Water quality regulations for swimming and recreation are enforced around the world to protect humans and the environment from hazardous pollution levels (Kirschner et al., 2009; Nevers et al., 2014; Reder et al., 2015). Although the methods used to quantify microbial water quality vary, common procedures include culturing of fecal microorganisms (e.g., Escherichia coli and enterococci) using indicator media. However, these culture-based methods require time and labor before a result is obtained.
This study investigated whether FCD could be estimated from physical or chemical parameters of river water which are easy and quick to measure. One parameter is electrical conductivity, and another is chloride ion concentration. Electrical conductivity is considered one of the predictive parameters for water hygiene, but its utility appears to vary from region to region. Guzman-Otazo et al. (2019) showed that electrical conductivity was positively associated with DNA concentration, number of gapA-positive bacteria and pathogenic E. coli in the Choqueyapu River in La Paz, Bolivia. In contrast, conductivity did not show a significant correlation with E. coli or total coliforms in the Red River basin of North Vietnam (Nguyen et al., 2016) and in the groundwater within the gold mining environment of Ghana (Armah, 2014). Chloride ion concentration is also considered as a physicochemical indicator of water pollution caused by organic waste from animals or industrial origins (Bhadra et al., 2003). David and Haggard (2011) showed that fecal bacterial concentrations were significantly correlated with dissolved chloride ion concentrations in the Illinois River in USA. In contrast, it was argued that chloride concentrations were unrelated to fecal contamination in the Mfoundi River Basin of Yaoundé, Cameroon (Djuikom et al., 2009). Our study also showed no significant correlation between chloride ion concentration and FCD in the Toga River, Japan. As described by Howard et al. (2004), chloride ions may be minimal as indicators of water pollution, because they are often found in water originating from sources other than that containing fecal matter. Chloride ions found in urban rivers may be due to various human activities such as road salt removers, fabric softeners and discharge of sewage containing food waste (Hunt et al., 2012). Kobe City’s location near the coast may also affect the seasonal or temporal change of chloride ion concentration. On the other hand, our data showed that electrical conductivity was an excellent parameter for estimating FCD in river water indicating its utility as a convenient and time-saving screening tool. Urban rivers flow through residential areas where agriculture, animal husbandry and wildlife are not potential sources of water pollution, therefore, the increase of conductivity will mostly reflect contamination from sewage and household drainage. Furthermore, in the case of the Toga River, possible causes of increased conductivity such as industrial effluent and groundwater contamination are rare, which seems to make electrical conductivity a reliable parameter.
The results of our study have revealed potential sources of fecal coliform contamination in the Toga River. As shown in Fig. 5, high FCDs were estimated at sites A and C, but were lower at upstream sites D and E. Site C showed the highest values at each sampling time, suggesting that affluent 2 is at least partly responsible for the pollution levels. The lower values at site B are probably due to the dilution of affluent 2 water in the main stream of the Toga River. Affluent 1 merges with the mainstream between sites A and B, but our previous investigation confirmed that this affluent zone is adequately isolated from the residential area and no significant fecal coliform pollution was observed at any time (data not shown). Therefore, it is unlikely that the inflow of contaminated water from affluent 1 caused high values at site A. Further study and detailed investigations are necessary to clarify the reason for the observation at site A.
Finally, Fig. 6 shows the correlation between the FCD measured at site C and the value estimated from the electrical conductivity of the same sample water. There is a good correlation between the measured and estimated values, indicating that electrical conductivity is a good indicator of fecal contamination. The FCD varied greatly depending on the month of measurement, indicating that fecal contamination occurred intermittently at the affluent 2 site. In July and September, the FCD was below the value set by the Ministry of the Environment of Japan as being suitable for swimming. The point where pollution occurs and the frequency of discharge of polluted water are still unknown, but in the future, an attempt will be made to identify the location of the source using electrical conductivity as an index.