MAC Educational Materials/ 1: Basic Groundwater Terminology

Groundwater— 8 Words to Know

Groundwater is an important natural resource.  In Illinois, nearly 1/3 of the state’s population and 90% of the state’s rural population rely on groundwater for its drinking water.  People understand how vital water is for daily life, but few people understand this hidden natural resource.  The first step in understanding a topic is learning the lingo.  These words are used in discussing groundwater— hydrologic cycle, porosity, hydraulic conductivity, confined and unconfined aquifers, aquitard, potentiometric surface, and water table.

Groundwater is part of the hydrologic cycle (Figure 1), the cycle that describes the global circulation of water from precipitation to runoff, recharge, groundwater flow, streamflow, and finally evaporation and transpiration.  Groundwater is water found in the subsurface, and can be found just a few inches below the ground surface during rainy periods to hundreds of feet in the subsurface.  Groundwater fills the pores between the grains of the subsurface materials.  Porosity defines how much water fills the pores in geologic materials and varies from less than 5% for some bedrock to over 40% for sand & gravels.  Hydraulic conductivity explains how easily water can move through a geologic material and is usually reported in units of cm per sec.  Sand and gravels tend to have high hydraulic conductivity (0.1 cm/sec) while fine grained materials like silts and clays have much lower hydraulic conductivity (0.000001 cm/sec).

Figure 1.  Arrows show the movement of water through a complete hydrologic cycle that includes precipitation, surface runoff into surface water, seepage into the ground, groundwater flow from recharge into discharge, evaporation and transpiration, and condensation.  The triangle indicates the position of the water table.

An aquifer is any saturated geologic material that yields useful quantities of water to wells or springs.  Thick deposits of sand and gravel, sandstone, and fractured bedrock are typically good aquifers.  Clay deposits limit water flow and are generally aquitards or confining layers.  Clay layers are used in landfills to contain wastes.  A sand and gravel deposit close to the ground surface can be an unconfined aquifer (Figure 2a).  The water level of an unconfined aquifer is known as the water table.  A confined aquifer (Figure 2b) differs from an unconfined aquifer in two ways.  First, the aquifer is covered by an aquitard or confining layer.  In addition, the water level of the aquifer is above the top of aquifer.  The water level of a confined aquifer is known as the potentiometric surface.

Figure 2.  Illustration of (a) an unconfined aquifer and (b) a confined aquifer.  The triangles indicate the water level of the well, the water table, and the potentiometric surface.