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Carolina bays are oriented, elliptical land forms of uncertain origin, present over much of the Atlantic coastal plain, and most heavily concentrated in the Carolinas. Ten Carolina bays in Horry County, South Carolina, were studied in an effort to determine their suitability for municipal effluent advanced treatment and disposal. Field studies included vegetation mapping, inventories of threatened and endangered species, substrate cores, and water quality analyses. Reported treatment efficiencies and characteristics of other natural effluent disposal sites were compared with the observed properties of the Carolina bays in order to predict effluent treatment potential and vegetational changes. The Horry County bays are typically peat-filled pocosins, however, a surprising heterogeneity in substrate depth and composition was discovered between adjacent Carolina bays. The nature of the bay substrate apparently plays a key role in determining both the water quality of overflow water from the bays, and their vegetative composition. At conservative hydraulic loading rates (between one to 5 centimeters per week of secondary effluent), the Carolina bays are predicted to provide excellent removal of biological oxygen-demanding substances, suspended solids, and total nitrogen. Some phosphorus removal in the bays is also anticipated because of the potential adsorptive capacity of the peat-based substrate.