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
Report
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Issue
2. SITE SETTING
The Village of Hartford is located in Madison County, Illinois on the east bank of the Mississippi River, approximately twelve miles northeast of St. Louis, Missouri. The definition of the site boundary is described in the Unilateral Administrative Order issued by the USEPA under Section 7003 of the Resource Conservation and Recovery Act (Docket No. RCRA-05-2010-0020). This component to the CSM does not seek to redefine the legal definition of the site boundary; however, discussions pertaining to the Hartford Site will focus on the current extent of petroleum related constituents in soil, groundwater, and soil. As such, references to the Hartford Site included herein will only include the area depicted on Figure 1 and described in the following bullets:
It should be noted that while the Hartford Site boundaries encompass the rights-of-way for the Norfolk and Western, Union Pacific, Kansas City Southern, and Norfolk Southern Railroads, further assessment of these four railroad rights-of-way has been limited due to access issues.
The railroad rights-of-way are therefore shown to be separated from the remaining portions of the Hartford Site on Figure 1 as an area that is 25 feet east and west of the centerline of the tracks.
2.1. SOURCES OF PETROLEUM HYDROCARBONS
Three refineries were constructed around the Hartford Site between 1907 and 1941, the Amoco Oil Refinery (currently the British Petroleum facility), the Clark Oil Refinery (currently the Premcor facility), and the Shell Oil Refinery (currently the Phillips66 facility). In addition, a bulk petroleum storage facility was constructed north of the Village of Hartford (currently the Hartford Wood River Terminal Oil Company facility). Refining, storage and transport of petroleum hydrocarbons continues to be conducted around the Village of Hartford associated with portions of these refineries. In addition, numerous underground and aboveground petroleum pipelines connected the refineries to the bulk storage terminal, loading and unloading facilities located on the Mississippi River, and to other entities.
Numerous releases of petroleum hydrocarbons have been documented within or immediately adjacent to Hartford.
Generally, the released hydrocarbons (referred to herein as LNAPL) migrated down through the subsurface under the influence of gravity until encountering the water table or less permeable layers (such as clays and silts). Due to capillary forces, some fraction of the LNAPL was retained in soil pore space in the unsaturated zone, whereas some fraction of the LNAPL reached the capillary fringe where it displaced water present in soil pore space. As the volume of LNAPL became sufficient to overcome hydrostatic forces, further lateral migration occurred. Vertical migration into deeper hydrostratigraphic units occurred where the less permeable layers were discontinuous or absent. The distribution of LNAPL stabilized as gravity and capillary forces approached equilibrium and natural smear zone depletion reduced the mass of hydrocarbons (notably along the vertical and horizontal margins of the smear zone).
2.2. INTERIM MEASURES
Interim measures performed at the Hartford Site since 1978 have primarily included skimming of LNAPL and operation of the SVE system. As documented in prior reports pertaining to the Hartford Site, between 1978 and 2013 approximately 2.25 million gallons of LNAPL has been recovered beneath the Hartford Site with 1.3 million gallons of LNAPL recovered via skimming (USEPA 2010, RAM 2013) and another 0.95 million gallons via operation of the SVE system (URS 2014). Over time, the recovery of hydrocarbons via skimming has diminished as LNAPL saturations in the shallow portions of the smear zone have been reduced (within the radius of influence of the well over time).
Conversely, vapor recovery rates fluctuate each year with the highest rate of recovery occurring in 2012, corresponding to a decrease in the water table during the second half of the year (Figure 2).
2.2.1. LNAPL RECOVERY
Between 1978 and 1979, Clark Oil Company installed two large diameter groundwater production wells (RW-001 and RW-002 shown on Figure 1) at the Hartford Site for the purpose of skimming LNAPL from the shallow portions of the smear zone. Between 1978 and 1990, LNAPL skimming was performed within these two production wells, with the exception of a period between 1983 and 1984 when operations were temporarily ceased. Approximately 1,162,000 gallons of LNAPL was recovered from these two wells through 1990. Skimming rates ranged from approximately 1,000 to 29,000 gallons per month (USEPA 2010).
An additional production well (RW-003 depicted on Figure 1) was installed at the Hartford Site by Premcor in 1993. From January 1994 through September 2002, Premcor reportedly recovered an additional 82,700 gallons of LNAPL from the three production wells installed across the Hartford Site (USEPA 2010).
Beginning in 2004, a consortium of oil companies (referred to as the Hartford Working Group) including Premcor, Shell, BP, and Sinclair Oil Corporation began managing interim measures, including LNAPL skimming. In 2004, the Hartford Working Group installed three additional wells (RW-004, RW-004A, and RW-005 shown on Figure 1) for the purpose of LNAPL recovery. Approximately 18,000 gallons of LNAPL were recovered via skimming activities between 2004 and 2009.
In addition, the Hartford Working Group performed a number of pilot tests over this five-year period to evaluate potential remedial technologies. These pilot tests primarily involved (1) MPE, which was defined as high vacuum recovery of vapor, groundwater and LNAPL using a stinger placed slightly above the LNAPL-air interface, and
(2) dual phase extraction (DPE), defined as LNAPL recovery augmented with limited groundwater extraction (maximum of 2.5 feet of drawdown was achieved during testing). An additional 6,000 gallons of LNAPL was recovered as part of performing these pilot tests by the Hartford Working Group.
In March 2009, routine operations, monitoring, and maintenance (OM&M) interim measures at the Hartford Site were transferred to Apex. Apex continued to conduct LNAPL skimming in two of the recovery wells (RW-002 and RW-004A) until December 2010 and recovered an additional 15,000 gallons of LNAPL. In addition, Apex conducted LNAPL skimming activities within the groundwater monitoring network beginning in late 2009 and recovered an additional 25,000 gallons of LNAPL through the end of 2012.
WSP Environmental & Energy (WSP) conducted a LNAPL recovery pilot test between October 2011 and January 2012 (the WSP pilot test) with the primary objective of evaluating previously selected technologies for LNAPL recovery including SVE, MPE, and DPE. As described in the Light Non-Aqueous Phase Liquid Recovery Pilot Test Interim Report (WSP 2012), groundwater and LNAPL were confined within the test well MPE-A001 throughout most of the WSP pilot test. Well MPE-A001 is located in Area A and screened across the top of the Main Sand Stratum. Immediately prior to testing, the LNAPL thickness in well MPE-A001 was 3.24 feet, greater than that typically observed in this well under unconfined conditions as shown on the figure provided as Appendix A (reproduced from the Light Non-Aqueous Phase Liquid Recovery Pilot Test Interim Report [WSP 2012]). The elevated LNAPL thickness observed in the test well prior to pilot testing was consistent with exaggerated LNAPL thicknesses observed in many of the wells under confined conditions across the Hartford Site (Table 1). The LNAPL-water interface was present within the screened interval of the well.
2.2.1.1. WSP PILOT TEST RESULTS
Although planned, SVE could not be tested as the screen became occluded once a vacuum was induced on well MPE-A001 during the pilot test. MPE was tested on November 7 through November 10, 2011. A drop tube was placed in the well with an applied vacuum for three hours the first day and near continuous thereafter. The drop tube diameter and elevation were varied during the testing, and airflow ranged from 13 standard cubic feet per minute (scfm) to 85 scfm. The applied vacuum achieved removal of fluids from well MPE-A001 with a maximum drawdown of 2.2 feet, but did not lower the fluid levels to below the top of the screen. Although a LNAPL thickness of 3.24 feet was measured prior to testing, no measurable LNAPL recovery was achieved during the test. Instead, approximately 6,900 gallons of groundwater were extracted. Pilot testing of DPE was planned, but based on the lack of significant drawdown during pilot testing of MPE, a pumping test was performed instead to assess achievable drawdown within the test well. Following a step test, a constant rate pump test was conducted at 20 gallons per minute (gpm) for 6.5 hours. Approximately 9 feet of drawdown was observed in the test well, exposing approximately 8 feet of the well screen. However, the LNAPL thickness in the well decreased from 2.89 feet to 0.14 feet. Fluid level monitoring within the nearby wells indicated some influence within 50 feet of the test well, but LNAPL thicknesses did not increase during the pump test. Overall, the pilot test resulted in no measureable LNAPL recovery using MPE, and insufficient drawdown in the well to expose the screen. Additionally, groundwater pumping did not affect LNAPL thickness in the test or nearby monitoring wells over the 6.5 hour test duration. The results suggest that MPE is not sufficient to achieve LNAPL recovery in Area A under confined conditions. The results also suggest that groundwater extraction may influence the piezometric surface in nearby wells. This supports the use of groundwater extraction as a possible means to change fluid levels in the formation and perhaps induce mobilization of LNAPL to wells under unconfined conditions. This is discussed further in Section 5.0.
2.2.2. SOIL VAPOR EXTRACTION
The original SVE system was installed by Clark Oil & Refining Corporation (now Premcor) and operated from approximately 1992 until it was upgraded in 2005. The original SVE system consisted of 12 vapor control boreholes, two 75- horsepower (HP) blowers with a combined capacity of approximately 1,500 standard cubic feet per minute (scfm) and a single thermal treatment oxidizer capable of treating up to 27 million British thermal units (BTU) per hour (URS 2013).
The original system was replaced in three phases beginning in 2005 by the Hartford Working Group and currently consists of a network of approximately 120 vapor extraction wells connected through a series of piping and valves to a single 12-inch pipe. The 12-inch pipe conveys the recovered vapors from the Hartford Site below the Union Pacific, Kansas City Southern, and Norfolk Southern Railroads rights-of-way located east of North Olive Street to four 75-HP blowers located on the Premcor facility. The four blowers have a total capacity of approximately 3,200 scfm. The recovered soil vapor is treated using between one and four thermal oxidizers, each capable of processing 9 million BTUs per hour.
Detailed records of hydrocarbon recovery rates have been documented for the SVE system since it was replaced by the Hartford Working Group in 2005. As shown on Figure 2, approximately 950,000 gallons of volatile petroleum hydrocarbons have been recovered through the SVE system between May 2005 and December 2013 (URS 2014). Vapor recovery has not reached asymptotic conditions - the highest daily recovery occurring in late 2012, as low water table conditions were observed beneath the Hartford Site.
2.3. GEOLOGIC SETTING
The Village of Hartford is located in the Springfield Plain of the Interior Plains Section of the Central Lowland Province. Specifically, the Village is situated within a shallow valley approximately 30 miles long and 11 miles across at its widest point and underlain by more than 100 feet of unconsolidated deposits created by alluvial and glacial processes during the Pleistocene period.
The Hartford Site is located along the historical edges of the Mississippi and Missouri River flood plains. Over the last 125,000 years, the Mississippi River has changed its course frequently through a process known as avulsion. An avulsion occurs when a river breaches its natural levee and then cuts a new channel in the adjacent floodplain. These frequent avulsions of the river have resulted in deposition of sediments with widely-varying grain size (including thick sequences of channel sands, lenticular splay sands, fine-grained levee sands, and finer-grained silty clay floodplain deposits) across a broad area creating a highly heterogeneous unconsolidated stratigraphy (USEPA 2010). These deposits are collectively referred to as the Cahokia Alluvium of Holocene Age. Underlying these alluvial deposits are a relatively thick sequence of sandy glacial outwash (between 60 and 150 feet thick) deposited during the Pleistocene Epoch, as the broad shallow valley was filled as part of a large outwash plain as the continental glaciers retreated. These sands are referred to as the Mackinaw Member of the Henry Formation. Locally these sands are referred to as the Main Sand hydrostratigraphic unit as described in Section 2.3.9.
These fluvial and glacial sediments are underlain by the Glasford Till or consolidated sedimentary bedrock more than 3,800 feet thick. These bedrock formations dip gently to the northeast from the Ozark Highlands toward the Illinois Basin and predominantly consist of limestone and dolomite with lesser amounts of sandstone and shale. Mississippian age bedrock believed to be the Renault Limestone underlies the Hartford Site. The Renault Limestone consists of relatively pure limestone and an upper sandy limestone (Clayton 2006). The limestone generally occurs more than 100 feet below ground surface (ft-bgs).
The following subsections describe the local geology beneath the Hartford Site with a focus on the alluvial and glacial units where petroleum hydrocarbons associated with releases from the refineries and pipelines may be present. Isopach maps and detailed descriptions of the shallow geology have previously been presented in the LNAPL Active Recovery System Conceptual Site Model (Clayton 2005).
2.3.1. A CLAY
The A Clay is the shallowest stratum beneath the Hartford Site. This clay unit ranges in thickness from 5 to 24 feet and is continuously present beneath Hartford, with the exception of areas where it has been removed as part of construction activities. As described in the LNAPL Active Recovery System Conceptual Site Model (Clayton 2005), this stratum is generally thickest: (1) near the intersection of West Date Street and North Delmar Avenue, and (2) north of West Rand Avenue along Illinois State Route 3. These areas are separated by relatively thin zones (less than 10 feet thick) that are generally situated: (1) near the intersection of West Date Street and Old St. Louis Road, (2) between West Watkins and West Forest Streets along Old St. Louis Road, (3) between East Rand Avenue and East Forest Street along North Olive Street, and (4) along East Forest Street between North Delmar Avenue and North Olive Street.
Geotechnical samples collected from the less permeable fine grained units, including the A Clay, beneath the Hartford Site contain mixtures of silt and clay ranging between 85% clay-15% silt and 20% clay-80% silt. Minor amounts of sand, generally less than 15% can be measured within these less permeable units (Clayton 2005).
2.3.2 NORTH OLIVE STRATUM
The North Olive stratum is encountered at depths ranging from approximately 8 to 15 ft-bgs and 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, as shown on Figure 3. The North Olive stratum is bounded by the A and B Clay and is comprised of approximately 12% sand, 71% silt, and 17% clay based on previously collected geotechnical samples (Clayton 2005). The North Olive stratum ranges from less than 1-foot to 10-feet thick and is generally thickest (1) along the southern portion of North Olive Street, (2) south of East Rand Avenue along North Olive Street, (3) at the intersection of North Delmar Avenue and West Birch Street, and (4) in the vicinity of the Hartford Community Center. The North Olive stratum is thinnest near the margins of the B Clay (Clayton 2005).
2.3.3 B CLAY
The B Clay, underlies and defines the extent of the North Olive stratum, where present, and overlies the Rand stratum. The B Clay is highly discontinuous and is generally absent beneath the central and southern portions of the Hartford Site as described in the previous section. The B Clay ranges in thickness from less than 1 foot to 12 feet and is generally thickest (1) near the intersection of East Elm and North Olive Streets, (2) between East Date and East Cherry Streets, (3) near the intersection of East Birch and North Market Streets, (4) between West Cherry and West Date Streets, and (5) near the intersection of East Rand Avenue and North Olive Street. The B Clay stratum is generally 6 feet thick east and west of North Delmar Avenue, near the Hartford Community Center (Clayton 2005).
2.3.4 RAND STRATUM
The Rand stratum is discontinuous and generally encountered at depths ranging from approximately 12 to 27 ft-bgs and is defined by the extent of the C Clay. Based on previously collected geotechnical samples, the Rand stratum is composed of approximately 10% sand, 70% silt, and 20% clay and is similar in composition to the North Olive stratum, and also that of the Main Silt in some areas (Clayton 2005). This stratum appears to extend across the majority of the northern and eastern portions of the Hartford Site as shown on Figure 4. To the south of Date Street, the Rand stratum grades laterally into the Main Sand (or Main Silt where present) and is locally absent near the intersections of West Rand Avenue and North Delmar Avenue, as well as West Rand Avenue and North Old St. Louis Road, where the Rand Stratum is absent and the B and C Clays are undifferentiated. The Rand stratum ranges in thickness from less than 1 foot to 11 feet and is thickest (1) near the intersection of East Birch and North Olive Streets, (2) south of East Rand Avenue along North Olive Street, and (3) between East Date and East Elm Streets. It is thinnest at the intersection of West Date Street and North Delmar Avenue (Clayton 2005).
2.3.5. C Clay
The C Clay defines the extent of the Rand stratum and ranges in thickness from less than 1 foot to approximately 8 feet. The C Clay is highly discontinuous and only present in the northern and eastern portion of the Site, with the edge of this stratum trending southeast from the west side of West Cherry Street to the east side of West Watkins Street. The C Clay is thickest in the area (1) near the intersection of East Cherry and North Olive Streets, (2) near the intersection of East Date and North Market Streets, and (3) between East Elm and East Forest Streets west of North Olive Street. The C Clay is thinnest near the intersection of North Market and East Forest Streets (Clayton 2005).
2.3.6 EPA STRATUM
As shown on Figure 5, the EPA stratum is only present in the northeastern portion of the Hartford Site at depths ranging from 27 to 46 ft-bgs. Based on prior geotechnical analyses, the EPA is composed of approximately 68% sand, 22% silt, and 10% clay (Clayton 2005). This stratum is defined from the Main Sand by the thin D Clay. The EPA grades laterally into the Main Sand south of a southeasterly trending line starting at the intersection of Old St. Louis Road and North Delmar Avenue to the intersection of East Date and North Olive Streets. Along this boundary, the EPA and Main Sand are hydraulically connected. The EPA stratum ranges from approximately 4 to 9 feet thick within the northeastern portion of Hartford and is thickest north of West Rand Avenue (Clayton 2005).
2.3.7 D CLAY
The D Clay underlies the EPA stratum and ranges in thickness from approximately 2 to 7 feet thick. This thin fine grained unit could be considered a discontinuous lens within the Main Sand, based on its relative thickness and limited extent. The D Clay stratum is thickest near the intersection of North Delmar Avenue and Old St. Louis Road and thins out along its western margin (Clayton 2005).
2.3.8 MAIN SILT STRATUM
The Main Silt is encountered at depths ranging from approximately 6 to 30 ft-bgs, where the B and/or C Clay are absent, along a northwest to southeast trending line across the central and southern portion of the Hartford Site as shown on Figure 6. Previous descriptions (Clayton 2005, Clayton 2006) of the extent of the Main Silt have been inconsistent due in part to the challenges in differentiating the stratum from the Rand and Main Sand. The Main Silt has been described as compositionally similar to the North Olive and Rand strata (approximately 25% sand, 64% silt, and 11% clay), and although compositionally different from the Main Sand, the gradational contact between the Main Silt and Main Sand makes discerning the units difficult (Clayton 2005). The interpretation of the lateral extent of the Main Silt is based on a review of historical isopach maps, geologic cross-sections, and lithologic logs from borings installed throughout the Village of Hartford.
In general, the top surface of the Main Silt is equivalent to the same horizon as the top of the North Olive stratum. The majority of the bottom surface of the Main Silt is equivalent to the same horizon as the top of the EPA stratum. The Main Silt ranges in thickness from approximately 2 to 19 feet and is thickest on North Delmar Avenue (1) between West Watkins and West Maple Streets, and (2) between West Elm and West Forest Streets. The Main Silt is thinnest in areas where the A Clay is thickest, such as between West Forest and West Watkins Streets along North Delmar Avenue. The nature and distribution of groundwater and LNAPL within the Main Silt is combined with descriptions regarding the Main Sand herein.
2.3.9 MAIN SILT STRATUM
The Main Sand is aerially extensive throughout the region. In Hartford, it is encountered at depths ranging from 19 and 45 ft-bgs based on the presence or absence of the overlying Clay strata. Although the Main Sand is primarily comprised of 90% sand, 7% silt, and 3% clay, discontinuous silty clay and clayey silt lenses of limited thickness and extent occur at various depths within the Main Sand (Clayton 2005). Gravels are also observed in lenses within the Main Sand stratum. The thickness of the Main Sand stratum ranges from 80 to 100 feet, with bedrock generally encountered at an elevation between 300 and 325 feet above mean sea level (ft-amsl).
2.4 HYDROGEOLOGIC SETTING
Groundwater present within the aerially extensive deposits of unconsolidated valley fill of the Mackinaw Member of the Henry Formation, extending across an area of approximately 175 square miles, is considered the most significant aquifer in the region. This aquifer, present in the Main Sand stratum beneath the Hartford Site, is commonly referred to as the American Bottoms aquifer. Natural groundwater movement within the American Bottoms aquifer is to the west, draining water from the limestone bluffs (along the east wall of the floodplain valley) into the Mississippi River (Engineering Science 1992).
There are three additional water bearing, or hydrostratigraphic units, located within the Cahokia Alluvium beneath the Hartford Site. These hydrostratigraphic units are generally present within the coarser grained silt and sand deposits including the North Olive, Rand, and EPA strata. Multiple flow directions have been observed in these shallower, less permeable hydrostratigraphic units. These shallow water-bearing zones are generally discontinuous and do not appear to have an effect on regional flow in the underlying Main Sand aquifer beneath the Hartford Site, with the exception of areas of recharge where the shallower units are contiguous within the Main Sand aquifer.
The Mississippi River is located less than a half mile from the Hartford Site and is hydraulically connected to the deeper hydrostratigraphic units (Rand, EPA, and Main Sand), where present, beneath the Hartford Site. Water level fluctuations in each unit are affected by changes in the Mississippi River stage. Since the river stage varies by more than 20 feet during a year, the groundwater conditions can fluctuate from unconfined to confined conditions throughout the year. There are some areas within the Main Sand where confined conditions may persist during low river stage, such as the northwest portion of the Hartford Site near the intersection of North Olive Street and West Rand Avenue where the D Clay is present.
2.4.1 NORTH OLIVE STRATUM
Groundwater within the North Olive stratum occurs as isolated areas of temporarily perched water on the surface of the underlying B Clay prior to migrating deeper into the subsurface. Therefore, it is not possible to generate a representative potentiometric surface map representing flow within this stratum. Figure 3 presents the groundwater elevation data where groundwater occurs within the monitoring wells and monitoring points screened in the North Olive stratum. Hydraulic conductivity estimates based on a limited number of slug tests performed by the Hartford Working Group in the North Olive stratum ranged from 2.9E-04 to 1.4E-06 centimeters per second (cm/s). These hydraulic conductivity estimates within the North Olive stratum were similar to the hydraulic conductivities measured in the over and underlying clay strata, which ranged from 1.7E-04 to 6.0E-09 cm/s (Clayton 2005).
2.4.2 RAND STRATUM
Groundwater in the Rand stratum, south of Rand Avenue, also represents localized areas of perched water. Groundwater elevations measured in October 2013 within the monitoring wells and monitoring points installed within this hydrostratigraphic unit are included on Figure 4. Hydraulic conductivity measured in the Rand stratum ranged from 7.9E-03 to 5.5E-05 cm/s (Clayton 2005). Similar to the underlying hydrostratigraphic units, groundwater within the Rand stratum is unconfined during periods of low Mississippi River stage, and becomes confined during times of high river stage. Groundwater within the Rand and underlying EPA stratum in northeast Hartford are hydraulically separate, although the C Clay located between these two units is discontinuous and leaky allowing vertical drainage between these hydrostratigraphic units.
2.4.3 EPA STRATUM
A groundwater divide is generally present within the EPA stratum, with groundwater flow on the southern side of the divide (beneath the northeastern portion of the Hartford Site) to the southwest. As groundwater flows southwesterly beyond the extent of the D Clay, it is hydraulically connected to the Main Sand and flows to the west and northwest. Groundwater elevations in the monitoring wells screened in the EPA stratum are shown on Figure 5. The average hydraulic conductivity of the EPA stratum was within the range of values for both the North Olive and the Rand strata. In the northeastern portion of Hartford where the EPA stratum is present, the hydraulic conductivity has been measured between 3.8E-04 and 1.5E-05 cm/s (Clayton 2005).
2.4.4 MAIN SAND STRATUM
Groundwater in the Main Sand aquifer within the Village of Hartford is generally unconfined during periods of drought and low Mississippi River stage which generally occurs for no more than several months each year, typically in the late Fall and Winter months. Groundwater becomes confined by the C and D Clay (where these finer units are present beneath the Hartford Site) during times of normal and high river stage, and usually extends throughout most of the year. Groundwater elevations within the Main Sand have fluctuated significantly over the past 50 years. Historical fluid level monitoring data indicate that groundwater elevations reached a high of approximately 415 ft-amsl during the early-1990s and have been as low as approximately 380 ft-amsl in the mid-1950s, which is typically 10 to 15 feet lower than conditions that have prevailed since 2004. The low groundwater elevations observed during the mid-1950s (that have not been observed since then) may be attributed to (1) the lowest mean Mississippi River stage as a result of extreme drought conditions, and (2) a period of maximum pumping of groundwater from the facilities adjacent to the Hartford Site (USEPA 2010). It should be noted that the Army Corp of Engineers constructed Dam No. 27 (a.k.a. the Chain of Rocks Dam), between 1959 and 1963, down-stream of the Hartford Site. This low water dam raised the minimum river stage to 9 feet within the Mississippi River from the dam up-stream to the Melvin Price Dam (which replaced Dam No. 26), which also may explain why groundwater elevations in the Main Sand have not reached the historical lows observed in the 1950s.
There is a significant difference between the hydraulic conductivity measured in the Main Sand aquifer and those of the overlying hydrostratigraphic units. The hydraulic conductivity for the Main Sand in the central portion of the Hartford Site determined via slug testing performed under unconfined conditions in wells screened across the upper portion of the hydrostratigraphic unit ranged from 1.6E-02 to 3.1E-02 cm/s. Hydraulic conductivities estimates reported via pump tests in the production wells installed on the Premcor facility were as high as 1.0E-01 cm/s (Clayton 2005). The Village of Hartford and adjacent refinery groundwater production wells are generally screened within deeper portions of the Main Sand stratum because of the elevated hydraulic conductivity and saturated thickness (between 80 and 100 feet), resulting in high groundwater transmissivity within this aquifer.
Natural flow of groundwater in the Main Sand aquifer has been locally altered beneath the Hartford Site due to pumping on the BP, Phillips 66, and Premcor facilities. In 2013, the pumping rate at the BP facility averaged 1,225 gpm, while pumping at the Premcor facility averaged 288 gpm with periods of pumping in excess of 500 gpm. Additional pumping wells located west and northwest of the Village of Hartford at the Phillips66 River Dock operated at rates between 6,300 and 7,100 gpm. In addition, groundwater production rates were reported between 3,000 and 3,800 gpm at the Phillips66 facility located northeast of the Village of Hartford (SJMA 2014).
The groundwater flow direction in the Main Sand is also influenced by the stage of the Mississippi River. During periods of high river stage groundwater flow is generally towards the east to northeast due to recharge from the river and bank storage within the Main Sand. During moderate river elevations, the groundwater flow direction is northward. During low river stages, groundwater flow trends westerly to northwesterly. As shown on Figure 7, groundwater flow during the fourth quarter 2013 was generally to the west and northwest and is attributed to the low water table combined with the high rate of pumping conducted within production wells on the Phillips 66 River Dock. There is also a small area of the Hartford Site along North Olive Street between East Date and East Watkins Streets, where flow is locally to the east (as influenced by pumping at the Premcor facility). Beneath the Premcor facility, groundwater within the Main Sand converges with groundwater flowing from the western limits of the EPA Stratum.
In the absence of groundwater production 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 2010). The Village of Hartford municipal wells are installed within the Main Sand aquifer to the south of the Hartford Site (Figure 1). The two most recently installed groundwater production wells (No. 3 and No. 4) have a total depth of approximately 105 ft-bgs and were constructed with between 20 and 35 feet of screen. Discontinuous pumping from these municipal wells (average of 150 gpm) is at a much lower rate than that performed on the various facilities located to the north of the municipal wells and does not affect flow direction within the Main Sand aquifer beneath the Hartford Site.