Groundwater

Movement and Storage of groundwater The Hydrosphere The Earth has about 1.3 billion km3 of water! All of the water at the Earth’s surface makes up the hydrosphere About 97% of the hydrosphere is contained in the oceans

Groundwater and Precipitation Water is constantly in motion in the hydrosphere. This movement can be traced by the water cycle When precipitation occurs bringing atmospheric moisture back to Earth’s surface, infiltration can occur Infiltration: process by which precipitation that has fallen on land trickles into the ground and becomes groundwater Groundwater slowly moves through the ground and eventually returns to the surface through springs and seepage into wetlands and streams, and then flows back to the oceans

Groundwater Storage Rocks have the ability to hold water. It is measured with porosity, or, the percentage of a material’s volume that is pore space that can hold water Pore spaces: small openings within the soil, sediment, and rock The greater the porosity, the easier water can flow through a material

Zone of Saturation Zone of Saturation: The region below Earth’s surface in which groundwater completely fills all the pores of a material Water table: The upper boundary of the zone of saturation Zone of aeration: area above the water table, materials are moist, but because they are not saturated with water, air occupies much of the pores

Water Movement Water can be classified as either gravitational or capillary Gravitational: water the trickles downward as a result of gravity Capillary: water that is drawn upward through capillary action above the water table and is held in the pore spaces of rocks and sediment because of surface tension

The Water Table The depth of the water table often varies Stream valleys- water table close to Earth’s surface, only a few meters deep Swamps- water table is at Earth’s surface Hilltops- water table can be hundreds of meters or more beneath the surface Water table fluctuates with seasonal and other weather conditions because of its dependence on precipitation Rises during wet seasons, and drops during dry seasons

Groundwater Movement Groundwater flows downhill in the direction of the slope of the water table Water has to flow through numerous tiny pores in the subsurface material Permeability: the tendency of a material to let water pass through Materials with large, connected pores, such as sand and gravel, have high permeability and permit relatively high flow velocities up to hundreds of meters per hour

Permeability Groundwater flows through permeable sediment and rock, called aquifers Pore spaces are large and connected Impermeable: pores are small so flow is minimal and slow Measured in millimeters per day Example: silt, clay, and shale Aquicludes: impermeable layer

Springs Groundwater moves slowly but continuously through aquifers and eventually returns to Earth’s surface Usually groundwater emerges wherever the water table intersects Earth’s surface Spring: The natural discharge of groundwater

Emergence of Springs Permeable & Impermeable layers coming together Perched water tables - Perched water table: zone of saturation that overlies an aquiclude and separates it from the main water table below Along faults - Fault brought two different types of bedrock together – porous and nonporous Karst Springs - Groundwater weathers through limestone bedrock, and the water in the underground caverns emerges at Earth’s surface

Temperature of Springs Hot springs: spring discharge water is much warmer that the average annual temperature Temperature increase due to igneous activity or from emerging from tremendous depths in Earth’s crust Geysers: Explosive hot springs Water is heated past its boiling point, causing it to vaporize The water vapor then builds up tremendous pressure – fuels the eruption Ex. Old Faithful in Yellowstone National Park, Wyoming

Dissolution by Groundwater Most of groundwater is slightly acidic due to carbonic acid Groundwater usually reacts with calcium carbonate rocks, like limestone When the groundwater evaporates, a precipitate or solid may crystalize out of the groundwater solution The process of dissolving and precipitating calcite both play a role in the formation of caves

Caves Cave (cavern): a natural underground opening with a connection to Earth’s surface Formed when groundwater dissolves limestone Groundwater infiltrates the cracks and joints of limestone formations and dissolves the adjacent rock. Passages enlarge and form an interconnected network of openings. Begins in the zone of saturation When water table lowers, the cave systems becomes filled with air Examples: Mammoth Cave in Kentucky & Carlsbad Caverns in New Mexico

Karst Topography Karst Topography: Sinkhole: depression in the ground caused by the collapse of a cave or by the direct dissolution of limestone by acidic water Disappearing Stream: surface stream drains into a cave system and continues to flow underground

Groundwater Deposits Dripstones: formations that build over time as water drips through caves, forming calcite minerals Stalactite: hangs from the cave’s ceiling like icicles and forms gradually Stalagmite: water drips to the floor of the cave and slowly builds a mound-shaped dripstone Dripstone column: stalactites and stalagmites growing together

Hard Water Hard water: water that contains high concentrations of calcium, magnesium, or iron Common in areas where the subsurface rock is limestone Can cause problems within the home – precipitate in water pipes and on the heating elements of appliances

Wells Wells: holes dug or drilled into the ground to reach an aquifer. 2 types: 1. Ordinary Wells Well dug or drilled below the water table As water is drawn out of a well, it is replaced by surrounding water in the aquifer Overpumping: when water is drawn out of the well at a rate faster than which it is replaced Drawndown: The difference between the original water-table level and the water level in the pumped well Recharge: the process in which water from precipitation replenishes the water content of an aquifer

Wells 2. Artesian Wells An aquifer’s that contains water under pressure When the rate of recharge is high enough, pressurized water can spurt above the land surface in the form of a fountain

Artesian Well Spring Aquifer Aquifer Water Table Aquifer Well Dry Well

Threats to Our Water Supply Freshwater is Earth’s most precious natural resource Estimates of water supplies are the result of a dynamic equilibrium between various factors Amount of Precipitation Amount of Infiltration Surface Drainage Porosity and Permeability of rock Volume of groundwater naturally discharged back to the surface Changes to groundwater supplies can lead to environmental issues such as a lowered water table, subsidence, and pollution

Threats to Our Water Supply Overuse If groundwater is pumped out at a rate greater than the recharge rate, the groundwater supply will decrease and the water table will drop. Subsidence Sinking of land The volume of water underground helps support the weight of the soil, sediment and rock above. Occurring along parts of the golf coast of Texas – average subsidence was 15 cm Pollution Confined aquifers are affected less frequently than water-table aquifers, because they are protected by an impermeable barrier Sources of pollution include sewage form septic tanks and farms, landfills, and other waste disposal sites, as well as chemicals, salt, and radon.

Protecting Our Water Supply How to protect and restore groundwater: Identify and eliminate pollution sources Build an impermeable underground barrier around the polluted area Pump out polluted groundwater for chemical treatment Educate human on their impact of the groundwater system