Cross-method assessment of coliphage detection methods in the framework of EPA regulatory standards

This study aimed to understand the performance and utility of US EPA-approved coliphage methods in comparison to fecal indicator bacteria (FIB) and molecular microbial source tracking (MST) methods in recreational waters. We used US EPA Method 1642 to quantify concentrations of coliphage along with culture- and molecular-based enumeration of E. coli and Enterococcus sp, and human fecal source marker HF183. We also conducted a feasibility assessment to determine the utility of US EPA Method 1642 for application to routine recreational water monitoring. Ten sampling events were conducted at three sampling sites over the duration of a year. Average concentrations of somatic (SC) (log10 1.48 PFU/100 mL) and male-specific (MSC) coliphages (log10 1.00 PFU/100 mL) at all sites were low with SC found across a broader range (0.3-3.1 log10 PFU/100 mL) of concentrations compared to MSC (non-detect-1.7 log10 PFU/100 mL). A feasibility assessment was conducted across US EPA Method 1642 for coliphage enumeration, culture-based FIB, defined substrate technology (DST) approaches Enterolert™ and Colilert®, and quantitative microbial source tracking (qMST) US EPA Method 1696. US EPA Method 1642 had the longest processing times, but also was moderate in cost, compared to the DST and qMST molecular methods. Given the poor correlations between MSC and SC with FIB and qMST markers in this study and the cumbersome nature of US EPA Method 1642, the method may not be the most applicable method for use in systems impacted predominantly by stormwater and other non-point source pollution. Findings from this study, however, provide guidance on the application of fecal indicator virus in ambient coastal surface waters.