The basin is underlain by rocks of the Conemaugh, Allegheny, and Pottsville Groups, all of Pennsylvanian age. The Conemaugh Group is further divided into the Casselman Formation and the Glenshaw Formation. All rock groups are composed primarily of flat layers of sandstones, siltstones, and shales that contain thin beds of limestone and coal. The coal has been extensively mined by surface and underground workings. Ground water is present in and moves through these consolidated rocks mostly within fractures.
Movement is from upland recharge areas to points of discharge at local springs, seeps, and streams.
Additionally, one site outside of the basin was sampled. The study area was divided into 40 blocks of about 6 square miles each, and a domestic well within each grid block was selected for sampling. Immediately before collecting a water sample, each well was purged until temperature, dissolved oxygen, specific conductance, and pH stabilized. Raw water samples were collected in the homeowner s plumbing system nearest to the well. All samples were analyzed by the USGS except bacteria, which was analyzed at the Pennsylvania Department of Environmental Protection.