Comparison of the chemical properties and water temperature

The Drava River and its HPP Dubrava are home to diverse plant and animal species. Water properties, such as dissolved oxygen, nitrites, nitrates, pH, and phosphates, affect these organisms. Research over six months shows that the drainage channel of the HPP Dubrava, with higher water flow, has more favorable chemical properties, including lower nitrate concentrations and slightly cooler temperatures compared to the lake. The drainage channel also experiences greater fluctuations in dissolved oxygen. However, differences in pH and phosphate data from Hrvatske vode and GLOBE measurements were found due to different measuring kits.

Students from Prelog High School regularly conduct field research along the Drava River, including the HPP Dubrava lake and drainage channel, focusing on physico-chemical parameters. The drainage channel, with higher water flow, is expected to have lower water temperature and higher dissolved oxygen. Despite wastewater inflows, the channel supports brown trout, which are restocked annually by the Međimurje Fishing Association according to the management plan.

The HPP Dubrava Lake, near Donja Dubrava, Croatia, was created by the hydroelectric power plant for electricity generation and serves recreational purposes like fishing, boating, and cycling (Sraka et al., 2010). Adjacent to it is the drainage channel, which plays a vital role in agriculture by managing water drainage, preventing erosion, and maintaining soil quality in water-prone areas (Prišlić, 2016).

The drainage channel, an artificial ditch, regulates groundwater levels that rose due to the filling of the accumulation lake. Although seepage water is minimal, artesian wells are used to reduce pressure and supply clean, cold, oxygen-poor, slightly alkaline water to the channel (Režek, 2003). With a flow rate of about 22 m³/s, the channel maintains low temperature fluctuations and supports abundant aquatic vegetation, particularly amphipods from the genus Gammarus, which are a food source for trout (GNP, 2018; Vuković, 2021).

The research aims to compare the concentrations of nitrites, nitrates, phosphates, pH values, and water temperature between the drainage channel and the HPP Dubrava lake. It also investigates whether the water flow in the drainage channel affects dissolved oxygen, chemical concentrations, and temperature. The hypothesis suggests that the drainage channel, with higher water flow, will have higher dissolved oxygen and lower temperatures than the lake. Due to wastewater inflows, the drainage channel is expected to have higher levels of nitrates, nitrites, and phosphates, which promote algae and plant growth (Nujić, Habuda-Stanić, 2017; Hach).

From August 6, 2023, to January 28, 2024, weekly measurements were taken at the drainage channel and HPP Dubrava lake. Water temperature was measured using a HANNA Instruments thermometer, and nitrate, nitrite, pH, and phosphate levels were tested with the VISOCOLOR - School Reagent Case. Dissolved oxygen was measured with a HANNA Instruments test kit, and flow rate was assessed with a table tennis ball, water level rod, and stopwatch.

The highest nitrate concentration in the HPP Dubrava lake was 5 mg/L, recorded in mid-November and January, with a minimum of 1 mg/L. In the drainage channel, nitrate levels were lower, peaking at 2 mg/L during summer, with a minimum of 0 mg/L recorded once (Figure 2).

Due to the lack of similar research, comparisons were made with Hrvatske Vode data for HPP Dubrava and previous measurements from 2010 for the drainage channel (Sraka et al., 2010). Differences in nitrite, nitrate, and phosphate levels between this study and Hrvatske Vode were likely due to different measurement instruments.

Key conclusions based on the research hypothesis:

• Flow Rate: The drainage channel had a flow rate of 22 m³/s, much lower than the 284 m³/s at HPP Dubrava, contradicting the hypothesis that higher flow leads to lower temperatures and higher dissolved oxygen. The low gradient and lack of barriers in the channel result in low water flow velocity.

• Nitrate Concentration: Nitrate levels were lower in the drainage channel, which aligns with data from Hrvatske Vode and the reduced use of agricultural chemicals in recent years.

• Nitrite Concentration: Nitrite levels were similar between the lake and drainage channel, with no significant difference.

• Phosphate Concentration: Phosphate levels were similar in both locations, with slight increases in the channel likely due to human activities.

• Water Temperature: Water in the drainage channel was generally cooler than in the lake, which aligns with official data, though the hypothesis that higher flow leads to cooler temperatures was not supported. A rise in temperature was noted in both summer and winter compared to previous years.

• Dissolved Oxygen: The drainage channel had lower dissolved oxygen levels, likely due to its groundwater-fed nature. These lower oxygen levels were not suitable for trout, contradicting the hypothesis that the channel would have higher oxygen levels.

Comparing data from Hrvatske Vode and the GLOBE school group revealed significant discrepancies in pH and phosphate measurements, suggesting unreliable water kits. Dissolved oxygen data showed minor deviations, likely due to the scale of the kit used. Hrvatske Vode’s more sensitive equipment yielded more accurate results. Overall, the findings highlight a complex interaction between water flow, chemical composition, and temperature in the drainage channel and HPP Dubrava lake, providing a basis for further research and water resource management.