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Between September 2000 and August 2001, 28 wells were installed including monitoring wells to measure perched water, instrumented boreholes to measure unsaturated moisture flux, and wells to perform geophysical measurements.

As of September 2001, scientists have a new field-scale research facility called the Vadose Zone Research Park. The park provides scientists with a one-square-mile facility to study the movement of water and solutes through unsaturated layers of basalt and sediment between the land surface and the underlying Snake River Plain Aquifer. The vadose zone is about 150 meters thick at the research park. Within the park are two new percolation ponds that will receive about a million gallons of equipment cooling water each day.

The Big Lost River also flows through the research area. The Big Lost River is a ’losing’ stream, losing water to the porous subsurface and eventually disappearing.

"The Vadose Zone Research Park gives us a facility where assumptions and hypotheses can be tested and validated under reasonably controlled conditions," said Subsurface Science Initiative geoscientist Larry Hull. "This is a tremendous opportunity to advance vadose zone research and significantly enhance INL’s subsurface science research capabilities."

Hypotheses developed in the laboratory must be tested at progressively larger scales to verify their applicability in the field. The various test beds required for complete testing range from bench-scale experiments conducted in a laboratory, through mesoscale experiments conducted in a high-bay, to field-scale studies where conditions are similar to those encountered at remediation sites.

Instruments were calibrated in the laboratory, clusters of instruments were assembled for installation, and instrument strings laid out prior to installation.

The research park, at the far end of the test bed spectrum, provides an area where conceptual models of fluid flow and contaminant transport can be evaluated, new instruments can be installed side-by-side and compared with existing technology, and investigations can be conducted to test hypotheses.

An improved understanding of vadose zone processes will be applied immediately to the INL’s environmental restoration, waste management and facility operations. For example, to prevent potential flooding of INL facilities, water is diverted out of the Big Lost River into old dry lake beds west of INL’s Radioactive Waste Management Complex. By improving their understanding of how groundwater and contaminants move in a fractured basalt subsurface, scientists will better understand the past effects of floodwaters on the areas of buried waste and the current impacts of diverting water from the Big Lost River. In addition, the knowledge will help scientists understand vadose zone issues at DOE sites with similar subsurface structures.

The new percolation ponds in the Research Park will replace percolation ponds located next to the Idaho Nuclear Technology and Engineering Center (INTEC) facility. Water seeping from these existing ponds may be contributing to the movement of soil contaminants at the facility. The new ponds are planned for operation in 2002.

Research area instrumentation

Instrument strings were placed into boreholes. As many instruments were installed as could be fit into each hole.

A line of monitoring wells and instrumented boreholes, extending almost 3,000 feet from the Big Lost River to the new percolation ponds, has been installed. The wells and boreholes will allow researchers to study the variable recharge from the Big Lost River, the relatively constant recharge from the percolation ponds, and the interactions between the two. The resulting information will allow researchers to develop an integrated picture of water movement in the subsurface.

Monitoring wells installed on each side of the Big Lost River, allow scientists to measure water levels and collect samples when perched water zones form. With this information, scientists will be able to study how perched water levels respond to the river’s variable flow.

The monitoring wells are equipped with an array of sensors that use electrical resistivity tomography to measure changes in the water’s movement. Mathematical inversion of the resistivity measurements can be used to derive a three-dimensional image of the electrical resistivity of the subsurface by taking advantage of water’s ability to conduct electricity much more readily than rock. Downhole geophysical instruments will also be used in the monitoring wells to remotely measure material properties.

The well holes were backfilled with bentonite seals between instruments to avoid vertical water movement through the borehole.

Boreholes have been drilled in areas where the basalt and sediments are not saturated. At the locations where perched water is expected to form, or where water was observed while drilling instrument packages were installed in the boreholes to monitor water movement outside the boreholes.

The instrument packages — typically containing a water content sensor, tensiometer, suction lysimeter and a gas port — have been installed at several depths in each instrumented borehole. The tensiometers and water content sensors will determine if water is moving through the sedimentary layers or around the sediments. The suction lysimeters and gas ports will monitor the liquid and gas phase chemistry in the unsaturated rocks.

A good connection between the instruments and surrounding rocks was established by packing a fine sand around the instruments. The boreholes were then sealed, by placing expanding clay between each set of instruments and the surface, to ensure that water cannot migrate down the boreholes.

The wells are now completed with stainless steel well boxes to protect the wiring and sampling ports. At each instrumented borehole, instrument data are collected automatically by data logger. The data loggers communicate with the INL computer network over radio link. Sampling ports for gas and water sampling are equipped with quick connects to facilitate sampling.

The research park and percolation ponds were completed in the summer of 2001. Data collection has begun so that information on background conditions in the subsurface can be gathered before the ponds are put into service. The background data will provide the basis for quantifying the changes that take place in the vadose zone as water from the percolation ponds is added to the subsurface.

For more information about the Vadose Zone Research Park, opportunities for collaborative research or use of the facility for proof of concept instrument testing, contact the researcher listed below.