APES Unit 6 – Land & Water Use

Groundwater Learning Targets I can explain how much water on Earth is useable and where it is located. I can explain how groundwater is formed. I can describe the major items that can affect groundwater around the Earth. Describe what an aquifer is; its benefits and how humans can damage them

Water Cycle Review

Water Facts Nearly 70% of Earth’s surface is covered by H 2 O % is found in oceans as salt water. 3% or less is fresh water. ~1/4 of the fresh water exists underground. The other ¾ is above ground, but most is in the form of ice and glaciers. Very little is actually in the atmosphere and in streams, rivers, wetlands and lakes.

Major Water Compartments Rivers and Streams –Precipitation that does not evaporate or infiltrate into the ground runs off the surface, back toward the sea. Best measure of water volume carried by a river is discharge –The amount of water that passes a fixed point in a given amount of time »Usually expressed as cubic feet per second

Major Water Compartments Lakes and Ponds –Ponds are generally considered small bodies of water shallow enough for rooted plants to grow over most of the bottom. –Lakes are inland depressions that hold standing fresh water year-round. Both ponds and lakes will eventually fill with sediment, or be emptied by an outlet stream.

Major Water Compartments Wetlands –Play a vital role in hydrologic cycle Lush plant growth stabilizes soil and retards surface runoff, allowing more aquifer infiltration. –Disturbance reduces natural water-absorbing capacity, resulting in floods and erosion in wet periods, and less water flow the rest of the year. –Half of U.S. wetlands are gone.

Major Water Compartments The Atmosphere –Among the smallest water reservoirs Contains < 0.001% of total water supply Has most rapid turnover rate Provides mechanism for distributing fresh water over landmasses and replenishing terrestrial reservoirs

Groundwater Facts Groundwater is the source of drinking water for about 1/3 of Virginia’s population Groundwater occurs in the voids between the soil and rock particles, referred to as an aquifer. Unconfined aquifers are porous rock covered by soil where water can flow in and out of. Confined aquifers are surrounded by a layer of impermeable rock or clay.

Zone of Aeration & Zone of Saturation

Aquifers The water table is the uppermost level at which the water in a given area fully saturates the rock or soil. Water from precipitation can percolate through the soil and work its way into an aquifer. This is called groundwater recharge. Aquifers serve as important sources of fresh water for many organisms. Some even naturally percolate up to the surface as springs.

Porosity & Permeability The rate that water can percolate into the ground to recharge an aquifer depends on its makeup. Average rate of groundwater movement in coarse sand is 0.3 m/day (360ft/yr) vs. average movement in clay is m/day (less than 1/2”/yr)

“Ancient Water” Humans discovered centuries ago that water can also be obtained from aquifers by digging a well. Groundwater within 1/2 mile of the surface is about 200 years old. Groundwater in deep aquifers may be 10,000 years old. Overuse of the groundwater may deplete water supply availability for many years.

Unconfined aquifer Confined aquifers

Wells Artesian wells are wells that are dug into a confined aquifer to release pressure, which allows the water to burst out of the aquifer and rise up the well. A pump may not be necessary to extract the water depending on the amount of pressure. The age of water in aquifers varies, as does the rate that aquifers are recharged. Largest U.S. aquifer is Ogallala aquifer

Troubled Wells When too much water is withdrawn and not balanced by recharge, the water table drops farther from the ground surface. Springs that once bubbled up no longer emerge and some shallow wells may not reach the water table. A cone of depression forms (an area where there is no longer any groundwater).

Troubled Wells cont. When pumping fresh water out faster than an aquifer can recharge, the water quality can be compromised, especially near coastlines. The pressure reduces on the aquifer due to the excessive pumping, which allows adjacent salt water to infiltrate the area, making the wells salty. This is known as saltwater intrusion.

Troubled wells Pumping wells influence the flow of groundwater movement.

Level of the Water Table Changes with the Topography

A side note about Floodplains

 The land adjacent to a river.  Experiences flooding during periods of high discharge.  Generally contain sediments of sand, gravel, loam, silt and/or clay, which are important for pre-filtering water as it seeps through.  Can support ecosystems rich in quantity and diversity.  Wetting the floodplain soil releases an immediate surge of nutrients (from the last flood) which benefits the ecosystem.  This makes floodplains particularly valuable for agriculture.

Groundwater Erosion Water and CO 2 from the air combine to form a weak acid, carbonic acid (H 2 CO 3 ) Carbonic acid dissolves limestone to form sinkholes and caverns. Caverns have formations in them such as: stalactites (from ceiling), stalagmites (from floor), columns, flow stones and curtains. Some of Virginia’s famous caverns are: Luray, Skyline, Shenandoah and Endless Caverns.

Caverns and Sinkholes

Threats to Virginia’s Groundwater, Virginia Water Resources Research Center, VPI&SU, 1988, p. 6. That Sinking Feeling in the Coastal Plain Withdrawals of ~73 mil. gal./day from well fields have caused two giant cones of depression to form in an extensive confined aquifer in VA and NC. Pumping by papermaking industries in Franklin and West Point VA is responsible for more than 60% of the daily withdrawal. In this area, such cones of depression can result in saltwater intrusion or in the collapse of the water-bearing formation and land above it. The numbers on the map represent height in ft. of the aquifer’s surface relative to mean sea level. Around Franklin, the depth to groundwater below the land surface has declined 165 feet since the 1930s but has recently stabilized.

Chemical Pollution Chemicals can flow either on top of or within the groundwater depending on its density.

Rock type can influence the rate of movement and the quality of groundwater

Groundwater Contamination Activity

Groundwater Learning Targets – So can you…. explain how much water on Earth is useable and where it is located. explain how groundwater is formed. describe the major items that can affect groundwater around the Earth. Describe what an aquifer is; its benefits and how humans can damage them