Chapter 17: Groundwater

6 percent of Earth's total water while representing 96 percent of accessible liquid freshwater. Groundwater stored in sediment pores and rock fractures supplies approximately half of U.S. drinking water, sustains agricultural systems, maintains streamflow during dry periods, and supports diverse ecosystems. The chapter establishes foundational concepts by distinguishing between the unsaturated zone above the water table, which contains soil moisture and the capillary fringe, and the zone of saturation below where pores are water-filled. Water table elevation fluctuates seasonally with precipitation patterns and topographical features, often mirroring surface relief. Critical hydraulic properties include porosity, the percentage of void space in rock and sediment, and permeability, which describes how well connected pores allow fluid movement. Permeable materials like sand and gravel function as aquifers that store and transmit water, while impermeable layers such as clay serve as aquitards that restrict flow. Groundwater movement obeys Darcy's Law, which quantifies discharge based on hydraulic gradient, hydraulic conductivity, and cross-sectional area. Flow paths extend from recharge areas where water infiltrates the subsurface to discharge areas including springs, streams, and wetlands. Confined aquifers under pressure create artesian systems where water rises above the aquifer without pumping, occasionally producing flowing wells. Natural discharge features include springs formed where water tables intersect the surface, hot springs heated by geothermal processes or magma, and geysers that erupt when superheated water converts to steam. The chapter addresses serious management challenges including groundwater mining in aquifers like the High Plains system, where extraction exceeds natural recharge rates causing severe depletion. Excessive pumping creates cones of depression that lower water tables, induces land subsidence as seen in California's San Joaquin Valley, and promotes saltwater intrusion in coastal regions. Contamination from septic systems, landfills, agricultural chemicals, and industrial storage presents persistent problems since remediation is difficult and expensive. As a geomorphic agent, groundwater dissolves limestone through carbonic acid action, creating cavern systems decorated with speleothems including stalactites, stalagmites, and columns. Cavern roof collapse produces sinkholes while widespread dissolution generates karst topography characterized by irregular surface drainage and distinctive landscapes.