Depth of the biologically active zone in upland habitats at the Hanford Site, Washington: Implications for remediation and ecological risk management

Abstract

Soil invertebrates,mammals, and plants penetrate and exploit the surface soil layer (i.e., the biologically active zone) to varying depths. As the US Department of Energy remediates radioactive and hazardous wastes in soil at the Hanford Site, a site-specific definition of the biologically active zone is needed to identify the depth towhich remedial actions should be taken to protect the environment and avoid excessive cleanup expenditures. This definition may then be considered in developing a point of compliance for remediation in accordance with existing regulations. Under the State of Washington Model Toxic Control Act (MTCA), the standard point of compliance for soil cleanup levels with unrestricted land use is 457cm(15ft) belowground surface. When institutional controls are required to control excavations to protect people, MTCA allows a conditional point of compliance to protect biological resources based on the depth of the biologically active zone. This study was undertaken to identify and bound the biologically active zone based on ecological resources present at the Hanford Site. Primary data were identified describing the depths to which ants, mammals, and plants may exploit the surface soil column at the Hanford Site and other comparable locations. Themaximumdepth observed for harvester ants (Pogonomyrmex spp.)was 270cm(8.9ft),with only trivial excavation below 244cm (8ft). Badgers (Taxidea taxus) are the deepest burrowing mammal at the Hanford Site, with maximum burrow depths of 230cm (7.6ft); all other mammals did not burrow below 122cm (4ft). Shrubs are the deepest rooting plants with rooting depths to 300cm (9.8ft) for antelope bitterbrush (Purshia tridentata). The 2 most abundant shrub species did not have roots deeper than 250cm(8.2ft). The deepest rooted forb had amaximumroot depth of 240cm(7.9ft). All other forbs and grasses had rooting depths of 200cm (6.6ft) or less. These data indicate that the biologically active soil zone in the Hanford Central Plateau does not exceed 300cm (9.8ft), themaximumrooting depth for the deepest rooting plant. The maximumdepth at which most other plant and animal species occur is substantially shallower. Spatial distribution and density of burrows and roots over depths were also evaluated. Althoughmaximum excavation by harvester ants is 270cm(8.9ft), trivial volume of soil is excavated below 150cm (~5ft). Maximum rooting depths for all grasses, forbs, and the most abundant and deepest rooting shrubs are 300cm (9.8ft) or less. Most root biomass (>50-80%) is concentrated in the top 100cm (3.3ft), whereas at the maximum depth (9.8ft), only trace root biomass is present. Available data suggest a limited likelihood for significant transport of contaminants to the surface by plants at or below244cm(8ft), and suggest that virtually all plants or animal species occurring on the Central Plateau have a negligible likelihood for transporting soil contaminants to the surface fromdepths at or below305cm (10ft).