Cost and Performance Report: Soil Vapor Extraction at the Hastings Superfund Site, Well Number 3 Subsite Hastings, Nebraska

Table of Contents

• Executive Summary
• Site Information
• Matrix Description
• Treatment System Description
• Treatment System Performance
• Treatment System Cost
• Observations and Lessons Learned
• References
• Appendix A- Treatability Study
  

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Treatment System Performance

Cleanup Goals/Standards [1, 5]

No cleanup levels were specified in the 1989 ROD. The remedial action at the Well Number 3 Subsite was completed as an interim measure for the purpose of controlling contaminant migration. In December 1992, EPA and the Nebraska Department of Environmental Quality established an extraction rate for carbon tetrachloride of 0.001 lbs/hr as a cutoff value for terminating operation of the SVE system. The rationale for the cutoff was supported by a cost comparison with groundwater extraction and treatment. For extraction rates less than the cutoff value, groundwater extraction and treatment at this site was found to be less expensive than SVE.

In addition, EPA determined that the system was to be operated until the field analytical results were verified through laboratory analysis and it was verified that no rebounding of CCl₄ was occurring.

Treatment Performance Data [2, 3, 6, 21, 22]

Treatment performance data for operation running time, air flow rates (Qs), mass extraction rate, and total mass removed for carbon tetrachloride for this SVE system are shown in Table 7. Figures 8 and 9 show the mass extraction rate and cumulative mass removed for carbon tetrachloride, respectively, plotted against time for the operation of the SVE system. These data are based on field analytical results. The rate and mass were calculated from the concentrations of extracted vapor samples collected at the carbon system inlet. Samples of the extracted vapor were collected weekly using a gas-tight syringe and analyzed on site with a gas chromatograph for carbon tetrachloride only. [2, 3, 6]

Table 7. SVE System Operation Log [6]

Figure 8. Carbon Tetrachloride Mass Extraction Rate vs. Time [Adapted from Reference 6]

Figure 9. Cumulative Mass of Carbon Tetrachloride Removed vs. Time [Adapted from Reference 6]

Table 8 presents the carbon tetrachloride concentrations measured at each wellhead and the carbon system inlet. Sampling occurred at each extraction well on a monthly basis with analyses performed on site. On a periodic basis, samples from the wellheads and carbon inlet were collected in stainless steel SUMMA^TM canisters and were analyzed by a Region VII laboratory for volatile organic compounds (VOCs). The canister sampling results, presented in Table 9, were used to compare with the syringe sample results and to quantify other VOCs that may be present.

Table 8. Carbon Tetrachloride Concentrations (µg/L) from On-site Analysis of Extracted Air Samples [6]

*S-101 is the carbon system inlet.
NS=Not sampled
Note: A correction factor of 1.85 was applied to on-site GC results obtained before August 12, 1992 to account for negative bias.

Table 9. Results of Canister Samples [6]
Canister Results for March 4, 1993

Table 9. (cont.) Results of Canister Samples [6]
Canister Results for March 6, 1993

Table 9. (Cont.) Results of Canister Samples [6]
Canister Results for May 1, 1993

Post-treatment sampling of the soil gas or soil borings was not performed because of difficulties with detecting VOCs in soils at the site. Soil samples collected during the previous investigations frequently showed non-detects in locations where significant levels of soil gas were found. Therefore, it was concluded that soil gas was a more reliable and easily measured indication of vadose zone contamination.

Performance Data Assessment

A review of the results in Table 7 and Figures 8 and 9 indicates that after approximately 3,600 hours of operation, the SVE system achieved the extraction rate cleanup goal of 0.001 lb/hr, with a corresponding mass of carbon tetrachloride removed equal to approximately 90 pounds. The results indicate that more than half of the mass removed occurred during the first 22 days of operation, and that the concentration of carbon tetrachloride at the wellheads sharply decreased after the first month of operation.

To verify that the carbon tetrachloride cleanup goal was achieved, the system was shut down for 2 months to assess the potential rebound in the carbon tetrachloride concentration. As shown in Table 7, there was no significant increase in the carbon tetrachloride concentrations after a 2-month shutdown.

The rapid decrease in carbon tetrachloride concentration is further supported by the information in Table 8, which shows a decrease in CCL₄ concentration by at least one order of magnitude from June 25 to July 28, 1992 for seven of eight sample locations, followed by a more gradual decrease in concentrations through June 17, 1993. The results in Table 9 show that, in addition to CCl₄, detectable levels of chloroform, benzene, trichloroethene, 1,1-dichloroethene (DCE), 1,1,1-trichloroethane (TCA), tetrachloroethene (PCE), and methylene chloride were present in the extracted vapors from wells 1D, 5D, and 5I. Also, as shown in Tables 8 and 9, the CCl₄ concentrations measured on May 1, 1994 using on-site analyses and canister samples were within 25% of each other for sampling locations S-101 (0.3 vs. 0.33 µg/L) and Well-5D (0.5 and 0.64 µg/L).

Performance Data Completeness

Data characterize concentrations of contaminants in soil vapors from each extraction well over the course of the treatment operation, and show how treatment performance varies with operating conditions of the SVE system.

Performance Data Quality [12]

A comparison of the on-site syringe results, performed in August 1992, with the canister results showed that the syringe results were biased low. The bias is believed to be a result of diffusion of the sample from the syringe prior to analysis. A larger sample injection volume was used to minimize the diffusion effect. A correction factor of 1.85 was developed for the syringe results based on studies done with larger injection volumes and the canister results.

Other exceptions noted by the vendor for this treatment application included:

In February 1993, a negative bias was also observed and verified by the March sampling. It was determined that the following reasons could have contributed to this bias:

• Simple sampling equipment (10-mL syringe versus 6-L SUMMA^TM canisters);
  
• Low levels of contaminants in the samples (at higher concentrations, small fluctuations are not so dramatic);
  
• Cold weather conditions; and
  
• The on-site laboratory and analytical methodology was limited, whereas the off-site analyses were performed by an EPA region laboratory.
  

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References

1. U.S. Environmental Protection Agency. Record of Decision, Hastings Groundwater, NE, September 1989.

2. Morrison-Knudsen Corporation. Preliminary Design Report for a Soil Vapor Extraction System, Well Number 3 Subsite, Revision 1, U.S. EPA ARCS, September 1991.

3. U.S. Environmental Protection Agency. Remedial Action Report for Source Control Operable Unit at the Well #3 Subsite, Hastings, Nebraska, June 17, 1993.

5. Letter from Diane Easley, EPA to Richard Schlenker, NDEQ, dated December 1, 1992.

6. Morrison-Knudsen Corporation. "Well Number 3 Source Control, Remedial Design/Remedial Action - May/June 1993 Monthly Status Report," U.S. EPA ARCS, July 7, 1993.

12. Morrison-Knudsen Corporation. "Well Number 3 Source Control, Remedial Design/Remedial Action - September 1992 Monthly Status Report," U.S. EPA ARCS, October 1, 1992.

21. Comments submitted by Morrison-Knudsen Corporation, on January 26, 1995.

22. Comments submitted by Diane Easley of EPA Region VII on February 9, 1995.