Interaction between percolating groludwaters and aragonitic carbonate sediments within the vadose zone of the coral cap of Barbados, West Indies, results in dissolution of aragonite and concurrent reprecipitation as low-magnesium calcite. Comparison of the ratios of strontium to calcium in groundwater, aragonitic carbonate, and recrystallized calcite indicates that locally the solution-reprecipitation process is operating at an efficiency greater than 90 percent.