Local View: A science-based approach to the Ogallala Aquifer

2011-09-24T23:45:00Z Local View: A science-based approach to the Ogallala AquiferBy JIM GOEKE JournalStar.com
September 24, 2011 11:45 pm  • 

Over the past 40 years, it has been my good fortune to focus my research on Nebraska's Sandhills and the Ogallala Aquifer. Thanks to the University of Nebraska's Conservation and Survey Division and to the U.S. Geological Survey, I have been able to drill more than 1,000 test holes into the Ogallala formation.

Many people and organizations have asked my opinion about the Keystone XL pipeline's route and its potential impacts on the Ogallala's water riches. My answer is twofold: First, a thank you for their openness to the science of the aquifer, and second, a reassurance that any leak would have minimal impact on the Ogallala Aquifer. Here's the background:

An aquifer usually is defined as any subsurface material or rock formation that stores and transmits water in usable amounts. Underground water by itself is not an aquifer; the definition must include the host material.

Nebraska has many different aquifers; the Ogallala/High Plains system is the largest. Wherever the pipeline is placed, it will go over or through some kind of aquifer. The Ogallala formation is the largest water-bearing unit in the state.

The Ogallala Formation is layered rock, not a lake or a sandpit. Some people say "the lake beneath my feet" when referring to the aquifer. Others think of it as loose sand identical in all directions. These are misconceptions. Our portion of the Ogallala/High Plains Aquifer is made of widely varied sediments eroded off the Rocky Mountains and then deposited in what is now Nebraska by streams and rivers similar to the Platte over a span of 5 million to 30 million years ago. Eventually, those sediments became layers of different types of rocks.

In the western reaches of the state, the Ogallala formation is exposed at the surface. Going eastward, the Ogallala and related rock units dip; the top of the aquifer can be as deep as 300 feet or so below the land surface.

Detailed test-drilling shows that those many layers of sediment that became rock vary tremendously in all directions. Some are heavily cemented siltstones and sandstones that impede the flow of water; others are highly porous sandstones and conglomerates, with the ability to contain vast amounts of moving water between the grains. No matter which direction you drill -- up, down, or sideways -- you'll go only a few hundred yards or so before hitting a different rock type.

In contrast, UNL environmental engineer John Stansbury's report (on worst-case consequences of a spill, released in July) makes the incorrect assumption that the Ogallala Aquifer is uniform sand in all directions and right below the surface. The calculations of a projected 15-mile plume (of leaked oil) did not take into consideration the geology of the aquifer.

The water movement within the Ogallala/High Plains Aquifer is persistently from west to east. Because of gravity and the gentle eastward slope of the rock formations, the aquifer's waters are in constant movement downgradient to the east. The average gradient of the water table within the Ogallala is 10 feet per mile with consistent flow rates of roughly 150 to 300 feet per year.

The pipeline alignment goes over a far eastern segment of the subsurface Ogallala. Roughly 75 to 80 percent of the aquifer is upgradient to the west. That unrelenting eastward water flow means that it would be a physical impossibility for any oil spill to reach the vast majority of the aquifer to the west.

Furthermore, in its report on a 25-year study of an oil spill near Bemidji, Minn., the USGS noted that "fine-grained layers impeded the infiltration and redistribution of oil." If there were a spill along the Keystone XL pipeline, and some of the spill did get into the Ogallala Aquifer, the variability of the aquifer's rock layers means that any spill would be contained within a very small area of that 25 percent of the aquifer to the east of the pipeline.

Yet another consideration is the depth to the water of the aquifer. Through most of its alignment, the pipeline is some 10 to 50 to 100 feet above the top of the Ogallala formation. It's questionable whether the leaked oil could work its way down through the overlying sediments; they contain interspersed seams of silt and clay.

Even so, no leak, large or small, is something to take lightly. For me, the area of greatest concern is Holt County, where the water table is at or near the surface. Any leaks there could go directly into the waters of the shallow aquifers, although not easily into the deeper Ogallala Aquifer. In this area, it is my understanding that TransCanada will build a protective, sealed cement conduit that will surround the pipeline.

If there is a spill, cleanups could require considerable effort, something that TransCanada has said it is prepared to undertake. It would make sense for the governor and Legislature to ask that the company provide a bond now for the costs of possible cleanups in the future.

These geology and hydrology realities, gleaned from over 75 years of research into Nebraska's subsurface, should put people's minds at ease about the fact that the overwhelming majority of the Ogallala/High Plains Aquifer is not at risk from a pipeline spill.

Jim Goeke is a research hydrogeologist and professor emeritus in the School of Natural Resources at the University of Nebraska-Lincoln. 

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