A well field drawing from an aquifer that is replenished by a lake or perennial stream is in a steady-state condition, provided that the aquifer is never dewatered no matter how long pumping continues. An economic problem, the common pool problem, is more complex than heretofore recognized for, although the private costs for any well fail to recognize the interference it causes to other wells, the interference of other wells is reflected in the costs of the first well. The excess of marginal social costs over marginal private costs can be corrected (to equate the two) by a pumpage tax on each well that is proportionate to the socially optimal offtake from other wells. (Author)