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K e y w o r d s : Natural background; Total nitrogen; Total phosphorus, RAC parameter, Macromodel DNS/SWAT. 1/2019 A R C H I T E C T U R E C I V I L E N G I N E E R I N G E N V I R O N M E N T 171 A R C H I T E C T U R E C I V I L E N G I N E E R I N G E N V I R O N M E N T The Si les ian Univers i ty of Technology No. 1/2019 d o i : 1 0 . 2 1 3 0 7 / A C E E 2 0 1 9 0 1 7 P. W i l k , P. O r l i ń s k a W o ź n i a k 172 A R C H I T E C T U R E C I V I L E N G I N E E R I N G E N V I R O N M E N T 1/2019 natural background of these pollutants can serve as a good reference system indicating the maximum achievable water quality [8, 9, 10]. A good example here is the US Environmental Agency (EPA), which recommends that individual US states use the 75th percentile values from regional distributions of background nutrient concentrations as the lower end of the appropriate range for choosing state criteria. In this case, the natural background of nutrients is calculated using data from long-term reference sites in these regions; these provide a potential source of information for developing regional background concentration distributions. The problem here is that such data often contain very large errors because in practice in industrialised countries there are no such things as reference sites. As already mentioned, in both Europe and North America, there are practically no catchments deprived of anthropopressure, so it is not possible to determine the concentrations and loads of the natural background of TN and TP in surface waters based on monitoring studies. Therefore, one of the solutions to this problem is research in catchments in poorly industrialised or sparsely populated countries located in South America, Africa and Asia, where human influence is as minimal as possible [11]. Such studies have shown, inter alia, that the amount of TN in such catchments is closely correlated with the size of surface runoff and, for example, the type of vegetation occurring in an analysed area. The problem is that these studies were mainly carried out in tropical areas, and the results are hard to translate directly into conditions such as those in northern Europe [3]. Currently, more and more advanced mathematical models are becoming helpful here – apart from the simulation of catchment hydrology and pollution movement, they also allow for the introduction of variant change scenarios in the use of a catchment area [12, 13, 14, 15, 16]. Such scenarios allow for a “virtual” change in the use of any previously calibrated, verified and validated [17] river basin, allowing the simulation of the conditions that existed in the river basin in the past [18]. This allows, for example – in either a highly urbanised catchment area or one dominated by intensive farming – for a scenario in which the entire catchment area is occupied by forest or natural meadows to be introduced [19, 20]. At the same time, the model will generate water quality data on any selected calculation profiles for such a scenario. This method allows one to freely modify not only the use of the catchment area, but also the climatic conditions, which offers great opportunities. Since 2012, the DNS Macromodel [21], which employs the SWAT module and allows the simulation of natural and anthropogenic phenomena occurring in river basins, has been developed in the Section of Modeling Surface Water Quality of the Institute of Meteorology and Water Management. This model has been used in the past to develop the method of River Absorption Capacity (RAC) [22]. So far, however, this parameter has not directly taken into account the natural background of TN and TP in the river. The paper presents a universal method for expanding the RAC parameter. The extension consists in taking into account the natural background of pollution that we encounter in each river basin. 2. MATERIALS AND METHODS 2.1. River absorption capacity – RAC Absorption capacity of a river is the difference between two loads: the first of these is the limit load calculated on the basis of a limit concentration determined in Poland for different types of water by the Regulation of the Minister of the Environment. The limit concentration is calculated based on monitoring data; the second is the actual load calculated based on the actual concentration at a selected river profile. Calculation profile is a plane created at the point of intersection of the river bed perpendicular to its current. When calculating both mentioned loads, the selected characteristic flow (flow values, based generally on a multi-year hydrogram flow – usually daily) is used. The absorption capacity of a river is calculated for each pollutant separately and should consider all potential sources of pollution (both point and nonpoint sources). Absorption capacity results are obtained for selected river profiles [22]. River absorption capacity RAC for a selected calculation profile is described with the equation:

USE OF THE MACROMODEL DNS/SWAT TO CALCULATE THE NATURAL BACKGROUND OF TN AND TP IN SURFACE WATERS FOR THE RAC PARAMETER