Lake Management Plan
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5.3 Ground Water
Groundwater is water flowing within aquifers below the water table. Within aquifers, the water flows through the pore spaces in unconsolidated sediments and the fractures of rocks. Groundwater is recharged from, and eventually flows to, the surface naturally; natural discharge often occurs at springs and seeps and can form oases or swamps. Groundwater is also often withdrawn for agricultural, municipal and industrial use through man-made wells. The study of the distribution and movement of groundwater is hydrogeology.
Fig 5.3 Illustration of how ground and aquifers flow.
Groundwater can be a long-term reservoir of the natural water cycle (with residence times from days to millennia), as opposed to short-term water reservoirs like the atmosphere and fresh surface water (which have residence times from minutes to years). The figure shows how deep groundwater (which is quite distant from the surface recharge) can take a very long time to complete its natural cycle. Groundwater is naturally replenished by surface water from precipitation, streams, and rivers when this recharge reaches the water table. It is estimated that the volume of groundwater is fifty times that of surface freshwater; the icecaps and glaciers are the only larger reservoir of fresh water on earth.
Usable groundwater is contained in aquifers, which are subterranean areas (or layers) of permeable material (like sand and gravel) that channel the groundwater's flow. Aquifers can be confined or unconfined. If a confined aquifer follows a downward grade from a recharge zone, groundwater can become pressurized as it flows. This can create artesian wells that flow freely without the need of a pump. The top of the upper unconfined aquifer is called the water table or phreatic surface, where water pressure is equal to atmospheric pressure.
Typically groundwater is thought of as liquid water flowing through shallow aquifers, but technically it can also include soil moisture, permafrost (frozen soil), immobile water in very low permeability bedrock, and deep geothermal or oil formation water. Groundwater is believed to provide lubrication and buoyancy which allow thrust faults to move. Nearly any point in the Earth's subsurface has water in it, to some degree (it may be very dry or mixed with other fluids).
5.3.1 Water Table
The water table is the upper limit of abundant groundwater. In the vadose zone, above the water table, the interstices between particles of earth are filled by air, or by air and water (with the exception of the capillary fringe). Below it, every available space is saturated with water. A large amount of water within a body of sand or rock below the water table is called an aquifer. A so-called "perched aquifer" (or perched water table) occurs when the descent of water percolating from above is blocked by a shelf of impermeable rock.
The water table is that surface beneath which all interconnected pore space in the rock is water-filled or saturated. Groundwater recharge is the process of adding new water to the groundwater body, as through infiltration of precipitation on the land surface. Discharge is the process of water escaping from the groundwater body, as when the water table intersects the land surface, allowing water to flow out from a spring.
Fig 5.4 Water table depth near Pelican Lake.
Recharge Potential
Pelican Lake has a moderate to high ground water recharge potential (from the Minnesota Environmental Atlas and based on aquifers in the region). The highest potential is at the east and west end of the lake where the Pelican River and Bob Creek inlets are found.
Fig 5.5 Pelican River watershed ground water recharge potential