Mantua Reservoir

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If you believe you or your pet have been exposed to a harmful algal bloom, call (800) 222-1222.

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Update November 7, 2019

Mantua Reservoir, October 30, 2019

The Bear River Health Department (BRHD) has removed the Danger and Warning Advisories on Mantua Reservoir. Toxin test results received from the Utah Public Health Lab (UPHL) for samples collected by the Division of Water Quality (DWQ) at the dock near the dam on October 25, 2019, and at the dock near the ramp on October 30, 2019, showed anatoxin-a and microcystin levels well below the recreation health-based threshold for a Warning Advisory.

The close of the monitoring season and removal of advisories doesn’t mean HABs are no longer present, but rather that monitoring data won’t be collected during the colder months to verify their presence. HABs can persist throughout the fall and winter and can pose a potential threat to humans and pets. It’s important for recreators to recognize the signs of a bloom and take appropriate precautions.

Recreators are advised to stay out of the water and avoid any contact with water or scum if they suspect a harmful algal bloom. Hunters and fishers should clean waterfowl and fish well and discard all guts.

Data Summary

Dock Near Ramp

  • Sample date: October 30, 2019
  • Sample type: Elbow-depth integrated composite sample
  • Microcystin: <2.0 micrograms per liter (µg/L)
  • Anatoxin-a: <0.10 µg/L

Dock Near Dam

  • Sample date: October 25, 2019
  • Sample type: Elbow-depth integrated composite sample
  • Microcystin: 0.17 micrograms per liter (µg/L)
  • Anatoxin-a: <0.10 µg/L

Update October 28, 2019

Toxin test results received from the Utah Public Health Lab (UPHL) for samples collected by the Division of Water Quality (DWQ) at the dock on Mantua Reservoir on October 9, 2019, showed anatoxin-a and microcystin levels well below the recreation health-based threshold for a Warning Advisory. Cyanobacteria cell-count concentration results are pending from PhycoTech.

A Danger Advisory indicates a high relative probability of acute health risk, cell-count density of >10 million cells per milliliter (cells/mL), microcystin levels >2,000 micrograms per liter (µg/L), or anatoxin-a levels >90 µg/L. A Warning Advisory indicates a moderate relative probability of acute health risk, cell-count density of 20,000 – 10 million cells per milliliter (cells/ml), microcystin levels of 4-2,000 micrograms per liter (µg/L), or anatoxin-a levels above non-detect.

The Danger Advisory remains in effect for North Beach, and the Warning Advisory remains in place for the rest of the reservoir pending cyanobacteria cell-count data from PhycoTech.

Data Summary

  • Sample date: October 9, 2019
  • Sample type: Elbow-depth integrated composite sample
  • Microcystin: <2.0 micrograms per liter (µg/L)
  • Anatoxin-a: <0.10 µg/L

Update September 25, 2019

Mantua Reservoir, September 16, 2019

Toxin test results received by the Division of Water Quality (DWQ) from the Utah Public Health Lab (UPHL) for samples collected on Mantua Reservoir on September 16, 2019, were below the advisory threshold. Cyanobacteria cell-count concentration results are pending from PhycoTech.

A Danger Advisory indicates a high relative probability of acute health risk, cell-count density of >10 million cells per milliliter (cells/mL), microcystin levels >2,000 micrograms per liter (µg/L), or anatoxin-a levels >90 µg/L. A Warning Advisory indicates a moderate relative probability of acute health risk, cell-count density of 20,000 – 10 million cells per milliliter (cells/ml), microcystin levels of 4-2,000 micrograms per liter (µg/L), or anatoxin-a levels above non-detect.

The Danger Advisory remains in effect for North Beach and the Warning Advisory remains in place for the rest of the reservoir pending cyanobacteria cell-count data from PhycoTech.

Data Summary

Mantua Reservoir Dike/North Beach

  • Sample date: September 16, 2019
  • Sample type: Elbow-depth integrated composite sample
  • Microcystin: 0.61 micrograms per liter (µg/L)
  • Anatoxin-a: <0.10 µg/L

Mantua Reservoir Boat Dock

  • Sample date: September 16, 2019
  • Sample type: Elbow-depth composite sample
  • Microcystin: 0.24 micrograms per liter (µg/L)
  • Anatoxin-a: <0.10 µg/L

Update September 10, 2019

Cyanobacteria cell-counts for four samples collected at Mantua Reservoir on September 10, 2019, were significantly lower than samples collected on August 26, 2019. Cell counts still exceeded the recreation health-based threshold for a Warning Advisory in the open water and North Beach but were well below the threshold for a Danger Advisory. An advisory can be downgraded or lifted after two consecutive weeks of sampling show cell counts below the Danger or Warning Advisory threshold.

Anatoxin-a levels in all water samples exceeded the recreation health-based threshold for a Warning Advisory according to toxin test results from the Utah Public Health Lab (UPHL). Microcystin levels were well below the recreation health-based threshold at all four sample locations.

A Danger Advisory indicates a high relative probability of acute health risk, cell-count density of >10 million cells per milliliter (cells/mL), microcystin levels >2,000 micrograms per liter (µg/L), or anatoxin-a levels >90 µg/L. A Warning Advisory indicates a moderate relative probability of acute health risk, cell-count density of 20,000 – 10 million cells per milliliter (cells/ml), microcystin levels of 4-2,000 micrograms per liter (µg/L), or anatoxin-a levels above non-detect.

The Danger Advisory remains in effect for North Beach and the Warning Advisory remains in place for the rest of the reservoir.

Data Summary

Mantua Reservoir Dike/North Beach

  • Sample date: September 10, 2019
  • Sample type: Surface sample
  • Microcystin: <2.0 micrograms per liter (µg/L)
  • Anatoxin-a: 0.14 µg/L
  • Cyanobacteria cell counts: 57,258 cells per milliliter (cells/mL)

Mantua Reservoir Boat Dock

  • Sample date: September 10, 2019
  • Sample type: Surface sample
  • Microcystin: 2.0 micrograms per liter (µg/L)
  • Anatoxin-a: 0.15 µg/L
  • Cyanobacteria cell counts: 17,034 cells per milliliter (cells/mL)

Mantua Reservoir Open Water

  • Sample date: September 10, 2019
  • Sample type: Surface sample
  • Microcystin: 2.5 micrograms per liter (µg/L)
  • Anatoxin-a: 0.17 µg/L
  • Cyanobacteria cell counts: 118,377 cells per milliliter (cells/mL)

Mantua Reservoir Open Water

  • Sample date: September 10, 2019
  • Sample type: Elbow-depth integrated composite sample
  • Microcystin: <2.0 micrograms per liter (µg/L)
  • Anatoxin-a: 0.14 µg/L
  • Cyanobacteria cell counts: 14,992 cells per milliliter (cells/mL)

Update September 10, 2019

The Bear River Health Department (BRHD) issued a Danger Advisory and closure for the North Beach at Mantua Reservoir after samples collected on August 26, 2019, showed cyanobacteria cell-count concentrations exceeding the recreation health-based threshold for a Danger Advisory. The rest of Mantua Reservoir remains under a Warning Advisory.

Cell counts at the Mantua Reservoir dike were approximately 14 million cells per milliliter (cells/mL). The primary taxon in the sample was Dolichospermum, a species that can produce potent liver toxins and neurotoxins.

A sampling crew will conduct follow-up sampling in the open water on September 10, 2019.

A Danger Advisory indicates a high relative probability of acute health risk, cell-count density of >10 million cells per milliliter (cells/mL), microcystin levels >2,000 micrograms per liter (µg/L), or anatoxin-a levels >90 µg/L. A Warning Advisory indicates a moderate relative probability of acute health risk, cell-count density of 20,000 – 10 million cells per milliliter (cells/ml), microcystin levels of 4-2,000 micrograms per liter (µg/L), or anatoxin-a levels above non-detect.


Update September 6, 2019

Cyanobacteria cell-count test results received by the Division of Water Quality (DWQ) for samples collected at two locations at Mantua Reservoir on August 26, 2019, exceeded the recreation health-based threshold for a Warning Advisory in the boat dock sample, and the recreation health-based threshold for a Danger Advisory in the dike sample. The primary taxon at the boat dock was Aphanizomenon, followed by Dolichospermum. The primary taxon at the dike was Dolichospermum, followed by Aphanizomenon. Both species are capable of producing microcystin and anatoxin-a.

Toxin test results from the dike sample exceeded the recreation health-based threshold for a Warning Advisory. Toxin test results from the boat dock sample were well below the advisory threshold.

A Warning Advisory indicates a moderate relative probability of acute health risk, cell-count density of 20,000 – 10 million cells per milliliter (cells/ml), microcystin levels of 4-2,000 micrograms per liter (µg/L), or anatoxin-a levels above non-detect. A Danger Advisory indicates a high relative probability of acute health risk, cell-count density of >10 million cells per milliliter (cells/mL), microcystin levels >2,000 micrograms per liter (µg/L), or anatoxin-a levels >90 µg/L.

The Warning Advisory for Mantua Reservoir remains in place.

Data Summary

Mantua Reservoir Dike

  • Sample date: August 26, 2019
  • Sample type: Surface sample
  • Microcystin: 12.92 micrograms per liter (µg/L)
  • Anatoxin-a: 0.16 µg/L
  • Cyanobacteria cell counts: 13,947,431 cells per milliliter (cells/mL)

Mantua Reservoir Boat Dock

  • Sample date: August 26, 2019
  • Sample type: Elbow-depth integrated composite sample
  • Microcystin: <0.10 micrograms per liter (µg/L)
  • Anatoxin-a: <0.10 µg/L
  • Cyanobacteria cell counts: 45,631 cells per milliliter (cells/mL)

Update August 30, 2019

Mantua Reservoir, August 20, 2019

Toxin test results received by the Division of Water Quality (DWQ) on August 29, 2019, for samples collected on Mantua Reservoir on August 26, 2019, exceeded the recreation health-based threshold for a Warning Advisory for microcystin and anatoxin-a. Toxin test results received by DWQ on August 23, 2019, from the Utah Public Health Lab (UPHL) for samples collected on August 20, 2019, were below the recreation health-based threshold with the exception of a detection of anatoxin-a in the surface sample on the North Shore. These test results show that microcystin levels in the reservoir rose considerably between August 20, 2019, and August 26, 2019.

Cyanobacteria cell-count concentration results are still pending from PhycoTech. Previous sampling on August 15, 2019, showed cell counts of approximately 11.5 million cells per milliliter (cells/mL)

A Warning Advisory indicates a moderate relative probability of acute health risk, cell-count density of 20,000 – 10 million cells per milliliter (cells/ml), microcystin levels of 4-2,000 micrograms per liter (µg/L), or anatoxin-a levels above non-detect.

The Warning Advisory for Mantua Reservoir remains in place.

Data Summary

Surface sample

  • Sample date: August 26, 2019
  • Sample location: Dike
  • Microcystin: 12.92 micrograms per liter (µg/L)
  • Anatoxin-a: 0.16 µg/L

Elbow-depth integrated composite sample

  • Sample date: August 26, 2019S
  • ample location: Boat Ramp
  • Microcystin: <0.10 micrograms per liter (µg/L)\Anatoxin-a: <0.10 µg/L

Surface sample

  • Sample date: August 20, 2019
  • Sample location: North Shore
  • Microcystin: 0.151 micrograms per liter (µg/L)
  • Anatoxin-a: 0.122 µg/L

Elbow-depth integrated composite sample

  • Sample date: August 26, 2019
  • Sample location: Dam
  • Microcystin: <0.10 micrograms per liter (µg/L)
  • Anatoxin-a: <0.10 µg/L

Update August 19, 2019

Mantua Reservoir, August 15, 2019

Mantua Reservoir, August 15, 2019

The Bear River Health Department (BRHD) issued a Warning Advisory for Mantua Reservoir on August 19, 2019, after samples collected in the reservoir last week showed high cyanobacteria cell-count densities. Health officials are waiting for follow-up samples before taking further action. BRHD has posted signs around the reservoir to alert the public about the Warning Advisory.

Cyanobacteria cell-count concentrations from samples collected by the Division of Water Quality (DWQ) at the public dock at Mantua Reservoir on August 15, 2019, were approximately 11.5 million cells per milliliter (cells/mL). These levels exceed the recreation health-based threshold for a Danger Advisory. Aphanizomenon and Dolichospermum were the primary taxa in the samples. Both are toxin-producing species, particularly for microcystin and anatoxin-a.

Toxin test results were detect for anatoxin-a, which exceeds the recreation health-based threshold for a Warning Advisory. Microcystin levels were well below the health advisory threshold.

A Warning Advisory indicates a moderate relative probability of acute health risk, cell-count density of 20,000 – 10 million cells per milliliter (cells/ml), microcystin levels of 4-2,000 micrograms per liter (µg/L), or anatoxin-a levels above non-detect.

Data Summary

Surface sample

  • Sample date: August 15, 2019
  • Microcystin: 0.33 micrograms per liter (µg/L)
  • Anatoxin-a: 0.17 µg/L
  • Cell counts: 11,447,127 cells per milliliter (cells/mL)

Elbow-depth integrated composite sample

  • Sample Date: August 15, 2019
  • Microcystin: <0.10 micrograms per liter (µg/L)
  • Anatoxin-a: <0.10 µg/L
  • Cell Counts: 116,859 cells per milliliter (cells/mL)
Mantua Reservoir, August 15, 2019

Mantua Reservoir, August 15, 2019


Update August 15, 2019

A possible harmful algal bloom (HAB) at Mantua Reservoir was observed by Brigham City staff the week of August 12, 2019. The Division of Water Quality (DWQ) sent a staff member to visit the reservoir on August 15, 2019, to collect samples and observe water conditions after DWQ was notified on August 14, 2019, that the reservoir might be experiencing a bloom.

DWQ won’t be able to confirm the presence of cyanobacteria in Mantua Reservoir until it receives sample results back from the Utah Public Health Lab (UPHL). Lab results for toxins are expected back on August 16, 2019, and for cyanobacteria cell count concentrations on August 19, 2019.

Mantua Reservoir, August 15, 2019

Brigham City notified city residents of the possible HAB via email and the city’s Facebook page. Information shared with residents noted that cyanobacteria and cyanotoxins have been detected periodically in the reservoir and included precautions to protect human and animal health. Users of Mantua Reservoir are advised by Brigham City to avoid areas of scum, not drink the water, keep pets and livestock away, and thoroughly clean fish and properly discard guts.

DWQ will post lab results as soon as they become available.