Watershed Protection is using lanthanum-modified bentonite as part of a pilot program to reduce the amount of phosphorus available to blue-green algae in two areas of Lady Bird Lake. Reducing phosphorus levels may help prevent the growth of harmful algae. Lanthanum-modified bentonite is safe for humans, the environment and wildlife, but boaters should still keep plenty of distance from the barge during applications. 

Results have been mixed. After the most recent application in June 2023, we have observed a reduced amount of blue-green algae in both locations. However, blue-green algae is still present and caution is still needed in these areas. In 2021, sediment testing showed a significant decline in the amount of phosphorus available to blue-green algae at Red Bud Isle. In 2022, east of I-35, sediment phosphorus levels remained static.

Red Bud Isle 

  • Total Amount: 20,000 pounds 
  • First Application: Monday, June 5, 2023 
  • Second Application: Monday, July 10, 2023
  • Third Application: Monday, August 14, 2023  

East of I-35

This includes the area from the north shore to the middle of Lady Bird Lake, between I-35 and the lagoon near the Festival Beach boat ramp. 

  • Total Amount: 40,000 pounds of lanthanum-modified bentonite
  • First Application: Tuesday, June 6, and Wednesday, June 7, 2023 
  • Second Application: Monday, July 10, 2023
  • Third Application: Monday, August 14, 2023

Please note that we did not include the south shore in this area. The south shore acts as a control area to help evaluate the effectiveness of the treatment. This area of the lake was chosen due to the presence of harmful algae with multiple toxins in 2021. 

How Lanthanum-Modified Bentonite Works 

Harmful algae thrive in warm, stagnant water with high nutrient levels. Lanthanum-modified bentonite binds to phosphorus making it unavailable to the algae. Phosphorus is a key nutrient for the growth of blue-green algae.


Watershed Protection began a pilot project in 2021 to try preventing the growth of harmful algae near Red Bud Isle. We tested whether lanthanum-modified bentonite would reduce phosphorus levels and the amount and/or toxicity of blue-green algae in the pilot project area.

In 2021, we applied 120,000 pounds of lanthanum-modified bentonite over 22 acres of Lady Bird Lake near Red Bud Isle. There were three applications in June, July and August. Initial chemical analysis in June 2021 showed about 40% of the phosphorus in the water around Red Bud Isle was mobile and available as a food source to blue-green algae. After application, the amount of mobile phosphorus declined to 18%, a significant decrease. Sediment testing in May 2022 continued to show mobile phosphorus fractions remained lower than the 2021 pre-application. In 2022, we expanded the applications  to include the Festival Park area. Sediment phosphorus contents did not respond as strongly as was observed at Red Bud Isle.  At Festival Beach, mobile sediment phosphorus contents remained similar to pre-application levels, suggesting that phosphorus levels did not increase.  

Safety Information 

Lanthanum-modified bentonite (LMB) is safe for humans, the environment and wildlife. It consists of a modified clay material that binds to phosphorus in surface water and sediment and is widely used to prevent harmful algae blooms.  


  • $300,000 per year for five years (2021-2025) 
  • Includes treatment and monitoring of sediments