With Joe Rousseau, Brennon Orr, and Brian Twining, and Steve Anderson of the USGS INL Project, and Pete Rose of the University of Utah Energy and Geoscience Institute, we have conducted a collaborative large-scale tracer test to assess long-range horizontal flow in the unsaturated zone through such features as basalt fractures, lava tubes, or rubble zones positioned above low-permeability layers of basalts or sediments. In June of 1999, we spread 675 kg of a polyaromatic sulfonate tracer (1,5 naphthalene disulfonate) in large artificial spreading areas where water accumulates seasonally due to snowmelt at higher elevations. The research group began a long-term sampling and tracer analysis program for ground water in perched zones and in the aquifer from boreholes within 3 km of the site of tracer application.
Tracer detections sugggest an average horizontal convective transport rate of 14 m/d or more along at least two flow paths, and demonstrate that perched water beneath the Subsurface Disposal Area (SDA) comes in part from subsurface flow of spreading-area water. The rapid horizontal flow suggested by this evidence occurred within the unsaturated zone more that 100 m above the water table under locally saturated or nearly saturated conditions.
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