Page 299 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

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(a) Centre-pivot irrigation system waters a wheat field.
(b) A pattern of quarter-section circular fields results from centre-pivot irrigation systems near Dalhart, Texas. In each field, a sprinkler arm pivots around a centre, delivering about 3 cm of High Plains Aquifer water per revolution.
▲Figure 9.1.2 Centre-pivot irrigation. [(a) gene alexander, USDa/nrCS. (b) USDa national agricultural imagery Program 2010.]
Pollution can enter groundwater from many sources: industrial injection wells (which pump waste into the ground), septic tank outflows, seepage from hazardous- waste disposal sites, industrial toxic waste, agricultural residues (pesticides, herbicides, fertilizers), and urban solid-waste landfills. An example is the suspected leak- age from some 10000 underground gasoline storage tanks
Wyoming
Colorado
New Mexico
0 75
South Dakota
Area of water-level change with few pre-development water levels
Nebraska
Kansas
Oklahoma
WATER-LEVEL CHANGE
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Areas of little or no saturated thickness
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Rise Decline
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Chapter 9 Water resources 263
Texas
150 KILOMETRES
mFigure 9.1.3 Water-level changes in the High Plains Aquifer, 1950 to 2011. The colour scale indicates widespread declines and a few areas of water-level rise. [adapted from “Water-level and storage changes in the High Plains aquifer, predevelopment to 2011 and 2009–2011,” by V. l. Mcguire, USgS Scientific investiga- tions report 2012–5291, 2013, Fig. 1; available at ne.water .usgs.gov/ogw/hpwlms/.]
at U.S. gasoline stations, thought to be contaminating thousands of local water supplies with cancer-causing gasoline additives. About 35% of groundwater pollution comes from point sources, such as a gasoline tank or sep- tic tank; 65% is categorized as nonpoint source, coming from a broad area, such as runoff from an agricultural field or urban community. Nitrates contaminate nearly all the