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.