TECHNICAL BMP MANUAL &
INFILTRATION FEASIBILITY REPORT:
Infiltration of Stormwater in Areas Underlain by Carbonate Bedrock within the Little Lehigh Creek Watershed
Lehigh Valley Planning Commission
Lehigh County, Pennsylvania
Cahill Associates developed a variety of special tools for the 107.5 square-mile Little Lehigh Creek Watershed in Lehigh County, as part of the expanded Act 167 Storm Water Management Plan project. Working for the Lehigh Valley Planning Commission, the first challenge was to develop technical guidance explaining how to accomplish infiltration of stormwater in carbonate geology - without creating subsidence and sinkholes that have plagued so much of the Watershed. A second important issue has been protection of both groundwater and surface water quality, given the extreme sensitivity of limestone aquifers to contamination.

Another major objective of this project was to develop specific engineering guidance that describes in as much detail as possible how to implement the various approaches to infiltration - specific Best Management Practices - which are available for development sites in the Little Lehigh Creek Watershed. Applicants need detailed, though straightforward, guidance - a BMP Infiltration Manual - which explains how to do what, where and when. Finally, all of this is tied together by a unifying watershed strategy, designed to achieve an ambitious "no increase" water quality objective in this Watershed already suffering from a variety of water quality problems.

Carbonate geology is both a blessing and curse. For some time, the fear of sinkhole creation has tended to convince many that water should simply be kept away from limestone, even though carbonate aquifers are wonderfully rich. Furthermore, the removal of water from carbonate, in whatever manner, ultimately weakens the rock and promotes subsidence itself and so is hardly a sound strategy. The management solution stated simply is to use infiltration BMPs carefully, in a way that mimics the natural hydrologic regime to the maximum extent through broad and even distribution of stormwater or wastewater effluent. If this approach is employed and other basic precautions are taken, sinkhole creation can be minimized and water quality can be protected.

Seven carbonate formations - of varying degrees of risk - underlie the Little Lehigh Creek Watershed.

Design guidance matrices for carbonate geologic formations in Little Lehigh Creek Watershed.