A Short Primer on Dilution and Flushing for Lake Management

Adding low nutrient water and keeping water moving through a lake can help prevent algae blooms, but there are caveats and limits. The Practical Guide to Lake Management in Massachusetts, available online from the DCR within the Mass.gov website, has a useful review of this technique. Lake waters that have low concentrations of an essential nutrient are unlikely to exhibit algal blooms. While it is preferable to reduce nutrient loads to the lake, it is possible to lower (dilute) the concentration of nutrients within the lake by adding sufficient quantities of nutrient-poor water from some additional source.  High amounts of additional water, whether low in nutrients or not, can also be used to flush algae out of smaller, linear impoundments faster than they can reproduce.  

When water low in phosphorus is added to the inflow, the actual phosphorus load will increase, but the mean phosphorus concentration should decrease. Dilution or flushing washes out algal cells, but since the reproductive rate for algae is high (blooms form within days to a few weeks), only extremely high flushing rates will be effective without a significant dilution effect. A flushing rate of 10 to 15% of the lake volume per day is appropriate to minimize algal biomass build-up. That means that the entire volume of the lake has to be exchanged in about a week, which will be a tall order in anything but a smaller pond with a substantial source of additional water.

Outlet structures and downstream channels must be capable of handling the added discharge for this approach to be feasible.  Qualitative downstream impacts must also be considered.  Water used for dilution or flushing should be carefully monitored prior to use in the lake.  Application of this technique is most often limited by the lack of an adequate supply of water.

The classic example of successful flushing is Moses Lake in Washington, which is not a small pond, but had a lot of river water available to flush it. Gene Welch and other studied this project over many years and concluded that flushing worked well for both water quality management and biological improvements. Examples in New England are much harder to come by. The lake in Look Park in Northampton MA has been successfully flushed by diverting a nearby river through it when necessary, but that is the only example that comes to mind.

Dilution is even harder to implement, as it requires very clean water. With phosphates often used to meet the anti-corrosion rule under the Safe Drinking Water Act, use of a public water supply may not result in lower phosphorus concentrations. The MWRA has refilled at least one emergency supply reservoir after winter drawdown with water that might otherwise have gone to consumers, and it has improved the phosphorus concentration. Few such cases are known, however. And even where clean dilution water is available, this may not prevent cyanobacteria from growing at the sediment-water interface using nutrients available there, then rising in the water column to form a bloom when available phosphorus near the surface is minimal.

If dilution or flushing seems like a viable alternative for a lake, the following information is needed to evaluate potential success:

  • Accurate hydrologic and nutrient budgets to allow evaluation of potential benefits
  • Assessment of probable in-lake effects and an evaluation of downstream impacts
  • Reliability of source water
  • Routing information for new water source
  • Monitoring program to track changes in detention time, nutrient levels and water clarity

Factors favoring the use of this technique include:

  • Actual reduction in nutrient inputs from identifiable sources is not practical, either for technical or jurisdictional reasons
  • Water level fluctuation will not differ greatly from pre-treatment conditions
  • Adequate water of a suitable quality is available for dilution or flushing
  • Downstream problems with water quantity or quality will not be caused

Permits for such use of water are typically required, and potential applicants should consult with their local and state environmental agencies.

Warning: Public water may contain high P!

Moses Lake in Washington, successful flushing.

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