Characteristics of water quality change during the 13th Five-Year Plan period and trend prediction during the 14th Five-Year Plan period in three major basins of Hainan Island

Based on the monitoring data of Hainan Province surface water environment quality network, the water quality, concentration of major pollutants, variation of composite pollution index in three major basins of Nandu River, Changhua River, Wanquan River in Hainan Island during the 13th Five-Year Plan period were analyzed, and Spearman rank correlation coefficient was used to research and judge the pollution changecharacteristic, and the mean GM(1,1) model (EGM) was used to predict the trend of water quality change during the 14th Five-Year Plan period for servicing the water ecological environment protection of Hainan Free Trade Port. The results showed that the water quality of three major basins in Hainan Island was getting better steadily during the 13th Five-Year Plan period. The proportion of Grade Ⅰ-Ⅲ water quality of Environmental Quality Standards for Surface Water (GB 3838-2002) in Nandu River basin and Wanquan River basin increased to 96.3% and 100%, respectively. The proportion of Grade Ⅰ-Ⅲ water quality in Changhua River basin remained at 100%. The average annual concentrations of permanganate index, chemical oxygen demand, ammonia nitrogen and total phosphorus were all at a low level, which were lower than the standard limits of Grade Ⅱ, and the average annual concentration of chemical oxygen demand was lower than the standard limits of Grade Ⅰ. The average annual concentration of ammonia nitrogen was decreased during the 13th Five-Year Plan period. The average annual concentrations of permanganate index, chemical oxygen demand and total phosphorus were increased or fluctuated during the 13th Five-Year Plan period, with an increase range of 6.7% to 23.8%. The composite pollution index of the mainstream declined with a range of 0.03 to 0.04. The composite pollution index of the main tributaries increased with fluctuations from upstream to downstream. EGM predicted that during 14th Five-year Plan period, the comprehensive pollution index of the mainstreams of three major basins and most of the tributaries showed an upward trend, with a maximum increase of 108.3%. The water environment of three major basins in Hainan Island would face high pressure, and especially the pollution load from the tributaries should not be neglected. According to the prediction results, in order to improve the water quality of the three major basins, it was necessary to implement measures such as “Governance of Six Waters” and “One River One Policy” in a systematic protection approach during the 14th Five-Year Plan.