A Unique Karst Geology: 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 characterized 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 Pennsylvania. Meaurements by the PGS (Pennsylvania Geological Survey) and the USGS (United States Geological Survey) show the following flow data in gallons per minute:
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). Flow is now gauged continuously by the U.S. Geological Survey.
Based on measured precipitation and evaporation, The surface basin’s recharge does not account for the tremendous flow of Big Spring. Fluorescent dye traces done since 2005 have begun to determine the source waters and areas for Big Spring, which extend far outside the surface basin toward Shippesnburg.
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 karst 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.