Title: Introduction Text: The cleanup of a site with a non-aqueous phase liquid (NAPL) source zone can be challenging because the NAPL acts as a continuing source of dissolved contamination to groundwater over a very long time period. Mass flux is a metric that is being proposed to better understand the impact of partial NAPL source removal and to assess the benefits versus costs of active source zone remediation efforts. Changes in mass flux before and after remediation could be used as one more tool to assess the effectiveness of a remedy. The US EPA Scientific Advisory Board has stated that: "measurements of mass flux of the contaminants and footprint parameters - not just concentrations - are necessary to document cause-and-effect and to assess long-term sustainability/permanence. Site-characterization and monitoring plans should be proactively designed to accommodate mass-flux estimates (US EPA, 2001)."

Title: NAPL Fate and Transport Concepts Text: Mass flux measurements can be used for sites with light non-aqueous phase liquid (LNAPL) or dense non-aqueous phase liquid (DNAPL) source zones. However, because of their complexity, it can be especially useful for DNAPL sites as described here. DNAPLs, such as chlorinated solvents, typically sink in groundwater until pooling above a low-permeability unit or becoming trapped as residual along the migration pathway. The DNAPL source zone is the portion of the subsurface that contains this pooled and/or residual DNAPL. Over time, typically over a period of decades, the DNAPL constituents gradually dissolve into groundwater. Because the pathway that the DNAPL follows depends on the unique heterogeneities of the site, it may be difficult to know where the DNAPL ends up. This often makes it difficult to accurately estimate the total mass of DNAPL present in the subsurface from soil and/or groundwater samples alone. Mass flux measurements can help to quantify the amount of DNAPL present in an aquifer and its impact on the dissolved phase plume.

Title: Mass Flux Concepts Text: Mass flux combines chemical data and groundwater flow velocity into a single measurement. Mass flux is a calculation of the mass of dissolved contaminants that passes through a cross-sectional area over time. Mass flux is expressed in the units of mass per time per area (e.g. lbs/hr-ft2). It provides an estimate of NAPL source strength and the rate of mass loading to the dissolved phase. In the figure shown here, note that there are areas where high concentrations correspond with high mass flux and low concentrations correspond with low mass flux. However, there are also areas where high concentrations do not correspond to high mass flux. These zones imply that the high concentrations are located in low velocity soils such as silts and clays. Furthermore, several moderate concentration zones exhibit high mass flux, which implies that they are in high velocity soils. This variation in mass flux is important to understand and points to an approach where remediation could be targeted to zones with high mass flux to improve effectiveness.

Title: Mass Flux Tools Text: There are several methods that have been proposed for measuring mass flux, but each has limited testing and their relative accuracies have yet to be determined. The three most common methods are: Using water quality data from groundwater well transects and groundwater velocity data Using passive flux meters with sorptive permeable media to intercept contaminated groundwater in downgradient wells and release tracers Using aquifer pump tests in downgradient wells These three methods are described in the following slides. In addition, click here for a detailed review of the advantages and limitations of these approaches as provided by the US EPA (2005).

Title: Mass Flux Transects (1 of 2) Text: One method of calculating mass flux is to use contaminant concentration data from a transect of groundwater wells and groundwater velocity data. The concentration data are multiplied by the corresponding flow data to give mass flux for a specific cross-sectional area of the plume. The results from all of the flow areas are summed together to obtain the total mass flux across the transect. This calculation can be done manually or by using computer software.

Title: Transect Calculation (2 of 2) Text: A Mass Flux Tool Kit has recently been developed as part of the Department of Defense (DoD) Environmental Security Technology Certification Program (ESTCP). Click here to learn more about this software. This software follows these steps: Plume Characterization: For a given transect, groundwater data are entered that defines the contaminant distribution in the subsurface including the full width and depth of the plume at that location. Single-level wells can be used, but multi-level monitoring points will provide a more detailed, three-dimensional characterization of the plume. Groundwater Flow Characterization: The specific discharge (q) is calculated for each transect given the hydraulic gradient (i) and the hydraulic conductivity (K) of the aquifer (where q = K x i). Subdivision of Transects: Transects are then divided into smaller subareas to better represent the different concentrations in the plume between each sampling location. Cumulative Mass Flux Calculation: The mass flux at a sampling location is the product of the contaminant concentration, specific discharge, and flow area. The total mass flux is the sum of all of the mass fluxes at all sampling locations across a given transect.

Title: Passive Flux Meter Text: Passive flux meters (PFMs) have been deployed for use in groundwater plumes with chlorinated solvents, petroleum hydrocarbons, pesticides, and inorganics (e.g., phosphate, nitrate, and metals). PFMs were developed by researchers at the University of Florida and are being studied under a current ESTCP project. Click here to learn more. The PFM is a permeable device that fits tightly in a screened well or boring and is placed in an aquifer for a period of days to months. Organic or inorganic contaminants present in the groundwater flow through it and are retained by the sorbents in the meter. These sorbents include a mixture of hydrophobic and/or hydrophilic materials. The mass of contaminant sorbed will be a function of the mass flux. The device also releases a soluble tracer into the groundwater and the loss of the tracer over time provides a measure of the groundwater flow rate. Given the time the device is placed, the mass flux is then calculated using the contaminant mass adsorbed and groundwater flow rate estimate.

Title: Aquifer Pump Test Text: Aquifer pump tests can be used to estimate mass flux from an extraction well or series of extraction wells located downgradient of the NAPL source zone. The concentration of the contaminant is measured in the pumping well as a function of time. The concentration versus time data are then used to determine the contaminant mass flux and estimated NAPL mass in the source zone using a mathematical fate and transport model. The assumptions of the modeling technique are as follows: The flow towards the pumping wells is radially symmetric and fully captures the contaminant plume. The aquifer is homogeneous with regard to porosity, hydraulic conductivity, and thickness. The concentration does not vary significantly in the capture zone of the well. The well is located far enough downgradient so that the groundwater flow conditions within the source zone are not significantly altered.

Title: Regulatory Considerations Text: The use of mass flux is not currently a widespread practice and therefore it has not been determined yet how mass flux can help to move toward site closure. Regulations do not address the reduction of mass flux as a remedial action objective and do not yet specify what amount of mass flux is acceptable. Remedial project managers (RPMs) planning to use reduction of mass flux as a remedial or performance objective should implement a rigorous data quality objective (DQO) evaluation process to specify precisely how mass flux data will be utilized to make management decisions. Mass flux measurements are not meant as a replacement for concentration-based values specified in regulations and the risk assessment process. Rather, mass flux estimates can serve as one more tool to characterize site conditions and to assess remedial action performance. Researchers have proposed the following ways to evaluate changes in mass flux before and after remediation of a NAPL source zone: Mass flux must be reduced enough to modify the dissolved plume behavior. Mass flux must be reduced to a level less than or equal to the “attenuation capacity” within the plume. Mass flux should be small enough so that flux-averaged concentrations at a down-gradient water supply well are below the regulatory limits (Annable, 2003).

Title: Case Studies: Mass Flux Measurements in Field Text: The results are presented here from two Navy case studies where mass flux measurements were determined in the field. Both projects were sponsored by ESTCP and were completed at Naval Base Ventura County (NBVC) in Port Hueneme, California. The studies were performed within a benzene and methyl tert-butyl ether (MTBE) plume at NBVC Port Hueneme. The groundwater plume at NBVC Port Hueneme is a result of a 1984 gasoline release from underground storage tanks (USTs) and fuel distribution lines at the Naval Exchange fuel service station.

Title: Case Study 1 Text:

Title: Case Study 1: Piezocone at NBVC Text: The objective of this ESTCP project was to demonstrate the use of an innovative direct push system for hydraulic assessments of contaminated aquifers. The system includes a high-resolution piezocone and a GeoVIS video microscope sensor to determine the direction and rate of groundwater flow in three-dimensions. Mass flux can then be calculated using the groundwater flow rate data and contaminant concentration data. As part of this project, groundwater flow rate and mass flux data measured by the piezocone were compared to data collected from nearby groundwater wells. It is anticipated that use of this technology could assist RPMs during the remedial action operation (RAO) and long term monitoring (LTM) phases of a project. Its use could help to prioritize areas for remediation and/or placement of wells in a monitoring network. Click here to learn more about this project.

Title: Case Study 1: Background Text: This slide explains how the high-resolution piezocone and GeoVIS sensors work. The high-resolution piezocone is a direct push sensor probe that provides data to determine soil type and hydraulic head as it is advanced into the subsurface. Click on the button to view an example of piezocone readings. A friction sleeve on the probe measures soil resistance which is correlated to soil classification (see first column of results). A pressure transducer mounted on the probe measures pore water pressure and is used to determine hydraulic head and dissipation curves that provide an estimate of hydraulic conductivity (see the second and third columns). Effective porosity can also be estimated based on the soil classification (see fourth column). The GeoVIS is a video microscope sensor that provides real-time images of the subsurface. Click on the button to view an example of a GeoVIS image. As shown here, the spatial arrangement of soil, groundwater, and free phase is used to estimate effective porosity.

Title: Case Study 1: Parez and Fauriel Relationship Text: This slide explains how the piezocone is used to estimate hydraulic conductivity by the Parez and Fauriel relationship. In saturated clays and silts, large excess porewater pressures are generated during initial penetration of the piezocone. Once penetration is stopped, these excess pressures decay with time and eventually reach equilibrium which corresponds to the hydrostatic head. This data of pressure decay with time is referred to as the pore pressure dissipation curve (shown on previous slide). As shown here, the hydraulic conductivity of the soil can be calculated using the Parez and Fauriel relationship and knowing the mean decay time.

Title: Case Study 1: Hydraulic Head Text: This plot shows an example of the three-dimensional hydraulic head measurements from the high-resolution piezocone. These results were compared to the hydraulic head measurements of wells in the same vicinity as the direct push locations. While some differences do exist between the high-resolution piezocone and control well data, it is important to note that gradient and direction as well as total range of values are in agreement. For instance, a total range of only 0.08 feet of head was observed across a transport distance of 25 feet by 15 feet of depth for each data set. In practice, piezocone data collection locations will be much farther apart, so this level of resolution should be extremely useful to modelers and remediation design engineers.

Title: Case Study 1: Hydraulic Conductivity Text: These results compare hydraulic conductivity (K) estimates and interpolations based on control well slug tests and high-resolution piezocone tests at approximately 11 feet below ground surface. Well K values were derived using slug tests, while the Mean K values represent estimates based on piezocone pressure dissipation tests. Lookup K values are based on a soil type conversion. Well K and Mean K values display similarities. On-going efforts will demonstrate that the control Well K values generally fall within range of piezocone derived K values. Soil Lookup values, although very useful, are only resolved to an order of magnitude in cm/s. However, one advantage of using the Soil Lookup K for deriving flux distribution models is the fact that a soil type value is generated every inch of each push.

Title: Case Study 1: Groundwater Velocity Text: These plots show the groundwater or seepage velocity estimated from the hydraulic gradient and hydraulic conductivity as measured by the piezocone and the effective porosity of the soil. The first row refers to a transect oriented perpendicular to the direction of flow, which for this case runs from the upper right towards the lower left of the top graphic. The centerline transect runs down the primary direction of flow within the multi-level well and push probe grids. Note the relative similarities between the calculated seepage velocity distribution along the centerline transect for both the control well derived estimates and the Mean K piezocone derived estimates.

Title: Case Study 1: Mass Flux Text: Mass flux is equal to the seepage velocity times the concentration at each location. As part of the project, a tracer was released and monitored as it moved through the well network. In order to predict the flux distributions, tracer transport was modeled to derive the concentration distributions over time. These slides show the predicted mass flux distribution 49 days after tracer release by multiplying the predicted concentration configurations by the velocity distributions depicted in the previous set of slides. Recent upgrades to GMS allow users to run these types of "spatial algebraic" calculations through the integration of high-resolution piezocone data with concentration data (obtained with sensor probes or well samples). For this example, well-derived hydraulic observations predict relatively more rapid movement of the tracer than the piezocone Mean K observations, so the modest to high flux zone is relatively larger (volumetrically) for the well-derived data set. In fact, after 49 days, the tracer was not predicted to reach the first row transect based on the piezocone derived data.

Title: Case Study 1: Concentration vs. Mass Flux Text: The top row displays three rotated views of the concentration distributions, while the bottom row depicts the same perspectives, but with flux distributions. Notice how the flux distributions cover much smaller volumes than the concentration distributions. This could be because low velocity soils bound the plume, thereby resulting in lower calculated flux values. Several conclusions can be drawn from this data. For example, high concentration does not always indicate high mass loading. Perhaps most importantly, a good remediation strategy could include defining high flux zones, followed by surgical removal or hydraulic isolation.

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Title: Case Study 2: Passive Flux Meter at NBVC Text: The objective of this ESTCP project was to demonstrate and validate the PFM as a tool for monitoring groundwater and contaminant mass fluxes. PFMs were tested at five sites with a wide variety of contaminants as listed below: NBVC Port Hueneme, CA - MTBE mass flux Hill Air Force Base, UT - DNAPL mass flux Canadian Forces Base - DNAPL mass flux NASA Launch Complex 34, FL - trichloroethene mass flux changes with bioaugmentation Naval Surface Warfare Center Indian Head, MD - perchlorate mass flux This case study will focus on the results obtained at NBVC Port Hueneme at the leading edge of an MTBE plume (shown here). Click here to learn more about the overall project.

Title: Case Study 2: Test Cell Text: The site-specific objective at NBVC Port Hueneme was to demonstrate the use of PFMs for the measurement of MTBE and groundwater fluxes in a cluster of wells. As shown here, the PFMs were deployed in several different wells using different installation techniques. The well designs included two different kinds of well construction techniques (direct push wells and hollow stem auger drilled wells), two well sizes (3/4-inch wells and 2-inch wells), as well as different slot sizes and filter pack materials.

Title: Case Study 2: Groundwater Velocity Text: This plot shows the depth-averaged groundwater flux for each of the five wells. The depth-averaged groundwater fluxes matched with previous slug test results except for the 2-inch drilled wells in which the PFM technique resulted in a much higher flux estimate. For these 2-inch hollow stem auger wells, the researchers indicated that the drilling process may have loosened the soil in the vicinity of the well and hypothesized that the PFM is more sensitive to local conductivity changes than the pneumatic slug test. These results highlight the large uncertainties that can be present in groundwater flow rate measurements and mass flux calculations.

Title: Case Study 2: Mass Flux Text: Two types of samples were collected for this study including groundwater samples from wells and sorbent samples from PFMs. Groundwater samples were collected in 40-ml VOA vials with zero headspace. Samples were pumped (or bailed) from the wells. The groundwater samples were analyzed for MTBE. Field samples of PFM sorbent were collected first to measure the initial concentrations of tracers present on the activated carbon. After 9-days of exposure, each flux meter was extracted from the well and sub-sampled in 5 to 30-cm vertical intervals. Sub-samples were homogenized and placed into 40-ml VOA vials each with approximately 20 g of sorbent extracted with 20 ml of alcohol. The extract from the sorbent samples were analyzed for MTBE. The flux-averaged MTBE concentrations from the PFM were found to be within a factor of 3 of the groundwater monitoring data.

Title: Conclusions Text: Mass flux reduction is not appropriate as the only performance metric for NAPL source zone remediation. However, it can serve as one tool among several others to assess the performance of NAPL source zone remediation technologies. Before widespread use of mass flux is implemented, further research is required to determine the accuracy and uncertainties associated with its measurement and to further quantify the relationship between NAPL mass removal and reductions in mass flux. ESTCP and others are conducting research into this area and references are provided on the next slide for further information.

Title: References Text: Annable, M.D. 2003. Flux Based Site Characterization and Remedial Performance Assessment. ESTCP. 2001. Demonstration and Validation of a Water and Solute Flux Measuring Device (ER-0114). ESTCP. 2004. Detailed Hydraulic Assessment Using a High-Resolution Piezocone Coupled to the GeoVIS (ER-0421). ESTCP. 2006. Field Demonstration and Validation of a New Device for Measuring Water and Solute Fluxes at Naval Base Ventura County (NBVC), Port Hueneme, CA. Farhat, S.K., Newell, C.J. and E.M. Nichols. 2006. User’s Guide: Mass Flux Tool Kit. US EPA. 2001. Monitored Natural Attenuation: USEPA Research Program - An EPA Science Advisory Board Review08-07-2001. US EPA. 2005. DNAPLs - Source Zone Behavior and Mass Flux Measurement. US Patent 6,208,940. March 27, 2001. Cone Tipped Cylindrical Probe for Use in Groundwater Testing, Kram, M.L. and Massey, J.A. US Patent 6,236,941. May 22, 2001. Cone Tipped Cylindrical Probe for Use in Groundwater Testing, Kram, M.L., and Massey, J.A. US Patent 6,317,694. November 13, 2001. Method and Apparatus for Selecting a Sand Pack Mesh for a Filter Pack and a Well Casing Slot Size for a Well, Kram, M.L. and Farrar, J.A.