3.0 LNAPL CHEMISTRY AND PHYSICAL PROPERTIES

 

Two types of data sets have historically been used to characterize LNAPL underlying the Site: (1) LNAPL chemistry data from laboratory analysis, and (2) LIF results. In 2004, 28 LNAPL samples were collected from the Main Sand stratum and characterized as primarily gasoline range hydrocarbons. The secondary product type, where present in these samples, was identified as diesel (Clayton 2005). A summary figure showing the distribution of these product types, reproduced from the Active LNAPL Recovery System Conceptual Site Model is provided as Appendix B. For the most part, LNAPL with gasoline reported as all or nearly all of the hydrocarbon makeup were identified in the central portions of the Hartford Site; while LNAPL with diesel as a secondary component tended to be located on the eastern and northern portions of the Site. A single LNAPL sample was also collected from the EPA (HMW-048C) and the Rand strata (MP-029C). LNAPL within these two hydrostratigraphic units was primarily characterized as diesel with lesser amounts of gasoline. The inferences from the LNAPL chemistry results were generally supported by LNAPL physical properties (i.e., molecular weight, density, and viscosity).

 

Twelve additional LNAPL samples were collected for laboratory analysis in 2006 and two samples were collected in 2009. These samples were collected from wells screened within the Main Sand stratum, with the exception of monitoring point MP-029C, which is screened within the Rand stratum. The laboratory analytical results for these samples are provided in Appendix B. LNAPL samples collected from the Main Sand generally consisted of low molecular weight hydrocarbons with chemical characteristics similar to a weathered gasoline. Conversely, the LNAPL sample collected from monitoring point MP-029C consisted of higher molecular weight hydrocarbons and was chemically more similar to a weathered diesel. The results for these samples are further discussed in Sections 3.2 and 3.3.

 

In 2004, 66 LIF borings were installed to evaluate the nature and distribution of hydrocarbons within the stratigraphic units beneath the Hartford Site using the Rapid Optical Screening Tool (ROST™). An additional 43 LIF borings were installed in 2005. The observed waveforms were categorized into three general LNAPL types including light-, mid-, and heavy-range hydrocarbons. Heavy-range hydrocarbons were primarily identified on North Olive Street between the intersections of East Date and East Watkins Streets within the Main Sand stratum. In this case, “heavy-range” generally corresponds to LNAPL with a higher proportion of diesel-range compounds than the light-range hydrocarbons which are comprised almost entirely of gasoline-range compounds (see Appendix B). Mid-range hydrocarbons were observed in the northern portion of the Hartford Site within the Main Sand and EPA strata, with several locations also showing similar LNAPL types in the overlying Rand and North Olive strata. Light-range hydrocarbons were identified within the central and southern portions of the Hartford Site within the Main Sand, with select locations showing similar light range hydrocarbons in the overlying EPA, Rand, and North Olive strata. Commingling of the three different LNAPL types was observed in some locations (Clayton 2004).

 

3.1 COMPARISON OF FLUORESCENCE WAVEFORMS AND LNAPL CHEMISTRY

In general, waveforms obtained by LIF are a qualitative indicator of LNAPL types, and not a quantitative measurement such as can be obtained by laboratory analysis of carbon distributions and individual constituents in LNAPL. Therefore, this section provides a comparison of the LNAPL chemistry results for the samples collected in 2006 to the historical LIF waveforms. This comparison was performed to increase the confidence of the LIF results as a direct indicator of LNAPL chemistry. The location of the seven monitoring points and wells where LNAPL samples were collected (total of ten samples), as well as the closest corresponding LIF borings, are shown on Figure 8.

 

In general, carbon distributions for LNAPL samples collected from the Main Sand stratum, included in Appendix D, peaked at C8 and have a shallower decrease in the direction of shorter chained constituents compared to longer chained constituents (in other words, there is an abundance of light-end constituents centered at the C8 carbon range). Several of the LNAPL samples collected from the Main Sand show a secondary peak in the carbon distribution centered at C5 (e.g., MP-029D, MP-047C, MP-079C). Both of these carbon distributions are characteristic of light-range hydrocarbons such as gasoline. The LNAPL chemistry results for samples collected from the Main Sand are consistent with the LIF data measured in the same stratum within the nearby borings, which show waveforms that are peaked at the first (or shortest) waveband. Two exceptions include LIF response within borings HROST-026 and HROST-130, which are collocated with LNAPL samples collected from monitoring points MP-047C and MP-029D, respectively.

 

The waveform for HROST-026 suggests a mid-range hydrocarbon with the peak at the second waveband, which is not consistent with the carbon distribution for the LNAPL sample collected from monitoring point MP-047C. The poor match between the LNAPL chemistry and the nearby LIF response may be an indication of a transition or mixing of LNAPL types along the eastern edge of the Site. In addition, the waveform observed within boring HROST-130 appears to be a mid-range hydrocarbon type (peak at the second waveband) which is inconsistent with the carbon distribution for the LNAPL sample collected from monitoring point MP-029D. The poor match between the LNAPL chemistry data and LIF response may be an indication of commingling of LNAPL types or heterogeneities in their distribution along the northern portions of the Hartford Site, where multiple releases are known to be present across the hydrostratigraphic units.

 

The carbon distribution for the LNAPL sample collected from the Rand stratum at monitoring point MP-029C peaked at C11, and does not reach a “tail” beyond the C15 carbon range. The carbon peak is suggestive of a mid-range hydrocarbon, though it is possible that other peaks are present above C15.

 

The LIF waveform observed within the Rand stratum from nearby boring HROST-130 shows the highest response within the second waveband, consistent with the interpretation of a mid-range hydrocarbon.

 

These comparisons generally confirm that the LIF waveforms are a reasonable indicator of LNAPL types at the Hartford Site. The LNAPL chemistry data set is strongest for light-range LNAPL with a gasoline-like carbon distribution and could be bolstered by collecting LNAPL samples from areas with mid- and heavy-range LNAPL types.

 

3.2 LNAPL PARTITIONING ESTIMATES

The LNAPL chemistry results reported for the samples collected in 2006 can also be used to estimate potential partitioning of individual constituents from the LNAPL to groundwater. Effective solubility estimates for benzene based on the LNAPL chemistry results are presented on Table 2, and compared to dissolved phase benzene concentrations measured in groundwater samples collected from nearby monitoring points and wells (Figure 9). The effective solubility estimates for benzene in the Main Sand stratum exceeded 10 milligrams per liter (mg/L) for each of the LNAPL samples, which are similar to the dissolved phase concentrations measured in groundwater samples collected from nearby wells, with a difference generally less than a factor of two. These results indicate general agreement between LNAPL effective solubility and dissolved phase concentrations and suggest that dissolution processes have reached equilibrium within the Main Sand stratum; therefore, measurements of individual hydrocarbon constituents in groundwater and LNAPL can be used somewhat interchangeably. This also means that depletion of constituents within the dissolved phase can be interpreted as depletion of individual constituents within the LNAPL, which will be discussed as part of the forthcoming dissolved phase component to the CSM.

 

3.3 LNAPL PHYSICAL PROPERTIES

The physical properties of the LNAPL can provide a supporting line of evidence regarding product type. This is especially the case for viscosity, which can vary over large ranges for different product types (e.g., less than 1 centipoise [cp] for gasoline and greater than 100 cp for motor oil). Physical properties of the LNAPL can also significantly affect the efficacy of various recovery approaches (e.g., high viscosity LNAPL is not amenable to recovery using traditional approaches). For example, moving from a 1 cp LNAPL to a 5 cp LNAPL can significantly decrease LNAPL recovery rates.

 

Figures 10 and 11 present measured kinematic viscosities for LNAPL samples collected from the Rand and Main Sand stratum, respectively. Viscosities for LNAPL samples collected from the Rand stratum range between 0.55 (MP-083B) and 3.39 centipoise (MP-029C). The higher viscosity at monitoring point MP-029C is indicative of a heavier-range hydrocarbon, which is consistent with the LNAPL chemistry and waveform from the nearby LIF boring.

 

Most measured viscosities for LNAPL samples collected from the Main Sand are less than 1 centipoise, with the lowest values generally observed within the central portions of the Hartford Site, consistent with other lines of evidence indicating predominantly light-range hydrocarbons within this stratum. The highest viscosity LNAPL in the Main Sand was measured in samples collected from wells HMW-045C and HMW-046C, located in the northwest corner of the Harford Site. These higher viscosity measurements may indicate a different LNAPL source, which is consistent with the LNAPL chemistry results. Slightly higher viscosities (but still below 1 centipoise) were measured in eastern and northeastern portions of the Hartford Site, possibly consistent with commingled LNAPL in these areas.

 

3.4 PROPOSED DATA COLLECTION AND UPDATES TO THE COMPREHENSIVE CSM

A robust data set for correlating carbon distributions with LIF response for the light-range hydrocarbons has previously been collected at the Hartford Site. However, the mid- and heavy-range LNAPL types have a smaller LNAPL chemistry data set for correlating carbon distributions with LIF responses. It is recommended that additional LNAPL samples be collected from groundwater monitoring wells and monitoring points HMW-044C, HMW-046C, and MP-029D screened in the Main Sand, as well as monitoring point MP-029C screened in the Rand stratum to bolster this data comparison.

 

Additional light-range LNAPL samples are also recommended from monitoring points MP-038C, MP-039C, MP-046C, MP-047C, MP-060C, and MP-079C screened within the EPA and Main Sand strata and monitoring points MP-053B and MP-054B screened in the Rand stratum. The light-, mid-, and heavy-range samples will be used to assess individual constituent and mass depletion of the smear zone beneath the Hartford Site.

 

The LNAPL samples will be collected from these wells using skimmer pumps, peristaltic pumps, or hydrophobic bailers. These samples will be collected and submitted for laboratory analysis of volatile and semivolatile organic constituents via USEPA Methods 8260 and 8270, respectively. Samples will also be analyzed for API gravity via ASTM Method D287, viscosity via ASTM Method D445, and simulated distillation by gas chromatography/flame ionization detector for carbon fraction ranges. The carbon distributions for the mid- and heavy-range LNAPL will be compared to the LIF results within the nearby borings. In addition, the mole fraction of benzene, as well as other select petroleum related constituents, will be estimated within the light-, mid-, and heavy-range LNAPL samples and compared to historical results to assess depletion of the LNAPL. These analyses will be provided within the Comprehensive CSM, if LNAPL samples are able to be collected and analyzed prior to preparing that document.