Trend, seasonality and relationships of aquatic environmental quality indicators and implications: An experience from Songhua River, NE China

To ensure safety of drinking water and verify effectiveness of environmental protection, it is essential to monitor aquatic environmental quality of rivers regularly, alongside hydrological variables. However, routine surveillance without in-depth analysis may prove insufficient to elucidate the changes, which is established issues receiving much attention. This study aimed to identify the trend, seasonality and relationships among the aquatic environmental quality indicators at a monitoring point of the mainstream of an important large river in Northeast China, the Songhua River. The study also endeavored to discuss the main factors impacting the quality and to verify the effectiveness of management. The aquatic environmental quality data and the hydrometeorological data were collected monthly from 2012 to 2018. Statistical analyses revealed that at the 5% significance level, SO42−, Cl− and total nitrogen (TN) showed an increasing trend over time, while F−, NH3-N and CODCr showed a decreasing trend. Chroma, SO42−, NH3-N, TN, Fe and dissolved oxygen showed relatively obvious seasonal changes. The water quality changed seasonally with the best occurring in summer and the worst in winter. And each season had its own typical indicators controlling the quality class. However, the randomness of the indicator changes cannot be ignored, which indicates the complexities of impacting factors and mechanism. There are both natural and anthropogenic factors and the latter is mainly the sewage discharge from industrial and agricultural production and residential life. The seasonality of natural impacts is obvious, while the trend or the randomness induced by anthropogenic impacts is more obvious. It remains imperative to ensure that the sewage discharge complies with the national discharge requirements and further reduce the discharge amount. It may be necessary for water plants to adjust the purification process according to the changes of river water quality, and for authorities to consider optimizing the existing monitoring program especially considering the monitoring indicators.