EFFECT OF P AND SE(IV) ON CALCITE PRECIPITATION INHIBITION.

We studied the role of inorganic P and Se(IV) in calcite precipitation inhibition in a Panoche Creek, California, soil. Calcite precipitation inhibition by inorganic P and Se(IV) was first examined in simple Ca^2+-Na^+-Cl^-HCO3- solutions. In these simple solutions, the presence of small quantities (~0.2 mg L^-1) of inorganic phosphate (similar to those typically found in soil solutions) completely inhibited calcite precipitation over the short time period examined. Se(IV) had a similar effect, though its concentration would need to be much higher to exhibit an equivalent degree of precipitation inhibition. A linear relation was found between relative precipitation rate decrease and P [or Se(IV)] concentration in solution when pH and pCO2 were held constant. The inhibition of calcite precipitation by P and Se(IV) was further investigated in more complex real and synthetic soil solutions. Calcite and gypsum saturation states in Panoche Creek, California, soil solutions were examined, and two soil horizons were found to be supersaturated with respect to calcite. Gypsum supersaturation was not observed. Calcite precipitation inhibition experiments using synthetic supersaturated Panoche Creek soil solutions indicated that soil solutions with (greater than)0.2 mg L^-1 inorganic P could completely inhibit calcite precipitation. Se(IV) concentrations in Panoche Creek soil solutions were too low to significantly inhibit calcite precipitation. Calcite precipitation inhibition by Mg, Si, S, Ba, and Sr in synthetic soil solutions was not observed.