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Aquifer Recharge
Figure 8: Map showing ground water ages in areas with Because of Arizona’s arid and semi-arid climate, on average, recharge to ground water is estimated to be 2% to 3% of the average annual rainfall (Uhlman, 2005). Most aquifer recharge occurs along the mountain fronts because that is where most of the rain falls, and also because the fractured rock of the mountains and the coarser grained materials along the margins of the alluvial basins allow water to infiltrate rapidly. Shallow wells near surface water or washes, with a water-table within a few feet of land surface, may exhibit dramatic seasonal variation in water table depth due to rapid infiltration of recharge following precipitation or stream flow. Most Arizona wells, however, are at a distance from their recharge source and are less likely to exhibit seasonal changes. In addition, most regional aquifers across the west, and in Arizona, have not received significant volumes of recharge for hundreds to thousands of years. Work done by the U.S. Geological Survey has age-dated ground water (see Figure 8). Age dating is accomplished by looking at carbon and oxygen isotopes within the water, and calculating when was the last time the water fell as rain or snow. For most of the west, the last time the climate was wet enough to fill up the aquifers was during the Ice Age over 10,000 years ago. |