Groundwater

Groundwater and Surface Water Are Critical Resources Zone of saturation Spaces in soil are filled with water Water table Top of zone of saturation. Aquifers Permeable layers of rock and sediment that store and carry ground water in enough quantity to supply wells.

Aquifer Depletion in the United States Ogallala aquifer: largest known aquifer. Covers portions of 8 states (174,000 square miles). Irrigates the Great Plains. Provides approximately 30% of the nation’s groundwater for irrigation. Also provides water to 82% of the people living with in the aquifer boundary. Very slow recharge Water table dropping Government subsidies to continue farming deplete the aquifer further Biodiversity threatened in some areas

Groundwater is Being Withdrawn Faster Than It Is Replenished Most aquifers are renewable Aquifers provide drinking water for half the world Water tables are falling in many parts of the world, primarily from crop irrigation

Groundwater is Being Withdrawn Faster Than It Is Replenished India, China, and the United States Three largest grain producers Over pumping aquifers for irrigation of crops India and China Small farmers drilling tube wells Effect on water table

Withdrawing Groundwater Trade-Offs Withdrawing Groundwater Advantages Disadvantages Useful for drinking and irrigation Aquifer depletion from overpumping Sinking of land (subsidence) from overpumping Exists almost everywhere Renewable if not overpumped or contaminated Pollution of aquifers lasts decades or centuries Figure 13.7: Withdrawing groundwater from aquifers has advantages and disadvantages. Questions: Which two advantages and which two disadvantages do you think are the most important? Why? Cheaper to extract than most surface waters Deeper wells are nonrenewable Fig. 13-7, p. 325

Natural Capital Degradation: Areas of Greatest Aquifer Depletion in the U.S. Figure 13.9: Natural capital degradation. This map shows areas of greatest aquifer depletion from groundwater overdraft in the continental United States. Aquifer depletion is also high in Hawaii and Puerto Rico (not shown on map). See an animation based on this figure at CengageNOW. Questions: Do you depend on any of these overdrawn aquifers for your drinking water? If so, what is the level of severity of overdraft where you live? (Data from U.S. Water Resources Council and U.S. Geological Survey) Fig. 13-9, p. 326

Subsidence in the San Joaquin Valley Figure 13.11: This pole shows subsidence from overpumping of an aquifer for irrigation in California’s San Joaquin Central Valley between 1925 and 1977. In 1925, the land surface in this area was near the top of this pole. Since 1977 this problem has gotten worse.

Groundwater Depletion Solutions Groundwater Depletion Prevention Control Waste less water Raise price of water to discourage waste Subsidize water conservation Tax water pumped from wells near surface waters Limit number of wells Figure 13.12: There are a number of ways to prevent or slow groundwater depletion by using water more sustainably. Questions: Which two of these solutions do you think are the most important? Why? Set and enforce minimum stream flow levels Do not grow water-intensive crops in dry areas Divert surface water in wet years to recharge aquifers Fig. 13-12, p. 327

Deep Aquifers Might Be Tapped May contain enough water to provide for billions of people for centuries Major concerns Nonrenewable Little is known about the geological and ecological impacts of pumping deep aquifers Some flow beneath more than one country Costs of tapping are unknown and could be high