ARCHIVED REPORTS PHASE II FIELD INVESTIGATION AND LNAPL MOBILITY EVALUATION REPORT

Home Browse Search

ARCADIS Phase II Field Investigation and LNAPL Mobility Evaluation Report Union Pacific Railroad Company Former Maintenance Facility Tracy,California 4.1.3 Bail-Down Testing ' The rate of LNAPL flow into the well is a function of LNAPL saturation, permeability of the surrounding formation to LNAPL, physical properties of the LNAPL(density, viscosity, interfacial tension between LNAPL and water), and the magnitude of the initial hydraulic gradient toward the well developed during LNAPL removal. LNAPL transmissivity values determined from bail-down testing are representative of the LNAPL transmissivity of the formation in the vicinity of the well being tested, under the conditions existing at the time of the test(for instance,transmissivity values obtained from bail-down testing conducted during periods of low water-table elevation in an unconfined aquifer will likely be greater than those obtained from tests conducted at high water-table elevation). If LNAPL is removed quickly to initiate bail-down testing, the test is analogous to a groundwater rising-head slug test. Bail-down test data can be analyzed using commercially available slug-test analysis software(such as AQTESOLVTm)to estimate LNAPL transmissivity and conductivity within the zone affected by the test. LNAPL transmissivity and LNAPL conductivity are proportional to rate of change in LNAPL volume during the test, which is proportional to the change in product thickness measured during the bail-down test under the Lundy and Zimmerman approach (Lundy 2002). The transmissivity values obtained from bail-down testing are representative of LNAPL flow from the formation rather than filter-pack drainage, because the significant LNAPL recovery during the test enabled fitting the model in the analysis to the proper portion of the recovery trend to correctly represent the formation response. LNAPL has low recoverability in the immediate vicinity of each well due to the relatively low transmissivity and conductivity values resulting from analysis of the bail-down tests and longevity of the tests. Bail-down testing data and AQTESOLVTm results are presented in Table 4. Appendix D presents the detailed bail-down testing results. The transmissivity data resulting from bail-down testing could not be used quantitatively ' because initial removal of LNAPL was not instantaneous. Therefore, LNAPL velocity calculations presented in Section 5 were not calibrated using site-specific LNAPL transmissivity. However, the bail-down tests provided a significant qualitative indication that the LNAPL has low recoverability since it took over two weeks for LNAPL to recover to an average of 90 percent of the initial thicknesses.Also, the Site has been thoroughly characterized through collection of site-specific data that were used in the MuoR+�vanm�mmmk��.zazoialaac ' 13