Most lake pollution problems are caused by nutrients, contaminants, and sediments carried into the lakes. Soil particles carry more than 90 percent of the organic nitrogen and phosphorus originating from upland agricultural practices. Sediments come from all over the land and thus have non point sources, whereas nutrients can have both non point and point sources. In a typical watershed, nutrients may come from sewage, wastewater, agricultural and urban runoff, and atmospheric fallout. In recent years, heavy use of fertilizers and pesticides and high rates of soil erosion have increased the severity of the problem.
High concentrations of nitrogen and phosphorus are the main causes of algal growth, which results in the deterioration of the lake water quality. Algae have a very high growth potential, many times more than any land-based plant system, and respond quickly to nutrient inputs which act as fertilizers.
Algae affect the quality and appearance of water. They cause taste and odor problems in municipal water supplies and excrete toxins into waters. Blooms of algae adversely affect recreational activities like swimming, boating and fishing. High algal growth leads to large masses of dead plants which settle to the bottom and cause dissolved oxygen deficiencies on decomposition. Fish die along with algae, and anaerobic decomposition releases ammonia, causing more fish kills. High sediment loads are unsuitable for fish because of the increased turbidity of the water. Because of relatively higher and more uniform temperatures in shallow lakes during summer, there is a reduction in the diversity of aquatic life.
Lake Restoration Methods
The various lake restoration methods can be considered under two broad categories: 1) preventive or indirect methods, and 2) ameliorative or direct methods. The preventive methods are basically the ones which identify the pollutants, reduce their rate of generation, and/or prevent them from reaching the lake. The ameliorative methods involve either the treatment of the wastes before discharge into the lake (outside-lake methods) or direct intervention in the lake (in-lake methods).
1. Drainage Basin Alterations
This approach is useful primarily for controlling non point sources of pollution which are incidental to the land use of the drainage basin of the lake. Land development activities, logging of forests, and other construction activities increase erosion and, thus, sediment in the runoff.
The drainage basin alterations involve structural and land treatment measures and interception of nutrients and sediments before they reach the lake. The main soil erosion management practices include terracing, contour farming, grassed waterways, conservation cropping systems, crop residue management or increases in the amount of residue left after harvesting, and creation of shelter belts.
2. Diversion of Wastes
This is a frequently-used solution for improving the water quality of eutrophic lakes. Before undertaking treatment by this method, it is necessary to account for all the sources of nutrients to the lake so that the impact of diverted nutrient sources can be estimated.
The effluents diverted from one watershed can end up in another ecosystem. The diversion is frequently criticized because the pollution is not eliminated but only transferred to another location. This solution is
reasonable if the wastewater is mainly of domestic origin, the ecosystem receiving the wastes is able to maintain the balance, and there is enough water in the former ecosystem to bear the loss.
3. Legal Control Measures
Some political jurisdictions can impose legal controls on the land use and discharge of nutrients so as to restrict uses with direct or indirect pollution potential or effects. Legal controls allow certain land-use practices, prohibit others, and control recreational use of water, location of industries, fertilizer use, etc.
In-Lake Physical Methods
1. Mechanical Harvesting of Biomass
Harvesting of the biomass may be useful if the rate of removal by existing machinery can be improved, making it an economical alternative.
2. Lake Deepening or Dredging
Sediment removal is resorted to for improving diminished recreational potential, reducing internal cycling of nutrients, removing any toxic sediments, and reducing nuisance aquatic macrophyte growth.
Sediment-regenerated phosphorus adds considerably to the phosphorus loading in lakes. Sediment removal for deepening shallow lakes may become necessary for restoring the lake to uses for which it was designed and built. Dredging operations may lead to some short-term environmental problems, such as resuspension of sediment, possible release of toxic substances, and destruction of the bottom benthic
3. Admission of Unpolluted Waters (Dilution/Flushing)
Admission of water or dilution results in lowering of nutrient concentration and a washout of algal cells, whereas flushing achieves only a washout of algal cells.
The effectiveness of this restoration technique increases with an increase in the difference between nutrient concentrations in the inflow and lake waters. The method is economical if an adequate supply of good quality water exists and the costs of facilities and their maintenance for delivering the water to the lake are not high.
4. Aeration of Water
Dissolved oxygen, DO, is very low in deep waters of stratified lakes during summer. The DO is needed by all the living organisms in the lake for their survival. The DO deficiency occurs mainly because of the decomposition of organic material in the hypolimnion and thermal stratification which restricts movement of cold, dense water in the hypolimnion to the overlying waters in the metalimnion and epilimnion. When the DO levels fall close to zero, more phosphorus is released from the sediments under the prevailing anaerobic conditions. This causes algal blooms, which produce more organic matter, requiring more dissolved oxygen. One ameliorative technique is to pump hypolimnetic water to the surface and allow it to mix with warm epilmnetic water, or force the epilimnetic water to the hypolimnion. The resulting mixing and disruption of the thermal stratification improve the DO in the deeper waters at the expense of the shallower waters. The other technique is to directly aerate the hypolimnetic water.
5. Hypolimnetic Drainage
In a thermally stratified lake, the cool and stagnant bottom layer (hypolimnion) is rich in nutrients and low in dissolved oxygen whereas the upper warmer layer (epilimnion) has fewer nutrients and high dissolved oxygen. By the withdrawal of water from the bottom layers instead of from the top, the nutrients in the lake are reduced and the dissolved oxygen condition is improved.
In-Lake Chemical Methods
1. Phosphorus Precipitation
This method involves successive applications of aluminum sulfate or alum to disrupt the internal phosphorus cycle. This is especially useful if the high phosphorus loading is not due to allochthonous nutrient inputs.
Many eutrophic lakes respond slowly following nutrient diversion because of long water retention times and the recycling of phosphorus from sediments and other internal sources. Restoration of such lakes with alum treatment is successful, with a few undesirable effects such as reduced planktonic microcrustacea species diversity.
Maximum alum dose is defined as that which reduces pH to 6 to form insoluble aluminum hydroxide. The technique is long-lasting (about 3 years) when properly applied. Alum controls the phosphorus in sediments through the sorptive capacity of the aluminum hydroxide floc formed in alkaline water. The maximum alum should be such that residual dissolved aluminum does not exceed 0.05 mg/l.
2. Use of Algicides and Herbicides
Chemical treatments have been used for control of nuisance algal blooms and dense growth of macrophytes. Copper sulfate has been widely used for control of blue-green algae.
The low concentration is applied to water with alkalinity less than 40 mg/l and the higher concentration to water with alkalinity exceeding 40 mg/l. The effectiveness of the copper ion in controlling algae is enhanced by using a chelating agent, such as citric acid.
Some of the other algicides used, though on a very small scale, are rosin, amines, triazine derivatives, mixture of copper sulfate and silver nitrate, quaternary ammonium compounds, organic acids, aldehydes, and ketones.
In-Lake Biological Methods
Species manipulation is the main in-lake biological method for lake restoration. The method considers the introduction or promotion of organisms that are inimical to the target organisms. In nature, predation
by zooplankton and fish species keeps a sort of control on algal populations.
Some case studies of Lake Restoration Projects
Bahia Del Mar Lake Restoration Case Study
For more details you can visit: https://vertexaquaticsolutions.com/bahia-del-mar-lake-restoration/
Huizhou West Lake, Huizhou
For more details you can visit: https://www.researchgate.net/publication/327588262_Successful_restoration_of_a_tropical_shallow_eutrophic_lake_Strong_bottom-up_but_weak_top-down_effects_recorded
Nalanda Sarovar, Indore, India
For more details you can visit: https://firstname.lastname@example.org/indore-leading-the-way-in-lake-restoration-initiatives-clean-water-d7052991c120
St. Petersburg, FL
For more details you can visit: https://www.solitudelakemanagement.com/lake-and-pond-management-success-stories-and-case-studies-lake-restoration-stpete-fl/
Subscribe to get exclusive updates in our daily newsletter
Follow us on twitter @DecodingBiosph1