Gypsum dissolution is not a universal integrator of 'water motion'

The dissolution of gypsum or plaster of Paris has been widely used as an in expensive integral measure of 'water motion' in the field and in laboratory tanks for studies of physical-biological interactions. Commonly, gypsum-di ssolution rates have been calibrated to steady flow speed or velocity in th e laboratory and the calibrations have been applied to dissolution (i.e., m ass-transfer) rates in the field or in tanks. We evaluated the,gypsum-disso lution technique in a steady-flow a fluctuating-flow, and a mixed-flow envi ronment by comparing dissolution rate to direct flow measurements with an a coustic Doppler velocimeter. We found that dissolution rates were related t o steady flow and to fluctuation intensity in the exclusively steady-how an d fluctuating-flow environments, respectively The relationships were weak i n the mixed-flow environment. Finally, dissolution and thus mass-transfer r elationships were different in each flow environment, and the effects of st eady flow and fluctuation intensity were not additive. Providing that it is rigorously checked and appropriately calibrated, the dissolution technique can be used to measure steady flow speed or fluctuation intensity in a ste ady-flow or fluctuating-flow environment, respectively. However, comparison s of dissolution rates between steady-flow, fluctuating-flow, and mixed-flo w environments or within environments that change over time to determine wa ter motion will be misleading. The gypsum-dissolution technique can be used as a good direct indicator of mass-transfer rates. However, mass-transfer rates are different in different flow environments. The gypsum-dissolution technique is not a universal integrator of 'water motion'.