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Evaluation of the conceptual flow model for a deep vadose zone system using advanced tensiometers

McElroy, D.L., Hubbell, J.M.
Vadose zone journal 2004 v.3 no.1
mathematical models, waste disposal, radionuclides, hazardous waste, equations, tensiometers, unsaturated flow, basalt, vadose zone, soil water movement, Idaho
Conceptual flow models provide a framework for predictive modeling of contaminant transport. This study tests the assumptions of steady-state flow and a unit hydraulic gradient in a 177-m-thick vadose zone beneath a mixed waste site, using a network of advanced tensiometers. The conceptual flow model at the waste site, located on the Idaho National Engineering Laboratory (INEEL), describes moisture movement through a geologically complex site comprising basalt flows intercalated with sedimentary interbeds. The presence of sedimentary interbeds is expected to dampen and store much of the episodic recharge, resulting in near steady-state conditions and unit gradient flow. Thirty advanced tensiometers in 18 wells provided field water potential data at depths ranging from 6.7 to 73.5 m below land surface (bls), beneath and adjacent to the waste site. Measured water potentials from February 2000 through August 2002 ranged from near saturation (-30 cm of water) to about -400 cm of water. Above 17 m, the observed long-term drying trends were presumed to be a response to the cumulative effect of lower than average annual precipitation for the last 3 yr (2000-2002). Below 17 m, steady-state conditions were observed at more than one-half of the monitored locations. However, long-term drying and wetting trends were also observed at 9 of the 25 monitored locations below 17 m, in contrast to the steady-state flow assumptions in the conceptual model. Long-term water potential changes ranged from about 20 to 200 cm of water. It is hypothesized that these drying trends are related to areas of focused infiltration, such as drainage ditches, and are a response to decreased runoff from three years of less than average precipitation. A unit gradient was indicated by aligning dispersed monitoring locations along a presumed vertical profile.