Summary #
Groundwater is water stored in underground aquifers — rock and soil formations that supply wells, support stream base flow, and serve as a primary drinking water source for Pennsylvania residents. Groundwater is vulnerable to contamination from coal mining, agriculture, and urbanization. Understanding groundwater is essential for assessing AMD impacts and watershed health.
Ground Water #
Groundwater is found underground. Aquifers carry most of it, though some is accessible at the natural water table. Groundwater is generally very pure, clear, and free of most sediment — the many layers of earth and rock it passes through effectively filter most pollutants. Some pathogens and dissolved solids such as calcium carbonate and iron may still be present.
Groundwater discharging into streams is what keeps streams flowing steadily year-round. Except during rainfall or snowmelt events, most of the water in a stream comes from groundwater seeping through stream banks and beds — this is called base flow.
Aquifers #
For groundwater to reach an aquifer, it must percolate down through layers of soil and rock (the zone of unsaturation) to reach the zone of saturation, where all spaces between rock particles are filled with water. The top of this zone is the water table.
Rock and soil layers that easily store and transmit water are called aquifers. Aquifers vary enormously — from a few acres to several states in size, and from a few feet to hundreds of feet in depth. Rock layers impermeable to water are called aquitards. An aquifer contained above and below by aquitards is a confined or artesian aquifer. One with no upper aquitard is unconfined.
Pennsylvania has four principal aquifer types: sand and gravel, sandstone and shale, carbonate rock, and crystalline rock. The state is predominantly sandstone and shale. Sandstone aquifers tend to produce softer water; shale tends toward harder water. Water hardness refers to calcium carbonate content.
Water Use #
Almost half of Pennsylvania’s groundwater use is for domestic water supply. Rural residents commonly rely on wells drawing either from the water table (shallow wells) or directly from an aquifer (artesian wells). Shallow wells are more vulnerable to running dry during droughts. Over-pumping of aquifers can reduce stream quality by decreasing groundwater available for base flow.
Land Use Threats to Groundwater #
Four categories of activity threaten groundwater: waste disposal, agriculture, resource extraction, and urbanization.
Coal mining is especially harmful. Mines often intersect aquifers, draining them — then that drainage seeps through the mine into other aquifers, carrying iron, manganese, and sulfate contamination. Many Pennsylvania aquifers are no longer suitable for drinking due to mine contamination.
Oil and natural gas drilling can release brine (salt water) into fresh water aquifers. Septic systems and waste disposal facilities can leach contaminants into groundwater. The primary agricultural threat is nitrogen from fertilizer runoff. Urbanization creates impervious surfaces — roads, roofs, parking lots — that prevent water from percolating into the ground, reducing aquifer recharge and increasing surface runoff.
For accessible explanations of groundwater topics, see Aquatic Resources of Pennsylvania from KARE (Keystone Aquatic Resource Education Program) and the PA Fish and Boat Commission.
Understanding Groundwater, University of Nebraska Cooperative Extension: http://ianrpubs.unl.edu/Water/g1128.htm#OCCOGRNDW
Related Pages #
- What is a Watershed?
- Water Cycle
- Drinking Water
- Impacts of AMD (AML and AMD Basics)
- Mining Methods and AMD (AML and AMD Basics)
Source and Last Reviewed #
Source: http://ianrpubs.unl.edu/Water/g1128.htm#OCCOGRNDW
Last reviewed: 2026-03 | Links may require verification — originally published pre-2010.
Tags: monitoring, assessment, practitioner, volunteer, pa
