Well Forecasting and Aquifers in Iowa
by Paul VanDorpe
What constitutes a good "well forecast"? Location: where is the property located on the land surface? -- a precise dot on a map is a good start. Location: what is the land surface elevation? -- this may be determined from a topographic map. Location: what areas of the property are available for well development? -- free from obstructions and sanitary impediments. Location: what aquifers are available and what are the properties of these aquifers? -- depth (and/or elevation), thickness, water level, potential yield, water quality, special circumstances. Use: what is the water being used for? -- domestic, livestock, irrigation, commercial, industrial, municipal, public -- each of these may have different concerns, such as desired yield, water quality, aquifer restrictions, or potential well interference issues.
The aquifer characteristics, as mentioned above, determine the usefulness of any geologic unit as an aquifer. In granular aquifers, such as sand and gravel and sandstone, water movement occurs in the spaces between the grains; this is called primary porosity. In carbonate rocks (limestone and dolomite), primary porosity is very low. Water movement is mainly along various fractures -- bedding planes, joints, faults, solutional openings and cavities; this is termed secondary porosity. Carbonates with low secondary porosity, shales, siltstones, and glacial till are aquitards; these are the confining units between aquifers. Aquifers are confined when there is a unit of lower permeability (ability to transmit water, not just pore space) above the aquifer.
What aquifers are available in Iowa? Alluvial aquifers, composed of sand and gravel, occur along most major rivers and streams. These aquifers are unconfined. Sand and gravel units also occur within the glacial drift, which occurs over almost all of the state. These aquifers are usually of limited aerial extent and are confined by the glacial drift. Buried channel aquifers are sand and gravel units that occur at the base of the glacial drift along some of the major bedrock valleys. These are confined and more extensive than other sand and gravel aquifers within the glacial drift. The Dakota Sandstone in the lower Cretaceous is called the Dakota aquifer; it is an important aquifer in western Iowa. Pennsylvanian System rocks, covering 40% to 45% of the state, contain sandstone and limestone units which are rarely utilized as aquifers. These are confined by shales, less permeable limestones, and siltstones. The Mississippian aquifer, from southeast to north-central Iowa, provides water through secondary porosity. The Devonian and Silurian aquifers are carbonate aquifers in hydrologic connection with each other. In eastern Iowa, these two aquifers may be considered as separate aquifers on a local scale. Ordovician carbonates are widely utilized in northeast Iowa where they are at or very near the land surface; in some places these aquifers are unconfined. The Cambrian-Ordovician aquifer, commonly called the Jordan aquifer, underlies most of the state. This aquifer is composed of a very thick carbonate sequence above the Jordan Sandstone, the Jordan Sandstone itself, and carbonates rocks below the sandstone. Cambrian sandstones, such as the Mt. Simon Sandstone, are utilized as aquifers in extreme east-central Iowa. These sandstones are deeper and much thicker than the Jordan.
Knowing the geology of the state, through both published and unpublished resources,
aids all researchers in determining which groundwater resources are available at any given
location. This is the crux of well forecasting -- knowing all the characteristics of
Adapted from an article in the Iowa Groundwater Quarterly, Summer 1999