Book excerpt: The purpose of this study is to provide tools for water-resource management and land-use planning; to accomplish this purpose we (1) characterize the relationship of geology to ground-water conditions in the Glen Canyon and the unconsolidated valley-fill aquifers, (2) classify the groundwater quality of the Glen Canyon (east of the valley only) and valley-fill aquifers to formally identify and document the beneficial use of ground-water resources, and (3) apply a ground-water flow model using a mass balance approach to determine the potential impact of projected increased numbers of septic-tank systems on water quality in the valley-fill aquifer and thereby recommend appropriate septic-system density requirements to limit water-quality degradation