Towards a rational strategy for monitoring of microbiological quality of ambient waters

Abstract

Water is one of the main sources of human exposure to microbiological hazards. Although legislation establishes regulatory standards in terms of fecal indicator bacteria to assess the microbiological quality of water, these do not necessarily predict the presence of pathogens such as parasites and viruses. Better surveillance and management strategies are needed to assess the risk of pathogens' waterborne transmission. We established a baseline dataset to characterize river water quality, identify changes over time, and design a rational monitoring strategy. Data from a year-long monthly monitoring campaign of the polluted Arenales River (Argentina), were analyzed to statistically correlate physicochemical and microbiological variables, the seasonal and longitudinal variations of the water quality and determine the similarity between study sites. The measured variables (sixteen) reflected the deterioration in the river quality through the city. Different viruses and parasites found did not correlate with the concentration of total and thermotolerant coliforms. There was significant seasonal variation for temperature, turbidity, conductivity, dissolved oxygen, enterococci, and norovirus. Strong correlations between some variables were found; we selected eight variables (dissolved oxygen, conductivity, turbidity, total and thermotolerant coliforms, Enterococcus, and adenovirus and Microsporidium as viral and parasitological indicators, respectively) for future monitoring. There was similarity between the monitoring locations, which were grouped into four clusters validated by cophenetic correlation and supported by discriminant analysis. This allowed us to reduce the number of sites, from eleven down to five. Sixty seven percent of the total variance and the correlation structure between variables were explained using five principal components. All these analyses led to a new long-term systematic monitoring scheme. A rational monitoring strategy based on the selection of the most suitable monitoring points and of the most significant variables to measure, will result in optimal use of the limited resources available to adequately protect the public and environmental health.

Figure 1

Geographical representation of the area under study: (a) Province of Salta in Argentina, (b) Capital Department in the Province of Salta, (c) Area of study in Salta City, main city of the Capital Department, (d) Monitoring points (P1-P11) along the Arias-Arenales River and wastewater treatment plant (WWTP). The ellipses correspond to points on the river while the rhombi correspond to points of discharges to the river.

Figure 2

Monitoring points located on the river (P1, P2, P6, P10, and P11) for wet and dry seasons. A. Concentration of dissolved oxygen. B. Concentration of thermotolerant (or fecal) coliforms. The bottom and top of the box are the 25th and 75th percentiles. The whiskers represent 5 and 95 percentiles. The black bar indicates the median and the black square shows the mean.

Figure 3

Detection of adenovirus (AdV), enterovirus (EV), and norovirus genotype II (NV) in samples from the monitoring points (P1-P11) under study. Positive results are expressed as ratios (positive samples/total tested samples) and as a percentage.

Figure 4

Cluster analysis (Ward’s hierarchical method) for the eleven monitoring sites based on microbial and physicochemical variables. Four mayor clusters (C1, C2, C3, and C4) were formed.