Reverse osmosis treated brackish water is the sole source of potable water supply for the City of Cape Coral, Florida. Two low pressure membrane treatment plants provide a total of approximately 14 MGD of treated water. The raw water supply originates in the Hawthorn and Suwanee aquifer systems. During the 14 year operating history of the wellfield, a number of problems have been encountered and solved. Several of the original production wells had to be modified in order to reduce the silt density index of water being received at the plant. Some encrustation of the open-hole portions of a few of the production wells has occurred. This crust had to be removed by treating the wells with hydrochloric acid. In addition, detailed monitoring of area water levels and water quality is conducted because of the potential for water quality deterioration from vertically adjacent aquifers or from anomalous poor water quality within the production formation. The supply of water from the Hawthorn and Suwannee aquifer systems is limited based on both drawdown of pressure and water quality. In order to quantify the "safe yield" of the aquifer system, a hydrogeologic assessment began in 1989. Preliminary results of the investigation show that the quality of water in the lower part of the Suwannee aquifer system in the western part of the city is substantially poorer than previously anticipated. Modeling results will show a slow, long-term increase in the dissolved solids concentration in the feedwater caused by vertical migration of poorer quality water.