1998 Conference Proceedings

The relationship Between Side-Slope Configuration, Precipitation, and Peripheral Recharge from Above-Grade Capillary Barriers

A. L. Ward, G. W. Gee
Pacific Northwest National Laboratory, Richland, Washington

Proceeding of the 1998 Fall Meeting of the American Geophysical Union, San Francisco, CA

Abstract

In designing above-grade surface barriers for the isolation of buried hazardous wastes, special design considerations are required to protect the extremities from water accumulation and erosion, and ultimately barrier failure. Yet, there is currently no consistent standard or practice for the design of protective side slopes for use on long-term (> 1000 yrs.) surface barriers. A field-scale (2.5 ha) prototype surface barrier was recently constructed over a decommissioned wastewater disposal facility at the semiarid Hanford Site in SE Washington, USA. The barrier is a vegetated, capillary barrier designed to limit recharge to # 0.5 mm yr^-1. Two different side slope configurations were incorporated for evaluation under conditions of ambient and elevated precipitation (P): (1) a relatively flat (10H:1V) clean-fill dike of pit-run gravel, and (2) a relatively steep (2H:1V) embankment of fractured basalt riprap. Test results show that both slopes drained much less than predictions based on field lysimeter measurements suggested. Over the last 3 yr, the irrigated gravel slope drained 384.5 mm (30% P) compared to 439 mm (34% P) drained by the irrigated riprap. These differences may be confounded by differences in water distribution on the slopes. However such an effect is absent on the unirrigated slopes. Over the same period, the unirrigated riprap drained 185 mm, (22% P) compared to 255 mm (31% P) drained by the unirrigated gravel slope. Drainage from the riprap also appears to be seasonally dependent; while drainage from the gravel was continuous, the riprap produced essentially no drainage in the summer, and drainage exceeded that from the gravel in the fall and winter. A conceptual model of side-slope drainage was developed and evaluated using the STOMP numerical simulator. The discrepancy in drainage amount and its seasonal dependence can be attributed to advective drying. This process also serves to reduce water storage in the adjacent soil layers of the capillary barrier.