Writer: Steve Hill, (979) 845-2895
Contact: Dr. Kevin McInnes, (979) 845-5986
COLLEGE STATION — Chemicals traveling through soil toward groundwater are like travelers without maps, clocks and speedometers: there’s no telling when they’ll arrive.
Texas Agricultural Experiment Station researchers, however, are developing a method they hope will help predict just how chemicals reach groundwater from the soil surface.
Using a unique grid of collecting cells buried at different levels in various soil types, the researchers have been able to confirm that water moving through clay soils tends to converge in fissures and cracks, said Dr. Kevin McInnes, an experiment station researcher.
The new method eventually may be usefully integrated with other knowledge and methods to make predictions about chemical movements. That’s because it can help researchers understand how water moves through soil at varying rates.
“Soil scientists have been collecting soil descriptions from all over the United States for more than 100 years,” said McInnes, who is also an assistant professor of soil and crop sciences at Texas A&M University. “But we don’t always know how fast water moves from the surface through soil to groundwater.”
Part of the problem, McInnes said, is that soil samples have often been taken in cores of two to four inches in diameter. Such cores may not reveal the fissures and cracks that channel water toward groundwater, just as rivers merge and run toward the sea.
“It is traditionally assumed that heavy clays don’t transmit water and solutes very quickly, but that’s because the samples have been from small cores, which might miss the channels that tend to move water more quickly,” he said.
To learn more about the process, McInnes and a team of experiment station researchers tested the method in two different Brazos Valley soils — Ships clay and Silawa sandy loam. The clay is what is called a “highly structured” soil, with cracks, fissures and pores; the Silawa has a finer, less pronounced structure.
The team also includes Drs. Larry Wilding and Tom Hallmark, both professors in A&M’s soil and crop sciences department, and departmental graduate research assistant Willem (over)
Heuvelman. The team conducted the research for the Texas Water Resources Institute, a special unit of the experiment station headquartered on the Texas A&M campus.
The device they developed was a grid of 7 by 14 inches made up of 98 individual cells. The cells collected water and bromide — a harmless, nondegradable and easily measured tracer applied to the soil surface.
As water percolated through the soil, it was collected in varying rates in each cell. The grids were buried in three different depths in each soil.
Most agricultural chemicals degrade naturally in soils, so the nondegradable bromide in each collection cell was expected to give some idea of the rate at which a solute could move with surface water through soil and into the groundwater.
The cells were actually connected to a complicated device that included secondary collection jars and a vacuum system. A series of statistical equations was used to analyze relationships between the amount of water and bromide found in collection jars from each depth of each soil type.
McInnes said the team found the method revealed useful information about how surface water converges with depth toward groundwater in clay, but did not yield as much information about how water flow is related to solute “travel time” in the soil.
The relationship between water flow, solute travel time and soil structure is a complicated one that will require much more study, both with the experiment station device and through other methods. McInnes said the team will continue working with the method to determine its viability as a research tool; the team also will focus on learning more about the chemical and physical characteristics of water flow paths in different types of soil.
He suspects the method and the information it yields will be very useful in geographic information systems. Those systems use many kinds of data — weather, soil characteristics, chemical characteristics, water-table depths, and other information — in computer models to solve various problems, including how certain chemicals reach groundwater in a given area.
That problem is a puzzle whose pieces have continued being collected by many scientists for many years. Many pieces must still be found and put together, McInnes said.
“You can’t go dig up all the soil everywhere to figure out what will happen,” McInnes said. “This method may really help when used with soil descriptions and other research.”
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