Computing SI and CCPP using spreadsheet programs

Lotus 1-2-3® worksheets for calculating the calcite saturation index (SI) and calcium carbonate precipitation potential of a water sample are described. A simplified worksheet illustrates the principles of the method, and a more complex worksheet suitable for modeling most potable water sources is briefly described. Both worksheets include ion pairing and adjustments of equilibrium constants for temperature and ionic strength. For several test cases with a wide range of temperature, alkalinity, pH, and calcium concentration, the worksheet SI values differed <1 percent from those computed by WATEQX. For water quality modeling, spreadsheets have several advantages compared with large equilibrium codes: flexible input and output, ease of maintaining the thermodynamic database, and relatively small memory and mass-storage requirements. Modeling of archived chemical data is possible through the combination of database management and computational capabilities of many commercially available programs. The worksheets can be obtained from the authors.