Groundwater Discharge

In a typical year, about 40 inches of precipitation falls upon Pennsylvania. This is equivalent to about 31 and a half trillion gallons of water. Of that amount, approximately one-third infiltrates the ground and begins its hydrologic journey. Typically, this journey is represented by groundwater flowing toward discharge zones or points at a nearby stream, where it provides the sustaining base flow. Discharge occurs where groundwater emerges at the Earth’s surface. This process takes place almost continuously but is restricted to springs, streams, wetlands, and areas where groundwater is close to the surface and can be tapped by surficial processes such as evaporation and vegetation growth. The geology can have a strong influence on how groundwater discharge is manifested. Below are some examples.

 Ice accumulation from the discharge of groundwater along U. S.  Route 322 in Dauphin County. Pa.

Ice accumulation from the discharge of groundwater from the Duncannon Member of the Catskill Formation along U. S. Route 322 near Clark's Ferry, Dauphin County. A diabase dike outcrops to the left of the ice in the roadcut. Groundwater flow through Peters Mountain is dammed by a diabase dike aquiclude and diverted toward the Susquehanna River water gap. Location of the diabase dike is shown below. In the foreground, the Susquehanna River is receiving much of its flow from groundwater.

--Photo by Gary Fleeger, PaGS


 Demarcation of the diabase area near the Rt 322 roadcut.

Photo of Sucking Spring, Centre County.


This karst spring in Centre County is locally known as Sucking Spring, because of its tendency to drain water from the stream or discharge water to the stream depending on the groundwater level.

-- Photo by Stuart Reese, PaGS


 Groundwater seeping from a rock fracture.

Groundwater seeps from a rock fracture in the Catskill Formation, Clinton County, Pa.

-- Photo by Stuart Reese, PaGS