What is source water?

The EPA broadly defines source water as, "Surface water (streams, rivers, and lakes) or ground water (aquifers)..." that serve as sources of drinking water.  This water "...provides...for public drinking water supplies and private water wells."


Source Water in Pennsylvania

Surface water vs. Groundwater

A visual representation of Pennsylvania's surface and groundwater supplies.     Image from DCNR. 

A visual representation of Pennsylvania's surface and groundwater supplies.

Image from DCNR. 

The water we rely upon to live our lives - the water that we use to brush our teeth, wash our cars, and fill our water bottles - is fresh water.  Pennsylvania has an incredible supply of fresh water, all of which is stored in one of two places: above ground or below the surface.  You are probably quite familiar with the fresh water that occurs above the surface, as it fills every pond and runs in every river; we see it daily.  However, you may be less familiar with the fresh water that occurs below the surface, also called groundwater.  Groundwater is normally found in the microscopic spaces between grains of sand or gravel that make up the bedrock deep beneath our feet.  The layers of rock that contain large quantities of water are referred to as aquifers.  Though it is rarely seen and hard to imagine, Pennsylvania's aquifers contain more than 30 times as much water as can be found on the surface.  If brought to the surface, this water would cover the entire state in over 8 feet of water*!

Of the approximately 12 million Pennsylvanians, approximately half have residential water sourced entirely from the surface supply (rivers, lakes, etc.).  The remaining population has at least a portion of their water supplied by groundwater*.

Source water in state college

the importance of groundwater

State College is located in the Spring Creek Watershed, a map of which can be seen below.  This watershed is primarily underlain by carbonate rocks (such as limestone) that are easily eroded by rain water, which is naturally slightly acidic.  The erosion of these rocks creates a subsurface that looks a little something like a block of Swiss cheese.  Geologists refer to this as "karst geology", and it has both positive and negative implications for those of us living in the area.  When enough of the bedrock erodes at a given location, the result is a sinkhole, which can be inconvenient at best, and quite dangerous at worst.  However, this pattern of consistent erosion also gives rise to beautiful caves (think about Penn's Cave and Lincoln Caverns), and helps to create large underground aquifers with substantial water storage capacities.  In fact, approximately 99% of the drinking water in our watershed is pumped from these groundwater aquifers.  

Connections to the surface

Unlike surface water, groundwater does not travel great distances between sources and sinks.  Instead, most of the groundwater aquifers near State College are fed by precipitation and surface water from the surrounding area.  This water moves from the surface to the subsurface via two major pathways: it can percolate through the soil, or it can move in much larger quantities through sinkholes and bedrock fractures.  In regions without karst geology, almost all aquifers are fed by the infiltration of precipitation through soil.  This process is generally quite slow, allowing the soil to act as a natural filter, removing any bacteria, nutrients, or toxins the water may have picked up while moving over the land surface.  In regions possessing karst geology (like central Pennsylvania), the soil's natural filter is often bypassed as the water is transported through sinkholes and fractures.  There are many places where the entirety of Slab Cabin Run disappears below the ground's surface!


Transport occurs in the opposite direction as well.  Water leaves the aquifer and returns to the surface via springs and direct inputs to surface water bodies.  In fact, Spring Creek is so-named because it is heavily influenced by spring flow: over 80% of the water in Spring Creek originates from a spring.  This spring water is generally harder, cooler, and higher quality than surface water, providing habitat for our robust trout populations and the macroinvertebrates they feed upon.


the spring creek watershed

The surface basin of the Spring Creek Watershed is delineated above by the dark black line.  Each of the smaller subwatersheds (Slab Cabin run, for example) are shown by the various colors.  The are that contributes groundwater is shown by the hashed black line.  Note that the area that contributes groundwater is quite a bit larger than the surface basin.

Image from Spring Creek Watershed Commission, Spring Creek Watershed Association, and ClearWater Conservancy.


*"The Geology of Pennsylvania's Groundwater", PA DCNR