Measurement and Prediction of Deep Drainage from Bare Sediments at a Semiarid Site

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.

Compendium of Data for the Hanford Site (Fiscal Years 2004 to 2008) Applicable to Estimation of Recharge Rates

“…Long-term lysimeter facilities were constructed, maintained, and monitored to measure recharge for several soil and vegetation covers (Gee 1987). This included the 200 East Lysimeters , Routson et al 1988) and later a lysimeter facility that was installed north of the 300 Area (Gee et al 2005). …”

“…Site-specific recharge measurements for areas of special interest were undertaken such as at the prototype Hanford Barrier to measure the effectiveness of proposed infiltration barriers . Data packages were prepared to support compliance assessments including the Solid Waste Landfill (SWL) (Gee et al 2005), the Integrated Disposal Facility (IDF) (Rockhold et al 1995, Fayer et al 1999, Fayer and Szecsody 2004 and the recharge data package for the RCRA Facility Investigation (Fayer and Keller 2007).…”

“…By the late 1990s tracer-based methods began to be employed in addition to water balance measurement based methods to estimate recharge (Murphy et al 1996, Prych 1998, Fayer et al 1999, Maher et al 2003, DePaolo et al 2004, Fayer and Szecsody 2004, Gee et al 2005, Singleton et al 2006, Keller et al 2007). …”

“…WFMs were installed keeping the layered soil column intact. Additional information about the Grass Site and the installation of monitoring equipment can be found in Keller and Gee (2005) 1 . The long-term average winter precipitation at HMS to date is 107 mm.…”

Recharge Data Package for Hanford Single-Shell Tank Waste Management Areas

“…For a nearly 22-year period, drainage from the 300 North Lysimeter averaged 62 mm/yr (Gee et al 2005a). Some of that period included the two wettest years (1995,1996) on record, so the average may be biased high relative to a long-term precipitation rate.…”