The indicator shows the level of nutrient and organic matter loading of rivers due to human activities. Nitrate and orthophosphate are representative parameters for nutrient loading, and biochemical oxygen demand and ammonium are representative parameters for organic loading. The parameters were measured in water samples taken in the course of regular monitoring of surface water quality.

Nitrate and ortophosphate are the main nutrients required for the growth of algae, mosses and aquatic plants in rivers. In non-polluted waters they are present at very low concentrations that mostly depend on geological properties and soil types in the catchment area. These so called natural or background values range for nitrate in Slovenian rivers between 0.2 and 0.9 mg NO₃-N/l depending on their ecological type. Background value for orthophosphate according to EEA is 0.01 mg PO₄-P/L (EEA). Nitrate and orthophosphate concentrations in water that exceed background values are a result of human activities, mainly agriculture and urban/industrial wastewater discharge. The surplus of nutrients in rivers leads to a deterioration in the normal functioning of the ecosystem, reduced biodiversity and eutrophication, which is problematic mainly in standing or slowly running waters. High nitrate content may also compromise the quality of drinking water, which is no longer suitable for drinking when nitrate concentration exceeds 10 mg NO₃-N/l (50 mg NO₃/l).

Biochemical oxygen demand (BOD) and ammonium are the main indicators of organic water pollution. The values of both parameters are usually elevated as a result of urban and industrial wastewater discharge and leaching of nutrients from agricultural land. The BOD is the amount of dissolved oxygen (in mg O₂/l) needed (i.e. demanded) by aerobic organisms to break down organic matter present in a given water sample at a certain temperature over a specific time period. It is usually expressed as BOD₅, which tells us how much oxygen is consumed per litre of sample over 5 days of incubation at 20°C. Background values for BOD₅ range between <1 and 1.4 mg O₂/l in Slovenian rivers. In an aquatic environment, ammonium also represents an increased demand for oxygen. In the nitrification process, ammonium is converted (through oxidation) to nitrite and then to nitrate, thus contributing to the eutrophication process. In small quantities, nitrite is toxic to water organisms as is ammonium, which is derived from ammonium under certain conditions (a combination of water temperature, salinity and pH-value). Ammonium represents a risk for drinking water sources as well. Background value for ammonium according to EEA is 0,015 mg NH₄-N/L.

Nitrate concentrations in the observed period were only slightly above the background values, which range between 0.2 and 0.9 mg N/l in Slovenian rivers depending on their ecological type. Increase in nitrate values ​​in the Mura river catchment since 2005 is observed as an increase in annual average concentrations at most of the sampling sites on Mura and its tributaries: Ledava with tributaries, Kučnica and Ščavnica. Orthophosphate values in rivers also differ among river catchments. In Soča river and Adriatic rivers orthophosphate is in the range of the background 0.01 mg P/L, but in the rivers of the Danube basin it is well above the background. In 2020, the values of the Mura river catchment are the lowest in the observed period. They are similar to the average values of other river catchments of the Danube basin. Nitrate values are also slightly lower. Nitrogen and phosphorus loading of rivers can be due to draining from agricultural areas or municipal and industrial sewage however, we cannot separate the sources based on monitoring data.

Organic loading of rivers decreased considerably from 1996 to 2006, and since then the BOD₅ values remain close to the range of the natural background, which ranges between <1 and 1.4 mg O₂/l in most Slovenian rivers. The decrease is larger in eastern part of Slovenia than in the western part, where the organic loading of rivers has also been lesser in the past. Urban wastewater is usually the main source of pollution with organic substances. As wastewater discharge and treatment has been improving in Slovenia for several years, observed changes in the parameters of organic load are expected. The indicator VD02: Wastewater treatment shows that the share of the population whose wastewater is treated by wastewater treatment plants is considerably higher since 2004. The percentage of secondary and tertiary wastewater treatment, which removes the majority of organic substances and some nutrients, is also increasing. Ammonium values are still above the natural background in all river catchments except the Adriatic river catchments.