Surface water and its piggybacking chemical riffraff seeps into the ground, traveling through a subsurface region called the vadose zone on its way to the aquifer. Little is known about the journey it takes through this zone. How the water and its baggage interact with the vadose zone will determine how much of the riffraff -- contaminants such as gasoline additives, agricultural chemicals, or buried nuclear waste leakage -- end up in our water supply.
Researchers from the Department of Energy's Idaho National Engineering and Environmental Laboratory set up monitoring equipment at the Department's Savannah River Site in South Carolina mid-April. To identify where and how fast water is moving before it mixes with groundwater at the water table, hydrologists Buck Sisson and Joel Hubbell are developing tools to follow water and its passengers traveling through the zone.
At the Savannah River Site, their stationary equipment will patrol the vadose zone below the site's radioactive waste disposal trenches, watching for water and the contaminant tritium. For years, waste management engineers built containment trenches for low-level radioactive waste. "Savannah River's objective is to demonstrate that there's no tritium coming out of the trenches and going into the groundwater," said Sisson.
The INEEL scientists dug three 8-inch-wide wells 60 feet deep around several of Savannah River Site's trenches, in which they planted soil monitoring equipment every 15 feet. The equipment will measure three things: the water content of the soil, the concentration of tritium, and the soil tension -- or how tightly the soil holds water. Soil tension is an indication of how fast the water will percolate through the ground.
This project is a first for vadose zone research. "We're monitoring at a depth and in an environment that's never been done before," said Hubbell. "And the sensors will give us the information we need to tell us what's going on."
Measuring soil tension at these depths requires an instrument the researchers have developed here at INEEL. This device, called the Advanced Tensiometer, can measure soil tension much deeper in the ground than the two or three feet to which conventional tensiometers are limited.
Conventional tensiometers, mostly used by farmers and researchers, are limited by their design to the shallow depth. They employ a column of water in a long tube above a porous ceramic cup in the soil, which connects the cup to a pressure gauge at the surface. The water pressure within the cup equilibrates with the water pressure in the soil, creating a pressure within the water column that is measured with a standard pressure gauge. The water column's height is limited by gravity and responds slowly to changes in soil tension.
Sisson and Hubbell eliminate the column of water by using an electronic pressure transducer resting on a small, porous ceramic cup. Only enough water is needed to fill the cup, and the water is retained fourfold as long as in a water column tensiometer. The transducer is connected via wires to electronics above ground -- this allows measurements at depths limited only by the length of the wire.
Many Advanced Tensiometers are planted and active in Idaho and elsewhere. At the INEEL, dozens of Advanced Tensiometers register soil tension near the Idaho Nuclear Technology and Engineering Center, the INEEL Research Center, nearby Hell's Half Acre and Box Canyon research sites, and the Radioactive Waste Management Complex.
Farming researchers at the University of California, Davis, are using them to determine how much excess water they are putting on their fields. At Oak Ridge National Laboratory in Tennessee, data collected from Advanced Tensiometers buried 12 to 25 feet down revealed unexpectedly big responses of the water table to small amounts of rainfall.
Sisson and Hubbell are eager to collect data such as that from Oak Ridge. The vadose zone here in Idaho extends 300 to 600 feet down, through volcanic rock and desert sediment, before it reaches the aquifer. In lush, water-laden areas such as Tennessee or South Carolina, the zone is much shallower and made up of soil that is very different from the Idaho desert's.
"We'll see how dynamic the water flow is at that location," said Hubbell.
The researchers will be collecting data from the wells in South Carolina for at least a year. The monitors buried below ground -- at 60, 45, 30, and 15 feet -- send their information to a data collector at the surface. The collectors feed into specialized cell phones, which can be called automatically by a computer in Idaho to download the data. Sisson and Hubbell will then be able to model the path water takes from the surface to the aquifer while researchers at the Savannah River Site monitor for their contaminant of interest -- tritium.
In addition to providing valuable information about the vadose zone, Sisson said the Advanced Tensiometer may change the way wastes -- be they nuclear waste or ordinary trash in sanitary landfills -- are disposed of. Historically, engineers have measured groundwater contaminants at the water table. Since there has never been a way to monitor the vadose zone, keeping an eye on it has never been required. As the usefulness of the Advanced Tensiometer is proven, and news of its use spreads, "it may set legal precedents," said Sisson, "if the technology is sophisticated enough."
INEEL celebrates its 50 year anniversary in 1999. The national laboratory is operated for the U.S. Department of Energy by Lockheed Martin Idaho Technologies Company.
Note to editors: Information on the award-winning Advanced Tensiometer can be found at http://inelext1.inel.gov/science/prestige.nsf/ineel/Tensiometer.