2019 NGWA Conference on Fractured Rock and Groundwater: Alphabetical Content Listing
Elements of Effective Conceptual Site Models I
Analytical and numerical modelling for conceptual site model refinement: A case study
Ranjeet Nagare, Ph.D., P.Eng.
Contaminated fractured bedrock site management benefits from a phased field investigation approach supported by groundwater modelling to continually enhance the conceptual site model (CSM). This study discusses learnings from a multi-year phased investigation at a site in Central Alberta, Canada. The site is underlain by fractured bedrock contaminated with salinity and hydrocarbons. The impacts have been contained by intercepting and pumping groundwater for 18 years. Groundwater modelling was used to refine field investigation targets and to test elements of the CSM. An equivalent porous medium (EPM) modelling approach successfully reproduced the flow field, but not the solute transport behaviour. A detailed discrete fracture network (DFN) model was able to closely reproduce both groundwater flow and solute transport behaviour at the site. An analytical solution was also developed based on mass recovery data and concentration rebound behaviour and was used to estimate initial conditions for the DFN model. The analytical solution was also successfully used as a screening tool to determine the effectiveness of remediation by pumping. The phased approach has led to an effective CSM and remedial action plan, and the simple analytical model has aptly supported the more detailed site and numerical investigations.
Characterizing Contaminant Transport in a Dual Aquifer System with Intervening Vadose Zone for Remedy Selection
Eric Dieck
The hydrogeologic conditions at an EPA Superfund site involve both an overburden and bedrock aquifer impacted with VOCs. The overburden aquifer is a continuing source to the bedrock aquifer, which are separated by 30 to 40 feet of bedrock vadose zone. Fractured bedding plane partings extend above the saturated zone of bedrock aquifer to the base of the perched overburden aquifer. The understanding of the hydraulic relationship between the aquifers is paramount in developing an effective remedy.
A CSM was further developed by implementing a 72-hour pumping test. The objectives of the pumping test were to refine the existing bedrock hydrostratigraphic model, determine orientation and extent of pumping influences, identify the hydraulic connection between the bedrock and overburden aquifers, and define an appropriate remedy for the site.
Analysis of the pumping test data was performed through curve matching techniques in Aqtesolv, and visualization of head change over time. Further development of the CSM informed remedy design for commingled plumes from various stakeholders across this multiple property site. Validation of the new CSM included various communications with all stakeholders and modification to the regulatory framework, and will result in developing an appropriate groundwater remedy for both the bedrock and overburden aquifer.
Comparison Of CSMs For Bedrock PFAS Impacts At A Former Manufacturing Facility And A Former Military Base
Peter Thompson
Per- and Polyfluoroalkyl Substances (PFAS) are perhaps the most significant and challenging emerging contaminants facing environmental professionals today. Both perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA), as well as other PFAS have been found in public and private water supplies and are present in biota as well as higher trophic level organisms. Due to their chemical structure they are resistant to biological degradation, are persistent in the environment and have a high potential for bioaccumulation and or magnification. This presentation will discuss the findings and CSM for release of PFOA and other PFAS from a secondary manufacturing source in New England where air deposition resulted in widespread soil contamination. Leaching to the water table and recharge to bedrock groundwater resulted in wide spread impacts to private potable water supplies in fractured bedrock. That CSM will be contrasted with multiple releases of PFOS and PFOA and other PFAS associated with use of AFFF at former Air Force bases in New England. Each Site occurs within carbonate fractured bedrock terrain. The nature of sources, residual sources, contaminant recycling, fate and transport characteristics at these different types of sites have considerable differences as well as important similarities which will be described.
Conceptual Site Model Development and Confirmation of a Bedrock Stratigraphic Preferential Pathway
Donald Maynard
A remedial investigation and feasibility study conducted for the United States Army Corps of Engineers at the former Glenburn Ground-to-Air Transmitter Station evaluated trichloroethylene in groundwater and private water supply wells. Bedrock geology data collected using surface geophysics, borehole geophysics of existing wells, outcrop mapping and previous geologic reports were combined with water quality data into a Conceptual Site Model used to develop a bedrock investigation. A relatively low cost approach using a water well air-hammer drill rig combined with borehole geophysics, packer tests, and discrete-interval well completions was used to define the nature and extent of contamination, and to correlate preferential pathways between the bedrock wells. The investigation documented a long narrow plume of trichloroethylene in the bedrock aquifer, the nature and extent of which was controlled by bedrock stratigraphy controlled preferential pathways. Pumping test results confirmed the elongated nature of the stratigraphic preferential pathways and documented hydrogeologic connections between specific vertical intervals in multiple bedrock wells. Subsequent years of water quality monitoring documented stable or declining concentrations in the groundwater. Natural attenuation, institutional controls, point-of-use water treatment and long-term monitoring were chosen as the primary remedial action elements in the Decision Document.
Developing a Robust Conceptual Site Model: A Learning Experience Case Study
David Sherman, PG
A former solvent recycling facility was used for disposal of distillation residues. Following remedial actions in the 1980s and 1990s to remove contaminated source materials, a Site characterization was conducted, which indicated perched groundwater zones and high concentrations of CVOCs were present in unconsolidated soil. Perimeter area monitoring wells were installed to evaluate karst bedrock groundwater.
From 2005 to 2015, soil vapor extraction (SVE) was implemented to treat impacted soil and reduce the CVOC concentrations in perimeter area groundwater; however, clean-up goals were not achieved. Following the SVE remedy, additional Site characterization indicated that groundwater is likely not perched, overburden material remains highly impacted, and DNAPL was observed in one SVE well. Groundwater underlying the source has CVOC concentrations that were much higher than the perimeter monitoring wells.
The initial characterization was not detailed and led to a remedy that could not complete cleanup of the Site. To develop a robust conceptual site model, the extent of contamination will be further delineated, and the processes that control transport of CVOCs will be evaluated, with an eye towards effective remedy design and implementation.
Development of a Conceptual Site Model for Comingled Chlorobenzene Plumes in Weathered Bedrock
Dariusz Chlebica
The presence of a relatively high hydraulic conductivity (K) weathered bedrock zone at the overburden/bedrock interface created challenges in the development of a Conceptual Site Model (CSM) for the fate and transport of chlorobenzene at a former industrial Site in Southeastern Massachusetts. The CSM formulation required a multi-line of evidence approach including a detailed evaluation of hydraulic gradients and chlorobenzene distribution across multiple units, the use of borehole geophysics to assess vertical gradients prior to bedrock well installation, and characterization of the weathered zone K and thickness across the Site. The bedrock chlorobenzene plume appears to discharge to weathered bedrock and shallow overburden in an area with artesian conditions in bedrock. The plume later becomes comingled with the overburden and weathered bedrock plumes beneath an encapsulated lagoon. Downward gradients beneath the lagoon drive groundwater flow into the weathered bedrock in a narrow area where its K is much higher than overburden. Downgradient of this area the plume appears to be confined to the weathered bedrock. A tracer was injected over a 12-hour period into a 100-ft deep bedrock well near the bedrock chlorobenzene source, and tracer concentrations detected in downgradient wells exceeded the background concentrations along the inferred flow path.
Employing Multiple Lines Of Evidence As A Remediation Decision-Making Tool At A Fractured Rock Site
Brent Aigler, P.G.
Multiple lines of evidence demonstrate that sustained elevated concentrations of dissolved volatile organic compounds (VOCs) and 1,4-dioxane in a fractured rock aquifer at a site in Rhode Island reflect stagnation of a detached plume rather than a separate nearby release that, if present, may warrant active remediation. Being able to substantiate a conceptual site model (CSM) of a detached groundwater plume is of critical importance at this site because it avoids the unwarranted expenditure of energy and resources to remediate a source area that is not present. This work presents the various data-collection methods and hydrogeologic assessments that were implemented at the site to iteratively develop a lines-of-evidence approach in support of a robust CSM that forms the basis of an exit strategy at the site. Lines of evidence supporting the CSM of a detached plume include: the history of chemical impacts to the study area; hydraulic influence on the study area by source-area remedies and groundwater extraction; results of borehole geophysical and geological assessments; the distribution of recently obtained per- and polyfluoroalkyl substances (PFAS) data; and an analysis of VOC fingerprinting and chemical gradients of the study area and the remediated source area.
Hydraulic Tomography: Estimating 3D Hydraulic Conductivity, Fracture Network, and Connectivity
Claire Tiedeman
Hydraulic tomography was performed in fractured mudstones at the former Naval Air Warfare Center, West Trenton, New Jersey, by conducting 47 pumping tests in packer-isolated intervals of seven closely-spaced wells. During each test, drawdown data were collected from 30-40 intervals throughout the wellfield. The 3D hydraulic conductivity (K) distribution was estimated at high resolution (K blocks < 1 m3) using the drawdown data, a heterogeneous continuum forward model, and geostatistical inversion. The 3D K estimate ranges from ~0.1 m/s (highest-K fractures) to ~10-13 m/s (unfractured mudstone). Important estimated features include: (a) a highly fractured zone (HFZ) consisting of a sequence of high-K bedding-plane fractures; (b) a low-K zone that disrupts the HFZ; (c) several secondary fractures of limited extent; and (d) large regions of very low-K rock matrix, particularly at depth. The 3D K estimate explains highly complex drawdown behavior observed in the field. Model fit to the drawdown data is best in the shallower part of the wellfield, with high density of observations and tests. The simulated drawdown progression during the tests, and particle tracking simulations in several directions across the wellfield, reveal a 3D fracture network within the estimated K distribution, and connectivity routes through the network.
Identifying a Trichloroethene Source Zone from Analyses of Rock Core in a Mudstone Aquifer
Allen Shapiro, Ph.D.
In fractured rock, Non-Aqueous Phase Liquid (NAPL) releases of trichloroethene (TCE) lead to complex contaminant distributions due to fracture connectivity and aperture variability. Continuous pool heights of NAPL can also result in TCE emplacement in both low permeability fractures and large pore throats in the rock matrix. Additionally, aqueous phase TCE diffuses into the rock matrix from fractures.
TCE analyses of rock core are an integral part of characterizing the magnitude and spatial distribution of contaminant mass in the rock matrix. Analyses of cores from seven closely spaced coreholes in a mudstone aquifer show a complex vertical and lateral TCE distribution. Phase partitioning calculations for TCE in the rock matrix show that most TCE is adsorbed to solid surfaces because of the large organic carbon fraction (foc) in the mudstone; however, large TCE content in some core samples also suggest the presence of NAPL in the rock matrix and proximity to a TCE source zone. A probability of NAPL occurrence, PNAPL, in core samples can be calculated by accounting for variability in matrix porosity and foc in the phase partitioning calculations. The distribution of PNAPL along the closely spaced coreholes identifies a configuration attributed to a TCE source zone.
Improving hydraulic aperture estimations to inform discrete fracture network models
Pat Quinn, PhD
Discrete fracture network numerical (DFN) models have long been available to simulate groundwater flow and transport in fractured rock, however these models are rarely applied at field sites because input fracture parameters are difficult to quantify. Because simulation results are strongly sensitive to fracture aperture there is need to decrease errors and uncertainty in aperture estimates. Hydraulic apertures derived using the cubic law using T values from straddle-packer tests are the most practicable way to obtain apertures. Over the last 10 years we have worked on improving our straddle-packer equipment design and test procedures to better understand the fluid mechanics of the water passing through the test equipment and in the fractures intersecting the tested interval. Insights concerning the nature of test conditions are obtained using a multiple-test approach to evaluate the inherent mathematical assumptions used to determine T, including validation of Darcian flow, cross-connection to the open borehole, and the steady flow assumption, to increase confidence in the calculated T values. However, further error in the calculated hydraulic aperture stems from uncertainty in the number of hydraulically active fractures in each test interval, and examples are provided showing how multiple datasets aid in the identification of the active fractures.
Keynote - Do You Know Where Your Aquitards Are?
Jessica Meyer, Ph.D.
Do You Know Where Your Aquitards Are? A Case for Hydraulically Calibrated Geology as the Foundation for Robust CSMs of Contaminated Fractured Rock Systems
Delineation of aquitard hydrogeologic units (HGUs) is critical to the development of conceptual site models (CSMs) for contaminated sites. However, aquitards are often delineated assuming a strong correlation between lithology and hydraulic conductivity. This assumption has not been rigorously tested in bedrock where fracture characteristics control bulk hydraulic conductivity. The objective of this research was to identify HGUs for a contaminated sedimentary rock site using a direct hydraulic dataset. Twelve boreholes were continuously cored to between 53 and 152 m bgs, comprehensively characterized, and instrumented with high-resolution multilevel systems. Head profiles were measured at each location several times per year over multi-year periods. The position/thickness of distinct vertical gradients occurred consistently in each profile indicating multiple, thin intervals of rock functioning as aquitards. The position of these aquitards was not predicted by lithostratigraphy but was strongly associated with sequence stratigraphy. The hydraulic contrasts identified by the head profiles were combined with sequence stratigraphy to delineate 6 aquitards and 4 aquifers. This hydraulically calibrated geologic framework was independently verified against the DNAPL and dissolved phase contaminant mass distributions. These HGUs now serve as the framework for numerical models which are needed to predict future conditions and evaluate remediation feasibilities and timeframes.
Paleo-Depositional Environment Controls Modern Day Groundwater Flow and Contaminant Migration
Ted Toskos
The anisotropy imparted by a heterogeneous aquifer complicated the development of a CSM for a former industrial facility in the Newark Basin, where past operations resulted in DNAPL releases to fractured bedrock. Understanding the heterogeneity was critical to understanding the control it exerted on contaminant transport and groundwater flow. The coarse grain size and poor sorting of the basal sandstones and conglomerates within upward fining sequences underlying the site, and the geometry of the sequences indicated paleo-deposition occurred in channels formed on terminal fans of steep gradient braided rivers. Conceptualizing this setting helped our interpretation of these small-scale sequences as being lens-shaped and laterally discontinuous in all directions, with overlapping depositional geometry. This interpretation and other investigative data allowed us to develop a reliable CSM of non-uniform, partially interconnected fracture networks (not continuous individual fractures) between sequences that extended down-dip beneath the site and offsite. The fracture networks within sequences pinched out like the depositional geometry of the small-scale lithologic sequences in which they developed. As a result, these fracture networks even if interconnected, occurred at different depths along strike and down dip. When made up of sufficiently interconnected openings these networks were groundwater bearing zones in which contaminants migrated.
Scaled Bioaugmentation Injection Strategy for Remediation of Mixed Chlorinated VOCs in a Fractured Shale Aquifer
Kevin Kelly, PG
Scaled bioaugmentation using an optimized strategy was applied to a mixed plume of chlorinated ethenes and methanes, primarily dissolved trichloroethene (TCE) and carbon tetrachloride (CTC), in a bedrock residual source area at the head of a 132-acre plume. We evaluated key fracture pathways, VOC degradation, bacterial census, CSIA, abiotic mechanisms, methane and ketone results to design the full-scale remedy, including addressing challenges of emerging contaminant 1,4-dioxane.
The mile-long commingled VOC plume in bedrock extends through a densely populated area located in the Newark Basin of northern New Jersey. Bioaugmentation was first applied in a 2015 pilot study primarily because of the lack of naturally-occurring bacteria in the bedrock aquifer. Three years of performance metrics showed significant mass reductions and informed the final 2019 remedial design; scaled 2019 bioaugmentation injections into eight bedrock wells, with geochemical reduction as an alternative. The injection design was optimized by modifying the volume of carbon substrate in bedrock to mitigate methane production, selecting of an alternative buffer to optimize pH, and the use of two different substrate products. We utilized geochemical conditions to degrade chlorinated solvents by biotic as well as abiotic methods (ferrous iron released from the matrix precipitated as iron sulfide).
Elements of Effective Conceptual Site Models II
A New Inexpensive Multi-Level System for Fractured Rock Sites and Sediments
Carl Keller
Boreholes in fractured rock are often instrument with a variety of multi-level systems (MLS) ranging from cluster wells, nested wells, CMTs, and several other systems which are installed with backfill between the casing and the borehole wall. The cost of the several systems varies dramatically depending on the materials and labor involved and also the sampling methods and the methods for measuring head at the several different elevations are variable. This presentation describes a different method devised by FLUTe which has two different aspects from the usual FLUTe methods of everting liners. A feature of the design is it can be used in highly unstable rock formations and even in sediments instead of only open stable boreholes. The new method, called a Cased Hole Sampler (CHS), has two forms: peristaltic sampling for shallow water tables and positive displacement pumping system for deep water tables. The CHS is not limited to cased holes and has been used in uncased holes. The new installation technique lowers the flexible liner assembly with the associated tubing, ports and spacers directly down the borehole or casing. In deep open stable holes a removable protective sheath avoids the possible abrasion of the liner.
Developing Effective Conceptual Site Models for Contaminated Bedrock Aquifers
Hank Andolsek, C.G.
Evaluating the nature and extent of groundwater contamination within a bedrock aquifer is complicated and expensive. Use of a conceptual site model (CSM) during preparation of a work plan ensures that data gaps are minimized and investigation objectives are met. The problem may be limited to the source area, or it may be much larger. CSMs are scale dependent, so understanding the potential breadth of the problem is important. Identifying the site-related flow field, both laterally and vertically helps frame the potential extent of the problem and determines the investigation approach. Groundwater flow in bedrock is controlled by hydraulic gradient and, where bedrock has limited primary porosity, the interconnected network of fractures. Published data are usually readily available for an area to begin to formulate a preliminary CSM before any borings have been installed. Understanding the contaminant chemistry, contaminant discharge mechanisms and duration of discharge, type, orientation and frequency of fractures, location of faults, location of potential groundwater discharge areas, vertical gradients, and hydraulic stresses within the flow field will enable development of a defensible CSM that can be used to develop a cost-effective bedrock investigation and later be used as a springboard for effective remedial designs.
Inspection of water level measurement data quality by means of a digital elevation model variogram analog
Joe Guggenberger, PhD, PE, RG
This presentation will describe a graphical technique for examining variance of ground water elevation values from unconfined aquifers and application of this technique to a case study at Fort Leonard Wood, Missouri. In evaluating quality of ground water elevation data for development of a ground water model, it may not be practical to consider every data point, particularly when working with the large datasets for a regional model. The new graphical technique expedites isolation of misrepresentative points in a large dataset, enabling users to examine the variance of ground water elevation values from an unconfined aquifer and to identify points with high variance. In the case study, potential outliers identified by the graphical variogram process, were evaluated by reviewing well borings, well installation records, and time series of water level measurements. Supplemental ground elevation data from a digital elevation model were used to create a threshold on the experimental variogram of ground water elevation data. This process was verified using a developed synthetic ground water dataset. The method facilitates identification of points that are justified for removal upon inspection of the available records, supports recommendations based on common causes of error, and serves as a preprocessing step prior to kriging.
Innovative/Cost-Effective Contaminant Assessment Strategies
1,4-Dioxane Assessment Strategies in a Fractured Rock Aquifer
Robert Bond, P.G.
The mass distribution and fate & transport of the emerging contaminant 1,4-dioxane in a sedimentary bedrock aquifer can be very different from the co-released chlorinated volatile organics (CVOCs) and requires a different strategy to assess. This presentation will detail our innovative assessment of 1,4-dioxane impacts in a fractured mudstone aquifer both in a source area and the distal plume. Two source areas had discharges of the 1,1,1-trichloroethane (1,1,1-TCA), which had been stabilized with 1,4-dioxane. The data set includes 20 years of data on CVOCs and 3 years on 1,4-dioxane. We used the 1,1,1-TCA/1,1-DCE/1,1-DCA forensic ratio method to date the releases of 1,1,1-TCA, and therefore also 1,4-dioxane, and used these estimates in fate & transport models. CSIA was also utilized on CVOCs and 1,4-dioxane to provide additional lines of evidence for sources. The network of 80+ monitoring wells, designed to characterize and monitor CVOCs in discrete highly transmissive bedding plane fractures, did not yield the same data set for the co-released 1,4-dioxane, which showed low concentrations over large areas of the 5,000-ft long plume. The more soluble and mobile 1,4-dioxane is depleted in monitored zones much faster than CVOCs, but is stored in the rock matrix, which informs the assessment strategy.
Direct Mass Flux Measurements for 1,4-Dioxane From an Industrial Site in a Karst Aquifer
Samuel Jacobson, BSc
Several sites are possible contributors to trace 1,4-dioxane detections in a municipal supply well in the karstic Upper Floridan aquifer. Efforts are underway to assess potential contribution from one nearby industrial site; however, high aquifer transmissivity and site dimensions create uncertainty in hydraulic gradients, thus stripping confidence from conventional Darcy’s law-based mass flux calculations. To avoid multiple measurement uncertainties, we advanced direct mass flux measurement techniques in two cored holes to characterize this site. Depth-discrete, high resolution rock core contaminant profiles were developed, and borehole geophysical and hydrophysical logs were collected. Physical caliper profiles captured the significant variability in borehole diameter, locating zones that could be sealed using FLUTe™ borehole liners. Hydraulically active features under natural hydraulic conditions were qualitatively identified using active distributed temperature sensing (A-DTS). These datasets informed placement of modified, fractured rock passive flux meters (PFM) in multiple, depth-discrete zones (1-2 m long), to quantify both water and contaminant flux. Vertical arrays of pressure and temperature sensors were co-deployed with the PFMs, external to the FLUTe™ liners, to capture transient hydraulic conditions. This presentation will discuss challenges and adjustments made for karstic boreholes as well as in-situ groundwater and contaminant flux distributions used to estimate mass discharge.
Evaluation of High Resolution Methods for Contaminant and Flux Distributions in Igneous / Metamorphic Rock Settings
Steven Chapman, M.Sc., P.Eng.
Characterization of contamination in fractured bedrock is a pervasive and challenging problem. While nearly all groundwater flow occurs in the fracture network, diffusion transfers mass to the matrix causing attenuation of downgradient transport, but also acting as an impediment to plume cleanup. High resolution rock core sampling using efficient and robust methods for sample collection, processing and lab extraction and analysis has been shown to be effective at many sedimentary bedrock (e.g. sandstone, mudstone) and a few crystalline bedrock sites for assessing contaminant distributions, while minimizing cross-connection effects that can confound assessment of groundwater concentrations. However in low matrix porosity igneous / metamorphic bedrock (e.g. granite, gneiss) with negligible matrix sorption, rock core sampling techniques are more challenging and may not always be diagnostic. This presentation will focus on insights from multi-method high resolution characterization at a chlorinated solvent contaminated site in Sweden involving rock core sampling, FLUTe FACT™ and depth-discrete groundwater sampling, along with complementary profiles from geophysical and hydrophysical logging. These datasets provide unique insights regarding method performance and biases (drilling disturbance, cross-connection and method sensitivity) for crystalline rock in contrast to sedimentary rock systems, and ultimately inform conceptual site models for transport to improve remediation decisions.
Integrated Use of Data and Numerical Models for Site Conceptual Model Development In Complex Hydrogeologic Systems
Donald Reeves, Ph.D.
Rainier Mesa is a tuffaceous plateau located on the Nevada National Security Site that has been subjected to a numerous subsurface nuclear tests conducted in a series of tunnel complexes. The tunnels are constructed near the middle of an 800 m thick Tertiary volcanic sequence of faulted, low-permeability welded and non-welded bedded, vitric and zeolitized tuff units that overlie a regional groundwater flow system within densely fractured carbonates. This presentation will focus on the multi-year development and iterative testing of a comprehensive site conceptual model that forms a basis for fluid flow and radionuclide transport simulations in support of the monitored natural attenuation strategy employed by the U.S. DOE. The hydrogeologic complexity of the site necessitated a comprehensive analysis of all available data to first explain and then properly develop numerical models that reasonably represent the dominant flow characteristics of the site which include an upper perched zone of saturated overlying a regional flow system separated by a thin unsaturated zone, and perplexing patterns and trends of fluid drainage from faults and fractures observed during tunnel excavation and operation. Emphasis will be placed on how seemingly disparate datasets and numerical modeling exercises were critical for successful site conceptualization.
Integration and Synthesis of Data at Different Scales of Investigation
Conceptual Site Model Evaluation to Support Focused Site Characterization – Rockland County, New York
Amberlee Clark
This presentation will discuss how refinement of a conceptual site model (CSM) led to a focused remedial investigation (RI). A former manufacturing facility with a deep bedrock production well used halogenated solvents and lubricants. Ten thousand gallons per day of non-contact cooling water generated during manufacturing was discharged to a leachfield for 30 years. Geology of the site consists of fractured sandstone bedrock overlain by dense glacial till deposits.
Bedrock production well testing in 1999 identified site contaminants of concern (COCs ). Site assessments were conducted which identified COCs above groundwater standards in overburden and bedrock groundwater. The original CSM indicated a sole source of contamination originating from the leachfield.
A RI was undertaken to evaluate and address data gaps identified in the original CSM. Data quality objectives and sampling techniques were selected to evaluate contaminant distribution under current site conditions.
Evaluation of the RI data indicates multiple source areas. COC distribution in the bedrock is influenced by the production well pulling contamination downward and against natural groundwater flow. Refinement of the CSM included targeted geophysical logging, aquifer hydraulic testing, and discrete sampling to evaluate fractures and flow potential in the bedrock including matrix diffusion of COCs as a continual source.
Bedrock production well testing in 1999 identified site contaminants of concern (COCs ). Site assessments were conducted which identified COCs above groundwater standards in overburden and bedrock groundwater. The original CSM indicated a sole source of contamination originating from the leachfield.
A RI was undertaken to evaluate and address data gaps identified in the original CSM. Data quality objectives and sampling techniques were selected to evaluate contaminant distribution under current site conditions.
Evaluation of the RI data indicates multiple source areas. COC distribution in the bedrock is influenced by the production well pulling contamination downward and against natural groundwater flow. Refinement of the CSM included targeted geophysical logging, aquifer hydraulic testing, and discrete sampling to evaluate fractures and flow potential in the bedrock including matrix diffusion of COCs as a continual source.
Correlation of Water-Bearing Fracture Zones with Stratigraphic Horizons in Sedimentary Rock
James Marolda, CPG, PG
In shallow to flat-dipping sedimentary rocks in the Northeast US, lateral groundwater flow is often controlled by open bedding plane fractures. If a readily recognized bed in a stratigraphic sequence (aka, a “marker bed”) can be identified proximal to a bedding plane fracture, that bed can be used to trace, project and predict the position of the fracture. Certain borehole geophysical tools can be used to identify the position of water-bearing fractures in a borehole. Others can be used to assess and correlate stratigraphy between locations. Together, the data from these logs may facilitate correlating water-bearing bedding plane fractures with marker beds. Connectivity of the fractures between locations can then be assessed using hydraulic monitoring.
At a former Manufactured Gas Plant (MGP) site in the northeast, a stratigraphic horizon was identified and correlated across the site using natural gamma logging; this horizon coincides with a water-bearing fracture zone. Hydraulic responses observed in nearby wells during drilling indicate a high degree of fracture connectivity within the zone, which serves as a primary migration pathway for MGP residuals and associated dissolved-phase contaminants. These findings yielded development of a reliable conceptual site model and will add efficiency to supplemental subsurface investigation efforts.
Focused Conceptual Site Model Evaluation for Remedial Action Selection – Colonie, New York
Nathan Vogan, PG
This presentation will discuss how refinement of a conceptual site model (CSM) led to an important remedial action decision. A former disposal site received steel manufacturing waste that was used to fill low lying areas. The site is underlain by impermeable glacial till overlying shale bedrock which slopes to the east and north with exposed bedrock.
A site characterization investigation identified contaminants of concern (COCs) including metals and polychlorinated biphenyls within the fill materials, overburden groundwater, and seeps on the steep slopes. A central bedrock knob was identified in direct contact with fill materials. A remedial investigation was conducted to evaluate the extent of contamination and refine the CSM. Water level measurements indicated radial horizontal flow consistent with topography, however overburden groundwater was not observed in the northern and eastern portions of the site. The contributing source of COCs identified at seeps on the steep slopes was undetermined.
A data gap investigation evaluated if bedrock groundwater is a source to the seeps. Geophysical logging identified transmissive fractures that would be unlikely contributing to seeps. The CSM and geochemical comparisons ruled out bedrock groundwater as a source and therefore, does not require remedial action which simplified site cleanup objectives.
Integrating Borehole-, Outcrop-, and Map-Scale Data to Construct a 3-D Model for PFOA Transport in Bennington, VT
Jonathan Kim
The fate and transport of PFOA is complex and involves emission as a waste product into the air, transport by wind, deposition on the ground surface, and downward eluviation through the vadose zone to the saturated zone and the fractured rock aquifer (FRA). In order to characterize the FRA in 3-D, we integrated data sets from borehole- to outcrop- to map-scale to construct a conceptual model for the contamination area.
Using acoustical borehole imaging, the orientations of structures in bedrock wells were determined. In addition, structures were identified as bedding, foliations, or fractures through borehole camera surveys and direct comparison with structures at nearby outcrops, including field measurements. By integrating borehole structural data with 1) temperature, conductivity, gamma, caliper, and heat-pulse flowmeter logs, 2) detailed structural data from outcrops, 3) the spatial analysis of well driller reports, and 4) bedrock geologic maps, the 3-D architecture of the FRA was constructed.
Significant findings are: A) Borehole structures strongly correlate with those at all scales and strongly influence the regional hydrogeology, B) One thrust fault is a barrier and the other is a conduit for groundwater flow, C) Steeply-dipping fracture zones bring up PFOA-free groundwater in areas surrounded by high-PFOA wells.
Tools and Techniques for Proper Characterization
Comparison of Near Replicate In Situ Trichloroethene Diffusion Tests in Fractured Sedimentary Rock Matrix
Richelle Allen-King, PhD
We developed a method to simultaneously measure sorption and diffusion coefficients for TCE and degradation products (DPs) in the low permeability matrix of fractured sedimentary rock in situ. We compare the results of two tests conducted in adjacent low permeability intervals of a single borehole in successive field seasons (2017 or 2018). The concentration patterns over time were qualitatively consistent for these two ‘near replicate’ tests building confidence in the method. Transport parameters in the matrix and biodegradation rate in the borehole were estimated using a radial diffusion model. The best-fit TCE sorption coefficients were greater than expected for low organic carbon content rocks, but consistent with independent laboratory measurements. The TCE biodegradation rates were very fast, greater than previous reports for this field site, and also consistent with laboratory experiments that used field samples. The porosities and tortuosity factors estimated are also reasonable. The best fit parameters estimated for each test differ modestly and differences may be attributable to lithologic variability and/or pre-test TCE concentration differences. Field and simulation sensitivity analysis combined suggest that abiotic TCE degradation had little effect on the TCE back diffusion rate.
Development of a Borehole Electrical Technology for Assessing Diffusion and Dual Domain Mass Transfer
Samuel Falzone
New technologies are needed to better understand the field-scale variations in effective diffusion coefficients, as well as the mass transfer parameters controlling contaminant transport, for low permeability zones (LPZs). Electrical geophysical measurements are theoretically related to the pore geometrical properties that control effective diffusion coefficients. Time lapse electrical measurements are sensitive to the transport of mass between LPZs and mobile zones, and can also sense sorption and desorption of mass from strongly sorbing matrix constituents. We have developed a borehole technology to interrogate diffusive mass transfer properties and processes on isolated sections of a borehole. The technology measures the complex (capturing both conduction and interfacial polarization mechanisms) electrical resistivity of a volume of sediment or rock mass adjacent to the borehole wall. Time-lapse measurements of the complex resistivity are recorded during injection of a tracer, from which diffusive mass transfer rates can be inferred. Preliminary measurements have been acquired at two Navy sites where back diffusion of contaminant mass from fractures into the rock matrix is documented. Laboratory tests on cores from the test sites validate the methodology for gaining insight into effective diffusion coefficients and mass transfer parameters in situ from a borehole.
Prototype Field Method for Diffusion and Sorption Coefficients and CVOC Reaction Rates in Low-Permeability Strata
Daniel J. Goode
The U.S. Geological Survey and the University at Buffalo, in cooperation with the U.S. Navy and the Strategic Environmental Research and Development Program, developed an in-situ field method for measurement of diffusion and sorption coefficients and reaction rates of chlorinated volatile organic compounds (CVOCs) in low-hydraulic-conductivity (low-K) sedimentary strata. A diffusion tracer test is initiated by replacing native groundwater between two packers, which isolate low-K strata from fractured strata, with native groundwater from which most of the CVOCs have been removed by sparging. Standard hydraulic tests prior to the test evaluate whether the isolated strata are sufficiently low-K such that advection will be insignificant. Groundwater samples are collected with a zero-net-volume, closed-loop protocol that further minimizes advection into and out of the isolated interval. Sample frequency ranges from sub-daily initially to at least weekly for three to four months. Biodegradation reaction rates and diffusion and sorption coefficients are estimated by inverse modeling of borehole CVOC and nonreactive tracer concentrations. The method is applicable to open intervals of wells that expose low-K strata, and have pre-test water quality data, including CVOC concentrations. A limitation is that groundwater levels must be shallow enough to allow peristaltic (suction) pumping from the interval.
Simulation of Borehole Test to Estimate in-situ CVOC Diffusion, Sorption, and Reaction Coefficients
Paul A. Hsieh
We developed a numerical model and coupled it with parameter estimation software to analyze data from a borehole test to estimate in-situ diffusion coefficients, sorption coefficients, and reactions rates of chlorinated volatile organic compounds (CVOCs) in low-permeability sedimentary rock. The model uses the finite-difference method to solve coupled solute-transport equations governing diffusion, linear sorption, and zero- or first-order sequential degradation of the parent CVOC and its degradation products. Simulation begins at the time when the borehole was drilled, exposing the borehole wall to CVOCs from contaminated fractures. Simulation of CVOC diffusion from the borehole into the rock matrix uses specified concentrations in the borehole as boundary conditions. During the field test, after concentrations of CVOCs in the borehole have been reduced by sparging, the model simulates concentration changes as CVOCs diffuse back out of the matrix and biodegradation occurs in the fully mixed borehole. PEST parameter estimation software identifies the reactive transport coefficients that yield the best match between simulated and measured concentrations in the borehole. Results of modeling borehole tests conducted in mudstone contaminated with trichloroethene showed that sorption limited the penetration of CVOCs into the matrix to just a few centimeters during several decades of contamination.