The effects of submarine groundwater discharge on fecal indicator bacteria in a fish culture zone in Tolo Harbour, Hong Kong

SGD can significantly alter salinity and temperature of coastal waters and also deliver abundant nutrients and anthropogenic substances, such as pathogens, toxins and other pollutants. Contaminated groundwater has been proven to be the sources of fecal indicator bacteria (FIB), which is the sum of total coliform, fecal coliform (i.e., E.coli) and Enterococcus. FIB is highly influential to the public health and fishery management. This study sought to investigate the effects of SGD on the E.coli concentrations in a representative surf zone. A total of 82 nearshore coastal water and 78 coastal groundwater samples were monthly collected from July, 2019 to January, 2020 for radon (Rn), E.coli and nutrients analyses, together with the measurements of concurrent water temperature and salinity. A good positive correlation between the coastal water Rn activity and E.coli concentrations was found with a coefficient of determination (R²) of 0.56, indicating that the SGD mounts a positive effect on the E.coli over the study period including wet and dry seasons. For each sampling period, the slope (K) of each regression model represents the increasing rate of E.coli per unit of Rn activity. The averaged E.coli concentrations (11.5 – 65.5 cfu/100 ml) for each month were inversely correlated to the averaged coastal water salinity (29.8 – 34.0 PSU) with a R² of 0.66; and the inhibited temperature of E.coli was observed to be 30 ℃. For groundwater, a stronger positive linear correlation between the Rn activity and E.coli concentrations was reported with a R² of 0.87. As the temperature and salinity of groundwater were almost constant, the K was not much changed. Also, compared to the coastal seawater, groundwater showed a better correlation as the temperature and salinity in groundwater were relatively constant. On the other hand, the regression model of Rn activity and E.coli in groundwater showed that the E.coli was not changed significantly when the Rn activity reached to near 1400 dpm L^-1, which means high SGD inputs may not always lead to lasting growth of E.coli.