1. Introduction
1.1. Purpose
2. Site Setting
3.1. LNAPL Recovery
4.1. LNAPL Properties
4.2. LNAPL Distribution
4.3. LNAPL Recoverability
4.4. LNAPL CSM Summary
6. Vapor Phase
8.1. Remediation Management Area No. 1
8.2. Remediation Management Area No. 2
8.3. Remediation Management Area No. 3
8.4. Remediation Management Area No. 4
8.5. Remediation Management Area No. 5
8.6. Remediation Management Area No. 6
8.7. Remediation Management Area No. 7
8.8. Remediation Management Area No. 8
8.9. Remediation Management Area No. 9
8.10. Remediation Management Area No. 10
9. References
Tables
Figures
Figure 1-1. Site Layout
Figure 2-1 North Olive Lithology Compare
Figure 2-2 North Market Lithology Compare
Figure 2-3 Isometric Compare
Figure 2-4. North Olive Stratum Extent And Isopach
Figure 2-5. Rand Stratum Extent And Isopach
Figure 2-6. Epa Stratum Extent And Isopach
Figure 2-7. Main Silt Extent And Isopach
Figure 3-1. Total Petroleum Hydrocarbons Recovered Since 1978
Figure 3-2. Vapor Collection System Layout And Production Well Locations
Figure 4-1. LNAPL Characterization And Viscosity Results
Figure 4-2. Benzene Effective Solubility And Dissolved Phase Concentrations
Figure 4-3. Laser Induced Fluorescence Boring And LNAPL Sample Locations
Figure 4-4. Three Dimensional LNAPL Distribution
Figure 4-5. LNAPL Thickness In The Rand Stratum, Low Groundwater
Condition, January 2006
Figure 4-6. LNAPL Thickness In The Rand Stratum, Low Groundwater
Condition, March 2015
Figure 4-7. LNAPL Thickness In The Rand Stratum, Average Groundwater
Condition, April 2007
Figure 4-8. LNAPL Thickness In The Rand Stratum, Average Groundwater
Condition, July 2017
Figure 4-9. LNAPL Thickness In The Rand Stratum, High Groundwater
Condition, July 2008
Figure 4-10. LNAPL Thickness In The Rand Stratum, High Groundwater
Condition, January 2016
Figure 4-11. LNAPL Thickness In The Main Sand Stratum, Low Groundwater
Condition, January 2006
Figure 4-12. LNAPL Thickness In The Main Sand Stratum, Low Groundwater
Condition, March 2015
Figure 4-13. LNAPL Thickness In The Main Sand Stratum, Average
Groundwater Condition, April 2007
Figure 4-14. LNAPL Thickness In The Main Sand Stratum, Average
Groundwater Condition, July 2017
Figure 4-15. LNAPL Thickness In The Main Sand Stratum, High Groundwater
Condition, July 2008
Figure 4-16. LNAPL Thickness In The Main Sand Stratum, High Groundwater
Condition, January 2016
Figure 4-17. Fluid Level Saturations For Soil Cores
Figure 4-18. Schematic Diagram Of Dual Optimal LNAPL Response Model
Figure 5-1. Saturated Thickness North Olive Stratum, Low Groundwater
Conditions, March 2015
Figure 5-2. Saturated Thickness North Olive Stratum, High Groundwater
Conditions, January 2016
Figure 5-3. Saturated Thickness Rand Stratum, Low Groundwater Conditions,
March 2015
Figure 5-4. Saturated Thickness Rand Stratum, High Groundwater Conditions,
January 2016
Figure 5-5. Potentiometric Surface Map Main Sand Stratum, Low Groundwater
Conditions, March 2015
Figure 5-6. Potentiometric Surface Map Main Sand Stratum, Average
Groundwater Conditions, July 2017
Figure 5-7. Hydraulic Head Analysis, Monitoring Point Mp-079d (Zone 1)
Figure 5-8. Hydraulic Head Analysis, Monitoring Point Mp-053c (Zone 5)
Figure 5-9. Hydraulic Head Analysis, Monitoring Point Mp-085d (Zone 6)
Figure 5-10. Detailed Potentiometric Surface Map Main Sand Stratum,
January 2016
Figure 5-11. Detailed Potentiometric Surface Map Main Sand Stratum,
April 2016
Figure 5-12. Detailed Potentiometric Surface Map Main Sand Stratum,
July 2016
Figure 5-13. Detailed Potentiometric Surface Map Main Sand Stratum,
October 2016
Figure 5-14. Historical Dissolved Phase Constituents Of Concern, Shallow
Hydrostratigraphic Units (2006 - 2008)
Figure 5-15. Dissolved Phase Constituents Of Concern, Shallow
Hydrostratigraphic Units (2013 - 2018)
Figure 5-16. Dissolved Phase Constituents Of Concern, Deep
Hydrostratigraphic Units (2013 - 2018)
Figure 5-17. Dissolved Phase Natural Attenuation Indicators, Shallow
Hydrostratigraphic Units (2013 - 2018)
Figure 5-18. Dissolved Phase Natural Attenuation Indicators, Deep
Hydrostratigraphic Units (2013 - 2018)
Figure 5-19. Dissolved Phase Constituents Of Concern And
Hydrogeochemical Indicator Summary Versus Distance
Figure 5-20. Assimilative Capacity Through Centerline
Figure 6-1. Vapor Intrusion Conceptual Site Model
Figure 6-2. Structures With Historical Fire And Odor Complaints
Figure 6-3. Typical Soil Vapor Extraction Wellhead Completion Detail
Figure 6-4. Typical Wellhead, Stinger, And Flowrate Measurement Device
Details For New And Modified Extraction Wells
Figure 6-5. Effectiveness Monitoring Network And Lines Of Section
Figure 6-6. Typical Stinger Detail
Figure 6-7. Total Volatile Petroleum Hydrocarbon Mass Recovery Rate By
SVE Effectiveness Zone
Figure 6-8. Distribution Of Benzene In Soil Vapor Under Low And High River
Stage (2004-2005)
Figure 6-9. Distribution Of Isopentane In Soil Vapor Under Low And High
River Stage (2004-2005)
Figure 6-10. River Stage Triggered Event Summary (2007 - 2011)
Figure 6-11. River Stage Triggered Event Summary (2012 - 2017)
Figure 6-12. Structures That Have Been Monitored And Mitigated
Figure 6-13. Select Constituents Of Concern Versus Total Volatile Petroleum
Hydrocarbons In Indoor Air
Figure 6-14. Vapor Intrusion Pathway Decision Flowchart
Figure 7-1. Site-Specific Attenuation Factors
Figure 8-1. Proposed Remediation Management Areas
Appendices
Appendix A
Appendix B
Appendix C
Appendix D
Appendix E
Appendix F
Appendix G
Appendix H
Appendix I
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Conceptual Site Model, March 2018
Hartford Petroleum Release Site
5. DISSOLVED PHASE
This section describes the dissolved phase component of the CSM and has been divided into the following subsections:
It is anticipated that the end-points described in the memo entitled, Proposed Multiphase Remedy Framework Remedial Objectives, Remediation Goals, and Performance Metrics, Hartford Petroleum Release Site, Hartford, Illinois (212 Environmental 2017c) for the dissolved phase petroleum hydrocarbons will likely be achieved after endpoints are reached for the other impacted media including LNAPL and vapor phase petroleum hydrocarbons, especially within the Main Sand stratum. Therefore, the hydrologic conditions, dissolved phase petroleum hydrocarbon distribution, and concentration trends were an important consideration in establishing the remediation management areas, as described in Section 8.0.
5.1. DISSOLVED PHASE INVESTIGATION SUMMARY
Several investigations have been conducted since 1978 to define the hydrogeologic setting, nature and extent of dissolved phase petroleum related hydrocarbons, as well as natural smear zone depletion processes within the saturated zone. The following provides a chronology of the various dissolved phase investigations that have been implemented at the Hartford Site.
The first evaluation of dissolved phase petroleum hydrocarbons was conducted in 1978 as part of a cooperative assessment by Amoco (currently British Petroleum), Clark Oil (currently Apex), Shell, and the Illinois EPA. This initial assessment included measuring fluid levels and inspecting groundwater conditions within: (1) the Village of Hartford public water supply wells, (2) ten groundwater monitoring wells that were installed by the Illinois EPA (IEPA-series wells), and (3) two monitoring wells installed by Shell (SHELL-series wells). A subsequent assessment was performed in 1982, to better define the nature and extent of LNAPL and dissolved phase hydrocarbons identified beneath the Harford Site during the initial assessment and included the installation of 35 groundwater monitoring wells (B-series wells) screened within the EPA and Main Sand stratum (Engineering- Science 1992). These cooperative assessments provided better definition of the hydrogeologic setting and distribution of LNAPL beneath the Hartford Site, primarily within the deeper hydrostratigraphic units (Engineering-Science 1992).
In December 1989, Shell completed an independent and limited evaluation of the groundwater occurrence and LNAPL distribution beneath the northeastern limits of the Hartford Site, as part of emergency response efforts related to a significant release of gasoline (approximately 290,000 gallons) along Rand Avenue (Engineering-Science 1992). Subsequently, in 1990 and 1991, the Illinois EPA required Clark Oil (currently Premcor) to develop a remedial action plan for recovering petroleum hydrocarbons present beneath the Hartford Site. In partial fulfillment of the Illinois EPA request, Clark Oil installed 24 groundwater monitoring wells (HMW-001 through HMW-024) within each of the hydrostratigraphic units between November 1990 and September 1991 (Clayton 2003a).
Following establishment of the Hartford Working Group, routine monitoring began within the five sentinel monitoring wells (wells HMW-025 through HMW-029) installed in December 2003 between the southern limits of dissolved phase petroleum hydrocarbons and the Village of Hartford 1,000- foot maximum setback zone for the drinking water production wells, which places the sentinel monitoring wells upgradient of the LNAPL smear zone and dissolved phase petroleum hydrocarbon plume. Between March and August 2004, an additional 55 monitoring wells (HMW-030 through HMW-052C) were installed by the Hartford Working Group as part of assessing the extent of LNAPL within the various hydrostratigraphic units. Several of these wells were installed as nested monitoring locations. Quarterly groundwater monitoring within the newly installed groundwater monitoring wells; in addition to the multiphase monitoring points installed to assess the effectiveness of the expanded SVE system was conducted between October and April 2005. Groundwater samples were collected for laboratory analysis from 78 wells and monitoring locations during these quarterly events, as well as the five sentinel groundwater monitoring wells.
The dissolved phase monitoring program was formalized in May 2005 with the approval of the first Dissolved Phase Groundwater Investigation Work Plan (Clayton 2005) by the USEPA and Illinois EPA. In addition to proposing a routine monitoring program, this initial work plan suggested: (1) collection and analysis of depth discrete groundwater samples via a direct push methodology, (2) collection of additional LIF and CPT data, and (3) in-situ hydraulic conductivity testing within select monitoring locations. The results of these additional investigation activities were submitted to the USEPA and Illinois EPA within the Dissolved Phase Groundwater Investigation Report (Clayton 2006) in January 2006.
Based on the 2005 investigation it was determined that additional modifications to the dissolved phase monitoring program were needed including: (1) installation of nested monitoring locations to the south and within the LNAPL smear zone, (2) reduction of fluid level gauging from monthly to quarterly, (3) modification of the frequency and locations where groundwater samples were collected, and (4) analysis of groundwater samples for natural attenuation indicators from select locations. Fluid level gauging and groundwater monitoring proceeded on a quarterly basis thereafter.
In March 2009, the Hartford Working Group submitted an updated Dissolved Phase Investigation Work Plan (URS 2009) in accordance with the Administrative Order of Consent (AOC) with the USEPA (Docket Number R7003-5-04-001). The work plan proposed: (1) installation of additional groundwater monitoring locations, (2) collection of additional LIF and CPT data, as well as (3) analysis of additional depth discrete groundwater samples during the installation of the LIF and CPT borings. Shortly after submitting the updated work plan, routine OMM of interim measures at the Hartford Site including assessment of dissolved phase conditions were transferred from the Hartford Working Group to Apex and the investigation activities described in the updated work plan were not conducted.
In June 2009, Apex submitted the Quarterly Groundwater Sampling and Gauging Sampling and Analysis Plan (Gannet Fleming 2009) to the USEPA. The sampling and analysis plan proposed a reduction in the frequency and number of locations for routine gauging and groundwater sampling, as well as suspending analysis of groundwater samples for natural attenuation indicators and dissolved metals. These modifications to the groundwater program were approved by the USEPA in July 2009, except for proposed changes to the frequency of monitoring within the sentinel groundwater wells. Subsequently on August 9, 2013, Apex submitted another update to the Dissolved Phase Investigation Work Plan (Trihydro 2013). The work plan proposed: (1) installation of additional LIF borings, (2) installation of pressure transducers along two west-east transects to provide a continuous record of groundwater fluid levels, and (3) collection of groundwater samples from select monitoring locations for the constituents of concern and, in some cases, natural attenuation indicators. Groundwater samples were proposed to be collected from an expanded monitoring network within the LNAPL smear zone, rather than focusing on those groundwater monitoring wells and multipurpose monitoring locations situated primarily beyond the smear zone limits. In addition, groundwater samples were proposed to be collected annually when the groundwater level was within the well screen and confined conditions were not present, which avoided collection of samples in which the dissolved phase constituents of concern were diluted because of confining conditions. It has been observed at the Hartford Site (as well as other petroleum release sites) that groundwater samples are reported with lower concentrations of constituents of concern and have a poor correlation to effective solubility estimates when groundwater elevations are present above the top of the well screen (such as during confined conditions) when samples are collected for laboratory analysis.
The results of the investigation activities were submitted to the USEPA within the Dissolved Phase Investigation Summary Report (212 Environmental 2016a) in July 2016. In addition to summarizing the monitoring activities conducted between September 1, 2013 and September 30, 2015, the Dissolved Phase Investigation Summary Report proposed additional modifications to the groundwater monitoring network to further target sampling within the LNAPL smear zone.
Most recently, in July 2016, Apex submitted a request to the USEPA proposing a reduction in the frequency of monitoring within the sentinel wells from quarterly to semiannually based upon: (1) thirteen years of data demonstrating that the sentinel wells had not been impacted by the dissolved phase hydrocarbons attributed to releases from the refineries beneath the Hartford Site, (2) groundwater flow direction within the Main Sand stratum being away from the Village water supply wellfield for more than 65 years, and (3) the multiple lines of evidence suggesting that the southern limit of the dissolved phase constituents beneath the Hartford Site has remained stable. The USEPA approved the reduction in monitoring frequency on July 22, 2016. The sentinel wells are currently monitored on a staggered semiannual basis (first quarter and third quarter during odd years and second quarter and fourth quarter during even years) to ensure that groundwater samples are collected under various hydraulic conditions.
5.2. HYDROGEOLOGY
As described in Section 2.3, five hydrostratigraphic units have been defined beneath the Hartford Site consisting of discontinuous, perched groundwater within the shallow portions of the subsurface described as the North Olive, Rand, and Main Silt strata; as well as the regionally extensive aquifer present deeper in the subsurface, which has been subdivided as the EPA and Main Sand strata. While it is recognized that this generalized stratigraphic description depicts an oversimplified summary of the heterogenous setting within the upper 40 feet of the subsurface, this interpretation represents a common and historically utilized framework for describing the setting beneath the Hartford Site. A discussion of groundwater occurrence and hydraulic conditions within each of these hydrostratigraphic units is provided in the following subsections, with the intent of further describing the heterogeneous hydrogeologic setting, particularly within the shallow, perched strata.
It should be noted that groundwater in the shallow and deeper strata are interconnected through: (1) more permeable lenses within the clay layers separating the strata, (2) fractures and discontinuities within the clay layers, and (3) along the lateral contact where the clay layers pinch out and the permeable units grade into one another (which has typically been described as the Main Silt strata). As described in the subsequent sections, groundwater can migrate downward from the perched hydrostratigraphic units into the deeper portions of the subsurface, as well as upwards from the regionally extensive Main Sand stratum, into the overlying Main Silt, Rand, and North Olive strata during flooding of the Mississippi River.
In order to consistently evaluate hydraulic conditions across the various strata, three fluid level gauging events were examined based upon the average elevation of groundwater within the Main Sand stratum. These three gauging events represent the 10th percentile (March 2015), 50th percentile (July 2017), and 90th percentile (January 2016) of the average groundwater elevation in the Main Sand stratum, estimated using data collected during each gauging event between 2004 and 2017.
Figures showing the screen interval, groundwater elevation, and saturated thickness under low (March 2015) and high (January 2016) groundwater conditions for wells screened in the North Olive and Rand strata were prepared for illustrative purposes and referenced in the subsequent sections. Figures are not provided showing the groundwater elevation or saturated thickness for the average (July 2017) groundwater conditions for the wells screened in the North Olive or Rand strata. A summary of the fluid level data for the March 2015, January 2016, and July 2017 gauging events is provided in Table 5-1.
There are numerous references to the Effectiveness Zones included within the remainder of this section. There are six Effectiveness Zones, which were previously established as a means of differentiating operations of the SVE system across the Hartford Site. The Effectiveness Zones are included on the figure in Appendix A, which depicts the monitoring locations, production wells, and SVE wells installed since interim remedial measures and investigation activities began in 1978.
5.2.1. NORTH OLIVE STRATUM
As previously stated in Section 2.1.2, the North Olive stratum is defined by the presence of the underlying B-Clay, such that the North Olive stratum is absent if the underling B-Clay is absent (Figure 2-4). LNAPL and groundwater in the North Olive stratum generally occur in isolated areas that are temporarily perched on the surface of the B-Clay before draining into the underlying stratum. There are 60 groundwater monitoring wells and multipurpose monitoring points screened across the North Olive stratum; however, not every location can be accessed during the routine gauging events (e.g., cars parked on monitoring location, well covered in gravel, well vault frozen). The maximum elevation of a well screen installed in the North Olive stratum is 430.11 ft-amsl and the minimum elevation was 407.91 ft-amsl. The average screen interval for wells installed in the North Olive stratum is between 416.5 and 420 ft-amsl. The primary source of recharge within the North Olive stratum is precipitation; however, there are infrequent occurrences when the Mississippi River stage exceeds 415 ft-amsl, the approximate elevation of the bottom of the North Olive stratum.
5.2.1.1. LOW GROUNDWATER CONDITIONS
During the gauging event conducted in March 2015 (representing the 10th percentile of average groundwater elevations within the Main Sand stratum), more than 75% of the monitoring locations screened in the North Olive stratum were dry2 (42 of 55 locations), and LNAPL was not detected within any monitoring location (Figure 5-1). Dry conditions are frequently encountered within monitoring locations screened within the North Olive stratum with approximately 15% of the well and multipurpose monitoring points not having measurable groundwater at any time between 2013 and 2017. These monitoring locations are distributed across the Hartford Site and demonstrate the spatial variability in groundwater conditions within this perched hydrostratigraphic unit. Local precipitation is the primary recharge mechanism within the North Olive stratum (212 Environmental 2016a). In the two months preceding the March 2015 gauging event, total precipitation at the Hartford Site measured less than 2.0 inches, which contributed to the dry conditions observed within the North Olive stratum.
The saturated thickness within the North Olive stratum was determined by comparing the elevation of groundwater measured within the monitoring location to the elevation of the top of the B-Clay based upon the borehole logs generated during the installation of the monitoring location. Saturated thicknesses in March 2015 ranged from 0.0 to 4.28 feet (0 to 63% of the stratum thickness) as shown on Figure 5-1 with none of the monitoring locations being fully saturated.
5.2.1.2. AVERAGE GROUNDWATER CONDITIONS
Fluid level gauging results collected from July 2017 (representing the 50th percentile of average groundwater elevations within the Main Sand stratum) were examined to evaluate the average hydraulic conditions within the North Olive stratum. Despite the higher groundwater elevations observed within the Main Sand stratum, more than 85% of the monitoring locations screened within the North Olive stratum remained dry (49 of 57 locations). LNAPL was not detected within any of the monitoring locations. Overall, the saturated thicknesses within the individual monitoring locations screened in the North Olive stratum in July 2017 were similar to those calculated in March 2015. These similarities suggest that even though total precipitation in the two months preceding the gauging event measured approximately 7.0 inches, it appears that the influence of precipitation on groundwater elevation within the North Olive stratum may be short in duration due to vertical drainage of groundwater into the deeper hydrostratigraphic units. Furthermore, while the groundwater elevations measured within the underlying Main Sand stratum were higher (50th percentile), conditions in the North Olive stratum were independent and remained largely unchanged.
5.2.1.3. HIGH GROUNDWATER CONDITIONS
During high groundwater conditions (representing the 90th percentile of average groundwater elevations within the Main Sand stratum) observed in January 2016, groundwater was detected within approximately 60% of the groundwater monitoring locations screened in the North Olive stratum (34 of 58 locations). LNAPL was detected within one monitoring location (MP-108B) screened within the North Olive stratum, which had an apparent LNAPL thickness of 0.16 feet. The increase in groundwater is attributed to the 15.5 inches of precipitation that occurred in the two months preceding the fluid level gauging event. Overall, saturated thicknesses increased across the North Olive stratum, ranging from 0.0 to 8.50 feet (0 to 100% of the stratum thickness), as shown on Figure 5-2. It should be noted that the saturated thickness for a monitoring location with 100% of the stratum thickness would be equivalent to the stratum thickness even if the groundwater elevation extends up into the overlying A-Clay (representing a confined condition). The North Olive stratum was fully saturated in five monitoring locations (MP-031B, MP-079A, MP-085A, MP-106B, and MP- 109B) during the January 2016 gauging event.
5.2.2. RAND STRATUM
The Rand stratum is defined by the presence of the underlying C-Clay, such that the Rand stratum is absent if the underlying C-Clay is absent as shown on Figure 2-5. The C-Clay is discontinuous and only present in the northern and eastern portions of the Hartford Site, which influences the distribution of groundwater and LNAPL within the hydrostratigraphic unit. There are 47 groundwater monitoring wells and multipurpose monitoring points that are screened within the Rand stratum that are routinely gauged. The maximum elevation of a well screen installed in the Rand stratum is 417.78 ft-amsl and the minimum elevation of a well screen is 400.22 ft-amsl. The average screen interval for wells installed in the Rand stratum is between 406 and 410.5 ft-amsl. Recharge of groundwater into the Rand stratum is from three potential sources: (1) precipitation and migration of rainwater infiltrate through the overlying North Olive stratum and clay lenses, (2) vertical migration of groundwater from the underlying EPA and Main Sand stratum under highly confined conditions, and (3) discharge of surface water from the Mississippi River directly into the Rand stratum during flood events.
5.2.2.1. LOW GROUNDWATER CONDITIONS
In March 2015, 18 of 47 (38%) monitoring locations screened in the Rand stratum were dry. The saturated thicknesses calculated in the Rand stratum in March 2015 ranged from 0.0 to 12.5 feet (0 to 100% of the stratum thickness) as shown on Figure 5-3. LNAPL was detected under unconfined conditions within four monitoring locations (MP-009D, MP-046B, MP-047B, and MP-053B), with thicknesses ranging from 0.03 to 0.23 feet.
The overall dry conditions observed in the Rand stratum in March 2015 were similar to conditions observed in the overlying North Olive stratum and believed to be attributed to both low precipitation rates over the previous two months, as well as the low elevation within the Mississippi River (below 400 ft-amsl), as measured at the Mel Price Lock and Dam. For reference, the bottom of the Rand stratum is generally at an elevation of 404 ft-amsl.
Despite these dry conditions, fully saturated conditions were observed within seven of the monitoring locations (HMW-048B, MP-028, MP-039B, MP-049B, MP-085B, MP-106C, and MP-109C) screened in the Rand stratum, with five of these locations installed in Effectiveness Zone 6. Further evaluation of the monitoring locations with fully saturated conditions indicated that groundwater elevations within two of the multipurpose monitoring points (MP-106C and MP-109C) installed in Effectiveness Zone 6, were higher than the bottom of the North Olive stratum. This could indicate potential upward vertical hydraulic gradients from the Rand stratum into the overlying North Olive stratum (Table 5-1).
5.2.2.2. AVERAGE GROUNDWATER CONDITIONS
During the July 2017 fluid level gauging event, more than 90% (44 of 48) of the monitoring locations contained groundwater. LNAPL was detected within four monitoring locations (MP-044C, MP-046B, MP-053B, and MP-054B) with thicknesses ranging from 0.30 to 0.99 feet. Two of these locations, MP-046B and MP-053B, contained LNAPL during the March 2015 and July 2017 gauging events. LNAPL thicknesses in these two monitoring locations increased as groundwater elevations increased. Similar to March 2015, LNAPL was unconfined within each monitoring location.
In July 2017, the saturated thickness measured in monitoring locations screened within the Rand stratum ranged from 0.0 to 14.62 feet (0 to 100% of the stratum thickness). There were 16 monitoring locations that displayed fully saturated conditions, with 13 of these situated within Effectiveness Zone 6. Groundwater was confined within five of these locations (HMW-045B, HMW- 048B, MP-085B, MP-106C, and MP-109C) with the groundwater elevation measured higher than base of the North Olive stratum.
The high frequency of monitoring locations screened in the Rand stratum that contained groundwater could be attributed to the 7.0 inches of precipitation that occurred in the two months preceding the gauging event; but is more likely related to major flooding of the Mississippi River which occurred two weeks prior to the May 2017 gauging event. The Mississippi River crested at 430.19 ft-amsl on May 6, 2017. It is likely that the rapid rise of the river resulted in both direct recharge within the Rand stratum, as well as vertical migration of groundwater from the underlying EPA and Main Sand stratum into the Rand stratum (described further in Section 5.2.5.3).
5.2.2.3. HIGH GROUNDWATER CONDITIONS
In January 2016, each of the 47 monitoring locations screened within the Rand stratum contained measurable groundwater. The saturated thicknesses across the Rand stratum generally increased with measurements ranging from 0.8 to 15.6 feet (between 51% and 100% of the stratum thickness), as shown on Figure 5-4.
LNAPL was detected within seven of the monitoring locations (MP-045B, MP-046B, MP-047B, MP- 051C, MP-053B, MP-055B, and MP-108C) with thicknesses ranging from 0.05 to 4.17 feet. Despite the increase in the number of monitoring locations with LNAPL and an overall increase in the apparent LNAPL thicknesses, LNAPL was unconfined within the seven multipurpose monitoring points.
There were 26 monitoring locations (55% of all monitoring locations) with fully saturated conditions within the Rand stratum. This included all the monitoring locations that displayed fully saturated conditions during the July 2017 fluid level gauging event, with the exception HMW-004, which could not be gauged in January 2016. Again, the majority of the monitoring locations that displayed fully saturated conditions (16 monitoring locations) were located within Effectiveness Zone 6. Groundwater was confined and measured above the bottom of the North Olive stratum in eleven of these monitoring locations. There were three monitoring locations, outside of Effectiveness Zone 6 (HMW-045B, MP-030B, and MP-080B), where groundwater was also measured above the bottom of the bottom of the North Olive stratum.
The high frequency of wells that contained groundwater and the elevated groundwater elevations observed in the Rand stratum in January 2016, can be attributed to high rates of precipitation in the two months preceding the gauging event (15.5 inches) and major flooding of the Mississippi River in the prior days (Mississippi River crested at 431.24 ft-amsl on January 1, 2016).
5.2.3. MAIN SILT STRATUM
The Main Silt stratum is predominantly located within the southwestern portion of the Hartford Site where the B- and/or C-Clay are absent. As previously stated in Section 2.1.8, the vertical extent of the Main Silt stratum is difficult to discern from the Main Sand stratum, as the contact is gradational. Due to the gradational contact with the underlying Main Sand stratum, it is not possible to estimate the saturated thickness within the monitoring locations screened in the Main Silt stratum, as such summary figures depicting the saturated thickness were not prepared for the Main Silt stratum. However, the inferred lateral extent and thickness of the Main Silt stratum is depicted on Figure 2-7. A summary of the fluid level measurement collected during the March 2015, January 2016, and July 2017 gauging events is provided in Table 5-1.
There are 51 monitoring locations screened within the Main Silt stratum. The maximum elevation of a well screen installed in the Main Silt stratum is 423.08 ft-amsl and the minimum elevation of a well screen is 394.21 ft-amsl. The average screen interval for wells installed in the Main Silt stratum is between 409 and 414 ft-amsl.
During the March 2015 gauging event (representing low groundwater conditions) and July 2017 gauging event (representing average groundwater conditions), 64% and 63% of the monitoring locations screened across the Main Silt stratum were dry. LNAPL was not detected within any of the monitoring locations during these gauging events. The high frequency of dry wells and low groundwater elevations observed within the Main Silt stratum during these two events reflects the direct hydraulic connection between the Main Silt and Main Sand strata.
During the January 2016 gauging event (representing high groundwater conditions), 16 of 47 (34%) of the monitoring locations remained dry. Two monitoring locations installed in Effectiveness Zone 2 (MP-038B and MP-048B) contained measurable LNAPL with thicknesses of 1.71 and 0.99 feet, respectively. These are the only two locations screened within the Main Silt stratum that have contained LNAPL over the past five years.
5.2.4. EPA STRATUM
As previously stated in Section 2.1.6, the EPA stratum is only present beneath the northeastern limit of the Hartford Site, as shown on Figure 2-6. The EPA stratum is defined by the presence of the D- Clay, which is essentially a thin lens within the Main Sand stratum. There are only four monitoring locations screened within the EPA stratum at the Hartford Site (HMW-048C, HMW-049C, HMW-003, and MP-085C). Groundwater measured within the EPA stratum generally remains confined across a range of hydraulic conditions, with unconfined groundwater observed in two locations (HMW-049C and MP-085C) under the low water table conditions recorded in March 2015.
Monitoring well HMW-048C typically contains LNAPL with apparent thicknesses ranging between approximately 6 and 17 feet during gauging events conducted since 2015. LNAPL is generally confined within this well and has not been unconfined since extremely low groundwater elevations were observed in January 2013.
5.2.5. MAIN SAND STRATUM
As described in Section 2.1.9, groundwater present in the Main Sand stratum is part of an extensive aquifer system commonly referred to as the American Bottoms aquifer. There are approximately 120 monitoring locations screened in the Main Sand stratum that are routinely gauged. The maximum elevation of a well screen for the locations installed in the Main Sand stratum is 411.27 ft-amsl and the minimum elevation of the well screen is 332.48 ft-amsl (including the HP-series wells installed south of the smear zone and dissolved phase limits to evaluate conditions within the deep portions of the Main Sand stratum within the Village of Hartford 1,000-foot maximum set back zone, which is depicted on Figure 1-1). The average screen interval for wells routinely gauged in the Main Sand stratum is between 386.5 and 398.75 ft-amsl.
5.2.5.1. LOW GROUNDWATER CONDITIONS
During the March 2015 fluid level gauging event (10th percentile of fluid level conditions within the Main Sand stratum), groundwater was generally unconfined (typically less than 400 ft-amsl) with flow to the north and northwest, as shown on Figure 5-5. The hydraulic gradient during the March 2015 event was generally between 0.002 and 0.0025 feet per foot across the Hartford Site. It is worth noting that, groundwater flow was locally altered along the eastern margins of the Hartford Site due to focused pumping (approximately 300 gpm) within Area A, as described in Section 4.3.5. This created a cone of depression around the production well (HPW-01) with an estimated radius of influence of approximately 700 feet (Trihydro 2015c). Hydraulic conductivity estimates for the Main Sand stratum were calculated at 109 feet per day (3.8E-02 cm/s) during the focused pumping test. This compared well to the previous hydraulic conductivity estimates for the Main Sand stratum which were reported between 45 to 88 feet per day (1.6E-02 to 3.1E-02 cm/s, Clayton 2005) in the central portions of the Hartford Site (determined via slug testing performed under unconfined conditions in partially penetrating wells screened across the upper portions of the aquifer). Additionally, hydraulic conductivity estimates reported via pumping tests in the production wells installed in the Main Sand stratum on the Premcor facility have been reported as high as 283 feet per day (1.0E-01 cm/s, Clayton 2005).
5.2.5.2. AVERAGE GROUNDWATER CONDITIONS
Groundwater elevations measured in July 2017 reflect the 50th percentile of average fluid level measurements within the Main Sand stratum. As shown on Figure 5-6, groundwater flow was predominantly towards the north in July 2017, with an approximate hydraulic gradient of 0.0025 feet per foot. The groundwater elevations measured within the Main Sand stratum were reported between 402.8 and 408.5 ft-amsl, with groundwater confined where the C- or D-Clay are present.
The hydraulic head within several of the monitoring locations screened within the Main Sand stratum was higher than the bottom of the overlying Rand stratum, as shown on Figure 5-7, Figure 5-8, and Figure 5-9, depicting groundwater elevations recorded using pressure transducers installed in monitoring locations MP-079D (located in Effectiveness Zone 1), MP-053C (Effectiveness Zone 5), and MP-085D (Effectiveness Zone 6). As discussed in Section 5.2.2.2, higher groundwater elevations in the Main Sand stratum could be attributed to the 7.0 inches of precipitation that occurred in the two months preceding the gauging event; but is more likely related to major flooding of the Mississippi River which occurred two weeks prior to the May 2017 gauging event. The Mississippi River crested at 430.19 ft-amsl on May 6, 2017.
5.2.5.3. HIGH GROUNDWATER CONDITIONS
In January 2016, groundwater elevations within the Main Sand stratum exceeded the 90th percentile of average fluid level measurements. As shown on Figure 5-10, groundwater flow was predominantly towards the north and northeast, with an approximate hydraulic gradient of 0.0024 feet per foot. The groundwater elevations measured within the Main Sand stratum were reported between 406.5 and 415.4 ft-amsl, with groundwater confined across much of the Hartford Site.
Much like the conditions observed in July 2017, groundwater elevations within the Main Sand stratum were measured above the bottom of the Rand stratum as shown on Figure 5-7, Figure 5-8, and Figure 5-9. As previously stated in Section 5.2.2.3, the high groundwater conditions observed in January 2016 are attributed to elevated rates of precipitation (15.5 inches) during the preceding two months and major flooding of the Mississippi River four days earlier (Mississippi River crested at
431.24 ft-amsl on January 1, 2016).
5.2.5.4. HYDRAULIC INFLUENCE ALONG PREMCOR FACILITY
As previously described in Section 2.3.4, the natural groundwater flow regime in the Main Sand stratum has been altered beneath the Hartford Site due to significant pumping for industrial use on the British Petroleum (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. Groundwater extraction from some of these facilities has been continuous since at least 1951 and results in flow that is generally to the north within the Main Sand stratum, across the full range of hydraulic conditions. Significant pumping from these facilities is anticipated into the future to support continued refining activities. In the absence of large scale groundwater extraction by the various facilities around the Hartford Site, groundwater flow within the Main Sand under typical river stage conditions may have been to the south and southwest, parallel to surface water flow within the Mississippi River (USEPA et al. 2010).
The four quarterly fluid level gauging events conducted in 2016 were further examined to determine the influence of inward hydraulic gradient controls at the Premcor facility along the southeastern boundary of the Hartford Site. Inward hydraulic gradient control at the Premcor facility is maintained utilizing three groundwater extraction wells, RPW-02, RPW-03, and P-6. Extraction wells RPW-02 and RPW-03 are referred to as shallow pumping wells and during 2016 had extraction rates that averaged 4 gpm and 35 gpm, respectively. Production well P-6 is screened within the Main Sand stratum and maintained an average pumping rate of 187.5 gpm (St. John-Mittelhauser & Associates 2017). Fluid level gauging data provided by Premcor was combined with the fluid level gauging results collected from the Main Sand stratum within the Village of Hartford to generate more detailed potentiometric surface maps (contour intervals reduced from 0.5 foot to 0.1 foot).
The first quarter 2016 fluid level gauging event occurred under high groundwater conditions (90th percentile for the average groundwater elevations collected from the Main Sand stratum), as previously described. Figure 5-10 shows the detailed potentiometric surface for the Main Sand stratum in January 2016, which depicts groundwater flow in a northeasterly direction, away from the Mississippi River as a result of flooding and recharge of water into the Main Sand stratum. During the January 2016 event, it appears that groundwater pumping (extraction rates of 4, 38, and 176 gpm from recovery wells RPW-02, RPW-03, and P-6, respectively) along the western boundary of the Premcor facility had limited influence on groundwater flow due to the overwhelming influence of the Mississippi River (St. John-Mittelhauser & Associates 2017).
During the second quarter 2016, groundwater elevations within the Main Sand stratum decreased (representing the 50th percentile of average groundwater elevations measured within the Main Sand stratum), and groundwater flow returned to a predominant north-northwesterly direction (Figure 5- 11). Along the southeastern boundary of the Hartford Site, the inward gradient controls on the Premcor boundary appeared to have limited influence on groundwater flow within the Main Sand stratum, with slight depressions observed in the vicinity of monitoring wells RMW-47C and RMW- 25B, which could be attributed to groundwater extraction from production well RPW-03, pumping at 38 gpm, and production well P-6, pumping at 184 gpm (St. John-Mittelhauser & Associates 2017).
Groundwater elevations during the third quarter 2016 were slightly higher than those observed during the second quarter (representing the 60th percentile of average groundwater elevations measured within the Main Sand stratum). As shown on Figure 5-12, groundwater flow was generally in a north-northwesterly direction with some local variations. Along the southwestern boundary of Premcor facility, the inward gradient controls appear to have had a more significant influence within the Main Sand stratum, with depressions centered around monitoring well RMW-25B, which is located near production well P-6, and to a lesser extent well RMW-47C, which is located near shallow pumping well RPW-02.
During the fourth quarter 2016, groundwater elevations were higher than those observed during the third quarter 2016 (representing the 75th percentile of average groundwater elevations measured within the Main Sand stratum). As shown on Figure 5-13, groundwater flow direction in the Main Sand stratum was predominantly towards the north and northwest. However, hydraulic controls along the Premcor boundary appear to have a greater influence along the southwestern limits of the Hartford Site. During the fourth quarter 2016 events, large cones of depression are clearly visible in the potentiometric surface, centered around monitoring wells RMW-38A, RMW-25B, and RMW-26D, which are all located near production well P-6.
Based upon the fluid level gauging events conducted in 2016, it appears that overall, the extraction wells located on the Premcor facility maintained inward gradient controls throughout the year. The conditions in 2016 represented above average hydraulic conditions, with significant flooding during the first quarter and groundwater flow predominantly to the northeast away from the Mississippi River, followed by a return to ambient conditions within the Main Sand stratum and bank discharge from the aquifer back towards the River (flow to the north and northwest). While the effectiveness of the inward controls at the Premcor facility was diminished during flooding of the Mississippi River during the first quarter 2016, groundwater flow directions were predominantly away from the Village of Hartford.
5.3. CONSTITUENTS OF CONCERN DISTRIBUTION AND TRENDS
The dissolved phase analytical results for groundwater samples collected over the past five years have been examined in the subsequent sections to determine the current distribution of petroleum related constituents of concern within each of the hydrostratigraphic units. The recent data was also compared to the historical groundwater analytical results (2006 to 2008) to evaluate spatial and temporal trends. The USEPA “Skinner List” for constituents of concern for wastes from petroleum processes was initially used as the analyte list for historical groundwater investigation activities (Clayton 2004a). Over time the analyte list for the dissolved phase constituents of concern has been truncated and routine groundwater samples are currently analyzed for benzene, toluene, ethylbenzene, total xylenes, and methyl tert-butyl ether. In many cases the analyses described in the following sections focus on dissolved phase benzene. Benzene was selected as it represents the constituent with the greatest potential risk to receptors when comparing the ratio of the constituent concentrations measured in groundwater samples to the USEPA and Illinois EPA potable water standards.
It should be noted that in accordance with the Dissolved Phase Investigation Work Plan (Trihydro 2013), over the past five years groundwater samples have only been collected from a monitoring location if: (1) LNAPL was not present and (2) the groundwater table was present within the screen interval ensuring that unconfined conditions were present. In order to compare the historical and more recent data, the conditions in each monitoring location were examined at the time that groundwater samples were collected to ensure that LNAPL or confining conditions were not present. Historical data collected from locations when LNAPL or confining conditions were present were excluded from any comparative analyses herein.
5.3.1. NORTH OLIVE STRATUM
Since 2013, groundwater samples were collected from four groundwater monitoring locations (HMW-048A, MP-056A, MP-085A, and MP-092C) screened within the North Olive stratum. Samples were also collected from two additional locations screened across the shallow portions of the subsurface including: groundwater monitoring well HMW-039A (screened within the shallow portions of the Main Silt stratum) and multipurpose monitoring point MP-089A (screened across the A-Clay and shallow portions of the Main Silt stratum). As previously stated in Section 2.1.8, the Main Silt stratum is compositionally similar to the North Olive and Rand strata. Monitoring locations installed in the Main Silt stratum are screened across similar elevations as monitoring locations installed in the North Olive and Rand strata. As such, it is appropriate to include discussions regarding groundwater analytical results collected from monitoring locations screened across the Main Silt stratum with the equivalent stratum (North Olive or Rand strata) based upon the elevation of the screen interval.
The small number of groundwater samples collected from the North Olive strata since 2013 is directly related to the limited availability of groundwater within this shallow hydrostratigraphic unit from both a spatial and temporal standpoint. As described in Section 5.2.1, under average groundwater conditions more than 75% of the monitoring locations screened in the North Olive stratum were dry. As shown on Figure 5-2, more than 40% of these monitoring locations remained dry even under the high groundwater conditions observed during the January 2016 gauging event. Furthermore, less than 30% (17 of the 58) of the monitoring locations contained more than 1-foot of groundwater in January 2017. The monitoring locations that can be sampled under these short duration, high water table conditions tend to be clustered together, such as the group of four locations installed north of the Hartford Community Center (locations HMW-047A, MP-074, MP-075, and MP-076), which contained more than 1-foot of water in January 2016. The majority of monitoring locations screened within the North Olive stratum are 1-inch in diameter with an average screen length of approximately 3.5 feet. It should be noted that historically (between 2004 and 2012), groundwater samples were only collected from five monitoring locations screened within the North Olive stratum (locations HMW-047A, HMW-048A, MP-031B, MP-083A, MP-092C).
Figure 5-14 and Figure 5-15 present the historical (2006-2008) and more recent (2013-2018) dissolved phase concentrations for constituents of concern reported in groundwater samples collected from the shallow hydrostratigraphic units. As shown on these two figures, the concentration of dissolved phase constituents of concern in groundwater samples collected from the North Olive stratum (as well as the locations installed in the A-Clay and shallow portions of the Main Silt stratum) have historically and more recently been measured at low concentrations or not detected above the laboratory reporting limits, with the exception of samples collected from monitoring well HMW-048A. This shallow well is located in the northeastern most portion of Effectiveness Zone 6. The groundwater samples collected from well HMW-048A were reported with elevated concentrations of benzene, ethylbenzene, and total xylenes.
The dissolved phase benzene concentration trend within monitoring well HMW-048A is provided in Appendix F-7. Since 2005, benzene concentrations have decreased by approximately two orders of magnitude at this location. However, as shown on Figures 5-14 and 5-15, the concentration of ethylbenzene and total xylenes have remained relatively stable over this same timeframe. As described further in Section 6.2.1.4, vapor recovery in this portion of Effectiveness Zone 6 has been ineffective as many of the SVE wells are occluded (screen interval remains submerged beneath the water table) throughout the year (212 Environmental 2016c). An expansion of the SVE system within Effectiveness Zone 6 was completed in June 2017 (212 Environmental 2017d). It has yet to be determined if the expanded system will improve vapor recovery in the northeast portion of Effectiveness Zone 6, near extraction well HMW-048A.
5.3.2. RAND STRATUM
As shown on Figure 5-14, there were six groundwater samples collected between 2006 and 2008 from monitoring locations screened within the Rand stratum, as well as monitoring locations installed within deeper portions of the Main Silt, B- and C-Clay that have screen intervals that are like locations installed in the Rand stratum. Concentrations of the constituents of concern within these historical samples were generally low or not detected above the laboratory reporting limits. The highest dissolved phase concentrations were reported in monitoring well HMW-045B, with benzene reported at 1.26 mg/L in 2008.
There were nine monitoring locations screened within the Rand stratum that were sampled between 2013 and 2018. In addition, groundwater samples were collected from four locations (HMW-039B, HMW-041B, HMW-052B, and MP-081B) installed within deeper portions of the Main Silt stratum and one additional monitoring location (HMW-049B) installed across the B- and C-Clays. Concentrations of the dissolved phase constituents of concern were generally low or not detected above the laboratory detection limits in samples collected from monitoring locations situated outside the LNAPL smear zone. The highest concentrations of the constituents of concern were reported in groundwater samples collected within the smear zone limits including monitoring point MP-083B located in Effectiveness Zone 1, monitoring point MP-056B installed in Effectiveness Zone 5, and groundwater monitoring well HMW-007 located in Zone 6. It should be noted that there was significant variability in the dissolved phase constituent concentrations in samples collected from the monitoring locations within the smear zone. For instance, within Zone 6, the concentration of benzene in groundwater monitoring wells HMW-007 was reported at 1.64 mg/L in 2017; however, benzene was not-detected in the groundwater sample collected from multipurpose monitoring point MP-085B (collected in 2013) located less than 250 feet from monitoring well HMW-007. These results reflect the highly discontinuous nature of groundwater and LNAPL within the Rand stratum.
Dissolved phase benzene degradation trends are presented in Appendix F for wells screened within the Rand and deeper portions of the Main Silt strata including well HMW-045B (located in Effectiveness Zone 1), well HMW-041B (located in Effectiveness Zone 3), monitoring point MP-056B (located in Effectiveness Zone 5), and multipurpose monitoring point MP-042B (located in Effectiveness Zone 6). Dissolved phase benzene concentrations have decreased by approximately two orders of magnitude in wells HMW-041B and HMW-045B, located in Effectiveness Zone 1 and Zone 3, and are currently less than 0.1 mg/L. However, dissolved phase benzene concentrations have decreased by less than an order of magnitude in monitoring point MP-056B and well HMW- 045B located in Effectiveness Zone 5 and Zone 6. Benzene concentrations have been measured above 1 mg/L in recent groundwater samples collected from these two wells. The lower depletion rates for benzene in these two wells could be attributed to:
5.3.3. EPA AND MAIN SAND STRATA
Since 2013, groundwater samples have been collected from 44 monitoring locations (208 groundwater samples) screened within either the EPA or Main Sand stratum including: (1) the five sentinel monitoring wells located between the limits of the LNAPL smear zone and the Village of Hartford 1,000-foot maximum setback zone for the municipal water supply wellfield, and (2) the seven monitoring locations sampled on a monthly basis as part of the additional LNAPL recovery pilot testing in Area A. As depicted on Figure 5-16, monitoring was conducted within 18 locations situated along the western and southern limits of the smear zone to delineate the extent of dissolved phase constituents of concern partitioning from LNAPL and confirm that the dissolved phase plume within the Main Sand stratum is stable. An additional 26 monitoring locations were sampled within the smear zone to better define the distribution of dissolved phase constituents and evaluate concentration trends since routine monitoring began in 2004. Changes in the dissolved phase constituents of concern can provide an indication of changes within the LNAPL source zone.
As depicted on Figure 5-16, concentrations of the dissolved phase constituents of concern were detected at low concentrations or not detected above the laboratory reporting limits beyond the western and southern margins of the smear zone. The dissolved phase plume appears to be stable beyond edge of the LNAPL smear zone. It should be noted that while monitoring locations are present and sampled along nearly the entire edge of the smear zone, there is a gap in the monitoring network between West Arbor and Birch Streets in the northwest portion of Effectiveness Zone 1. As described in Section 4.2.3, LNAPL thicknesses in monitoring well HMW-046C (approximately 200-feet east of the smear zone limit) have been stable or slightly increasing over time depending on the hydraulic conditions within the Main Sand stratum), in addition, dissolved phase concentrations within monitoring well HMW-038C (located at the edge of the LNAPL smear zone) have remained relatively unchanged since 2004. As described in Section 7.0, two additional monitoring locations screened across the upper portion of the Main Sand stratum are proposed to define the limits of the dissolved phase constituents of concern in the northwest portion of Effectiveness Zone 1.
Within the smear zone, concentrations of the constituents of concern were elevated within nearly every location sampled in the Main Sand stratum, with the sole exception of well HB-032. Dissolved phase benzene concentrations were typically twice as high, and in many cases more than an order of magnitude higher, than concentrations of the other dissolved phase constituents of concern. Benzene was generally measured at concentrations exceeding 1 mg/L, and often concentrations
exceeded 10 mg/L within most of the groundwater samples collected from the Main Sand stratum inside the smear zone.
Concentrations of the dissolved phase constituents of concern were significantly lower in the groundwater samples collected from the two monitoring locations screened within the EPA stratum (monitoring point MP-085C and well HMW-049C). Benzene concentrations in these samples were not significantly different from the concentration of the other constituents of concern. It should be noted that methyl tert-butyl ether was only detected in groundwater samples collected from one location screened within the Main Sand or EPA strata (monitoring well HMW-049C). Monitoring well HMW-049C is located north of the boundary of the Hartford Site and screened in the EPA stratum.
Dissolved phase benzene concentration trends for the monitoring locations screened in the deeper hydrostratigraphic units are included in Appendix F. Concentration trends are provided for monitoring locations that are within the smear zone and where at least three representative groundwater samples have been collected since 2004. The following bullets summarize the dissolved phase benzene concentration trends observed within each of the Effectiveness Zones.
Benzene concentration trends were not prepared for Effectiveness Zone 3, as most of monitoring locations are outside of the smear zone limits and dissolved phase benzene is generally not detected above the laboratory detection limits in groundwater samples collected in the Main Sand stratum.
Dissolved phase concentration trends could not be prepared for monitoring locations installed in Effectiveness Zone 6, as a limited number of representative samples have been collected due to the persistence of confining conditions, as well as LNAPL within the monitoring locations screened within the Main Sand stratum.
The dissolved phase benzene concentration trend for one additional location situated to the north of the Hartford Site is provided in Appendix F-8. Dissolved phase benzene concentrations for groundwater monitoring well HMW-049C (screened in the EPA stratum) have decreased over time.
Overall, the concentration of the dissolved phase constituents of concern are mostly unchanged within the Main Sand stratum in monitoring locations installed within the smear zone. Although decreasing concentration trends can be observed within the monitoring locations situated along the edges of the smear zone (such as well HMW-042B and monitoring point MP-063C), where dissolved phase constituents of concern were below 1 mg/L when routine groundwater monitoring began in 2004.
5.4. GEOCHEMICAL INDICATORS OF NATURAL ATTENUATION
Characterization of geochemical indicators in the shallow and deeper hydrostratigraphic units provides evidence of natural processes that may be attenuating petroleum hydrocarbons in the smear zone. While measuring biodegradation directly is challenging, it is possible to measure changes in geochemical parameters that can be related qualitatively and quantitatively to natural attenuation process. Geochemical species serve as electron acceptors and are reduced during microbial degradation (i.e., oxidation) of petroleum hydrocarbons.
The primary mechanism for natural attenuation is through the metabolic processes of petrophilic microorganisms that are ubiquitous in the subsurface. Within the saturated zone, aerobic biodegradation of petroleum hydrocarbons will proceed until dissolved oxygen is depleted and anaerobic conditions prevail. Typically, there are numerous potential electron acceptors besides oxygen that are available to support microbial respiration. Microorganisms preferentially use the electron acceptor that is thermodynamically most favorable, as follows:
Where significant hydrocarbon mass exists, methanogenesis can become the dominant long-term degradation pathway as more thermodynamically favored electron acceptors become depleted within the saturated zone. The primary by-products of aerobic and anaerobic biodegradation are methane and carbon dioxide. These gases can be transferred from the saturated zone to the vadose zone by partitioning into soil gas, as well as formation of gas bubbles and ebullition.
A summary of the hydrogeochemical indicator concentrations measured in groundwater samples collected from the Rand, Main Silt, Main Sand, and EPA strata is provided on Figure 5-17 and 5-18, respectively.
5.4.1. RAND STRATUM
Between 2013 and 2017, groundwater from three monitoring locations screened within the Rand stratum (multipurpose monitoring points MP-034B, MP-042B, and MP-049B) and one location screened within the deeper portions of the Main Silt stratum (well HMW-041B) were sampled for geochemical indicators of natural attenuation. The concentrations of the dissolved phase constituents of concern in groundwater samples collected from monitoring point MP-049B were relatively low or not detected above the laboratory reporting limits. Concentrations of the dissolved constituents of concern, were slightly higher in the samples collected from monitoring well HMW- 041B (total concentration between 0.1 and 0.7 mg/L). Whereas, the total concentration of dissolved phase constituents of concern was measured at greater than 1 mg/L in samples collected from monitoring points MP-034B and MP-042B, as shown on Figure 5-15.
As depicted on Figure 5-17, the concentrations of electron acceptors including sulfate and nitrate were higher in groundwater samples collected from monitoring point MP-049B and well HMW-041B (where groundwater contained no or relatively low concentrations of dissolved phase constituents of concern) compared to monitoring points MP-034B and MP-042B (where groundwater samples were reported with elevated concentrations of dissolved phase constituents of concern). Additionally, the concentrations of electron donors (or byproducts of microbial degradation of petroleum hydrocarbons) including ferrous iron, dissolved manganese, and dissolved methane were generally higher in monitoring locations MP-034B and MP-042B and were lowest within monitoring point MP- 049B. Dissolved phase carbon dioxide was relatively high in all four locations, indicative of complete mineralization of petroleum hydrocarbons within the saturated portions of the Rand stratum.
Overall, the distribution of electron acceptors and donors measured within the four locations screened in the Rand and deeper portions of the Main Silt strata indicate that intrinsic processes are actively reducing the LNAPL source. It should be noted that it is not possible to infer the rate of natural smear zone depletion from these processes without measuring the vertical and horizontal flux of the natural attenuation indicators across the saturated and unsaturated portions of the Rand stratum.
5.4.2. MAIN SAND STRATUM
Geochemical indicators of natural attenuation were assessed within groundwater samples collected from 16 locations screened in the Main Sand and EPA stratum since 2013. The spatial distribution of electron acceptors and byproducts are displayed on Figure 5-18. Concentrations of the electron acceptors including nitrate and sulfate were significantly higher in groundwater samples collected within Effectiveness Zone 4 (up-gradient portions of smear zone) compared to monitoring locations in Effectiveness Zones 1, 2, 5, and 6, except for well HMW-040C, located outside the smear zone and dissolved phase plumes in Zone 5. These results provide evidence denitrification and sulfate reduction of petroleum hydrocarbons in the Main Sand stratum.
Concentrations of geochemical byproducts including ferrous iron and dissolved manganese were either not detected or measured at very low concentrations in locations situated outside the western and southern limits of the smear zone and increased significantly (typically by an order of magnitude) in groundwater samples collected inside the smear zone, which is indicative of iron and manganese reduction. It is interesting to note that methane and/or carbon dioxide were measured at elevated concentrations within all the monitoring locations, suggesting reduction of dissolved and vapor phase petroleum hydrocarbons via anaerobic biodegradation processes both within and outside the smear zone.
A summary of the hydrogeochemical indicator concentrations versus distance for monitoring performed since 2008 is displayed on Figure 5-19. Select hydrogeochemical concentrations are compared to the concentrations of the dissolved phase constituent of concern through the centerline of the smear zone. The centerline selected (from up-gradient of the smear zone in Effectiveness Zone 4 extending north past the boundary of the Hartford Site on West Rand Avenue) is shown on Figures 5-16 and 5-18. As shown on Figure 5-19, nitrate and sulfate reduction occurs within approximately 100 and 1,000 feet down-gradient of the smear zone boundary where available electron receptors are fully reduced. There is a rapid increase in the dissolved phase constituents of concern including benzene that is accompanied with a rapid increase in the concentrations of geochemical byproducts including ferrous iron, dissolved manganese, dissolve methane, and carbon dioxide within approximately 100 feet of the smear zone boundary. Methanogenesis appears to be the dominant process degrading hydrocarbons within the smear zone. It should be noted that while the concentrations of the dissolved phase constituents of concern decrease outside the boundary of the Hartford Site, the concentrations of the elector acceptors remain reduced and the concentration of the geochemical byproducts remain elevated.
The assimilative capacity of groundwater within each of the monitoring locations included in the centerline analysis is depicted on Figure 5-20. The assimilative capacity is estimated by summing the concentrations of electron acceptors and subtracting geochemical byproducts, scaled to stoichiometric coefficients for attenuation of hydrocarbons (Johnson et al. 2006). As shown on Figure 5-20, the assimilative capacity within groundwater collected from the monitoring locations in Effectiveness Zone 4 (locations HMW-042B, MP-062C, MP-065C, and MP-088C) is positive indicating that groundwater at these locations has the potential to further degrade petroleum hydrocarbons within the saturated zone. Whereas, the assimilative capacity calculated for groundwater collected in the Main Sand stratum down-gradient of Effectiveness Zone 4 is negative, indicating that additional intrinsic biodegradation within the smear zone may be rate limited by the availability of electron acceptors within the saturated zone.
5.5. DISSOLVED PHASE SUMMARY
This subsection provides a summary of the dissolved phase component of the CSM for each hydrostratigraphic unit. This summary includes a description of the (1) LNAPL presence, (2) groundwater occurrence, (3) dissolved phase petroleum hydrocarbon distribution, and (4) dissolved phase concentration trends. This information was used to establish the proposed remedial management areas described in Section 8.0.
5.5.1. NORTH OLIVE STRATUM
The North Olive stratum ranges from less than 1-foot up to 10-feet thick. Groundwater within the North Olive stratum is perched upon the B-Clay, which is highly discontinuous and generally absent beneath the central and southern portions of the Hartford Site.
5.5.2. RAND STRATUM
The Rand stratum is discontinuous and ranges in thickness from less than 1 foot to 16 feet. Groundwater within the Rand stratum is perched upon the C-Clay. The Rand stratum grades laterally into the Main Silt stratum and is locally absent north and south of West Rand Avenue between North Delmar Avenue and North Old St. Louis Road.
5.5.3. MAIN SILT STRATUM
The Main Silt stratum is present along a southeasterly trending line where the B-and/or C-Clay are absent. Definition of the horizontal and lateral extent of the Main Silt stratum have been inconsistent due in part to the challenges in differentiating the stratum from the Rand and Main Sand strata.
5.5.4. EPA STRATUM
The EPA stratum is only present in the northeastern portion of the Hartford Site and grades laterally into the Main Sand stratum along a southeasterly trending line starting at the intersection of North Old St. Louis Road and North Delmar Avenue extending to the intersection of East Date Street and North Olive Avenue. Along this boundary, the EPA and Main Sand strata are hydraulically connected. The EPA stratum ranges in thickness from approximately 4 to 9 feet.
5.5.5. MAIN SAND STRATUM
The Main Sand stratum consists of a relatively thick sequence of sand and gravel glacial outwash (between 80 and 100 feet thick locally) that was deposited as a broad shallow valley was filled as the continental glaciers retreated during the Pleistocene Epoch. The Main Sand stratum is an extensive aquifer throughout the region (aka, American Bottoms aquifer).