Solid As A Rock. Maybe.
A lot of people think that a rock is a pretty solid thing. But on closer examination, many rocks have cracks and holes that allow water to exist in and pass through a rock – an astonishing amount of water in some cases. It is the holes and cracks – called porosity and permeability – that determine how fast the water moves and how much water can come out of a water well. Think of a sponge – it is the holes in the sponge that fills with water (porosity) and the interconnected pores (permeability) that allow the water to go through the sponge.
Geology plays an important part in the amount of water a well can produce. Again, go back to the sponge analogy. If the holes in the sponge are large and connected, water will flow through faster than isolated small holes. Different rocks will in general have different types of porosity and permeability.
Limestone and dolomite rocks can occasionally have large porosity and permeability – enough to produce underground caverns on occasion and approximate an underground stream. This happens very rarely, though, and only in areas where the geology is dominantly carbonate, such as the Rush Springs Aquifer. Limestones also can have areas of huge porosity and no permeability, or good permeability and poor porosity, depending on how the rock has been fractured. Good water wells may be localized only in certain areas of limestone formations.
Sandstones generally have good porosity and permeability, making a good well. Shales have poorer permeability and well yields are poorer. Geology plays an important part in the area of influence of a well and the spacing of two or more wells. The more porous and permeable the rock, the smaller the radius of influence of the well at a given rate of pumping. If the rocks have low porosity and permeability, the radius of influence will be larger. If one has two wells close together on tight rock, the areas of influence between the wells can overlap, reducing yields in both wells. In central Oklahoma, the Garber-Wellington aquifer can provide groundwater at rates averaging between 150-250 gallons per minute (gpm). Yields on the Hennessey Shale are much lower, usually less than 100 gpm. Alluvial wells, which are usually very shallow, can produce up to 500 gpm.
John Harrington, P.G., CFM
Director, Water Resources Division