1. Statistical Evaluation of Attenuation Factors at Lowry Air Force Base, CO Helen E. Dawson, PHD Regional Superfund Hydrogeologist US EPA Region VIII Denver, CO 303-312-7841 dawson.helen@epa.gov AEHS VI Workshop October 2004

2. LAFB TCE Plume Study Area

3. LAFB Cross-Section Depth to groundwater: 6 m Vadose zone: fining upwards sequence; sand and gravels to sandy silts. Saturated zone: sands and gravels Residential AreaBase

4. LAFB Soil Gas Study Area

5. LAFB Indoor Air & Foundation Vapor

6. TCE Concentrations Regulatory Levels Background Lowry Air Force Residences Full Basement ½ Basement ½ Crawlspace Slab

7. Sampling Methods Indoor and ambient air: 24-hour Summa; TO15S Subslab: Summa after 2 L purge; TO15S Groundwater: Bailer; 8260S

8. Chemicals 11DCA1,1-Dichloroethane 12DCA1,2-Dichloroethane 11DCE1,1-Dichloroethene C12DCEcis-1,2-Dichloroethene T12DCEtrans-1,2-Dichloroethene PCETetrachloroethene 111TCA1,1,1-Trichloroethane 112TCA1,1,2-Trichloroethane TCETrichloroethene VCVinyl Chloride

9. Attenuation Factors Calculated Groundwater to indoor air(GW-IA) Groundwater to subslab (GW-SS) Subslab to indoor air(SS-IA)

10. Data Quality Filters Data excluded if: GW, SS, or IA value < detection limit IA conc. > subslab conc. IA conc. < OA conc. (IA – OA)/IA < 20% Chemicals filtered out: 12DCA, T12DCE, 112TCA

11. Groundwater to Subslab Attenuation Factors

13. Subslab to Indoor Air Attenuation Factors

15. Groundwater to Indoor Air Attenuation Factors

16. Q4 Generic Alpha Q5 Alpha

17. Comparison with Figure 3(b) of EPA’s VI Guidance