Using satellite data to monitor the quality of inland lakes
Pollutants are known to enter rivers, lakes and coastal waters, damaging and threatening drinking water and crucial habitats. Unfortunately, in situ water quality sampling is costly, time consuming and often unrealistic over vast areas. The EU-funded GLaSS project has developed a system to help monitor global lakes and water reservoirs remotely.
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Eutrophication is affecting many of our lakes and rivers. This occurs when fertilisers upset the natural balance of aquatic and marine ecosystems by increasing the levels of nutrients, such as nitrogen and phosphorus, in the water. Phytoplankton and algae start to grow prolifically, sometimes producing toxins and using up the oxygen dissolved in the water during decay. Ultimately, aquatic life starts to die, and without clean water our entire existence becomes vulnerable.
Every year, water filled with hazardous chemicals also breaks through the walls of mining tail ponds. Many of these thousands of dams are in remote areas and have been abandoned, placing nearby areas at risk. “Locating these pools via satellite is the first step to preventing future disasters,” reports GLaSS project manager Annelies Hommersom.
Recognising these and other serious issues affecting water quality, the EU has implemented the Water Framework Directive (WFD) that all EU member states must now follow. Each EU country must submit detailed reports about its water basin monitoring activities. To help ecologists and water managers comply with WFD reporting, the EU has funded the development, testing and validation of the GLaSS methods.
Assessing water quality from space
The GLaSS system — tools, algorithms and applications — enables ecologists and water managers to use earth observation (EO) data from satellites for water quality monitoring and management purposes. “GLaSS is one of the first projects to prepare to work with large quantities of data from the Copernicus satellites Sentinel-2 (S2) and Sentinel-3 (S3). When combined, the data from S2 and S3 provide unprecedented monitoring capabilities for inland waters,” reports Hommersom.
The satellites include optical sensors that detect the water’s colour (the reflectance per wavelength). With its high spatial resolution, S2 is good for inland lake monitoring, while S3 captures optical images in lower spatial resolution, enabling scientists to work with more wavelength bands — differences in shades of colour. For example, the data from S3 enables researchers to detect blue green algae, which are potentially toxic, as opposed to green algae, which are food for animals, such as fish. Also mining tailing ponds appear as distinctive colours.
Commercially available from providers in Germany, the Netherlands and Sweden, the GLaSS system first ingests and processes raw data from S2 and S3. It uses a pre-classification tool to select the category of water and uses algorithms to best match the water sensed remotely to the spectrum of colours. The pre-classification tool applies algorithms to the colour data to determine what substances, such as algae, sediment, and chemicals, are in the water. And what’s in the water determines the water quality.
Assessing water quality through field studies
The GLaSS project studied four categories of lakes: eutrophic lakes, deep clear lakes, shallow lakes with high suspended matter, and highly absorbing lakes. It used satellite and in situ data, as well as the GLaSS tools and adjusted algorithms to study individual lakes, and to validate the tools.
Researchers chose four algorithms to monitor eutrophication by its proxy Chlorophyll and cyanobacteria abundance in eutrophic lakes. “We found that eutrophication is taking place in some deep clear lakes, such as Lakes Maggiore and Constance, whereas overall chlorophyll concentration is decreasing in other lakes, such as Lake Garda and Tanganyika,” reports Hommersom.
Wind and dredging — clearing the bed of a harbour, river, or lake by scooping out mud, weeds, and debris — impacts water quality too. Those natural and manmade processes affect the concentration of suspended matter, as measured by the GLaSS project. For glacial lakes, measuring these concentrations can predict dangerous glacial lake run-off events.
The GLaSS system can also monitor polluted water from oil refinery activities. “We developed an algorithm to detect the concentration of oil contaminants in Azerbaijan,” reports Hommersom. In a large restoration project, the government built dams to contain the polluted water. GLaSS researchers tracked improvements in the quality of the water in the lake. These results have been used as an example for other oil refineries and construction companies to follow.
The GLaSS project didn’t stop there, however. Results have been used to develop training material for masters-level students and water managers. Together, the findings and the training materials could mark a turning point in global water quality management; combining the advantages of remote monitoring from satellites with field studies on the Earth’s surface could mean cleaner water for all.