A contamination problem caused by Ca⁺² ions, usually occurring in fresh water, seawater and other low-salinity and low-hardness mud systems. Soluble calcium comes into a mud from various sources: gypsum- or anhydrite-bearing strata, unset cement and hardness ions in make-up water or from an influx of formation water. Ca⁺² can flocculate colloidal clays and precipitate large anionic polymers that contain carboxylate groups, such as an acrylate polymer. On the other hand, some mud types tolerate calcium, in which case calcium is not considered a contaminant.