Bedrock Matrix Research Articles

Numerical model development for investigating hydrologic pathways in shallow fluviokarst

Karst pathways and fluvial pathways control hydrology in shallow fluviokarst basins, and numerical modelling of fluviokarst is seldom reported. In this study, we developed a combined discrete-continuum fluviokarst numerical model by simulating surface river routing, in-stream swallet sources and sinks, epikarst storage and dynamic transfer, matrix bedrock interactions, and closed conduit phreatic flow. We applied the model to the Cane Run-Royal Spring basin in Kentucky, USA. Model evaluation indicated that spring discharge alone inadequately constrained model pathways and uncertainty. Instead multi-objective calibration, integrating riverine discharge and well-head data from multiple locations, assisted in identifying sensitive parameters (p < 0.05). Multi-objectives improved representation of stream-cave connectivity and limited prior knowledge biases of the system but was computationally expensive with 168 h required on a high-performance cluster. Results provided evidence for a mature fluviokarst basin with well-defined fracture-conduit network and phreatic aquifer. Residence times of karst pathways vary by five orders of magnitude, ranging from less than one hour in vertical swallets, 12.7 h in longitudinal conduits, 12.7 days in the vadose zone and epikarst, and 142.7 days in the bedrock matrix. Results suggest arrival of source waters to the subsurface systems is disconnected in time from the springflow response. Model simulations show a dimensionless vertical to longitudinal conveyance ratio helps predict swallets linking the fluvial and karst systems. Transferability of the developed model, and other karst models, is discussed relative to availability of information for karst basins.

Chalk-glacitectonite, an important lithology in former glaciated terrains covering chalk and limestone bedrock

A glacitectonite is defined as a brecciated sediment or a cataclastic sedimentary rock formed by glaciotectonic deformation (Pedersen 1988). The term tectonite was initially introduced by Sander (1912), mainly for tectonically brecciated metamorphic rocks in the Alps. In the classic work on cataclastic rocks, Higgins (1971) stated that the term covered all rocks with fabric displaying coordinated geometric features related to continuous flow during deformation.Therefore brecciated lithologies formed by glaciotectonic deformations can be termed tectonites. Banham (1977) suggested the prefix glaci- to clarify the relation to glacial dynamics. Furthermore, Pedersen (1988) suggested the application of the bedrock prefix. Thus, a chalk-glacitectonite is a brecciated chalk formed by shear deformation during a glacial advance over an exposed bedrock surface of chalk (Fig. 1). Hence the term describes a sedimentary rock in which the primary structures are so disturbed that they cannot be continuously traced, and a glacitectonic fabric developed as joint fractures or shear surfaces superimposed on the lithology. The significance of recognising chalk-glacitectonite from chalk and limestone bedrock is the difference in textural properties, which is fundamental in geological modelling. In areas dominated by glaciotectonic complexes, which include thrust sheets of pre-glacial sedimentary rocks, the sheets are subject to shearing and dragged along the sole of the ice during its movement over the glaciotectonic complex. Due to truncation and shear-drag, the glacitectonite forms at the base of the deformational layer in a lodgement till. From the source area, which typically is a detachment anticline, the glacitectonite thins out in the direction of transport from 1–2 m (Fig. 2) to a thin shear zone only a few centimetres thick over a distance of one to a few kilometres (Pedersen 1996). Moreover, brecciation of thrust sheets displaced by glacial thrusting occurs within glaciotectonic complexes. The deformation ranges from initially anastomosing jointing (Figs 1, 3) to brecciation with bedrock clasts in crushed bedrock matrix (Fig. 4). The tectonic breccia distributed from the décollement zone at the base to the truncating glacial unconformity at the top may additionally be termed glacitectonites. Here we describe the occurrence and identification of chalk-tectonites.

Identifying and assessing key parameters controlling heat transport in discrete rock fractures

Numerical modeling of heat transfer in low porosity fractured rock is challenging because of the complexity associated with the interactions between fractures, bedrock matrix, groundwater, and heat sources. Possible influential parameters include the heat source configuration, the thermal conductivity of the matrix, the velocity of the fluid, the thermal dispersivity in the fracture, and the aperture of the fracture. In this investigation we use factorial analysis (2K) to define which of the five parameters, or combinations thereof, significantly influence heat migration in a single fracture setting assuming a parallel plate condition. A 3-D numerical model based on the control-volume finite element method is used for the simulations. For the parameter ranges investigated, results indicate that the most influential factor controlling heat propagation in a single fracture setting is the velocity of the fluid in the fracture. The interaction between the thermal conductivity of the matrix and the velocity of the fluid, and between the thermal conductivity of the matrix and the aperture of the fracture, dominantly control the attenuation of the thermal front migration. Depending on the particular system, one or more of these parameters should be given better consideration during site characterization in fractured rock and in the compilation of site-specific models intended to predict heat propagation in fractured systems.

Determination of Cr(III) solids formed by reduction of Cr(VI) in a contaminated fractured bedrock aquifer: Evidence for natural attenuation of Cr(VI)

Hexavalent chromium Cr(VI) is toxic and can be highly mobile in many aquifer systems. Redox reactions with naturally occurring minerals and organic compounds can reduce Cr(VI) to Cr(III), forming labile Cr(III) oxyhydroxide precipitates, which is a natural attenuation process. In fractured bedrock aquifers, reduction of Cr(VI) in the rock matrix can enhance attenuation beyond that from matrix diffusion only, and potentially reduce back diffusion if concentrations in fractures decline following source reduction via natural processes or engineered remediation. In this study, we develop an extraction method for labile Cr(III) precipitates from Cr(VI) reduction using 5% hydrogen peroxide (H2O2). Combining Cr(III) extractions with an established sodium hydroxide (NaOH) method for determination of Cr(VI) concentrations in rock porewater, a measure of the labile Cr(III) and Cr(VI) fractions in geologic samples is achieved. The methods were applied to cores from a contaminated groundwater system in fractured porous bedrock in order to assess the effectiveness of natural attenuation and whether Cr(VI) mass that diffused into the bedrock matrix was undergoing reduction. Detailed vertical distributions display two depth intervals with corresponding elevated concentrations of Cr(VI) in the porewater and extractable total Cr. The correspondence of Cr(VI) and labile Cr(III) provides evidence for reduction of Cr(VI) contamination in the bedrock matrix. Mineralogical analysis suggests that Fe(II)-bearing minerals, chlorite and biotite are the most likely candidates for natural reductants. This study provides evidence for the natural attenuation of anthropogenic Cr(VI) contamination in the porewater of a fractured bedrock aquifer, and it outlines a quantitative method for evaluating the effectiveness of natural attenuation in groundwater systems.

Vadose zone processes delay groundwater nitrate reduction response to BMP implementation as observed in paired cultivated vs. uncultivated potato rotation fields

Nitrate leaching from crop production contributes to groundwater contamination and subsequent eutrophication of the receiving surface water. A study was conducted in a 7-ha potato-grain-forages rotation field in Prince Edward Island (PEI), Canada during 2011–2016 to link potato rotation practices and groundwater quality. The field consists of fine sandy loam soil and is underlain by 7–9 m of glacial till, which overlies the regional fractured “red-bed” sandstone aquifer. The water table is generally located in overburden close to the bedrock interface. Field treatments included one field zone taken out of production in 2011 with the remaining zones kept under a conventional potato rotation. Agronomy data including crop tissue, soil, and tile-drain water quality were collected. Hydrogeology data including multilevel monitoring of groundwater nitrate and hydraulic head and data from rock coring for nitrate distribution in overburden and bedrock matrix were also collected. A significant amount of nitrate leached below the soil profile after potato plant kill (referred to as topkill) in 2011, most of it from fertilizer N. A high level of nitrate was also detected in the till vadose zone through coring in December 2012 and through multilevel groundwater sampling from January to May 2014 in both cultivated and uncultivated field zones. Groundwater nitrate concentrations increased for about 2.5 years after the overlying potato field was removed from production. Pressure-driven uniform flow processes dominate water and nitrate transport in the vadose zone, producing an apparently instant water table response but a delayed groundwater quality response to nitrate leaching events. These data suggest that the uniform flow dominated vadose zone in agricultural landscapes can cause the accumulation of a significant amount of nitrate originated from previous farming activities, and the long travel time of this legacy nitrate in the vadose zone can result in substantially delayed responses of groundwater quality to field management adjustments. The delayed effects should also apply to the transport of other contaminants. This study also suggests that management practices should be optimized to reduce soil nitrate build-up during the non-growing season (when plant N uptake is diminishing and the soil contains excessive moisture, for example, after the potato harvest period in PEI) in order to protect groundwater quality.

Using isotopes of dissolved inorganic carbon species and water to separate sources of recharge in a cave spring, northwestern Arkansas, USA Blowing Spring Cave

Blowing Spring Cave in northwestern Arkansas is representative of cave systems in the karst of the Ozark Plateaus, and stable isotopes of water (δ18O and δ2H) and inorganic carbon (δ13C) were used to quantify soil-water, bedrock-matrix water, and precipitation contributions to cave-spring flow during storm events to understand controls on cave water quality. Water samples from recharge-zone soils and the cave were collected from March to May 2012 to implement a multicomponent hydrograph separation approach using δ18O and δ2H of water and dissolved inorganic carbon (δ13C–DIC). During baseflow, median δ2H and δ18O compositions were –41.6‰ and –6.2‰ for soil water and were –37.2‰ and –5.9‰ for cave water, respectively. Median DIC concentrations for soil and cave waters were 1.8 mg/L and 25.0 mg/L, respectively, and median δ13C–DIC compositions were –19.9‰ and –14.3‰, respectively. During a March storm event, 12.2 cm of precipitation fell over 82 h and discharge increased from 0.01 to 0.59 m3/s. The isotopic composition of precipitation varied throughout the storm event because of rainout, a change of 50‰ and 10‰ for δ2H and δ18O was observed, respectively. Although, at the spring, δ2H and δ18O only changed by approximately 3‰ and 1‰, respectively. The isotopic compositions of precipitation and pre-event (i.e., soil and bedrock matrix) water were isotopically similar and the two-component hydrograph separation was inaccurate, either overestimating (>100%) or underestimating (<0%) the precipitation contribution to the spring. During the storm event, spring DIC and δ13C–DIC decreased to a minimum of 8.6 mg/L and –16.2‰, respectively. If the contribution from precipitation was assumed to be zero, soil water was found to contribute between 23 to 72% of the total volume of discharge. Although the assumption of negligible contributions from precipitation is unrealistic, especially in karst systems where rapid flow through conduits occurs, the hydrograph separation using inorganic carbon highlights the importance of considering vadose-zone soil water when analyzing storm chemohydrographs. Keywords: carbon, stable isotopes, cave, hydrograph, Arkansas. DOI: 10.3986/ac.v42i2-3.667

Using PHREEQC to Simulate Solute Transport in Fractured Bedrock

The geochemical computer model PHREEQC can simulate solute transport in fractured bedrock aquifers that can be conceptualized as dual-porosity flow systems subject to one-dimensional advective-dispersive transport in the bedrock fractures and diffusive transport in the bedrock matrix. This article demonstrates how the physical characteristics of such flow systems can be parameterized for use in PHREEQC, it provides a method for minimizing numerical dispersion in PHREEQC simulations, and it compares PHREEQC simulations with results of an analytical solution. The simulations assumed a dual-porosity conceptual model involving advective-reactive-dispersive transport in the mobile zone (bedrock fracture) and diffusive-reactive transport in the immobile zone (bedrock matrix). The results from the PHREEQC dual-porosity transport model that uses a finite-difference approach showed excellent agreement compared with an analytical solution.

A Field Study of Water Flow and Virus Transport in Weathered Granitic Bedrock

Where soils are shallow, effluents from on-site wastewater disposal systems (OSWDS) can be dispensed onto underlying weathered bedrock. Viruses contained in the effluents pose a threat to groundwater quality if the weathered bedrock materials do not possess the properties necessary to treat the effluents before they reach groundwater. The extent and pathways of water flow and virus transport in fractured, weathered granitic bedrock were investigated at a field site in southern California. A suspension containing MS-2 bacteriophage, sodium bromide, and blue dye was ponded at the soil–weathered bedrock interface and allowed to infiltrate for 9 h. A trench was excavated, and bedrock samples were collected and assayed for water, bromide, and MS-2 content. Distributions of dye, bromide, and MS-2 indicate that joint fractures facilitated the channeling of water and of viruses to depths >105 cm. Infiltration data suggest that fracture channeling occurred primarily during the first 50 min, after which time vertical convective flow through the bedrock matrix was the primary infiltration and transport process. The transition from fracture to matrix flow was the result of a decrease in fracture aperture caused by the swelling of pedogenic clay in the weathered bedrock matrix. Bromide and MS-2 concentration profiles suggest that the lower extent of matrix flow was 45 cm, below which water flow and virus transport occurred solely via fracture channeling. These results indicate that caution should be used when operating OSWDS on weathered granitic bedrock in California, and emphasize the need to collect morphologic and hydraulic data prior to their installation.

A Field Study of Water Flow and Virus Transport in Weathered Granitic Bedrock

Where soils are shallow, effluents from on‐site wastewater disposal systems (OSWDS) can be dispensed onto underlying weathered bedrock. Viruses contained in the effluents pose a threat to groundwater quality if the weathered bedrock materials do not possess the properties necessary to treat the effluents before they reach groundwater. The extent and pathways of water flow and virus transport in fractured, weathered granitic bedrock were investigated at a field site in southern California. A suspension containing MS‐2 bacteriophage, sodium bromide, and blue dye was ponded at the soil–weathered bedrock interface and allowed to infiltrate for 9 h. A trench was excavated, and bedrock samples were collected and assayed for water, bromide, and MS‐2 content. Distributions of dye, bromide, and MS‐2 indicate that joint fractures facilitated the channeling of water and of viruses to depths &gt;105 cm. Infiltration data suggest that fracture channeling occurred primarily during the first 50 min, after which time vertical convective flow through the bedrock matrix was the primary infiltration and transport process. The transition from fracture to matrix flow was the result of a decrease in fracture aperture caused by the swelling of pedogenic clay in the weathered bedrock matrix. Bromide and MS‐2 concentration profiles suggest that the lower extent of matrix flow was 45 cm, below which water flow and virus transport occurred solely via fracture channeling. These results indicate that caution should be used when operating OSWDS on weathered granitic bedrock in California, and emphasize the need to collect morphologic and hydraulic data prior to their installation.

Influence of hydrological and geochemical processes on the transport of chelated metals and chromate in fractured shale bedrock

Field-scale processes governing the transport of chelated radionuclides in groundwater remain conceptually unclear for highly structured, heterogeneous environments. The objectives of this research were to provide an improved understanding and predictive capability of the hydrological and geochemical mechanisms that control the transport behavior of chelated radionuclides and metals in anoxic subsurface environments that are complicated by fracture flow and matrix diffusion. Our approach involved a long-term, steady-state natural gradient field experiment where nonreactive Br − and reactive 57Co(II)EDTA 2−, 109CdEDTA 2−, and 51Cr(VI) were injected into a fracture zone of a contaminated fractured shale bedrock. The spatial and temporal distribution of the tracer and solutes was monitored for 500 days using an array of groundwater sampling wells instrumented within the fast-flowing fracture regime and a slower flowing matrix regime. The tracers were preferentially transported along strike-parallel fractures coupled with the slow diffusion of significant tracer mass into the bedrock matrix. The chelated radionuclides and metals were significantly retarded by the solid phase with the mechanisms of retardation largely due to redox reactions and sorption coupled with mineral-induced chelate–radionuclide dissociation. The formation of significant Fe(III)EDTA − byproduct that accompanied the dissociation of the radionuclide–chelate complexes was believed to be the result of surface interactions with biotite which was the only Fe(III)-bearing mineral phase present in these Fe-reducing environments. These results counter current conceptual models that suggest chelated contaminants move conservatively through Fe-reducing environments since they are devoid of Fe-oxyhydroxides that are known to aggressively compete for chelates in oxic regimes. Modeling results further demonstrated that chelate–radionuclide dissociation reactions were most prevalent along fractures where accelerated weathering processes are expected to expose more primary minerals than the surrounding rock matrix. The findings of this study suggest that physical retardation mechanisms (i.e. diffusion) are dominant within the matrix regime, whereas geochemical retardation mechanisms are dominant within the fracture regime.

Recharge and aquifer response: Northern Guam Lens Aquifer, Guam, Mariana Islands

The Northern Guam Lens Aquifer is an island karst aquifer in uplifted young, highly conductive limestone. Calculations of recharge based on differences between daily rainfall and daily pan evaporation suggest that the maximum annual mass of water delivered to the freshwater lens is about 67% of mean annual rainfall. Hydrographs of daily well-level responses plotted against daily rainfall indicate that the rate at which water is delivered to the lens is a function of rainfall intensity and the relative saturation of the vadose zone. Together, these variables determine the degree to which stormwater is shunted into fast flow through preferred pathways that bypass the bedrock matrix, rather than percolating slowly through the bedrock matrix. Data from the 40-year interval from 1956 to 1995 show that some 17% of rainfall on northern Guam arrives in small amounts (<0.6 cm/day). Most of this light rainfall is probably lost to evapotranspiration. At least another 20% of total rainfall on Guam arrives at very high intensities (>5.0 cm/day), which tend to promote fast flow at the expense of percolation. Rapid recovery of the water table from rapid recharge suggests that the lens either takes such recharge into storage very rapidly, discharges it rapidly without taking it into storage, or some combination of both. Significant vadose buffering of recharge to the lens is indicated by the fact that simulations assuming that the recharge from precipitation received in any given month is transmitted to the lens during the same month consistently over-predict observed peak mean monthly water levels and under-predict the minima.

A Field Study of Water Flow and Virus Transport in Weathered Granitic Bedrock

Where soils are shallow, effluents from on-site wastewater disposal systems (OSWDS) can be dispensed onto underlying weathered bedrock. Viruses contained in the effluents pose a threat to groundwater quality if the weathered bedrock materials do not possess the properties necessary to treat the effluents before they reach groundwater. The extent and pathways of water flow and virus transport in fractured, weathered granitic bedrock were investigated at a field site in southern California. A suspension containing MS-2 bacteriophage, sodium bromide, and blue dye was ponded at the soil–weathered bedrock interface and allowed to infiltrate for 9 h. A trench was excavated, and bedrock samples were collected and assayed for water, bromide, and MS-2 content. Distributions of dye, bromide, and MS-2 indicate that joint fractures facilitated the channeling of water and of viruses to depths >105 cm. Infiltration data suggest that fracture channeling occurred primarily during the first 50 min, after which time vertical convective flow through the bedrock matrix was the primary infiltration and transport process. The transition from fracture to matrix flow was the result of a decrease in fracture aperture caused by the swelling of pedogenic clay in the weathered bedrock matrix. Bromide and MS-2 concentration profiles suggest that the lower extent of matrix flow was 45 cm, below which water flow and virus transport occurred solely via fracture channeling. These results indicate that caution should be used when operating OSWDS on weathered granitic bedrock in California, and emphasize the need to collect morphologic and hydraulic data prior to their installation.

Soil and Weathered Bedrock

Virtually all of the commercial forests in the southern Sierra Nevada are on granitic terrain, where bedrock may be weathered to depths &gt;15 m while soils are &lt;1 m thick. Because plant‐available water is depleted in these thin soils by midsummer, study objectives were to characterize the edaphic role of the weathered bedrock relative to the soil. The site was a 30‐yr‐old Jeffrey pine (Pinus jeffreyi Grev. &amp; Balf.) plantation growing on relatively thin soils (75 cm in depth) overlying weathered granitic bedrock. The average depth to hard bedrock was 350 cm. A trench was excavated and physical and chemical properties of the soil and bedrock were evaluated. Cation‐exchange capacities (CEC) were lower in the weathered bedrock (Cr1 horizon = 4.6 cmol kg−1) than in the soil (A horizon = 13.4 cmol kg−1), but pH values were similar (4.6–5.5). Organic C content was negligible in the weathered bedrock matrix (&lt;0.1%), but was higher within joint fractures (3.7%), where roots were concentrated, than within the soil A horizon (2.7%). Carbon/N ratios were much lower in the soil A horizon (19.6) than in the bedrock fractures (62.0). Saturated hydraulic conductivities (Ksat) of the soil and the weathered bedrock were similar and high (8–11 cm h−1). Mean root length density (RLD) was greater within the joint fractures than within the soil, but on a whole rock basis bedrock RLD was much lower (&lt;0.08 cm cm−3). Total plant‐available water storage capacity of 48.8 cm was calculated for the 350 cm thickness of regolith, with 14.7 cm (30%) contributed by soil and 34.1 cm (70%) by weathered bedrock. Weathered bedrock underlying soils is critical to the survival of forest ecosystems, particularly with regard to water supply, and should not be neglected in ecosystem site evaluations and models.

Quantifying diffusive mass transfer in fractured shale bedrock

A significant limitation in defining remediation needs at contaminated sites often results from an insufficient understanding of the transport processes that control contaminant migration. The objectives of this research were to help resolve this dilemma by providing an improved understanding of contaminant transport processes in highly structured, heterogeneous subsurface environments that are complicated by fracture flow and matrix diffusion. Our approach involved a unique long‐term, steady state natural gradient injection of multiple tracers with different diffusion coefficients (Br, He, Ne) into a fracture zone of a contaminated shale bedrock. The spatial and temporal distribution of the tracers was monitored for 550 days using an array of groundwater sampling wells instrumented within a fast flowing fracture regime and a slow flowing matrix regime. The tracers were transported preferentially along strike‐parallel fractures, with a significant portion of the tracer plumes migrating slowly into the bedrock matrix. Movement into the matrix was controlled by concentration gradients established between preferential flow paths and the adjacent rock matrix. Observed differences in tracer mobility into the matrix were found to be a function of their free‐water molecular diffusion coefficients. The multiple tracer technique confirmed that matrix diffusion was a significant process that contributed to the overall physical nonequilibrium that controlled contaminant transport in the shale bedrock. The experimental observations were consistent with numerical simulations of the multitracer breakthrough curves using a simple fracture flow model. The simulated results also demonstrated the significance of contaminant diffusion into the bedrock matrix. The multiple tracer technique and ability to monitor the fracture and matrix regimes provided the necessary experimental constraints for the accurate numerical quantification of the diffusive mass transfer process. The experimental and numerical results of the tracer study were also consistent with indigenous contaminant discharge concentrations within the fracture and matrix regimes of the field site. These findings suggest that the secondary source contribution of the bedrock matrix to the total off‐site transport of contaminants is relatively large and potentially long‐lived.