Characterization of a TCE-Contaminated Aquifer Using Tritium-Helium Ages and Geochemical Tracers
A deltaic sand aquifer is contaminated with trichloroethylene (TCE). A numerical groundwater model is being developed to better define dissolved TCE transport and guide the implementation of remediation strategies. A geochemical characterization was carried out to further contrain the model with independent data. Tritium- helium (3H-3He) dating is a reliable method of determining groundwater residence times in shallow groundwater systems, which can be directly compared with simulated model ages [1, 2]. Changes in groundwater chemistry provide a way to trace dissolved TCE to its origin and identify groundwater flow paths . In the studied aquifer, groundwater samples were obtained along flowlines originating from two distinct TCE source zones, which converge to form a single plume. Isotopic ages were compared to simulated ages using a range of values for porosity to determine the best match. In certain locations, anomalously old 3H-3He ages with high concentrations of terrigenic helium indicate areas where groundwater from the underlying proglacial unit flows upward into the deltaic sand aquifer through aquitard windows. The upflow locations correspond with increased TCE concentrations, suggesting significant TCE provenance from the proglacial unit and a potential third source of TCE contributing to the plume. Geochemical signatures associated with each TCE source zone were identified by examining spatial patterns in groundwater chemistry. Geochemical tracers such as dissolved ions, stable isotopes and other parameters were used to distinguish the relative TCE contribution from each source zone, and support the possibility of a third TCE source. This original approach of characterizing groundwater flow and contaminant history can lead to more reprensentative numerical models of groundwater flow and contaminant transport.  Solomon et al. (1992) Wat. Resour. Res. 28, 741-755.  Dunkle-Shapiro et al. (1998) Wat. Resour. Res. 34, 1165-1180.  Glynn & Plummer (2005) Hydrogeol. J. 13, 263-287.
A Single Well Tracer Test for Aquifer Characterization
Effective and efficient contaminated site remediation requires site-specific knowledge of physical, chemical and biological properties of the aquifer (e.g., hydraulic conductivity, porosity, ion exchange capacity, redox capacity, and biodegradation potential). Aquifer property measurement techniques for groundwater transport and reactions are too costly or not-representative of in situ conditions and therefore there is an over-reliance on literature values or model assumptions. This results in overly uncertain predictions of in situ performance and therefore unnecessarily cautious risk assessment and costly remediation strategies. Therefore, cost-effective site investigative tools that have the capability of producing high quality characterization data are required. A single well tracer test called the dipole flow and reactive tracer test (DFRTT) is proposed as an alternative to current parameter estimation methods. This test circulates groundwater between isolated injection (source) and extraction (sink) chambers within a single well. Once steady-state flow has been reached, conservative and reactive tracers are added to the injected solution and the concentration of the tracers and their reaction products can be monitored in the extracted solution. These tracer breakthrough curves are analyzed by a an interpretation model to obtain parameter estimates. A dipole probe prototype has been constructed at the University of Waterloo and more than 50 field tests have been conducted in the unconfined sand aquifer at CFB Borden near Alliston, ON. Four characteristic type or response curves were observed. The DFRTT showed good repeatability between tests and captures both the response of the disturbed zone and formation. The DFRTT response profiles were determined to be scalable to some of the key system design parameters. Estimates of the hydraulic conductivity of the aquifer were within literature values, but variability at the sensing scale of the dipole tool was observed. This presentation focuses on the tracer data collected to date, model interpretation efforts, and outstanding issues that need to be addressed for this technology to be used widely.
Response of Alum Rock Springs to the October 30, 2007 Alum Rock Earthquake and Implications for the Origin of Increased Discharge after Earthquakes
The origin of increased stream flow and spring discharge following earthquakes have been the subject of controversy, in large part because there are many models to explain observations and few measurements suitable for distinguishing between hypotheses. On October 30, 2007 a magnitude 5.5 earthquake occurred near the Alum Rock springs, California, USA. Within a day we documented a several-fold increase in discharge. Over the following year, we have monitored a gradual return towards pre-earthquake properties, but for the largest springs there appears to be a permanent increase in the steady discharge at all the springs. The Alum Rock springs discharge waters that represent a mixture between modern ("shallow") meteoric water and old ("deep") connate waters expelled by regional transpression. After the earthquake, the increased discharge at the largest springs was accompanied by a small decrease in the fraction of connate water in the spring discharge. Combined with the rapid response, this implies that the increased discharge has a shallow origin. Increased discharge at these springs occurs for earthquakes that cause static volumetric expansion and those that cause contraction, supporting models in which dynamic strains are responsible for the subsurface changes that cause flow to increase. We show that models in which the permeability of the fracture system feeding the springs increases after the earthquake are in general consistent with the changes in discharge. The response of these springs to another earthquake will provide critical constraints on the changes that occur in the subsurface.
Field Studies and Models of Hydrofracture Propagation in Layered Fractured Rocks
Hydrofractures are fractures generated at least partly by internal fluid pressure, including mineral veins and many joints. Together with shear fractures, hydrofractures contribute significantly to the permeability of fractured rocks. Here, field measurements of mineral veins and joints in sedimentary rocks are presented and compared with the results of analytical and numerical models on the propagation and aperture variation of hydrofractures in layered fractured rocks. The focus is on the effects of mechanical layering, primarily the stiffness (Young's modulus) of the host rock, on the emplacement mechanisms and geometries of hydrofractures. The field studies were carried out at four localities of different lithologies: (1) mudstones (Upper Triassic) with gypsum veins; (2) limestone and marl (Lower Jurassic) with calcite veins; (3) jointed sandstones (Lower Triassic) and jointed limestones (Middle Triassic) in England and Germany. In all study areas mineral veins occur almost exclusively in the cores and damage zones of faults, indicating paleofluidtransport along the faults. Analytical models indicate that hydrofractures with any significant overpressures generate high theoretical crack-tip tensile stresses. Therefore, in homogeneous, isotropic rocks, buoyant hydrofractures should normally propagate to the surface. However, field observations show that in heterogeneous, anisotropic rocks, most hydrofractures do not reach the surface but rather become arrested. Hydrofracture arrest is primarily controlled by local variations in the stress field, mainly due to three factors: discontinuities, changes in host rock mechanical properties and stress barriers. These factors are related in that changes in stiffness and stress barriers are common at contacts (discontinuities) between different rock types. Numerical models indicate that, for fluid overpressure as the only loading, soft layers suppress tensile stresses and blunt hydrofracture tips, thereby favouring their arrest, whereas stiff layers concentrate tensile stresses, sharpen hydrofracture tips and ease their propagation. By contrast, when the rock is subject to compression, stiff layers concentrate compressive stresses and may act as stress barriers. In a rock where most hydrofractures become stratabound (layerbound), it is less likely than in one with non- stratabound hydrofractures that interconnected fracture systems can develop. Thus, rocks with mostly arrested and stratabound hydrofractures may have a difficulty in reaching the percolation threshold needed for significant permeability.
Numerical and Experimental Modeling of Multiple Fluid Flows in a Rough-Walled Fracture in Sandstone
Within many low-permeability rocks, fractures exist that can act as natural fluid conduits. Understanding how multiple fluids move within natural rock fractures is important for the prediction of fluid transport within many underground locations. Our study examined experimentally and numerically the motion of immiscible fluids as they were transported through models of a natural, rough-walled fracture in Berea sandstone. The natural fracture geometry was initially scanned using micro-computerized tomography (CT) at a fine volume-pixel resolution. This CT scanned fracture was converted into a numerical mesh for two-phase flow calculations using the commercial finite finite-volume CFD solver FLUENT and the volume-of-fluid method. Additionally, a translucent experimental model was constructed using stereolithography. The numerical model was shown to agree well with experiments for the case of a constant rate injection of air into the initially water-saturated fracture. The invading air moved intermittently, quickly invading large-aperture regions of the fracture. Relative permeability curves were developed to describe the fluid motion. The fluid properties were than varied in the CFD model to show the variation in fluid motion due to different viscosity fluids and different fluid-solid interactions. Low interfacial tensions and low contact angle were shown to change the observed flow patterns significantly, with more of the fracture volume filled by the invading fluid.
Modeling flow through the sand pack: implications for groundwater sampling from multi- level monitoring wells in fractured bedrock aquifers.
Multi-level piezometers are often used in groundwater studies to monitor multiple zones within a single borehole. In the fractured rock setting the monitoring intervals are typically designed to isolate discrete fracture features (single fractures or fracture zones). This can be very useful for determining vertical connectivity and the distribution of a contaminant within a fractured rock aquifer. A simple and inexpensive method for completing a bedrock borehole as a multi-level piezometer is to use PVC screen and riser, a sand pack around the screened section, and bentonite to isolate each interval. Flow into the borehole is dominantly confined to the intersecting discrete fracture features. The objective of this study is to examine the nature of the flow through the sand pack and screen slots as water travels from the fracture to the pump intake under pumping conditions. Our conceptual model suggests only a portion of the sand pack in the vicinity of the fractures should be hydraulically active in this scenario. Thus, portions of the wellbore may remain stagnant during pumping depending on the location of the pump intake with respect to the fractures. Flow paths in the sand pack may be controlled by the relationship between the transmissivity of the fracture and screen slots. HydroGeoSphere, a numerical model for flow and solute transport in discrete fractures and porous media, will be used to validate the conceptual model and define the head and velocity profiles in a multi-level interval under various pumping rate and discrete fracture aperture scenarios. The results of this study could have implications for defining a "well volume" in sampling protocols designed for multi-level piezometers in bedrock aquifer systems. The results may also be a useful tool for interpreting the significance of the sand pack as a source of bacteria in water quality monitoring studies that use multi-level piezometer construction of this sort.
Hydraulic Testing of Silurian and Ordovician Strata at the Bruce Site
Ontario Power Generation is proposing a Deep Geologic Repository (DGR) for the long-term management of its Low and Intermediate Level Radioactive Waste (L&ILW) within a Paleozoic-age sedimentary sequence beneath the Bruce Site near Tiverton, Ontario, Canada. The concept envisions that the DGR would be excavated at a depth of approximately 680 m within the Ordovician Cobourg Formation, a massive, dense, argillaceous limestone. A key attribute of the Bruce site is the extremely low permeabilities associated with the thick Ordovician carbonate and argillaceous bedrock formations that will host and enclose the DGR. Such rock mass permeabilities are thought sufficiently low to contribute toward or govern a diffusion-dominated transport regime. To support this concept, hydraulic testing was performed in 2008 and 2009 in two deep boreholes at the proposed repository site, DGR-3 and DGR-4. The hydraulic testing was performed using a straddle-packer tool with a 30.74-m test interval. Sequential tests were performed over the entire open lengths of the boreholes from the F Unit of the Silurian Salina Formation into the Ordovician Gull River Formation, a distance of approximately 635 m. The tests consisted primarily of pressure-pulse tests, with a few slug tests performed in several of the higher permeability Silurian units. The tests are analyzed using the nSIGHTS code, which allows the entire pressure history a test interval has experienced since it was penetrated by the drill bit to be included in the test simulation. nSIGHTS also allows the model fit to the test data to be optimized over an n-dimensional parameter space to ensure that the final solution represents a true global minimum rather than simply a local minimum. The test results show that the Ordovician-age strata above the Coboconk Formation (70+ m below the Cobourg) have average horizontal hydraulic conductivities of 1E-13 m/s or less. Coboconk and Gull River hydraulic conductivities are as high as 1E-11 m/s. Hydraulic conductivities of the Silurian-age strata generally range from 1E-13 to 1E-11 m/s, with discrete test values approaching 1E-7 m/s associated with portions of the A2 and A1 Units of the Salina Formation and with the Guelph Formation.
Lithostratigraphy of Nigeria An Overview
Nigeria lies very close to the equator (hot country) West coast Africa between latitude 4 N and 14 N degree and longitude 2 E and 15 E degree. The country is located at the Northern end of Eastern branch of west coast of Africa rift system. Nigeria geological set up comprises broadly sedimentary formation and crystalline basement complex, which occur more or less in equal proportion all over the country. The sediment is mainly Upper Cretaceous to recent in age while the basement complex rocks are thought to be Precambrian. The studied area lies between latitude 12.4" and 11.11"W and longitude 13.81" and 14.13" S. The studied area is underlain by Precambrian basement complex of southern western Nigeria .The major rock in the area is charnokite and granite rock. The granite rock which are member of the older granite suite occupy about 65% of the total area .The principal granite is petrographic variety are recognized .The fine grained biotite-granite medium-coarse, non porphyritic biotite -hornblende granite and coarse-porphyritic biotite -hornblende granite. Also three main textural type of Charnokitic rock are also distinguished are coarse grained, massive fine grained and gneissic fine grained .The mode of occurrence of rock is three (1) core of the granite rock as exemplified by study area and few smaller bodies (2) Margin of the granite bodies as seen in Ijare and Uro edemo-idemo Charnokitic bodies and (3) Discrete bodies of the gneissic fine grained Charnokitic rock within the country gneisses as seen in Ilaro and Iju and Emirin village. All the charnokite in the region are dark-greenish to greenish-gray rocks with bluish quartz and greenish feldspar
The Hydrogeologic Properties of Potsdam Sandstone in the Chateauguay River Basin and Their Implications Regarding U.S.-Canada Transboundary Water Issues
Proper management of water resources across shared boundaries requires a fundamental understanding of the hydrogeologic system underlying the region. In the case of the Chateauguay River Basin, a transboundary watershed with roughly half of its surface area located in northern New York State (USA) and half in southern Quebec Province (Canada), the Potsdam Sandstone forms a regional fractured-rock aquifer that extends across international jurisdictions. Evidence gathered from geophysical well logs, pumping tests, and numerous field observations on both sides of the border indicates that ground-water flow is confined to discrete and laterally extensive fracture zones that are associated with subhorizontal bedding planes. During aquifer tests, drawdown was detected in boreholes at distances greater than 1 km along bedding but with negligible drawdown across bedding. In addition, flow zones were delineated at the same stratigraphic horizon in two boreholes spaced more than 4 km apart. Vertical flow, including cascading water, was common in boreholes that penetrated multiple transmissive fracture zones. Accordingly, hydraulic-head differences between shallow and deep flow zones were found to range from 1 m to more that 20 m, thereby demonstrating the highly anisotropic nature (horizontal versus vertical) of flow paths. Springs are ubiquitous in this area and occur where water-bearing fractures intercept terrain topography. Focused discharges at rates from 1,000 to 5,000 L/min were measured at several major springs, indicating that the aquifer is supported by a broad, regional collection system. These results reveal that subhorizontal hydraulic communication through these fractured rocks is pervasive and, in some cases, may extend relatively unimpeded for hundreds of meters to kilometers. As such, the distinct hydrogeological properties of the Potsdam Sandstone emphasize the interdependence of national resources in this transborder basin where ground water is highly coveted for bottling, fruit processing, and fish-hatchery use because of its exceptional quality. These aquifer characteristics should have a direct impact on issues regarding recharge areas to wells and springs, well interference, quarry dewatering, and contaminant pathways.