How to Clear Muddy Ponds
Galen Jons, Fisheries Biologist

Muddy or turbid ponds are common throughout Texas. Muddy ponds are not aesthetically pleasing and may inhibit fish growth by reducing the amount of plankton available to drive the food chain. Reduced visibility also makes it difficult for sight-feeding predators such as largemouth bass to effectively feed on forage fish.

There are generally four conditions which cause a pond to be muddy. They are:

Watershed - Check the watershed (the area where rainwater runs off to collect in a pond) to make sure there is no barren ground. Ground cover such as grass, shrubs and brush help to hold the soil and prevent it from being washed into the pond during heavy rains. Grasses such as coastal and common Bermuda can usually be started with just a few sprigs of grass planted in the area.

Rainfall/Wind - Heavy rainfall will usually muddy up a pond, but it should clear up after a few days. In larger ponds, wind and wave action can keep mud stirred up and also erode shorelines. In such cases, it is a good idea to provide grass or wetland cover on the shorelines if possible, or at least add some large rock or rip-rap to the water's edge to reduce erosion. Earthen jetties or large floating tubes can be placed in open water to help reduce wave action.

Rough fish - Abundant populations of rough fish such as bullheads or carp can keep the bottom mud stirred up. If this is the case, the pond owner may consider renovating the pond by draining it or using the chemical rotenone to poison the fish, then start over with desirable species.

Suspended clay particles - This is usually the most common cause of turbid ponds in Texas. Clay particles carry a negative charge, thus repelling each other and staying suspended in the water. Addition of positively-charged particles will cause these clay particles to clump together so they become heavier than water and settle to the bottom.

One of the most effective ways to clear ponds is to add gypsum or alum. To determine the amount of gypsum or alum needed, you will need to know the volume of your pond (acre-feet) and several clear gallon jars filled with water from the pond.

 

To determine pond volume, you will need to calculate surface area and average depth. For example, if a pond is 300 feet long x 200 feet wide and averages five feet deep, it's volume would be 6.9 acre-feet. (Note: 43,560 sq. ft. in one acre)

(300x200)/43560 è 60000/43560 è 1.37 acres X 5 feet deep è 6.9 acre-feet

Gypsum
Set one jar aside with no gypsum added for comparison. In the other jars, add various rates of gypsum. For example, if you have three other jars, add 2 measured tablespoons of gypsum to one jar, 4 tbsp. gypsum to the second jar and 6 tbsp. gypsum to the third jar. Wait 12 hours.

If the jar containing no gypsum has cleared, then the addition of gypsum may not help clear your pond. This means that either rough fish or wind/wave action is keeping the mud stirred up in your pond. Treat those symptoms.

If the jar with no gypsum has not cleared, but the one with 2 tbsp. has cleared, then you will need to run the test again with 1 tbsp. added to see if that clears the jar in 12 hours. By the same token, if 2 won't clear the water, but 4 will, then re-run the test with 3 tbsp. gypsum added to a jar. To do this, you can just add 1 tbsp. gypsum to the jar that has 2 tbsp. gypsum already added to it.

If no jars have cleared, run the test again with higher amounts of gypsum.

Once you have determined how many tablespoons of gypsum are needed to clear one jar of pondwater within 12 hours, you can determine the amount of gypsum needed for your pond.

For each tablespoon of gypsum added to the gallon jar, you will need to add 80 pounds of gypsum to each acre-foot of water.

Using the example above of 6.9 acre-feet, let's assume that 3 tablespoons of gypsum cleared the water. This would mean we would have to add 1656 pounds of gypsum to the pond to clear it. As you can see, the cost can add up quickly, which is the reason for doing the tests above to determine the exact rate you will need!

Alum (aluminum sulfate)
The same tests above can be used to determine the amount of alum needed to be added to the pond. In this case, however, for each tablespoon of alum, you would add 30 pounds of alum for each acre-foot of water. If you're adding more than 50 pounds of alum per acre-foot, then you will also want to add approximately 15 pounds of hydrated lime for each acre-foot to get the best results.

Whether you use gypsum or alum will depend on the costs of each (don't forget the hydrated lime to be used with alum), keeping in mind you will use less than half as much alum as you would gypsum.

     
     

Texas Parks and Wildlife Dept.
5325 N. 3rd
Abilene, TX 79603
(915) 692-0921
tpwd1b@sbcglobal.net

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