extended terranes: An evaluation of the sources of discharge at Ash Meadows, Nevada

Summary Ash Meadows, Nevada, USA is a site of major groundwater discharge (∼38,000 L/min) in the arid Mojave Desert, and hosts a number of endemic and threatened wetland species. In addition to these resources, Ash Meadows may also represent the future discharge location of radionuclide-laden waters from nuclear weapons testing at the Nevada Test Site. More importantly, however, Ash Meadows provides the opportunity to understand the controls on water transfer between basins through fractured bedrock. 4000+ solute analyses were assembled from the literature into a single database. The data were screened for spatial distribution, completeness, charge balance, and elevated temperatures (⩾20 °C and within regional flow systems), with 246 candidate up-gradient water remaining distributed among six potential source areas in addition to and Ash Meadows itself. These potential sources include both carbonate, volcanic and perhaps valley-fill aquifer systems. These waters were characterized by cluster analysis in order to sort similar waters in an objective fashion into potential flow paths and to establish representative endmember waters for inverse geochemical models and other modes of analysis. Isotopic tracers, both conservative and those reflecting water–rock interaction, all suggest that waters at Ash Meadows are derived by southward flow from volcanic terranes, parallel to the preferred permeability structure induced by active regional east–west extension. Solute balances support this conclusion. However, this runs counter to the prevailing model that waters at Ash Meadows are derived from easterly and northeasterly flows from the Spring Mountains and Pahranagat Valley areas by interbasin flow through a continuous fractured carbonate aquifer. This work suggests that carbonate aquifer systems in extended terranes are more compartmentalized than previously appreciated and that anisotropy in fracture permeability is key to compartmentalization and the control of flow directions.

Simulation and validation of concentrated subsurface lateral flow paths in an agricultural landscape

Abstract. The importance of soil water flow paths to the transport of nutrients and contaminants has long been recognized. However, effective means of detecting concentrated subsurface flow paths in a large landscape are still lacking. The flow direction and accumulation algorithm based on single-direction flow algorithm (D8) in GIS hydrologic modeling is a cost-effective way to simulate potential concentrated flow paths over a large area once relevant data are collected. This study tested the D8 algorithm for simulating concentrated lateral flow paths at three interfaces in soil profiles in a 19.5-ha agricultural landscape in central Pennsylvania, USA. These interfaces were (1) the interface between surface plowed layers of Ap1 and Ap2 horizons, (2) the interface with subsoil water-restricting clay layer where clay content increased to over 40%, and (3) the soil-bedrock interface. The simulated flow paths were validated through soil hydrologic monitoring, geophysical surveys, and observable soil morphological features. The results confirmed that concentrated subsurface lateral flow occurred at the interfaces with the clay layer and the underlying bedrock. At these two interfaces, the soils on the simulated flow paths were closer to saturation and showed more temporally unstable moisture dynamics than those off the simulated flow paths. Apparent electrical conductivity in the soil on the simulated flow paths was elevated and temporally unstable as compared to those outside the simulated paths. The soil cores collected from the simulated flow paths showed significantly higher Mn content at these interfaces than those away from the simulated paths. These results suggest that (1) the D8 algorithm is useful in simulating possible concentrated subsurface lateral flow paths if used with appropriate threshold value of contributing area and sufficiently detailed digital elevation model (DEM); (2) repeated electromagnetic surveys can reflect the temporal change of soil water storage and thus is a useful indicator of possible subsurface flow path over a large area; and (3) observable Mn distribution in soil profiles can be used as a simple indicator of water flow paths in soils and over the landscape; however, it does require sufficient soil sampling (by excavation or augering) to possibly infer landscape-scale subsurface flow paths. In areas where subsurface interface topography varies similarly with surface topography, surface DEM can be used to simulate potential subsurface lateral flow path reasonably so the cost associated with obtaining depth to subsurface water-restricting layer can be minimized.

Detection and Characterization of Preferential Flow Paths in the Downstream Area of a Hazardous Landfill

We have used surface-based electrical resistivity tomography to detect and characterize preferential hydraulic pathways in the immediate downstream area of an abandoned, hazardous landfill. The landfill occupies the void left by a former gravel pit and its base is close to the groundwater table and lacking an engineered barrier. As such, this site is remarkably typical of many small- to medium-sized waste deposits throughout the densely populated and heavily industrialized foreland on both sides of the Alpine arc. Outflows of pollutants lastingly contaminated local drinking water supplies and necessitated a partial remediation in the form of a synthetic cover barrier, which is meant to prevent meteoric water from percolating through the waste before reaching the groundwater table. Any future additional isolation of the landfill in the form of lateral barriers thus requires adequate knowledge of potential preferential hydraulic pathways for outflowing contaminants. Our results, inferred from a suite of tomographically inverted surfaced-based electrical resistivity profiles oriented roughly perpendicular to the local hydraulic gradient, indicate that potential contaminant outflows would predominantly occur along an unexploited lateral extension of the original gravel deposit. This finds its expression as a distinct and laterally continuous high-resistivity anomaly in the resistivity tomograms. This interpretation is ground-truthed through a litholog from a nearby well. Since the probed glaciofluvial deposits are largely devoid of mineralogical clay, the geometry of hydraulic and electrical pathways across the pore space of a given lithological unit can be assumed to be identical, which allows for an order-of-magnitude estimation of the overall permeability structure. These estimates indicate that the permeability of the imaged extension of the gravel body is at least two to three orders-of-magnitude higher than that of its finer-grained embedding matrix. This corroborates the preeminent role of the high-resistivity anomaly as a potential preferential flow path.

Integration of hydrological and ecological modelling for the assessment of a nuclear waste repository

The Swedish Nuclear Fuel and Waste Management Co. (SKB) is currently investigating two sites in Sweden as potential locations for underground disposal of spent nuclear fuel. The site investigations cover geology, chemistry, hydrology and ecology, among several other disciplines. Strong emphasis is put on the characterisation of properties and processes affecting the water-born transport and retention of radionuclides along potential flow paths from repository depth (ca. 500 m below ground) to ground surface, and the dispersal and accumulation of the radionuclides in the surface system. The modelling performed to support the site descriptions and the safety assessments utilises a wide range of hydrological modelling tools. These include tools primarily developed for modelling groundwater flow in fractured rock, and modelling tools that handle surface and subsurface flows, water uptake in vegetation and the interactions with the atmosphere. The couplings between the hydrogeology, the near surface hydrology and the surface ecosystems are especially important in this integrated modelling. This paper highlights some ecohydrological aspects of this coupled modelling and provides example results from site descriptive and safety assessment modelling.

Integration of local–global upscaling and grid adaptivity for simulation of subsurface flow in heterogeneous formations

We propose a methodology, called multilevel local–global (MLLG) upscaling, for generating accurate upscaled models of permeabilities or transmissibilities for flow simulation on adapted grids in heterogeneous subsurface formations. The method generates an initial adapted grid based on the given fine-scale reservoir heterogeneity and potential flow paths. It then applies local–global (LG) upscaling for permeability or transmissibility [7], along with adaptivity, in an iterative manner. In each iteration of MLLG, the grid can be adapted where needed to reduce flow solver and upscaling errors. The adaptivity is controlled with a flow-based indicator. The iterative process is continued until consistency between the global solve on the adapted grid and the local solves is obtained. While each application of LG upscaling is also an iterative process, this inner iteration generally takes only one or two iterations to converge. Furthermore, the number of outer iterations is bounded above, and hence, the computational costs of this approach are low. We design a new flow-based weighting of transmissibility values in LG upscaling that significantly improves the accuracy of LG and MLLG over traditional local transmissibility calculations. For highly heterogeneous (e.g., channelized) systems, the integration of grid adaptivity and LG upscaling is shown to consistently provide more accurate coarse-scale models for global flow, relative to reference fine-scale results, than do existing upscaling techniques applied to uniform grids of similar densities. Another attractive property of the integration of upscaling and adaptivity is that process dependency is strongly reduced, that is, the approach computes accurate global flow results also for flows driven by boundary conditions different from the generic boundary conditions used to compute the upscaled parameters. The method is demonstrated on Cartesian cell-based anisotropic refinement (CCAR) grids, but it can be applied to other adaptation strategies for structured grids and extended to unstructured grids.

Hydrogeochemical processes, mixing and isotope tracing in hard rock aquifers and surface waters from the Subarnarekha River Basin, (east Singhbhum District, Jharkhand State, India)

Geochemical observations, including major ion and trace element analysis, and isotopic tracing have been carried out in the Subarnarekha River system (northeastern India) during a surface-water- and groundwater-monitoring program aimed at evaluating impacts of mining. The aquifer is of fracture type. Groundwater flow conditions and pollutant transfer were observed through a network of 69 wells. δ18O and δ2H results suggest that transfer from rainfall towards groundwater storage through soils and the unsaturated zone is fast, without any major transformation like evaporation. The scatter of 87Sr/86Sr signatures in surface water and groundwater are explained by three end-members. One is compatible with rainwater inputs. The most mineralised end-member represents anthropogenic inputs (agricultural practices and ore processing). The third end-member, characterised by a high 87Sr/86Sr signature, is believed to be controlled by natural geochemical processes, although affected by human activities (e.g. drainage of mine waste). Potential flow paths, investigated north of the area, reveal that all groundwater types seem to evolve more in pockets than along a flow path. The limited extent of transfer and the predominance of natural phenomena help to explain the moderate level of groundwater contamination and the characteristics of surface water contamination by mining and the metallurgy industry.

Analysis of Water Flow Paths: Methodology and Example Calculations for a Potential Geological Repository in Sweden

Safety assessment related to the siting of a geological repository for spent nuclear fuel deep in the bedrock requires identification of potential flow paths and the associated travel times for radionuclides originating at repository depth. Using the Laxemar candidate site in Sweden as a case study, this paper describes modeling methodology, data integration, and the resulting water flow models, focusing on the Quaternary deposits and the upper 150 m of the bedrock. Example simulations identify flow paths to groundwater discharge areas and flow paths in the surface system. The majority of the simulated groundwater flow paths end up in the main surface waters and along the coastline, even though the particles used to trace the flow paths are introduced with a uniform spatial distribution at a relatively shallow depth. The calculated groundwater travel time, determining the time available for decay and retention of radionuclides, is on average longer to the coastal bays than to other biosphere objects at the site. Further, it is demonstrated how GIS-based modeling can be used to limit the number of surface flow paths that need to be characterized for safety assessment. Based on the results, the paper discusses an approach for coupling the present models to a model for groundwater flow in the deep bedrock.

Geochemical interpretation of groundwater flow in the southern Great Basin

The study of geochemical processes and integrated water flow can help identify groundwater sources and improve predictions of contaminant fate and transport in groundwater systems. Understanding groundwater flow paths in and around the Nevada Test Site (NTS) is important due to the possible migration of contaminated groundwater to the neighboring communities. A total of 118 groundwater samples from the NTS and surrounding area (e.g., Oasis Valley, Ash Meadows, Death Valley, the Spring Mountains, and Pahranagat Valley) were collected and analyzed for trace elements and major solutes. Cluster analysis and principal component analysis (PCA), along with geographical information systems (GIS), were used to interpret the resulting hydrogeochemical data. Cluster analysis was used to group the ground-water samples into four major clusters. PCA was used to reduce the data into three components that describe differences in ionic strength, groundwater compositions, reflecting interaction with volcanic and/or carbonate rock aquifers, and redox characteristics. Twelve potential flow paths, characteristic of those reported in earlier studies, were identified.

Hydrogeology of Palm Valley, central Australia; a Pleistocene flora refuge?

The Palm Valley Oasis (Finke Gorge National Park) in arid central Australia is characterised by large stands of red cabbage palm trees ( Livistona mariae). How these unique plants, over 1000 km away from nearest relatives in the tropical parts of northern Australia persist, has long fascinated visitors. The hydrogeology of this area helps explain this phenomenon. Stable isotope (δ 2H, δ 8O) analyses shows groundwater to have a uniform composition that plots on or near a local meteoric water line. Carbon-14 results are observed to vary throughout this aquifer from effectively dead (<4%) to 87% modern carbon. Ratios of chlorine-36 to chloride range from 130 to 290×10 −15 36Cl/Cl. In this region atmospheric 36Cl/Cl ratio is around 300×10 −15. Thus an age range of around 300 ka is indicated if, as is apparent radioactive decay is the only significant cause of 36Cl/Cl variation within the aquifer. The classic homogenous aquifer with varying surface topography flow model is the simplest conceptual model that need be invoked to explain these data. Complexities, associated with local topography flow cells superimposed on the regional gradient, may mean groundwater with markedly different flow path lengths has been sampled. This potential flow path complexity, which is also evidenced by slight variation in groundwater cation ratios, can account for the distribution of isotope age data throughout the aquifer. Given the likely very slow travel times indicated by this aquifer's hydraulic properties, age differences of the magnitude indicated from chlorine-36 data are feasible. The likely slow travel times (>100 ka) along some flow paths indicate groundwater discharge would endure through arid phases associated with Quaternary climate oscillations. Such a flow system can explain the persistence of this population of Palms and also highlight the possibility that Palm Valley has acted as a flora refuge since at least the mid Pleistocene.

Sorption-desorption behavior of strontium-85 onto montmorillonite and silica colloids

Strontium-90 ( 90Sr) is one of the major radioactive contaminants found in DP Canyon at Los Alamos, New Mexico, USA. Radioactive surveys found that 90Sr is present in surface water and shallow alluvial groundwater environments in Los Alamos National laboratory (LANL). Colloids may influence the transport of this radionuclide in surface and groundwater environments in LANL. In this study, the authors investigated the sorption/desorption behavior of radioactive Sr on Ca-montmorillonite and silica colloids, and the effect of ionic strength of water on the sorption of Sr. Laboratory batch sorption experiments were conducted using 85Sr as a surrogate for 90Sr. Groundwater, collected from Well LAUZ-1 at DP Canyon and from Well J-13 at Yucca Mountain, Nevada, and deionized water, were used. The results show that 92–100% of the 85Sr was rapidly adsorbed onto Ca-montmorillonite colloids in all three waters. Adsorption of 85Sr onto silica colloids varied among the three waters. The ionic strength and Ca 2+ concentration in groundwater significantly influence the adsorption of 85Sr onto silica colloids. Desorption of 85Sr from Ca-montmorillonite colloids is slower than from silica colloids. Desorption of 85Sr from silica colloids was faster in LAUZ-1 groundwater than in J-13 groundwater and deionized water. The results suggest that clay and silica colloids may facilitate the transport of Sr along potential flowpaths from DP Canyon to Los Alamos Canyon.

On Fracture Structure and Preferential Flow in Unsaturated Chalk

AbstractThe mechanisms controlling fluid flow through fractures intersecting chalk in the vadose zone were studied through water percolation experiments in natural discrete fractures and by close examination of the inner Structure of fracture voids. The percolation experiments showed that the flow is focused in dissolution channels along the fracture plane, and that fluxes and flow trajectories within that net vary in both time and space. The locations of the dissolution channels, the main potential flow paths within the fracture plane, were generally associated with fracture Intersections. The flow through these channels was governed primarily by the mineralogical composition of the filling material and the inner structure of the fracture voids. Salt dissolution, solid‐particle migration, and clay swelling were found to be the predominant processes controlling flow through the dissolution channels. These physical changes in the structure of the filling material in the dissolution channels accounted for the observed unstable flow behavior. Our results suggest that models aimed at simulating water percolation through fractures in unsaturated chalk should consider the mapping of fracture intersections in addition to the commonly used mapping of fracture lineaments. Moreover, the detailed characterization of fracture apertures may not be the key parameter determining fracture flow, because in such formations the flow is controlled primarily by filling material. These materials undergo significant physical variations during wetting and drying cycles.

In situ pneumatic testing at Yucca Mountain

The Yucca Mountain site in Nevada is a potential site for a geologic high-level nuclear waste repository. The proposed repository location is in the unsaturated zone in the mountain. Fluid flow through the fractured tuff repository rock is the potential transport mechanism for radionuclides from nuclear waste to the environment. This flow is believed to be predominantly confined to the fractures, which means that an understanding of how fracture systems transport fluids is essential to determining how fast and in what quantities fluids will travel through the rock. As part of an ambient testing program for the site characterization project at Yucca Mountain, the unsaturated zone is under investigation to examine the fluid flow and transport characteristics of the fractured tuff. The size and number of connections between fractures are characteristics that affect flow and how flow can travel in networks of fractures. To determine these potential flow paths in the fracture system, site investigators use automatically controlled packer assemblies to seal clusters of boreholes drilled into the rock, enabling injection of air into specific locations while monitoring pressure responses at other locations. Tests have been performed both before and after major excavations near the test locations, enabling study of the mechanical effects of construction on local permeability. To date, an estimated 3500 separate air injections have been undertaken and nearly a quarter of a million pressure-response curves have been logged in the study. The number of tests lends itself to visualization and statistical examination of the flow connections and the distributions of permeability in the rock mass.

Southern hemisphere Miocene bottom-water circulation: a palaeobathymetric analysis

Abstract Palaeobathymetric reconstructions of the southern hemisphere are used to assess the potential flow paths and extent of proto-Antarctic Bottom Water (proto-AABW) for three time slices during the Miocene: 20 Ma (early), 15 Ma (middle) and 10 Ma (late). The depth ranges of fluctuations in this water mass were derived from its waxing/waning curve, based on an analysis of hiatuses within drill sites from the Indian Ocean. Proto-AABW was likely to be expressed at depths below −4400 m, −5500 m and −4700 m at 20 Ma, 15 Ma and 10 Ma respectively. These depth ranges were extended into the South Atlantic and South Pacific Oceans to allow a first approximation of potential proto-AABW flow paths. An understanding of modem oceanography and the distribution of suspended sediment particle concentrations within the nepheloid layer and sediment drifts were used to constrain flow paths. This interpretation was made on the basis that proto-Antarctic Bottom Water was an analogue of it modern equivalent and was likely to have followed similar flow paths. The palaeobathymetric analysis reveals that at 20 Ma (early Miocene) the deep basins of the southern hemisphere were well connected and proto-AABW flow paths were likely to have been widespread. By 15 Ma (middle Miocene), however, the waning of proto-AABW led to the disconnection of the deep ocean basins and this water mass was probably restricted to the Antarctic Basin. Waxing of proto-AABW during the late Miocene allowed the reconnection of the deep basins but not to the same extent as at 20 Ma (early Miocene).