Quality Control of Chemical and Isotopic Analyses of Geothermal Water Samples

Chemical and isotopic analyses of geothermal water samples must meet certain levels of accuracy and reliability to be useful for identifying geochemical processes in hydrothermal systems. Quality control is largely a concern for the analytical laboratory, but the geochemist or reservoir engineer using the chemical data must also be concerned with analytical quality. To test accuracy and reliability of analyses available from laboratories, splits of seven water samples were sent to four stable-isotope laboratories, and splits of five water samples were sent to four chemical laboratories. The analyses of each sample were compared among laboratories, and the differences in analyses were evaluated using criteria developed for this comparison. Isotopic compositions were considered reliable if they deviated from mean values by less than 277, for hydrogen and by less than 0.15%., for oxygen. Concentrations of each chemical component were considered reliable if they differed from mean values by less than 10%. Chemical analyses were examined for internal consistency by calculating the error in ionic charge balance and the error between ionic charge and electrical conductivity. To be considered internally consistent, chemical analyses must have less than 5% error in charge balance and less than 10% error in conductivity balance. Three isotope laboratories gave consistent compositions of all samples. No chemical laboratory gave consistent analyses of all samples. Recommendations are made that provide the user of isotopic and chemical data with the ability to better evaluate the quality of analyses.