17.3- Groundwater Diversion and Depletion 1. Describe- What causes a water table to drop? 2. Summarize- What is subsidence, and why is it irreversible? 3. Why is a coastal area susceptible to saltwater intrusion when the aquifer is overused as a source of fresh water?

Let's break down each of the questions related to groundwater diversion and depletion. ### 1. What causes a water table to drop? The water table can drop due to several factors: - **Over-extraction of groundwater**: When water is pumped from wells faster than it can be replenished by rainfall or surface water, the water table declines. - **Drought conditions**: Extended periods of low precipitation can reduce the amount of water that infiltrates the ground, leading to a drop in the water table. - **Increased evaporation**: In areas with high temperatures, increased evaporation can lead to less water being available to recharge the groundwater. - **Land use changes**: Urbanization and agricultural practices can alter the natural flow of water, reducing the amount that reaches the aquifer. - **Climate change**: Changes in climate patterns can affect precipitation and evaporation rates, impacting groundwater recharge. ### 2. What is subsidence, and why is it irreversible? **Subsidence** is the gradual sinking or settling of the Earth's surface due to various factors, including: - **Groundwater extraction**: When groundwater is removed, the support that water provides to soil and rock layers is lost, causing them to compact and sink. - **Soil compaction**: The removal of water from saturated soils can lead to a decrease in pore space, resulting in compaction and subsidence. - **Natural processes**: Some areas may experience subsidence due to natural geological processes, such as the settling of sediments. **Irreversibility**: Subsidence can be irreversible because once the soil and rock layers have compacted, they may not return to their original state even if groundwater levels are restored. The loss of pore space and structural integrity can lead to permanent changes in the landscape, making it difficult or impossible for the area to recover fully. ### 3. Why is a coastal area susceptible to saltwater intrusion when the aquifer is overused as a source of fresh water? Coastal areas are particularly vulnerable to **saltwater intrusion** due to the following reasons: - **Hydraulic gradient**: Freshwater in aquifers is less dense than saltwater. When freshwater is over-extracted, the hydraulic gradient that keeps saltwater at bay is disrupted, allowing saltwater to move inland. - **Reduced pressure**: Over-extraction lowers the pressure in the aquifer, making it easier for saltwater to intrude into the freshwater supply. - **Natural balance**: Coastal aquifers typically maintain a balance between freshwater and saltwater. When freshwater is depleted, this balance is disturbed, leading to the encroachment of saltwater. - **Limited recharge**: Coastal aquifers often have limited recharge areas, making it difficult for them to recover from over-extraction, further exacerbating the problem of saltwater intrusion. In summary, the combination of over-extraction of freshwater, changes in hydraulic pressure, and the natural dynamics of coastal aquifers makes these areas particularly susceptible to saltwater intrusion.