PNNL

Abstract

Pacific Northwest National Laboratory made in situ flow measurements in groundwater monitoring wells at the U.S. Department of Energy (DOE) Hanford Site to determine the flow direction in an aquifer with a flat water table. Given the total errors in water level elevations, flow directions based on the potentiometric surface are ambiguous at best. The colloidal borescope was used because it allows direct, real time observation of mobile colloidal particles in the open interval of a water well and thus, avoids the use of water level data. The results characterize a complex groundwater flow pattern under several buried waste storage tank farms. The aquifer, artificially high due to large volume liquid discharges to the soil column from Hanford's nuclear production era, is currently receding to original conditions. The aquifer lies in unconsolidated gravel beds overlying an impermeable basalt surface that has a plucked, flood-scoured, scabland structure. The current aquifer thickness is similar to the relief on the basalt basement. Thus the groundwater must flow around the impermeable basalt structures producing a complicated flow pattern under the waste storage unit. The original monitoring network was designed for northwest flow when the water table was held artificially high. Proper locations for new wells are dependent on our knowledge of the flow direction. The results of the colloidal borescope investigation agree with the southerly direction indicated from hydrographs, contaminant trends, other direct flow data and the general concept of a receding aquifer draining off the southern limb of a basalt anticline. Flow in the aquifer is diverted by irregular local structural highs of very low permeability basalt.