Comparison of three techniques to measure unsaturated-zone air permeability at Picatinny Arsenal, NJ

Abstract

The purpose of this study is to compare three techniques to measure the air permeability of the unsaturated zone at Picatinny Arsenal, NJ and to examine the effects of moisture content and soil heterogeneity on air permeability. Air permeability was measured in three ways: laboratory experiments on intact soil cores, field-scale air pump tests and calibration of air permeability to air pressures measured in the field under natural air pressure conditions using a numerical airflow model. The results obtained from these three methods were compared and found to be similar. Laboratory experiments performed on intact cores measured air permeability values on the order of 10 −14 to 10 −9 m 2. Low-permeability cores were found between land surface and a depth of 0.6 m. The soil core data were divided into two layers with composite vertical permeability values of 1.3×10 −13 m 2 from land surface to a 0.6-m depth and 3.8×10 −10 m 2 for the lower layer. Analyses of the field-scale pump tests were performed for two scenarios: one in which the entire unsaturated zone was open to the atmosphere and one assuming a cap of low permeability extending 0.6 m below land surface. The vertical air permeability values obtained for the open scenario ranged from 1.2×10 −9 to 1.5×10 −9 m 2, and ranged from 3.6×10 −9 to 6.8×10 −9 m 2 in the lower layer, assuming an upper cap permeability of 6.0×10 −14 m 2. The results from the open scenario are much higher than expected and the possible reasons for this ambiguity are discussed. The results from the capped scenario matched closely with those from the other methods and indicated that it is important to have background information on the study site to correctly analyze the pump test data. The optimized fit of the natural subsurface air pressure was achieved with an intrinsic permeability value of 3.3×10 −14 m 2. When the data were refitted to the model assuming two distinct layers of the unsaturated zone, the optimized fit was achieved with intrinsic air permeabilities of 1.6×10 −14 m 2 for the upper, low-permeability region and 8.4×10 −9 m 2 for the lower region. Vertical air-permeability predictions from the three methods are similar provided that soil cores are collected from all representative depths to account for heterogeneous layers and that the correct assumptions are made when analyzing field-scale pump test data.