Can Well Water Harm Pond Environments and Fish Populations?

According to Dr. Billy Higginbotham, an Extension fisheries and wildlife specialist, well water that is perfectly safe for human consumption can actually be detrimental to fish in ponds. Even during a normal summer, the most likely cause of fish death is oxygen depletion.
The Dangers of Well Water in Ponds
Well water, while clear and clean, can pose several risks to pond environments and fish populations if not properly managed. The two main issues are quality and quantity of the well water being added to the pond.
Quality Concerns
The most critical factor is the difference in quality between the pond water and the well water. Sudden changes in temperature or pH can be harmful to fish. Dr. Higginbotham explains that a change of more than one pH unit, up or down, within a short period of time can be negative for fish. Abrupt temperature changes of 10 degrees Fahrenheit or more, up or down, can also put fish at risk.

Pond Stratification and Turnover

Another quality-related issue is the phenomenon of pond stratification and turnover. During the summer, ponds become thermally stratified, with a warmer, oxygen-rich upper layer and a cooler, oxygen-depleted lower layer. A sudden weather event, such as a heavy rain or strong wind from a summer thunderstorm, can cause the pond to “turn over” as the cooled top layer sinks and mixes with the lower layer. This pushes dead organic matter from the bottom up to the surface, where it undergoes rapid decomposition that consumes oxygen and can result in fish kills.
Pumping large amounts of cool well water into the surface of a stratified pond can replicate this pond turnover effect, even if the pond’s pH and temperature are similar to the well water. This is because well water typically has no appreciable dissolved oxygen and may have high levels of carbon dioxide.

Aeration and Agitation

To remedy the risk of oxygen depletion, Dr. Higginbotham recommends “breaking up” the well water before it enters the pond. This process of agitation and increased exposure to air can be done through various methods, such as letting the water run over rocks, hardware cloth, or through a series of boxes or rock piles arranged in a stair-step or waterfall configuration. Anything that induces aeration before the water reaches the pond can help ensure the well water is properly oxygenated.
Quantity Concerns
The second consideration is the quantity of well water being added to the pond. Dr. Higginbotham explains that if a garden hose is used to slowly add well water to the pond, the change in water quality will be gradual, and the fish will have time to adapt. However, if a large-diameter pipe is used to pump large quantities of well water into the pond, the change in quality can happen much more quickly, potentially shocking the fish and leading to fish kills. It’s not just the change itself, but the rate of change, that can be problematic.
Preventing Fish Kills in Ponds
To help prevent fish kills in ponds due to well water additions or other factors, Dr. Higginbotham offers the following recommendations:

Never enter the summer season with more than 1,000 pounds of fish per surface acre of the pond. This is the recommended maximum carrying capacity to avoid oxygen depletion.

If aquatic weed control is necessary, only treat 15-20% of the pond’s surface area at a time, with one-week breaks between treatments. This will lessen the likelihood of creating an oxygen depletion.

Keep a close eye on the weather, especially during periods of hot, still, and cloudy days, as these conditions can reduce photosynthesis and oxygen production in the pond.

If an oxygen depletion is suspected, check the pond at daybreak, as oxygen levels will be at their daily low at sunrise. If fish are seen “piping” (swimming at the surface, gasping for air), the pond owner must react quickly to save the fish.

Consider investing in a pond aeration system to help prevent future oxygen depletion issues, especially for ponds that have experienced fish losses due to low oxygen levels in the past.

By understanding the potential risks of well water, properly aerating and introducing it to the pond, and closely monitoring pond conditions, pond owners can help protect their valuable fish populations and maintain a healthy pond ecosystem.