New Beginning for Lake Velnezers: Monitoring water quality improvements following the ALUM treatment

Following best practice, lake monitoring should be continued beyond the duration of the restoration project. Prior to treatment, Lake Velnezers was extensively monitored to gather necessary data to ensure a safe and effective ALUM treatment. Post-treatment monitoring serves a different, yet equally important purpose. The collected data is used to evaluate the effectiveness of the restoration efforts, supports adaptive management of the lake, and contributes to the development and refinement of the ALUM method itself.

Post-treatment monitoring of Lake Velnezers, conducted within the frames of the TRUST ALUM project, has been ongoing since November 2024 and continued until the project conclusion in July 2025. Every monitoring trip, the LHEI team gathered, packed an inflatable boat along with all necessary equipment, and headed out to lake.

Figure 1. Lake Velnezers water quality before and after the ALUM treatment.

To understand the lake system, data must be collected from different seasons and water depths (Figure 1). Post-treatment monitoring is conducted once per season. During each monitoring trip, water samples were taken at three different depths, representing the surface (0.5 m), middle (3.5 m) and bottom (5.0 m) sections of the water column. These samples were analyzed in the laboratory to determine the concentrations of phosphorus, nitrogen, aluminum, and other components of lake water. Key water parameters, such as: transparency, pH, temperature and oxygen levels were also measured in the water column.

Since the ALUM treatment in October 2024, Lake Velnezers was monitored four times – in November 2024 and in January, April and July 2025. What have we learnt? As seen on the graphs below, the markers of lake water quality showed substantial improvement. The average concentration of all combined forms of phosphorus – total phosphorus – dropped by up to 91%, with the average concentration of biologically available form of phosphorus – orthophosphate – decreasing by 95 – 99% depending on month. Within a month after treatment water transparency increased from 0.8 to 2.6 m. In the following seasons, it began gradually decreasing, remaining however, above pre-treatment levels. During monitoring, we also kept a close eye on aluminum concentrations in water. As expected, levels increased following the treatment, but remained safely below the limit for drinking water. At present, aluminum concentrations are steadily declining toward pre-treatment levels.

Changes in Lake Velnezers Ecological Quality Classification

Pre-treatment the yearly average concentration of total phosphorus in surface water of Lake Velnezers amounted to 0.07 mg/L. According to the Latvian method of ecological quality classification based on physical and chemical parameters, such high value classified the lake at the boundary between moderate and bad ecological quality. Post-treatment the value of this parameter decreased to 0.03 mg/L, which together with the yearly average concentration of total nitrogen of 1.1 mg/L indicates that currently Lake Velnezers has good ecological quality.

Ongoing studies will show how the water quality improvement achieved by the reduction of internal phosphorus loading in Lake Velnezers is reflected in the diversity and health of aquatic life. Aquatic organisms require slightly more time to recover from decades of harsh living conditions in a phosphorus-polluted lake. However, with time, the improved water quality will support the development of a healthy urban habitat where fish and other aquatic life can thrive, and the local community can enjoy!

More information about Lake Velnezers and the latest monitoring results can be found on a new, dedicated website.

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