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Groundwater contimced from l~rr~ious pace <br />10% for public needs. The importance <br />of groundwater for agriculture can't be <br />overstated. Without irrigation, the sand <br />plains left by retreating glaciers <br />thousands of years ago would be <br />worthless for agriculture. <br />Pesticides and herbicides used in <br />agriculture become a hazard to those <br />who use groundwater for drinking <br />water. Other threats to groundwater <br />include ruptured underground storage <br />tanks, improper household waste <br />disposal, failing septic tanks, and even <br />the improper burial of bodies. <br />The U.S. Geological Survey has <br />identified 14 principal aquifers in <br />Minnesota, one of them directly under <br />the Twin Cities. The groundwater in this <br />aquifer is held by a geologic formation <br />shaped like a bowl, with its western <br />edge chopped off by one of the glaciers. <br />As a result, water in the western <br />metropolitan area-like Lake <br />Minnetonka-flows into the artesian <br />basin beneath the Twin Cities. This <br />explains the low level of Lake <br />Minnetonka during the drought of the <br />1930s and early 1940s. During that 12 <br />year period of drought, all of the lakes <br />and rivers in the area experienced <br />severe declines in water levels, but Lake <br />Minnetonka was more than eight feet <br />below today's level. <br />Withdrawing more water from <br />aquifers than is recharged each year <br />causes drops in the level of the land as <br />well. A sign in the middle of the <br />Imperial Valley in California, nailed to <br />the top of a 30-foot post, marks the <br />original level of the land before <br />intensive irrigation in the valley led to <br />soil compaction. The water level in a <br />well near Pierre, South Dakota, dropped <br />300 feet over 35 years because of excess <br />water drawn from the aquifer. <br />According to John Tester, in his <br />book Minnesota's Natural Heritage, the <br />precipitation that recharges our <br />groundwater varies widely across the <br />state. It averages about 20 inches in the <br />northwest to 32 inches in the southeast. <br />The average precipitation in the Twin <br />Cities is about 27 inches. About 70% of <br />Minnesota's precipitation falls between <br />May and September. Snow may contain <br />up to 90% air, so seven to 15 inches of <br />snow equals only about an inch of <br />precipitation. Precipitation is our main <br />source of "new" water in our aquifers. <br />Excessive withdrawal from groundwater <br />affects all of us. <br />We can't afford that. <br />~~G ~r~ <br />FACETS December 2003 <br />