What is Groundwater?

What is Groundwater?

Groundwater is but one stage, or form, through which water passes in the earth's hydrologic cycle. The hydrologic cycle is the continual movement of water over, in, and though the earth and its atmosphere as it changes from one form (solid, liquid, gas) to another.

The water you use today may have evaporated from an ocean, traveled through the atmosphere, fallen back to the earth's surface, gone underground, and from there moved to streams that lead back to the seas. Water is readily visible in many forms (clouds, rain, snow, fog, lakes, streams, oceans, polar ice caps), but as groundwater it is, by definition, out of sight. Our understanding of groundwater and its role in the hydrologic cycle has been hindered by the difficulty of observing and measuring the properties and extent of groundwater.

Long-standing misconceptions about groundwater's origin, occurrence, and movement have by no means prevented people from using it. Groundwater supplies have been tapped for thousands of years, but only recently have we started to understand their characteristics. Much remains to be discovered about groundwater, but wider public awareness of its nature and properties is an important first step.

Water Underground

When water falls to the earth as rain or snow, most of it seeps into the ground. It first passes through the unsaturated zone, where soil pores are filled partly with air and partly with water. Plant roots, bacteria, fungi, insects, and burrowing animals are found in the unsaturated zone. The water flows downward through the unsaturated zone into the saturated zone, where all pores are filled with water. The upper boundary of the saturated zone is called the water table. The water table rises when water enters the saturated zone; the water table falls when water is discharged from the saturated zone (e.g., springs, lakes, or rivers) or by pumping. Water in the saturated zone is commonly referred to as groundwater.


The process by which water (from rainfall, snowmelt, and other sources) flows into a waterbearing geologic formation (aquifer) is known as recharge. Recharge of the saturated zone occurs as water seeps down through the unsaturated zone. The unsaturated zone is important to the groundwater underlying it because incoming water seeps down through the unsaturated zone into the saturated zone removing impurities and helping to cleanse the water. Both the quantity and quality of groundwater can be affected by the condition of the unsaturated zone in a recharge area.

Septic Tank Systems

Septic tank systems are the largest of all contributors of wastewater to the ground and are among the most frequently reported sources of groundwater contamination in the United States. Wastewater from septic tank systems may include many types of contaminants such as nitrates, harmful bacteria, and viruses. Chemical substances commonly used by homeowners such as pesticides, paints, varnishes, and thinners can also end up in the groundwater. Chemical contamination is especially dangerous since it may be permanent. Some chemicals, even in small amounts, are almost impossible to remove from the groundwater once they reach the saturated zone.

Evidence indicates that bacteria and viruses generally are removed in the unsaturated zone. However, in fractured rock where groundwater flow rates can be high, these bacteria and viruses may be transported very rapidly and could contaminate nearby drinking water supplies. It is critical that your well casing is sealed and is separated the required distance from the septic tank system's absorption field area. This will keep the contaminated water from seeping into and mixing with your drinking water.

Distances and Dimensions

A septic tank system must be located a certain distance away from wells, streams, lakes, and houses. In addition to spacing, minimum dimensions have been established for absorption field trenches and the distance between the trenches is specified in local regulations. In order to maintain aerobic digestion processes and effectively remove microorganisms, the absorption field must be separated from groundwater. This is known as the separation distance.

Water use in rural houses can be predicted from the house plans, depending on the number of bedrooms, water using appliances, and potential additions.

Although the actual number of residents determines water use in a house, the house plan suggests the potential number of residents and wastewater flow, which varies from 200 gallons per day to 1,000 or more. This flow estimate, plus the soil permeability estimates (i.e., how easily water moves through the soil), is used to determined the absorption field area needed for your house. Installing a sufficient drainfield will help your septic tank system function longer.

How to Tell if Contaminants Are Reaching the Water

Look for these symptoms to determine if waste from your septic tank system is reaching the surface water:
  • Excessive weed or algae growth in the water near your shore. Nutrients leaking from septic tank systems could be a major cause of this type of growth.
  • Unpleasant odors, soggy soil, or liquid waste flow over the land surface. These symptoms often indicate failure of the system and the need for repairing, expanding, or replacing the absorption field.
  • Water sample test results indicate the presence of biological contamination. These tests may show the presence of harmful bacteria in the water. Although wastes from septic tanks are not the only source of these contaminants, they are likely suspects.
  • Indicator dye put into your septic tank reaches nearby ditches, streams, or lakes. Special dyes are available that may help to find the problems that otherwise are difficult to notice. This method can help verify the other symptoms listed above.
How to prevent problems
  • Regularly inspect, pump, and maintain your septic tank system.
  • Conserve water in your home.
  • Redirect surface water flow away from your absorption field.
  • Do not drive vehicles or heavy equipment over the absorption field.
  • Plant a greenbelt (grassy strip or small, short rooted vegetation) between your absorption field and the shoreline.
  • Keep chemicals and other hazardous wastes out of the septic tank system.