FIGURE 4.1 Distribution of the world’s water.

SOURCE: Courtesy “Earth Update” CD-ROM, Rice University and the Houston Museum of Natural Science; used with permission.

creased pressures and temperatures due to burial and compaction. The increased pressure and temperature result in enrichment of aqueous solutions within the rocks by soluble chemical species, especially salts. Such interstitial brines may be several times saltier than the world’s oceans, and thus fresh groundwater is typically limited to near-surface reservoirs.

Within the earth’s hydrosphere, fresh water comprises only 3% of the total water in the earth system. Because most fresh water is held in glaciers and polar ice caps, only ~30% of fresh water reserves are available as surface water or groundwater for human use (Dingman, 2002; see Figure 4.1). In many arid areas of the world, and even in some more humid locations, groundwater extraction rates by humans exceed natural recharge rates, and the available water stored in aquifers is decreasing. Agriculture



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