Keep in mind that the more alum that is applied per acre foot of water the more the need for buffering of lime or natural lime carbonates. I have applied alum in ponds with fish and Higher alkalinity and hardness (100-250ppm, mg/L) and found it will buffer most suggested applications of alum (100-400lbs/ac) without the need for lime addition. Below 90mg/L alkalinity, I suggest you monitor the pH when adding alum.
As already noted: after - during alum application, there is a chemical reaction that impacts the pH (acidity) of the water. The reaction produces small amounts of sulfuric acid which can decrease pH significantly in some lower alkalinity buffered waters to levels harmful to aquatic life. You can treat with a suggested or minimal amount of alum and wait a day or two to determine if more alum is needed to achieve water clarity 18"-2ft. Therefore, alkalinity and pH should be tested prior to application. Alkalinity should exceed 100 mg/l and pH should be greater than 7.0.
I wouldn't want to suggest trying to "reduce" turbidity if caused by colloidal clay. Alum doesn't dissolve into water unless the pH is 5.4 or lower, and that pH is not good. Above 5.4, Alum sinks to the bottom, and adding more will need to be at a higher application rate than the original application...it isn't a cumulative effect. i.e. If a 1000# rate is required to completely flocculate the colloidal clay, but only 600# is applied on day one... You will likely need 1000# still on day 2's application, for a total of 1600# instead of the original 1000 that would have been needed.
Be sure to ensure you have high quality, guaranteed analysis Hydrated Lime, applied at 40-50% of what alum is applied or you may do what just happened to me...Smoke a pond from a pH drop and be replacing a pond full of fish...
