Yesterday we talked about the porosity, or open space, you need to make an oil reservoir. It’s the same concept as you have for an aquifer, a subsurface reservoir for water. Across most of Iowa and parts of Wisconsin and Michigan, the Silurian and some Devonian rocks in the subsurface form an important groundwater aquifer. They are mostly porous carbonates – dolomites and limestones.
Some of the water wells in the Silurian aquifer in Iowa have yielded more than a million gallons per day for 40 years, but flows are pretty variable and a few hundred thousand gallons per day per well is probably more common. In the 1980s in Iowa, especially northeastern Iowa, two-thirds of the water from this aquifer was used for domestic and commercial water supplies, and about a quarter went to agricultural use including irrigation. The total was about 130 million gallons a day in 1985.
—Richard I. Gibson