U.S. Water News Online
LINCOLN, Neb. -- Nebraska's saline wetlands -- a rare and vanishing plant and animal habitat in the southeastern corner of the state -- stand a far better chance of being managed properly thanks to recent findings by a University of Nebraska researcher.
Saline wetlands -- unique ecosystems characterized by salt-tolerant plants growing in salty soils and surface waters -- have been reduced to about 10 percent of their original area in Nebraska. Only 1,200 acres of saline wetlands remain in the state.
Jerry Ayers, groundwater geologist with the Conservation and Survey Division of NU's Institute of Agriculture and Natural Resources, is studying the saline wetlands area of the Rock Creek watershed in Lancaster and Saunders counties. Little is known about the physical makeup of the wetlands, Ayers said, an ignorance which has led to problems with their preservation.
"The number of saline wetlands has decreased substantially, partially because we've never known much about them.," said Ayers. "You can't manage anything if you don't know how it works."
These wetlands, also called salt marshes, are home to many plant and animal species that are specially adapted to thrive in that kind of ecosystem, said Randy Stutheit with the Nebraska Game and Parks Commission.
Plants such as saltwort and sea blite grow in saline wetlands, Stutheit said. And the rare Salt Creek tiger beetle makes its home only in the saline environments of the Salt Creek watershed.
Stutheit said two types of saline wetlands are recognized in Nebraska. Eastern salines are found in the southeastern part of the state, and their salinity is thought to be derived primarily from sodium salts -- probably deposited by an ancient sea that once covered the state.
"The theory is that a vast interior sea covered the Midwest and left behind prehistoric ocean salts," said Stutheit.
The other type of saline wetland is the western alkaline, found in western Nebraska. Their salinity is thought to come from magnesium, sodium, and calcium salt deposits, Stutheit said.
Ayers' research project on the saline wetlands in Rock Creek began when The Nature Conservancy bought the critical habitat and turned it over to the Nebraska Game and Parks Commission, which then began to establish management procedures for the area. The commission requested help from the U.S. Natural Resource Conservation Service (formerly the Soil Conservation Service), which enlisted Ayers and Anne Matherne, former CSD surface-water researcher, to help determine how the saline wetland developed.
Ayers wanted to discover the source of salinity in the wetland. While it was once thought this was primarily caused by salts concentrating through evaporation at the ground surface, Ayers thought the real answer would most likely lie below the surface.
The subsurface of the Rock Creek site consists of a top layer of silt, 35 feet deep, a sand and gravel layer, 10 to 40 feet deep, and another layer, 82 to 120 feet deep, consisting of silt and clay at the top with course sand and fine gravel underneath. This last layer is known as the Dakota Group, a geological unit of Cretaceous-age deposits from more than 70 million years ago.
In his examination of apparent movement of underlying groundwater, Ayers observed water flowing upward toward the surface quite rapidly through the sand in the Dakota Group. From this, he concluded that the sand is probably the main source of the wetland's salinity.
While every wetland is different, Ayers said, the key to wetland formation in southeastern Nebraska appears to be the presence of a source layer coupled with a high water table that has an upward gradient -- a tendency for groundwater to flow toward the surface rather than down. Upward flow can then carry salt concentrations to the surface.
Also supporting the Dakota sand as the source of the salinity is the fact that some areas of the wetland have higher saline concentrations than others. For the most part, the highest concentrations, Ayers said, existed where there was a confining layer of dense clay above the Dakota sand -- a layer that was thin, or even absent, elsewhere.
Without that layer, water from the Dakota sand would have a much easier time reaching the surface in many other places.
"All indications point to the Dakota sand as the source, which gives us a good foundation for further research," Ayers said.
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