Pond Boss Forums Thread Like Summary - Alum vs. Gypsum for Clearing Clay Turbidity

We find it difficult to watch the ongoing debate regarding gypsum vs. alum for clearing clay turbidity. It is important to consider the information provided by professionals who have used both methods, many times, in many scenarios, and have settled on a certain method for solid reason. Most of our muddy water samples have low hardness, less than 50mg/liter. When we raise hardness to around 100mg/liter, most samples clear up. Muddy water samples that also have high hardness are often the result of physical disturbance, not water chemistry.


Gypsum Pros
>Improves water quality by raising calcium hardness. Benefits entire ecosystem
>Relatively non-toxic. Even gypsum from power-plant fly ash has to pass a safety standard to be released for agricultural/industrial purposes
>Long residual effect, as extra product can be applied safely and economically. Residual gypsum will settle to the pond or lake bottom and supply calcium for a longer period of time than with alum
>Easy to apply, small jobs with shovel and trailer, or large jobs with proper equipment
>Will not harm fish, wildlife, humans
>Will not cause a pH shift
>Can easily be tested on a water sample for feasibility and dosage calculation. 1.4grams per gallon = 1000 lbs per acre ft.
>Can be applied old school with shovels, or easily with the right equipment and strategy. We have a proprietary application method that works very well.

Gypsum Cons
>May require significant product, 1000-2000 lbs per acre ft. Current cost is $195/ton. Your typical 1 acre pond with 6’ avg depth may require 6 tons x $195 = $1170
>May take longer to clear water, depending on chemistry, dosage, amount used, application method

Alum Pros
>Requires less product than gypsum. At what hidden cost? Is this the only potential advantage? According to SRAC publications, you may need up to 120lbs of product per acre ft of water. So your 1 acre, 6’ avg depth pond at 6 acre-ft may require up to 720 lbs of alum, and will require 300lbs of hydrated lime. Hey Rex Rains what is your charge on alum per lb or per 50lb bag? What about hydrated lime?
>Works fast once you reach the threshold.

Alum Cons
>Aluminum is a heavy metal and has toxic effects if absorbed through skin or inhaled as dust. Aluminum has a 3+ positive charge, making it very difficult to remove from the body. See this Stephanie Seneff on Toxicity link and go to the 16min mark for in depth discussion of aluminum toxicity. Feel free to check her qualifications and educational background.
>Alum will cause a pH drop when enough is used to clear turbidity, which can stress or kill fish. Due to this fact, hydrated lime has to be used to counter this pH shift. Hydrated lime is also hazardous, can cause skin burns and irritation, blindness, and can also stress fish as it is applied. Hydrated lime can not be applied along with alum, so has to be done as a separate step, while checking pH. If you are on a large job, you may need to alternate the application, bouncing back and forth with alum and lime, to prevent an extreme shift.
>Alum is highly reactive, resulting in tremendous heat production as it is dissolved.
>Alum does nothing to provide for long term residual effect. If you experience a runoff event that flushes the pond or lake, you lose some of your product, and may experience turbidity again in short order.



- Todd Overton

Those of us who are considering gypsum or alum treatment can easily do a simulation.

Take 3, 1 gallon samples, in clear or semiclear plastic jugs. Use one as a control, dont treat it, just leave it alone and watch it for a week or so. If the control sample settles, you have a physical disturbance issue. Physical disturbance is often the culprit if you have high hardness (100 mg/liter+) and also excessive turbidity. Treat one sample with gypsum. 1.4 grams per gallon sample is equivalent to 1000lbs per acre ft. You can break that down into smaller increments if you want to be precise in determining threshold dosage. Some samples dont clear, and they are easily ruled out as candidates. Also do the alum simulation, .067grams per gallon is equal to 50lbs per acre ft of pond water. Small gram scales are easy and cheap. No need to do the slurry method suggested in some of the publications.

The simulation can save you money and give you insight. Make sure to do your gypsum simulation with product from your would-be supplier. Same with alum. Sone samples are not good candidates. Different types of clay respond differently. Often the chocolate milk samples are easiest to clear, and the slightly turbid samples are stubborn or ruled out as candidates. Often folks confuse stained water with clay turbidity.

Hope this helps