Water supply reservoirs in urban areas need protection from potential loading of toxic chemicals associated with industrial and commercial land uses, as well as transportation spills. As it is infeasible to eliminate all uses of toxic chemicals from developed watersheds, management strategies attempt to eliminate potential sources from the most sensitive areas. In many jurisdictions this goal is implemented through zoning restrictions which prohibit commercial/industrial activities within a certain fixed distance of the reservoir pool. This simple approach, however, often imposes unnecessary restrictions on some landowners, while failing to provide uniform protection to the water supply. Land use restrictions are never popular, and should be targeted toward the most sensitive potential source areas. A quantitative, geographic assessment of relative risk from all points within the watershed can be used to maximize protection from zoning while minimizing restrictions on economic growth. A steady-state modeling approach, as presented here, provides a rational basis for developing zoning restrictions for source water protection.