Underground Water

Groundwater and the Hydrologic Cycle

        Groundwater is water lying below the water table (an imaginary surface marking the depth below which all pore spaces are filled with water). The subsurface is one reservoir for water in the hydrologic cycle. Most groundwater comes from infiltrated precipitation. Groundwater eventually returns to the surface by flowing into lakes, streams or the ocean.

        Most of the Earth's unfrozen, fresh water supply lies below the surfase. More than 65% of the groundwater used in the US goes for irrigation, with industry the second largest user, and domestic consumption third. There are potential problems related to its distribution and use:

1. Overuse may severely depleted groundwater supplies.
2. Pollution may make some groundwater sources unusable.
3. Ground subsidence and salt water contamination may result from excessive groundwater withdrawal.

As water infiltrates into the ground, it passes through several zones:

Zone of Aeration is where pore spaces in soil and rock contain air and water, and water is prevented from moving deeper by water tension (suspended water). This zone can be subdivided into:

1. Belt of soil moisture where water is held near the surface and used by plants. Some is evaporated back to the atmosphere.
2. Intermediate belt lies below the belt of soil moisture.
3. Capillary Fringe or the saturated zone in the zone of aeration resulting from the capillary pull of water upward into pore spaces.

Zone of Saturation occurs where pore spaces in soil and rock contain water (groundwater). Upper limit of zone is marked by the water table. The water table tends to mimic the shape of the land surface and is marked by the level of streams and lakes in humid areas. It tends to be flat in arid and semi-arid regions, and lies below the level of river valleys. Various factors can cause the position of the water table to shift upward and downward:

1. Precipitation - Water table rises in periods of high rain and drops during droughts.
2. Groundwater withdrawal - Water table lowers in and around pumped wells.

        Groundwater generally moves very slowly because it must travel through the pore spaces of rock and soil. Average groundwater velocities are a few cm/day. Groundwater is replenished (recharged) through both natural processes and artificial methods. The amount and availability of groundwater depends on:

1. Porosity is the percentage of void space in a material, and determines the amount of water a rock can hold. Void space can be from fractures, vesicles, dissolution channels, or pore spaces between elastic grains. Porosity depends on the size, shape and arrangement of the material composing the rock. In general, porosity of less than 5% is low, 5-15% is moderate, and porosity over 15% is considered high.
2. Permeability is the ability of a material to transmit water (a measure of the connectivity of the pore spaces). Clay may have a high porosity, but has low permeability because the pore spaces are isolated. The term aquifer is applied to materials that have good porosity and permeability and serve as water sources in sufficient quality for drinking. Aquicludes are materials that have low penneability and restrict the movement of water.
3. Darcy's Law expresses the velocity of water movement through subsurface materials:

V = K(h.g.)

Velocity depends upon the permeability of the material (K is hydraulic conductivity, measured in units of ft/day or m/day) and the hydraulic gradient (h.g.) or slope of the water table (elevation change/length of flow). For homogeneous material, velocity increases as the hydraulic gradient increases. Water moves from areas of high hydraulic head (high elevation) to areas of low hydraulic head (low elevation) under the influence of gravity. Flow path is as downward-curved arcs, not directly down the water table surface.

        Subsurface materials are generally not homogeneous. Heterogeneity can lead to the channeling and
trapping of groundwater.

• Springs represent natural discharge of groundwater occuning where the water table intersects the Earth's surface. Most commonly occur along valley walls where streams have eroded below the level of the water table. A perched water table occurs where an impermeable layer of rock (aquiclude) in the zone of aeration prevents water from reaching the water table below. Water moves laterally until it reaches the surface.

• Wells are artificial openings dug or drilled into the zone of saturation. Wells will fill to the level of the water table. Most wells must be pumped to bring groundwater to the surface. Pumping creates a cone of depression, which lowers the groundwater level around the well.

• Artesian systems occur where goundwater under pressure rises above the surface of the aquifer to the artesian pressure surface (the level to which groundwater will naturally rise due to the confining pressure in the aquifer). An artesian well will flow without pumping if the well head lies below the pressure surface. Conditions necessary for an artesian system include:

1. water is contained in an inclined aquifer whose upward end receives water.
2. aquicludes are present above and below the aquifer to trap the water.
3. there is suffcient recharge to keep the aquifer filled.

        Groundwater is recharged from precipitation falling on aquifer rock outcrops or by precipitation infiltrating through the zone of aeration. Factors controlling natural recharge:

1. Amount of moisture used by vegetation.
2. Nature of precipitation event.
3. Presence of any subsurface barriers to now.
4. Amount of groundwater used by humans.

        Chemical weathering causes dissolution of carbonate rock (limestone and dolostone) in contact with groundwater containing carbonic acid (produced from carbon dioxide in the air and from organic decay). Chemical weathering by groundwater produces several features in areas with carbonate bedrock:

• Caves and Cave Deposits form at or below the water table in limestone and dolostone through dissolution. Caves can be brought above water table by uplift and/or erosion. More than 17,000 caves are known in the US.

• Caves are a naturally formed subsurface opening that is generally connected to the surface and is large enough for a person to enter. A cavern is a very large cave or system of interconnected caves.
• Cave deposits or dripstones are deposits of calcium carbonate that form as groundwater evaporates in caves. Kinds of dripstone formations include:
1. Stalactites - hang down from ceiling of cave.
2. Stalagmites - grow upward from floor of cave.
3. Columns - form when a stalactite and stalagmite meet.
4. Drip curtains - vertical sheets of dripstone formed by water seeping through crack in cave ceiling.
5. Travertine terraces - layers of dripstone formed by water flowing across the floor of a cave.
• Karst topography represents landscapes shaped by the dissolution of underlying limestone or dolomite by groundwater. Karst forms in humid and temperate climates. Caves and springs are common, and other features include:

1. Sinkholes form by the collapse of an underground cavity or by dissolution of soluble rock below a soil layer. Sinkholes can fill with water to form a lake.
2. Solution valleys are caused by the coalescence of sinkholes, and locally contain disappearing streams (stream flows into a sinkhole).

        Groundwater currently provides about 20% of all water used in the US. Human modification of the groundwater system can have several long-lasting consequences:

• Lowering of the water table occurs where groundwater is withdrawn faster than it can be recharged. It can cause wells to dry up; for example: in some areas of the mid-West, 2-10x more water is being withdrawn from the High Plains aquifer than is being recharged.

• Saltwater incursion involves contamination of freshwater aquifers with saltwater. Fresh water floats as a lens on denser salt water. If too much fresh water is removed, a cone of depression is created in the fresh water lens. Lowering the water table by 1 foot results in raising the level of salt water by 40 feet. This situation occurs primarily in island or coastal communities, but is also threatening the Salinas Valley in Californfa. It can be counteracted by:

1. Reducing groundwater withdrawal.
2. Reinjecting treated wastewater into recharge wells.
3. Construction of recharge ponds.

• Land subsidence occurs where excessive pumping of groundwater removes ground support, particularly in areas of unconsolidated sediments and sedimentary rocks. Removal of water causes sediment compaction. The weight of buildings can also cause compaction and subsidence.

• Pollution - Sewage is the most common source of groundwater pollution. Landfills, underground storage tanks, and hazardous waste disposal sites are other sources of contamination. Surface sources of pollution can affect groundwater where the ground is very permeable or where conduits to the water table are present. Pollution spreads with the flow of groundwater. Cleanup of contaminated groundwater is extremely difficult and expensive. High-level nuclear waste disposal may threaten groundwater quality fn the future.

        These features are produced by groundwater percolating into areas heated by igneous activity or by deeply circulating water heated by the geothermal gradient.

1. Hot springs are springs that bring water to the surface that is at least 6.5 degrees C higher than the mean air temperature. Most US hot springs are located in the West, and are produced by igneous activity.
2. Geysers are hot springs which periodically emit columns of water and steam with great force. They occur where groundwater percolates into underground chambers that exist in hot rock. Water eventually boils under great pressure and forces its way to the surface. Cooler groundwater seeps back in, and the cycle is repeated.

        This is energy produced from steam and hot water trapped in the Earth's crust. Relatively non-polluting power sources, geothermal areas are often protected, typically remote from population centers, and have a limited lifetime.