PRELIMINARY RESULTS OF VERTICAL PRESSURE PROFILES USING MULTILEVEL MONITORING SYSTEMS IN BASALT AND INTERBEDDED SEDIMENT AT THE IDAHO NATIONAL LABORATORY, IDAHO

The U.S. Department of Energy's Idaho National Laboratory (INL) overlies a west-central portion of the eastern Snake River Plain aquifer (ESRPA), which is a major source of water for agricultural, industrial, and domestic uses in southeastern Idaho. For more than 50 years, radioactive and chemical wastes were discharged to the unsaturated zone and the aquifer, leaving residual contaminants in the aquifer. The U.S. Geological Survey (USGS) has been monitoring the movement of these contaminants, mostly in open boreholes. Open borehole construction is good for maximum water-production rates, for identifying the arrival of contaminant plumes, and for delineating the horizontal extent of contaminants. However, it is not conducive to identifying the vertical distribution of contaminants or pressure gradients.

Starting in 2005, the USGS, in collaboration with the INL, deployed four modular multilevel systems to examine vertical differences in both piezometric head and water chemistry. The systems were adapted for high-resolution, depth-discrete monitoring in cored boreholes with depths ranging from 800 ft to 1,400 ft. Each borehole was equipped with multiple measurement ports (up to 23), inflatable packers, and pumping ports that are accessed through a single 4 to 5 inch borehole. Boreholes were continuously cored through basalt and layered sediment, and then geophysical logs and borehole videos were obtained. These data were used for borehole characterization and to identify areas of competent basalt, necessary for packer placement.

Several piezometric head profiles were collected over the past two years to examine daily and seasonal variation. Preliminary piezometric-head change ranges from a 1 to 8 ft among the four boreholes, and it displays a relatively uniform rise averaging 0.5 to 1 ft from August 2006 to March 2007. In general, changes in piezometric head agree with geophysical flowmeter data recorded prior to the installation of the multilevel monitoring systems. Geologic features, such as basalt and sediment layer thickness and composition, appear to influence piezometric-head profiles. Sediment layers seem to force horizontal flow where low vertical gradients exist. In several boreholes, reversals in vertical flow direction were recorded immediately above or below sediment layers.