Changes in Escherichia coli to Cryptosporidium ratios for various fecal pollution sources and drinking water intakes

Highlights

• High Escherichia coli/Cryptosporidium ratios for sewage show E. coli was a suitable indicator.

• Low E. coli/Cryptosporidium ratios for rural runoff show E. coli was a poor indicator.

• E. coli explains Cryptosporidium concentrations at DWI impacted by municipal sewage.

• E. coli was not a good indicator of Cryptosporidium for agriculture impacted water.

Abstract

Assessing the presence of human pathogenic Cryptosporidium oocysts in surface water remains a significant water treatment and public health challenge. Most drinking water suppliers rely on fecal indicators, such as the well-established Escherichia coli (E. coli), to avoid costly Cryptosporidium assays. However, the use of E. coli has significant limitations in predicting the concentration, the removal and the transport of Cryptosporidium. This study presents a meta-analysis of E. coli to Cryptosporidium concentration paired ratios to compare their complex relationships in eight municipal wastewater sources, five agricultural fecal pollution sources and at 13 drinking water intakes (DWI) to a risk threshold based on US Environmental Protection Agency (USEPA) regulations. Ratios lower than the USEPA risk threshold suggested higher concentrations of oocysts in relation to E. coli concentrations, revealing an underestimed risk for Cryptosporidium based on E. coli measurements. In raw sewage (RS), high ratios proved E. coli (or fecal coliforms) concentrations were a conservative indicator of Cryptosporidium concentrations, which was also typically true for secondary treated wastewater (TWW). Removals of fecal indicator bacteria (FIB) and parasites were quantified in WWTPs and their differences are put forward as a plausible explanation of the sporadic ratio shift. Ratios measured from agricultural runoff surface water were typically lower than the USEPA risk threshold and within the range of risk misinterpretation. Indeed, heavy precipitation events in the agricultural watershed led to high oocyst concentrations but not to E. coli or enterococci concentrations. More importantly, ratios established in variously impacted DWI from 13 Canadian drinking water plants were found to be related to dominant fecal pollution sources, namely municipal sewage. In most cases, when DWIs were mainly influenced by municipal sewage, E. coli or fecal coliforms concentrations agreed with Cryptosporidium concentrations as estimated by the meta-analysis, but when DWIs were influenced by agricultural runoff or wildlife, there was a poor relationship. Average recovery values were available for 6 out of 22 Cryptosporidium concentration data sets and concomitant analysis demonstrated no changes in trends, with and without correction. Nevertheless, recovery assays performed along with every oocyst count would have enhanced the precision of this work. Based on our findings, the use of annual averages of E. coli concentrations as a surrogate for Cryptosporidium concentrations can result in an inaccurate estimate of the Cryptosporidium risk for agriculture impacted drinking water intakes or for intakes with more distant wastewater sources. Studies of upstream fecal pollution sources are recommended for drinking water suppliers to improve their interpretation of source water quality data.

Introduction

Cryptosporidium is one of the most frequently identified etiologic agents associated with drinking waterborne illness in the United States (US) (Craun et al., 2006) and in some European countries such as England and Wales (Smith et al., 2006). In reaction to the threats posed by this pathogen, increasing rigorous regulations on its surveillance and removal were promulgated in the US and in England (USEPA, 2005a, USEPA, 2005b, USEPA, 1998, Lake et al., 2007). Oocyst quantification through USEPA Method 1623 (USEPA, 2005b) has so far been the reference method but challenges remain, such as widespread affordability (USEPA, 2005a). Consequently, most regulations rely on widely recognized fecal indicators to assess the microbiological quality of surface water (Tallon et al., 2005, Ashbolt et al., 2001). Several studies have shown positive correlations in different types of aquatic systems between Cryptosporidium oocysts and fecal indicators, turbidity and rainfall (Table S1). Significant correlations have been reported for somatic coliphage (Fu et al., 2010), Escherichia coli, fecal streptococci and total coliforms in sewage (Reinoso et al., 2008) and for Clostridium perfringens spores (Atherholt et al., 1998, Payment and Franco, 1993), E. coli (McGuire et al., 2002, Wilkes et al., 2009) and rainfall events (Kistemann et al., 2002, Schets et al., 2008) for surface water. However, in most cases, studies conducted on surface water and wastewater have shown that the concentrations of Cryptosporidium oocysts were poorly or uncorrelated with those of traditional indicators such as E. coli, total and fecal coliforms and enterococci (Fu et al., 2010, Gibson et al., 1998, Isaac-Renton et al., 2005, Wohlsen et al., 2006, Payment et al., 2000, World Health Organization WHO, 2009, Payment and Locas, 2010, Rose et al., 2004, Wu et al., 2011a) or with turbidity or rainfall (Wilkes et al., 2009, Isaac-Renton et al., 2005). Reasons for poor correlations may be inherent to limited data sets and statistics using Cryptosporidium counts below detection limits for linear correlations (Wu et al., 2011a). As a consequence, costly Cryptosporidium monitoring in source water (USEPA, 2003) and in treated drinking water (Drinking Water Inspectorate (DWI) 2000) and increased treatment requirements for high risk systems have been implemented as regulatory requirements in the US and in England. However, for many other countries, E. coli is still accepted as the best (and affordable) surrogate of contamination by Cryptosporidium (WHO, 2006).

Relationships between E. coli and Cryptosporidium presence and concentrations have been studied with the help of an important survey conducted on American raw waters during the regulatory development process (Information Collection Rule: ICR and its Supplemental Survey: ICRSS). Results supported the decision not to require expensive Cryptosporidium monitoring for small systems under the USEPA Long Term 2 Enhanced Surface Water Treatment Rule (LT2) if they had low densities of E. coli (in lakes or reservoirs: <10; in flowing streams: <50 CFU 100 ml⁻¹) (USEPA, 2003). These levels were considered too stringent for small systems and were increased up to 100 E. coli 100 ml⁻¹ in the 2010 revisions (USEPA, 2010). This E. coli level is considered to safely predict densities of Cryptosporidium below 0.075 oocysts L⁻¹ in source waters.

A better understanding of the implications of using indicators to define pathogen concentrations could be obtained by examining paired E. coli/Cryptosporidium ratios, further referred to as Ec/Cr, in different types of pollution sources, namely raw sewage (RS), treated wastewater (TWW) and agricultural runoff (AgrRO). To help comparisons, a reference ratio Ec/Cr of 1.3 × 10⁴ can be calculated from these values.

The main objective of this study was to nuance the significance of E. coli originating from various water sources as a surrogate for Cryptosporidium. A meta-analysis was performed using published and local data to calculate Ec/Cr ratios for the primary sources of fecal contamination, namely raw sewage, treated wastewater and manure contaminated agricultural runoff. Ratios were shown to be very different for wastewater effluents and agricultural runoff. The influence of various and mixed fecal pollution sources on the ratio were investigated at 13 drinking water intakes (DWI). Monitoring recommendations are proposed for DWI where the ratio is insufficiently conservative.