Lake Water Improvement Project Summary

Lake Mission Viejo Association is committed to providing a premier recreational lake that our membership can enjoy – be it for swimming, sailing, fishing or simply enjoying the magic of a vista of sparkling blue water.

Accomplishing this requires ongoing research and work by our lake management team and significant funding because the issues, if left unchallenged, could be detrimental to our lake continued health.

Through our continued diligence and proactively addressing numerous regional assaults that other lakes in our area have had to deal with, we have protected our most valuable asset.  One such challenge we have successfully avoided due to our proactive focus on our lake biology, is a quagga mussel infestation.  Quagga mussels rapidly reproduce and could potentially foul boats, docks, master water valves and lake equipment, and could cost substantial amounts of money to address.  LMV implemented a boat inspection program and 30 day quarantine, with regular testing of our lake water 8 years ago. Through this inspection program and continuous water testing we have avoided irreparable damage due to a quagga mussel infestation. Other lakes in our area have not been so fortunate.

Another major issue we have successfully addressed in the last 6 years is the California drought and the threat of not having lake refill water available to maintain our lakes biology or its recreational availability.  We worked with numerous entities, including Santa Margarita Water District and we were able to build the Advanced Purified Water (APW) plant to secure lake refill water permanently.  This APW intervention has eliminated the threat of having the lake go dry due in future droughts.  APW was a monumental undertaking and is a grand accomplishment.  The lakes future water refill is now secure.

Golden algae is the most challenging of these issues and is a national problem killing entire fish populations across the country with no defined solution for eradication.  Golden algae is named in the July 8, 2020 U.S. Congressional Research Report on Freshwater Harmful Algal Blooms and Congress has committed to ongoing research dedicated to finding a solution to control and eliminate golden algae. The Lake Association and its management team – staff members, scientific consultants and the LMVA Board of Directors has determined through our intense research that golden algae is reactive to pH and salinity.  Through LMVA’s proactive and solution-oriented focus on how to manage golden algae we are moving forward with an action plan in 2021.

We are providing this update on what we’ve learned, what we’ve done already, and a preview of the road ahead.


The Challenges: Golden Algae and Water Clarity

The golden algae that decimated the lake’s fish population appears to be an intractable problem, not just in Lake Mission Viejo but across American and, indeed, the world.

The US Congress and world scholars have addressed harmful algae blooms and specifically golden algae and have found no known cure or formula for eradication of golden algae. 

Our Association-funded research by Dr. David Caron, Chief Science Officer for Aquatic EcoTechnologies, LLC and a professor in the Department of Marine Sciences at USC and a world leader in research into harmful algae blooms, has shifted our focus from irradicating golden algae – which no one has been able to do anywhere in the world – to reducing its toxicity and the frequency of its blooms. Research carried out in labs and in the lake indicates that the way to do this is by reducing pH and total dissolved solids (TDS, common measure of salinity) levels in the lake.

Lowering pH and TDS, as discussed further below, can be done by using lower-pH and TDS water for lake refill.  This could be facilitated by increasing the flushing of lake water so there is more room for lower-TDS water. Our research also showed that alum, a chemical that is commonly used in water treatment, can be effective in reducing the lake’s pH level,  at least for a period of time, much like shock treatments to a swimming pool but on a larger scale.

The clarity of the lake’s water dropped rapidly after 2014’s golden algae bloom, possibly because the algae bloom and loss of fish threw off the lake’s ecosystem. High TDS levels  alone can’t be blamed for clarity issues because the oceans can be exceedingly clear even though seawater’s TDS levels are about 16 times greater than the lake’s. But it appears that the same approach we will employ to fight golden algae, should help the lake’s ecosystem, and encourage the return of more favorable aquatic plants and a plankton ecology that  should have a positive impact on clarity.


Overall Strategy

The research we have conducted leads to the conclusion that the lake’s pH and TDS levels need to be reduced. For TDS, this will return the water to levels of salinity similar to what it had years ago. Unlike TDS, the lake’s pH level has been consistent over the years, so it will need to be reduced below its historic levels. Accomplishing this will be a significant challenge, given the volume of water in the lake, and the costs of the various approaches to dropping TDS and pH levels.


Why the Lake’s Salinity Has Increased

Natural freshwater lakes continuously flush their salts because water flows into and out of them, carrying salts away. As an engineered lake, Lake Mission Viejo is different. It is impractical to have water flow through it because nearly all the water used to keep the lake’s water level within its engineered requirements must be purchased at considerable expense. We have three sources of water to keep the lake at its required level: Water from the Advanced Purified Water (APW) plant, that has very low TDS but is very expensive, low-cost well water, which has TDS levels higher than our target level for the lake, and free rain that falls directly onto the lake, which has almost no TDS. (All other rainfall is carried away from the lake by storm drains to protect its water quality from runoff pollutants like automobile oils and brake dust, fertilizers and insecticides, and animal waste.)

As water evaporates from the lake, salts are left behind and gradually build up to certain limiting amounts. Our records show that each year, about 450 acre-feet of water, 150 million gallons, evaporate from the lake’s 124-acre surface. Historically we could do little to stop this build-up because most of the water needed for lake re-fill came from SMWD’s potable (drinking) water system, which most often provides water from the Colorado River that has a relatively high salt content.


Actions We’ve Already Taken to Reduce Salinity and pH

The first major step in reducing the lake’s TDS levels came in 2017, when we stopped using SMWD’s potable water to maintain the lake’s water level and began using their Advanced Purified Water (APW) instead. APW water is produced by running water through several filtering systems, until its TDS level drops to just 20 parts per million, approaching the purity of rainwater. Initially, we were adding about 200 acre-feet of APW water a year, which had an immediate but small positive effect on TDS levels.

Next, we stopped using well water for lake refill, allowing us to replace its volume with ultra-low TDS APW water. This doubled the amount of APW water we can put in the lake, to over 400 acre feet a year. This was a costly decision because the well’s water is much less expensive than APW water.  We hope to be able to use well water again in the future, since it is so much less expensive than APW water.

Finally, we are targeting lowering the lake’s pH levels. pH should naturally decrease as the lake water’s alkalinity is decreased as part of the efforts to reduce salinity, since alkalinity resists pH decreases. Also, APW water now has a low pH level, which should help to lower lake pH, and should do even more if more APW water can be added. Finally, we are working with Dr. Caron on experiments with alum, both in a laboratory setting and in contained “corrals” in the lake. These experiments are helping us develop the most effective application protocols for alum treatments.


Looking Forward: Future Water Improvement Efforts

A finalized plan for moving beyond the initial work just described is evolving and LMVA’s staff and consultants are at work now preparing recommendations for the Board’s consideration. The recommendations detail options for speeding the lowering of TDS and pH in the lake. The lake’s considerable volume, 1.2 billion gallons or 3,700 acre-feet, means these efforts will tend to be slow and expensive, especially since evaporation will continue concentrating salts as we work to reduce them.

The Association’s challenge, then, is to find the best balance between our desire to rapidly reduce TDS and pH and our need to proceed within our budgetary means. We are considering several alternatives:

  • Expanding the capacity of the APW plant.We have begun working with SMWD to determine the feasibility and costs of various APW plant expansion options. Permitting, building, and operating a larger plant all will come at considerable expense, but having more APW water than currently available will create the opportunity to speed the lowering of TDS and pH levels.
  • Temporary Reverse Osmosis Units. Renting, leasing or purchasing a packaged temporary water treatment plant or plants that can remove salts from lake water is an alternative to APW plant expansion – or, possibly, a supplement to an expansion. These plants come in many sizes, have varying capabilities and varying visual impacts, so we are gathering the information needed to consider how, or if, these plants will fit into our final program.
  • Flushing. We cannot put new, low-TDS water into the lake unless there’s capacity for it. Thus far, we can add no more APW water than the volume of lake water lost to evaporation and seepage. One way we could increase the lake’s capacity to receive more APW water is to let water flow out of the lake into Oso Creek, but this would require a very difficult permitting process and may ultimately not be feasible. Nevertheless, we are looking into it. An option that would not require these difficult permits involves running lake water through an expanded APW plant or through temporary reverse osmosis units, then returning it to the lake.
  • The Board has directed staff to help procure a design engineer (or firm) to prepare a preliminary design report on both the above-noted additional treatment and lake flushing.  This effort would confirm an array of initial planning assumptions, further develop important facility requirements and details, and provide refined estimations of various project costs. 
  • Alum applications. Alum applications will be part of the adopted program, following protocols that will be developed through the experiments now underway. 


Dr. Caron’s research has helped us to identify the “sweet spot” levels for pH and TDS that we should strive to reach.  He recently finalized findings that indicate the sweet spot will involve lowering TDS to 1,000 to 1,250 parts per million (ppm) from the lake’s current level of approximately 2,200 ppm and its pH to 7.0 from its current level of approximately 8.3. At those levels, golden algae blooms should be less toxic and less frequent, which will help us to restore fish populations. The new TDS and pH levels will also provide a water environment that should encourage healthy lake flora and fauna and ultimately a plankton ecology that could foster greater water clarity.

While we have a way to go, the amount of information we now have is orders of magnitude greater than what we started with in 2014. It will soon be possible to intelligently weigh the options before us and choose the best way forward for the Association and the lake. Still, the lake’s dynamics are complicated and even mysterious, and we’ve learned it’s best not to predict how golden algae will respond to what we do, or to believe the timelines we set will align with how the lake ultimately responds. So, as we move forward, it would be best to think in terms of what the action plan “should” do, not what it “will” do.

We are committed to doing what’s scientifically indicated and financially feasible to improve our Members’ enjoyment of Lake Mission Viejo. Your Board will keep you updated as work on this high-priority work continues.