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

near-surface environment. Also, many of the complicating effects found in the near-surface environment will Approaches for estimating liquid flux in the shallow (0–2 m) vadose be damped or eliminated with increasing depth. For zone are hindered by the high degree of spatial and temporal variability present near the land surface. It is hypothesized that high-frequency these reasons, flux measurement at depth would appear variations in flux will be damped with depth. This study was conducted to be an attractive alternative at such sites. However, to estimate deep liquid flux using the Darcian approach at a waste borehole instruments for direct measurement of deep disposal site in south-central Idaho that is underlain by a complex flux do not exist at this time. Environmental tracers sequence of unsaturated basalt flows intercalated with thin sedimen(e.g., Scanlon et al., 1997; Phillips, 2001) may be used tary layers. Flux is estimated by combining in situ water potential to provide information on average historical flux at measurements from sedimentary interbeds located at depths of 34 some sites, but this approach is not conducive to moniand 73 m below land surface (bls) with laboratory estimates for the toring activities. Conversely, Darcian approaches will unsaturated hydraulic conductivity. Tensiometer data at seven locahave a more widespread applicability and are amenable tions indicated nearly constant conditions for 30 mo, while nine of to monitoring. the other 10 sites showed small gradual trends. Assumption of a unit hydraulic gradient led to flux estimates ranging from 0.2 to 10 000 cm Darcian approaches are founded on an assumption of yr 1. Estimates in the 34-m interbed ranged across four orders of one-dimensional vertical flow. One then needs sufficient magnitude while flux estimates for the 73-m interbed ranged three information on either the in situ moisture content or orders of magnitude. While the tensiometer data appear to reflect in water potential to calculate flux from laboratory-derived situ conditions and are a sensitive indicator of hydrologic conditions in unsaturated hydraulic conductivity. Previous applicathe deep vadose zone, the laboratory-developed hydraulic properties tions of this approach have used tensiometers (Stephens introduce a high degree of uncertainty, potentially affecting predicand Knowlton, 1986), thermocouple psychrometers tions by orders of magnitude. There is a need to develop techniques (Andraski, 1997), or heat dissipation sensors (Montazer for assessing flux rates for the range of applicable field conditions to et al., 1986) to measure water potential gradients along improve the confidence in deep flux estimates. boreholes at depths of 2, 5, and 200 m, respectively. Tensiometric data have a distinct advantage in that it is a direct measure of water potential, whereas the other F the transport of waterborne contamitwo methods are calibration-dependent, indirect meanants through the vadose zone requires estimates sures. Available techniques for in situ measurement of for liquid flux between the land surface and the water moisture content are not only calibration dependent, table. While there is an extensive body of literature but also physically difficult to install at depths beyond regarding the estimation of flux at shallow depths, the a few meters. deeper vadose zone has received much less attention. Conventional tensiometers require a continuous waInstruments used for direct measurement of flux in the ter column that extends from the measurement point near-surface environment (0–2 m) include pan lysimeto the sensor location at or near the land surface. The ters (e.g., Jordan, 1968), tension lysimeters (Byre et al., vaporization of water in the water column limits the 1999), and vadose zone flux meters (Wagenet, 1986; depth of emplacement to about 8 m and has precluded Gee et al., 2002). There are also Darcian approaches in the use of tensiometer data for Darcian estimates of which shallow measurements of moisture content or flux below that depth. This problem was recently overwater potential ( ) are combined with laboratory develcome by development of the advanced tensiometer oped relations for the unsaturated hydraulic conductiv(Hubbell and Sisson, 1998), which has been successfully ity to estimate flux (Stephens and Knowlton, 1986). deployed to make direct measurements of water potenHowever, the utility of all such measurements is limited tial at depths up to 145 m. The advanced tensiometer has by the inherent spatial and temporal variability of flux two parts, a permanently installed porous cup assembly in the shallow vadose zone (Wagenet, 1986). with casing that extends to land surface and a removable At sites with thick vadose zones, flux estimates obelectronic pressure transducer assembly for installation tained at depth may be more representative of mass from land surface. Positioning the sensor close to the transfer to the water table than those obtained from the measurement point (porous cup) eliminates the need for a water column extending to land surface, thus reJ.M. Hubbell, J.B. Sisson (ret.), and D.L. McElroy, Idaho National moving the restriction on depth of emplacement. Engineering and Environmental Laboratory, Geosciences Research Here, we present a first attempt to estimate flux at Department, P.O. Box 1625, MS 2107, Idaho Falls ID 83415; M.J. Nicholl, Geosciences Dep., Univ. of Nevada, Las Vegas, NV 89122. depth using long-term monitoring data obtained from Received 10 July 2003. Special Section: Uncertainty in Vadose Zone the deployment of advanced tensiometers. Instruments Flow and Transport Properties. *Corresponding author (jmh@ inel.gov). Abbreviations: bls, below land surface; ESRP, Eastern Snake River Plain; INEEL, Idaho National Engineering and Environmental LaboPublished in Vadose Zone Journal 3:560–569 (2004).  Soil Science Society of America ratory; RWMC, Radioactive Waste Management Complex; SDA, Subsurface Disposal Area. 677 S. Segoe Rd., Madison, WI 53711 USA