What is an aquifer?

An aquifer is an underground layer of soil, sand, gravel or rock saturated with water. Water is held there in natural storage. Aquifers generally hold sufficient water to be used as a water supply. There are two basic types of aquifers in the Great Miami River Watershed:

  • Buried valley aquifers – a glacial deposit largely consisting of sand and gravel with water contained in the spaces.
  • Bedrock aquifers – a solid formation of rock lying beneath the surface of the soil containing enough water in its cracks and small spaces to provide water for wells.

The Great Miami River and some of its tributaries are located along the path of the buried valley aquifer. Most cities in the watershed depend on the high volume wells that are drilled into the permeable sand and gravel material of the buried valley aquifer for their drinking water.

Bedrock aquifers are the source of many private drinking water wells in the Great Miami River Watershed and generally produce enough water for residential home use.

There are some important terms that describe the different areas that water can be underground. Great Miami Buried Valley Aquifer and other unconsolidated aquifers are made up of loose, coarse sediments such as sand and gravel. In this type of aquifer, groundwater moves between the sediment particles. In general, groundwater flows from high areas to lower areas and generally mimics the direction of flow of the streams and rivers that lay above it. In bedrock aquifers, groundwater can move between the sediment particles (in cases like sandstone) or it can move along fractures within the rock. In the Great Miami River Watershed, you would not find underground caves filled with flowing water, though there are a few places in Ohio where you would find this type of geology.

One frequently used term is water table. The water table marks the highest level of the groundwater beneath the ground’s surface.

The area between the ground’s surface and the water table is called the unsaturated zone, or zone of aeration. In this zone, you will find some water, but you will also find air filling the spaces between soil and rock particles. Any water in this area, called soil moisture, could be used by plant roots. The entire area below the water table is the saturated zone. This zone marks the area where every pore or crack in soil and rock is completely filled in (saturated) with water.