Evaluation of the Conceptual Flow Model for a Deep Vadose Zone System Using Advanced Tensiometers

Abstract

moisture contents. Flint et al. (2001) described the evolution of the conceptual flow model for Yucca Moun-Conceptual flow models provide a framework for predictive modeltain, an arid site under consideration as a high-level ing of contaminant transport. This study tests the assumptions of steady-state flow and a unit hydraulic gradient in a 177-m-thick vadose waste repository, during 15 yr. The primary driver bezone beneath a mixed waste site, using a network of advanced tensiom-hind the evolution of the conceptual flow model was eters. The conceptual flow model at the waste site, located on the the accumulation of site-specific data from neutron log-Idaho National Engineering Laboratory (INEEL), describes moisture ging of boreholes, bomb-pulse isotopes, perched water movement through a geologically complex site comprising basalt flows analyses, and thermal analyses.intercalated with sedimentary interbeds. The presence of sedimentary Collecting site-specific temporal data in deep vadose interbeds is expected to dampen and store much of the episodic zone systems present unique challenges because of the recharge, resulting in near steady-state conditions and unit gradient difficulty and expense of installing and maintaining inflow. Thirty advanced tensiometers in 18 wells provided field water strumentation for extended time periods (years). Howpotential data at depths ranging from 6.7 to 73.5 m below land surface ever, the in situ measurement of water or matric poten-(bls), beneath and adjacent to the waste site. Measured water potentials from February 2000 through August 2002 ranged from near tials provides data needed to characterize flow processes, saturation (Ϫ30 cm of water) to about Ϫ400 cm of water. Above 17 m, track infiltration or drainage, and estimate deep percothe observed long-term drying trends were presumed to be a response lation. to the cumulative effect of lower than average annual precipitation Thermocouple psychrometers, heat-dissipation senfor the last 3 yr (2000-2002). Below 17 m, steady-state conditions sors, and tensiometers measure components of the total were observed at more than one-half of the monitored locations. Howenergy potential of water. The total energy potential of ever, long-term drying and wetting trends were also observed at 9 of water is the sum of the contributions of the gravitational, the 25 monitored locations below 17 m, in contrast to the steady-state pressure, and osmotic potentials (Hillel, 1980). Thermoflow assumptions in the conceptual model. Long-term water potential couple psychrometers measure water (matric and oschanges ranged from about 20 to 200 cm of water. It is hypothesized motic) potential in the Ϫ2000 to Ϫ80 000 cm of water that these drying trends are related to areas of focused infiltration, such as drainage ditches, and are a response to decreased runoff from Abbreviations: bls, below land surface; INEEL, Idaho National Engi-

Characterising deep vadose zone water movement and solute transport under typical irrigated cropland in the North China Plain

“…Schindler and Müller () concluded that unit gradient occurred only in sand rather than in loam beneath the 2‐m soil depth. Using data monitored by advanced tensiometers, at depths ranging from 6.7 to 73.5 m below land surface, McElroy and Hubbell () revealed that unit gradient condition exists in the 6.7‐ to 73‐m layer beneath the subsurface disposal area, located in the southwest portion of the INEEL, in southeastern Idaho. However, the shallower depth (<6.7 m) was not investigated, and hence the exact minimum depth satisfying the unit gradient condition could not be revealed.…”

Application of a Darcian Approach to Estimate Liquid Flux in a Deep Vadose Zone

“…Water potential measurements obtained from advanced tensiometers targeting the 34‐m and 73‐m interbeds are shown in Fig. 3 for February 2000 to August 2002 (further details in McElroy and Hubbell, 2003a, 2003b). Measurements ranged between −21 and −388 cm of water (Fig.…”

“…In addition to the apparent steady‐state behavior at most of our measurement sites, supporting evidence for a near unit vertical gradient within the interbeds comes from evaluation of tensiometric data at this site as a whole, and from measurements at a nearby analog site. McElroy and Hubbell (2003b) evaluated water potential measurements from 30 locations around the SDA, including the 17 considered here and additional sites within the basalt. They concluded that the correlation between measured water potential and elevation of the measurement suggested a unit hydraulic gradient in the upper 73 m of the unsaturated zone.…”

“…Of the 19 vadose zone wells that have been installed in or around the RWMC (see McElroy and Hubbell, 2003a, 2003b), 16 were completed near the tops of the 34‐m or 73‐m interbeds (one was completed in both) and are thus relevant to this study. Ten of those wells are inside the SDA, and the remaining six are within 0.5 km of the perimeter (Fig.…”

“…Instruments were installed within two laterally extensive sedimentary interbeds at depths of 34 and 73 m for the purpose of monitoring hydrologic activity within a complex sequence of unsaturated basalt flows that underlie a waste disposal facility in south‐central Idaho. At most of the 17 measurement sites, data collected during a 30‐mo period (McElroy and Hubbell, 2003b) indicated near steady‐state conditions. From Richards' equation for vertical flow, steady‐state flow occurs for a unit hydraulic gradient, where flux is given by the in situ unsaturated hydraulic conductivity.…”