Source apportionment of nutrients in the small Lake Baikal tributary on the basis of monitoring data, using water quality index and positive matrix factorization model

The object of the study was a stream, characterised by high nutrient concentrations in water. The water quality subindex (WQSI) values calculated for nutrients were used for the identification of pollution events, the positive matrix factorisation (PMF) model was used for source apportionment, and total dissolved solids (TDS) concentrations were used as the proxies of direct flow/base flow ratios in stream flow. As the result of PMF analysis, three sources were apportioned. It was found that contributions of one source were positively correlated with TDS, contributions of the second source were negatively correlated with TDS, and the contributions of the third source were uncorrelated with TDS. It was suggested that these sources were deep groundwater, shallow groundwater, and wastewater, respectively. According to the PMF results, deep groundwater was likely the source of phosphates and nitrates, and wastewater was likely the source of ammonium and nitrites. However, the verification of results of PMF analysis by establishing the correlation between PMF-derived source contributions and WQSI values calculated for PO43- showed that wastewater may also be a significant source of phosphates. Shallow groundwater, apparently, did not contribute significantly to the concentration of any nutrient ion in stream water.