Ogallala Aquifer vs. Keystone XL Pipeline

Figure 1

The Ogallala is the largest freshwater aquifer in the world. The maps above show the saturated thickness (vertical distance between the water table and the aquifer floor) of the Ogallala in 1996-97 and the water level changes between 1980 and 1997. The aquifer is depleted in parts of northern Texas and west central Kansas. A large share of Ogallala water lies beneath the Nebraska Sandhills, where the resource remains largely untapped because crop irrigation is uneconomic.

(McGuire et al., 1999)

The Ogallala is composed primarily of unconsolidated, poorly sorted clay, silt, sand, and gravel with groundwater filling the spaces between grains below the water table . The Ogallala was laid down about 10 million years ago by fluvial deposition from streams that flowed eastward from the Rocky Mountains during the Pliocene epoch

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The Ogallala is an unconfined aquifer, and virtually all recharge comes from rainwater and snowmelt. As the High Plains has a semiarid climate, recharge is minimal. Recharge varies by amount of precipitation, soil type, and vegetational cover and averages less than 25 millimeters (1 inch) annually for the region as a whole. In a few areas, recharge from surface water diversions has occurred. Groundwater does flow through the High Plains Aquifer, but at an average rate of only 300 millimeters (12 inches) per day.
The depth to the water table of the Ogallala Aquifer varies from actual surface discharge to over 150 meters (500 feet). Generally, the aquifer is found from 15 to 90 meters (50 to 300 feet) below the land surface. The saturated thickness also varies greatly. Although the average saturated thickness is about 60 meters (200 feet), it exceeds 300 meters (1,000 feet) in west-central Nebraska and is only one-tenth that in much of western Texas. Because both the saturated thickness and the areal extent of the Ogallala Aquifer is greater in Nebraska, the state accounts for two-thirds of the volume of Ogallala groundwater, followed by Texas and Kansas, each with about 10 percent.