Characteristics and trends of water quality in underground caverns in island

Underground caverns can be used as a supplementary water resource in those islands away from the continent. The groundwater quality is then becoming an important condition for ensuring the safety of water supply. Based on the water quality monitoring data from April 2017 to July 2019, the trend of water quality indices such as pH, COD_Mn(permanganate index), NH₃-N(ammonia nitrogen), turbidity, TN(total nitrogen), TP(total phosphorus) and chloride were analysed by Kendall-trend-test method. The Pearson-correlation-method is also used to analyse the relationship among these indices in this study. The results show that: (1) For different forms of underground caverns, the concentration of pollutants and the number of bacteria in fully closed cavern is relatively low compared to the semi-closed cavern, the total number of bacteria is only 9 CFU/mL, and the DO(the dissolved oxygen) is as low as 5.12 mg/L. The concentration of pollutants in underground caverns with surface water is higher than the rainwater, which demonstrates that the groundwater quality is actually worse than the water source. (2) The water quality of underground caverns will change variously with time. The concentrations of COD_Mn and NH₃-N will decrease gradually with time, the TN will increase gradually, the concentration of TP will decrease significantly in fully closed caverns, while it will not change too much in semi-closed caverns. (3) Correlation analysis showed that there was no significant relationship among TP, TN and chlorophyll in different forms of caverns. In order to ensure the safety of water quality, the underground caverns should be disinfected, sterilized, and moreover, the sludge should also be cleaned up regularly. (paper)