In the Interior Highlands of western and northern Arkansas ground-water supplies are more limited than in the Coastal Plain. Much of the Ozark Plateaus region is underlain by carbonate rocks, which are quite soluble in the presence of water. Solution by ground water has caused many large openings through which water passes so quickly that contaminants from the surface can not be filtered out. Signs of these openings are caves, sink holes, springs and lost stream segments. As a consequence, the water in shallow wells may not be suitable for human consumption without treatment. However, there are two important aquifers at greater depth – the Roubidoux Formation and the Gunter Member of the Gasconade Formation. Both are permeable sandstone and carbonate units of Ordovician age. These aquifers serve as the principal source of high-quality water for many communities in northern Arkansas.
Dependent upon the constancy of the water source (rainfall or snowmelt that infiltrates the earth), a spring may be ephemeral (intermittent) or perennial (continuous).
Water issuing from an artesian spring rises to a higher elevation than the top of the confined aquifer from which it issues. When water issues from the ground it may form into a pool or flow downhill, in surface streams. Sometimes a spring is termed a seep.
Minerals become dissolved in the water as it moves through the underground rocks. This may give the water flavour and even carbon dioxide bubbles, depending upon the nature of the geology through which it passes. This is why spring water is often bottled and sold as mineral water, although the term is often the subject of deceptive advertising. Springs that contain significant amounts of minerals are sometimes called 'mineral springs'. Springs that contain large amounts of dissolved sodium salts, mostly sodium carbonate, are called 'soda springs'. Many resorts have developed around mineral springs known as spa towns.
Water emanating from karst topography is another type of spring, often called a resurgence as much of the water may come from one or more sinkholes at a higher altitude. Karst springs generally are not subjected to as great a degree of ground filtering as spring water which may have continuously passed through soils or a porous aquifer.
Springs are often classified by the volume of the water they discharge. The largest springs are called "first-magnitude," defined as springs that discharge water at a rate of at least 2800 L/s. The scale for spring flow is as follows:
Magnitude Flow (ft³/s, gal/min, pint/min) Flow (L/s)
1st Magnitude> 100 ft³/s 2800 L/s
2nd Magnitude 10 to 100 ft³/s 280 to 2800 L/s
3rd Magnitude 1 to 10 ft³/s 28 to 280 L/s
4th Magnitude 100 US gal/min to 1 ft³/s (448 US gal/min) 6.3 to 28 L/s
5th Magnitude 10 to 100 gal/min 0.63 to 6.3 L/s
6th Magnitude 1 to 10 gal/min 63 to 630 mL/s
7th Magnitude 1 pint to 1 gal/min 8 to 63 mL/s
8th Magnitude Less than 1 pint/min 8 mL/s
0 Magnitudeno flow (sites of past/historic flow)
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1. What is the magnitude of this spring?
2. Is this spring ephemeral (intermittent) or perennial (continuous)?
3. What mineral do you think is present in this water? Why?
4. How many layers of rock are above the pipe vertically and how many rocks are above the pipe horizontally?