Illinois State Library

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• Yields of shallow dolomite wells in northern Illinois

Description

• In northern Illinois large quantities of ground water are withdrawn from wells in shallow dolomite aquifers of Silurian and Ordovician age. The Niagaran and Alexandrian Series of Silurian age and the Galena-Platteville Dolomite of Ordovician age yield moderate to large quantities of ground water. Dolomite beds of the Maquoketa Formation of Ordovician age yield small quantities of water to wells. Silurian rocks are usually encountered at depths of between 10 and 300 feet in northeastern Illinois and between 30 and 880 feet in northwestern Illinois. These rocks exceed 450 feet in thickness at places and are often overlain by glacial drift. The average depth of shallow doIomite weIIs is about 140 feet, and most wells of recent design are finished 12 to 16 inches in diameter. About 1000 well-production tests were made, 1921-1961, on more than 800 shallow dolomite wells. Statistical analysis of specific-capacity data provided a basis for determining 1) the role of individual shallow dolomite aquifers or formations, uncased in wells, as contributors of water; 2) whether or not significant relationships exist between the yields of wells and geohydrologic controls; and 3) the effects of acid treatment on the productivities of wells. It is concluded that the Niagaran Series, Alexandrian Series, and Galena-Platteville Dolomite all have similar moderate to high yields and inconsistency of yields in areas throughout northern Illinois where these rocks directly underlie glacial drift. These Silurian and Ordovician rocks have similar low yields and inconsistency of yields in areas where these rocks are overlain by bedrock. On the other hand, the Maquoketa Formation and rocks of Devonian age yield very little water to wells. Most water-yielding openings occur in the upper one-third of the shallow dolomite aquifers. There is a good connection between glacial drift and the upper part of the shallow dolomite aquifers. Highest yielding wells are found in bedrock upland areas, in areas where the glacial drift immediately overlying the shallow dolomite aquifers is composed of sand and gravel, and in areas where reefs and associated strata are present. Most dolomite wells treated with acid show significant improvement in yield; largest improvements are recorded for rehabilitated wells. Yields are increased because water-yielding openings are enlarged and fine drill cuttings or incrustations are removed from openings. Probable ranges in yields of shallow dolomite wells in undeveloped areas are estimated from specific-capacity frequency graphs, aquifer thickness and area1 geology maps, and waterlevel data.

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1. Yields of shallow dolomite wells in northern Illinois (20061006183959_ISWSRI-46.pdf).
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