Average Groundwater Velocity Research Articles

Longevity of colloidal activated carbon for in situ PFAS remediation at AFFF‐contaminated airport sites

AbstractA review of state per‐ and polyfluoroalkyl substances (PFAS) guidelines indicates that four long‐chain PFAS (perfluorooctanesulfonic acid [PFOS] and perfluorooctanoic acid [PFOA] followed by perfluorohexanesulfonic acid [PFHxS] and perfluorononanoic acid [PFNA]) are the most frequently regulated PFAS compounds. Analysis of 17 field‐scale studies of colloidal activated carbon (CAC) injection at PFAS sites indicates that in situ CAC injection has been generally successful for both short‐ and long‐chain PFAS in the short‐term (0.3–6 years), even in the presence of low levels of organic co‐contaminants. Freundlich isotherms were determined under competitive sorption conditions using a groundwater sample from an aqueous film‐forming foam (AFFF)‐impacted site. The median concentrations for these PFAS of interest at 96 AFFF‐impacted sites were used to estimate influent concentrations for a CAC longevity model sensitivity analysis. CAC longevity estimates were shown to be insensitive to a wide range of potential cleanup criteria based on modeled conditions. PFOS had the greatest longevity even though PFOS is present at higher concentrations than the other species because the CAC sorption affinity for PFOS is considerably higher than PFOA and PFHxS. Longevity estimates were directly proportional to the CAC fraction in soil and the Freundlich Kf, and were inversely proportional to the influent concentration and average groundwater velocity.

Active distributed temperature sensing to assess surface water–groundwater interaction and river loss in braided river systems

Braided river systems are continuously changing environments, comprising heterogeneous gravel-forming meandering channels and bars, and dynamic surface water–groundwater (SW–GW) interactions. In alluvial aquifers associated with braided rivers, river leakage is volumetrically the most important source of groundwater recharge, yet neither the leakage process or recharge rates are well understood. Insights into how braided river systems recharge underlying braidplain aquifers during different hydrological conditions, and how this can be measured, is of critical concern for river management practices. Innovative field techniques using heat as a tracer can be applied to spatially map and quantify the temporal changes of SW–GW interaction and the processes and mechanisms of groundwater recharge for different flow regimes. Horizontal directional drilling was used to construct two 100 m long drillholes at a depth of ∼ 5 m beneath and perpendicular to the river channel at a field site on the Waikirikiri Selwyn River, South Island, New Zealand. The two drillholes were completed with a hybrid fibre optic cable, containing four multi-mode fibres and 2x18AWG copper conductors. A Silixa XT-DTS™ distributed temperature sensor unit combined with a Silixa Heat Pulse System, was used to collect active distributed temperature sensing (A-DTS) measurements along both cables. Results from two A-DTS surveys showed distinct temperature changes across the active river channel indicating spatial variability in river loss and preferential groundwater recharge pathways to the shallow braidplain aquifer. The average groundwater velocity was 14 m d−1. The average calculated river loss at the survey sites was 21 m3 d−1 m−1 compared to 25 – 26 m3 d−1 m−1 that was measured using differential flow gauging conducted during the same time period. The horizontal installation of the fibre optic cable and the A-DTS method provided valuable insights into SW–GW exchange between the Waikirikiri Selwyn River and shallow braidplain aquifer and how the river leakage rates varied spatially across the river channel.

Sulfur Hexafluoride and Potassium Bromide as Groundwater Tracers for Managed Aquifer Recharge.

Sulfur hexafluoride (SF6 ) is an established tracer for use in managed aquifer recharge projects. SF6 exsolves from groundwater when it encounters trapped air according to Henry's law. This results in its retardation relative to groundwater flow, which can help determine porous media saturation and flow dynamics. SF6 and the conservative, nonpartitioning tracer, bromide (Br- added as KBr), were introduced to recharge water infiltrated into stacked glacial aquifers in Thurston County, Washington, providing the opportunity to observe SF6 partitioning. Br- , which is assumed to travel at the same velocity as the groundwater, precedes SF6 at most monitoring wells (MWs). Average groundwater velocity in the unconfined aquifer in the study area ranges from 3.9 to 40 m/d, except in the southwestern corner where it is slower. SF6 in the shallow aquifer exhibits an average retardation factor of 2.5 ± 3.8, suggesting an air-to-water ratio on the order of 10-3 to 10-2 in the pore space. Notable differences in tracer arrival times at adjacent wells indicate very heterogeneous conductivity. One MW exhibits double peaks in concentrations of both tracers with different degrees of retardation for the first and second peaks. This suggests multiple flowpaths to the well with variable saturation. The confining layer between the upper two aquifers appears to allow intermittent connection between aquifers but serves as an aquitard in most areas. This study demonstrates the utility of SF6 partitioning for evaluating hydrologic conditions at prospective recharge sites.

Groundwater modeling of Musi basin Hyderabad, India: a case study

In general, groundwater flow and transport models are being applied to investigate a wide variety of hydrogeological conditions besides to calculate the rate and direction of movement of groundwater through aquifers and confining units in the subsurface. Transport models estimate the concentration of a chemical in groundwater which requires the development of a calibrated groundwater flow model or, at a minimum, an accurate determination of the velocity and direction of groundwater flow that is based on field data. All the available hydrogeological, geophysical and water quality data in Musi basin, Hyderabad, India, were fed as input to the model to obtain the groundwater flow velocities and the interaction of surface water and groundwater and thereby seepage loss was estimated. This in turn paved the way to calculate the capacity of the storage treatment plants (STP) to be established at the inlets of six major lakes of the basin. The total dissolved solid was given as the pollutant load in the mass transport model, and through model simulation, its migration at present and futuristic scenarios was brought out by groundwater flow and mass transport modeling. The average groundwater velocity estimated through the flow model was 0.26 m/day. The capacities of STP of various lakes in the study area were estimated based on the lake seepage and evaporation loss. Based on the groundwater velocity and TDS as pollutant load in the lakes, the likely contamination from lakes at present and for the next 20 years was predicted.

Borehole characterization of hydraulic properties and groundwater flow in a crystalline fractured aquifer of a headwater mountain watershed, Laramie Range, Wyoming

Fractured crystalline aquifers of mountain watersheds may host a significant portion of the world’s freshwater supply. To effectively utilize water resources in these environments, it is important to understand the hydraulic properties, groundwater storage, and flow processes in crystalline aquifers and field-derived insights are critically needed. Based on borehole hydraulic characterization and monitoring data, this study inferred hydraulic properties and groundwater flow of a crystalline fractured aquifer in Laramie Range, Wyoming. At three open holes completed in a fractured granite aquifer, both slug tests and FLUTe liner profiling were performed to obtain estimates of horizontal hydraulic conductivity (Kh). Televiewer (i.e., optical and acoustic) and flowmeter logs were then jointly interpreted to identify the number of flowing fractures and fracture zones. Based on these data, hydraulic apertures were obtained for each borehole. Average groundwater velocity was then computed using Kh, aperture, and water level monitoring data. Finally, based on all available data, including cores, borehole logs, LIDAR topography, and a seismic P-wave velocity model, a three dimensional geological model of the site was built. In this fractured aquifer, (1) borehole Kh varies over ∼4 orders of magnitude (10−8–10−5 m/s). Kh is consistently higher near the top of the bedrock that is interpreted as the weathering front. Using a cutoff Kh of 10−10 m/s, the hydraulically significant zone extends to ∼40–53 m depth. (2) FLUTe-estimated hydraulic apertures of fractures vary over 1 order of magnitude, and at each borehole, the average hydraulic aperture by FLUTe is very close to that obtained from slug tests. Thus, slug test can be used to provide a reliable estimate of the average fracture hydraulic aperture. (3) Estimated average effective fracture porosity is 4.0 × 10−4, therefore this fractured aquifer can host significant quantity of water. (4) Natural groundwater velocity is estimated to range from 0.4 to 81.0 m/day, implying rapid pathways of fracture flow. (5) The average ambient water table position follows the boundary between saprolite and fractured bedrock. Groundwater flow at the site appears topography driven.

Hydrogeological characterization of flow system in a karstic aquifer, Seymareh dam, Iran

In order to determine the characteristics of the flow system in a karstic aquifer, an extensive hydrogeological study includes dye tracing test was conducted. The aquifer suited left abutment of Seymareh Dam, in Ravandi Anticline and discharges by more than 50 springs in the southern flank. Flow system in the aquifer is mainly controlled by the reservoir of Seymareh Dam. Time variations of the spring discharge and water table in the observation wells were highly correlated with the reservoir water level. The average groundwater velocity ranges from 0.2 to more than 14 m/h based on the dye tracing test. The probable flow paths were differentiated in two groups including the flow paths in the northern and southern flanks of Ravandi Anticline. Types of groundwater flow in the proposed flow paths are determined as diffuse or conduit flow type considering groundwater velocity and shape of the breakthrough curves. An index is proposed for differentiation of diffuse and conduit flow system based on relationship of groundwater velocity and hydraulic gradient. Dominant geometry of the flow routs (e.g., conduit diameter and fracture aperture) is estimated for the groundwater flow paths toward the springs. Based on velocity variations and variance coefficient of the water table and discharge of springs on map view a major karst conduit was probably developed in the aquifer. This research emphasizes applying of an extensive hydrogeological study for characterization of flow system in the karst aquifer.

Soil characterization in and around a proposed waste disposal facility at Nuclear Fuel Complex, Hyderabad, India

Generation of site-specific hydrogeological parameters such as porosity, bulk density, hydraulic conductivity, pore water velocity, etc. are the pre-requisites to carry out studies on the low probability event of migration of radionuclides along with groundwater due to long-term leaching from a waste disposal site. In the present work, the hydrogeological parameters are estimated for soil samples collected from various locations in and around a proposed solid waste disposal site at the Nuclear Fuel Complex, Hyderabad, India. The groundwater velocity is estimated using Darcy’s law. The range of values for hydraulic conductivity, porosity, and bulk density are determined to be 16.24–76.90 m day−1 (average 48.12 m day−1), 0.42–0.53 (average 0.47), and 1.01–1.43 g cm−3 (average 1.24 g cm−3), respectively. Using the estimated values of hydraulic conductivity, porosity, and the average value of hydraulic gradient between various wells at the site, the average groundwater velocity has been estimated to be around 1.0 m day−1 which matches fairly well with the groundwater velocity estimated using radioactive tracer techniques. These site-specific results will be useful in the radiological impact assessment of a proposed near surface waste disposal facility at Nuclear Fuel Complex, Hyderabad, India.

Predicting longevity of iron permeable reactive barriers using multiple iron deactivation models

In this study we investigate the model uncertainties involved in predicting long-term permeable reactive barrier (PRB) remediation efficiency based on a lab-scale column experiment under accelerated flow conditions. A PRB consisting of 20% iron and 80% sand was simulated in a laboratory-scale column and contaminated groundwater was pumped into the column for approximately 1year at an average groundwater velocity of 3.7E−1m d−1. Dissolved contaminants (PCE, TCE, cis-DCE, trans-DCE and VC) and inorganic (Ca2+, Fe2+, TIC and pH) concentrations were measured in groundwater sampled at different times and at eight different distances along the column. These measurements were used to calibrate a multi-component reactive transport model, which subsequently provided predictions of long-term PRB efficiency under reduced flow conditions (i.e., groundwater velocity of 1.4E−3m d−1), representative of a field site of interest in this study. Iron reactive surface reduction due to mineral precipitation and iron dissolution was simulated using four different models. All models were able to reasonably well reproduce the column experiment measurements, whereas the extrapolated long-term efficiency under different flow rates was significantly different between the different models. These results highlight significant model uncertainties associated with extrapolating long-term PRB performance based on lab-scale column experiments. These uncertainties should be accounted for at the PRB design phase, and may be reduced by independent experiments and field observations aimed at a better understanding of reactive surface deactivation mechanisms in iron PRBs.

The alluvial aquifer of the Lower Magra Basin (La Spezia, Italy): conceptual hydrogeochemical–hydrogeological model, behavior of solutes, and groundwater dynamics

The alluvial aquifer of the Lower Magra Basin (La Spezia Province, Italy) is mainly recharged by two main rivers of the region, Magra and Vara, with a subordinate contribution from rain waters infiltrating in the eastern horst adjacent to the graben of the Lower Magra River. This conceptual model is consistent with the constraints provided by both piezometric maps and chemical and isotopic data of local groundwaters and fluvial waters. Chemical data also suggest that, in the considered system, Cl and SO4 have mobile (conservative) behaviour, and Na, K, and Sr have close-to-mobile behaviour; in other words, the contents of these chemical components are essentially fixed by mixing relationships between Magra and Vara river waters. In contrast, Mg, Ca, Ba, HCO3, and SiO2 are largely contributed by water–rock interaction. Unexpectedly, Li has non-mobile behavior owing to either incorporation into secondary minerals or sorption onto solid phases or both. The discovery of two natural sources, one of Cl and the other of SO4, in the basin of the Aullella Creek, a tributary of the Upper Magra River, made possible a key understanding of groundwater dynamics. Every year, during the summer, a mass of Cl and SO4-rich waters originating from the Aullella Creek enters the alluvial aquifer of the Lower Magra Basin. Average groundwater velocity and average permeability, representative of the entire alluvial aquifer of interest, were derived based on the movement of this high-salinity water mass.

Transient Heterogeneity in an Aquifer Undergoing Bioremediation of Hydrocarbons

Localized, transient heterogeneity was studied in a sand aquifer undergoing benzene, toluene, ethylbenzene, and xylene bioremediation using a novel array of multilevel, in situ point velocity probes (PVPs). The experiment was conducted within a sheet-pile alleyway to maintain a constant average flow direction through time. The PVPs measured changes in groundwater velocity direction and magnitude at the centimeter scale, making them ideal to monitor small-scale changes in hydraulic conductivity (K). Velocities were shown to vary nonuniformly by up to a factor of 3 when a source of oxygen was established down-gradient of the petroleum spill. In spite of these local variations, the average groundwater velocity within the 7 m × 20 m sheet-piled test area only varied within ± 25%. The nonuniform nature of the velocity variations across the gate indicated that the changes were not due solely to seasonal hydraulic gradient fluctuations. At the conclusion of the experiment, microbial biomass levels in the aquifer sediments was approximately 1 order of magnitude higher in the oxygen-amended portion of the aquifer than at the edge of the plume or in locations up-gradient of the source. These data suggest that the transient velocities resulted, at least in part, from enhanced biological activity that caused transient heterogeneities in the porous medium.

Predictions of long-term performance of granular iron permeable reactive barriers: Field-scale evaluation

Long-term performance is a key consideration for the granular iron permeable reactive barrier (PRB) technology because the economic benefit relies on sustainable operation for substantial periods of time. However, predictions on the long-term performance have been limited mainly because of the lack of reliable modeling tools. This study evaluated the predictive capability of a recently-developed reactive transport model at two field-scale PRBs, both having relatively high concentrations of dissolved carbonate in the native groundwater. The first site, with 8 years of available monitoring data, was a funnel-and-gate installation, with a low groundwater velocity through the gate (about 0.12 m d − 1 ). The loss in iron reactivity caused by secondary mineral precipitation was small, maintaining relatively high removal rates for chlorinated organics. The simulated concentrations for most constituents in the groundwater were within the range of the monitoring data. The second site, with monitoring data available for 5 years, was a continuous wall PRB, designed for a groundwater velocity of 0.9 m d − 1 . A comparison of measured and simulated aqueous concentrations suggested that the average groundwater velocity through the PRB could be lower than the design value by a factor of two or more. The distribution and amounts of carbonate minerals measured in core samples supported the decreased groundwater velocity used in the simulation. The generally good agreement between the simulated and measured aqueous and solid-phase data suggest that the model could be an effective tool for predicting long-term performance of granular iron PRBs, particularly in groundwater with high concentrations of carbonate.

Measurements of groundwater velocity in discrete rock fractures

Estimating groundwater velocity in fracture networks using a Darcy or cubic law calculation is complicated by the wide distribution of fracture aperture often found in these systems and by the difficulty in measuring hydraulic head in discrete fracture features. Although difficult to conduct in a fractured rock setting, the point dilution method can be utilized to collect direct measurements of groundwater velocity in individual fractures. To compare measured against calculated velocities, more than 100 point dilution experiments were conducted within a 35 × 35 m area of a single fracture and in discrete fracture features within a fracture network at a larger scale. The dilution experiments were conducted by isolating a fracture feature in a borehole, measuring the hydraulic aperture, and measuring the decay of an injected tracer due to the advective groundwater flux across the fracture. Groundwater velocity was estimated using the hydraulic aperture and the rate of decay of the injected tracer. Estimates of the local hydraulic gradient were calculated via the cubic law using the velocity estimate and the hydraulic aperture. The results of the tests conducted in the single fracture show variable (1 to 33 m/day) but on average higher velocities in comparison to that measured during a natural gradient tracer experiment conducted previously (in which the effects of matrix diffusion were accounted for) and to that which would be calculated using the cubic law. Based on these results, it was determined that the best estimate of the average groundwater velocity, at the scale of the measurement area used for the cubic law calculations, could only be obtained using the largest apertures in the aperture distribution. Variability of the velocity measurements was also observed over time. Increases in velocity were attributed to the effect of rainfall although concurrent increases in hydraulic gradient were not detected (likely within the tolerance of the measuring devices). The groundwater velocities measured in the fracture network varied over a wider range than at the scale of the single fracture (from 2 to 388 m/day). No correlation, however, was observed between the size of the fracture aperture and measured velocity.

Hydrogeologic and hydrochemical framework of the shallow groundwater system in the southern Voltaian Sedimentary Basin, Ghana

The southern Voltaian Sedimentary Basin underlies an area of about 5000 km2 in east-central Ghana. Groundwater in the basin occurs in fractures in highly consolidated siliciclastic aquifers overlain by a thin unsaturated zone. Aquifer parameters were evaluated from available aquifer-test data on 28 shallow wells in the basin. Hydraulic-conductivity values range from 0.04–3.6 m/d and are about two orders of magnitude greater than the hydraulic conductivity calculated using Darcy's Law and the average groundwater velocity estimated from carbon-14 dating. Linear-regression analysis of the transmissivity and specific-capacity data allowed the establishment of an empirical relationship between log transmissivity and log specific capacity for the underlying aquifers.

Hydrogeological investigation of a groundwater contamination site in southern Taiwan

Groundwater samples taken from wells adjacent to a food machinery manufacturing plant in southern Taiwan indicate that there is a serious phenolic contamination. To understand the hydrogeological properties, and to prepare for remedial action, a series of hydrogeological investigations were conducted. Investigative work included collecting background information, analyzing existing data, measuring the groundwater, and conducting a slug test, pumping and recovery test, aerial photography analysis and electrical resistivity survey. Results from these investigations show that the local groundwater aquifer may be classified as an unconfined or confined formation, depending on the thickness of the interbedded clay layer. The direction of local groundwater flow is from southwest to northeast, with high transmissivity. The contaminant moves much more slowly than the average groundwater velocity, and it is limited to an area centered around the plant. The local geology of the contaminated area exhibits significant heterogeneity; it is not likely to have been formed by natural sedimentation. Data from the field aerial photography analysis and electrical resistivity survey also suggest that this shallow formation may result from artificial back-filling.

Colloid and suspended particle migration experiments in a granite fracture

To determine the mobility of colloids (0.001–0.45 μm) and suspended particles (> 0.45 μm) in granite fractures, laboratory particle-migration and conservative tracer studies have been carried out in a natural fracture within a large granite block, with overall dimensions of 83×90×60 cm. Flow fields within this horizontal fracture were controlled through a set of 9 boreholes drilled orthogonally to the fracture. Laboratory experiments were performed using a range of average water velocities which contained values low enough to closely approximate the natural flow velocities of < 2 m yr −1 in plutonic rocks of the Canadian Shield. The particles used had diameters between 0.02 and 22 μm, and included latex spheres, glass spheres and colloidal silica. Migration experiments were carried out with a filtered groundwater, ionic strength of 0.01 mol kg −1, obtained from a granite fracture within the Whiteshell Research Area of Manitoba. Flushing experiments showed that suspended particles as large as 40 μm could be mobilized from the fracture surface. The mobility of suspended particles was significantly less than that of colloids. However, within the size range of colloids used in these studies (0.022–0.090 μm), colloid size did not affect colloid migration. Although, in general, colloids eluted ahead of the conservative tracer, colloid mobility was significantly reduced when the average groundwater velocity dropped below between 32 and 240 m yr −1. Colloid transport was found to be very sensitive to flow path and flow direction.

Simulation of atrazine and metabolite transport and fate in a sandy-till aquifer

In a 2.5-yr field study we determined the distribution of atrazine and its metabolites in a sandy-till aquifer overlying Cambrian sandstone in a dairy farming area. Flow paths are predominantly downward at piezometer nests where vertical hydraulic gradients dominate. Atrazine and desethylated atrazine concentrations along those flow paths generally decrease with increasing estimated groundwater travel time to the monitoring points. A one-dimensional contaminant transport model is developed in FORTRAN incorporating major chemical processes and dispersion along simulated flow paths. Separate simulations of atrazine and desethylated atrazine transport to individual piezometers are made, each with an appropriate average groundwater velocity. Regression equations are developed, based on an extensive literature review, to estimate acceptable sorption and dispersivity coefficients for the transport model. The one-dimensional simulations are calibrated using the field concentration-travel time relationships for atrazine and desethylated atrazine. The calibration procedure provides estimates of atrazine and desethylated atrazine degradation rates in groundwater corresponding to half-lives of 3470 and 2770 days, respectively. Although uncertain, the estimates provide evidence of much slower degradation in the aquifer than indicated by laboratory experiments. Using the calibrated transport parameter values, simulations of long-term steady-state leaching to the water table demonstrate that even with such slow degradation rates, steady-state concentrations at most piezometers are reached within ∼ 20 yr. Concentrations in the underlying sandstone aquifer are therefore not expected to increase substantially over time due to continued atrazine use. However, the slow degradation rates mean that even with the most optimistic condition, i.e. all input to the aquifer ceases in response to a cessation of atrazine application, it may take more than a decade for concentrations deeper in the aquifer to drop to half of what they were during the period of atrazine use.

Heat flow and subsurface temperature as evidence for basin-scale ground-water flow, North Slope of Alaska

Research Article| May 01, 1992 Heat flow and subsurface temperature as evidence for basin-scale ground-water flow, North Slope of Alaska DAVID DEMING; DAVID DEMING 1U.S. Geological Survey, 345 Middlefield Road, M.S 923, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar JOHN H. SASS; JOHN H. SASS 2U.S. Geological Survey, 2255 North Gemini Drive, Flagstaff, Arizona 86001 Search for other works by this author on: GSW Google Scholar ARTHUR H. LACHENBRUCH; ARTHUR H. LACHENBRUCH 1U.S. Geological Survey, 345 Middlefield Road, M.S 923, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar ROBERT F. DE RITO ROBERT F. DE RITO 1U.S. Geological Survey, 345 Middlefield Road, M.S 923, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1992) 104 (5): 528–542. https://doi.org/10.1130/0016-7606(1992)104<0528:HFASTA>2.3.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation DAVID DEMING, JOHN H. SASS, ARTHUR H. LACHENBRUCH, ROBERT F. DE RITO; Heat flow and subsurface temperature as evidence for basin-scale ground-water flow, North Slope of Alaska. GSA Bulletin 1992;; 104 (5): 528–542. doi: https://doi.org/10.1130/0016-7606(1992)104<0528:HFASTA>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract In conjunction with the U.S. Geological Survey's exploration program in the National Petroleum Reserve, Alaska (NPRA) several high-resolution temperature logs were made in each of 21 drillholes between 1977 and 1984. These time-series of shallow (average 600-m depth) temperature profiles were extrapolated to infinite time to yield equilibrium temperature profiles (±0.1 °C). Thermal gradients are inversely correlated with elevation, and vary from 22 °C/km in the foothills of the Brooks Range to as high as 53 °C/km on the coastal plain to the north. Shallow temperature data were supplemented with 24 equilibrium temperatures (±3-5 °C) estimated from series of bottom-hole temperatures (BHTs) measured near the bottom of petroleum exploration wells. A total of 601 thermal conductivity measurements were made on drill cuttings and cores. Near-surface heat flow (±20%) is inversely correlated with elevation and ranges from a low of 27 mW/m2 in the foothills of the Brooks Range in the south, to a high of 90 mW/m2 near the north coast. Subsurface temperatures and thermal gradients estimated from corrected BHTs are similarly much higher on the coastal plain than in the foothills province to the south. Significant east-west variation in heat flow and subsurface temperature is also observed; higher heat flow and temperature coincide with higher basement topography. The observed thermal pattern is consistent with forced convection by a topographically driven ground-water flow system; alternative explanations are largely unsatisfactory. Average ground-water (Darcy) velocity in the postulated flow system is estimated to be of the order of 0.1 m/yr; the effective basin-scale permeability is estimated to be of the order of 10-14 m2. Organic maturation data collected in other studies indicate that systematic variations in thermal state may have persisted for tens of millions of years. The ground-water flow system thought to be responsible for present heat-flow variations conceivably has existed for the same period of time, possibly providing the driving mechanism for petroleum migration and accumulation at Prudhoe Bay. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Ground Water Flow Velocity Derived from Tritium Measurements at the Gloucester Landfill Site, Gloucester, Ontario

Abstract Ground water contaminated by organic compounds from a special waste disposal area at the Gloucester Landfill site is migrating through a confined glaciofluvial aquifer toward water supply wells. The distribution of tritium in ground water samples, collected in October 1982, indicates that tritium has originated from two distinct sources: atmospheric nuclear weapons testing in the 1950's and 60's and on-site waste disposal. By comparing the tritium concentration pattern in ground waters to the 1953 to 1982 record for tritium in precipitation at Ottawa, broad zones of the aquifer corresponding to different recharge periods may be defined. Based on the distribution of these zones, an average groundwater velocity of 5.5 cm/day is estimated. A zone of high tritium concentration immediately below the waste disposal compound is attributed to the disposal of tritiated wastes.

Migration of contaminants in groundwater at a landfill: A case study: 5. Cation migration in the dispersion test

A shallow natural-gradient tracer test was conducted in an unconfined sandy aquifer at the Borden landfill site in June 1978. The tracer injected was a chloride solution containing approximately equimolar concentrations of the major cations at levels significantly above those in the background groundwater. Samples collected during monitoring of the tracer were analysed for calcium, magnesium, sodium and potassium. The results showed an order of attenuation for major cations of K > Mg > Na. Ca, which was the dominant species on the exchange sites before injection of the tracer solution, was liberated during flow of the solution. This resulted in the development of a calcium halo at the leading edge of the tracer solution during flow. A one-dimensional mixing-cell model which includes a chemical equilibria subroutine to appraise the effects of major-ion complexing, calcite dissolution and precipitation, and cation exchange was used to simulate the results of the injection experiment. Both the field and model results show that the chemical composition of the background groundwater is changed due to exchange reactions as it flows in the path of the injection solution. These influences continue until all the exchange sites along the flow path re-equilibrate to their former pre-injection values by the passage of background groundwater. The mixing-cell model was calibrated to the dispersive characteristics of the aquifer by varying average groundwater velocity, mixing-cell size and dispersivity to match the chloride distribution. Model results provide simulated cation distributions which are in close agreement with the field results including a “Ca-halo” produced due to the liberation of Ca 2+ from exchange sites. The migration of Na + in the flow system is observed to closely parallel the movement of Cl − although the maximum concentrations are decreased over the concentration at injection. Mg 2+ migrates downflow at a rate slower than Cl −, and at a much reduced concentration reflecting a strong tendency for sorption onto exchange sites. Both model and field results show pronounced K + attenuation during flow of the injection solution. Concentrations throughout the flow system during the monitoring program were always very close to background values. The bulk of the injected potassium was sorbed very close to the injection wells and is slowly liberated by interaction of background groundwater following the passage of the injection solution.

Origin, age and movement of pore water in argillaceous Quaternary deposits at four sites in southwestern Ontario

Hydrogeologic information in the form of textural properties, field- and laboratory-determined hydraulic conductivity values, vertical hydraulic gradients from piezometer nests, and major-ion and isotopic contents ( 18O, 2H, 3H, 13C, 14C), was obtained from four sites located in southwestern Ontario on thick deposits of clayey till and glaciolacustrine clay. At each site the piezometers were installed from the water-table zone, situated 1–4 m below the surface, to a maximum depth between 20 and 35 m below the surface. Values of hydraulic conductivity determined in the laboratory by consolidometer and triaxial methods are in the order of 10 −8–10 −7 cm/s and are similar to values calculated from rates of water-level rise in piezometers. Mean values of average vertical groundwater velocity, calculated with the measured values of hydraulic conductivity, porosity and hydraulic gradient, using the Darcy equation for a saturated non-fractured porous medium, range from 0.13 to 0.26 cm/yr. Tritiated pore waters only occur within 3–6 m of ground surface, thereby indicating that groundwaters below this depth recharged prior to at least 1952. 18O in the pore waters exhibit a distinctive regular shift with depth from water-table values of −9 to −10‰ (SMOW), characteristic of present-day precipitation, to values between −14 and −17‰ at depths of 20–40 m, which are characteristic of much cooler waters. Corrected 14C ages of the groundwater are greater than 8000 yr. B.P., thereby suggesting that the groundwaters at depth entered these deposits during or after the formation of these deposits approximately 11,000 to 14,000 yr. ago. 18O, 2H and Cl − concentration profiles were simulated with a one-dimensional model for transport by advection and diffusion in a saturated porous medium. Reasonable agreement between the model and the field profiles was obtained with values of effective diffusion coefficients of 3.0 · 10 −6 cm 2/s for 18O, 2H and Cl − and with groundwater velocities between 0.03 to 0.05 cm/yr. Based on these results, it is concluded that the pore water in these deposits is a mixture of late Pleistocene and modern waters and that the distribution of 18C, 2H and Cl − in these deposits is influenced predominantly by molecular diffusion, apparently more so than by hydraulic flow.