tac18-1251_fig01

U.S. Department of the Interior

U.S. Geological Survey

Data Series 1101

National Water Quality Program

Point-Source Nutrient Loads to Streams of the 

Conterminous United States, 2012
Cover: Map showing SPARROW regions in the conterminous United States during 2012.
Point-Source Nutrient Loads to Streams of the 

Conterminous United States, 2012

By Kenneth D. Skinner and Molly A. Maupin

National Water Quality Program

Data Series 1101

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

David Bernhardt, Acting Secretary

U.S. Geological Survey

James F. Reilly II, Director

U.S. Geological Survey, Reston, Virginia: 2019

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Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials 

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Suggested citation:

Skinner, K.D., and Maupin, M.A., 2019, Point-source nutrient loads to streams of the conterminous United States, 

2012: U.S. Geological Survey Data Series 1101, 13 p., https://doi.org/10.3133/ds1101.

ISSN 2327-638X (online)
iii

Foreword

Sustaining the quality of the Nation’s water resources and the health of our diverse ecosystems 

depends on the availability of sound water-resources data and information to develop effective, 

science-based policies. Effective management of water resources also brings more certainty and 

efficiency to important economic sectors. Taken together, these actions lead to immediate and 

long-term economic, social, and environmental benefits that make a difference to the lives of 

the almost 400 million people projected to live in the United States by 2050.

In 1991, Congress established the National Water-Quality Assessment (NAWQA) to address 

where, when, why, and how the Nation’s water quality has changed, or is likely to change 

in the future, in response to human activities and natural factors. Since then, NAWQA has 

been a leading source of scientific data and knowledge used by national, regional, state, and 

local agencies to develop science-based policies and management strategies to improve and 

protect water resources used for drinking water, recreation, irrigation, energy development, 

and ecosystem needs (https://water.usgs.gov/nawqa/applications/). Plans for the third decade 

of NAWQA (2013–23) address priority water-quality issues and science needs identified by 

NAWQA stakeholders, such as the Advisory Committee on Water Information and the National 

Research Council, and are designed to meet increasing challenges related to population growth, 

increasing needs for clean water, and changing land-use and weather patterns.

Federal, state, and local agencies have invested billions of dollars to reduce the amount of 

pollution entering rivers and streams that millions of Americans rely on for a variety of water 

needs and biota rely on for habitat. Understanding the sources and transport of pollution is 

crucial for designing strategies to improve water quality. The U.S. Geological Survey’s SPAtially 

Referenced Regressions On Watershed attributes (SPARROW) model was developed to aid 

in the understanding of sources and transport of pollution across large spatial scales. The 

SPARROW model is calibrated by statistically relating watershed sources and transport-related 

properties to monitoring-based water-quality load estimates. This report describes how total 

nitrogen and phosphorous loads were estimated for 5,430 major point-source facilities and 

11,537 minor wastewater treatment facilities discharging to streams in the conterminous United 

States during 2012. Point-source data were compiled from the U.S. Environmental Protection 

Agency’s (EPA) Permit Compliance System (PCS) and Integrated Compliance Information 

System (ICIS) databases. The report also describes a new method to calculate typical pollutant 

concentrations for use in the absence of point-source nutrient concentrations.

We hope this publication will provide insights and information to meet water-resource needs 

and will foster increased citizen awareness and involvement in the protection and restoration of 

our Nation’s waters. The information in this report is intended primarily for those interested in or 

involved in resource management and protection, conservation, regulation, and policymaking at 

the regional and National levels.

Dr. Donald W. Cline

Associate Director for Water

U.S. Geological Survey
iv

Contents

Foreword ........................................................................................................................................................iii

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................1

Purpose and Scope ..............................................................................................................................2

Methods...........................................................................................................................................................2

Data Sources.........................................................................................................................................2

Permit Compliance System and Integrated Compliance Information System ..................2

Clean Watershed Needs Survey ...............................................................................................2

Point-Source Data Quality Assurance and Quality Control...........................................................4

Updated Methods from Previous Studies.........................................................................................4

Effluent Nutrient Load Calculation.....................................................................................................6

Point-Source Nutrient Loads .......................................................................................................................7

Summary........................................................................................................................................................12

References Cited..........................................................................................................................................12

Figures

1. Map showing major point-source facilities and minor wastewater treatment

facilities in the conterminous United States, 2012 ..................................................................3

2. Map showing example of state expansion levels for determining typical pollutant

concentration values ...................................................................................................................5

3. Graphs showing comparison of nutrient loads from 2012 data derived from the

Maupin and Ivahnenko (2011) method and the varying concentration and facility

typical pollutant concentration method....................................................................................6

4. Graphs showing percentage of measured and typical pollutant concentration

values used to determine nutrient loads to streams by state, 2012 .....................................8

5. Graphs showing total nitrogen (A) and phosphorous (B) loads in streams by state,

2012 .................................................................................................................................................9

6. Pie graphs showing total nitrogen (A) and total phosphorous (B) loads by standard

industry classification code (SIC) and treatment level for 2012 ........................................10

7. Graphs showing total nitrogen (A) and total phosphorous (B) loads by major and

minor wastewater treatment facilities in states for 2012.....................................................11
v

Conversion Factors

Inch/Pound to International System of Units

Multiply By To obtain

Length

mile (mi) 1.609 kilometer (km)

Flow rate

million gallons per day (Mgal/d) 0.04381 cubic meter per second (m3/s)

International System of Units to U.S. customary units

Multiply By To obtain

Mass

kilogram (kg) 2.205 pound avoirdupois (lb)

kilogram per year (kg/yr) 2.205 pound per year (lb/yr)

Datum

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Altitude, as used in this report, refers to distance above the vertical datum.

Supplemental Information

Concentrations of chemical constituents in water are given in milligrams per liter (mg/L).
vi

Abbreviations

CWNS Clean Watershed Needs Survey

DMR discharge monitoring report

EPA U.S. Environmental Protection Agency

ICIS Integrated Compliance Information System

MRB major river basin

NHDPlus National Hydrography Dataset version 2

NPDES National Pollutant Discharge Elimination System

PCS Permit Compliance System

PSLoadEsT Point Source Load Estimation Tool

SIC standard industry classification code

SPARROW SPAtially Referenced Regressions On Watershed attributes

TN total nitrogen

TP total phosphorous

TPC typical pollutant concentration

USGS U.S. Geological Survey

WWTF wastewater treatment facility
Point-Source Nutrient Loads to Streams of the 

Conterminous United States, 2012

By Kenneth D. Skinner and Molly A. Maupin

Abstract

Total nitrogen and phosphorous loads were estimated 

for 5,430 major point-source facilities (all types) and 11,537 

minor wastewater treatment facilities discharging to streams 

in the conterminous United States during 2012. Facilities 

classified as a major discharger are typically a facility that 

discharges greater than one million gallons of water per day 

however some industrial facilities are classified as a major 

based on specific criteria developed by the U.S. Environmental 

Protection Agency (EPA) and the National Pollutant Discharge 

Elimination System state program. Data documenting 

discharge information from point sources were obtained from 

the EPA’s Integrated Compliance Information System (ICIS) 

and Permit Compliance System (PCS). When available, actual 

nutrient concentration measurements were used to calculate 

point-source loads. In the many cases in which concentration 

data were not available in either the ICIS or PCS databases, 

typical pollutant concentrations (TPCs) were developed using 

data from similar facilities. A new method for calculating 

TPCs was implemented that allows varying amounts of 

nutrient concentration data and (or) varying numbers of 

facilities to determine TPCs. This new method minimized 

the effect that any single facility discharging extremely large 

nutrient concentrations had on resultant TPC values. Because 

of the smaller TPC values from this new TPC method, the 

total nutrient load for many states was reduced compared to 

previous TPC methods.

Major wastewater treatment facilities are the largest 

contributor of nutrient loads to streams even though there 

are almost three times as many minor wastewater treatment 

facilities. Specifically, 4,218 major wastewater treatment 

facilities account for 94 percent of the total nitrogen load 

for the conterminous United States, whereas 11,397 minor 

wastewater treatment facilities contribute 6 percent of the total 

nitrogen load. Total phosphorous loads are similarly divided 

among major (93 percent) and minor (7 percent) wastewater 

treatment facilities. Total nitrogen loads, including all facility 

types, primarily are from wastewater treatment facilities and 

some petroleum refining facilities. Total phosphorous loads 

also are primarily from wastewater treatment facilities, but 

several manufacturing facilities such as corn milling, pulp and 

paper mills, and industrial chemical facilities also contribute to 

total phosphorous loads.

Introduction

The U.S. Geological Survey (USGS) developed a spatial 

water-quality model called SPAtially Referenced Regressions 

On Watershed attributes (SPARROW) to estimate the major 

sources and environmental factors that affect the long-term 

supply, transport, and fate of contaminants in the Nation’s 

streams (Smith and others, 1997; Schwarz and others, 

2006; Preston and others, 2011). The SPARROW model 

relates in-stream water-quality data to spatially referenced 

characteristics of watersheds, including contaminant sources 

and factors influencing terrestrial and aquatic transport. Based 

on SPARROW modeling, one of the main nutrient sources to 

streams is point-source facilities such as municipal wastewater 

treatment facilities (WWTFs) that discharge directly to 

streams (Alexander and others, 2008, Preston and others, 

2011, Robertson and Saad, 2011).

Consistent and reliable data about point-source 

dischargers are difficult to acquire, especially at the national 

scale (Maupin and Ivahnenko, 2011). Point-source flow 

and effluent concentrations are available from the U.S. 

Environmental Protection Agency’s (EPA) Permit Compliance 

System (PCS) and Integrated Compliance Information System 

(ICIS) databases as required by the Clean Water Act of 1972 

to monitor facility compliance. However, individual states 

provide the data stored in the EPA databases and participation 

in submitting data and the quality of the submitted data is often 

highly variable. Maupin and Ivahnenko (2011) documented 

methods derived from the U.S. Environmental Protection 

Agency (2006a, 2006b) and McMahon and others (2007) to 

address data gaps and to develop a consistent dataset with 

established quality-control and quality assurance procedures. 
2 Point-Source Nutrient Loads to Streams of the Conterminous United States, 2012

Purpose and Scope

This report provides estimates of annual total nitrogen 

(TN) and total phosphorous (TP) loads for all major pointsource facilities and minor wastewater treatment facilities 

discharging to streams in the conterminous United States 

during 2012. It also describes an updated method of 

determining typical pollutant concentrations (TPC) used in the 

absence of measured TN or TP concentration data.

Methods

The methods used to compile, check, and calculate pointsource nutrient loads closely followed those of McMahon and 

others (2007) and Maupin and Ivahnenko (2011). Point-source 

loads were estimated for all major point-source facilities as in 

the previous work. However, instead of including all minor 

facilities of any type as did Maupin and Ivahnenko (2011), 

only minor municipal WWTFs were considered because only 

minor WWTFs were thought to be the largest contributor 

of the nutrient load. A point-source facility is classified as 

a major discharger by an EPA regional administrator or in 

conjunction with a state director if the facility is in a state with 

an approved National Pollutant Discharge Elimination System 

(NPDES) program. Major municipal dischargers include all 

facilities (WWTF or other) with designed flows of greater 

than one million gallons per day (Mgal/d) and facilities with 

EPA/state-approved industrial pretreatment programs. Major 

industrial facilities are determined based on specific ratings 

criteria developed by EPA and the state if an approved NPDES 

state program exists. Locations and estimates of annual 

total nitrogen (TN) and total phosphorous (TP) loads for all 

major point-source facilities and minor wastewater treatment 

facilities discharging to streams in the conterminous United 

States during 2012 are shown in figure 1.

Point-source facility, flow, and concentration data were 

compiled from several sources. Flow and concentration data 

for facilities were limited to the discharge point from the 

facility to a stream. Other measurement locations such as the 

input to a facility were not used. The concentration data used 

to calculate loads were TN as nitrogen and TP as phosphorous. 

The calculated load measured in weight per time (kilograms 

per year) represents the entire amount of total nitrogen and 

total phosphorous being contributed to a stream.

Data Sources

Permit Compliance System and Integrated 

Compliance Information System

Point-source discharge facility data including facility 

descriptions, monthly effluent flow, and monthly nutrient 

concentration data were compiled from several sources. 

A copy of EPA’s PCS and ICIS databases was received 

directly from EPA in autumn 2013. The EPA PCS database 

provides information such as compliance and enforcement 

status on facilities that have been issued permits to discharge 

wastewater into streams (U.S. Environmental Protection 

Agency, 1990). The EPA ICIS database was implemented as a 

modernization of the PCS database and provides information 

for the NPDES program, including permit information and 

discharge monitoring data. The ICIS database differs from the 

PCS database because it also includes compliance information 

for air quality and hazardous waste facilities.

A review of the 2012 PCS/ICIS database revealed that 

a large amount of data were either questionable or missing. 

Routine downloads from the EPA Discharge Monitoring Report 

(DMR) Loading Tool (https://echo.epa.gov/trends/loadingtool) that allows searches for point-source discharge facility 

data, indicated that states continued to submit additional 

2012 NPDES DMR data to the PCS/ICIS database, (U.S. 

Environmental Protection Agency, 2018a, 2018b, 2018c) for 

some time after 2012. In areas where NPDES facilities were 

known to exist but data were missing from the initial PCS/ICIS 

download or subsequent website downloads, individual states 

were contacted to access the NPDES DMR data.

Clean Watershed Needs Survey

The EPA Clean Watershed Needs Survey (CWNS) was 

another source of point-source discharge facility information, 

including facilities not identified in the EPA PCS/ ICIS 

database downloads and state-provided information. The 

CWNS also provided treatment level information for WWTFs. 

The CWNS is a survey done every 4 years to assess the 

financial requirements needed to meet the water-quality 

goals of the Clean Water Act (U.S. Environmental Protection 

Agency, 2018a). The CWNS collects information on publicly 

owned wastewater collection and treatment facilities, 

stormwater and combined sewer overflow control facilities, 

nonpoint source pollution control projects, and decentralized 

wastewater management. Only CWNS facilities discharging 

effluent to streams were used in this data compilation.
Methods 

3tac18-1251_fig01

EXPLANATION

SPARROW region

MidwestNortheastPacificSoutheastSouthwest

Point-source facility discharge type

MajorMinor

Figure 1. Major point-source facilities and minor wastewater treatment facilities in the conterminous United States, 2012.
4 Point-Source Nutrient Loads to Streams of the Conterminous United States, 2012

The CWNS provides treatment level information for 

WWTFs ranging from raw discharge to advanced treatment 

for nitrogen or phosphorous removal. Treatment levels 

were recoded for use in the SPARROW point-source load 

compilation according to CWNS treatment levels:

• Raw sewage discharge, coded as “0”.

• Primary treatment, coded as “1”—effluent treatment 

by screening and sedimentation. The WWTP also 

has a 5-day biochemical oxygen demand (BOD5) 

concentration greater than 45 milligrams per liter 

(mg/L).

• Advanced primary treatment, coded as “1A”—effluent 

treatment whereby chemicals are used to further treat 

primary effluent and increase the amount of solid 

matter removed. The BOD5 concentration is greater 

than 30 but less than 45 mg/L.

• Secondary treatment, coded as “2”—effluent treatment 

including aeration and activated sludge. This is the 

minimum level of treatment required for publicly 

owned wastewater treatment facilities and requires 

minimum removal standards for BOD (less than or 

equal to 30 mg/L), total suspended solids, and pH.

• Advanced treatment, coded as “2A”—effluent 

treatment producing a significant reduction in 

nonconventional or toxic contaminants and has a BOD 

less than or equal to 20 mg/L.

• Advanced treatment for phosphorous, coded as 

“3P”—a WWTF with advanced treatment emphasizing 

phosphorous removal.

• Advanced treatment for nitrogen, coded as “3N”—a 

WWTF with advanced treatment emphasizing the 

removal of nitrogen.

• Advanced treatment for nitrogen and phosphorous, 

coded as “3NP”—a WWTF with advanced treatment 

emphasizing the removal of nitrogen and phosphorous.

The 2012 CWNS was the primary source of facility 

treatment level information. Because the CWNS is a voluntary 

survey, not all WWTFs chose to participate. For example, no 

facilities in South Carolina participated in the 2012 CWNS. 

In this instance and when WWTFs from other states did not 

participate in the CWNS, the 2008 CWNS was used to fill in 

any data gaps from the 2012 CWNS.

Point-Source Data Quality Assurance and 

Quality Control

The point-source facility data were in various levels of 

quality and completeness and often contained incorrect data. 

Facility location information or georeferenced data were 

evaluated and corrected where needed. Facility location data 

often referred to a facility’s main office address; because 

this study focused on point-source inputs to streams, each 

facility location coordinates were changed to the outfall 

from the facility that contributes to a stream. After an outfall 

location to a stream was verified, the corresponding National 

Hydrography Dataset (NHDPlus) unique stream identifier 

(ComID) (McKay and others, 2012) of the stream was 

assigned to the facility.

Additionally, facilities records often were either missing 

or had an incorrect Standard Industry Classification code, SIC 

code (U.S. Department of Labor, 2018), which is a four-digit 

numerical code used for classifying industries. The SIC codes 

were used in this study to group point-source facilities by 

similar levels of nutrient discharge. If a facility was missing a 

SIC code, one was assigned based on the name and regulated 

discharge constituents of the facility. Municipal WWTFs 

are some of the largest nutrient dischargers (Alexander and 

others, 2008, Preston and others, 2011, Robertson and Saad, 

2011), so facilities were checked for incorrectly assigned 

SIC codes related to WWTF. Often the facility name would 

include “WWTP” but be assigned a SIC code related to a 

different industry. These codes were then changed to the SIC 

code related to sewerage systems (SIC 4952). For example, 

many point-source facilities had a SIC code for a school or 

correctional facility when the discharge was for a WWTF 

at the school or correctional facility. Updating the SIC code 

according to type of discharge from a facility allows for the 

proper assignment of TPC values if necessary and the proper 

grouping when summarizing nutrient loads by SIC.

Updated Methods from Previous Studies

The methods used in this study to compile, check, and 

calculate point-source nutrient loads followed the methods 

of McMahon and others (2007) and Maupin and Ivahnenko 

(2011) with some differences. One difference was the use 

of the treatment level used at a WWTF. Increased treatment 

levels at a WWTF can result in decreased discharged nutrient 

concentrations (Ivahnenko, 2017). The SIC codes for each 

WWTF were amended with the treatment levels described in 

section, “Clean Watershed Needs Survey” to create a new code 

for use in load calculation (for example, 4952_1A, 4952_2, 
Methods 5

4952_3NP). Each updated SIC code was treated as a separate 

unique SIC code during load calculations and, where needed, 

was used to create a TPC value appropriate for a WWTF with 

a given level of treatment.

In the two previous studies (McMahon and others, 2007; 

Maupin and Ivahnenko, 2011), a median seasonal flow and 

nutrient concentration was calculated from 3 years of data 

(1992, 1997, and 2002) when a facility met the condition 

of flow data for less than 12 months of the year and at least 

for three or four seasons of the year, a stipulation for using a 

TPC value. This report uses data from 2012 only, so seasonal 

medians were derived from the months of the season for 2012. 

Seasons were defined as summer (June–August), autumn 

(September–November), winter (December–February), and 

spring (March–May).

The other difference occurred when a TPC value was 

derived from facilities in a specific geographical area. In 

the studies by McMahon and others (2007) and Maupin and 

Ivahnenko (2011), if a median seasonal concentration could 

not be derived, TPC values were calculated using nutrient 

concentration data from other facilities of the same size (based 

on flow) and type (SIC code) from within the same major 

river basin (MRB). If that still did not provide enough nutrient 

concentration data, then the median calculation was expanded 

to include all facilities of the same size and type in all MRBs. 

For this report, a MRB was not used as a geographical unit, 

but instead all geographic aggregation was based on states. 

Specifically, TPC values were calculated from facilities of the 

same size and type in the state in which the facility resides. If 

this did not result in enough concentration data to calculate a 

TPC, then the geographical area was expanded to include all 

surrounding states. A second expansion to the second layer of 

bordering states occurred if still more concentration data were 

required to calculate a TPC; if that was still not enough data, 

then the geographical area was expanded to the conterminous 

United States (fig. 2).

The last difference was the number of sources of effluent 

nutrient concentration data required to calculate a TPC value. 

In the previous studies, a minimum of five effluent nutrient 

concentration values from at least one facility were required to 

determine a TPC value. However, for this study the minimum 

of five concentration values had to come from at least five 

different facilities to meet the minimum requirements to 

determine a TPC value. In comparing the two methods, 

some states had considerably lower nutrient loads using the 

new method because this approach decreases the positive or 

negative bias that a single facility can have on the TPC value 

(fig. 3).

EXPLANATION

State where facility is located

State expansion level 1

State expansion level 2

State expansion level 3

Figure 2. Example of state expansion levels for determining typical pollutant concentration values.
6 Point-Source Nutrient Loads to Streams of the Conterminous United States, 2012

tac18-1251_fig 03

0

10,000,000

20,000,000

30,000,000

40,000,000

50,000,000

60,000,000

70,000,000

80,000,000

90,000,000

Total nitrogen load, in kilograms

0

5,000,000

10,000,000

15,000,000

20,000,000

25,000,000

30,000,000

35,000,000

40,000,000

Total phosphorous load, in kilograms

AL

AR

AZ

CACO

CT

DC

DE

FL

GA

IAID

IL

IN

KS

KY

LA

MAMD

ME

MI

MN

MO

MS

MT

NC

ND

NE

NH

NJ

NM

NV

NY

OH

OKOR

PA

RI

SC

SD

TN

TX

UT

VA

VT

WA

WI

WV

WY

AL

AR

AZ

CACO

CT

DC

DE

FL

GA

IAID

IL

IN

KS

KY

LA

MAMD

ME

MI

MN

MO

MS

MT

NC

ND

NE

NH

NJ

NM

NV

NY

OH

OKOR

PA

RI

SC

SD

TN

TX

UT

VA

VT

WA

WI

WV

WY

State

A. Total nitrogen load

B. Total phosphorus load

Five facilities typical pollutant 

concentration method

Original typical pollutant 

concentration method

EXPLANATION

EXPLANATION

Five facilities typical pollutant 

concentration method

Original typical pollutant 

concentration method

Figure 3. Comparison of nutrient loads from 2012 data derived from the Maupin and Ivahnenko (2011) method and the 

varying concentration and facility typical pollutant concentration method.

Effluent Nutrient Load Calculation

Annual point-source nutrient loads were calculated using 

the methods of McMahon and others (2007) and Maupin and 

Ivahnenko (2011) with the changes described in the previous 

section, “Updated Methods from Previous Studies.” The loads 

were calculated as the effluent flow in million gallons per day 

(Mgal/d) multiplied by the number of days in the month and 

the nutrient effluent concentration (in milligrams per liter), 

converted to kilograms per month and summed for the year. 

If less than 12 months of flow data were available at a facility 

for the year but flow data for three or four seasons were 

available, then it was presumed that the facility discharged 

each month of the year and the flow data were missing and 

could not be recovered. In this case, seasonal median flow 

and nutrient concentrations were calculated from available 

data and seasonal loads were calculated by using the number 

of days in the season instead of monthly loads by using 

the number of days in the month. Annual loads were then 

calculated by summing the seasonal loads. If a facility had less 

than three seasons of effluent flow data, then monthly loads 

were calculated for the available months only and summed 

for an annual load. In this case, it was thought that the facility 

discharged only for part of the year.
Point-Source Nutrient Loads 7

Like McMahon and others (2007) and Maupin and 

Ivahnenko (2011), a hierarchical approach to assign TPC 

values for facilities lacking effluent nutrient concentrations 

was used for this study. Assigned TPC values have a 

geographic priority over facility similarity. Effluent nutrient 

concentration assignments by decreasing priority were:

1. Use measured effluent nutrient concentrations for all 

12 months in 2012.

2. If one or two months of a season were missing effluent 

nutrient concentrations, then a seasonal median value 

was calculated for effluent data.

3. If no effluent nutrient concentration data were available 

for a season, then monthly TPC values were used. A TPC 

value was derived from similar facilities in the state of 

interest based on decreasing facility similarities. This 

study used a minimum of five concentration values from 

at least five different facilities to calculate a TPC value.

a. The TPC was derived from facilities of the same 

size (flow class), type (SIC code plus treatment 

level if a WWTF), and season from within the 

state.

b. If not enough effluent nutrient concentration 

data were available to meet the requirement 

described in number 3.a., then a TPC was 

derived from facilities of the same size and type 

from within the state.

c. If not enough effluent nutrient concentration 

data were available to meet the requirement 

described in number 3.b., then a TPC was 

derived from facilities of the same type from 

within the state.

4. If not enough effluent nutrient concentration data were 

available to meet the conditions described in number 3, 

then the same evaluation process would continue within 

the state expansion level 1 (fig. 2). The evaluation steps 

a–c in number 3 are repeated at state expansion level 1.

5. If not enough effluent nutrient concentration data were 

available to meet the requirement described in number 

4, then the same evaluation process continues within the 

state expansion level 2 (fig. 2). The evaluation steps a–c 

in number 3 are repeated at state expansion level 2.

6. If not enough effluent nutrient concentration data were 

available to meet the requirement described in number 5, 

then the same evaluation process continues (steps a–c) 

within the conterminous United States (fig. 2).

7. If none of these options provided a TPC nutrient 

concentration, a national SIC-specific TPC value was 

used. Maupin and Ivahnenko (2011) described this 

dataset as SIC-specific TPC values.

These methods were incorporated into the Point Source 

Load Estimation Tool (PSLoadEsT) (Gorman-Sanisaca and 

others, 2018). PSLoadEst was written using the open-source 

programming language R and has an easy-to-use interface 

written in Visual Basic for Applications® within a Microsoft 

Access® database file that guides the user through the options 

to estimate point source loads using the methods described in 

this report.

Point-Source Nutrient Loads

Point-source nutrient loads were estimated for all 

major point-source facilities and all minor WWTFs in the 

conterminous United States. Point-source facility, discharge, 

and nutrient concentration data and all load estimates are 

available from Skinner and Wise (2018). Nutrient loads were 

calculated for 5,430 major point-source facilities and 11,537 

minor WWTFs for a total of 16,967 point-source facilities 

contributing nutrient loads to streams in the conterminous 

United States in 2012. Of the 16,967 facilities, 20 percent 

of the TN values are measured concentrations and the 

other 80 percent are TPC-derived concentrations. For TP, 

37 percent were measured concentrations and 63 percent 

were TPC-derived concentrations. These ratios of measured 

concentrations and TPC values vary by state. For example, 

the facilities in the District of Columbia all have measured 

TN and TP concentrations (no TPC values), whereas Indiana 

and Wisconsin have the lowest percentage of measured TN 

concentrations (4 percent) and Iowa and Texas have the lowest 

percentage of measured TP concentrations (9 percent) (fig. 4).

The TN and TP loads also vary by state (fig. 5). The 

three states with the highest TN loads are the same as those 

for TP loads: California, Illinois, and New York. The largest 

contributor of nutrient loads to streams are WWTFs of various 

treatment levels, SIC code 4952 plus treatment level (fig. 6). 

Petroleum refining (SIC 2911) also is a large contributor 

of TN loads to streams; however, it does not contribute a 

significant amount of TP loads to streams. Several types of 

manufacturing facilities (for example, facility types SIC 2011, 

2034, 2041, and so forth) contribute TP but not TN loads. 

These facility types include: wet corn milling (SIC 2046), pulp 

mills (SIC 2611), paper mills (SIC 2621), industrial inorganic 

chemicals, not classified elsewhere (SIC 2819), and industrial 

organic chemicals, not classified elsewhere (SIC 2869). 

WWTFs (SIC 4952) contribute most nutrient loads to streams 

(fig. 6). However, even though there are 11,396 minor WWTF 

dischargers (73 percent of the total WWTFs) and 4,218 major 

WWTF dischargers (27 percent of the total WWTFs), the 

major discharging WWTFs contribute 94 percent of the TN 

load and 93 percent of the TP load to streams (fig. 7).
8 Point-Source Nutrient Loads to Streams of the Conterminous United States, 2012

tac18-1251_fig 04 AL ARAZ

CACO

CT

DC

DE

FL

GA

IAID

IL

IN

KS

KY

LA

MAMD

ME

MI

MN

MO

MS

MT

NC

ND

NE

NH

NJ

NM

NV

NY

OH

OKOR

PA

RI

SC

SD

TN

TX

UT

VA

VT

WA

WI

WV

WY

AL

AR

AZ

CACO

CT

DC

DE

FL

GA

IAID

IL

IN

KS

KY

LA

MAMD

ME

MI

MN

MO

MS

MT

NC

ND

NE

NH

NJ

NM

NV

NY

OH

OKOR

PA

RI

SC

SD

TN

TX

UT

VA

VT

WA

WI

WV

WY

0

10

20

30

40

50

60

70

80

90

100

110

10

20

30

40

50

60

80

90

100

110

0

State

A. Total nitrogen load

B. Total phosphorus load

Load delivered from measured concentrations

Load delivered from typical pollutant concentrations

Load delivered from measured concentrations

Load delivered from typical pollutant concentrations

EXPLANATION

EXPLANATION

Percentage of measured and typical pollutant concentration Percentage of measured and typical pollutant concentration

Figure 4. Percentage of measured and typical pollutant concentration values used to determine nutrient loads to streams 

by state, 2012.
Point-Source Nutrient Loads 9

tac18-1251_fig 05

423

451

52

229

424

105

1

23

169

374

567

189

903

568

138

309

489

137258

149

479

406

1,168

366

146

611

59

237

72

186

54

7

668

1,511

298

155

698

23

220

105

278

1,734

51

279

92

315463

251

77

Number of facilities

0

5,000,000

10,000,000

15,000,000

20,000,000

25,000,000

30,000,000

35,000,000

40,000,000

45,000,000

50,000,000

Total nitrogen, in kilograms per year

AL

AR

AZ

CACO

CT

DC

DE

FL

GA

IAID

IL

IN

KS

KY

LA

MAMD

ME

MI

MN

MO

MS

MT

NC

ND

NE

NH

NJ

NM

NV

NY

OH

OKOR

PA

RI

SC

SD

TN

TX

UT

VA

VT

WA

WI

WV

WY

AL

AR

AZ

CACO

CT

DC

DE

FL

GA

IAID

IL

IN

KS

KY

LA

MAMD

ME

MI

MN

MO

MS

MT

NC

ND

NE

NH

NJ

NM

NV

NY

OH

OKOR

PA

RI

SC

SD

TN

TX

UT

VA

VT

WA

WI

WV

WY

0

1,000,000

2,000,000

3,000,000

4,000,000

5,000,000

6,000,000

7,000,000

8,000,000

Total phosphorous, in kilograms per year

State 

A

B

Figure 5. Total nitrogen (A) and phosphorous (B) loads in streams by state, 2012.
10 Point-Source Nutrient Loads to Streams of the Conterminous United States, 2012

tac18-1251_fig 06

SIC 4952

Treatment level 3NP

SIC 4952

Treatment level 3P

SICs 4953–9999; 1011–2899

SIC 2911

SICs 2951 – 4941

SIC 4952

Treatment level 2A

SIC 4952

Treatment level 3N

B. Total phosphorous load, in kilograms, by facility type (Standard Industry Classification) 

and treatment level if SIC = 4952 

SIC 4952

Treatment level 2

SIC 4952

Treatment level 2

SIC 4952

Treatment level 2A

SICs 2873–4941

SIC 4952

Treatment level 3N

SIC 2869

SIC 2819 SICs 2821–2865

SIC 2046

SIC 2611 SICs 2631–2816 SIC 2621

SICs 2047–2493

SICs 4953–9999; 1011–2037 SIC 4952

Treatment level 3P

SIC 4952

Treatment level 3NP

A. Total nitrogen load, in kilograms, by facility type (Standard Industry Classification) 

and treatment level if SIC = 4952 

Figure 6. Total nitrogen (A) and total phosphorous (B) loads by standard industry classification 

code (SIC) and treatment level for 2012.
Point-Source Nutrient Loads 11

tac18-1251_fig 07

0

5,000,000

10,000,000

15,000,000

20,000,000

25,000,000

30,000,000

35,000,000

40,000,000

45,000,000

50,000,000

AL

AR

AZ

CACO

CT

DC

DE

FL

GA

IAID

IL

IN

KS

KY

LA

MAMD

ME

MI

MN

MO

MS

MT

NC

ND

NE

NH

NJ

NM

NV

NY

OH

OKOR

PA

RI

SC

SD

TN

TX

UT

VA

VT

WA

WI

WV

WY

AL

AR

AZ

CACO

CT

DC

DE

FL

GA

IAID

IL

IN

KS

KY

LA

MAMD

ME

MI

MN

MO

MS

MT

NC

ND

NE

NH

NJ

NM

NV

NY

OH

OKOR

PA

RI

SC

SD

TN

TX

UT

VA

VT

WA

WI

WV

WY

Total nitrogen load, in kilograms

0

1,000,000

2,000,000

3,000,000

4,000,000

5,000,000

6,000,000

7,000,000

8,000,000

Total phosphorous load, in kilograms

State 

Total nitrogen load for major 

wastewater treatment facilities

Total nitrogen load for minor 

wastewater treatment facilities

EXPLANATION

A

B

Figure 7. Total nitrogen (A) and total phosphorous (B) loads by major and minor wastewater treatment facilities in 

states for 2012.
12 Point-Source Nutrient Loads to Streams of the Conterminous United States, 2012

Summary

Total nitrogen (TN) and total phosphorous (TP) loads 

were estimated for 5,430 major point-source facilities and 

11,537 minor wastewater treatment facilities (WWTFs) 

discharging to streams in the conterminous United States 

during 2012. Facilities classified as a major discharger are 

typically a facility that discharges greater than one million 

gallons of water per day however some industrial facilities 

are classified as a major based on specific criteria developed 

by the U.S. Environmental Protection Agency (EPA) and 

the National Pollutant Discharge Elimination System state 

program. This study used a new method for determining 

typical pollutant concentrations (TPC) in the absence of 

measured TN or TP concentration data. This new method 

allows for a varying amount of nutrient concentration data 

and, varying number of facilities from which the concentration 

data is derived, or both. For this study, concentration values 

were used from a minimum of five facilities to determine a 

TPC value for that type of facility. This is an important feature 

because it decreases the possibility of any single facility with 

large nutrient concentrations affecting the loads derived by 

TPC concentrations, especially because most of the nutrient 

loads are determined from TPC concentrations.

Major WWTFs with varying levels of treatment were the 

largest contributor of nutrient loads to streams even though 

there were almost three times as many minor WWTFs. TN 

loads primarily were from WWTFs and some petroleum 

refining facilities. TP loads also primarily were from WWTFs, 

but several manufacturing facilities such as corn milling, 

pulp and paper mills, and industrial chemical facilities also 

contributed TP loads.

References Cited

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Publishing support provided by the U.S. Geological Survey

Science Publishing Network, Tacoma Publishing Service Center

For more information concerning the research in this report, contact the

Director, Idaho Water Science Center

U.S. Geological Survey

230 Collins Rd

Boise, Idaho 83702-4520

https://www.usgs.gov/centers/id-water
Skinner and Maupin—Point-Source Nutrient Loads to Streams of the Conterminous United States, 2012—Data Series 1101

ISSN 2327-638X (online)

https://doi.org/10.3133/ds1101