About 43 million gallons per day of ground water was used in 1985 by the 150,000 people living in the 10 valleys of central Pennsylvania that are underlain by Cambrian and Ordovician carbonate rocks. Noticeable effects on water levels from withdrawals occur in the vicinity of the Borough of State College where an average of 8.1 million gallons per day is pumped. The carbonate aquifer system is heterogeneous and anisotropic parallel to the regional strike. The Nittany and Gatesburg Formations are the most prolific sources of water and will yield 1,000 gallons per minute or more to properly located and constructed wells. The Coburn through Nealmont Formations are the poorest sources of water but wells in these rocks will yield about 50 gallons per minute. Of the 42 large perennial springs identified, 25 have flows that commonly exceed 1,000 gallons per minute, and flows of 3 of the largest springs at times exceed 10,000 gallons per minute. Water budgets for Kishacoquillas and Spring Creeks, which are representative of all the valleys, indicate that an annual average of 0.62 and 0.80 million gallons per day per square mile, respectively, are available for water supply. Nitrate, chiefly from agricultural sources, averages from 3.3 to 5.9 milligrams per liter in ground water from individual valleys. Triazine herbicides at extremely low concentrations were present in 11 of 20 well and spring water samples.