PNNL

Abstract

In desert environments, non-vegetated (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 lead to groundwater contamination. 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 23-year drainage record for bare, coarse sand was tested, and the calibration developed for the past 10 years (1994 to 2004) was found to reliably predict drainage from the previous 12 years. The texture models were also compared against Darcy’s Law estimates (i.e., unsaturated hydraulic conductivity) for coarse sand and found to agree within 15% of the long-term drainage average. For cases where unsaturated hydraulic-property estimates were not available, the texture models reasonably predicted annual drainage rates for bare sediments containing significant fines (materials less than 50 microns). Clean rock and gravels with little or no fines yielded less drainage (more evaporation) than gravels with fines, and this was attributed to advective drying in the open work gravel and rock. Because of advective drying, no single empirical texture function could be found to predict drainage from all the coarse sediments tested. However, drainage rates from sediments ranging from silts to coarse sediments mixed with fines appear to be predictable with the models tested. To minimize drainage at desert waste sites, only modest quantities of fines need to be added to the coarse sediments to substantially reduce the potential for groundwater contamination.