Big Spring Watershed Association 2007 by Eugene P. Macri Jr., Aquatic and Environmental Scientist

The geology of Big Spring Creek is known as Karst. Karst topography or geology as it is called is usually found in large limestone or dolomite formations. Karst topography is characterised by sinkholes, springs, caves, karst windows, and underground streams. These features are directly related to the ground-water system. The massive flow of Big Spring is the result of these karst features. Big Spring is the fifth largest spring in the Cumberland Valley. Meauresments by the PGS (Pennsylvania Geological Survey) and the USGS (United States Geological Survey) show the following flow data in gallons per minute:

Maximum	Medium	Minimum
15000	12500	6000

In 1986 Smith of the PGS measured the flow of Big Spring at 17,000 gallons per minute. In that study (1986-1987) the lowest flow was measured at 11,000 gallons per minute which occurred in late September and early October. In 2002 due to a prolonged drought the flow of Big Spring was measure below 6000 gallons per minute (DEP).

Root and Becher investigated the hydrology and flow characteristics of the Big Spring watershed and came to some startling “revelations.” The basin’s recharge rate cold not account for the tremendous flow of Big Spring. Root and Becher came to the following conclusions which were backed up by Smith and Kochanov (PGS, 1985) and Chichester (USGS, 1996): 1) a north-south conduit transfers water to Big Spring and 2) Big Spring steals water from the Yellow Breeches. “ A portion of the Yellow Breeches Creek south of Big Spring flows acrosss its own groundwater divide into the Big Spring groundwater basin. A well-know aspect of carbonate hydrogeology is the difficulty in determining groundwater-flow dirctions and recharge areas (Smith and Kochanov, 1985).” This is one of the reasons that portions of the Breeches near Walnut Bottom and elsewhere sometimes goes dry in the summer.

But even these conclusions do not account for all the flow of Big Spring. From the USGS and PGS data Macri (2001) analyzed and calculated that local recharge accounts for only 20% percent of the flow into Big Spring. The other 80% comes from the combination of the Breeches, the north-south conduit and other areas near the flank of coluvium of South Mountain. Further evidence of the small local recharge area is found in specfic conductance measurements of local wells in the Big Spring area. Well data did not show similar specific conductance with that of Big Spring which would indicate local recharge. In other words, Big Spring has a different “specific conductance signature.” The greater the local recharge the more similar the specific conductance.

The diagram below illustrates some of the features of karst topography. Karst topography is vulnerable to all types of pollution. This contamination can come from a variety of sources including farms, industry, construction, spills, septic systems, landfills, quarrying, stormwater, and other pertubations. Since these pockets of water are not filtered as in a sandstone base the complexity of these problems can be lethal to wells and springs in the area. The flow velocities of these underground conduits approach that of surface streams. The pollutants may also stay in areas for years until disturbed by blasting or natural subsidence. Furthermore, the withdrawl of water, development, and anything that would interfere with the underground flow poses a threat to this spring creek. The karst geology makes Big Spring one of the great streams in the world but the same karast topography is also its vulneribility.

This is one of the reasons that the watershed association is pursuing dye trace tests, to discover any vulnerabilities, and suggest remediations if needed to further protect Big Spring.