Pollutant Category Development

On this page:

• Approach for Grouping Pollutants into Categories
• Pollutant Categories

Approach for Grouping Pollutants into Categories

The Water Pollutant Loading Tool calculates mass loadings for individual pollutants. To enhance querying capabilities, EPA developed a methodology to calculate pollutant loadings and TWPE for broader pollutant categories. The Water Pollution Search calculates pollutant category loadings by assigning pollutant categories to pollutant parameters and aggregating the loads based on specified priorities. The DMR_LOADINGS_PARM_CAT_XWALK table assigns relevant parameters to pollutant categories and priorities for aggregating the category loads.

Database Table Relationships for Pollutant Categories

The Water Pollutant Loading Tool uses two levels of aggregation:

• The first level of aggregation groups parameters representing the same pollutant. For example, the Loading Tool groups all copper parameters (i.e. dissolved copper, total copper, total recoverable copper, etc.) together into the group labeled CU as shown in the Parameter Group column.
• The second level of aggregation categorizes all pollutant groups for a particular pollutant category, e.g., metals (MET) and priority pollutants (PP). To perform the category aggregation, the Water Pollutant Loading Tool follows three steps, which are described in the sections that follow.

Identify Parameters Included in Category

The Water Pollutant Loading Tool links the DMR_LOADINGS_EZ_SRCH_CALCS table with the DMR_LOADINGS_PARM_CAT_XWALK table by parameter code. The tool then searches the DMR_LOADINGS_PARM_CAT_XWALK table for parameter codes that are assigned to the pollutant category queried by the user.

For categories where the Loading Tool calculates pollutant loadings, EPA excluded any parameters that are reported in units of measure that could not be converted into the standard units for the pollutant category (e.g., percent, visual observation, severity).

Aggregate Loads by Parameter Group

For each facility in the query output, the Water Pollutant Loading Tool selects one parameter from each parameter group by searching the Group Priority column for the parameter code with the minimum priority value.  For example, if a facility reports two copper parameters: one with a priority 2 (“Copper”) and one with a priority 4 (“Copper, total recoverable”), the Loading Tool will select the parameter with the priority of 2 (“Copper”) to represent the copper load for that facility. Generally, the Water Pollutant Loading Tool gives preference to pollutant parameters that express total discharges (e.g., total copper) over parameters that express partial discharges (e.g., total recoverable copper).

Aggregate Category Loads by Facility

The Water Pollutant Loading Tool aggregates the loads for the parameter groups by searching the Category Priority column for parameter groups with the minimum category priority. It then sums the loads for the selected parameter groups. The Water Pollutant Loading Tool sums all loads for all metal parameter groups, and all parameter groups for metals are assigned a category priority of 1. For other pollutant categories, the Water Pollutant Loading Tool prioritizes certain parameter groups over others and does not simply sum the loads for all groups. These category-specific aggregation methodologies are described in the sections that follow.

Pollutant Categories

The pollutant categories in the Water Pollutant Loading Tool include chemicals of interest for most water programs and frequently correspond to common causes of impairments in receiving water bodies:

• Nutrients: Nitrogen and Phosphorus
• Organic Enrichment
• Suspended Solids
• Metals
• Clean Water Act Priority Pollutants
• CERCLA Hazardous Substances
• TRI Chemicals
• Radionuclides
• Per- and Polyfluoroalkyl Substances (PFAS)

In addition to the categories above, there are other categories available in the Water Pollution Seach, including temperature and wastewater flow, that cannot be converted into a “load”:

• Pathogen Indicators
• Temperature
• Wastewater Flow
• General Radioactivity
• Color
• Whole Effluent Toxicity

Nutrients

Refer to Nutrient Aggregation for EPA’s approach to calculating the total aggregated nitrogen and phosphorus loads.

Organic Enrichment

Parameters Included

All biochemical oxygen demand (BOD) and chemical oxygen demand (COD) pollutants are selected to represent organic enrichment.

Parameter Grouping/Category Priorities

BOD5 is the standard method for estimating the mass of oxidizable carbon and nitrogen. BOD5 is measured over a period of 5 days at 20°C. All parameters that relate to BOD5 are assigned to a group labeled BOD5. All of these parameters are assigned a category priority of 1, since BOD5 is the first choice for calculating organic enrichment. If no parameters for BOD5 are recorded, the Water Pollutant Loading Tool uses other parameters. Parameters are assigned category priorities according to the following hierarchy:

1. COD. COD measures the total oxidizable content. Because the COD method relies on a chemical oxidant rather than bacteria, other compounds such as metals and organics that may be oxidized in addition to carbon and nitrogen. Typically, COD measurements are slightly higher than BOD5 measurements.
2. BOD. BOD represents methods with nonstandard conditions that use bacteria. Because such variations make BOD parameters inconsistent, BOD parameters are given a higher category priority number than COD parameters.
3. CBOD5 and NBOD. Carbonaceous or nitrogenous BOD (C or NBOD) estimate only the mass of oxidizable carbon or nitrogen respectively. Because both values are needed to calculate BOD accurately, these parameters are given higher category priority numbers than BOD parameters.
4. CBOD. Carbonaceous BOD parameters are given the highest category priority numbers because NBOD is not an available parameter, and both parameters are necessary to accurately estimate organic enrichment.

Organic Enrichment Load Aggregation

If available, the parameter in the BOD5 group with the minimum group priority is used as the total organic enrichment load. Otherwise, the parameter with the minimum category priority is used as the total organic enrichment load. There is one exception; if no BOD5 or COD parameters are available, the Water Pollutant Loading Tool adds the parameter for NBOD5 to the parameter with the minimum group priority in the CBOD5 group to calculate the total organic enrichment load.

Suspended Solids

Parameters Included

The suspended solids category includes all suspended or settable solids, but does not include pollutant specific solids (e.g., suspended copper).

Parameter Grouping/Category Priorities

All parameters for total suspended solids are grouped together under the pollutant group labeled TSS. Category priorities for all parameters in the TSS group are assigned a ‘1’. Some solids are not included in the TSS group, so they are not assigned a group priority number. However, these solids are still included in the suspended solids category and are assigned a category priority. The category priority hierarchy is established by assigning a higher priority to parameters that represent the total release of a pollutant rather than the partial release.

Suspended Solids Load Aggregation

The parameter in the TSS group with the lowest priority number is used as the suspended solids category load. If there is no reported TSS pollutant, the parameter with the lowest category priority number is used as the suspended solids category load. If two parameters have the same category or group priority number, the Water Pollutant Loading Tool adds the two values. If a priority 1 category parameter is not available, and both total non-settleable and settleable residue are reported, then the Water Pollutant Loading Tool will sum the two parameters together to calculate the total suspended solids category load. If only one of the two parameters is reported, then the Water Pollutant Loading Tool will use that parameter’s load to represent the total suspended solids category load.

Metals

Parameters Included

All metals in their elemental form are included in this category. Hexavalent or trivalent metals and metals in ionic form (e.g., hexavalent chromium and aluminum, ion) are included, but specific metallic compounds (e.g., calcium chloride) are not.

Parameter Grouping/Category Priorities

All parameters for a specific metal are grouped together (e.g. all aluminum-related parameters are assigned to the pollutant group labeled AL). EPA assigns each parameter a group priority number using the hierarchy shown in the table below. Metal parameters that express total discharges are given a higher priority (lower number) than parameters that express partial discharges. Category priorities for all metal parameters are assigned a ‘1’ because all groups in the metal category are included in the calculation of the total metal load.

Metal Load Aggregation

The pollutant parameter in each metal group with the minimum group priority number represents each individual metal load. If two parameters have the same minimum group priority number, the Water Pollutant Loading Tool adds them together. If only one of the two parameters is reported, then the Water Pollutant Loading Tool uses the reported parameter’s load to represent the total copper load. Because all parameters for metals have a category priority of 1, the Water Pollutant Loading Tool sums all of the individual metal loads to calculate the total metal load.

Priority Pollutants

Parameters Included

Priority pollutants are regulated chemical pollutants that can be tested by established analytical methods. EPA included all 126 pollutants defined as priority pollutants in this category. Wherever possible, synonyms for the priority pollutants are incorporated into the DMR_LOADINGS_PARM_CAT_XWALK table. Pollutants must exactly match the priority pollutant to be included in the Water Pollution Search. The priority pollutant category includes metals. EPA used the same methodology to define the metals priorities and hierarchies as described in the previous section.

Parameter Grouping/Category Priorities

All parameters for a specific priority pollutant are grouped together. Each parameter in the group is assigned a group priority using the hierarchy shown in the table below. Some pollutants contain additional parameters and are handled on a case-by-case basis. Category priorities for all priority pollutants are assigned a ‘1’ because all groups in the priority pollutant category are included in the calculation of the total priority pollutant load.

Priority Pollutant Load Aggregation

The pollutant parameter in each priority pollutant group with the minimum group priority number is selected to represent each individual priority pollutant load. For category aggregation, all priority pollutant loads are summed together to calculate the total priority pollutant load.

Pathogen Indicators

Parameters Included

All pathogen parameters are considered pathogen indicators in the Water Pollutant Loading Tool, except for:

• Pathogen parameters reported in measurement units that cannot be expressed in units of counts per 100 milliliters (e.g., percent of fecal coliform samples).
• Sludge-related parameters.

Parameter Grouping/Category Priorities

Parameters in the pathogen indicator category are organized into three groups: enteric viruses (EV), fecal streptococci (FS), and total coliform (TC). Group priorities are assigned to each of these parameters.

Since E. coli is a subset of fecal coliform, which is a subset of total coliform, total coliform group priority numbers are assigned using the following hierarchy:

1. Total Coliform
2. Fecal Coliform
3. E. coli

Since enterococci is a subset of streptococci, fecal streptococci group priority numbers are assigned using the following hierarchy:

1. Fecal Streptococci
2. Enterococci

All parameters in the pathogen indicator category are assigned a category priority of 1, because all groups are included in the total pathogen load.

Pathogen Indicators Load Aggregation

The load for each group is calculated by selecting the parameter in each group that has the minimum group priority number. The Water Pollutant Loading Tool calculates the total pathogen indicators load by summing the total coliform load, fecal streptococci load, and the enteric viruses load.

CERCLA Hazardous Substances

Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) hazardous substances are substances that are considered severely harmful to human health and the environment. Many are commonly used substances which are harmless in their normal uses, but are quite dangerous when released. They are defined in terms of those substances either specifically designated as hazardous under the Superfund law, or those substances identified under other laws, including:

• Any element, compound, mixture, solution, or substance designated as hazardous under section 102 of CERCLA.
• Any hazardous substance designated under section 311(b)(2)(a) of the Clean Water Act (CWA), or any toxic pollutant listed under section 307(a) of the CWA. There are over 400 substances designated as either hazardous or toxic under the CWA.
• Any hazardous waste having the characteristics identified or listed under section 3001 of the Resource Conservation and Recovery Act. Any hazardous air pollutant listed under section 112 of the Clean Air Act, as amended. There are over 200 substances listed as hazardous air pollutants under the Clean Air Act (CAA).
• Any imminently hazardous chemical substance or mixture which the EPA Administrator has "taken action under" section 7 of the Toxic Substances Control Act.

See EPA's Clean Water Act (CWA) and CERCLA Hazardous Substance Lists page for more information about CERCLA Hazardous Substances.

For a list of the NPDES parameters identified as CERCLA Hazardous Substances, download the "DMR Pollutant Parameters Used and Not Used by the Loading Tool" CSV file on the Water Pollutant Loading Tool - Resources page. The pollutants with the "Y" flag in the "CERCLA_HAZ_SUBST" column are classified in this pollutant category.

TRI Chemicals

TRI Chemicals are chemicals and chemical categories that are covered by EPA's Toxics Release Inventory (TRI) Program. Refer to EPA's TRI-Listed Chemicals page for the most recent list, historical changes, and more information about chemical classifications under TRI.

For a list of the NPDES parameters identified as TRI chemicals, download the "DMR Pollutant Parameters Used and Not Used by the Loading Tool" CSV file on the Water Pollutant Loading Tool - Resources page. The pollutant parameters with the "Y" flag in the "TRI_CHEMICALS" column are part of this pollutant category.

Radionuclides

The Loading Tool calculates annual pollutant loadings for specific radioactive isotopes or radionuclides, as listed in the table below. Radionuclides can be measured in units of mass or radioactivity (in curies (Ci)). For all measurements, the Loading Tool first converts to standard units, either Ci or grams. For measurements of radioactivity, the specific activity of the radionuclide is used to convert from curies to mass.

Source: Turner, James E. (1992). Atoms, Radiation, and Radiation Protection. Mc-Graw Hill.

^aSource: International Atomic Energy Agency (IAEA) International Nuclear Structure and Decay Data Network. Evaluated Nuclear Structure Data File (ENSDF) DatabaseExit
^bRadioactivity (Ci/d)/[SA (Ci/g)*(1000 g/kg)] = Mass (kg/d)
^cRadioactivity (Ci/L)*(1000 mg/g)/(SA (Ci/g)) = Concentration (mg/L)

Parameters Included

All parameters measuring radioactivity of a specific radionuclide are included in this category.

Parameter Grouping/Category Priorities

All parameters with the same pollutant name are grouped together as shown in the "Group Priority" column of the table above. The radionuclide category does not have a category-level aggregation hierarchy and includes all radionuclides with equal priority. Therefore the category priorities for all radionuclide parameter codes are assigned a '1'.

Pollutant Load Aggregation

For category aggregation, all pollutant loads are summed together to calculate the total load.

Per- and Polyfluoroalkyl Substances (PFAS)

The Loading Tool provides data for measurements of  Per- and Polyfluoroalkyl Substances (PFAS) in wastewater. PFAS are a group of man-made chemicals that includes PFOA, PFOS, GenX, and many other chemicals. The Loading Tool calculates pollutant loadings for individual pollutants within this category.

Parameters Included

The list of parameters is provided in the table below.

Parameter Grouping/Category Priorities

There is no grouping or prioritization in the PFAS pollutant category.

Pollutant Load Aggregation

There is no load aggregation in the PFAS pollutant category.

Temperature

The Convert Module of the Water Pollutant Loading Tool aggregates temperature measurements in DMR data and stores them in the DMR_LOADINGS_EZ_SRCH_CALCS table. The Convert Module selects one temperature measurement for each outfall, monitoring location, and date, and converts all temperatures to degrees Fahrenheit. 

The Water Pollution Search aggregates the temperature measurements by facility by calculating the average and the maximum temperature measurements per facility per year.

Wastewater Flow

Because a method for calculating total annual wastewater flow has already been developed in the Convert and Load Calculator Modules, the Water Pollution Search did not require a new method for calculating annual wastewater flow.

General Radioactivity

The table below provides the pollutant group and category priority for the parameters in the General Radioactivity category.

Parameters Included

All parameters measuring gross radioactivity or radioactivity of more than one radionuclide are included in this category. Pollutant loadings are not calculated for these parameters.

Parameter Grouping/Category Priorities

All parameters with the same pollutant name are grouped together as shown in the "Group Priority" column of the table above. The general radioactivity category does not have a category-level aggregation hierarchy and includes all radioactive pollutants with equal priority. Therefore the category priorities for all general radioactivity parameters are assigned a '1'.

Color

The Loading Tool provides data for measurements of color in wastewater. Color measurements may be reported using a variety of units, such as Platinum Cobalt (Pt-Co) units or pound/ton produced. The Loading Tool presents the measurements in the units reported on DMRs, and does not convert the measurements into standard units.

Parameters Included

 All parameters measuring color are included in this category. A complete list of the color parameters is provided in the table below.

Parameter Grouping/Category Priorities. 

There is no grouping or prioritization in the color pollutant category.

Pollutant Load Aggregation. 

There is no load aggregation in the color pollutant category.

Whole Effluent Toxicity 

The Loading Tool provides data for wastewater parameters measuring WET. WET refers to the aggregate toxic effect to aquatic organisms from all pollutants contained in a facility’s wastewater. WET tests measure a wastewater's effects on specific test organisms' ability to survive, grow and reproduce. WET measurements may be reported using a variety of units, such as percentages or pass/fail. The Loading Tool presents the measurements in the units reported on DMRs, and does not convert the measurements into standard units.

See Whole Effluent Toxicity Methods for more information about Clean Water Act WET test methods.

Parameters Included in the WET Category. All parameters measuring WET are included in this category. A complete list of the WET parameters is provided in the DMR Pollutant Parameters Used and Not Used by the Loading Tool (CSV) (650 K).

Parameter Grouping/Category Priorities. There is no grouping or prioritization in the WET pollutant category.

Pollutant Load Aggregation. There is no aggregation in the WET pollutant category.

References

U.S. EPA. Technical Support Document for the Annual Review of Existing Effluent Guidelines and Identification of Potential New Point Source Categories. EPA-8 21 -R-09-007. Washington, DC. EPA-HQ-OW-2008-0517 DCN 06557.

U.S. EPA Effluent Limitation Guidelines: Frequent Questions. https://www.epa.gov/eg/learn-about-effluent-guidelines.