Lake Restoration
Using phosphorus fixing chemicals
The issue
- 224 lakes are monitored by the Irish EPA enabling a comprehensive national overview of the ecological and chemical status of Irish lakes.
- Note the cluster of lakes on the map (below) with “Poor” and “Moderate” water quality around southern border regions.
- One of the main factors responsible for lakes with moderate/poor ecological status is nutrient enrichment – specifically addition of phosphorus and nitrogen from anthropogenic sources.
- Most work which tries to improve ecological status of lakes have sensibly aimed to reduce the nutrient load to catchments – from both sewage treatment and agriculture.
- In some circumstances, the nutrient load to lakes has been so high, for such a long period, that even with significant reduction, the lake sediment will continue to enrich the overlying water for many years to come.
- This project assessed the feasibility of using chemical salts to remove phosphorus from the water column, locking it away into the sediment of a lake and in turn increasing the lake’s ecological status as measured by the Water Framework Directive.
Border counties are dominated by a drumlin landscape, with numerous small lakes scattered within the agricultural catchments.
As an example, Greagh Lough in Co Monaghan is a water reservoir, but yet has to contend with agricultural pressures of nutrient enrichment directly from the land. Greagh Lough borders the road on the bottom of this picture.
Greagh Lough is on the right hand side of the road, the small pond on the left is typical of a highly nutrient enriched environment, which flows directly into the water reservoir.
Scoping
Numerous lakes in the border counties within the CatchmentCARE area of study were initially visited and sampled to assess if they were nutrient enriched, and if they were, do they fulfil criteria to enable restoration via chemicals. Eventually 5 lakes in Fermanagh and Monaghan were selected which fulfilled all dosing criteria.
1. Phosphorus
Lake water total phosphorus concentration greater than 0.05 mg P/L was used so that the lake is eutrophic enough to require nutrient reduction.
2. Alkalinity
The addition of chemical salts lowers the pH of the water, but this is buffered if the alkalinity is high enough. If the alkalinity of the lake water is greater than 1 meq/L the pH is virtually unaffected.
3. Lake Size
This is mainly a cost criterion as larger lakes require more chemical salts for immobilization compared to smaller ones. A criterion of 10 ha or less was used.
4. Boat Access
The lake must be accessible to a boat. A sizeable boat needed to be launched in order to sample the lake and treat it with chemical salts.
5. Lake use
Lakes in constant use for leisure pursuits would be less suitable for this type of remediation due to the disruption of use during dosing activity
Before any dosing was attempted, the project had to review the scientific literature to produce a formal risk assessment for phosphorus fixing chemicals in Irish lakes and liaised with stakeholders to seek approval for lake dosing to proceed.
Numerous discussions were undertaken with representatives from:
- DEARA
- Irish EPA
- IFI
- Irish Water
- Monaghan County Council
This group agreed (with some reservations) that the dosing experiments could proceed within a tightly controlled dosing regime and with extensive pre and post water sampling to monitor lake changes.
ALUM (aluminium sulphate), as used extensively within the drinking water industry, was the metal salt finally chosen for dosing.
A 61 page scientific report “Report on phosphorus fixing chemicals in lakes” produced and distributed to all stakeholders. The report reviewed sixteen Aluminium and Iron chemical remediation case studies, >900 lakes.
''The weight of evidence of this review of the ecological effects of immobilizing phosphorus in lakes using aluminium and iron salts, based on whole lake case studies supplemented by mesocosms and major general work, was that there were almost none, based on zooplankton, benthic macroinvertebrates, fish and macrophytes.”
Dosing with Alum took place between October 2021 and March 2022.
Pre and post monitoring included parameters:
- Soluble Aluminium
- Iron
- Manganese
- Alklalinity
- pH
- Total Phosphorus
- Chlorophyll a
- Turbidity
- Zooplankton
- Fish surveys
- Dated sediment cores
Safety of staff was a concern as the undiluted ALUM has a very low pH (acidic). Suitable pumps, piping and general protective gear were integral to ensure the dosing proceeded without problems.
Left: Grove Lough, Co Monaghan before dosing with ALUM
During dosing
Alum was pumped into the tank on the boat, then pumped into the lake under a specific flow rate. The propellor of the boat mixed the Alum.
Grove Lough, Co Monaghan during dosing with ALUM.
Note the plum of ALUM precipitating and removing solids from the water column to settle into the sediment.
Corcaghan Lough, Co Monaghan during dosing with ALUM, note the plum of ALUM precipitating and removing solids from the water column to settle into the sediment.
Boat and equipment retrieval from Grove Lough, Co Monaghan
Boat and equipment retrieval from Grove Lough, Co Monaghan
Results
Would be hoping to see longterm decrease of Total Phosphorus and Chlorophyll a after the dosing.
So far we have seen a decreases, monitoring will continue to investigate the effectiveness of the restoration.
Grove pre-dosing and post dosing data
Greagh pre-dosing and post dosing data
Corcaghan pre-dosing and post dosing data
Project Legacy and Transferable Knowledge
- Chemical remediation is regularly used as a lake management technique in the United States, New Zealand and parts of Europe.
- However, this is only the second time it has been performed in Ireland (Bob Foy on White Lough, Co Tyrone in 1980). Post monitoring continued for 3 years after addition.
- The addition of Alum was delayed in this project by both Covid and approval from stakeholders – however post monitoring of the lakes will continue after the project end as Ulster University has funded a PhD student (Juan Bennett) who will continue monitoring and perform fish toxicity studies (from pre and post dosing collected fish).
- Chemical remediation in lakes is not a “fix all” technique. However, the CatchmentCARE team has shown that it can be performed on carefully selected lakes in Ireland.
2. Initial results do perhaps show a decrease in both Chlorophyll a and Phosphorus – key parameters in assessing ecological status of lakes. However, monitoring over the next few years will be needed to truly assess if the technique works.
3. The Alum dose used on the 3 lakes was deliberately low (<1mg Al/l) – this was to address the concerns of the stakeholders and also to fully understand the processes at work. Remediation management in the United States have used concentrations in excess of 10mg Al/l on a regular basis.
Alum does not just fix phosphorus, it has the potential to be used to deal with other problems, for example Alum has previously been used to help control elevated Manganese concentrations – a problem evident in a Co Monaghan water reservoir just last week (Sep 2, 2022).
Remainder of Project and beyond
- Monthly sampling of the 3 lakes will continue until the project end and beyond.
2. Dated sediment cores will be taken over the next few months to ascertain changes in sediment chemical concentration i.e. Fe, Al, Mn and TP.
3. A large amount of laboratory chemical analysis still needs to be carried out on stored water samples. This will be performed over the next months.
4. Final report to be produced outlining the findings of the Ulster CatchmentCARE team. This will be published and disseminated to all relevant stakeholders for possible future use in Irish lakes.