signed: August 23, 2007 

MEMORANDUM 

SUBJECT: Analytical Methods for Mercury in National Pollutant Discharge Elimination 

System (NPDES) Permits 

FROM: James A. Hanlon, Director 

Office of Wastewater Management 

TO: Water Division Directors, Regions 1 B 10 

The purpose of this memorandum is to inform you of EPA’s March 12, 2007, approval of 

Method 245.7 for measurement of mercury and modified versions of approved analytical 

methods for mercury as well as the impact of their approval on the NPDES permitting process. 

While several different methods are currently approved under 40 CFR Part 136 for the analysis 

of mercury, some of these methods have much greater sensitivities and lower quantitation levels 

than others. This memorandum clarifies and explains that, in light of existing regulatory 

requirements for NPDES permitting,1 only the most sensitive methods such as Methods 1631E 

and 245.7 are appropriate in most instances for use in deciding whether to set a permit limitation 

for mercury and for sampling and analysis of mercury pursuant to the monitoring requirements 

within a permit. 

BACKGROUND 

Section 301 of the Clean Water Act (CWA) requires NPDES permits to include effluent 

limitations that are as stringent as necessary to meet water quality standards. Thus, under the Act 

and EPA regulations, each permit must include, as necessary, requirements in addition to or 

more stringent than technology-based effluent limitations established under section 301 of the 

CWA in order to achieve water quality standards. 40 C.F.R. ' 122.44(d)(1). The regulations 

require limitations to control all pollutants that the NPDES program director determines are or 

may be discharged at a level that Awill cause, have the reasonable potential to cause, or 

contribute to an excursion above any state water quality standard,@ including both narrative and 

This memorandum is based on existing legal requirements and authorities. It does not impose any new, 

legally binding requirements on EPA, states, or the regulated community. 

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numeric criteria. 40 C.F.R. ' 122.44(d)(1)(i). If the program director determines that a discharge 

has the reasonable potential to cause or contribute to such an excursion, the permit must contain 

water quality-based effluent limitations for the pollutant. 40 C.F.R. ' 122.44(d)(1)(iii). Thus, a 

prospective permittee may need to measure various pollutants in its effluent at two stages: first, 

at the permit application stage so that the program director can determine whether “reasonable 

potential” exists and establish appropriate permit limits; and second, where a permit limit has 

been established, to meet the monitoring requirements within the permit. The following 

discussion explains which analytical methods permit applicants and permittees should use to 

make these measurements when mercury is the pollutant at issue. 

Approved Analytical Methods 

Measurements included on NPDES permit applications and on reports required to be submitted 

under the permit must generally be made using analytical methods approved by EPA under 40 

CFR Part 136. See 40 CFR 136.1, 136.4, 136.5, 122.21(g)(7), and 122.41(j). For mercury, there 

are three methods commonly used in the NPDES program that EPA has approved under Part 

136: Method 245.1, Method 245.2, and Method 1631E. Methods 245.1 and 245.2 were approved 

by EPA in 1974 and can achieve measurement of mercury down to 200 parts per trillion (ppt). 

Additionally, EPA approved Method 1631 Revision E in 2002. Method 1631E has a quantitation 

level of 0.5 ppt, making it 400 times more sensitive than Methods 245.1 and 245.2. In fact, the 

sensitivity of Methods 245.1 and 245.2 are well above the water quality criteria now adopted in 

most states (as well as the criteria included by EPA in the Final Water Quality Guidance for the 

Great Lakes System) for the protection of aquatic life and human health, which generally fall in 

the range of 1 to 50 ppt.2

 In contrast, Method 1631E, with a quantitation level of 0.5 ppt, does 

support the measurement of mercury at these low levels. 

In addition to Methods 245.1, 245.2, and 1631E listed above, EPA approved Method 245.7 as 

well as modified versions of other EPA-approved methods on March 12, 2007. See 72 FR 

11200. Method 245.7 has a quantitation level of 5.0 ppt, making it 40 times more sensitive than 

Methods 245.1 and 245.2. Additionally, modified versions of EPA-approved methods may also 

be used for the measurement of mercury. Methods approved under Part 136, such as 245.1 and 

245.2, may be modified to achieve lower quantitation levels than can be achieved by the method 

as written.3 Modifications to an EPA-approved method for mercury that meet the method 

2 Many states have adopted mercury water quality criteria of 12 ppt for protection of aquatic life and 50 ppt 

for the protection of human health, and for discharges to the Great Lakes Basin, the applicable water quality criteria 

for mercury are 1.3 ppt for the protection of wildlife and 1.8 ppt for the protection of human health. In 2001, EPA 

issued new recommended water quality criteria guidance for the protection of human health. This new guidance 

recommends adoption of a methylmercury water quality criterion of 0.3 milligrams of methylmercury per kilogram 

(mg/kg) in fish tissue. EPA is currently developing implementation guidance to assist states in implementing the 

criterion, and Draft Guidance for Implementing the January 2001 Methylmercury Water Quality Criterion (EPA823-B-04-001) was released for public comment in August 2006. 

3 Examples of such modification may include changes in the sample preparation digestion procedures such as 

the use of reagents similar in properties to ones used in the approved method, changes in the equipment operating 

parameters such as the use of an alternate more sensitive wavelength, adjusting the sample volume to optimize 

method performance, and changes in the calibration ranges (provided that the modified range covers any relevant 

regulatory limit). 

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performance requirements of Part 136.6 are considered to be approved methods and require no 

further EPA approval. See 72 FR 11239-40 (March 12, 2007). For analytical method 

modifications that do not fall within the flexibility of Part 136.6, the modified methods may be 

approved under the alternate test procedure program as defined by Parts 136.4 and 136.5. 

ACTIONS RESULTING FROM THE MARCH 12, 2007, RULEMAKING 

To implement the March 12, 2007, rule, the Office of Wastewater Management (OWM) provides 

the following guidance: 

Monitoring Data Submitted as Part of NPDES Permit Applications 

As noted, most states have adopted water quality criteria for the protection of aquatic life and 

human health that fall in the range of 1 to 50 ppt, and Methods 245.1 and 245.2, as written, do 

not detect or quantify mercury in this range. A Adid not detect@ result using Method 245.1 or 

Method 245.2 would show only that mercury levels are below 200 ppt but would not establish 

that they are at or below the applicable water quality criterion. Therefore, when a permit writer 

receives a permit application reporting mercury data analyzed with Method 245.1 or Method 

245.2 as Adid not detect@ results, the permit writer in reality may lack the information needed to 

make a Areasonable potential@ determination. In contrast, Method 1631E is able to detect and 

quantify mercury concentrations at these low levels. 

EPA therefore expects, in general, that all facilities with the potential to discharge mercury will 

provide with their NPDES permit applications monitoring data for mercury using Method 1631E 

or another sufficiently sensitive EPA-approved method. For purposes of permit applications, a 

method for mercury is “sufficiently sensitive” when (1) its method quantitation level is at or 

below the level of the applicable water quality criterion for mercury or (2) its method 

quantitation level is above the applicable water quality criterion, but the amount of mercury in a 

facility’s discharge is high enough that the method detects and quantifies the level of mercury in 

the discharge.4

 Accordingly, EPA strongly recommends that the permitting authority determine 

that a permit application that lacks effluent data analyzed with a sufficiently sensitive EPAapproved method such as Method 1631E is incomplete unless and until the facility supplements 

the original application with data analyzed with such a method. See 40 CFR 122.21(e) (a permit 

application is determined to be complete at the discretion of the permitting authority) and 40 

CFR 122.21(g)(13) (the applicant shall provide to the Director, upon request, such other 

information as the Director may reasonably require to assess the discharge). Such data would 

allow the permitting authority to characterize the effluent to determine whether the discharge 

causes, has the reasonable potential to cause, or contributes to an excursion of state water quality 

standards for mercury and would consequently allow the permitting authority to determine 

whether a water quality-based effluent limit for mercury is necessary in the permit. 

To illustrate the latter, if the water quality criterion for mercury in a particular state is 2.0 ppt, Method 245.7 

(with a quantitation level of 5.0 ppt) would be sufficiently sensitive where it reveals that the level of mercury in a 

facility’s discharge is 5.0 ppt or greater. In contrast, Method 245.7 would not be sufficiently sensitive if it resulted in 

a level of non-detect for that discharge because it could not be known whether mercury existed in the discharge at a 

level between 2.0 and 5.0 (less than the quantitation level but exceeding the water quality criterion). 

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Monitoring Requirements in Permits 

Where a permit authority establishes a permit limit for mercury, it also needs to consider 

specifying an analytical method that the permittee must use to monitor for mercury during the 

term of the permit. Methods 245.1 and 245.2, as written, are not likely to be sensitive enough to 

detect or quantify the concentration of mercury in the discharge at a level that matches the 

limitation for mercury in the permit. EPA therefore expects the permitting authority to require 

the use of a sufficiently sensitive EPA-approved method for monitoring under the permit in order 

to ensure that the sampling and measurements required are Arepresentative of the monitored 

activity@ (as required by 40 CFR 122.41(j)(1)). For purposes of monitoring under a permit, a 

method for mercury is “sufficiently sensitive” when (1) its method quantitation level is at or 

below the level of the mercury limit established in the permit or (2) its method quantitation level 

is above the mercury limit in the permit, but the amount of mercury in a facility’s discharge is 

high enough that the method detects and quantifies the level of mercury in the discharge.5 

EPA Permit Review and Objection to State Issued Permits 

For NPDES-authorized states, EPA regions are expected to review state permits and should 

strongly consider objecting to permits that are issued based on analytical data collected and 

analyzed using an EPA-approved method that is not sufficiently sensitive or that do not require 

use of a sufficiently sensitive EPA-approved method for monitoring when the permit includes a 

limit for mercury. OWM is expecting to undertake a permit quality review of a small 

representative number of permits with respect to mercury limitations and other conditions. 

If you have questions concerning the content of this memorandum, please contact Linda 

Boornazian, Director of the Water Permits Division, at 202-564-0221 or have your staff contact 

Marcus Zobrist of the State and Regional Branch at 202-564-8311 or zobrist.marcus@epa.gov. 

cc: NPDES Branch Chiefs Regions 1 B 10 

bcc: Paul Banger, OGC

Barbara Pace, OGC

Tom McCully, OW

Mary Smith, OST

See footnote 4. 

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