Filling Ponds with Well Water Can Kill Fish

Robert Burns, 903-834-6191,rd-burns@tamu.edu

OVERTON – Landowners seeking to replenish their drought-depleted farm ponds with well water could kill their fish if they're not careful, said an expert with Texas Cooperative Extension. "We can kill fish with well water that's perfectly good to drink," said Dr. Billy Higginbotham, Extension fisheries and wildlife specialist.

Because of the ongoing drought, water levels of many of Texas' estimated 1 million farm ponds are dropping.

With dropping water levels, the surface area of a pond is also likely to shrink, Higginbotham said. And when the surface area – the area of water exposed to the air – shrinks, then the pond water's ability to absorb oxygen is reduced as well.

Even during a normal summer, the most likely cause of fish death is oxygen depletion, Higginbotham said. The hot, windless days often experienced in summer reduce the ability of ponds to absorb and hold oxygen. Also, cloudy days in summer can slow down photosynthesis of aquatic plants, which release oxygen, compounding the problem. "Oxygen depletion problems account for about 85 percent of all fish die-offs in Texas farm ponds," he said.

But with reduced surface areas, the risk is increased, Higgenbotham said. Many pond owners know this, and have been calling asking about adding water from their wells.

But though the pond water may be muddy and green, while the well water is clear and clean, it's still not a good idea to rush in, he said. "There are two issues -- quality and quantity of the well water -- that must be taken into consideration," he said.

The most critical is quality. By "quality," Higginbotham meant a difference in quality between the pond and well water. "Either a too large of a change in temperature or pH can shock the fish and result in fish death," he said.

With pH, a change of more than one unit, either up or down, can endanger fish, if occurring in a short period of time. With temperature, a quick change of 10 degrees or more, also either up or down, can put the fish at risk, he said.

Which leads to the second consideration: quantity, Higginbotham said.

"If we've got a garden hose running into a pond, then the change in (water) quality is going to be small over a long period of time, particularly in a larger pond," he said. "The fish will have time to adapt.

"But with a 6-inch line pumping at fire hose quantities into a fairly small pond, the change in quality will happen much more quickly. It's not just the change; it's the rate of change." Even if the pH is similar, a wholesale temperature change can also result in fish death, Higginbotham said.

Such a wholesale change can happen naturally during the summer if a period of high temperatures is followed by a heavy rain, he said. This is because ponds can become stratified. The top stratum of water will be warmer because warm water, like warm air, rises. The lower strata, including the bottom stratum, remains cooler. Moreover, dead organic matter will build up on at the lowest levels. Deprived of oxygenated water, this organic matter will remain in a kind of stasis, not decaying, he said.

But a cool rain or strong wind from a summer thunderstorm can cause the pond to "turn over" as the now-cooled top layer sinks and mixes with the warmer lower layer. This causes the dead organic matter to be brought to the surface, he said.

With the dead organic matter now exposed to oxygenated water, a natural decay process can proceed at a vigorous rate, and so reduce oxygen levels that results in fish kills. Pumping large amounts of cool water into the surface of a stratified pond can duplicate the process of "pond turnover," he said.

But even if pH and temperature of the pond water and well water are similar, it's still possible to deplete the level of oxygen if the landowner isn't careful, Higginbotham said. This is because well water has no appreciable absorbed oxygen whatsoever, he said, and it may have a high level of carbon dioxide.

To remedy this, Higginbotham advised "breaking up the well water" before letting it enter the pond. Breaking up the water – agitating it and exposing more of its surface to the air – can be done by various methods.

The simplest is just to let the pond water run over some balled-up hardware cloth on its way to the pond's surface. More complicated methods, he said, involve dropping the water from a series of boxes arranged in stair-step fashion.

"Anything that serves to aerate the water before it enters the pond should work," he said. A small flow of water is easier to break up than a large one, he noted.