INFLUENCE OF HYDROGEOLOGIC SETTING, INCLUDING LINEAMENTS, ON WATER WELL YIELD IN LEBANON AND DAUPHIN COUNTIES, PENNSYLVANIA

This report summarizes highlights of the M.S. Geology thesis of Jeremy Robinson. The study area is in southeastern Pennsylvania, and lies mostly within the Great Valley, Valley and Ridge, and Piedmont physiographic provinces. About 1,100 water supply wells were investigated for impacts of hydrogeologic setting on driller's well yield. The wells were subdivided into three different populations based on cumulative frequency probability plots of the common log of well yield. The two largest well groups, for domestic or agriculture (DA) supply wells and for business and large- diameter public (BLDP) supply wells, were statistically analyzed for yield effects. BLDP supply wells have the highest median yield (50 gpm) which is ~4 times that (12 gpm) for DA supply wells. This probably reflects optimum well siting and drilling depth associated with BLDP wells with their higher water supply demands.

Clastic rock terranes have twice the average well yield of carbonate (Cambrian-Ordovician) rock terranes, for both BLDP and DA well groups. Topographic setting affects BLDP well yield, with much higher yield in valleys than on hill tops, for limestone and clastic rock terranes, but not for dolomite terranes. Shallow wells (<400 ft deep) have lesser yield than deep wells for BLDP wells, but the opposite is true for DA wells where strata >200 ft deep produce little water. Faults are strongly associated with higher well yield, especially strike-slip faults in carbonate terranes and thrust faults in clastic terranes.

Short straight lineaments (SSLs), which are <2km long (similar to fracture traces), are strongly associated in close proximity with higher well yield for most rock terrane types, but only for BLDP wells. The best SSLs for wells are perennial stream segments. DA well yield does not relate to SSL proximity. Curvilineaments generally have weaker well yield trends than SSLs. LANDSAT lineaments generally relate strongly to higher well yield in close proximity, except for DA wells in clastic terranes. Both Harold Meisler and Richard Parizek appear to be right concerning the usefulness of SSLs for high well yield in the study area; Meisler said fracture traces are not useful (generally the case for DA wells) whereas Parizek said that fracture traces are useful (the case for BLDP wells).

Higher cave density and low karst depression density associate best with high well yields.