I suggested to Tracy that we take a sample of his water and do the alum experiment, but he did not provide a sample. So we are going off of his numbers here.
OK, lets determine exactly what 4 grams per gallon equates to in lbs per acre-ft.
4g/454 = .0088lbs
.0088lbs per gallon x 325829 (gallons/ac-ft ) = 2867lbs per ac-ft = 57 50# bags per ac-ft.
Laboratory and pond setting are different of course. Most importantly, in lab setting you are able to be precise and are able to mix 100% of the alum in a gallon sample. Also there is no physical disturbance to interfere. However in a pond setting, some product may settle to the bottom and not dissolve 100%, plus there is physical disturbance in the equation. So in order to achieve the same results that Tracy measured in his sample, he may need slightly more than the experiment suggested.
I do not have immediate access to my water quality records for Tracy, but his alkalinity is high and hardness is low, and there is a significant difference between those values. Tracy please fill in here with your water quality values if you like.
Here is an excerpt from SRAC Publication#4604, Managing High pH in Freshwater Ponds, Balance the hardness and alkalinity
Problems with high pH seem to occur
most often in ponds where total alkalin- ity (the amount of bicarbonate and car-
bonate in the water) far exceeds water hardness (the amount of calcium and magnesium in the water). For example, it is common for freshwater prawn ponds at the Mississippi State Uni- versity aquaculture unit in Starkville
to have high pH in late spring. The groundwater supply for these ponds has a hardness of about 30 mg/L as CaCO3 and an alkalinity of about 90 mg/L. An even wider disparity between hard- ness and alkalinity is found in many other waters, particularly those in the southeastern coastal plains where many groundwaters have alkalinities exceed- ing 150 mg/L and hardness values of less than 10 mg/L.
De ciencies in hardness relative to alkalinity can be corrected by adding gypsum (calcium sulfate). The effective- ness of gypsum treatment in reducing pH is subject to debate; at best, it is a preventive procedure rather than an emergency treatment. Hardness de - ciencies should, therefore, be corrected before stocking, preferably as soon as the pond is lled in the spring.
The amount of gypsum needed to roughly balance hardness and alkalinity can be calculated by subtracting hard- ness from alkalinity and multiplying that value by two. For example, if hard- ness is 30 mg/L as CaCO3 and alkalinity is 90 mg/L as CaCO3, then 120 mg/L
of gypsum will be needed. This would require about 2,500 pounds of gypsum in a 2-acre pond that is 4 feet deep. This is a large amount of gypsum, but the re- sults of treatment should be long-lasting because calcium is lost from ponds only when waters are diluted by excessive rainfall or by the addition of water with a low calcium content.
Increasing the calcium level in a pond by adding gypsum may help reduce
the occurrence of high pH and bene t animals by helping them respond bet- ter physiologically to pH extremes and other environmental stressors. Rela- tively high levels of calcium also help crustaceans, such as freshwater prawns, to replace calcium lost during molting.
Add alum or an
organic substance
It is dif cult to reduce pH signi cantly by adding an acid to the water because
pond waters are usually buffered by bases of the alkalinity system. Rela- tively large amounts of acid are there- fore needed to achieve a meaningful decrease in pH. Also, adding an acid
to water is only a short-term solution because it addresses the result rather than the cause of the problem, which is rapid plant growth.
An emergency treatment that quickly reduces high pH is the application of alum (aluminum sulfate). This is a safe, relatively inexpensive chemical that reacts in water to form an acid. Besides reducing pH, alum also occulates and removes algae by sedimentation, thus decreasing algal biomass and reducing photosynthesis. Alum may also help
to reduce pH indirectly by removing phosphorus�an important nutrient for plant growth.
Alum does not have a permanent effect and it may need to be applied more than once until plant or algal growth decreases. A precise reduction of pH through the addition of alum is dif- cult because response is in uenced by a number of conditions in the pond, especially the water�s total alkalinity. Overtreatment with alum can cause a dramatic decrease in pH, possibly to levels more dangerous than the original high pH problem.
Experience dictates a cautious ap- proach, starting with an initial dose of 10 mg/L alum (27 pounds of alum per acre-foot of water) followed by ad- ditional applications in 5- to 10-mg/L increments as needed. Alum should not be used in waters with total alkalinities of less than 20 mg/L as CaCO3 because even small amounts may reduce pH to dangerous levels.
A safer, longer lasting way to reduce high pH is to add carbon dioxide, which acts as an acid in water. Carbon dioxide levels can be increased by adding or- ganic matter such as cracked corn, soy- bean meal or cottonseed meal to ponds. As organic matter decays, it releases carbon dioxide. This method does not reduce pH immediately, but it is a safe and relatively dependable practice that yields results rather quickly. Generally, applying about 15 pounds per acre daily for about 1 week should prevent pH from rising to undesirable levels. This
amount would be in addition to any daily application of organic fertilizer already planned. The total daily applica- tion of organic matter should not exceed 50 pounds per acre. The decay process that releases carbon dioxide into the water also uses dissolved oxygen, so adding too much organic matter could reduce dissolved oxygen concentrations to dangerous levels. Dissolved oxygen concentrations must therefore be mea- sured regularly and the pond aerated,
if necessary, to maintain satisfactory oxygen levels.
