Gypsum (CaSO4) is a water soluble mineral that can be found in fertilizers and mixed in many other products. It is pH neutral, therefore, its application does not have the risk associated with alum treatments. Gypsum works for water clarification by attracting clay particles together to form clumps or what we call floccules. As the clay particles continue to clump they eventually increase in weight and settle to the bottom of the pond of lake.
Gypsum is typically applied at a rate of 1,000-2,000lbs per Surface acre.
Apply gypsum so that it is completely and quickly mixed with the pond water. It should be premixed and pumped or sprayed as a surface slurry or distributed into the propeller wash of an outboard motorboat driven at high speed around the pond. If effective, the treatment should clear the pond in a few days and it should remain clear for months. If not effective, increase the dosage and frequency of gypsum applications.
Since the exact application rate varies, concentrations for each pond can be determined by completing an experimental treatment of pond water samples. Samples can be held in jars or buckets. (small scale test) Then, you take the level of gypsum that caused the clay particles in the bucket or jar to precipitate within a day to calculate the treatment of the pond volume. Gypsum will provide only temporary relief if the source of the problem, eroded soil particles, is not eliminated by proper land management practices.
Source Virginia Tech
For more details on maintaining ponds and water testing methods see the research papers at the top of this page.
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