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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4.0 LNAPL DISTRIBUTION
LNAPL distribution is affected by several factors including but not limited to the lithology, groundwater fluctuations, release history, and LNAPL recovery efforts. As detailed in Section 2.2, approximately 2.25 million gallons of LNAPL have been recovered from the Hartford Site, with approximately half of that mass removed from the Main Sand via LNAPL skimming and the other half from the overlying strata via vapor extraction. Recent investigation and routine monitoring activities indicate that while the extent and mass of petroleum hydrocarbons has been reduced over the past three decades, LNAPL remains beneath many portions of the Hartford Site.
4.1 3D VISUALIZATION MODEL
Historical investigations have documented that LNAPL is primarily distributed within the more permeable strata including the North Olive, Rand, EPA, and Main Sand hydrostratigraphic units. Because historical LIF data provide information about the horizontal and vertical extent of LNAPL, as well as hydrocarbon type across the Hartford Site, these data were incorporated into a 3D visualization model. This model also incorporates lithologic information from 379 soil borings. In addition, cone penetrometer testing conducted contemporaneous to ROST™ within the same boring was used to verify the lithologic descriptions. Ground surface elevations for each boring and monitoring location were also incorporated into the model where available. It should be noted that some information from off-site borings and monitoring locations were incorporated into the 3D model to improve interpretations along the lateral limits of the Hartford Site. Figure 12 presents the data that was incorporated into the 3D model.
Leapfrog Hydro 4.0™ was used to integrate the LIF, lithology, and ground surface data for display in three dimensions. Lithology is generated as a 3D mesh, with zones between data points using all adjacent borings for interpolation. Contaminant distribution, in this case LNAPL, is developed by krigging a dataset and displaying the 3D shape of interpolated values. The krigging parameters can be adjusted by the modeler and this was done to develop LNAPL bodies consistent with the general understanding of LNAPL morphology. For instance, LNAPL tends to spread along the contact between the more permeable, hydrostratigraphic units and the less permeable strata.
A total of 379 locations providing subsurface lithology were incorporated into the 3D model. When interpolated between locations, a 3D depiction of the various clays layers is generated, as shown in the oblique views on Figure 13. In addition, LIF data from approximately 109 borings installed in 2004 and 2005 were incorporated into the model, in a manner consistent with historical conventions and interpretations (Clayton 2004). Interpretations from two of the historical LIF borings were adjusted slightly based on observations within nearby locations. Specifically, LNAPL present within the lower portions of the smear zone in the Main Sand stratum (approximately 40 to 45 ft-bgs) within borings HROST-002 and HROST-003 was previously identified as a mid-range hydrocarbon. However, these waveforms were “borderline” with a heavy-range LNAPL type. More recent borings HROST-087 and HROST-090 indicated heavy-range LNAPL with this portion of the Main Sand; therefore, LNAPL within the lower portion of the Main Sand within borings HROST-002 and HROST-003 were changed from a mid-range to heavy-range LNAPL.
An oblique view of the LIF boring locations and depiction of the various LNAPL types beneath the Hartford Site are shown on Figure 14. If the waveforms indicated more than one LNAPL type within a boring, these different LNAPL types were depicted within the model. Figure 14 includes the vertical and horizontal limits of the smear zone where LNAPL shares pore space (whether residual or mobile) with groundwater and soil vapor (if within the vadose zone). While the model indicates the presence or absence of LNAPL (i.e., normalized fluorescence measurements above background), it does not provide any indication of LNAPL saturations or potential recoverability. In addition, Figure 14 shows edges of the model that are beyond the boundaries of the Hartford Site where less data is available (Figure 12). The model algorithm interpolates areas between data points and often extrapolates them to the domain boundaries. Thus, observations for areas beyond the Site boundary should be considered in the context of the density of data integrated into the model in the area of interest.
4.2 LNAPL PRESENCE IN NORTH OLIVE STRATUM
Figure 15 displays the B Clay, as well as LNAPL present within the overlying North Olive stratum. It should be noted that the North Olive stratum is defined by the presence of the underlying B Clay, such that the North Olive is absent if the underlying B Clay is absent. The North Olive stratum extends across the majority of the Hartford Site, with the most notable absence in the central area of the Site along North Delmar Avenue and North Market Street. Within the North Olive stratum, all three LNAPL types described in Section 3.1 are identified.
Light-range LNAPL is present in the northeast corner of the Site along East Rand Avenue. This LNAPL type is also present in localized areas along: (1) North Old St. Louis Road between West Birch and West Cherry Streets, (2) near the intersection between East Forest Street and North Olive Street, and (3) on the eastern portion of the Hartford Site, between East Cherry and East Date Streets. A body of light-range LNAPL is also present in the central portion of the Site, along the southern edge of the B Clay stratum. Light-range LNAPL is also present immediately below these locations within the Rand and Main Sand strata which may indicate vertical migration downward as LNAPL reached the edges of the B and C Clay.
Mid-range LNAPL is present in the northeast corner of the Hartford Site, flanking the light-range LNAPL in this area. Two localized bodies of the mid-range LNAPL are observed in the northwest portion of the Site (one south of West Rand Avenue and the other at the corner of West Arbor Street and North Old St. Louis Road) and two are present on the eastern side of the Site (both along East Elm Street).
Heavy-range LNAPL is present at the eastern edge of the Site along North Olive Street. These heavy-range hydrocarbons are not identified in other portions of the Site within the North Olive stratum.
The above describes the distribution of LNAPL within the shallowest hydrostratigraphic unit beneath the Hartford Site. LNAPL and groundwater generally occur in isolated areas that are temporarily perched on the surface of the underlying B Clay before draining into underlying stratum. LNAPL is generally not measured in wells screened in the North Olive stratum. There are only two groundwater monitoring wells (HMW-013 and HMW-044A) and two monitoring points (MP-055A and MP-108B) screened within the North Olive stratum where LNAPL has been measured (thickness greater than 0.01 feet), with the most recent occurrence reported in monitoring point MP-108B in April 2011. LNAPL has only been measured 22 times in these four locations and only under unconfined conditions. It is possible that LNAPL is currently present at residual saturations based on the lack of LNAPL thickness measured in wells screened in this stratum since April 2011.
4.3 LNAPL PRESENCE IN RAND STRATUM
Figure 16 displays the C Clay, as well as LNAPL present within the overlying Rand stratum. The C Clay is highly discontinuous and only present in the northern and eastern portion of the Site, with the edge of this clay stratum trending southeast from the west side of West Cherry Street to the east side of West Watkins Street. Similar to the North Olive stratum, the Rand stratum is absent south of Watkins Street, although it generally covers a smaller footprint than the North Olive stratum. All three LNAPL types have been observed within the Rand stratum.
Light-range LNAPL is present in a localized area along North Old St. Louis Road, south of West Birch Street and along West Cherry Street and appears continuous with light-range LNAPL within the North Olive stratum but with a larger lateral extent. This, combined with the localized absence of B Clay in the area (Figure 15), suggests a release within or above the North Olive stratum that migrated downward to the top of the C Clay. Light-range LNAPL is also present to the south near the edge of the C Clay. This light-range LNAPL is also present immediately above in the North Olive and below in the Main Sand and appears to have migrated downward as it reached the edges of the B and C Clay.
Mid-range LNAPL is present primarily at the northeast corner of the Hartford Site in the Rand stratum. This mid-range LNAPL overlaps with that present in the overlying North Olive stratum, suggesting a similar source. However, mid-range LNAPL in the Rand extends across a wider footprint compared to that in the North Olive, and again could be an indication of lateral LNAPL spreading that occurred at the top of the C Clay. A localized body of mid-range LNAPL is present in the northwest portion of the Site, near the corner of West Arbor Street and North Old St. Louis Road. A localized mid-range LNAPL has also been identified to the south along East Elm Street and might be an indication of commingling of the heavy-range and light-range LNAPL bodies in this area.
Heavy-range LNAPL is present at the eastern edge of the Site along North Olive Street. This LNAPL type is not identified in other portions of the Site within the Rand stratum.
The above summarizes LNAPL present at intermediate depths beneath the Hartford Site. In general, water and LNAPL are discontinuous and perched within the monitoring wells and points screened in this stratum. As shown on the comparison of LNAPL thickness and groundwater elevations over time for select wells screened within the Rand Stratum (Figure 17), the frequency and occurrence of LNAPL at thicknesses greater than 0.1 feet in groundwater monitoring wells and multipurpose monitoring points has been decreasing over the past five years.
4.4 LNAPL PRESENCE IN MAIN SILT, EPA, AND MAIN SAND STRATA
Figure 18 displays the D Clay, as well as LNAPL present within the overlying EPA stratum, and underlying Main Sand where the D Clay is absent. The D Clay could be considered a thin lens within the Main Sand stratum and is only present in the northeastern most portion of the Hartford Site. This means that the EPA stratum is limited in aerial extent in the Village of Hartford and that the Main Sand is present beneath most of the Site. All three LNAPL types are observed within these strata across a wider footprint than the shallower hydrostratigraphic units. The majority of the LNAPL mass is currently present within the EPA and Main Sand strata (Clayton 2005).
Light-range LNAPL is present throughout much of the central portions of the Site within the Main Sand. These light- range hydrocarbons are similar to that present in the Rand and North Olive strata; however, they extend across a larger aerial extent within the Main Sand. This morphology suggests that LNAPL released from within or above the North Olive stratum migrated downward with distribution controlled by the multiple factors previously discussed in Section 2.1. LNAPL occurs across the greatest vertical interval within the Main Sand. This thickness is variable, measuring less than one foot along the southern and western limits of the smear zone and as much as 29 feet within the central portions of the Hartford Site. It appears that the predominant LNAPL mass is present in the Main Sand, which is consistent with fluid level measurements and the volume of LNAPL recovered from the various strata via skimming.
The large vertical extent over which the smear zone is observed within the Main Sand can be attributed to the volume of hydrocarbons present at the time of the release(s), the period over which hydrocarbons were released, as well as the magnitude of water table fluctuations within this deeper hydrostratigraphic unit. Localized and discontinuous bodies of light-range LNAPL also appear to be present northwest of the Site.
Mid-range LNAPL is present at the northeast corner of the Hartford Site and appears continuous with mid-range hydrocarbons present in the overlying Rand and North Olive strata, suggesting a similar source. However, LNAPL present in the EPA and Main Sand has a larger footprint than either of the above hydrostratigraphic units and could be an indication of LNAPL spreading that occurred along less permeable zones (e.g., D Clay or Main Silt), as well as along the groundwater table. A localized body of mid-range LNAPL is also present at the eastern edge of the Site at the intersection of East Elm Street and North Olive Street and might be an indication of commingling of heavy-range and light-range LNAPL bodies.
Heavy-range LNAPL is present at the eastern edge of the Site along North Olive Street. This LNAPL is not present in other portions of the Site within the EPA and Main Sand strata. A discontinuous body of heavy-range LNAPL is also present to the northwest of the Site.
The above describes LNAPL present within the deeper saturated strata beneath the Hartford Site. Groundwater and LNAPL present in these strata can occur under confined or unconfined conditions depending on the fluid level elevation and occurrence of overlying less permeable strata including the D Clay to the northeast, the C Clay within the central and eastern portions of the smear zone, and sometimes the Main Silt present in the western and southern portions of the Hartford Site. As shown on Figure 19, LNAPL is generally not observed at a high frequency nor at significant thickness within monitoring locations screened within the Main Silt (e.g., MP-038B and MP-048B) compared to locations screened in the Main Sand.
There are locations where the Main Silt appears to have a confining effect on LNAPL thickness measurements in the Main Sand; however, this is only reflected in the routine gauging results from a few locations such as monitoring point MP-038C. In other locations, the Main Silt does not appear to have any effect on apparent LNAPL thicknesses measurements, such as monitoring point MP-048C (where LNAPL thicknesses do not substantially change relative to the groundwater and LNAPL elevations) or monitoring point MP-049C (where the overlying fine grained unit appears to result in confined LNAPL and groundwater conditions but not the Main Silt).
LNAPL is only measured above 0.01-foot thickness in two monitoring locations (HMW-048 and MP-085C) screened within the EPA. As shown of Figure 20, LNAPL occurrence within monitoring point MP-085C is relatively thin and infrequent. LNAPL thickness measurements in monitoring well HMW-048C have generally been decreasing over the past two years with the exception of periods when water and LNAPL are unconfined, and groundwater elevations are measured below approximately 395 ft-amsl.
LNAPL has been measured within many of the groundwater monitoring wells and monitoring points screened in the Main Sand; therefore LNAPL skimming has historically been focused within wells screened within these strata. As shown on Figures 21A and 21B depicting fluid level elevations in selected monitoring locations screened in the Main Sand stratum, LNAPL thicknesses have generally decreased since 2004 under both confined and unconfined conditions. These decreases in LNAPL thickness may suggest an overall reduction in the mass or saturations of LNAPL near these wells. These reductions may be attributable to mass recovery via manual LNAPL skimming, redistribution of LNAPL with fluctuating groundwater elevations, and natural smear zone depletion. Losses attributed to skimming performed within the groundwater monitoring wells and monitoring points since 2009 may be localized to portions of the stratum immediately adjacent to the well screen. A discussion regarding the influence of LNAPL skimming on the radius of capture near the monitoring locations is provided in Sections 5.2 and 5.3.
Figures 22 through 24 show the maximum thickness of LNAPL measured within groundwater monitoring wells and multipurpose monitoring points screened within the Main Sand Stratum over three time periods including 2003 through 2005, 2007 through 2009, and 2011 through 2013. These figures present the maximum LNAPL thickness measured within the monitoring locations over each two year span. LNAPL thicknesses were only considered when the fluid levels were present within the screen interval of the monitoring location. In general, the lateral extent of monitoring locations where LNAPL has been measured at thicknesses less than one foot under unconfined conditions has been consistent beneath the western and northern portions of the Hartford Site, providing evidence that the smear zone is stable. The thickness of LNAPL measured in the monitoring locations screened in the Main Sand along the southern limits of the Hartford Site are generally decreasing suggesting that the saturations may be decreasing in this portion of the smear zone. While LNAPL thicknesses generally appear stable or decreasing along the edges, redistribution of LNAPL is occurring within the interior portions of the smear zone. This redistribution results in increasing thicknesses observed in some areas with decreases in LNAPL thicknesses observed in other portions of the smear zone over time. This redistribution could be attributed to LNAPL recovery efforts including skimming which have resulted in LNAPL gradients towards recovery wells, hydraulic controls from nearby facilities, and fluctuating fluid level elevations over time.
4.5 COMPARISON OF HISTORICAL AND RECENT LIF RESULTS
A total of 24 LIF borings were installed in September 2013 across the Hartford Site using an Ultraviolet Optical Screening Tool (UVOST™). As shown on Figure 25, fourteen borings were installed at previous ROST™ monitoring locations within the six proposed remediation areas (Areas A, B1, B2, B3, B4, and C) described in the LNAPL Active Recovery System 90% Design Report (Clayton 2006). These fourteen LIF borings were installed to assess changes in the LNAPL distribution within the hydrostratigraphic units targeted for remediation. To assess changes in the lateral and vertical distribution of LNAPL along the western and southern limits of the smear zone, ten additional borings were installed at previous LIF borings installed in 2004 and 2005 (including borings HROST-007, -013, -019, -028, -049, -066, -068, -072, -090, and -099). Each boring was installed to a minimum of five feet below the vertical smear zone limits in the Main Sand. It should be noted that a proposed LIF boring at location HROST-123 could not be completed in September 2013; multiple attempts to install an LIF boring at this location resulted in refusal at approximately 3 to 5 ft-bgs.
Both the ROST™ and UVOST™ make use of fluorescence and data acquisition systems developed wholly or in part by Dakota Technologies. These two methods differ primarily in the laser and associated wavelength used to excite polycyclic aromatic hydrocarbons (PAHs) within the LNAPL (290 and 308 nanometer wavelengths, respectively). The PAH mixtures within the LNAPL emit photons of a distinctive wavelength irrespective of the excitation wavelength, although the intensity of the response may vary. By sampling the total fluorescence at different wavelength channels (which are nearly identical for both tools), a multi-wavelength waveform is generated. The waveform allows simultaneous description of the spectral and temporal qualities of the fluorescence with depth and can be used to identify different product types. The waveform data are referenced and displayed as a percent of the response compared to the calibration reference emitter (RE). The RE is similar to a calibration gas used in a flame ionization or photoionization detector, and is placed on the sapphire probe window before collecting fluorescence data at each boring. The same RE is used for the ROST™ and UVOST™ (that is to say, the RE produces the same multi- wavelength waveform). Fluorescence measurements generated in the borings are normalized to the RE measurements which allows for spatial and temporal comparisons of the fluorescence results despite changes in the optics, laser energy drift, window, mirror, etc.
Both the ROST™ and UVOST™ readily detect most light- to mid-range product types including diesel and gasoline. The fluorescence response for these product types are generally linear, with higher concentrations of PAHs within a given product type resulting in a greater percent response relative to the RE (excluding any matrix interferences described below). With respect to gasoline, ROST™ will potentially have an advantage over UVOST™ since its laser system produces a shorter wavelength. But much of this advantage may be normalized through comparison of the LIF results from ROST™ and UVOST™ to the same RE. This is generally observed in the waveforms for the ROST™
borings installed in 2004 and 2005 when compared to the UVOST™ borings installed at the Hartford Site in 2013. The fluorescence results from the 24 collocated borings are presented as mirror images on the figures included in Appendix E. The scale for the total waveform from the ROST™ was adjusted in the horizontal direction (i.e., stretched or compressed) so the percent fluorescence response (%RE) was equivalent to that of the corresponding scale for the UVOST™ waveform.
This comparison of the ROST™ and UVOST™ waveforms is semi-qualitative and may be affected by changes in the distribution or weathering of the LNAPL within the hydrostratigraphic units due to groundwater fluctuations, remedial system operation, and natural smear zone depletion. These results are semi-qualitative as there are several sources of variation with respect to fluorescence response beyond the aforementioned differences in the ROST™ and UVOST™. For instance, only the relative fraction of LNAPL that is optically accessible at the sapphire window of the probe can contribute to the fluorescence response. Therefore, significant heterogeneities in the lithologic setting and LNAPL distribution within the soil matrix can affect the fraction of LNAPL present within a few centimeters of the window. In addition, the method used to install the borings (e.g., cone penetrometer, direct push) can result in differing physical response of the soils and LNAPL such that the diameter of probe, push speed, and other factors combine to influence how much LNAPL gets preferentially drawn towards or pushed away from the sapphire window.
These figures provided in Appendix E show the current vertical distribution of LNAPL within the hydrostratigraphic units at these 24 locations compared to the historical results for borings installed in 2004 and 2005. In addition, a comparison of the vertical extent of LNAPL, as well as the depth and degree of maximum fluorescence response is included in Table 3. Temporal changes in the vertical extent of the LNAPL and maximum fluorescence response within a location may indicate preferential depletion of the smear zone due to a combination of interim measures, redistribution due to fluctuating groundwater elevations, and natural smear zone depletion processes. Apparent temporal changes (subject to the differences in the ROST™ and UVOST™, inherent variation associated with LIF, and subsurface heterogeneity described above) were most prevalent within the North Olive stratum and within the deeper hydrostratigraphic units along the western and southern boundaries of the smear zone.
4.5.1 SMEAR ZONE DEPLETION IN THE NORTH OLIVE STRATUM
At those locations where LNAPL was identified in 2004 and 2005 via a fluorescence response (Figures D-1, D-2, D-4, D-12, D-13, D-14, D-19, and D-21), there was either no response or a significantly reduced response observed within the North Olive stratum in 2013. This includes locations situated along the margins, as well as the interior portions of the smear zone. Petroleum hydrocarbons within this shallowest hydrostratigraphic unit are being targeted for recovery using the SVE system. Natural smear zone depletion may also be occurring within the North Olive stratum via (1) volatilization and subsequent biodegradation within the vadose and (2) nutrient delivery within rainwater infiltrate and subsequent oxidation by petrophyllic bacteria in the saturated zone. Additional evaluation of the effects of the SVE system and natural smear zone depletion processes will be considered as part of future components to the CSM.
4.5.2 SMEAR ZONE DEPLETION IN THE DEEPER HYDROSTRATIGRAPHIC UNITS
A comparison of the historical and more recent LIF results for boring installed along the western (Figures D-5, D-6,
D-8, D-18, and D-20) and southern (Figures D-13 and D-17) edges of the smear zone provides evidence of depletion of the smear zone within the deeper hydrostratigraphic units (Rand and Main Sand strata). Similar depletion of the smear zone within the deeper hydrostratigraphic units was not observed in the LIF comparisons for collocated borings installed along the northern and eastern portions of the Hartford Site (Figure D-1, D-2, D-12, and D-14). Additional evidence of smear zone depletion along the southern and western limits of the smear zone will be considered in the forthcoming components to the CSM.
It should be noted that significant decreases in the maximum fluorescence intensity between the historical and recent LIF borings was observed at four locations within the interior portions of the smear zone (Figures D-9, D-10, D-22, and D-23). However, these decreases in the maximum fluorescence intensity were not coupled with significant decreases in the vertical thickness of the smear zone observed via the LIF response.
4.6 LNAPL DISTRIBUTION ACROSS THE HYDROSTRATIGRAPHIC UNITS
Figure 26 provides the full lithologic and LNAPL sequence observed beneath the Hartford Site through various oblique cross sections. The lines of section are oriented northwest to southeast and parallel to the edge of the C Clay (bottom of Rand stratum) and spaced approximately 200 feet apart to show a progression of the LNAPL distribution and lithologic sequence.
Light-range LNAPL is the predominant hydrocarbon type observed within each segment shown on Figure 26, except for Oblique Cut 1 showing the northeastern portion of the Hartford Site (Segments A and E). These light-range hydrocarbons have the broadest footprint and occur across the greatest vertical interval, extending from several feet below the displayed water table in the EPA and Main Sand strata (January 2004) upward nearly to the bottom of the A Clay. The submerged portions of the smear zone in the EPA and Main Sand may be attributed to historical low water table conditions present prior to the construction of Dam No. 27 by the Army Corp of Engineers between 1959 and 1963. The light-range LNAPL, as well as the other distillates, are acting as a continuous source for petroleum related constituents in groundwater as described in Section 3.2.
Two additional LNAPL bodies that are notable include the mid-range hydrocarbons in the northeastern portion of the Site and heavy-range LNAPL along the eastern boundary. In both cases, the largest lateral extent is observed in the EPA and Main Sand strata, with a decreased footprint of these LNAPL types observed in the overlying Rand and North Olive strata. An isolated heavy-range LNAPL body is also observed northwest of the Hartford Site, and again is identified in multiple strata above the piezometric surface. It is worth noting that there are also multiple localized LNAPL bodies, especially in the northern portions of the Hartford Site. While these may not stand out on a large-scale rendering of the Site, these smaller LNAPL bodies may be important in providing a source of volatile petroleum hydrocarbons partitioning to soil vapor beneath individual structures.
The largest identified LNAPL bodies beneath the Hartford Site are present in multiple strata suggesting that LNAPL has migrated through gaps and fractures within the discontinuous finer grained layers (e.g. B Clay, C Clay, D Clay) or along the margins where these layers pinch out. Still, the LNAPL morphologies suggest lateral spreading of the LNAPL bodies along the contacts of the finer grained layers, as well as the groundwater table within the deeper hydrostratigraphic units, especially when groundwater was unconfined within the Main Sand. In either case, vertical migration of LNAPL would have been restricted due to elevated water content within the pore space of the unconsolidated sediments, leading to components of flow in horizontal directions.
The lines of section provided on Figure 26 highlight the variability in the vertical sequence of the finer grained less permeable deposits depending on location. On Oblique Cut 1, at least three fine grained layers are evident across the full line of section, and a fourth layer, the D Clay, is present beneath the northeastern most segment (Section E). Also along this line of section, the B and C Clay merge in areas, meaning that the Rand stratum is thin or absent. Oblique Cut 1 shows the largest variety of LNAPL types across the various strata. While lithology is likely not the only reason for this variety (the release history itself can also be of importance, for instance), the presence of the multiple fine grained layers likely enhanced lateral migration of LNAPL within the various strata. On Oblique Cuts 2 through 4, fewer fine grained layers are present and a single light-range LNAPL body is observed, with little evidence of other LNAPL types. The less frequent occurrence of the fine grained layers in these areas could have meant that the water table was the primary mechanism limiting vertical migration of LNAPL and may have facilitated lateral spreading, smearing, and mixing of different LNAPL releases, resulting in an inferred homogeneous LNAPL type in these areas.
4.7 PROPOSED DATA COLLECTION AND UPDATES TO THE COMPREHENSIVE CSM
The data sets reviewed in this section are historical and mapped LNAPL distributions are consistent with interpretations presented in previous reports (Clayton 2004, 2005, 2006). The most pertinent update for the current CSM is the 3D model, which was noted as an important gap in the CSM (USEPA 2010). While the model makes the overall data visualization simpler by displaying all LNAPL bodies at once rather than on multiple maps and cross sections, its greatest use may be in future remedial system optimization. Lines of section can be produced and rotated for any portion of the Hartford Site, allowing for a close inspection of an area of interest. For instance, this may be useful for siting future recovery wells (e.g., soil vapor or LNAPL), evaluating potential sources for vapors present beneath specific structures, or determining dissolved phase hydrocarbon longevity within groundwater at specific monitoring locations.
In addition, select dissolved and vapor phase analytical results will be incorporated into the 3D model to better understand partitioning of petroleum related constituents from LNAPL to groundwater and soil vapor. These updates will be made as part of the dissolved and vapor phase components to the CSM. For example, dissolved phase benzene results collected over several timeframes within the various hydrostratigraphic units may be incorporated into the model. Timeframes that could be considered include (1) 2003 through 2005, (2) 2007 through 2009, and (3) 2011 through 2013. Data would be evaluated for representativeness (e.g., samples collected when the groundwater table was within the vertical extent of the well screen, LNAPL not present in the monitoring location). Fluid level results (groundwater and LNAPL elevations) may also be incorporated into the model including periods when the water table is elevated and confining conditions are present and periods when groundwater is seasonally low and unconfined conditions are present in the Main Sand stratum. The 3D model may also be updated to include select volatile petroleum related hydrocarbon and fixed gas concentrations within the vadose zone for high and low water table conditions. Pressure readings, fixed gas concentrations (including oxygen, carbon dioxide and methane), and total organic vapor concentrations from selected nested vapor monitoring wells and multipurpose monitoring points may be incorporated into the model. Vapor phase concentrations for select petroleum related hydrocarbons including benzene and hexane could also be incorporated into the model, where sufficient data is available. This could also include field and analytical data for sub-slab soil gas (measured as part of the in-home monitoring program) for select monitoring events.