Summary: A conceptual model of the
ground water flow system
The boundary of the
region referred to as the Tunkhannock Creek Watershed is delineated based on
surface topography. Hilltops along the perimeter of the watershed act
as surface water divides. Water on one side of the divide will flow
toward the Tunkhannock Creek, and water on the opposite side will flow into
an adjacent watershed. The topography, geology, and size of the
Tunkhannock Creek Watershed suggest that the surface water divide also
approximates a ground water divide. Ground water recharge on the
Tunkhannock Creek side of the divide will ultimately discharge into the
Tunkhannock Creek. Surface water and ground water flow out of the
watershed at the confluence of the Tunkhannock Creek and Susquehanna River.
During an average
year, 299 billion gallons of water in the form of rain, snow, and sleet
fall within the 413 square mile Tunkhannock Creek Watershed; 127 billion
gallons are returned to the atmosphere by evaporation and transpiration.
Fifty-three billion gallons move as surface water runoff into Tunkhannock
Creek. Approximately 110 billion gallons enter the ground water
reservoir. About 91 billion gallons of ground water eventually discharge
into Tunkhannock Creek, where it accounts for 63 percent of surface water
flow from the watershed. The vast quantity of water passing over the
surface and through the subsurface of the watershed on an annual basis is
only a small fraction of the approximately 3 trillion gallons stored in the
ground water reservoir.
A dendritic drainage
network densely dissects the flat-lying sandstone, siltstone, and shale
bedding of the Catskill Formation that underlies the watershed surface.
Till partially fills many stream valleys and armors most hillsides
throughout the watershed. The till cover in conjunction with short
distances between hilltops and valley streams suggests that most groundwater
flow paths are local (i.e., shallow and short in length and duration).
The largest volume of ground water flow and storage is in bedding plane
parallel fractures and orthogonal sets of near-vertical joints in the
Catskill Formation. It is probable that this secondary porosity decreases
significantly below a depth of about 400 feet. More than 99 percent of the
more than 1400 wells in the watershed draw water from the Catskill
formation. Typical well and aquifer characteristics included well depth =
257 feet, depth to bedrock = 50 feet, yield = 23 gallons per minute,
specific capacity = 0.6 gallons per minute per foot, and transmissivity =
700 gallons per day per foot.
In addition to
sustaining stream flow, ground water plays the important role of sustaining
much of the human population within the watershed, as indicated by the
intensity of ground water development. Detailed aquifer studies
aimed at examining the immediate or potential effects of potable water
withdrawal from and disposal of wastewater into the ground water system are
lacking. Long-term regional changes to the ground water system in
response to climate change are certain but presently poorly constrained by
existing climate models.
