Abstract

In desert environments, nonvegetated (bare) soils and sediments can act as recharge basins, allowing water infiltration but restricting evaporation. When such sediments are located over buried wastes, drainage can transport vadose zone contamination downward to groundwater. Lysimeters were used to quantify drainage from bare sediments at the U.S. Department of Energy's Hanford Site in Washington state, USA. Drainage varied widely from zero to more than half of the annual precipitation for sediments ranging from fine silts to coarse rock fragments. Decade-long drainage records were used to develop two empirical models relating annual drainage and textural properties of bare sediments. A 22-yr drainage record for bare, coarse sand was tested, and the calibration developed for the past 10 years (1995–2004) was found to reliably predict drainage from the previous 12 years. The texture models were also compared against Darcy's Law drainage estimates (i.e., unsaturated hydraulic conductivity) for coarse sand and found to outperform Darcy's Law estimates of the long-term drainage average. The texture models reasonably predicted annual drainage rates for bare sediments containing significant fines (materials less than 50 μm), but significantly overpredicted drainage rates for clean rock and gravels with little or no fines. The failure of the textural models with coarse materials containing minimal fines was attributed to advective drying. Drainage predictions using the textural models indicate that to minimize drainage only modest quantities of fines need to be added to the coarse sediments to substantially reduce the potential for groundwater contamination.

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