PNNL

Abstract

Effective monitoring of surface barriers intended to isolate and protect waste from the accessible environment requires techniques to assess their performance. Quantifying drainage conditions at any point below the barrier is difficult because field-scale drainage measurements are not well suited for assessing spatial drainage heterogeneity. Measurements of water storage, however, can provide an indirect measure of impending drainage. We measured seasonal water content at a vegetated capillary barrier on the Hanford Site in southeastern Washington State to determine effective water-content monitoring methods. Measurements were made using ground-penetrating radar (GPR) techniques. Simultaneous measurements using time-domain reflectometry (TDR) and neutron-scattering probe (NP) at multiple depths were used to determine the depth of influence of the GPR ground wave. The GPR depth of influence increased only slightly with decreasing moisture content, ranging from 33 cm to 37 cm. TDR measurements were made using permanently emplaced arrays, eliminating the need for repeated ground disturbance. All three methods showed similar seasonal responses, with the highest water contents observed during the winter months decreasing through the summer. Unlike TDR and NPs, which are point measurements, GPR exhibited much greater spatial variability across the barrier surface. Our results indicate that GPR, in conjunction with automated TDR depth profiles can provide an effective and minimally invasive method for providing high-resolution estimates of soil water content within an engineered barrier.