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HOW IS WATER FILTERED IN NATURE?

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Confined aquifers are bound on the top by impermeable material, such as clay. Water in a confined aquifer is normally under pressure and can cause the water level in a well to rise above the water table. If the water rises above the ground surface it is designated a flowing artesian well. A perched water table occurs when water is held up by a low permeability material and is separated from a second water table below by an unsaturated zone. In the saturated zone, groundwater flows through the pores of the soil or rock both laterally and vertically. Water moving from an aquifer and entering a stream or lake is called groundwater discharge, whereas any water entering an aquifer is called recharge. In Michigan, groundwater typically discharges from aquifers to replenish rivers, lakes, or wetlands. An aquifer may receive recharge from these sources, an overlying aquifer, or more commonly from precipitation followed by infiltration. The recharge zone is that area, either at the surface or below the ground, which provides water to an aquifer and may encompass most of the watershed.

Glacial and Bedrock Geology.

The ease with which water moves through the ground is influenced by the glacial and bedrock geology of an area. In Michigan, glaciers covered much of the land surface and left behind till, outwash, and lacustrine (lake) deposits. Till is a mixture of soil and rock ranging in size from clay particles to boulder-size rocks. Tills generally have low permeability due to the presence of clay. Outwash consists primarily of highly permeable sand and gravel that allow groundwater to flow easily. Lacustrine deposits can be clay, silt, or sand, and their permeability depends on the sediment type. The type of bedrock formations under glacial deposits also influences groundwater movement. Sandstone can transmit water if the pores between grains are connected, giving the rock a high permeability. Limestones fractured with many connecting cracks can also transmit water easily. Fine grained rocks such as shale and slate generally have a low permeability. Groundwater is all the water that has penetrated the earth's surface and is found in one of two soil layers. The one nearest the surface is the "zone of aeration", where gaps between soil are filled with both air and water. Below this layer is the "zone of saturation", where the gaps are filled with water. The water table is the boundary between these two layers. As the amount of groundwater water increases or decreases, the water table rises or falls accordingly.

When the entire area below the ground is saturated, flooding occurs because all subsequent precipitation is forced to remain on the surface. The amount of water that can be held in the soil is called "porosity". The rate at which water flows through the soil is its "permeability". Different surfaces hold different amounts of water and absorb water at different rates. Surface permeability is extremely important for hydrologists to monitor because as a surface becomes less permeable, an increasing amount of water remains on the surface, creating a greater potential for flooding. Flooding is very common during winter and early spring because the frozen ground has no permeability, causing most rainwater and meltwater to become runoff. Water that infiltrates the soil flows downward until it encounters impermeable rock, and then travels laterally. The locations where water moves laterally are called "aquifers". Groundwater returns to the surface through these aquifers, which empty into lakes, rivers, and the oceans.

Under special circumstances, groundwater can even flow upward in artesian wells. The flow of groundwater is much slower than runoff, with speeds usually measured in centimeters per day, meters per year, or even Aquifer: A formation, group of formations, or part of a formation that contains sufficient saturated, permeable material to yield significant quantities of water to wells and springs. It is difficult to visualize water underground. Some people believe that ground water collects in underground lakes or flows in underground rivers. In fact, ground water is simply the subsurface water that fully saturates pores or cracks in soils and rocks. Ground water is replenished by precipitation and, depending on the local climate and geology, is unevenly distributed in both quantity and quality.

When rain falls or snow melts, some of the water evaporates, some is transpired by plants, some flows overland and collects in streams, and some infiltrates into the pores or cracks of the soil and rocks. The first water that enters the soil replaces water that has been evaporated or used by plants during a preceding dry period. Between the land surface and the aquifer water is a zone that hydrologists call the unsaturated zone. In this unsaturated zone, there usually is at least a little water, mostly in smaller openings of the soil and rock; the larger openings usually contain air instead of water. After a significant rain, the zone may be almost saturated; after a long dry spell, it may be almost dry. Some water is held in the unsaturated zone by molecular attraction, and it will not flow toward or enter a well. Similar forces hold enough water in a wet towel to make it feel damp after it has stopped dripping.

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