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 ammonia 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 water 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 (natural background value). The values of nitrate and ortophosphate in water that exceed the natural background 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 ammonia 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. In a water environment, ammonia also represents an increased demand for oxygen. In the nitrification process, ammonia 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 ammonia under certain conditions (a combination of water temperature, salinity and pH-value). Ammonia represents a risk for drinking water sources as well.

Nitrate concentrations in the observed period were only slightly above the natural background values, which range between 0.2 and 0.9 mg N/l in Slovenian rivers depending on their ecological type. Values ​​vary over the years due to natural factors as well as due to the varying sampling points in the annual monitoring programme (the latter applies to all the parameters shown). Nitrate values ​​in the Drava river catchment, Sava river catchment and especially in the Mura river catchment are higher than in the Soča river catchment and Adriatic rivers. Orthophosphate values in rivers also differ among river catchments. In Soča river and Adriatic rivers orthophosphate is in the range of the natural background 0.01 mg P/L (EEA), but in the rivers of the Danube basin it is well above the natural background, from which Mura with Kučnica, Ledava and Ščavnica stand out with the highest values​​.

Trends analysed for individual sampling sites with longer historical data sets (Figure VD10-5) show that nitrate and orthophosphate concentrations are already in the range of the natural background at one third of the sampling points in the analysis. Trends are downward for about 15 to 20 % of the sampling points, however at the remaining 50 % of sample sites improvement cannot be expected, as the trend analysis does not show any future changes or it even shows that a slight increase in values can be expected.

Nitrogen and phosphorus loading of rivers is mostly due to draining from agricultural areas, which is confirmed by the highest measured values ​​of nutrients in ​​the Mura river catchment, where the agricultural activity in Slovenia is the most intensive, however municipal and industrial sewage can also be a significant source. The indicator VD07 Phosphorus in lakes also indicates a similar nutrient loading of waters in NE Slovenia, where the highest values ​​of phosphorus accumulations are observed.

In the observed period of 1996-2017, the organic load of rivers decreased considerably. Particularly noticeable is the decrease in BOD5 values from 2006 onwards, when the values occasionally approached the levels of the natural background, which ranges between <1 and 1.4 mg O2/l in most Slovenian rivers. The decrease is larger in east of Slovenia than in the west, where the organic loading of rivers has also been lesser in the past. Ammonia levels show similar improvement.

Urban wastewater is 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 today than it was in 2004. The percentage of secondary (biological) wastewater treatment, which removes the majority of organic substances and some nutrients, is also much higher today than it was in 2004.