1

Calculations Performed in Developing the Proposed 2012 RWQC Values:

This memo presents the calculations and process of rounding values performed by EPA in the 

process of developing the 2012 RWQC values. 

1. The Recommended 2012 RWQC values

Table 1. Recommended 2012 RWQC.

Criteria 

Elements

Estimated Illness Rate (NGI): 36 

per 1,000 primary contact 

recreators

OR

Estimated Illness Rate (NGI): 

32 per 1,000 primary contact 

recreators

Magnitude Magnitude

Indicator 

GM

(cfu/100 mL)a

STV

(cfu/ 100 mL)

a

GM

(cfu/100 mL)a

STV

(cfu/ 100 mL)a

Enterococci –

marine and 

fresh 35 130 30 110

OR

E. coli – fresh 126 410 100 320

Duration and Frequency: The waterbody GM should not be greater than the selected GM 

magnitude in any 30-day interval. There should not be greater than a ten percent excursion 

frequency of the selected STV magnitude in the same 30-day interval. 

a

EPA recommends using EPA Method 1600 (U.S. EPA, 2002a) to measure culturable enterococci, or another 

equivalent method that measures culturable enterococci and using EPA Method 1603 (U.S. EPA, 2002b) to measure 

culturable E. coli, or any other equivalent method that measures culturable E. coli.

Enterococci (fresh and marine waters)

Culturable enterococci GMs of 30 and 35 cfu per 100 mL are based on the EPA’s analysis of the 

1986 epidemiological data and the NEEAR combined fresh and marine water quality data as 

described in the 2012 RWQC document. STVs for those GMs were calculated as follows:

STV = 130 cfu per 100 mL for a GM of 35 cfu per 100 mL

 Assuming a log10 standard deviation1of 0.44, the 90th percentile of the distribution with 

a GM of 35 cfu per 100 mL is given by Equation (1): 

STV= antilog10 [log10(GM)+Z90*log10SD] (1)

where Z90=1.28 is the one-sided standard normal z statistic for a 90% confidence interval

(CI). 

 The STV is 128 cfu per 100 mL. EPA rounded up to 130 cfu per 100 mL.

 

1 The standard deviations are based on water quality data from EPA epidemiological studies. Standard deviations 

based on: the 1986 freshwater E. coli studies was 0.40; the NEEAR studies, using the marine and freshwater beaches 

combined (not including Surfside or Boquerón) for culturable enterococci was 0.44; and the NEEAR studies, using 

the marine and freshwater beaches combined (not including Surfside or Boquerón) for Enterococcus qPCR Method 

1611 was 0.49.
2

STV = 110 cfu per 100 mL for a GM of 30 cfu per 100 mL

 Using Equation (1) and a log10 standard deviation of 0.44, the STV for a GM of 30 cfu 

per 100 mL is 110 cfu per 100 mL.

E. coli (fresh waters)

GM = 126 cfu per 100 mL

 Based on the results of the 1986 freshwater analyses and the analyses presented in section 

3.2.4 of the 2012 RWQC, EPA assumes that 35 cfu per 100 mL culturable enterococci 

corresponds to 36 NGI per 1,000 primary contact recreators (8 HCGI per 1,000 primary

contact recreators).

 Using the 1986 fresh water relationship between swimming-associated illness (Equation 

(2))

2

:

Swimming associated HCGI illness = − 11.74 + 9.397 (mean log10 E. coli per 100 mL) (2)

EPA computed that 8 HCGI per 1,000 primary contact recreators corresponds to an E. 

coli density of 126 cfu per 100 mL.

GM = 100 cfu per 100 mL 

 Based on the analyses presented in section 3.2.4 of the 2012 RWQC, EPA estimated that 

30 cfu per 100 mL culturable enterococci corresponds to an estimated illness range of 27-

36 NGI per 1,000 primary contact recreators (Figure 4 in the 2012 RWQC document).

EPA used the midpoint of this range (31.5 NGI per 1,000 primary contact recreators) as 

the estimated illness corresponding to 30 cfu per 100 mL. The illness rate of 31.5 NGI 

per 1,000 primary contact recreators corresponds to 7 HCGI per 1,000 primary contact 

recreators.

 Using the 1986 fresh water relationship between swimming-associated illness (Equation 

(2)), 7 HCGI per 1,000 primary contact recreators corresponds to an E. coli density of 99 

cfu per 100 mL. EPA rounded up to 100 cfu per 100 mL.

STV = 410 cfu per 100 mL for a GM of 126 cfu per 100 mL

 Using Equation (1) and assuming a log10 standard deviation of 0.40, the STV is 410 cfu 

per 100 mL.

STV = 320 cfu per 100 mL for a GM of 100 cfu per 100 mL

 Using Equation (1) and assuming a log10 standard deviation of 0.40, the STV 

corresponding to the GM of 100 cfu per 100 mL is 325 cfu per 100 mL. EPA rounded 

down to 320 cfu per 100 mL. 

 

2 U.S. EPA 1986. EPA’s Ambient Water Quality Criteria for Bacteria – 1986. U.S. Environmental Protection 

Agency: Washington, DC. EPA440/5-84-002
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2. Beach Action Values (BAV)

Table 2. Beach Action Values (BAV).

Indicator

Estimated Illness Rate 

(NGI): 36 per 1,000 

primary contact 

recreators

OR

Estimated Illness Rate 

(NGI): 32 per 1,000 

primary contact 

recreators

BAV

(Units per 100 mL)

BAV

(Units per 100 mL)

Enterococci – culturable 

(fresh and marine)a70 cfu 60 cfu

E. coli – culturable (fresh)b235 cfu 190 cfu

Enterococcus spp. – qPCR 

(fresh and marine)c 1,000 cce 640 cce

a Enterococci measured using EPA Method 1600 (U.S. EPA, 2002a), or another equivalent method that measures 

culturable enterococci.

b

E. coli measured using EPA Method 1603 (U.S. EPA, 2002b), or any other equivalent method that measures 

culturable E. coli.

c EPA Enterococcus spp. Method 1611 for qPCR (U.S. EPA, 2012b). See section 5.2. 2012 RWQC.

Enterococci (marine and fresh waters)

BAV of 70 cfu per 100 mL for a GM of 35 cfu per 100 mL

 Assuming a log10 standard deviation of 0.44, the 75th percentile of the distribution is given 

by Equation (1), where Z75=0.675 is the one-sided standard normal z statistic for a 75% CI.

 The BAV is 69 cfu per 100 mL. EPA rounded up to 70 cfu per 100 mL. 

BAV of 60 cfu per 100 mL for a GM of 30 cfu per 100 mL

 Using Equation (1) with a Z75=0.675 and log10 standard deviation of 0.44, the BAV is 59

cfu per 100 mL. EPA rounded up to 60 cfu per 100 mL. 

E. coli (fresh waters)

BAV of 235 cfu per 100 mL for a GM of 126 cfu per 100 mL

 Using Equation (1) with a Z75=0.675 and log10 standard deviation of 0.40, the BAV is 

235 cfu per 100 mL.

BAV of 190 cfu per 100 mL for a GM of 100 cfu per 100 mL

 Using Equation (1) with a Z75=0.675 and log10 standard deviation of 0.40, the BAV is 

186 cfu per 100 mL. EPA rounded up to 190 cfu per 100 mL

Enterococcus qPCR (fresh and marine waters)

BAV of 1,000 cce per 100 mL for a GM of 470 cce per 100 mL

 Using Equation (1) with a Z75=0.675 and log10 standard deviation of 0.49, the BAV is 

1,007 cce per 100 mL. EPA rounded down to 1,000 cce per 100 mL.
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BAV of 1,000 cce per 100 mL for a GM of 300 cce per 100 mL

 Using Equation (1) with a Z75=0.675 and log10 standard deviation of 0.49, the BAV is 

642 cfu per 100 mL. EPA rounded down to 640 cce per 100 mL

3. Equivalent Enterococcus qPCR Method 1611 values (fresh and marine waters)

Table 3. Values for qPCR.

Element

Estimated Illness Rate 

(NGI): 36 per 1,000 

primary contact 

recreators

OR

Estimated Illness Rate 

(NGI): 32 per 1,000 

primary contact 

recreators

Magnitude Magnitude

GM

(cce per

100 mL)

STV

(cce per

100 mL)

GM 

(cce per 

100 mL)

STV 

(cce per

100 mL)

qPCRa470 2,000 300 1,280

Duration and Frequency: The waterbody GM should not be greater than the selected 

GM magnitude in any 30-day interval. There should not be greater than a 10 percent 

excursion frequency of the selected STV magnitude in the same 30-day interval.

a

EPA Enterococcus spp. Method 1611 for qPCR (U.S. EPA, 2012b).

Enterococcus – qPCR Method 1161(fresh and marine waters) 

qPCR-based GM values of 470 cce per 100 mL and 300 cce per 100 mL

 Using EPA’s NEEAR combined relationship between swimming-associated illness 

(Equation (3) below) 

Swimming associated NGI = -27.31 + 23.73 (mean log10 qPCR cce per 100 mL) (3)

 31.5 NGI per 1,000 primary contact recreators corresponds to a qPCR GM of 301 cce per 

100 mL.

 36 NGI per 1,000 primary contact recreators corresponds to an exact qPCR GM of 466 

cce per 100 mL.

 EPA rounded 301 to 300 cce per 100 mL, and 466 to 470 cce per 100 mL.

qPCR-based STV values of 2,000 and 1,280 cce per 100 mL

 Using Equation (1) with a Z90=1.28 and assuming a log10 standard deviation of 0.49,

o The STV value for a GM of 470 cce per 100 mL is 1,992 cce per 100 mL. EPA 

rounded up to 2,000 cce per 100 mL. 

o The STV value for a GM of 300 cce per 100 mL is 1,272 cce per 100 mL. EPA 

rounded up to 1,280 cce per 100 mL.