1. Introduction
1.1. Purpose
2. Site Setting
4.1 3D Visualization Model 4-1
4.2 LNAPL Presence in North Olive Stratum 4-2
4.3 LNAPL Presence in Rand Stratum 4-3
4.4 LNAPL Presence in Main Silt, EPA, and Main Sand Strata 4-4
4.5 Comparison of Historical and Recent LIF Results 4-7
4.6 LNAPL Distribution Across the Hydrostratigraphic Units 4-9
4.7 Proposed Data Collection and Updates to the Comprehensive CSM
5.1 Soil Petrophysical Analysis 5-3
5.2 LNAPL Transmissivity Measurements 5-7
5.3 Historical LNAPL Recovery within the Main Sand Stratum 5-9
5.4 Historical LNAPL Recovery at the Premcor Facility 5-11
5.5 Proposed Data Collection and Updates to the Comprehensive CSM
7. REFERENCES
Tables
Figures
2. Volatile Petroleum Hydrocarbons Recovered Via Soil Vapor Extraction System
3. Fourth Quarter 2013 Groundwater Elevations, North Olive Hydrostratigraphic Unit
4. Fourth Quarter 2013 Groundwater Elevations, Rand Hydrostratigraphic Unit
5. Fourth Quarter 2013 Groundwater Elevations, EPA Hydrostratigraphic Unit
6. Fourth Quarter 2013 Groundwater Elevations, Main Silt Hydrostratigraphic Unit
7. Fourth Quarter 2013 Groundwater Elevations, Main Sand Hydrostratigraphic Unit
8. Collocated LNAPL Sample and LIF Boring Locations
9. Collocated Benzene Effective Solubility and Dissolved Phase Concentrations
10. LNAPL Viscosity in the Rand Stratum
11. LNAPL Viscosity in the Main Sand Stratum
12. Data Points Used to Generate the 3D Model
13. Boring Locations and 3D Lithology Data Display
14. LIF Locations and 3D LNAPL Data Display
15. LNAPL Distribution in the North Olive Stratum
16. LNAPL Distribution in the Rand Stratum
17. Fluid Level Elevations Within Select Wells Screened in the Rand Stratum
18. LNAPL Distribution in the EPA and Main Sand Strata
19. Fluid Level Elevations Within Select Wells Screened Within or Across the Main Silt Stratum
20. Fluid Level Elevations Within Select Wells Screened in the EPA Stratum
21A. Fluid Level Elevations Within Select Wells Screened in the Main Sand Stratum (Areas A, B1, and B2)
21B. Fluid Level Elevations Within Select Wells Screened in the Main Sand Stratum (Areas B3/B4 and C)
22. Maximum LNAPL Thickness in the Main Sand (2003-2005)
23. Maximum LNAPL Thickness in the Main Sand (2007-2009)
24. Maximum LNAPL Thickness in the Main Sand (2011-2013)
25. Lased Induced Fluorescence Boring Locations
26. LNAPL Distribution within the Hydrostratigraphic Units
27. Schematic Diagram of the Dual Optimal LNAPL Response Model
28. Historical LNAPL Soil Core Sample Locations
29. Fluid Saturations for Soil Cores Below the Water Table in the Main Sand
30. Mobile and Residual LNAPL Saturations Based on Lab Centrifuge
31. LNAPL Transmissivity Summary for 2005 High Vacuum Recovery Pilot Testing
32. LNAPL Transmissivity Summary for 2011 Multiphase Extraction Pilot Testing
33. Locations Considered as Part of Evaluating the Dual Optimal LNAPL Response Model
34. Historical LNAPL Recovery From the Main Sand Stratum
35. Schematic Diagram of Low Flow Dual Phase Extraction and Focused Pumping
Appendices
Appendix A
Appendix B
Appendix C
Appendix D
Appendix E
Appendix F
Appendix G
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Conceptual Site Model, May 2014
Hartford Petroleum Release Site
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6.0 SUMMARY OF THE LNAPL COMPONENT TO THE CSM
The Comprehensive CSM will integrate data previously collected at the Hartford Site with new analyses and visual presentation of the existing data to provide a comprehensive depiction of current conditions that will serve as the baseline for understanding the distribution of the LNAPL and subsequent partitioning to the dissolved and vapor phases, as well as the pathways for potential exposure to receptors. The Comprehensive CSM will guide: (1) evaluation of the current remedial activities (e.g., SVE, LNAPL skimming), (2) the remedy selection process, and (3) expectations for evaluating remedy performance in the future.
The Comprehensive CSM for the Hartford Site is being prepared in a step-wise fashion, starting with this LNAPL component to the CSM. The next two deliverables will provide updates to the CSM for dissolved and vapor phase petroleum hydrocarbons partitioning from the LNAPL. The final deliverable, the Comprehensive CSM, will compile all the information presented within the LNAPL, dissolved phase, and vapor phase components to the CSM, as well as additional information gathered as part of resolving data gaps.
6.1 SETTING
The Hartford Site is located along the historical edges of the Mississippi and Missouri River flood plains within a shallow valley approximately 30 miles long and 11 miles across at its widest point, and underlain by more than 100 feet of unconsolidated deposits created by alluvial and glacial processes during the Pleistocene period. Over the last 125,000 years, the Mississippi River has changed its course frequently resulting in deposition of sediments with widely-varying grain size across a broad area creating a highly heterogeneous unconsolidated stratigraphy (USEPA 2010). As a result, the lithology beneath the Hartford Site consists of alternating alluvial deposits of clay and silt overlying a regionally extensive sand deposit referred to as the Main Sand stratum. The Main Sand stratum consists of alluvial sands and coarse grained glacial outwash that ranges from 80 to 100 feet in thickness. The permeable zones of alluvial deposits overlying the Main Sand are locally known (in descending order) as the North Olive, the Rand, and the EPA hydrostratigraphic units. These permeable zones are bounded by discontinuous clay deposits identified as (in descending order) the A, B, C, and D Clay strata.
The A Clay stratum is continuously present beneath the Hartford Site, with the exception of areas where it has been removed as part of construction activities. The B and C Clay strata are highly discontinuous and of limited aerial extent. The B and C Clay strata define the extent of the North Olive and Rand hydrostratigraphic units, respectively. The North Olive and Rand strata laterally grade into and are hydraulically connected with the Main Sand (and Main Silt where present under the western and southwestern portions of the Hartford Site), where the B and C Clay strata are absent. Groundwater within the North Olive and Rand strata generally occurs as isolated areas of perched water on the surface of the underlying clay.
The D Clay stratum underlies and defines the limits of the EPA stratum. The D Clay stratum could be considered a discontinuous lens within the Main Sand stratum based on its relative thickness (thickness between approximately 2 to 7 feet) and limited extent (only present in the northeastern portion of the Hartford Site). The EPA stratum grades laterally into the Main Sand to the south of a southwesterly trending line extending from the intersection of Old St. Louis Road and North Delmar Avenue to just north of the intersection of East Date Street and North Olive Street. Along this boundary, the EPA and Main Sand strata are hydraulically connected with flow in the EPA stratum towards the southwest.
Groundwater present in the Main Sand stratum is part of an extensive aquifer system commonly referred to as the American Bottoms aquifer. Groundwater flow in the Main Sand stratum has been altered beneath the Hartford Site due to pumping on the BP (approximately 1,225 gpm), Phillips66 (more than 6,000 gpm along the river dock and 3,000 gpm on the refinery), and Premcor (approximately 300 gpm) facilities. The groundwater flow direction in the Main Sand is also influenced by the stage of the Mississippi River. During periods of high river stage groundwater flow is generally towards the east to northeast due to recharge from the river and bank storage within the Main Sand. During moderate river elevations, the groundwater flow direction is northward. During low river stages, groundwater flow trends westerly to northwesterly.
The Mississippi River is located less than a half mile from the Hartford Site and is hydraulically connected to the two deeper hydrostratigraphic units (EPA and Main Sand), and on occasion during very high river stages, the groundwater surface in the Main Silt and Main Sand can reach the Rand stratum. Water level fluctuations in the EPA stratum and Main Sand correspond to changes in the Mississippi River stage. Since the river stage varies by more than 20 feet during a year, the groundwater conditions can fluctuate from unconfined to confined conditions throughout the year.
6.2 LNAPL DISTRIBUTION
Petroleum hydrocarbon releases occurred from the former refineries and related facilities located to the north and east of the Village of Hartford, as well as from pipelines connecting these refineries and facilities with terminal operations on or near the Mississippi River (Figure 1). Released hydrocarbons (LNAPL) migrated down through the subsurface under the influence of gravity until encountering the groundwater or less permeable layers. Due to capillary forces, some fraction of the LNAPL was retained in soil pore space in the unsaturated zone, whereas some fraction of the LNAPL reached the capillary fringe where it displaced water present in soil pore space. As the volume of LNAPL became sufficient to overcome hydrostatic forces, further lateral migration occurred. Vertical migration into deeper hydrostratigraphic units occurred where the clay layers are discontinuous or absent. The distribution of LNAPL stabilized as gravity and capillary forces approached equilibrium.
Vertical smearing of the LNAPL occurred over time as a result of fluctuation of the groundwater elevations within the hydrostratigraphic units beneath the Hartford Site, leaving some LNAPL within the soil pore spaces below and above the water table. The smear zone describes the horizontal and vertical extent of LNAPL (including residual and mobile) beneath the Hartford Site. The nature and extent of the LNAPL smear zone has been previously defined, at least in part, across the various hydrostratigraphic units using LIF, soil core analyses, and routine fluid level monitoring (Clayton 2004, 2005, 2006a). The bottom of the "smear zone" is roughly coincident with the historical low groundwater elevation in the Main Sand stratum (10 to 20 feet lower than typical groundwater elevations observed over the past decade). The thickness of the smear zone is variable measuring only a few inches at the periphery, to tens of feet in locations near historical releases and along the boundaries of the clay strata. The vertical and lateral distribution of the smear zone also varies due to heterogeneities in the lithology.
As part of the preparation of this component to the CSM, a 3D visualization model was developed, which had been noted as an important data gap in previous analyses (USEPA 2010). The 3D model integrates the lithology, LNAPL distribution, and hydrocarbon types across the Hartford Site. Figure 12 presents the data that was incorporated into the 3D model. The 3D model indicates that LNAPL is present within each of the hydrostratigraphic units, with the greatest lateral and vertical extent observed in the Main Sand stratum. LNAPL is continuous through the shallower hydrographic units and into the Main Sand stratum, indicating historical releases at shallower depths with subsequent lateral migration along the tops of the clay layers and vertical migration where these clay layers are discontinuous or absent. All three LNAPL types are observed within the Main Sand stratum, with light-range LNAPLs having the largest distribution beneath the Site. Mid- and heavy-range LNAPLs are primarily observed in the northeast and eastern edge of the Hartford Site, respectively. Several disconnected and smaller localized releases of light-range and mid-range LNAPLs are also observed in the shallow hydrostratigraphic units. These may be indicative of smaller, isolated sources in the shallow subsurface.
6.3 LNAPL CHEMISTRY
Historical LNAPL samples collected from the Main Sand stratum have been characterized as primarily gasoline range hydrocarbons with a secondary product type, where present, identified as diesel (Clayton 2005). For the most part, LNAPL with gasoline reported as all or nearly all of the hydrocarbon makeup were identified in the central portions of the Hartford Site; while LNAPL with diesel as a secondary component tended to be located on the eastern and northern portions of the Site. LNAPL samples collected from the EPA and the Rand strata have been primarily characterized as diesel with lesser amounts of gasoline. LNAPL samples collected beneath the Hartford Site generally have viscosities below one centipoise, consistent with gasoline and diesel mixtures. A comparison of the LIF waveforms to the LNAPL analytical results indicates that the LIF results are a reasonable indicator of LNAPL types at the Hartford Site. The LNAPL chemistry data set is strongest for light-range LNAPL with a gasoline-like carbon distribution, and could be bolstered by collecting additional LNAPL samples from areas with mid- and heavy-range LNAPL types.
6.4 LNAPL RECOVERABILITY
Approximately 2.25 million gallons of LNAPL has been recovered beneath the Hartford Site via interim measures between 1978 and 2014. LNAPL recoverability is a function of water table elevation which has changed through time. The DOLR model (H2A 2006) developed for the Hartford Site explains the occurrence and potential recoverability of LNAPL under various hydraulic conditions. In summary, the DOLR model predicts that LNAPL thickness in wells will be high under confined conditions, with initial high LNAPL recovery rates that may decrease over time because the mass of mobile LNAPL is minimal and much of the LNAPL is submerged (although optimization of recovery under confining conditions is possible under certain lithologic conditions). Under intermediate unconfined conditions, LNAPL thicknesses may be smaller, and recovery rates may be relatively low because the wells are no longer acting as “pressure relief” points and much of the LNAPL continues to be submerged. Under highly unconfined conditions, relatively high recovery rates may be attained because the largest vertical interval of LNAPL is unsubmerged.
LNAPL recovery performed under confined and intermediate unconfined conditions over the last decade at the Hartford Site have generally supported predictions of the DOLR model, with a decrease in the rate of LNAPL recovery and LNAPL transmissivity observed within the wells where skimming has been conducted. However, the DOLR model has not been tested under all anticipated hydraulic conditions beneath the Hartford Site. Specifically, the DOLR model predicts that when groundwater elevations are within the lower portions of the smear zone, LNAPL recovery rates would be most sustainable over time. Water table elevations have not approached these lower portions of the smear zone since installation of Dam No. 27 down-stream of the Village of Hartford between 1959 and 1963.
Therefore, in order to observe and confirm LNAPL recovery under low water table conditions, an additional pilot test has been developed to evaluate groundwater extraction at higher rates within a focused portion (Area A) of the Hartford Site, when groundwater within the Main Sand is approaching seasonally low and unconfined conditions. The pilot test will be conducted pursuant to the approved Final Light Non-Aqueous Phase Liquid Recovery Pilot Test Work Plan Addendum (Trihydro 2013a). Through the pilot test, LNAPL recoverability can be observed in the vicinity of the groundwater extraction well. The pilot testing will assess whether inducing unconfined conditions in the vicinity of a groundwater production well can increase LNAPL recovery rates within the Main Sand stratum. The results from this pilot test represent the largest remaining data gap within this component to the CSM and will be incorporated into the Comprehensive CSM.
6.5 PROPOSED DATA COLLECTION AND UPDATES TO THE COMPREHENSIVE CSM
The most significant data gap remaining in the LNAPL CSM is related to LNAPL recoverability. This data gap will be addressed via implementation of an additional LNAPL recovery pilot test as described within the Comprehensive Conceptual Site Model Framework and Timeline, Hartford Area Hydrocarbon Plume Site (Trihydro 2013c). In addition, it is recommended that additional light-, mid-, and heavy-range LNAPL samples be collected from groundwater monitoring wells and monitoring points screened in the Rand, EPA, and Main Sand strata. These LNAPL samples will be used to further correlate the hydrocarbon types to the LIF results, as well as to assess individual constituent and mass depletion of the smear zone beneath the Hartford Site. Finally, as part of preparing the dissolved and vapor phase components to the CSM, the 3D visualization model will be updated with dissolved and vapor phase analytical results to better understand partitioning of petroleum-related constituents from the LNAPL to groundwater and soil vapor, as well as to assess mass losses due to natural attenuation processes. Thus, a more complete understanding of the distribution of the LNAPL, chemical partitioning to the dissolved and vapor phases, pathways for potential exposure to receptors, and LNAPL recoverability will be secured when this additional data is obtained, analyzed, and integrated into the Comprehensive CSM.