Appendix A.2: Evaluation of Multiple Indicator Combinations to Develop 

Quantifiable Relationships

COMPLETION NOTICE

Evaluate multiple indicator/method combinations to develop quantifiable relationships 

(P15) 

Summary of the Study 

The study examines the quantifiable relationships between combinations of multiple fecal 

indicator bacteria (FIB) and analytical methods (AM) as they relate to gastrointestinal illness 

(GI) risks in swimmers (i.e., linkage approaches). Specifically, the study evaluates quantitative 

polymerase chain reaction (qPCR) and membrane filtration (MF) method comparisons, using 

available Enterococcus, Bacteroidales and E. coli data. Two different linkage approaches are 

explored: Risk Link and Water Quality Link. The Risk Link approach uses FIB-GI relationships 

generated from two epidemiological studies to determine densities of alternative FIB/AM 

combinations corresponding to the same GI risk level for swimmers. The Water Quality Link 

approach links an indicator-method combination, for which there is no health effects relation, to 

an indicator-method combination with a health effects relation via a quantifiable relationship 

between their measured fecal indicator bacteria densities. As part of this approach, the uses of 

paired water quality data, rather than the linkage of health effects curves alone, were explored. 

Summary of Findings 

This study created quantifiable relationships between multiple FIB/AM combinations and GI 

risks, using both the Risk Link and Water Quality Link approaches. Three demonstrations were 

explored using the Risk Link approach. The first demonstration illustrated the linkage of 

Bacteroidales and Enterococcus, as measured by qPCR. Health effects curves for both of those 

indicator-method combinations were developed concurrently as part of the NEEAR marine 

studies. The second demonstration illustrated a technique for assessing health effects curves from 

different epidemiology studies with similar study designs, but conducted at different times and 

settings. In this linkage, epidemiological studies by Dufour

1 

and Marion et al.

2 

were evaluated as 

they both show a relationship between GI and E. coli, as measured by MF. Health effects curves 

for the aforementioned studies pooled statistically, indicating that while the epidemiology studies 

differ temporally and spatially, the relationships between indicators and health effects are 

similar. The third demonstration illustrates a harmonization of health effects curves from two 

epidemiological studies using different illness definitions and analytical methods. In this 

demonstration, a comparison of the FIB-GI slopes generated from Cabelli

3 

and the NEEAR 

marine studies (Fairhope, Goddard and Biloxi) allowed for the generation of Enterococcus 

density values, as measured by either MF or qPCR, at the same risk levels. 

1 

Dufour, A.P. 1984. Health Effects Criteria for Fresh Recreational Water. EPA-600/1-84-004. 

Research Triangle Park, NC: USEPA. 
2 

Marion, J.W., Lee, J., Lemeshow, S., Buckley, T.J. 2010. Association of illness and recreational 

water exposure during advisory and non-advisory conditions at an inland U.S. beach. Water 

Research 44(16): 4796-4804. 

3 

Cabelli, V.J. 1983. Health Effects Criteria for Marine Recreational Waters. EPA-600/1-80-031. 

Research Triangle Park, NC: USEPA. 
Use of the paired (qPCR and MF) water quality data, from the epidemiological studies in the 

Great Lakes as part of EPA’s NEEAR study, allowed for an exploration of the Water Quality 

Link approach. For purposes of this analysis, paired water quality data could not be pooled 

across beaches, indicating that the Water Quality Link approach may work only on a site-specific 

basis. 

This study has been completed as of December 15, 2010.
U.S. Environmental Protection Agency

EVALUATION OF MULTIPLE INDICATOR COMBINATIONS 

TO DEVELOP QUANTIFIABLE RELATIONSHIPS

U.S. Environmental Protection Agency 

Office of Water 

December 2010 

EPA 822-R-10-004
U.S. Environmental Protection Agency

Table of Contents 

Executive Summary.......................................................................................................................1

1 Introduction ............................................................................................................................3

1.1 Purpose, Objectives, and Importance................................................................................3 

1.2 Approaches for Developing Water Quality Measures Corresponding to Equivalent 

Risks..................................................................................................................................4 

1.2.1 Risk Link.....................................................................................................................4 

1.2.2 Water Quality Link .....................................................................................................6 

2 Background.............................................................................................................................8

2.1 Review and Assessment of Relevant Epidemiology Studies and Datasets.......................8 

2.1.1 Prospective Cohort Studies.........................................................................................8 

2.1.2 Randomized Control Trials.........................................................................................9 

2.1.3 Studies that have Generated Health Effects Relationships.........................................9 

2.1.4 Factors that Preclude Direct Comparison of Epidemiology Studies ........................10 

2.2 Differences in Indicator Performance for Indicators as Measured by CultureBased and qPCR-Based Methods ...................................................................................11 

2.2.1 General Method Performance...................................................................................11 

2.2.2 Factors Impacting Indicator/Method Performance ...................................................16 

2.2.3 Uncertainty of Indicator/Method Combinations at Low Density .............................20 

3 Analyses.................................................................................................................................21

3.1 Risk Link.........................................................................................................................21 

3.1.1 Review of NEEAR Study and Other Health Effects Relationships for qPCR 

and Culturable FIB....................................................................................................22 

3.1.2 Direct Application of the Risk Link Approach: Linking Bacteroidales

Measured by qPCR to Enterococcus Measured by qPCR........................................24 

3.1.3 Linking E. coli Densities as Measured by MF via Health Effects Curves from 

Different Epidemiology Studies ...............................................................................24 

3.1.4 Application of the Risk Link Approach for Health Effects Curves Based on 

Different Illness Definitions .....................................................................................27 

3.2 Water Quality Link Approach.........................................................................................29 

3.2.1 Datasets Used in Water Quality Link Demonstrations.............................................29 

3.2.2 Linear Models of Log-Transformed Indicator Data .................................................30 

3.2.3 Demonstration of the Water Quality Linkage Approach using Linear 

Regression Models....................................................................................................35 

3.2.4 Broken Stick Models of Log-Transformed Indicator Data.......................................38 

4 Discussion..............................................................................................................................45

5 References .............................................................................................................................46

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List of Tables 

Table 1. Epidemiology study differences that limit direct comparison of health effects 

curves ................................................................................................................................6

Table 2. Epidemiology studies that have established relationships between FIB density and 

excess GI illness due to swimming.................................................................................10

Table 3. Summary of factors affecting culture-based and/or qPCR data for fecal indicator 

bacteria............................................................................................................................11

Table 4. Summary of the target organisms and primer sets used for a selection of qPCR 

assays ..............................................................................................................................15

Table 5. Summary of health effects relationships from USEPA NEEAR studies and marine 

studies conducted to support development of the current RWQC..................................23

Table 6. Risk-linked Bacteroidales and Enterococcus densities..................................................24

Table 7. Results from ANCOVA comparing data from the USEPA (1984) and Marion 

et al. (2010) epidemiology studies..................................................................................26

Table 8. Equivalent Criteria in Terms of HCGI and NGI Definitions of Illness..........................28

Table 9. Examples of equivalent criteria values for marine waters via the statistical 

linkages approach............................................................................................................29

Table 10. Results of ANCOVAs for sets of data for Huntington Beach with different 

collection times ...............................................................................................................34

Table 11. Summary of models resulting from retention of data pairs with culture counts 

above three thresholds.....................................................................................................44

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iv 

List of Figures 

Figure 1. Illustration of the risk link approach ...............................................................................5

Figure 2. Illustration of the Water Quality Linkage approach........................................................7

Figure 3. Paired qPCR and culture data for Enterococcus: Huntington Beach............................12

Figure 6. Correlation of qPCR Enterococcus densities from replicate samples ..........................14

Figure 7. Trend lines for dilutions of laboratory cultures of E. coli and C. jejuni enumerated 

using qPCR and culture methods....................................................................................17

Figure 8. Comparison of bacterial data from culture-based and multiplex real-time PCR 

methods, and statistical sample description....................................................................18

Figure 9. Health Effects Data and Trend Lines from the USEPA and Marion Studies................25

Figure 10. Typical plot of paired Enterococcus densities as measured by qPCR and culturebased methods.................................................................................................................31

Figure 11. Paired qPCR and culture Enterococcus data, all NEEAR study Great Lakes 

beaches............................................................................................................................32

Figure 12. Paired culture and qPCR Enterococcus data, all samples from Huntington Beach ....33

Figure 13. Paired qPCR and culture Enterococcus data, Huntington Beach: 8:00 AM 

samples only....................................................................................................................33

Figure 14. Paired qPCR and culture Enterococcus data, Huntington Beach: 3:00 PM 

samples only....................................................................................................................34

Figure 15. Linear model, data from all NEEAR freshwater beaches ...........................................36

Figure 16. Linear model, data from Huntington Beach................................................................37

Figure 17. Linear model, 8:00 AM data from Huntington Beach ................................................38

Figure 18. Huntington Beach broken stick model fits for all data and 8:00 AM data..................39

Figure 19. West Beach broken stick model fits for all data and 8:00 AM data............................40

Figure 20. Silver Beach broken stick model fits for all data and 8:00 AM data ..........................40

Figure 21. Washington Park Beach broken stick model fits for all data and 8:00 AM data ........41

Figure 22. Model resulting from retention of all data above the “kink” in the broken stick........42

Figure 23. Model resulting from retention of all data above 80 CFU/100 mL.............................43

Figure 24. Model resulting from retention of all data above 104 CFU/100 mL...........................43
U.S. Environmental Protection Agency

Executive Summary 

This report was written to meet the EPA Critical Path Science Plan1 element P15. The 

objectives of this work were to compare the fecal indicator bacteria and health effect 

relationships for multiple indicator/method combinations used in epidemiology studies and to 

evaluate multiple indicator/method combinations to develop quantifiable relationships. To meet 

those objectives, two approaches were employed to relate the different indicator/method 

combinations to gastrointestinal (GI) illness risks, namely the Risk Link and Water Quality Link. 

In the Risk Link approach, indicator-method combinations are linked via health effects curves2

generated by epidemiology studies. Three demonstrations of the Risk Link approach are 

presented. In the first demonstration, linkages are established between indicator densities for 

multiple indicators used in the same epidemiology studies. This demonstration is a 

straightforward implementation of the Risk Link approach and entails linkage of Enterococcus

density as measured by qPCR with Bacteroidales density as measured by qPCR. The second 

demonstration illustrates an assessment of differences between epidemiology studies conducted 

at different places or times, using an analysis of covariance (ANCOVA) to compare data from 

(Marion et al.3

) and (USEPA4) EPA recreational water epidemiology studies in freshwater. The 

analyses indicate that although the two sets of studies were conducted at different times, the 

slopes of their health effects curves are statistically similar. 

The third Risk Link was performed using health effects relations from epidemiology studies with 

Enterococcus and GI relationships, measured by either qPCR or culture-based method. The EPA 

recreational water epidemiology study in marine waters5

 and the EPA’s National 

Epidemiological and Environmental Assessment of Recreational (NEEAR) Water Studies 

(marine) were linked to determine comparable Enterococcus culture and qPCR indicator 

densities at various GI health risk levels. To enable the calculation of comparable Enterococcus

densities for the two studies, results from the 1983 USEPA study were first translated such that 

the GI illness definition matched that of the NEEAR GI illness (NGI) definition. Although 

numeric qPCR-based water quality criteria have not been established at this time, assumptions 

about the way those criteria will likely be developed can be used to demonstrate potential qPCR 

and culture-based criteria that are consistent with the same level of risk (table below). 

 

1

 USEPA. 2007. Critical Path Science Plan for the Development of New or Revised Recreational Water Quality 

Criteria. U.S. Environmental Protection Agency, Offices of Water and Research and Development, Washington, 

DC. 

2

 A health effects curve refers to a mathematical relationship between fecal indicator bacteria (FIB) density and 

observed illness in epidemiology studies of recreational waters. All health effects curves referenced in this report 

relate the incidence of gastrointestinal (GI) illness to the log-transformed FIB density 3

 Marion, J.W., Lee, J., Lemeshow, S., Buckley, T.J. 2010. Association of illness and recreational water exposure 

during advisory and non-advisory conditions at an inland U.S. beach. Water Research 44(16): 4796-4804. 4

 USEPA 1984. Health Effects Criteria for Fresh Recreational Water. EPA-600/1-84-004. Research Triangle Park, 

NC: USEPA. 

5

 USEPA 1983. Health Effects Criteria for Marine Recreational Waters. EPA-600/1-80-031. Research Triangle 

Park, NC: USEPA. 

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Tolerable 

attributable 

illness level (as 

HCGI‡

 per 1000 

swimmers) 

Tolerable 

attributable 

illness level (as 

NGI*

 per 1000 

swimmers) 

Hypothetical 

qPCR 

Enterococcus

density 

(CCE§

/100 mL) 

Hypothetical 

geometric mean 

membrane filtration 

Enterococcus density 

(CFU/100 mL) 

75th

percentile 

value†

(CFU/100 

mL) 

8 35 427 11 33 

10 43 610 14 42 

19 82 3460 35 104 

†

 The 75th percentile value for Enterococcus density based on the calculated geometric mean 

Enterococcus density, assuming Enterococcus densities are log-normally distributed, and 

assuming a typical standard deviation of log-transformed Enterococcus density for marine sites 

of 0.7. 

‡ Highly Credible Gastrointestinal Illness, as defined in the epidemiology studies conducted in 

support of the 1986 water quality criteria 

*

 NEEAR study GI illness (NGI, per the definition used in the NEEAR epidemiology studies) § Calibration cell equivalent; using a calibration sample containing a known concentration of the 

target sequence, CCEs are the normalized values of the test sample cell equivalents 

The Water Quality Link approach links an indicator-method combination for which there is no 

health effects relation to an indicator-method combination with a health effects relation via a 

quantifiable relationship between their measured fecal indicator bacteria densities. In short, the 

Water Quality Link Approach uses paired water quality data, rather than the linkage of health 

effects curves alone, as the basis for an alternative method for establishing culture-based criteria. 

Demonstrations of this approach were made using densities of Enterococcus enumerated by 

qPCR and by membrane filtration (MF) and linked to the health effects relationship from EPA’s 

NEEAR freshwater studies. It was found that while the Water Quality Link may be useful on a 

site specific basis, in this particular demonstration the relationships between paired Enterococcus

data, measured by culture and qPCR, were not consistent among NEEAR study freshwater 

beaches when simple linear and broken stick (segmented) regression models were employed. 

Further, the regression fits to this dataset exhibited heteroskedasticity (uneven distribution of 

residuals) and both the simple linear regression and broken-stick models resulted in comparable 

Enterococcus culture criteria values far in excess of the current criteria. 

The exploration of the Risk Link and the Water Quality Link approaches provides a proof of 

concept with the currently available data. It is likely that new health effects data or improved 

models of the co-occurrence of indicators will allow for additional or improved linkages to be 

established. 

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U.S. Environmental Protection Agency

1 Introduction 

1.1 Purpose, Objectives, and Importance 

Pathogens within fecal matter arising from human and animal sources that enter coastal and 

inland waters pose risks to recreational swimmers. Because it is not feasible to monitor for all 

pathogens that may occur in ambient waters, monitoring strategies currently involve the 

detection of indicator organisms that are present in fecal material in greater numbers than the 

pathogenic organisms (NRC 2004). For decades, EPA has relied on the use of epidemiology 

studies to assess the risk of gastrointestinal (GI) illness in swimmers exposed to increasing 

densities of fecal indicator bacteria (FIB).6 Previous epidemiology studies used only culturebased methods for enumerating FIB. More recent epidemiology studies have evaluated 

associations between GI illness and a wider range of indicator organisms, using both improved 

culture techniques and molecular-based (rapid) methods. 

The primary purpose of this report is to help meet one of the elements (Project P15) in the U.S. 

Environmental Protection Agency (EPA or the Agency) Critical Path Science Plan for 

Development of New or Revised Recreational Water Quality Criteria (CPSP or science plan) 

(USEPA 2007). The science plan is a key component of EPA’s overall process to develop new 

or revised Section 304(a) Ambient Water Quality Criteria (RWQC) for recreational waters that 

will be used by States, Territories, and Tribes to develop their own water quality standards 

(WQS7

). 

The objectives of CPSP Project P15 are as follows: 

 To compare the GI illness response to exposure relationship for multiple fecal indicator 

organism/method combinations; and

 To develop quantifiable relationships between the results from the various 

indicator/method combinations. 

More generally, P15 commissions scientific studies to explore correlations and linkages between 

available indicator organisms (FIB) and methods. These linkages are established to ensure that 

equal protection is provided for any future criteria associated with each indicator-method 

combination. 

Section 2 provides a background review and assessment of relevant epidemiology studies, as 

well as a review of differences in FIB performance for indicators enumerated culture and 

quantitative polymerase chain reaction (qPCR). The background supports the analyses described 

in Section 3 that analyze available epidemiology datasets using both the “Risk Link” and the 

 

6

 “Traditional” FIB include culturable total coliforms, fecal (thermotolerant) coliforms, Escherichia coli (an 

important member of the coliform group), and Enterococcus (enterococci). Although the presence of FIB indicates 

the presence of fecal matter, and the potential presence of (enteric) pathogens, FIB are not pathogenic (NRC 2004; 

WHO 2003). 

7

 Under the Clean Water Act (CWA) States, Territories, and Tribes are required to adopt new or revised WQS for 

those pathogens and pathogen indicators for which EPA’s new or revised criteria have been developed. Once 

approved by EPA, WQS are used for various CWA purposes and programs and are the effective (enforceable) 

standards to protect waters for specified designated uses, such as “primary contact recreation.” 

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“Water Quality Link” approaches. Both of these approaches are described in detail in later 

sections of this report. Section 4 provides a discussion of the analyses, including limitations and 

assumptions. 

1.2 Approaches for Developing Water Quality Measures Corresponding to Equivalent 

Risks 

At the core of RWQC is a quantitative relationship between a measure of fecal pollution in water 

and the risk of adverse health outcomes (health effects curves) arising from primary recreational 

contact (e.g., swimming) in the polluted water. Although the expense, complexities, and time 

associated with conducting epidemiology studies are significant, several studies have been 

completed and are ongoing in the U.S. and abroad since the issuance of the current (1986) 

RWQC. Many of those studies were conducted in direct support of the development of RWQC, 

standards, and guidance. In addition, new technologies, such as rapid molecular-based detection 

methods, have been developed for identifying and enumerating FIB and other (alternate) 

indicator organisms that are fundamentally different from traditional culture-based methods. 

None of these approaches (new methods, alternative indicators, or alternate risk assessment 

methods) involves direct measurement of the pathogen(s) suspected of causing illness in 

recreational water users. Therefore, linkages are made between the indicators that are being 

measured and the adverse health effects observed in relevant and available epidemiology studies. 

The health effects curves from some of the epidemiology studies may not be compared without 

prior analysis and transformations. Differences that necessitate transformations include, for 

example, differences in study design (prospective cohort [PC] vs. randomized control trial 

([RCT]) (see more below), differences in definitions of GI illness, and differences in the settings 

(marine vs. freshwater environments). Ongoing epidemiology studies may yield health effects 

curves for additional indicator/method combinations and these health curves may be suitable for 

use in developing linkages between indicator/method combinations. 

Further, incorporation of molecular-based methods into RWQC based on culture methods (or 

incorporation of culturable methods into criteria based on molecular methods) is complicated by 

the differences in their targets. Because different methods measure different targets with 

different abundances in environmental samples, it is not a given that health effect relationships 

developed for one particular indicator-method combination are valid for another indicatormethod combination. This report places emphasis on the linkages between (1) Enterococcus

measured by qPCR and Bacteroidales measured by qPCR for publicly owned (sewage) treatment 

works (POTW)-impacted marine settings; (2) E. coli measured by membrane filtration (MF) for 

studies conducted at different times and in different settings; and (3) Enterococcus measured by 

qPCR and Enterococcus measured by MF for studies of marine beaches conducted at different 

times. 

1.2.1 Risk Link 

1.2.1.1 Approach Description 

The most direct route to meeting the CPSP P15 objectives is through use of health effect-FIB 

density curves to determine quantifiable relationships for indicator/method combinations that 

correspond to the same risk level for swimmers. This approach, termed the Risk Link, is 

illustrated in Figure 1. For this approach to be feasible, all indicator-method combinations 

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U.S. Environmental Protection Agency

should have an associated relationship between indicator density and health effects (the solid 

lines in both plots in Figure 1). At present, all such health effects curves have been developed 

based on epidemiology studies of surface water recreation sites. Future relationships may be 

developed using QMRA, watershed modeling approaches, or other methods developed 

specifically for that purpose. Assuming the indicator densities and illness rates for the standard 

and alternative indicator health curves are comparable (and this assumption is addressed below), 

the indicator density corresponding to a selected acceptable level of risk is calculated using the 

health effects relationship for each of the indicator/illness combinations. 

Figure 1. Illustration of the risk link approach 

The rationale for using a Risk Link approach is that it maximizes use of direct measurements of 

health effects from relevant and available epidemiology studies. Over the recreational season, 

the pathogens to which swimmers are exposed and the densities of fecal indicators are variable. 

Further, the characteristic pathogens and FIB densities at different beaches likely differ, 

depending on the alignment of the beach with fecal pollution sources and other factors such as 

climate and rainfall. Given this variability, measurement of the association of adverse health 

effects with FIB through epidemiology studies appears to be the most direct and reliable means 

for relating water quality to health effects. 

1.2.1.2 Harmonizing Data from Disparate Studies 

For health effects curves to be comparable, they should relate to risks for the same illness. 

Further, the statistical measure used for characterizing water quality (e.g., geometric mean of 

multiple samples on a single day vs. the water quality from a single sample taken in a zone and at 

a time where swimming occurs) should be taken into account. An extreme example of curves 

that would not be comparable is curves that correspond to excess GI illness in swimmers and 

excess respiratory infection in non-swimmers. A summary of potential differences between 

epidemiology studies that could prevent direct comparison of health effects curves is presented 

in Table 1. 

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U.S. Environmental Protection Agency

Table 1. Epidemiology study differences that limit direct comparison of health effects curves 

Cause Differences in data construct 

Different 

epidemiology 

study design 

(PC vs. RCT) 

 Exposures in prospective cohort (PC) studies are not prescribed and have a much 

wider range of durations and ingestions typical of recreation events; exposures in 

randomized control trial (RCT) studies are controlled and have more consistent 

ingestion volume, but less variability in exposure. 

 RCT studies control the location of exposure far more than in PC studies; thus, the 

impact of this element of study design is site specific. 

 Water quality associated with illness incidence in PC studies is based on an 

average indicator (usually FIB) density for the recreation site and over the entire 

study day; water quality associated with illness incidence in RCT studies is based 

on the indicator density for a sample taken at the same time and location as 

swimmer exposure. Proponents of RCT designs state that this feature reduces 

bias, but this claim may not be true depending on the magnitude of short-term 

variability. 

Different 

definition of 

human health 

outcome 

Epidemiology studies differ in their choices in health endpoints and their definitions of 

those endpoints, including GI illness. 

Different settings Indicators may be associated with different risks at sites with different 

 fecal pollution sources; 

 level of treatment of fecal pollution; 

 loading characteristics (continuous vs. event); and 

 proportion of FIB resuspended in sediments. 

Examples of pairs of studies that may have significant setting-related effects include 

 studies conducted in inland and coastal waters, 

 studies conducted in the United States and Europe, and 

 studies conducted at sites in (sub)tropical and temperate climates. 

Regulation 

changes and 

technological 

advances 

Regulations and advances in technology have changed wastewater (including POTW) 

treatment practices and the resulting indicator and pathogen loads to recreation sites. 

The most important of these changes are disinfection of wastewater, development of 

improved animal waste treatment technologies, and regulations on the land application 

of biosolids (treated wastewater sludge) and animal wastes. 

1.2.2 Water Quality Link 

Many indicator/method combinations are not, at present, associated with health effects curves or 

have not been applied directly in epidemiology studies. In such cases, a less direct approach for 

linking indicator/method combinations can be applied. The Water Quality Linkage approach 

relates paired data from two indicator/method combinations and links the method/indicator 

combinations to each other through their relationship, rather than through a direct linkage to 

health effects curves. This approach is illustrated in Figure 2, where in the top graph, a criterion 

density for a standard indicator/method combination is established based on a selected level of 

tolerable risk and a health effects curve. A quantifiable relationship for an alternative 

method/indicator combination is established through a model relating the density of the standard 

indicator/method combination to the density of the alternative method/indicator combination. 

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U.S. Environmental Protection Agency

Figure 2. Illustration of the Water Quality 

Linkage approach 

As with the Risk Link approach, the Water Quality Link should be employed when paired water 

quality data are from comparable studies. Further, a Water Quality Linkage at one site may not 

be applicable to another site—even if the sites have the same primary fecal pollution impact. A 

critical component in the application of the Water Quality Link is the development of a useful 

statistical model that relates FIB-method combinations. At present, models relating culturable 

and qPCR indicator counts are either site-specific (e.g., Byappanahalli et al. 2010; Lavender and 

Kinzelman 2009) or have been developed based on pooling of datasets under the assumption that 

datasets are similar and may be pooled (Haugland et al. 2005; Whitman et al. 2010). In this 

report, linear, and “broken stick” models for relating log-transformed culture and qPCR indicator 

densities are explored. It is possible that other statistical models linking these types of analytical 

methods will be proposed and evaluated in the future. 

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U.S. Environmental Protection Agency

2 Background 

Ordinarily, FIB themselves do not cause illness, but their densities can provide estimates overall 

levels of fecal contamination. As a result, their densities cannot usually be used to directly 

estimate health risks through a risk assessment approach. The relationship between indicators 

and health risks are best and most commonly established by epidemiology studies (NRC 2004; 

WERF 2009). Epidemiology studies (1) establish microbial water quality, typically via FIB 

density measurements, with a sufficient number and timing of samples to establish a 

characteristic indicator density during swimming; and (2) associate the FIB density with the 

adverse health effects observed among the population swimming compared to a non-swimming 

control population. This section includes a review and assessment of relevant and available 

epidemiology studies, as well as a review of differences in FIB performance for culture and 

qPCR indicators. 

2.1 Review and Assessment of Relevant Epidemiology Studies and Datasets 

Since the 1950s, numerous epidemiology studies have been conducted in the United States and 

abroad, most commonly at beaches impacted by sewage/wastewater effluent (e.g., POTWs), to 

evaluate the association between recreational water quality and adverse health outcomes. In 

these studies, attempts were made to relate a quantitative microbial indicator of water quality to 

health effects (usually some form of GI illness) using log-transformed data or a geometric mean 

to characterize exposure to water quality indicators. However, eye infections; skin irritations; 

ear, nose, and throat infections; and respiratory illness have also been evaluated. Many of these 

recreational water epidemiology studies are reviewed in one or more of the metaanalyses/systematic reviews of Prüss (1998), Wade et al. (2003), and Zmirou et al. (2003). More 

recent studies are reviewed in WERF (2009), which notes that all recreational epidemiology 

studies identified higher rates of at least some self-reported health end points (usually GI illness) 

in relation to water exposure (usually swimmers vs. non-swimmers). That is, recreational water 

contact by its nature is associated with increased risk of adverse health effects—even if the 

excess risk is not correlated with increases in fecal indicator organisms. 

Both PC and RCT (also called prospective randomized exposure studies [Fleisher et al. 2010]) 

epidemiology study designs have been used to evaluate recreational waters. The primary 

difference between RCT and PC design is that in RCT studies the volunteers are randomly 

assigned to swim in a selected area where the water quality is measured during the timed 

swimming exposure. A brief overview of each design is provided below. 

2.1.1 Prospective Cohort Studies 

EPA’s current (1986) recreational water quality are based on the observed occurrence of GI 

illness associated with swimming in fresh or marine recreational waters receiving point sources 

of effluent from POTWs as determined through several PC studies conducted in the 1970s 

through the early 1980s. Over the past several years, the EPA conducted a PC study called 

National Epidemiological and Environmental Assessment of Recreational (NEEAR) Water 

Study. This series of epidemiology studies evaluated sites at four POTW-impacted Great Lakes 

beaches and five marine recreational beaches (four POTW-impacted and one non-POTWimpacted). The results of the freshwater and marine studies have been published (e.g., Wade et 

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U.S. Environmental Protection Agency

al. 2006, 2008, 2010). Further, there have been relatively few studies of inland (non-Great 

Lakes) waters or of nonpoint source-impacted recreational sites. 

2.1.2 Randomized Control Trials 

The other major epidemiology study design used for evaluating health risks associated with 

recreational exposures is the RCT. This design has been used extensively in Europe, and more 

recently in the United States to determine the association between microbial water quality and 

increased risk of adverse health effects in swimmers vs. non-swimmers in both marine and fresh 

recreational waters. The WHO Guidelines for Safe Recreational Water Environments (WHO, 

2003) apply to both fresh and recreational waters and are largely based on RCT studies 

conducted in the late 1980s and early 1990s at POTW-impacted marine recreational beaches in 

the United Kingdom (UK) (Fleisher et al. 1996; Kay et al. 1994). In 2006, the European Union 

adopted a new directive for the management of bathing water quality that is also based, in large 

part, on the same UK marine recreational water studies. It also includes the results of a more 

recent RCT study of POTW- and nonpoint source-impacted fresh recreational waters in Germany 

(Wiedenmann et al. 2006). Epibathe (2009a, 2009b) describes the results of a series of RCT 

studies conducted at marine and fresh recreational waters in Europe in 2006 and 2007. Finally, 

the results of an RCT study of a non-POTW-impacted marine beach in Miami, Florida have been 

recently published (Fleisher et al. 2010). 

2.1.3 Studies that have Generated Health Effects Relationships 

A number of epidemiology studies have established a significant relationship between indicator 

organism density and increased GI illness (Table 2; see also WERF, 2009). Highlighted rows in 

Table 2 indicate relevant and available epidemiological datasets that were obtained and evaluated 

in this report (see Section 3). It is beyond the scope of this report to list all epidemiology studies 

and datasets of recreational waters that have been conducted. 

Among the studies listed in Table 2, three have established relationships for inland waters, and 

all of those studies were of waters that are likely predominantly impacted by human fecal 

pollution sources. A number of the unique features for the studies that resulted in health effects 

relations are the following: 

 the USEPA (1983, 1984) studies related illness rates to season-averaged culture indicator 

densities; 

 the USEPA (1984) study health effects relationship was developed using pooled data 

collected for both inland and Great Lakes beaches; 

 the USEPA (1983, 1984) studies used different definitions of GI illness than the NEEAR 

studies; 

 the study by Marion et al. (2010) was conducted on a relatively small inland lake, though 

as shown in Section 3.1, the health effects observed in that study were similar to those 

observed by USEPA (1984). 

In contrast to studies of waters potentially impacted primarily by POTW effluent, epidemiology 

studies of marine and freshwater non-POTW impacted-recreational waters tend to yield weak 

associations between increased densities of traditional indicators (Enterococcus and E. coli) and 

health risk. Calderon (1991) found no association of indicator density with incidence of adverse 

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health effects at a non-POTW-impacted pond with no known human fecal pollution effects. 

Similar results were reported by Colford et al. (2007) for a California coastal beach suspected to 

be affected primarily by birds, and for a subtropical coastal marine beach most likely affected by 

dog and human nonpoint sources of fecal pollution (Abdelzaher et al. 2010; Fleisher et al. 2010; 

Sinigalliano et al. 2010). Results from a study of stormwater impacts on GI illness rates are 

difficult to interpret (Haile et al. 1999). It is possible that densities of alternative (nontraditional) indicators might be associated with risk and that relationships may be established as a 

result of ongoing epidemiology studies. 

Table 2. Epidemiology studies that have established relationships between FIB density and 

excess GI illness due to swimming 

Study(s) 

Indicator and primary 

detection method Setting and sources 

Study 

type 

Marion et al. 

(2010) E. coli culture 

U.S. impounded freshwater beaches, 

human and other sources PC 

NEEAR 

Density of Bacteroidales 

measured by qPCR U.S. marine beaches, POTWs PC 

NEEAR (Wade et 

al. 2006, 2008) 

Density of Enterococcus 

spp. measured by qPCR 

US freshwater beaches (Great Lakes), 

POTWs PC 

NEEAR (Wade et 

al. 2006, 2008) 

Density of Enterococcus 

spp. measured by qPCR U.S. marine beaches, POTW PC 

Wiedenmann et 

al. (2006) 

Enterococcus and E. coli

chromogenic substrate

German, freshwater beaches with one of 

more point- (including POTWs) and 

nonpoint sources RCT 

Fleisher et al. 

(1996) and Kay et 

al. (1994) Enterococcus culture U.K. marine beaches, POTWs RCT 

USEPA (1984) 

Enterococcus and E. coli

culture 

U.S. Great Lakes and inland freshwater 

beaches, POTWs PC 

USEPA (1983) Enterococcus culture U.S. marine beaches, POTWs PC 

2.1.4 Factors that Preclude Direct Comparison of Epidemiology Studies 

Although epidemiology studies may be able to identify a general association between a given 

fecal pollution source and indicator organism densities by estimating incidence of disease, a 

major and ongoing concern is that their results may be limited to describing risk only for beaches 

similar to those evaluated in the epidemiology studies (e.g., similar fecal sources). The ability to 

conduct valid comparisons between PC and RCT epidemiology studies depends on several key 

and potentially interrelated factors. These factors relate to several critical differences in the 

details of RCT and PC studies, and include the following: 

 the manner in which microbial water quality is associated with illnesses; 

 the specific exposure that swimmers experience; 

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 the definition of (GI) illness and the duration of follow-up; 

 the age and make up (e.g., tourists vs. locals) of the subject pool; 

 the source of fecal indicators and pathogens in the recreational water, their distribution, 

and temporal-spatial variability of those distributions; and 

 the methods used to enumerate the indicators and the corresponding relationships 

between the indicators and the pathogens, as measured by those methods. 

2.2 Differences in Indicator Performance for Indicators as Measured by Culture-Based 

and qPCR-Based Methods 

Cell counts obtained by qPCR are generally greater, often by orders of magnitude, than those 

provided by MF analyses of the same samples (e.g., as observed by He and Jiang, 2005). This 

section begins with a review of select studies illustrating the differences between qPCR and MF 

methods. Common causes of variations in FIB counts via the different methods, as shown in the 

literature, are listed below by category and subcategory and are discussed subsequently. Results 

from studies on paired comparisons of different indicator/method combinations are also 

highlighted throughout the section. 

2.2.1 General Method Performance 

The performance of selected analytical methods and the fate and transport of cells and genomic 

material can be influenced by a number of factors, as summarized in Table 3. 

Table 3. Summary of factors affecting culture-based and/or qPCR data for fecal indicator bacteria 

Influencing factor Variables Outcome Reference(s) 

Time of day for 

sample collection 

Exposure to sunlight 

(UV radiation) 

Diurnal variation in FIB 

density 

Haugland et al. (2005) 

Variations in method 

performance 

 qPCR amplification 

efficiency 

 Limits of detection 

 Inter-laboratory 

variability 

 Choice of target 

gene(s) 

 Sample preparation 

 Concentration 

factor 

Inconsistencies in data 

generation and 

interpretation 

Haugland et al. (2005) 

Khan et al. (2007) 

Kinzelman et al. (2010; 

preliminary report) 

Bacterial viability Enumeration of live, 

dead, and/or VNBC 

FIB 

Under-representation of 

VNBC FIB using MF; overrepresentation of viable 

bacteria using qPCR 

Nocker and Camper (2006) 

Nocker et al. (2006) 

Bae and Wuertz (2009) 

Varma et al. (2009) 

With regards to the sampling approach, the time of day at which samples are collected has been 

shown to influence the relationship between culture-based and qPCR data (e.g., Haugland et al. 

2005). During EPA’s NEEAR studies conducted at freshwater sites in the Great Lakes, water 

samples were tested for Enterococcus by qPCR and MF. Samples were collected three times per 

day (8:00 AM, 11:00 AM, and 3:00 PM), at multiple depths (waist, shin, and knee), and along 

three transects at all four beaches being studied. A model relating enterococci qPCR and culture 

data can be developed using results from all the sites and times, or using data that are selected 

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because they appear to have similar underlying trends. All samples collected from one of these 

beaches are shown in Figure 3, along with a 45° line showing perfect agreement. Although one 

may observe a general trend at high FIB density, at densities below ~500 CFU/100 mL, no trend 

is apparent and there is no obvious choice for the form of a model relating enterococci density 

derived from qPCR methods to culturable enterococci density. The influence of sample 

collection time on the relationship between culture and qPCR FIB density is explored in Section 

3.2.1. 

Figure 3. Paired qPCR and culture data for Enterococcus: 

Huntington Beach 

Variations in the data because of the performance of the analytical method have been 

demonstrated in a number of studies. The factors affecting the analytical performance include 

the efficiency of the qPCR method, the influence of detection limits, and variations in interlaboratory performance. Haugland et al. (2005) compared a qPCR-based enumeration method 

for Enterococcus to EPA Method 1600 (MF enumeration). The qPCR method employed in that 

NEEAR-related study demonstrated very high amplification efficiency (0.99, or 99% of 

amplicons doubling with each cycle) for DNA in dilution water. A modest level of false positive 

results from qPCR (19% of 217 negative control samples) was speculated to be the result of 

aerosolized DNA contamination that occurred in analytical laboratories. DNA recoveries in 

calibration samples (from seeded filters) from the two beaches sampled in the study were 82% 

and 51% in beach samples, respectively. Results of qPCR and MF enumerations were 

reasonably well correlated (R2

 = 0.68) and log-normal distributions described the densities of 

Enterococcus for samples collected at each beach and for both qPCR and MF enumerations. The 

authors performed linear regression of qPCR results (as cell equivalents) against MF results (as 

CFU) for samples collected on multiple days and at multiple locations on two beaches. The 

resulting relation: 

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log10   CE  log53.056.1 10 CFU [1] 

was proposed to describe the variation in qPCR cell counts with those from MF. 

Khan et al. (2007) evaluated qPCR enumeration of E. coli in waters of agriculture-dominated 

watersheds against MF methods, finding as did Haugland et al. (2005), that (1) qPCR-based 

enumerations yielded consistently higher estimates of density than culture methods (attributed to 

lack of discrimination between DNA from live and dead cells); and (2) that standard curves (in 

this case, based on both dilution water and autoclaved agricultural water) had high coefficients of 

variation. Khan and colleagues noted that the detection limit in agricultural waters was 10 cells, 

whereas in dilution water it was 1 cell, and that sample preparation had a profound impact on the 

viability of the qPCR method. Overall, qPCR enumerations were found to be consistently higher 

than those from MF methods—the cell counts for all samples analyzed via MF ranged from 1.0 

to 2800 CFU/100 mL, whereas the range for qPCR counts was 15 to 9900 cells/100 mL. The 

authors concluded that qPCR results were less variable than MF results; that qPCR could be used 

effectively in diverse agricultural watersheds; and that qPCR is more rapid, producing 

meaningful results far faster than culture-based indicator methods. Application of qPCR 

methods and interpretation of qPCR results may require consideration of the source of indicator 

organisms as well as the physiological status of FIB at the time of sampling. For example, the 

selection of target gene plays an important role in qPCR enumeration. The number of target 

genes per bacteria cell is dependent upon physiological state of the cell and, for example, during 

the bacterial logarithmic-growth phase up to 36 rrn genes/cell (range: 12-36 copies/cell) have 

been reported for E. coli (Bremer and Dennis 1996). 

Inter-laboratory variability was assessed by Kinzelman et al. (2010; preliminary report) through 

the analysis of replicate samples in multiple labs to assess the uncertainty and recovery of qPCR. 

Archived (MF) filters were used to develop replicate samples and qPCR was used to determine 

Enterococcus calibrator cell equivalents (CCEs) in the replicate samples. Multiple laboratories 

conducted the study and all reported similar findings. A typical plot of the paired samples is 

presented in Figure 4. In all cases there was very little difference between replicate samples at 

high indicator density and higher scatter at lower densities. The scatter at low densities appears 

to be Poisson distributed (analysis not shown) and attributable to sampling uncertainty. The 

relatively tight distribution of points around the 45° line indicates a consistent and relatively high 

recovery. 

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Figure 4. Correlation of qPCR Enterococcus densities from 

replicate samples (Source: Kinzelman et al. 2010) 

Comparing some of the key papers used to review general method performance, it is important 

that the choice of target gene(s) and reaction conditions for the qPCR assays be accounted for 

when comparing research approaches. As shown in Table 4, there are a variety of bacteria 

targeted in these key papers, some targeting the genus as a whole (e.g., Enterococcus spp.), 

others targeting a particular species (e.g., E. coli), and others targeting a particular strain of a 

species (e.g., E. coli O157:H7). 

Accounting for bacterial viability is an important consideration when comparing culture-based 

methods with qPCR assays. Because of its sensitivity, qPCR will amplify DNA regardless of 

whether or not it is contained within an intact cell. However, successfully culturing bacteria 

requires the organism to be intact. There is an intermediate phase termed viable but nonculturable (VBNC), where bacteria appear to not be culturable but retain the ability to reproduce 

under appropriate conditions. By their nature, culture-based methods will discriminate against 

free DNA and VNBC bacteria, whereas qPCR methods will quantify genetic material from cells 

in all states of existence. In the case of detection of DNA from dead cells, this ability is a 

significant and widely recognized shortcoming of qPCR methods and a potential cause of false 

positives (NRC 2004). 

Several researchers have explored PCR techniques that enable discrimination between DNA 

from live and dead cells. For example, Nocker and Camper (2006) used a chemical reaction of 

DNA from dead cells with ethidium monoazide (EMA) to prevent reaction of DNA from dead 

cells with PCR reagents. Thus, prior to qPCR determination, EMA was added to sample water. 

EMA, which bonds strongly with DNA and is inactivated in water, can enter only bacterial cells 

with compromised cell walls. Following this step, DNA was extracted from the live cells using 

conventional techniques and qPCR was performed for determination of E. coli O157:H7 and 

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Table 4. Summary of the target organisms and primer sets used for a selection of qPCR assays 

Reference 

Target 

organism Target gene Primers 

ECST748F 

Haugland et al. (2005) Enterococcus 23R rRNA 

ENC854R 

IEC-UP 

Khan et al. (2007) E. coli 

Internal transcribed 

spacer (ITS) region 

between the 16S–23S 

rRNA IEC-DN 

E. coli stx1-forward 

O157:H7 Shiga toxin 1 

stx1-reverse 

invA2-F Nocker and Camper (2006) 

Salmonella 

enterica invA gene 

invA2-R 

stx1-forward 

Nocker et al. (2006) E. coli 

O157:H7 Shiga toxin 1 

stx1-reverse 

BacUni-520f 

BacUni-690r1 

BacUni-690r2 

BacHum-160f 

Bae and Wuertz (2009) Bacteroidales 16S rRNA 

BacHum-241r 

Enterococcus ECST748F 

spp.

 16S RNA 

Varma et al. (2009) ENC854R 

Bacteroidales 16S rRNA In Siefring et al. (2008) 

Salmonella. The authors suggested that use of EMA with DNA-based methods, if refined, could 

be a viable alternative to the use of more complicated RNA methods (RNA degrades rapidly 

after cell death) in developing cell density estimates that exclude dead cells. In a different study, 

Nocker et al. (2006) suggested that propidium monoazide (PMA) may be a better reagent for use 

in preventing DNA from dead cells from being amplified in the PCR reaction. In that study, 

EMA was found to penetrate live cells of some microbiological species, preventing amplification 

of DNA from the penetrated cells and reducing the accuracy of qPCR estimates of cell density. 

In contrast, PMA was observed to be selective only for dead cells for the organisms tested in that 

study. 

Similarly, Bae and Wuertz (2009) used PMA to inhibit amplification of DNA from dead cells. 

The use of PMA (rather than EMA) was suggested because, and as noted by Nocker et al. 

(2006), EMA is believed to cause degradation of some DNA from viable cells for E. coli

O157:H7, Campylobacter jejuni, Listeria monocytogenes, and perhaps other pathogenic and 

indicator bacteria. Bae and Wuertz (2009) determined PMA reaction conditions (PMA 

concentration and light exposure time) that optimized removal of target DNA (host-specific 

Bacteroidales genetic markers) from dead cells in wastewater plant influent and effluent. For 

samples from the wastewater plant effluent, gene copies from qPCR with PMA were only 30% 

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of those from qPCR without PMA. The difference between qPCR with and without PMA was 

greater than two orders of magnitude for samples of wastewater plant effluent. 

Varma et al. (2009) investigated the difference between PMA-qPCR and qPCR without PMA for 

Enterococcus and Bacteroidales in wastewater. Objectives of the study included assessment of 

water matrix effects on the efficiency of the PMA reaction and exploration of use of qPCR for 

analysis of wastewater treatment plant operation. Some of the conclusions from that study were 

the following: 

 high levels of biomass or suspended solids in water samples appeared to interfere with 

the ability of the PMA-qPCR method to specifically detect live cells, and 

 standard POTW chlorine disinfection practices resulted in substantially greater reductions 

in fecal indicator bacteria CFU densities than those observed for PMA-qPCR detectable 

target sequences. 

When reviewing general method performance, the influence of variability in the data generated 

from different methods and their influence on health effect relations has also been described. 

Similar to Haugland et al. (2005), albeit for groundwater samples, Lleo et al. (2005) found that 

qPCR estimates of E. coli and Enterococcus faecalis density were significantly and consistently 

higher than those obtained using MF methods. The authors ascribed the difference to the ability 

of qPCR to detect VBNC cells and suggested that the qPCR method provides estimates of FIB 

density that are more protective of human health—particularly given the potential for VBNC 

cells to resuscitate under appropriate conditions. 

In the course of EPA’s NEEAR studies conducted at Great Lakes beaches, Wade et al. (2008) 

observed that, in contrast to counts from culture methods, qPCR counts of enterococci were 

relatively constant during the day. In contrast, counts of enterococci from culture methods show 

a distinctive diurnal variation, with densities for early morning samples being as much as two 

orders of magnitude higher than for late afternoon samples. In addition to observing less hourly 

variation in indicator density measurements using qPCR compared to culturable methods, Wade 

and colleagues also noted that increasing qPCR densities (expressed as cell equivalents) were 

associated with excess risk of GI illness in swimmers vs. non-swimmers. 

2.2.2 Factors Impacting Indicator/Method Performance 

2.2.2.1 Choice of Target Organism 

Although this report focuses on the methods used to detect and quantify Enterococcus in relation 

to health effects, it is important to consider how different FIB compare to one another when 

detected in recreational waters by various methods. Differences in the comparison of qPCR and 

culture signal exist for different microorganisms, as demonstrated by Rothrock et al. (2009). In 

that study, triplicate analyses were performed on samples inoculated with dilutions of laboratory 

cultures of E. coli and C. jejuni. Plots of qPCR counts vs. MF counts for the two organisms are 

presented in Figure 5, where the dashed line is a 45° line (exact concordance between methods). 

In general, the qPCR and culture counts are 1:1, with better agreement at the higher counts 

(lower dilutions) than at lower counts. The curve for C. jejuni has a slope of 1:1, but is shifted 

above the 45° line. This shift is potentially due to low recoveries typical for this bacterium via 

culture methods. As suggested in Figure 5, the impact of difference in recovery between the 

methods is manifested as a shift of the trendline above or below the hypothetical line of perfect 

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agreement. Differences in recovery among MF counts may results from damage or shock to 

bacteria during sample preparation, clumping of cells, or difficulty in maintaining growth 

conditions for some organisms. Differences in recovery among qPCR counts may arise from the 

presence of substances inhibiting the PCR reaction and differences in specificity, efficiency, and 

sensitivity for PCR assays targeting different microorganisms. 

Figure 5. Trend lines for dilutions of laboratory cultures of 

E. coli and C. jejuni enumerated using qPCR and culture 

methods (SOURCE: Rothrock et al. 2009) 

Simultaneous detection of Enterococcus and Bacteroidales in recreational marine water was 

undertaken by Elmir et al. (2009) using MF and qPCR detection methods. Two pools, one large 

and one small, were filled with local offshore marine water and recreational users (adults and 

toddlers) were required to be in the water for pre-determined periods. Using three analytical 

methods, Enterococcus and Bacteroidales were enumerated from the water samples before and 

after users were in contact with the water. Microbial concentrations in the source water were 

generally low (MF = 5 [std. dev. ± 7] CFU, qPCR = 29 [± 49] genomic equivalent units). 

Bacteroidales was detected only using qPCR human markers (UCD and HF8), yielding genomic 

equivalent units (GEU) of 45 (± 18 3) and 3 (± 10) GEU/100 mL, respectively. The 

concentrations of Enterococcus and Bacteroidales in both pools were variable following 

recreational use. Enterococcus densities calculated using MF ranged from 1.8104 to 2.0106, 

whereas values calculated using qPCR ranged from 3.8105 to 5.5106. Bacteroidales values 

ranged from below the limit of detection (1.4103

 GEU) to 1.3106. The authors concluded that 

the bathers appeared to release significant amounts of FIB via shedding from their bodies and 

into the water column. For this study, an added significance of the study is that culture and 

qPCR counts for enterococci from bathers were generally in the same range. By comparison, 

effluent from POTWs employing chlorination may have culture and qPCR counts that differ by 

many orders of magnitude (e.g., He and Jiang 2005, Bolster et al. 2005) and qPCR and culture 

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counts in POTW-impacted coastal waters may differ by smaller margins than observed in 

chlorinate POTW effluent (Byappanahalli et al. 2010). 

A suite of FIB from brackish water was analyzed by Ortega et al. (2009). Eighteen sampling 

events were conducted over five sampling trips in the St. Lucie River Estuary, South Florida. 

Enterococci were enumerated using MF, whereas E. coli were assayed using the most probable 

number (MPN) method. The values for enterococci and E. coli were similar (101

 to 102 CFU or 

MPN/100 mL) and an R2 value of 0.53 was reported when correlating the two fecal indicators. 

Finally, Agudel et al. (2010) used MF and multiplex real time PCR to compare the 

concentrations of Bacteroides spp., E. coli, and enterococci in a variety of environmental 

matrices in Barcelona, Spain. Seventy-four samples from rivers, wells, urban groundwater, and 

wastewater were analyzed for FIB counts using standardized MF methods for E. coli, total 

coliforms and enterococci, and qPCR assays for total Bacteroides spp. and enterococci. Based 

on the authors’ statistical analyses, Figure 6 summarizes the enumeration of the fecal indicators. 

The authors concluded that bacterial quantification data were more homogeneous when using 

PCR than when using conventional culture microbiology. 

Figure 6. Comparison of bacterial data from culture-based and multiplex 

real-time PCR methods, and statistical sample description (SOURCE: 

Agudel et al. 2010)

2.2.2.2 Water Matrix 

Inhibition of the qPCR amplification reaction is an important factor to consider when comparing 

performance of qPCR with the Enterococcus MF technique. Duprey et al. (1997) hypothesized 

possible seawater inhibition of the qPCR assay through secretion of substances by phytoplankton 

and zooplankton, while Haugland et al. (2005) also noted frequent inhibition of qPCR in 

undiluted Great Lakes water. However, such inhibition is reduced or eliminated by performing 

serial dilutions of the sample (Ahmed et al. 2009). 

Characteristics of the water matrix also impact quantification assays. Sinton et al. (2002) showed 

that salinity increased bacterial cells rate of decay, which would widen the already demonstrated 

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gap between culture-based and qPCR results in marine waters. Duprey et al. (1997) noted in 

their seawater study that DNA persistence was lower during the summer for free and dead cell 

DNA, and could extend up to 55 days in the winter for dead cell DNA. The persistences of 

naked DNA and a small inoculum of dead cell DNA were the lowest at two days. Walters et al. 

(2009) reported that in sewage microcosms exposed to sunlight, culturable Enterococcus

concentration fell below detection limit within 5 days while qPCR signal persisted for at least 28 

days. These observations are consistent with the general observation of higher qPCR-based 

bacterial densities (enumerated in cell equivalents) than culture-based counterparts in both 

marine and freshwaters (Byappanahalli et al. 2010; Bower et al. 2005; Haugland et al. 2005; 

Lavender and Kinzelman 2009; Morisson et al. 2008; Whitman et al. 2010)—except for Elmir et 

al. (2009) who found comparable densities from both techniques in a marine setting and Noble et 

al. (2010) who found underestimation of bacterial density via qPCR for marine settings. 

The two studies by Noble et al. (2010) were interpreted by the authors as demonstrating an 

underestimation of Enterococcus by qPCR with regards to culture-based methods in marine 

water. However, on this point there are several concerns in interpretation that need to be 

considered. The primer set used was not specifically described in the study. In the discussion it 

was indicated that the primers used were designed for high specificity for E. faecalis and E. 

faecium (which would detect a narrower population of Enterococcus) in this study than in studies 

that used the EPA method. The use of a single species calibrator standard could also result in 

disagreement between the techniques, as different species of enterococci may have different 

numbers of the target gene. Finally, the authors pointed that the relative quantification method 

with a calibrator and a salmon testes DNA control used by other teams expressing results in 

CCEs could explain the observed discrepancy between underestimation and overestimation. 

They discarded the possibility of error originating from PCR chemistry by showing no 

significant difference between the Taqman and Scorpion assays for Enterococcus. However, this 

team confirmed the previously published findings of significant correlation between qPCR and 

culture-based assays for both Enterococcus and E. coli, with a stronger agreement for E.coli than 

for Enterococcus in terms of beach management decisions (88% vs. 94%). With regards to 

technique implementation, the authors showed that microbiologists with little qPCR training 

produced similar results as their experienced counterparts, while finding DNA extraction 

complex and time-consuming. This surprising finding was attributed to a simplified PCR 

preparation protocol. 

The forgoing findings illustrate the complexities of applying qPCR methods, particularly since 

this technique can amplify all DNA present in a sample, regardless of its viability state. This 

implies that ambient background DNA (i.e., naked and dead cell DNA; see Lavender and 

Kinzelman 2009) needs to be assessed when employing qPCR methods, regardless of the water 

matrix. Lavender and Kinzelman (2009) successfully demonstrated the use of a site-specific 

corrective factor to correlate culture and qPCR counts and highlighted its importance for the 

prediction of beach microbial water quality status. 

2.2.2.3 Primary Fecal Pollution Source on Indicator/Method Performance 

Fecal pollution sources can include point- and nonpoint sources. Common point sources are 

POTW effluents and stormwater outfalls, while nonpoint sources include wildlife, human 

shedding and soils and sediments. Although most U.S. POTW effluents are disinfected, some 

are not, and in some facilities seasonal disinfection is employed. Disinfection usually yields 

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minimal to null culture-based results for FIB, while qPCR results can still remain high, 

accounting for the VBNC and background fractions of enterococci and other FIB. However, 

POTW monitoring results typically represent human-derived enterococci, with direct relevance 

to public health. Comparatively, indicators in stormwater outfalls can originate from a variety of 

sources, in which case qPCR primers are chosen based on an understanding of the origin of the 

fecal pollution. Such human/nonhuman speciation would be especially necessary in rural 

settings with agriculturally developed watersheds but could be recommended at all recreational 

sites due to potential contribution of native Enterococcus to beach advisories (Yamahara et al. 

2009). Simultaneous probing with Enterococcus and Bacteriodales primer sets can provide such 

speciation (Converse et al. 2009; Elmir et al. 2009; Shanks et al. 2009). The human shedding of 

enterococci from bathers was considered minimal in marine waters during initial load or bathing 

cycle (about 4%) and proportional to the body’s surface area (Elmir et al. 2009). These 

researchers noted that additional bathing cycles could increase the predominance of human 

shedding of FIB. 

2.2.2.4 Treatment and Environmental Factors 

Lavender and Kinzelman (2009) and Varma et al. (2009) investigated the effects of wastewater 

treatment on culture-based and qPCR results for enterococci. Their similar findings showed that 

culture-based numbers are strongly reduced (2 to 5 orders of magnitude), especially by 

secondary treatment and disinfection, while qPCR numbers experience smaller reductions or 

remain unchanged. As noted previously, this is probably due to the strong impact of disinfection 

on culturable cells. Environmental factors (e.g., rain events, UV light) can also significantly 

affect qPCR and MF results differently. Lavender and Kinzelman showed that culture-based 

Enterococcus densities at stormwater outfalls were significantly different depending on rain 

events and location, while no significant differences were observed for qPCR. These researchers 

also showed that ambient background DNA appeared insignificant during stormwater events. 

Varma et al. (2009) also found that qPCR results were virtually unaffected between normal and 

stormflow operations. The contribution of FIB from undeveloped watersheds to reference 

beaches studied by Griffith et al. (2010) implies that native sands are at least partially responsible 

for some FIB exceedances of water quality thresholds. Furthermore, Walters et al. (2009) 

showed that sunlight seems to inhibit the cultivability of cells and accelerate their degradation, 

reducing persistence of enterococci by half for culture-based results. Neither dead cell nor naked 

DNA (ambient background DNA) appeared to be affected by sunlight. Given the nature of 

qPCR and the relative small fraction of viable and culturable FIB, qPCR are not greatly affected 

by insolation while culture-based methods experience strong reduction by photoinactivation 

during sunlight exposure. 

2.2.3 Uncertainty of Indicator/Method Combinations at Low Density 

As noted previously, qPCR measurements of FIB exhibit higher variability at low densities 

(Haugland et al. 2005). This trend is probably explained by the presence of ambient background 

DNA, the abundance of which is site-specific. Lavender and Kinzelman (2009) proposed to 

reconcile qPCR counts with culture counts by introducing a site-specific corrective factor in the 

calculation of qPCR-derived densities. Bae and Wuertz (2009) proposed a more universal 

solution by modifying the traditional qPCR approach with the introduction of PMA, allowing 

qPCR to distinguish viable from non-viable cells. This approach was found to be promising and 

was used successfully by Varma et al. (2009). 

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3 Analyses 

This section describes and demonstrates analyses comparing the illness response to exposure 

relationship for multiple indicator/method combinations used in epidemiology studies and 

evaluating multiple indicator/method combinations to develop quantifiable relationships. The 

approach taken to meet the report’s objectives is through the use of available data to provide a 

proof of concept for the application of various statistical techniques to develop equivalence 

between indicator densities for different indicator-method combinations. These initial analyses 

illustrate the most direct approaches for linking indicator-method combinations and the steps that 

can be undertaken to effect meaningful comparison of results across disparate datasets. 

Two approaches for developing quantifiable relationships for “linking” indicator/method 

combinations, the Risk Link approach and Water Quality Link approach, are demonstrated in 

this section. 

The Risk Link approach is evaluated first using three different statistical linkage analyses. The 

first is a straight-forward linkage of Enterococcus density as measured by qPCR to Bacteroidales

density as measured by qPCR. The second demonstration shows analyses that can be conducted 

prior to linkage of indicators using curves from different studies. That demonstration links 

curves (Marion et al. 2010 and USEPA 1984) based on the same indicator-method combination 

(E. coli by MF), but from epidemiological studies conducted in different settings, with different 

samples sizes, and at different times. The third demonstration illustrates the linkage of 

Enterococcus enumerated by MF to Enterococcus enumerated by qPCR, both from studies 

conduted in POTW-impacted marine waters. That demonstration also illustrates the translation 

of GI definitions between studies, prior to conducting the Risk Link analysis. Comparisons 

beyond the three Risk Links demonstrated here are possible, and additional linkages may be 

conducted as alternative statistical analyses or datasets become available. 

Secondly, the Water Quality Linkage approach is evaluated using two statistical models, simple 

linear regression and broken stick (segmented) regression. Using paired water quality data from 

the NEEAR freshwater dataset, the culture-qPCR relationships appear to vary significantly 

among beaches. The linkage results in translated culture Enterococcus criteria values generally 

significantly higher than the existing culture criteria for those specific beaches. Other sitespecific models, or models that incorporate data beyond paired indicator-method data, may be 

developed to overcome the limitations of the evaluated regression models in the future. 

3.1 Risk Link 

Several Risk Link demonstrations are provided in this section. The simplest and perhaps 

strongest application of the Risk Link approach is the linkage of densities of indicators for 

multiple indicator-method combinations via health effects curves generated in the same 

epidemiology study. In this comparison, illness definitions, sampling strategies, statistical 

interpretations and fecal pollution sources are the same, and no conversions or intermediate data 

analyses are necessary. However, Risk Linkages can also be made using health effects curves 

from different epidemiology studies. In that event, analyses must be conducted prior to the 

linkage of indicator densities, resulting in linkages that are less direct than those established 

using multiple health effects curves from a single epidemiology study. 

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The Risk Linkages demonstrated in this section illustrate both the direct application of the Risk 

Link using health effects relationships from the same study, and the less direct Risk Link, that 

requires preliminarily conversions or analyses. The first demonstration illustrates the linkage of 

Bacteroidales as measured by qPCR and Enterococcus as measured by qPCR. Health effects 

curves for both of those indicator-method combinations were developed in NEEAR study marine 

beach epidemiology studies in which both indicator densities were measured concurrently and 

associated with the same illness levels. The second demonstration illustrates techniques for 

assessing the differences in health effects associations with indicator densities for a common 

indicator-method combination (E. coli by MF) for epidemiology studies with similar designs, but 

conducted at different times and in different settings. The two studies from which health effects 

curves were drawn are a recent study of illness-indicator density associations at a small inland 

lake (Marion et al. 2010), and the studies conducted as a prelude to establishment of the current 

RWQC (USEPA 1984). The third demonstration illustrates the harmonization of health effects 

curves, from two epidemiology studies using different illness definitions, prior to the application 

of the Risk Link approach. In that study, qPCR-based Enterococcus health effects curves from 

the NEEAR study marine beaches are linked to health effects curves based on Enterococcus as 

measured by MF in the USEPA (1983) epidemiology studies. Implicit in the third demonstration 

is an assumption that the conditions at the beaches studied in the two epidemiology studies were 

sufficiently similar to allow meaningful comparison of their health effects curves. 

3.1.1 Review of NEEAR Study and Other Health Effects Relationships for qPCR and 

Culturable FIB 

The NEEAR studies provide the best current association between water quality measures and 

illness rates (i.e., health effects curves), as they are the most recent, largest, and most carefully 

designed epidemiology studies conducted by EPA. In this section, health effects relations from 

the NEEAR studies are presented. Those relationships may be used in development of new 

qPCR-based criteria. Note that the results presented in this report are only a small portion of the 

findings of the NEEAR studies. More detailed results and full descriptions of the studies can be 

found in Wade et al. (2006, 2008, 2010). 

To date, EPA’s NEEAR recreational water epidemiology studies have employed both qPCR and 

culture-based methods in determining FIB density. Further, the studies have yielded health 

effects relationships between Enterococcus density as measured by qPCR, Bacteroidales density 

as measured by qPCR, and health effects (GI illness) for both marine and Great Lakes POTWimpacted sites. The development of associations of illnesses with those indicator-method 

combinations is particularly significant because qPCR is a rapid method. Health effects curves 

generated from observed indicator densities in the NEEAR study marine beaches and for the 

USEPA (1983) study are presented in Table 5. Candidate NEEAR study health effects relations 

for marine beaches were provided in a personal communication from T. Wade (2010). 

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Table 5. Summary of health effects relationships from USEPA NEEAR studies and marine studies 

conducted to support development of the current RWQC 

Setting Health effects relationship Indicator and method Reference 

Great Lakes N  log0214.01000 10 CENT   00918.0

Enterococcus

measured by qPCR 

(CE/100 mL) 

Wade et al. 

(2006) 

Marine 

beaches N  log0517.01000 10 CENT   101.0

Enterococcus

measured by qPCR 

(CCE/100 mL) 

T. Wade, 

personal 

communication, 

unpublished 

data, 2010 

Marine 

beaches N  log00406.01000 10 CENT  0144.0

Enterococcus 

measured by MF 

(CFU/100 mL) 

T. Wade, 

personal 

communication, 

unpublished 

data, 2010 

Marine 

beaches N  log0368.01000 10 CBAC  0962.0

Bacteroidales

measured by qPCR 

(CCE/100 mL) 

T. Wade, 

personal 

communication, 

unpublished 

data, 2010 

Marine 

beaches N  log0219.01000 10 CENT   01485.0

Enterococcus

measured by MF 

(CFU/100 mL)

USEPA (1983) 

In the EPA study of marine beaches impacted by POTW effluent, Wade et al. (T. Wade, personal 

communication, unpublished data, 2010) found that swimmers experienced more GI illness than 

non-swimmers on days when Enterococcus density (as measured by MF) exceeded the current 

geometric mean guidelines. However, the association among swimmers was not statistically 

significant. The authors contrast this lack of association with those for Enterococcus and 

Bacteroidales enumerated by qPCR, both of which were associated with a consistent trend of 

excess GI illness among swimmers. Note that a weak association of Enterococcus density as 

measured by MF with health effects (GI illness) was also observed in EPA’s Great Lakes 

epidemiology studies (Wade et al. 2006, 2008). 

For a rate of swimming-associated GI illness of 35 per 1000 swimmers (risk levels similar to 

those underlying the USEPA1986 criteria) the Great Lakes health effects curve indicates an 

indicator density of 116 qPCR Enterococcus CCE/100 mL. For a swimming-associated GI 

illness rate of 35 per 1000 swimmers, the marine health effects curve based on qPCR 

enumeration of enterococci yields a corresponding indicator density of 427 qPCR Enterococcus 

CCE/100 mL. The culture-based NEEAR study health effects curve for marine waters produces 

a high FIB density corresponding to a swimming-associated illness rate of 35 per 1000 

swimmers. 

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3.1.2 Direct Application of the Risk Link Approach: Linking Bacteroidales Measured by 

qPCR to Enterococcus Measured by qPCR 

Application of the Risk Link approach to determine densities of Enterococcus measured by 

qPCR and Bacteroidales measured by qPCR that are consistent with comparable risks is 

straightforward. The Enterococcus and Bacteroidales health effects curves presented in Table 5

were generated using water quality and attributable illness rate data collected concurrently at the 

same beaches and from the same cohorts. Thus the health effects curves (Table 5) may be used 

without prior analyses to establish risk-linked indicator densities. Risk-linked Enterococcus and 

Bacteroidales densities for marine waters at three risk levels of interest are presented in Table 6 

Table 6. Risk-linked Bacteroidales and Enterococcus densities 

Tolerable attributable 

illness level (as NGI†

per 1000 swimmers) 

Enterococcus density, 

qPCR enumeration 

(CCE/100 mL) 

Bacteroidales density, 

qPCR enumeration 

(CCE/100 mL) 

35 427 3680 

43 610 6060 

82 3460 69,600 

†

 NEEAR study definition of GI illness 

3.1.3 Linking E. coli Densities as Measured by MF via Health Effects Curves from Different 

Epidemiology Studies 

In the second application of the Risk Link approach, health effects curves based on densities of 

E. coli measured by MF are used to demonstrate the viability of comparing results from studies 

conducted at different times and in different settings. The analyses performed in this 

demonstration are intended to illustrate techniques for assessing whether epidemiology study 

findings differ between studies and provide insights regarding the combinability of data from 

multiple studies. The demonstration relates to the P15 objectives in that it illustrates methods for 

comparing the GI illness response to exposure relationships, though in this case the relationships 

are for the same indicator-method combination. Note that the analyses presented are not the only 

ones that could be performed. Other meta-analyses that have explored the combination of health 

effects data from disparate epidemiology studies are summarized in a text box concluding this 

section. 

A recent epidemiology study conducted by Marion et al. (2010) resulted in a health effects curve 

relating E. coli density via MF to both HCGI and GI illness. Limitations of this study’s use in 

developing general, quantifiable relations are that the study was relatively small (minimum 

number of swimmers on a study day was 11 and the maximum was 88); the study was conducted 

for an inland waterbody (impounded reservoir); the dominant fecal pollution source was not 

clearly POTW discharge; and E. coli by MF was the sole indicator/method investigated. Despite 

these limitations, the analyses seek to evaluate whether the health effects curves generated in the 

Marion study are comparable to those generated in the epidemiology studies conducted by 

USEPA (1984) on freshwaters in support of the 1986 criteria. Although not conclusive, the 

comparison indicates that for the beaches studied by USEPA (1984) and Marion et al. (2010), 

FIB densities are associated with similar risks. 

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The Marion et al. (2010) and USEPA (1984) studies differ in the following respects: 

 the decade in which the study was conducted, 

 the manner in which water quality and health effects are characterized, and

 the settings at which data were collected. 

Plots of health effects vs. indicator densities for the two studies are presented in Figure 7. Data 

from the USEPA studies are blue diamonds and the regression line through the data is blue. The 

data from Marion and colleagues are presented in two different ways. In that study, illness rates 

are provided for ranges of FIB densities. Those data are plotted as purple crosses. Data for each 

study day were provided by the author in a personal correspondence (Marion, personal 

communication, 2010) and those data and associated regression line are shown as red circles and 

a red line. Individual study day data from the Marion et al. (2010) study show wide scatter, 

which is not surprising given the overall low rates of attributable illness and the relatively small 

number of swimmers on each study day. Overall, the ranges of attributable illness rates for the 

two sets of studies are similar (based on the binned data from the Marion study) and the slopes of 

the health effects curves also appear similar. 

Figure 7. Health Effects Data and Trend Lines from the USEPA and 

Marion Studies 

To test whether the data from the two studies indicated the same or different trends, an analysis 

of covariance (ANCOVA) for the individual study day and USEPA datasets was performed. The 

ANCOVA tests the hypothesis that the slope and intercept estimates for regression lines are the 

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same, accounting for covariation of the slopes and intercepts. The results of the ANCOVA are 

presented in Table 7. In ANCOVA analyses, the p-value is the probability that the parameter is 

the same for both models and a p-value above a value of 0.05 indicates that the null hypothesis is 

accepted. For the two epidemiological datasets the p-values for both the slopes and intercept are 

significantly above 0.05. Thus, it is concluded that the health effects curves are not different 

from each other and that the data may be pooled. 

Table 7. Results from ANCOVA comparing data from the 

USEPA (1984) and Marion et al. (2010) epidemiology studies 

Parameter P-value for comparison between models 

Slope 0.85 

Intercept 0.10 

The finding that the health effects models for the USEPA and Marion epidemiology studies are 

not significantly different has two ramifications. First, the study by Marion et al. (2010) was 

conducted at an inland lake (impounded reservoir) with mixed fecal pollution sources, one of 

which was discharge from small POTWs into tributaries to the lake. The studies conducted by 

USEPA (1984) took place on Lake Erie and at a freshwater inland lake (Keystone Lake, 

Oklahoma). While not conclusive, the ability to pool health effects data from these sites 

indicates that health effects at POTW-impacted inland waters are similar to those for POTWimpacted coastal waters. Second, the ANCOVA supports health effects relations for the two 

studies being similar despite the long time period between the studies. This finding cannot, 

however, be used to assert that water quality at all sites has not changed in the intervening years 

between the studies. However, the similarity in the health effects curve for the Marion et al. 

(2010) and USEPA (1984) studies may indicate that the earlier relationship is indicative of 

indicator-health effects relationships that may be observed for some types of current recreational 

sites. 

Again, it is noted that the comparison of the USEPA (1984) and Marion et al. (2010) studies is 

conducted to demonstrate techniques for establishing Risk Links. In this case, the analyses are 

conducted to establish that indicator densities are associated with similar illness levels in 

different studies. Analyses such as these may be required to establish linkages in the event 

straightforward linkages with data from a single study or set of studies are not possible. 

Finally, we note that meta-analyses of epidemiology studies have evaluated either implicitly or 

explicitly the similarities and differences in observed health effects and in health effects 

associations with indicator densities for different beaches and times. The objectives of those 

meta-analyses are in some respects similar to the analyses conducted to establish that the USEPA 

(1983) health effects curves are generally applicable today. Brief descriptions of other metaanalyses that have been performed are provided in the text box below. 

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Prüss (1998) conducted a systematic review following discussions between the WHO Regional Office 

for Europe and WHO Headquarters to initiate development of new guidelines for recreational use of 

the water environment. The comprehensive review of 22 published studies on sewage pollution of 

recreational water and health outcomes concluded that there was a causal association between GI 

illness symptoms and increased bacterial indicator density (i.e., enterococci for marine, enterococci 

and E. coli for fresh) in recreational waters. 

A meta-analysis of 18 published studies (Zmirou et al. 2003) provided a scientific basis for 

establishing new standards for the microbial quality of marine and fresh recreational waters to replace 

the 30 year-old European Union bathing water quality guidelines. The researchers provided four 

major results: (1) increased concentrations of fecal coliforms or E. coli and enterococci in both fresh 

and marine recreational waters are associated with increased risks of acute GI illness, with 

enterococci eliciting four to eight times greater excess risks than fecal coliforms or E. coli at the same 

concentrations; (2) GI illness risks associated with enterococci occur at lower concentration in marine 

versus fresh recreational waters; (3) increased concentrations of total coliforms have little or no 

association with GI illness risk; and (4) no evidence exists of a threshold of indicator density below 

which there would be no GI illness risk to bathers. 

Wade et al. (2003) conducted a systematic review and meta-analysis of 27 published studies to 

evaluate the evidence linking specific microbial indicators of recreational water quality to specific 

health outcomes under non-outbreak (endemic) conditions. Secondary goals included identifying and 

describing critical study design issues and evaluating the potential for health effects at or below the 

current regulatory criteria (USEPA, 1986). The researchers concluded that (1) enterococci and to a 

lesser extent E. coli are adequate indicators (predictors) of GI illness in marine recreational waters, 

but fecal coliforms are not; (2) the risk of GI illness is considerably lower in studies with enterococci 

and E. coli densities below those established by EPA (1986), thus providing support for their 

regulatory use; (3) E. coli is a more reliable and consistent predictor of GI illness than enterococci or 

other indicators in fresh recreational waters; and (4) based on heterogeneity analyses, studies that 

used a non-swimming control group and that focused on children found elevated GI illness risks. 

3.1.4 Application of the Risk Link Approach for Health Effects Curves Based on Different 

Illness Definitions 

Two steps are required to demonstrate the Risk Linkage between Enterococcus measured by MF 

in studies of POTW-impacted marine sites conducted by USEPA (1983) and Enterococcus

measured by qPCR in NEEAR marine water studies: 

 illness rates from the USEPA (1983) studies are translated to equivalent rates reflecting 

different illness definition used in NEEAR studies, and 

 demonstration that the association of indicator densities and health effects observed in the 

two studies are similar. 

Direct comparison of health risks associated with water quality published in 1986 and the health, 

water quality relationships developed by Wade (2009 to 2010) cannot be done because the case 

definitions underlying the relationships are not the same. Wymer (L. Wymer, personal 

communication, 2010) suggested a translation algorithm for converting the health criteria 

developed in 1986, based on a HCGI case definition, to an equivalent health criteria that uses 

NEEAR study data based on a NGI case definition. The translation would be independent of 

water quality data and uses health data from the 1972 to 1981 and 2002 to 2009 EPA studies. 

Conversion of the 1986 criteria to potential new criteria requires only the illness rates (using 

their respective case definitions) from the non-swimming populations from both study periods 

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U.S. Environmental Protection Agency

and the 1986 criteria value. The algorithm uses the HCGI illness rate (IR) in non-swimmers and 

the acceptable HCGI illness rate from the criterion, and calculates a relative risk (RR) value as: 

1986

1986 1986

1986 swimmer-Non

Criteria acceptable swimmer-Non

IR

IR IR RR   [2] 

An equivalent criterion value (ECV) can be calculated by multiplying the relative risk from the 

1986 data by the 2002-2009 non-swimmer illness rate (NGI) and subtracting the 2002-2009 nonswimmer illness rate (NGI) from that value. 

  1986 2009 2009 RRECV  swimmer-Non IR  swimmer-Non IR [3] 

Non-swimmer illness rates observed in the epidemiology studies conducted to support the1986 

criteria and in the NEEAR studies were 13.6/1000 swimmers and 59/1000 swimmers, 

respectively (USEPA 1986). Based on the translation and observed non-swimmer illness rates, 

equivalent criteria values for three risk levels of interest are provided in Table 8. 

Table 8. Equivalent Criteria in Terms of HCGI and NGI 

Definitions of Illness 

HCGI criterion (per 1000 

swimmers) 

ECV (NGI equivalent) 

(per 1000 swimmers) 

8 35 

10 43 

19 82 

At present, EPA has not selected new RWQC based on qPCR or MF measurement of 

Enterococcus. Therefore, this section concludes with illustration of the risk linkage approach for 

a range of risk levels and approaches for selecting criteria. Reasonable levels of risk for EPA to 

evaluate when selecting new criteria are the tolerable illness rates from the 1986 RWQC for 

recreation in freshwaters (8 HCGI attributable illnesses per 1000 swimmers), the tolerable illness 

rate for recreation in marine waters (19 HCGI attributable illnesses per 1000 swimmers), or some 

intermediate risk level (e.g., 10 HCGI attributable illnesses per 1000 swimmers). 

The FIB densities used in the USEPA (1983) health effects relationship are geometric mean 

values for indicator densities taken over entire recreation seasons. Recognizing that water 

quality varies over the recreation season, current criteria for single sample maximums for 

designated beaches are based on the 75th percentile of a log-normal distribution whose geometric 

mean corresponds to the indicator density from the USEPA health effects relationship and whose 

log-transformed standard deviation is 0.7. New or revised criteria for Enterococcus using MF 

methods can be based on the 75th percentile value or other values selected based on improved 

characterization of the temporal variability of Enterococcus densities in typical marine waters. 

Considering all of these and other factors, example criteria values for qPCR and culture-based 

datacorresponding to equivalent risk levels are presented in Table 9. The qPCR example criteria 

were calculated using the NEEAR study health effects relationship for all marine POTW28 
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impacted beaches combined. Note that all alternatives to the current culture-based criteria value 

for designated beaches (104 CFU/100 mL) are significantly lower than the current RWQC. 

Table 9. Examples of equivalent criteria values for marine waters via the statistical 

linkages approach 

Tolerable 

attributable 

illness level (as 

HCGI per 1000 

swimmers) 

Tolerable 

attributable 

illness Level (as 

NGI per 1000 

swimmers) 

Enterococcus 

density 

measured by 

qPCR (CCE/100 

mL) 

Geometric mean 

Enterococcus

density measured 

by MF (CFU/100 

mL) 

75th percentile 

value†

(CFU/100 mL) 

8 35 427 11 33 

10 43 610 14 42 

19 82 3460 35 104 

†

 The 75th percentile value for Enterococcus density based on the calculated geometric mean 

density and assuming a typical standard deviation of log-transformed density for marine sites 

of 0.7. 

3.2 Water Quality Link Approach 

The Water Quality Link approach requires two steps: (1) development of a model relating 

indicator density as measured by one indicator-method combination to indicator density as 

measured by another and (2) establishment of equivalent values of indicator densities based on 

two indicator-method combinations using the resulting model and a health effects curve for one 

of the indicator-method combinations. As noted in Section 2, approaches for establishing 

models relating indicator-method pairs are in development. Recently published studies have 

included models based on simple linear regression (e.g., Noble et al. 2010; Whitman et al. 2010) 

and suggested correction factors that account for site- or time-specific conditions influencing the 

ratio of indicator counts by different methods (e.g., Lavender and Kinzelman 2009). 

This section demonstrates both the modeling component and the linkage component of the Water 

Quality Link approach. First, general features of the paired data used in the Water Quality 

Linkage are presented. Next, development of the statistical models (functional form of the 

relationship between the densities by the different methods and data used to determine the model 

parameters) for linking indicator-method density pairs is demonstrated. The demonstration 

includes techniques for determining which data to use in generating the model and alternative 

model forms for NEEAR study freshwater beach data pairs for Enterococcus density as 

measured by qPCR and Enterococcus density as measured by MF. ANCOVA (described in 

Section 3.1.3) is suggested as a potential means for screening data prior to model development. 

Simple linear regression and broken stick (segmented) regression models are fit to the data and 

compared. The broken stick regression models were investigated because simple linear 

regression models of the NEEAR study data exhibited distributions of residuals that indicate the 

simple linear models may not adequately describe the relationship between the indicator method 

combinations. 

3.2.1 Datasets Used in Water Quality Link Demonstrations 

The NEEAR study freshwater data are used in demonstrating the Water Quality Link approach. 

Briefly, the NEEAR study water quality data include paired data for Enterococcus as measured 

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by MF, and Enterococcus as measured by qPCR for samples collected at multiple transects at 

four beaches and at three sample collection times for each sample day. The four beaches 

included Huntington Beach (Lake Erie), Washington Park Beach (Lake Michigan), Silver Beach 

(Lake Michigan) and West Beach (Lake Michigan). Samples were collected along multiple 

transects on each sample day at 8:00 AM, 11:00 AM, and 3:00 PM and at shin and waist depths. 

Other water quality and environmental data were collected during the NEEAR studies, but those 

data were not used in the current demonstration. 

3.2.2 Linear Models of Log-Transformed Indicator Data 

The most critical component of the Water Quality Linkage approach is establishing a model 

relating the densities of paired water quality samples for different indicator/method 

combinations. Several researchers have proposed linear models for relating log-transformed 

Enterococcus qPCR densities and culture counts (e.g., Haugland et al. 2005; Noble et al. 2010; 

Whitman et al. 2010;). This section describes a process by which a linear model of logtransformed data may be developed. Model development includes regression of data to find 

model parameters (slope and intercept). Additionally, the data undergo a selection process by 

which similar data are included, and dissimilar data are excluded, in the analysis. 

To introduce the Water Quality Link approach and modeling of paired indicator density data, a 

plot of paired data (Enterococcus density as measured by MF and Enterococcus density as 

measured by qPCR, Figure 8) is presented and described. The data and fits shown in this 

example provide background for development of the linear regression model and orient the 

reader to general features of the relationship between densities of Enterococcus measured by MF 

and Enterococcus measured by qPCR. The data shown in Figure 8 were collected and analyzed 

by Ferretti et al. (2008) in a study of multiple beaches along the New Jersey shore. They show a 

general trend toward a 1:1 correspondence between cell equivalents (CE) and CFUs at high 

indicator densities, with significantly greater scatter, and possibly a different trend, at low 

indicator densities. These trends are both expected and, as described in Section 2, are likely due 

to the following: 

 the association of each live, culturable cell with genetic material measured via qPCR; 

 high uncertainty in qPCR at low densities due to analysis of very small samples volumes 

(in terms of original sample volume); 

 divergence in the culture and qPCR signals as organisms age; and 

 the difference in the culture to qPCR ratio arising from uneven loading of culture and 

qPCR targets to recreational sites. 

In Figure 8, paired Enterococcus densities for two New Jersey beaches, in Monmouth County 

(red circles) and Ocean County (blue circles), are illustrated. A dashed green regression line 

shows the best fit of a linear model to the raw data (not log-transformed) while a dashed red 

regression line shows a linear regression fit to the log-transformed data. The black 45° line is a 

hypothetical line showing perfect correspondence between Enterococcus densities when 

measured via the two methods. The paired data for these two beaches are similar to those 

reported for other beaches—high variability and uncertainty in the qPCR counts at low density, 

with a correlation approaching a 1:1 ratio at higher indicator density. In this example, it is not 

obvious whether the data from the two beaches are sufficiently similar to be pooled and used to 

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develop a single regression model for linking the indicator/method combinations. It is also not 

clear which water quality model is the best for fitting the data, though both have the potential for 

application. These two observations (i.e., uncertainty regarding data pooling and uncertainty 

regarding model form) indicate that statistical tests should be used to evaluate the similarity 

between datasets prior to their inclusion in the model, and that alternative functional forms 

should be evaluated for the model relating Enterococcus density as measured by culture (MF) 

methods to Enterococcus density as measured by qPCR. 

Figure 8. Typical plot of paired Enterococcus densities as measured by 

qPCR and culture-based methods 

ANCOVA, a statistical method for assessing differences in model slopes and intercepts for data 

corresponding to different factors (e.g., different beaches or different times of day), can be used 

to assess whether trends observed at different locations or times are similar, and to help ensure 

that only related data are used in model development. In the event that paired water quality data 

are determined to be similar, the data may be pooled and model parameters may be estimated 

using the pooled dataset. If models using datasets that are determined to be dissimilar, it is 

assumed that there are differences in the systems that generated the data and those differences 

are not accounted for in the model relating the two datasets. The use of ANCOVA for 

developing datasets is illustrated with the following example. 

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As part of EPA’s NEEAR studies conducted in the Great Lakes, Enterococcus densities as 

measured by MF and by qPCR were collected (Wade et al. 2006, 2008). Samples were collected 

three times per day (8:00 AM, 11:00 AM, and 3:00 PM), at multiple depths (waist, shin, and 

knee), and along three transects at each of four beaches studied. A model relating the paired 

Enterococcus densities as measured by both qPCR and MF can be developed from all the sites 

and times, or from data that are selected because they appear to have similar underlying trends. 

All the paired water quality data collected are shown in Figure 9, along with a 45° line showing 

perfect agreement. Although one may observe a general trend at high indicator densities, no 

trend is apparent at densities below roughly 500 CFU/100 mL. Additionally, there is no obvious 

choice for the form (equation relating Enterococcus density as measured by qPCR and as 

measured by membrane filtration) of a model relating the data from the two analytical methods. 

Figure 10 shows paired water quality data from Huntington Beach only, one of the four Great 

Lakes beaches studied. Again, the FIB densities appear linearly related at high densities and 

scattered at low densities, though the scatter is considerably less than that observed when data for 

all the beaches are plotted together. 

Figure 9. Paired qPCR and culture Enterococcus data, all 

NEEAR study Great Lakes beaches 

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Figure 10. Paired culture and qPCR Enterococcus data, all 

samples from Huntington Beach 

Figure 11 and Figure 12 show data for only 8:00 AM and 3:00 PM, respectively, along with 45 ° 

lines and regression fits developed via linear regression of the log-transformed densities. Note 

that the trend lines for the 8:00 AM and 3:00 PM samples are markedly different and that 

segregation of data by time of the day markedly reduces the scatter in the plots. 

Figure 11. Paired qPCR and culture Enterococcus data, 

Huntington Beach: 8:00 AM samples only 

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Figure 12. Paired qPCR and culture Enterococcus data, Huntington 

Beach: 3:00 PM samples only

ANCOVAs can be used to assess whether the differences in the regression lines observed for the 

8:00 AM and 3:00 PM data are statistically significant. ANCOVAs were run to compare the 

regression lines for sets of data corresponding to each of the collection times at Huntington 

Beach. As shown in Table 10, the ANCOVA indicates that there are significant differences in 

the models for the 8:00 AM and 11:00 AM dataset, and for the 8:00 AM and 3:00 PM datasets, 

but not for the 11:00 AM and 3:00 PM datasets. This finding is not surprising given that the 8:00 

AM and 3:00 PM samples do not correspond to the same model, and because solar radiation is 

known to reduce culture counts for afternoon samples while qPCR counts remain relatively 

constant throughout the day. 

Table 10. Results of ANCOVAs for sets of data for 

Huntington Beach with different collection times 

P-value 

Collection times Intercepts Slopes 

8:00 AM and 

11:00 AM <0.0001 0.002 

8:00 AM and 3:00 

PM <0.0001 0.01 

11:00 AM and 

3:00 PM 0.52 0.60 

Additional ANCOVAs were performed to compare models for all of the NEEAR freshwater 

study datasets and to determine whether sample water depth, time of day, and beach location 

altered the model relating qPCR and culture-based datasets. Findings from the ANCOVAs are 

as follows: 

 Samples corresponding to different sample collection depths can be pooled. 

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 Models for different times of day differ for two beaches (Huntington and West Beach) 

but do not differ for the other two beaches (Silver Beach and Washington Park Beach). 

Agreement between models for different times of day for Silver Beach and Washington 

Park Beach might be the result of extreme scatter in the data for those two beaches. 

Based on these findings and knowledge of the variation in culture and qPCR signals 

during the day, 8:00 AM Enterococcus measured by MF are fit by different models than 

samples from other times of day.. 

 For these data, models for different beaches are dissimilar and pooling of the data may be 

inconsistent with simple linear models. 

3.2.3 Demonstration of the Water Quality Linkage Approach using Linear Regression Models 

In this section, the Water Quality Link approach is demonstrated using NEEAR study Great 

Lakes Enterococcus density data (as measured by both MF and qPCR), the NEEAR study 

freshwater health effects curve (presented in equation form in Table 5), and linear models 

relating log-transformed data of Enterococcus density as measured by qPCR and MF. As noted 

previously, these linear models may not adequately characterize the correlation between qPCR 

and culture Enterococcus density for some datasets. Therefore, the illustration presented below 

may be used for developing an understanding of the Water Quality Linkage approach but not for 

developing specific equivalences between densities of Enterococcus as measured by MF and as 

measured by qPCR. The Water Quality Link demonstration entails 

1. selection of an Enterococcus density based on the NEEAR study freshwater beaches 

health effects curve; 

2. use of linear regression to develop several candidate models relating density of 

Enterococcus as measured by qPCR to Enterococcus density as measured by MF; and 

3. use of the linear relationships developed in step 2 to establish candidate Enterococcus

densities as measured by MF to the Enterococcus density selected in step 1. 

After the Water Quality Linkage approach is demonstrated the results are critically evaluated and 

used to suggest alternative Water Quality Linkage approaches. 

The NEEAR study health effects curve for Great Lakes beaches (see Table 5) indicates that an 

Enterococcus density of about 125 qPCR CE/100 mL relates to a risk of about 45 NGI illnesses 

in 1000 swimmers. The rate 45 NGI illnesses in 1000 is approximately equivalent to a risk of 8 

HCGI illnesses in 1000 swimmers—the level at which current (1986) freshwater criteria are 

based (see Section 3.1.4). Thus for this demonstration, the qPCR level of interest is selected to 

be 125 qPCR CE. This selection does not imply that new or revised criteria will be based on the 

Great Lakes health effects curve or on a risk level of 45 NGI illnesses in 1000 swimmers. It is 

selected for demonstration purposes only and because it is based on risk in a range consistent 

with current (USEPA 1986) criteria. 

The choice of a model relating the Enterococcus density data from qPCR to density data from 

MF has a profound influence on the outcome of a Water Quality Linkage analysis. This is 

illustrated using the NEEAR study data. When data from all the NEEAR study freshwater 

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beaches are used to develop a model relating Enterococcus qPCR and culture densities (the 

purple line in Figure 13), the regression line has a slope <1 and falls significantly below the 45° 

line, for the culture-based density above about 100 CFU/100 mL. The model mean value for 

CFU equivalent to CE for 45 illnesses/1000 swimmers is 1212 CFU/100 mL and the confidence 

interval for CFU equivalent to CE for 45 illnesses/1000 swimmers is <519 CFU/100 mL, 4628 

CFU/100mL>. Thus, these Enterococcus densities, measured by MF, are clearly very high and 

inconsistent with the selected level of tolerable risk. 

Figure 13. Linear model, data from all NEEAR freshwater beaches 

When only Huntington Beach data are used, the results appear to be more reasonable. The 

model line (the purple line in Figure 14) still has a slope <1, which is inconsistent with expected 

trends at high indicator density, but the slope is steeper than that of the model for data from all 

beaches. The model mean value for CFU equivalent to CE for 45 illnesses/1000 swimmers is 

10.8 CFU/100 mL, and the confidence interval around that value is <7.1 CFU/100mL, 15.4 

CFU/100mL>. These equivalent values appear unrealistically low, though more reasonable than 

the equivalent values derived using the model for data from all beaches. 

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Figure 14. Linear model, data from Huntington Beach 

The most reasonable equivalent culture densities result from a model developed using only 8:00 

AM data from Huntington Beach. The model, equivalent value, and confidence interval are all 

shown in Figure 15. The model mean value for CFU equivalent to CE for 45 illnesses/1000 

swimmers is 29.4 CFU/100 mL and the confidence interval around that value is <17.8 

CFU/100mL, 45.9 CFU/100mL>. 

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Figure 15. Linear model, 8:00 AM data from Huntington Beach 

Based on the analysis of paired water quality NEEAR study Enterococcus density data 

measured by qPCR and MF and using simple linear model of log-transformed variables, several 

observations may be made, though the extent to which these may apply to datasets other than the 

ones studied is unknown. First, model selection (inclusive of data selection) profoundly impacts 

the Water Quality Linkage approach. For this dataset, when all data are pooled and a water 

quality model is generated, the resulting densities of Enterococcus as measured by MF that are 

equivalent to the density of Enterococcus as measured by qPCR appear very high in comparison 

to the current criteria. When only data from morning samples at a single beach are used to 

develop the linear model, the resulting equivalent Enterococcus densities for MF are closer to the 

current criteria. The non-robust performance of linear models of log-transformed indicator 

densities in this demonstration suggests exploration of alternative models that may be capable of 

modeling more complex phenomena and providing an adequate fit to a larger set of data. 

3.2.4 Broken Stick Models of Log-Transformed Indicator Data 

The relationship between paired Enterococcus densities as measured by MF and Enterococcus

densities as measured by qPCR data might not be fit best by a simple linear regression model of 

log-transformed data. An alternative model evaluated in the development of the Water Quality 

Linkage approach is the broken stick regression model, also known as the segmented model 

(Draper and Smith 1998). In broken stick regression models, the data are divided in two parts, 

each of which is fit with a different linear model. The two linear models meet at a “kink” in the 

regression line. Broken stick regression models have four parameters (two slopes and the 

coordinates of the point where the segments of the broken stick meet) that may be determined via 

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optimization, whereas simple linear regression models have two parameters (slope and 

intercept). The broken stick model appears a good choice for analyzing paired densities of 

Enterococcus as measured by MF and Enterococcus as measured by qPCR because the data 

appear to behave differently at low and high indicator densities. 

Broken stick regression models were developed for each of the four NEEAR Great Lakes study 

beaches separately. Two models were developed for each beach—one for all data for that beach 

pooled and one for only data from samples collected at 8:00 AM. The resulting curves are 

shown in Figure 16 to Figure 19. 

Figure 16. Huntington Beach broken stick model fits for all data and 8:00 AM data 

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Figure 17. West Beach broken stick model fits for all data and 8:00 AM data 

Figure 18. Silver Beach broken stick model fits for all data and 8:00 AM data 

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Figure 19. Washington Park Beach broken stick model fits for all data and 8:00 AM data 

Broken stick regression models of Huntington and West Beach data both exhibit slopes closer to 

1.0 in the high FIB density portion of the broken stick, compared to the low density portion. For 

the Washington Park and Silver Beach datasets, the presence of water quality data pairs with 

relatively high Enterococcus culture density and very low Enterococcus qPCR density appear to 

exert an undue influence on the slope portion of the broken stick in the high culture density data 

range. For the four beaches, the location of the “kink” in the stick varies within the range 12–

100 CFU/100 mL for models of data including all sample times, and in the range 28–62 

CFU/100 mL for the 8:00 AM samples of the Huntington and West Beach data. 

Given the empirical and theoretical support for different correlation of qPCR and culture counts 

at high and low densities, the broken stick model offers advantages over models of simple linear 

regression of log-transformed densities, including 

 better fits to data, 

 reduction in heteroskedasticity of regression fits, and 

 consistency with observed relationships between qPCR and culture densities. 

Regression analysis of correlations between the qPCR and culture densities, as well as the 

literature, support different relationships between the two water quality measures at low and high 

indicator counts. At low indicator densities, the relationship of the culture indicator with the 

fecal pollution source becomes tenuous and the qPCR counts become highly uncertain. At 

higher densities, the abundance in targets for the two methods is correlated. These findings both 

support and suggest performing regression analysis only for those data pairs for which culture 

densities exceed a threshold value. 

Three regression analyses were performed to evaluate whether using only data above such 

threshold values yielded robust models for estimating culture densities that are equivalent to 

qPCR densities. In all of the analyses, only FIB density data collected in the morning (8:00 AM 

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sample time) were included, due to potential discrepancies between enumeration methods at later 

times. In the first analysis, only points above the “kink” in the stick were retained for analysis 

for each of the four Great Lakes beaches at which EPA’s NEEAR epidemiology studies were 

conducted. The data retained for each beach were determined by the “kink” location for each of 

the four beaches. In the second and third analyses, a threshold common to all beaches was 

chosen as the basis for retaining data. Two thresholds were explored—80 CFU/100mL and 104 

CFU/100 mL. The value 104 CFU/100 mL was selected based on the current single sample 

maximum for designated beaches; 80 CFU/100 mL was selected because it lies between the 

current criteria and the geometric mean on which it is based (35 CFU/100 mL). 

Plots showing the resulting three regression models and the culture equivalent to a qPCR density 

of 126 CE/100 mL are shown in Figure 20 (data above the kink in the stick), Figure 21 (data 

above 80 CFU/100 mL), and Figure 22 (data above 104 CFU/100 mL). The qPCR density 

chosen for this illustration (126 CE/100 mL) is a plausible value for qPCR-based criteria (Table 

9). In all cases, the new regression models and qPCR value of interest result in equivalent 

culture criteria exceeding the current single sample maximum value for infrequently-used fresh 

waters of 151 CFU enterococci / 100 mL. Regression model lines and culture equivalents to a 

potential qPCR criterion of 126 CE/100 mL are provided in Table 11. 

Figure 20. Model resulting from retention of all data above 

the “kink” in the broken stick 

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Figure 21. Model resulting from retention of all data 

above 80 CFU/100 mL 

Figure 22. Model resulting from retention of all data above 

104 CFU/100 mL 

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Table 11. Summary of models resulting from retention of data pairs with culture counts above 

three thresholds 

Treatment Regression line 

Equivalent culture 

density of interest 

(CFU/100 mL) 

Retain data for which CFU count 

is above the break in the stick     10 qPCR CFU10 log C   log759.0393.0 C 178 

Retain data for which CFU count 

is above 80 CFU/100 mL     10 qPCR CFU10 log C    log944.0056.0 C 192 

Retain data for which CFU count 

is above 126 CFU/100 mL     10 qPCR CFU10 log C    log358.1289.1 C 246 

Several observations can be made regarding development of a regression model based on data 

only above thresholds. First, the model results appear quite sensitive to the selection of the break 

point above which data are retained. In this regard, there does not appear to be a widely accepted 

methodology for selecting the appropriate break point. Both Lavender and Kinzelman (2009) 

and Byappanahalli et al. (2010) determined that the qPCR-culture relationship varies with 

environmental conditions which, in turn, vary differently at different sites. Second, in all cases, 

data pairs with much lower qPCR densities than culture densities appear to have a pronounced 

effect on the regression models. At present, we have no explanation for the occurrence of those 

data pairs. Because the pairs occur above the cutoff value for culture densities, it is unlikely that 

sampling error (i.e., small qPCR sample size in terms of original sample volume) is the cause. In 

summary, regression models based on the broken stick or threshold approach were dependent on 

a cut-off value, the choice of which could be viewed as arbitrary. Further, culture densities 

equivalent to potential qPCR densities were higher than the current single sample maximum for 

infrequently-used freshwater beaches when the Water Quality Link approach is applied to the 

NEEAR study freshwater indicator dataset, using a broken stick regression model to relate 

densities of Enterococcus as measured by MF and qPCR. 

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4 Discussion 

Two approaches for linking indicator-method combinations to a common risk level were 

demonstrated—the Risk Link approach and the Water Quality Link approach. Using currently 

available data and techniques, the exploration of these methods meets both the objective of 

developing quantifiable relationships between indicator-method combinations and provides 

practical experience in the complexities underlying the two approaches. 

Applying the Risk Link approach can be relatively straightforward when multiple indicator 

health relationships data are available from a single epidemiology study, as demonstrated in the 

Bacteroidales (measured by qPCR) and Enterococcus (measured by qPCR) Risk Link 

comparison. However, when directly comparable data are not available, additional steps are 

added to the Risk Link approach. These steps may include, but are not limited to, the 

harmonization of GI definitions used in two epidemiology studies or the generality of health 

effects relations established for sites with specific sources of fecal pollution. 

Regarding the Water Quality Link approach, generating a relationship between indicator 

densities, as measured by qPCR and culturable methods, is more complex than simple linear or 

broken stick regression models. In our demonstration, the relationship, between paired water 

quality data corresponding to different indicator-method combinations, appears to vary from 

beach to beach. The Water Quality Link may be a useful tool for the development of sitespecific standards by states; but as formulated in this report, is not necessarily useful for the 

development of National criteria. 

It is possible that additional analyses or datasets might allow for alternative quantifiable 

relationships to be explored using both Link approaches. For example, if statistical techniques 

are developed that allow for the direct comparison of results between RCT and PC epidemiology 

study designs, additional health effects relationships, such as those from the Epibathe studies, 

could be evaluated. 

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