United States

Environmental Protection

Agency

Office of Water

4304

EPA 822-B-00-008

December 2000

Ambient Water Quality

Criteria Recommendations

Information Supporting the Development

of State and Tribal Nutrient Criteria

Lakes and Reservoirs in 

Nutrient Ecoregion VI
EPA-822-B-00-008

AMBIENT WATER QUALITY CRITERIA RECOMMENDATIONS

INFORMATION SUPPORTING THE DEVELOPMENT OF STATE AND TRIBAL

NUTRIENT CRITERIA

FOR

LAKES AND RESERVOIRS IN NUTRIENT ECOREGION VI

CORN BELT AND NORTHERN GREAT PLAINS

including all or parts of the States of:

South Dakota, North Dakota, Nebraska, Kansas Minnesota, Iowa, Illinois, Wisconsin, Indiana,

 Michigan, Ohio

and the authorized Tribes within the Ecoregion

U.S. ENVIRONMENTAL PROTECTION AGENCY

OFFICE OF WATER

OFFICE OF SCIENCE AND TECHNOLOGY

HEALTH AND ECOLOGICAL CRITERIA DIVISION

WASHINGTON, D.C.

DECEMBER 2000
ii

FOREWORD

This document presents EPA’s nutrient criteria for Lakes and Reservoirs in Nutrient

Ecoregion VI. These criteria provide EPA’s recommendations to States and authorized Tribes

for use in establishing their water quality standards consistent with section 303(c) of CWA. 

Under section 303(c) of the CWA, States and authorized Tribes have the primary responsibility

for adopting water quality standards as State or Tribal law or regulation. The standards must

contain scientifically defensible water quality criteria that are protective of designated uses. 

EPA’s recommended section 304(a) criteria are not laws or regulations – they are guidance that

States and Tribes may use as a starting point for the criteria for their water quality standards. 

The term “water quality criteria” is used in two sections of the Clean Water Act, Section

304(a)(1) and Section 303(c)(2). The term has a different impact in each section. In Section 304,

the term represents a scientific assessment of ecological and human health effects that EPA

recommends to States and authorized Tribes for establishing water quality standards that

ultimately provide a basis for controlling discharges or releases of pollutants or related

parameters. Ambient water quality criteria associated with specific waterbody uses when

adopted as State or Tribal water quality standards under Section 303 define the level of a

pollutant (or, in the case of nutrients, a condition) necessary to protect designated uses in ambient

waters. Quantified water quality criteria contained within State or Tribal water quality standards

are essential to a water quality-based approach to pollution control. Whether expressed as

numeric criteria or quantified translations of narrative criteria within State or Tribal water quality

standards, quantified criteria serve as a critical basis for assessing attainment of designated uses

and measuring progress toward meeting the water quality goals of the Clean Water Act.

EPA is developing section 304(a) water quality criteria for nutrients because States and

Tribes consistently identify excessive levels of nutrients as a major reason why as much as half of

the surface waters surveyed in this country do not meet water quality objectives, such as full

support of aquatic life. EPA expects to develop nutrient criteria that cover four major types of

waterbodies – lakes and reservoirs, rivers and streams, estuarine and coastal areas, and wetlands –

across fourteen major ecoregions of the United States. EPA’s section 304(a) criteria are

intended to provide for the protection and propagation of aquatic life and recreation. To support

the development of nutrient criteria, EPA is publishing Technical Guidance Manuals that describe

a process for assessing nutrient conditions in the four waterbody types. 

EPA’s section 304(a) water quality criteria for nutrients provide numeric water quality

criteria, as well as procedures by which to translate narrative criteria within State or Tribal water

quality standards. In the case of nutrients, EPA section 304(a) criteria establish values for causal

variables (e.g., total nitrogen and total phosphorus) and response variables (e.g., turbidity and

chlorophyll a). EPA believes that State and Tribal water quality standards need to include

quantified endpoints for causal and response variables to provide sufficient protection of uses and

to maintain downstream uses. These quantified endpoints will most often be expressed as

numeric water quality criteria or as procedures to translate a State or Tribal narrative criterion

into a quantified endpoint.
iii

EPA will work with States and authorized Tribes as they adopt water quality criteria for

nutrients into their water quality standards. EPA recognizes that States and authorized Tribes

require flexibility in adopting numeric nutrient criteria into State and Tribal water quality

standards. States and authorized Tribes have several options available to them. EPA

recommends the following approaches, in order of preference:

(1) Wherever possible, develop nutrient criteria that fully reflect localized conditions and

protect specific designated uses using the process described in EPA’s Technical Guidance

Manuals for nutrient criteria development. Such criteria may be expressed either as

numeric criteria or as procedures to translate a State or Tribal narrative criterion into a

quantified endpoint in State or Tribal water quality standards.

(2) Adopt EPA’s section 304(a) water quality criteria for nutrients, either as numeric

criteria or as procedures to translate a State or Tribal narrative nutrient criterion into a

quantified endpoint.

(3) Develop nutrient criteria protective of designated uses using other scientifically

defensible methods and appropriate water quality data.

 

Geoffrey H. Grubbs, Director

Office of Science and Technology
iv

DISCLAIMER

This document provides technical guidance and recommendations to States, authorized

Tribes, and other authorized jurisdictions to develop water quality criteria and water quality

standards under the Clean Water Act (CWA) to protect against the adverse effects of nutrient

overenrichment. Under the CWA, States and authorized Tribes are to establish water quality

criteria to protect designated uses. State and Tribal decision-makers retain the discretion to adopt

approaches on a case-by-case basis that differ from this guidance when appropriate and

scientifically defensible. While this document contains EPA’s scientific recommendations

regarding ambient concentrations of nutrients that protect aquatic resource quality, it does not

substitute for the CWA or EPA regulations; nor is it a regulation itself. Thus it cannot impose

legally binding requirements on EPA, States, authorized Tribes, or the regulated community, and

it might not apply to a particular situation or circumstance. EPA may change this guidance in the

future.
v

EXECUTIVE SUMMARY

 

Nutrient Program Goals

EPA developed the National Strategy for the Development of Regional Nutrient Criteria

(National Strategy) in June 1998. The strategy presents EPA=s intentions to develop technical

guidance manuals for four types of waters (lakes and reservoirs, rivers and streams, estuaries and

coastal waters, and wetlands) and produce section 304(a) criteria for specific nutrient ecoregions

by the end of 2000. In addition, the Agency formed Regional Technical Assistance Groups

(RTAGs) which include State and Tribal representatives working to develop more refined and

more localized nutrient criteria based on approaches described in the waterbody guidance

manuals. This document presents EPA=s current recommended criteria for total phosphorus, total

nitrogen, chlorophyll a, and turbidity for lakes and reservoirs in Nutrient Ecoregion VI (Corn Belt

and Northern Great Plains) which were derived using the procedures described in the Lakes and

Reservoirs Nutrient Criteria Technical Guidance Manual (U.S. EPA, 2000a). 

EPA’s ecoregional nutrient criteria are intended to address cultural eutrophication-- the

adverse effects of excess nutrient inputs. The criteria are empirically derived to represent

conditions of surface waters that are minimally impacted by human activities and protective of

aquatic life and recreational uses. The information contained in this document represent starting

points for States and Tribes to develop (with assistance from EPA) more refined nutrient criteria.

In developing these criteria recommendations, EPA followed a process which included, to

the extent they were readily available, the following elements critical to criterion derivation: 

! Historical and recent nutrient data in Nutrient Ecoregion VI

 Nutrient Data from Legacy STORET (from 1990-1998) were used to assess historical

and recent nutrient conditions and develop reference conditions in Ecoregion VI

! Reference sites/reference conditions in Nutrient Ecoregion VI

Reference sites/reference conditions in Nutrient Ecoregion VI were based on the lake

population distribution approach using a representative sample of all lakes within the

Ecoregion (see Nutrient Criteria Technical Guidance Manual-Lakes and Reservoirs, April

2000, EPA-822-B00-001. States and Tribes are urged to determine their own reference

sites for lakes and reservoirs within the ecoregion at different geographic scales and to

compare them to EPA’s reference conditions.

! Models employed for prediction or validation

EPA did not identify any specific models used in the ecoregion to develop nutrient criteria. 

States and Tribes are encouraged to identify and apply appropriate models to support

nutrient criteria development.
vi

! RTAG expert review and consensus

 EPA recommends that when States and Tribes prepare their nutrient criteria, they obtain

the expert review and consent of the RTAG.

! Potential Downstream effects

EPA encourages the RTAG to assess the potential effects of the proposed criteria on

downstream water quality and uses.

In addition, EPA followed specific QA/QC procedures during data collection and

analysis: All data were reviewed for duplications. All data are from ambient waters that were not

located directly outside a permitted discharger. The following States indicated that their data

were sampled and analyzed using either Standard methods or EPA approved methods: 

Illinois, Iowa, Indiana, Nebraska, Minnesota, Ohio and Michigan. 

The following tables contain a summary of Aggregate and level III ecoregion values for

TN, TP, water column chl a, and turbidity:

BASED ON 25th PERCENTILE ONLY

Nutrient Parameters Aggregate Nutrient Ecoregion VI

Reference Conditions

Total phosphorus (μg/L) 37.5

Total nitrogen (mg/L) 0.781

Chlorophyll a (μg/L) (Fluorometric method) 8.59

Secchi depth (meters) 1.356

For subecoregions 46, 47, 48, 54, 55, and 57 the ranges of nutrient parameter reference

conditions are:

BASED ON 25th PERCENTILE ONLY

Nutrient Parameters Range of Level III Subecoregions

Reference Conditions

Total phosphorus (μg/L) 10-90 

Total nitrogen (mg/L) 0.63-1.43

Chlorophyll a (μg/L) (Fluorometric method) 4-14.4

Secchi depth (meters) 0.87-2.6
vii

NOTICE OF DOCUMENT AVAILABILITY

 This document is available electronically to the public through the INTERNET at:

(http://www.epa.gov/OST/standards/nutrient.html). Requests for hard copies of the document

should be made to EPA’s National Service Center for Environmental Publications (NSCEP),

11029 Kenwood Road, Cincinnati, OH 45242 or (513) 489-8190 or toll free (800) 490-9198. 

Please refer to EPA document number EPA-822-R-00-008.
viii

ACKNOWLEDGMENTS

The authors thankfully acknowledge the contributions of the following State and Federal 

reviewers: EPA Regions 5, 7, and 8; The States of South Dakota, North Dakota, Nebraska,

Minnesota, Iowa, Illinois, Wisconsin, Indiana, Michigan and Ohio and the Tribes within the

Ecoregion. EPA Headquarters personnel from the Office of Wetlands, Oceans and Watersheds,

Office of Wastewater Management, Office of General Counsel, Office of Research and

Development, and the Office of Science and Technology. EPA also acknowledges the external

peer review efforts of Eugene Welch (University of Washington), Robert Carlson (Kent State

University), Steve Heiskary (Minnesota Pollution Control Agency), Greg Denton and Sherry

Wang (Tennessee Department of Environment and Conservation), and Gerhard Kuhn (U.S.

Geological Survey). 
ix

LISTS OF TABLES AND FIGURES

Figures

Figure 1 Aggregate Ecoregion VI ............................... 7

Figure 2 Aggregate Ecoregion VI with level III ecoregions shown ...... 9

Figure 3 Sampling locations within each level III ecoregion .......... 11

Figure 4a Illustration of data reduction process for lake data ........... 20

Figure 4b Illustration of reference condition calculation ............... 21

Tables

Table 1 Lake and reservoir records for Aggregate Ecoregion - Corn Belt

and Northern Great Plains ............................. 12

Table 2 Reference conditions for Aggregate Ecoregion VI lakes ...... 14

Table 3a-f Reference conditions for level III ecoregion lakes ........... 15
x

TABLE OF CONTENTS

Foreword ................................................................. ii

Disclaimer ............................................................... iv

Executive Summary .......................................................... v

Notice of Document Availability ............................................... vii

Acknowledgments ........................................................ viii

List of Tables and Figures .................................................... ix

Table of Contents ........................................................... x

1.0 Introduction ............................................................ 1

2.0 Best Use of this Information ............................................... 4

3.0 Area Covered by This Document (waterbody type and ecoregion) .................. 6

3.1 Description of Aggregate Ecoregion VI–Corn Belt and N. Great Plains ........ 6

3.2 Geographical Boundaries of Aggregate Ecoregion VI ...................... 6

3.3 Level III Ecoregions within Aggregate Ecoregion VI ...................... 7

4.0 Data Review for Lakes and Reservoirs in Aggregate Ecoregion VI .................. 9

4.1 Data Sources .................................................... 10

4.2 Historical Data from Aggregate Ecoregion VI (TP, TN, Chl a, Turbidity) ....... 10

4.3 QA/QC of Data Sources ............................................ 10

4.4 Data for All Lakes/Reservoirs within Aggregate Ecoregion VI ............... 10

4.5 Statistical Analysis of Data .......................................... 13

4.6 Classification of Lake/Reservoir Type ................................. . 19

4.7. Summary of Data Reduction Methods .................................. 19

5.0 Reference Sites and Conditions in Aggregate Ecoregion VI ...................... 22

6.0 Models Used to Predict or Verify Response Parameters ......................... 22

7.0 Framework for Refining Recommended Nutrient Criteria for Lakes and Reservoirs in 

 Aggregate Ecoregion VI .................................................... 22

7.1 Example Worksheet for Developing Aggregate Ecoregion and Subecoregion Nutrient 

 Criteria .......................................................... 23

7.2 Tables of Refined Nutrient Water Quality Criteria for Aggregate Ecoregion VI and 

 Level III Subecoregions ............................................. 24

7.3 Setting Seasonal Criteria ............................................ 25

7.4 When Data/Reference Conditions Are Lacking ........................... 25
xi

7.5 Site-Specific Criteria Development .................................... 26

8.0 Literature Cited ....................................................... 26

9.0 Appendices ........................................................... 27
1

1.01 INTRODUCTION

Background

Nutrients are essential to the health and diversity of our surface waters. However, in

excessive amounts, nutrients cause hypereutrophication, which results in overgrowth of plant life

and decline of the biological community. Excessive nutrients can also result in potential human

health risks, such as the growth of harmful algal blooms - most recently manifested in the

Pfiesteria outbreaks of the Gulf and East Coasts. Chronic nutrient overenrichment of a waterbody

can lead to the following consequences: low dissolved oxygen, fish kills, algal blooms,

overabundance of macrophytes, likely increased sediment accumulation rates, and species shifts of

both flora and fauna. 

Historically, National Water Quality Inventories have repeatedly shown that nutrients are a

major cause of ambient water quality use impairments. EPA’s 1996 National Water Quality

Inventory report identifies excessive nutrients as the leading cause of impairment in lakes and the

second leading cause of impairment in rivers (behind siltation). In addition, nutrients were the

second leading cause of impairments reported by the States in their 1998 lists of impaired waters. 

Where use impairment is documented, nutrients contribute roughly 25-50% of the impairment

nationally. The Clean Water Act establishes a national goal to achieve, wherever attainable, water

quality which provides for the protection and propagation of fish, shellfish, and wildlife and

recreation in and on the water. In adopting water quality standards, States and Tribes designate

uses for their waters in consideration of the Clean Water Act goals, and establish water quality

criteria that contain sufficient parameters to protect those uses. To date, EPA has not published

information and recommendations under section 304(a) for nutrients to assist States and Tribes in

establishing numeric nutrient criteria to protect uses when adopting water quality standards.

In 1995, EPA gathered a set of national experts and asked the experts how to best deal

with the national nutrient problem. The experts recommended that the Agency not develop single

criteria values for phosphorus or nitrogen applicable to all water bodies and regions of the country. 

Rather, the experts recommended that EPA put a premium on regionalization, develop guidance

(assessment tools and control measures) for specific waterbodies and ecological regions across the

country, and use reference conditions (conditions that reflect pristine or minimally impacted

waters) as a basis for developing nutrient criteria.

With these suggestions as starting points, EPA developed the National Strategy for the

Development of Regional Nutrient Criteria (National Strategy), published in June 1998. This

strategy presented EPA’s intentions to develop technical guidance manuals for four types of waters

(lakes and reservoirs, rivers and streams, estuaries and coastal waters, and wetlands) and,

thereafter, to publish section 304(a) criteria recommendations for specific nutrient ecoregions. 

Technical guidance manuals for lakes/reservoirs and rivers/streams were published in April 2000

and July 2000, respectively. The technical guidance manual for estuaries/coastal waters will be

published in spring 2000 and the draft wetlands technical guidance manual will be published by

December 2001. Each manual presents EPA’s recommended approach for developing nutrient

criteria values for a specific waterbody type. In addition, EPA is committed to working with

States and Tribes to develop more refined and more localized nutrient criteria based on approaches

described in the waterbody guidance manuals and this document. 
2

Overview of the Nutrient Criteria Development Process

For each Nutrient Ecoregion, EPA developed a set of recommendations for two causal

variables (total nitrogen and total phosphorus) and two early indicator response variables

(chlorophyll a and some measure of turbidity). Other indicators such as dissolved oxygen and

macrophyte growth or speciation, and other fauna and flora changes are also deemed useful. 

However, the first four are considered to be the best suited for protecting designated uses.

The technical guidance manuals describe a process for developing nutrient criteria that

involves consideration of five factors. The first of these is the Regional Technical Assistance

Group (RTAG), which is a body of qualified regional specialists able to objectively evaluate all of

the available evidence and select the value(s) appropriate to nutrient control in the water bodies of

concern. These specialists may come from such disciplines as limnology, biology, natural

resources management-- especially water resource management, chemistry, and ecology. The

RTAG evaluates and recommends appropriate classification techniques for criteria determination,

usually physical within an ecoregional construct.

The second factor is the historical information available to establish a perspective of the

resource base. This is usually data and anecdotal information available within the past ten-twenty

five years. This information gives evidence about the background and enrichment trend of the

resource.

The third factor is the present reference condition. A selection of reference sites chosen to

represent the least culturally impacted waters of the class existing at the present time. The data

from these sites is combined and a value from the distribution of these observations is selected to

represent the reference condition, or best attainable, most natural condition of the resource base at

this time.

A fourth factor often employed is theoretical or empirical models of the historical and

reference condition data to better understand the condition of the resource.

The RTAG comprehensively evaluates the other three elements to propose a candidate

criterion (initially one each for TP, TN, chl a, and some measure of turbidity).

The last and final element of the criteria development process is the assessment by the

RTAG of the likely downstream effects of the criterion. Will there be a negative, positive, or

neutral effect on the downstream waterbody? If the RTAG judges that a negative effect is likely,

then the proposed State/Tribal water quality criteria should be revised to ameliorate the potential

for any adverse downstream effects.

While States and authorized Tribes would not necessarily need to incorporate all five

elements into their water quality criteria setting process (e.g., modeling may be significant in only

some instances), the best assurance of a representative and effective criterion for nutrient

management decision making is the balanced incorporation of all five elements, or at least all

elements except modeling.
3

Because some parts of the country have naturally higher soil and parent material

enrichment, and different precipitation regimes, the application of the criterion development

process has to be adjusted by region. Therefore, an ecoregional approach was chosen to develop

nutrient criteria appropriate to each of the different geographical and climatological areas of the

country. Initially, the continental U.S. was divided into 14 separate ecoregions of similar

geographical characteristics. Ecoregions are defined as regions of relative homogeneity in

ecological systems; they depict areas within which the mosaic of ecosystem components (biotic

and abiotic as well as terrestrial and aquatic) is different than adjacent areas in a holistic sense. 

Geographic phenomena such as soils, vegetation, climate, geology, land cover, and physiology that

are associated with spatial differences in the quantity and quality of ecosystem components are

relatively similar within each ecoregion. 

The Nutrient ecoregions are aggregates of U.S. EPA=s hierarchal level III ecoregions. As

such, they are more generalized and less defined than level III ecoregions. EPA determined that

setting ecoregional criteria for the large scale aggregates is not without its drawbacks - variability

is high due to the lumping of many waterbody classes, seasons, and years worth of multipurpose

data over a large geographic area. For these reasons, the Agency recommends that States and

Tribes develop nutrient criteria at the level III ecoregional scale and at the waterbody class scale

where those data are readily available. Data analyses and recommendations on both the large

aggregate ecoregion scale as well as more refined scales (level III ecoregions and waterbody

classes), where data were available to make such assessments, are presented for comparison

purposes and completeness of analysis. 

Relationship of Nutrient Criteria to Biological Criteria

Biological criteria are quantitative expressions of the desired condition of the aquatic

community. Such criteria can be based on an aggregation of data from sites that represent the

least-impacted and attainable condition for a particular waterbody type in an ecoregion,

subecoregion, or watershed. EPA’s nutrient criteria recommendations and biological criteria

recommendations have many similarities in the basic approach to their development and data

requirements. Both are empirically derived from statistical analysis of field collected data and

expert evaluation of current reference conditions and historical information. Both utilize direct

measurements from the environment to integrate the effects of complex processes that vary

according to type and location of waterbody. The resulting criteria recommendations, in both

cases, are efficient and holistic indicators of water quality necessary to protect uses.

States and authorized Tribes can develop and apply nutrient criteria and biological criteria

in tandem, with each providing important and useful information to interpret both the nutrient

enrichment levels and the biological condition of sampled waterbodies. For example, using the

same reference sites for both types of criteria can lead to efficiencies in both sample design and

data analysis. In one effort, environmental managers can obtain information to support assessment

of biological and nutrient condition, either through evaluating existing data sets or through

designing and conducting a common sampling program. The traditional biological criteria

variables of benthic invertebrate and fish sampling can be readily incorporated to supplement a

nutrient assessment. To demonstrate the effectiveness of this tandem approach, EPA has initiated

pilot projects in both freshwater and marine environments to investigate the 
4

relationship between nutrient overenrichment and apparent declines in diversity indices of benthic

invertebrates and fish.

2.0 BEST USE OF THIS INFORMATION

EPA recommendations published under section 304(a) of the CWA serve several purposes,

including providing guidance to States and Tribes in adopting water quality standards for nutrients

that ultimately provide a basis for controlling discharges or releases of pollutants. The

recommendations also provide guidance to EPA when promulgating Federal water quality

standards under section 303(c) when such action is necessary. Other uses include identification of

overenrichment problems, management planning, project evaluation, and determination of status

and trends of water resources. 

State water quality inventories and listings of impaired waters consistently rank nutrient

overenrichment as a top contributor to use impairments. EPA’s water quality standards

regulations at 40 CFR §131.11(a) require States and Tribes to adopt criteria that contain sufficient

parameters and constituents to protect the designated uses of their waters. In addition, States and

Tribes need quantifiable targets for nutrients in their standards to assess attainment of uses,

develop water quality-based permit limits and source control plans, and establish targets for total

maximum daily loads (TMDLs).

EPA expects States and Tribes to address nutrient overenrichment in their water quality

standards, and to build on existing State and Tribal initiated efforts where possible. States and

Tribes can address nutrient overenrichment through establishment of numerical criteria or through

use of new or existing narrative criteria statements (e.g., free from excess nutrients that cause or

contribute to undesirable or nuisance aquatic life or produce adverse physiological response in

humans, animals, or plants). In the case of narrative criteria, EPA expects that States and Tribes

establish procedures to quantitatively translate these statements for both assessment and source

control purposes. 

The intent of developing ecoregional nutrient criteria is to represent conditions of surface

waters that are minimally impacted by human activities and thus protect against the adverse effects

of nutrient overenrichment from cultural eutrophication. EPA’s recommended process for

developing such criteria includes physical classification of waterbodies, determination of current

reference conditions, evaluation of historical data and other information (such as published

literature), use of models to simulate physical and ecological processes or determine empirical

relationships among causal and response variables (if necessary), expert judgement, and evaluation

of downstream effects. To the extent allowed by the information available, EPA has used elements

of this process to produce the information contained in this document. The values for both causal

(total nitrogen, total phosphorus) and biological and physical response (chlorophyll a, turbidity)

variables represent a set of starting points for States and Tribes to use in establishing their own

criteria in standards to protect uses.

In its water quality standards regulations, EPA recommends that States and Tribes establish

numerical criteria based on section 304(a) guidance, section 304(a) guidance modified to reflect

site-specific conditions, or other scientifically defensible methods. For many pollutants, such as

toxic chemicals, EPA expects that section 304(a) guidance will provide an appropriate level of
5

protection without further modification in most cases. EPA has also published methods for

modifying 304(a) criteria on a site-specific basis, such as the water effect ratio, where site-specific

conditions warrant modification to achieve the intended level of protection. For nutrients,

however, EPA expects that, in most cases, it will be necessary for States and authorized Tribes to

identify with greater precision the nutrient levels that protect aquatic life and recreational uses. 

This can be achieved through development of criteria modified to reflect conditions at a smaller

geographic scale than an ecoregion such as a subecoregion, the State or Tribe level, or specific

class of waterbodies. Criteria refinement can occur by grouping data or performing data analyses

at these smaller geographic scales. Refinement can also occur through further consideration of

other elements of criteria development, such as published literature or models.

The values presented in this document generally represent nutrient levels that protect

against the adverse effects of nutrient overenrichment and are based on information available to the

Agency at the time of this publication. However, States and Tribes should critically evaluate this

information in light of the specific designated uses that need to be protected. For example, more

sensitive uses may require more stringent values as criteria to ensure adequate protection. On the

other hand, overly stringent levels of protection against the adverse effects of cultural

eutrophication may actually fall below levels that represent the natural load of nutrients for certain

waterbodies. In cases such as these, the level of nutrients specified may not be sufficient to

support a productive fishery. In the criteria derivation process, it is important to distinguish

between the natural load associated with a specific waterbody and current reference conditions,

using historical data and expert judgement. These elements of the nutrient criteria derivation

process are best addressed by States and Tribes with access to information and local expertise. 

Therefore, EPA strongly encourages States and Tribes to use the information contained in this

document and to develop more refined criteria according to the methods described in EPA’s

technical guidance manuals for specific waterbody types.

To assist in the process of further refinement of nutrient criteria, EPA has established ten

Regional Technical Advisory Groups (experts from EPA Regional Offices and States/Tribes). In

the process of refining criteria, States and authorized Tribes need to provide documentation of

data and analyses, along with a defensible rationale, for any new or revised nutrient criteria they

submit to EPA for review and approval. As part of EPA’s review of State and Tribal standards,

EPA intends to seek assurance from the RTAG that proposed criteria are sufficient to protect uses.

In the process of using the information and recommendations contained in this document,

as well as additional information, to develop numerical criteria or procedures to translate narrative

criteria, EPA encourages States and Tribes to:

C Address both chemical causal variables and early indicator response variables. Causal

variables are necessary to provide sufficient protection of uses before impairment occurs

and to maintain downstream uses. Early response variables are necessary to provide

warning signs of possible impairment and to integrate the effects of variable and potentially

unmeasured nutrient loads.

C Include variables that can be measured to determine if standards are met, and variables that

can be related to the ultimate sources of excess nutrients.

C Identify appropriate periods of duration (i.e., how long) and frequency (i.e., how often) of

occurrence in addition to magnitude (i.e., how much). EPA does not recommend
6

identifying nutrient concentrations that must be met at all times, rather a seasonal or annual

averaging period (e.g., based on weekly measurements) is considered appropriate. 

However, these seasonal or annual central tendency measures should apply each season or

each year, except under the most extraordinary of conditions (e.g., a 100 year flood).

3.0 AREA COVERED BY THIS DOCUMENT

The following sections provide a general description of the aggregate ecoregion and its

geographical boundaries. Descriptions of the level III ecoregions contained within the aggregate

ecoregion are also provided.

3.1 Description of Aggregate Ecoregion VI- Corn Belt and Northern Great Plains

The rolling plains and flat lake beds of Region VI are dominated by extensive, highly

productive cropland. Moist, fertile soils are characteristic and are often more nutrient-rich than

those of Regions IV, VII, VIII, and IX. Perennial streams, lakes, and concentrations of seasonal

wetlands occur. Corn, soybean, and livestock farming is common and feedlots occur. Many

urban, suburban, and industrial areas are also found in Region VI. This land use mosaic differs

from the rangeland of Region IV, the winter wheat, grain sorghum, and feedlots of Region V, the

dairying and silage corn of Region VII, and the forest, cropland, and pastureland of Region IX.

The regions nutrient-rich soils significantly influence surface and subsurface water quality. 

Elevated concentrations of nitrate and phosphorus are significant water quality problems in many

basins and are a byproduct of nutrient-rich agricultural runoff and wastewater treatment effluent;

dissolved oxygen depletion occurs in sluggish, warm rivers that have high nutrient levels. 

Pesticides are widely used on cropland in the Corn Belt and Northern Great Plains (VI) and have

contaminated surface waters. High concentrations of suspended sediment are found in many

streams especially those in flat, agricultural areas with clayey soils and artificial drainage. Fecal

coliform bacteria levels in streams have been elevated by feedlots, municipal wastewater effluent,

urban runoff, and livestock operations. Lakes occur especially in the northern part of the region;

they are used for fishing and recreation and are important wildlife habitat. Those that are found in

Region VI range from mildly eutrophic to hypereutrophic. Eutrophic conditions are also found in

southwestern portions of Lake Michigan and Lake Erie.

3.2 Geographical Boundaries of Aggregate Ecoregion VI

As this ecoregion’s name suggests, it encompasses areas of the northern midwest where

corn production dominates land usage. The eastern portions of North and South Dakota and

Nebraska form the northwest boundary of the ecoregion. A small corner of northeast Kansas is

included in the region. Also, most of Iowa, the northern half of Illinois, central Indiana, the

southern and western portions of Minnesota and western Ohio are included in ecoregion VI. In

addition, very small segments of Wisconsin and Michigan are included in this ecoregion. 
7

Figure 1. Aggregate Ecoregion VI. 

3.3 Level III Ecoregions Within Aggregate Ecoregion VI

46. Northern Glaciated Plains

The Northern Glaciated Plains ecoregion is characterized by a flat to gently rolling landscape

composed of glacial till. The subhumid conditions foster a transitional grassland containing

tallgrass and shortgrass prairie. High concentrations of temporary and seasonal wetlands create

favorable conditions for waterfowl nesting and migration. Though the till soils are very fertile,

agricultural success is subject to annual climatic fluctuations.

47. Western Corn Belt Plains

Once covered with tallgrass prairie, over 75 percent of the Western Corn Belt Plains is now used

for cropland agriculture and much of the remainder is in forage for livestock. A combination of

nearly level to gently rolling glaciated till plains and hilly loess plains, an average annual

precipitation of 63 - 89 cm which occurs mainly in the growing season, and fertile, warm, moist

soils make this on of the most productive areas of corn and soybeans in the world. The region is

also one of major environmental concerns regarding surface and groundwater contamination from

fertilizer and pesticide applications as well as livestock concentrations.

48. Lake Agassiz Plain

Glacial Lake Agassiz was the last in a series of proglacial lakes to fill the Red River valley in the

three million years since the beginning of the Pleistocene. Thick beds of lake sediments on top of

glacial till create the extremely flat floor of the Lake Agassiz Plain. The historic tallgrass prairie
8

has been replaced by intensive row crop agriculture. The preferred crops in the northern half of

the region are potatoes, beans, sugar beets and wheat; soybeans, sugar beets, and corn

predominate in the south. 

54. Central Corn Belt Plains

Extensive prairie communities intermixed with oak hickory forests were native to the glaciated

plains of the Central Corn Belt Plains; they were a stark contrast to the hardwood forests that grew

on the drift plains of ecoregions to the east. Ecoregions to the west were mostly treeless except

along larger streams. Beginning in the nineteenth century, the natural vegetation was gradually

replaced by agriculture. Farms are now extensive on the dark, fertile soils of the Central Corn Belt

Plains and mainly produce corn and soybeans; cattle, sheep, poultry, and especially hogs are also

raised, but they are not as dominant as in the drier Western Corn Belt Plains to the west. 

Agriculture has affected stream chemistry, turbidity, and habitat.

55. Eastern Corn Belt Plains

The Eastern Corn Belt Plains is primarily a rolling plain with local end moraines; it had more

natural tree cover and has lighter colored soils than the Central Corn Belt Plains. The region has

loamier and better drained soils than the Huron/Erie Lake Plain, and richer soils than the

Erie/Ontario Hills and Lake Plain. Glacial deposits of Wisconsin age are extensive. They are not

as dissected nor as leached as the pre-Wisconsin till which is restricted to the southern part of the

region. Originally, beech forests were common on Wisconsin soils while beech forests and elm-ash

swamp forests dominated the wetter pre-Wisconsin soils. Today, extensive corn, soybean, and

livestock production occurs and has affected stream chemistry and turbidity.

57. Huron/Erie Lake Plain

The Huron/Erie Lake Plain is a broad, fertile, nearly flat plain punctuated by relic sand dunes,

beach ridges, and end moraines. Originally, soil drainage was typically poorer than in the adjacent

Eastern Corn Belt Plains, and elm-ash swamp and beech forests were dominant. Oak savanna was

typically restricted to sandy, well-drained dunes and beach ridges. Today, most of the area has

been cleared and artificially drained and contains highly productive farms producing corn,

soybeans, livestock, and vegetables; urban and industrial areas are also extensive. Stream habitat

and quality have been degraded by channelization, ditching, and agricultural activities.
9

Figure 2. Aggregate Ecoregion VI with level III ecoregions shown

Suggested Ecoregional subdivisions or adjustments.

EPA recommends that the RTAG evaluate the adequacy of EPA nutrient ecoregional and

subecoregional boundaries and refine them as needed to reflect local conditions. 

4.0 DATA REVIEW FOR LAKES AND RESERVOIRS IN AGGREGATE

ECOREGION VI

The following section describes the nutrient data EPA has collected and analyzed for this

Ecoregion, including an assessment of data quantity and quality. The data tables present the data

for each causal parameter-- total phosphorus and total nitrogen (both reported and calculated from

TKN and nitrite/nitrate), and the primary response variables-- some measure of turbidity and

chlorophyll a. These are the parameters which EPA considers essential to nutrient assessment

because the first two are the main causative agents of enrichment and the two response variables

are the early indicators of system enrichment for most of the surface waters 

(See Chapter 5 of the Lakes and Reservoirs Nutrient Criteria Guidance Manual (U.S. EPA, 2000a)

for a complete discussion on choosing causal and response variables.)
10

4.1 Data Sources

Data sets from Legacy STORET were used to assess nutrient conditions from 1990 to

1999. The following States submitted nutrient data to Legacy STORET: North Dakota, South

Dakota, Minnesota, Nebraska, Iowa, Illinois, Michigan, Indiana, Ohio and Kansas. This data is

from January 1990 to September 1998. EPA recommends that the RTAGs identify additional data

sources that can be used to supplement the data sets listed above. In addition, the RTAGs may

utilize published literature values to support quantitative and qualitative analyses.

4.2 Historical Data from Aggregate Ecoregion VI ( TP, TN, Chlor a and Secchi Depth)

 EPA recommends that States/Tribes assess long-term trends observed over the past 50

years. This information may be obtained from scientific literature or documentation of historical

trends. To gain additional perspective on more recent trends, it is recommended that States and

Tribes assess nutrient trends over the last 10 years (e.g., what do seasonal trends indicate?)

4.3 QA/QC of data sources

An initial quality screen of data was conducted using the rules presented in Appendix C. 

Data remaining after screening for duplications and other QA measures (.e.g., poor or unreported

analytical records, sampling errors or omissions, stations associated with outfalls, storm water

sewers, hazardous waste sites) is the data used in statistical analyses.

States within Ecoregion VI were contacted regarding the quality of their data. The

following States provided information on the methods used to sample and analyze their waters:

Illinois, Indiana, Nebraska, Minnesota, Ohio, and Michigan. In all cases, States indicated a

Standard method or an approved EPA method was used. S. Dakota, North Dakota and Kansas

did not provide information prior to the publication of this document.

4.4 Data for all lakes/reservoirs within Aggregate Ecoregion VI 

The map in Figure 3 shows the location of the sampling stations within each sub ecoregion.

 Table 1 presents all data records for all parameters for Aggregate Ecoregion VI and

subecoregions within the Aggregate Ecoregion. The map indicates that stations are present in all

subecoregions and somewhat uniformly scattered within the subecoregions. There is a degree of

concentration of stations near what are presumed to be population centers.
11

Figure 3. Map of sampling locations within each level III ecoregion
12

 Table 1. Lake records for Aggregate Ecoregion VI - Corn Belt and Northern Great Plains

Aggregate

Ecoregion

IV

Sub

ecoR 46

Sub

ecoR 47

Sub

ecoR 48

Sub

ecoR 54

Sub

ecoR 55

Sub

ecoR 57

# of Lakes 531 103 125 18 224 58 10

# of Lake Stations 1320 172 208 23 813 93 11

Key Nutrient Parameters

(listed below)

 - # of records for Secchi

depth

33, 073 2463 6060 134 24,114 301 1

- # of records for Chlorophyll

a (all methods)

8741 302 1892 18 5378 1151 0

- # of records for Total

Kjeldahl Nitrogen (TKN)

8277 1166 2088 170 4014 780 59

- # of records for Nitrate +

Nitrite (NO2 + NO3)

10,455 898 2704 145 5660 989 59

- # of records for Total

Nitrogen (TN)

74 67 0 7 0 0 0

- # of records for Total

Phosphorus (TP)

11,139 1440 2770 190 5672 1008 59

Total # of records for key

nutrient parameters

71,759 6336 15,514 664 44,838 4229 178
13

Definitions used to complete Table 1:

1. # of records refers to the total count of observations for that

parameter over the entire decade (1990-1999) for that particular

aggregate or subecoregion. These are counts for all seasons over

that decade.

2. # of lake stations refers to the total number of lake and

reservoir stations within the aggregate or subecoregion from

which nutrient data were collected. Since lakes and reservoirs can

cross ecoregional boundaries, it is important to note that only

those portions of a lake or reservoir (and data associated with

those stations) that exist within the ecoregion are included within

this table.

4.5 Statistical Analysis of Data/Potential Reference Conditions

EPA’s Technical Guidance Manual for Developing Nutrient Criteria for Lakes and

Reservoirs describes two ways of establishing a reference condition. One method is to choose the

upper 25th percentile (75th percentile) of a reference population of lakes. This is the preferred

method to establish a reference condition. The 75th percentile was chosen by EPA since it is likely

associated with minimally impacted conditions, will be protective of designated uses, and provides

management flexibility. When reference lakes are not identified, the second method is to determine

the lower 25th percentile of the population of all lakes within a region. The 25th percentile of the

entire population was chosen by EPA to represent a surrogate for an actual reference population. 

Data analyses to date indicate that the lower 25th percentile from an entire population roughly

approximates the 75th percentile for a reference population. See case studies for Minnesota lakes

in the Lakes and Reservoirs Nutrient Criteria Technical Guidance Document (U.S. EPA, 2000a)

and the case study for Tennessee streams in the Rivers and Streams Nutrient Criteria Technical

Guidance Document (U.S. EPA, 2000b), and the letter from Tennessee Department of

Environment and Conservation to Geoffrey Grubbs [TNDEC, 2000]). New York State has also

presented evidence that the 25th percentile and the 75 percentile compare well based on user

perceptions of water resources (NYSDEC., 2000)

The following tables 2 and 3a-f, present the potential reference conditions for both the

aggregate ecoregion and the subecoregions using both methods. However, the reference lake

column is left blank because EPA does not have reference data and anticipates that States will

provide information on reference lakes. Appendix A provides a complete presentation of all

descriptive statistics for both the aggregate ecoregion and the level III subecoregion.
14

Table 2. Reference conditions for aggregate ecoregion VI lakes.

 Parameter

No. of

Lakes

 N ++

Reported values 25th Percentiles based on all

seasons data for the Decade

Reference Lakes **

Min Max P25* all seasons + P75 all seasons

TKN (mg/L) 313 .0945 4.355 .765

NO2 + NO3 (mg/L) 368 0 9.513 .016

TN (mg/L) - calculated NA .0945 13.867 .781

TN (mg/L) - reported 3 1.68 2.445 1.68

TP (ug/L) 393 2.5 1000 37.5

Secchi (meters) 387 .089 5.03 1.356

Chlorophyll a (ug/L) - F 1 18.8 18.8 18.8

Chlorophyll a (ug/L) - S 224 .32 147.45 8.59

Chlorophyll a (ug/L) - T 164 0.66 134.97 17.26

 P25: 25th percentile of all data

P75: 75th percentile of all data

** as determined by the Regional Technical Assistance Groups (RTAGs)

+ Median for all seasons’ 25th percentiles. E.g. this value was calculated from four seasons’ 25th

percentiles. If the seasonal 25th percentile (P25) TP values are - spring 10ug/L, summer

15ug/L, fall 12ug/L, and winter 5ug/L, the median value of all seasons P25 will be 11ug/L. 

++ N = largest value reported for a decade / Season.

TN calculated is based on the sum of TKN + NO2+NO3. 

TN reported is actual TN value reported in the database for one sample.

F Chlorophyll a measured by Fluorometric method with acid correction.

S Chlorophyll a measured by Spectrophotometric method with acid correction.

T Chlorophyll a b c measured by Trichromatic method.

NA Not Applicable 

Table(s) 3a.-f. present the potential reference conditions for lakes and reservoirs in the

Level III subecoregions within the Aggregate Ecoregion. The footnotes for Table 2 apply to tables

3a-f.
15

Table 3a. Reference conditions for level III ecoregion 46 lakes.

 Parameter

No. of

Lakes

 N ++

Reported values 25th Percentiles based on all

seasons data for the Decade

Reference Lakes **

Min Max P25* all seasons + P75 all seasons

TKN (mg/L) 66 0.69 3.89 1.41

NO2 + NO3 (mg/L) 58 .003 .635 .022

TN (mg/L) - calculated NA 0.69 4.525 1.43

TN (mg/L) - reported 2 z 1.83 2.45 1.83

TP (ug/L) 69 15 970 90

Secchi (meters) 55 0.2 3.0 1.46

Chlorophyll a (ug/L) - F ----- ----- ------ ------

Chlorophyll a (ug/L) - S 24 2.84 40.63 6.5

Chlorophyll a (ug/L) - T 1 z 19 19 19 zz

Table 3b. Reference conditions for level III ecoregion 47 lakes.

 Parameter

No. of

Lakes

 N ++

Reported values 25th Percentiles based on all

seasons data for the Decade

Reference Lakes **

Min Max P25* all seasons + P75 all seasons

TKN (mg/L) 81 0.15 4.23 0.957

NO2 + NO3 (mg/L) 79 0 5.76 .0065

TN (mg/L) - calculated NA 0.15 9.99 .9635

TN (mg/L) - reported ------ ------ ----- ------

TP (ug/L) 92 15 847.5 55

Secchi (meters) 108 0.175 4.195 1.23

Chlorophyll a (ug/L) - F 1 z 18.8 18.8 18.8

Chlorophyll a (ug/L) - S 57 3.25 129.5 14.6

Chlorophyll a (ug/L) - T 16 4.0 128 14
16

Table 3c. Reference conditions for level III ecoregion 48 lakes.

 Parameter

No. of

Lakes

 N ++

Reported values 25th Percentiles based on all

seasons data for the Decade

Reference Lakes **

Min Max P25* all seasons + P75 all seasons

TKN (mg/L) 8 0.59 1.76 0.62

NO2 + NO3 (mg/L) 8 .006 .06 .007

TN (mg/L) - calculated NA .60 1.82 .63

TN (mg/L) - reported 1 z 2.14 2.14 2.14 zz

TP (ug/L) 11 22.5 325 51.25

Secchi (meters) 6 0.61 2.75 1.98

Chlorophyll a (ug/L) - F -------- -------- ------- -----------

Chlorophyll a (ug/L) - S 3 z 4 15.5 4

Chlorophyll a (ug/L) - T ------- ------- ------ ---------

Table 3d. Reference conditions for level III ecoregion 54 lakes.

 Parameter

No. of

Lakes

 N ++

Reported values 25th Percentiles based on all

seasons data for the Decade

Reference Lakes **

Min Max P25* all seasons + P75 all seasons

TKN (mg/L) 102 0.18 2.3 0.62

NO2 + NO3 (mg/L) 167 .003 7.85 0.015

TN (mg/L) - calculated NA 0.18 10.15 0.63

TN (mg/L) - reported -------- -------- ------- -----------

TP (ug/L) 165 5.75 656.25 20

Secchi (meters) 197 0.114 4.43 1.44

Chlorophyll a (ug/L) - F ------- -------- ------- --------

Chlorophyll a (ug/L) - S 133 0.57 134.6 7.85

Chlorophyll a (ug/L) - T 133 0.66 125.9 7.64
17

Table 3e. Reference conditions for level III ecoregion 55 lakes.

 Parameter

No. of

Lakes

 N ++

Reported values 25th Percentiles based on all

seasons data for the Decade

Reference Lakes **

Min Max P25* all seasons + P75 all seasons

TKN (mg/L) 47 0.188 1.83 0.525

NO2 + NO3 (mg/L) 47 0.025 6.197 0.257

TN (mg/L) - calculated NA 0.213 8.03 0.782

TN (mg/L) - reported ------- ------- ------ -------

TP (ug/L) 47 3.75 455.88 35

Secchi (meters) 20 0.229 1.06 0.87

Chlorophyll a (ug/L) - F ------ ------- ------ --------

Chlorophyll a (ug/L) - S 7 2.75 21.66 5.27

Chlorophyll a (ug/L) - T 14 1.4 28.89 5.45

Table 3f. Reference conditions for level III ecoregion 57 lakes.

 Parameter

No. of

Lakes

 N ++

Reported values 25th Percentiles based on all

seasons data for the Decade

Reference Lakes **

Min Max P25* all seasons + P75 all seasons

TKN (mg/L) 9 0.4 1.05 0.5

NO2 + NO3 (mg/L) 9 0.025 9.13 0.535

TN (mg/L) - calculated NA 0.425 10.18 1.04

TN (mg/L) - reported ------ ------- ------- -------

TP (ug/L) 9 8 140 10

Secchi (meters) 1 z 2.6 2.6 2.6 zz

Chlorophyll a (ug/L) - F ------- ------ ------- -------

Chlorophyll a (ug/L) - S ------- -------- ------ ------

Chlorophyll a (ug/L) - T ------- ------- ------- -------
18

Definitions used in filling Tables 2 and 3 - Reference Condition tables

1. Number of Lakes in Table 2 refers to the largest number of lakes and

reservoirs for which data existed for a given season within an aggregate nutrient

ecoregion.

2. Number of Lakes in Table 3 refers to the number of lakes and reservoirs for

which data existed for the summer months since summer is generally when the

greatest amount of nutrient sampling is conducted. If another season greatly

predominates, notification is made (s=spring, f=fall, w=winter).

3. Medians. All values (min, max, and 25th percentiles) included in the table are

based on waterbody medians. All data for a particular parameter within a lake

for the decade were reduced to one median for that lake. This prevents overrepresentation of individual waterbodies with a great deal of data versus those

with fewer data points within the statistical analysis.

4. 25th percentile for all seasons is calculated by taking the median of the 4

seasonal 25th percentiles. If a season is missing, the median was calculated with 3

seasons of data. If less than 3 seasons were used to derive the median, the entry is

flagged (z).

5. A 25th percentile for a season is best derived with data from a minimum of 4

lakes/season. However, this table provides 25th percentiles that were derived with

less than 4 lakes/season in order to retain all information for all seasons. In

calculating the 25th percentile for a season with less than 4 lake medians, the

statistical program automatically used the minimum value within the less-than-4

population. If less than 4 lakes were used in developing a seasonal quartile and or

all-seasons median, the entry is flagged (zz).

Preferred data choices and recommendations when data is missing

1. Where data are missing or are very low in total records for a given parameter, use 25th

percentiles for parameters within an adjacent, similar subecoregion within the same aggregate

nutrient ecoregion or when a similar subecoregion can not be determined, use the the 25th

percentile for the Aggregate ecoregion or consider the lowest 25th percentile from a subecoregion

(level III) within the aggregate nutrient ecoregion. The rationale being that without data, one may

assume that the subecoregion in question may be as sensitive as the most sensitive subecoregion

within the aggregate.

2. TN calculated: When reported Total Nitrogen (TN) median values are lacking or very low in

comparison to TKN and Nitrate/Nitrite-N values, the medians for TKN and nitrite/nitrate-N were

added, resulting in a calculated TN value. The number of samples (N) for calculated TN is not
19

filled in since it is represented by two subsamples of data: TKN and nitrite/nitrate-N. Therefore,

N/A is placed in this box. 

3. TN reported: This is the median based on reported values for TN from the database.

4. Chlorophyll a: Medians based on all methods are reported, however, the acid corrected medians

are preferred to the uncorrected medians. In developing a reference condition from a particular

method, it is recommended that the method with the most observations be used. Fluorometric and

Spectrophotometric are preferred over all other methods. However, when no 

data exist for Fluorometric and Spectrophotometric methods, Trichromatic values may be used.

Data from the variance techniques are not interchangeable.

5. Periphyton: Where periphyton data exist, record them separately For periphyton-dominated

streams, a measure of periphyton chlorophyll is a more appropriate response variable than

planktonic chlorophyll a. See Table 4, p. 101 of the Rivers and Streams Nutrient Technical

Guidance Manual (U. S. EPA, 2000b) for values of periphyton and planktonic chlorophyll a

related to eutrophy in streams.

6. Secchi depth: The 75th percentile is reported for Secchi depth since this is the only variable for

which the value of the parameter increases with greater clarity. (For lakes and reservoirs only.)

7. Turbidity units: All turbidity units from all methods are reported. FTUs and NTUs are

preferred over JCUs. If FTUs and NTUs do not exist, use JCUs. These units are not

interchangeable. Turbidity is chosen as a response variable in streams since it can be an indicator

of increasing algal biomass due to nutrient enrichment. See pages 32 -33 of the Rivers and

Streams Nutrient Technical Guidance Manual for a discussion of turbidity and correlations with

algal growth.

8. Lack of data: A dash (-) represents missing, inadequate, or inconclusive data. A zero (0) is

reported if the reported median for a parameter is 0 or if the component value is below detection. 

4.6. Classification of Lake/Reservoir Type

It is anticipated that assessing the data by lake type will further reduce the variability in the data

analysis. There was no readily available classification data in the National datasets used to develop these

criteria to make this assessment for this ecoregion and set of subecoregions, but it is strongly encouraged

that this assessment be made before a State or Tribe develops a final criterion.

4.7. Summary of Data Reduction Methods

All descriptive statistics were calculated using the medians for each lake within ecoregion

VI, for which data existed. For example, if one lake had 300 observations for phosphorus over the

decade or one year’s time, one median resulted. Each median from each lake was then used in

calculating the percentiles for phosphorus for the aggregate nutrient ecoregion/subecoregion (level

III ecoregion) by season and year (Figure 4a & b).
20

Rainy Lake Median

Fish Reservoir Median

Swan Lake Median

Moon Lake ...

Timber ...

Rainy Lake Median

Fish Reservoir Median

Swan Lake Median

Moon Lake ...

Timber ...

Rainy Lake Median

Fish Reservoir Median

Swan Lake Median

Moon Lake ...

Timber ...

Rainy Lake Median

Fish Reservoir Median

Swan Lake Median

Moon Lake ...

Timber ...

Figure 4a. Illustration of data reduction process for lake data.

Pine

Lake

Data

Ashley

Lake

Data

Clear

Lake

Data

Swan

Lake

Data

Moon

Lake

Data

Fish

Reservoir

Data

Green

Reservoir

Data

Marsh

Lake

Data

Bear

Reservoir

Data

Sandy

Reservoir

Data

Timber

Lake

Data

Ecoregion

Observations for All Lakes/Reservoirs

Winter Spring Summer Fall

Data Reduced

to

Median Value

for each

Lake/Reservoir

by

Season
21

Select 25th Percentile

from Distribution

of Median

Values

25th 25% 25% 25%

Winter Spring Summer Fall

Calculate Median

Value of the

25th Percentiles

for the Four Seasons

Half values

Below Median

Half values

Above Median

25%

Season D

25%

Season A

25%

Season B

25%

Season C

Median = Reference Condition for the Ecoregion

TP

TN

TKN

NO2+NO3

DO

SECCHI

} 25%

} 75%

Figure 4b. Illustration of reference condition calculation.
22

5.0 REFERENCE SITES AND CONDITIONS IN AGGREGATE ECOREGION VI

Reference conditions represent the natural, least impacted conditions or what is considered

to be the most attainable conditions. This section compares the different reference conditions

determined from the two methods and establishes which reference condition is most appropriate.

A priori determination of reference sites. The preferred method for establishing reference

condition is to choose the upper percentile of an a priori population of reference lakes. States and

Tribes are encouraged to identify reference conditions based on this method.

Statistical determination of reference conditions (25th percentile of entire database.) See Tables 2

and 3a-f in section 4.0. 

RTAG discussion and rationale for selection of reference sites and conditions in Ecoregion VI.

The RTAG should compare the results derived from the two methods described above and present

a rationale for the final selection of reference sites. 

6.0 MODELS USED TO PREDICT OR VERIFY RESPONSE PARAMETERS

The RTAG is encouraged to identify and apply relevant models to support nutrient criteria

development. The following are three scenarios under which models may be used to derive criteria

or support criteria development.

C Models for predicting correlations between causal and response variables

C Models used to verify reference conditions based on percentiles

C Regression models used to predict reference conditions in impacted areas

7.0 FRAMEWORK FOR REFINING RECOMMENDED NUTRIENT CRITERIA FOR

LAKES AND RESERVOIRS IN AGGREGATE ECOREGION VI

Information on each of the following six weight of evidence factors is important to refine

the criteria presented in this document. All elements should be addressed in developing criteria, as

is expressed in our nutrient criteria technical guidance manuals. It is our expectation that EPA

Regions, States, and Tribes (as RTAGs) will consider these elements as States/Tribes develop their

criteria. This section should be viewed as a work sheet (sections are left blank for this purpose) to

assist in the refinement of nutrient criteria. If many of these elements are ultimately unaddressed,

EPA may rely on the proposed reference conditions presented in Tables 3a-f and other literature

and information readily available to the HQ nutrient team to develop nutrient water quality

recommendations for this ecoregion.
23

7.1 Example Worksheet for Developing Aggregate Ecoregion and Subecoregion Nutrient

Criteria

C Literature sources

C Historical data and trends

C Reference condition

C Models

C RTAG expert review and consensus

C Downstream effects
24

7.2 Tables of Refined Nutrient Water Quality Criteria for Aggregate Ecoregion VI and

Level III Subecoregions for TP, TN, Chl a, Turbidity (where sufficient data exist)

Aggregate Ecoregion VI- Corn Belt and N.

Great Plains

Proposed Criterion

Total Phosphorus (μg/L)

Total Nitrogen (mg/L)

Chlorophyll a (μg/L or mg/m2)

Secchi depth (meters)

Other (Index; other parameter such as DO)

C Literature sources

C Historical data and trends

C Reference condition

C Models

C RTAG expert review and consensus
25

C Downstream effects

Ecoregion #46-Northern Glaciated Plains Proposed Criterion

Total Phosphorus (μg/L)

Total Nitrogen (mg/L)

Chlorophyll a (μg/L or mg/m2)

Secchi depth (meters)

Other (Index; other parameter such as DO)

7.3 Setting Seasonal Criteria

The criteria presented in this document are based in part on medians of all the 25th

percentile seasonal data (decadal), and as such are reflective of all seasons and not one particular

season or year. It is recommended that States and Tribes monitor in all seasons to best assess

compliance with the resulting criterion. States/Tribes may choose to develop criteria which reflect

each particular season or a given year when there is significant variability between seasons/years

or designated uses that are specifically tied to one or more seasons of the year (e.g., recreation,

fishing). Using the tables in Appendix A and B, one can set reference conditions based on a

particular season or year and then develop a criterion based on each individual season. Obviously,

this option is season-specific and would also require increased monitoring within each season to

assess compliance. 

7.4 When Data/Reference Conditions are Lacking

When data are unavailable to develop a reference condition for a particular parameter(s)

within a subecoregion, EPA recommends one of three options: 1. Use data from a similar

neighboring subecoregion. E.g., If data are few or nonexistent for the northern cascades, consider

using the data and reference condition developed for the cascades; or 2. Use the 25th perecentiles

for the Aggregate ecoregion or 3. Consider using the lowest of the yearly medians for that

parameter calculated for all the subecoregions within the Aggregate Ecoregion.
26

7.5 Site-specific Criteria Development 

Criteria may be refined in a number of ways. The best way to refine criteria is to follow the

critical elements of criteria development as well as to refer to the Lakes and Reservoirs technical

guidance manual (U. S. EPA, 2000a).

The Lakes and Reservoirs Nutrient Criteria Technical Guidance Manual presents sections

on each of the following factors to consider in setting criteria

- refinements to ecoregions (Chapter 3)

- classification of waterbodies (Chapter 3)

- setting seasonal criteria to reflect major seasonal climate differences (Chapter 7)

- accounting for significant or cyclical rainfall events - high flow/low flow conditions (Chapter 7)

- setting criteria for reservoirs only (The technical guidance manual recommends that data be

separated for lakes and reservoirs and treated independently if possible because of differing

physical conditions that occur in lakes and reservoirs. In this document all data from both

reservoirs and lakes were considered together since STORET does not allow for the

differentiation of data except by waterbody name.) 

8.0 LITERATURE CITED

NYSDEC (New York State Department of Environment and Conservation). 2000. 

Memorandum from Scott Kishbaugh to Jay Bloomfield, September 26, 2000, regarding

reference lakes for nutrient criteria.

TNDEC (Tennessee Department of Environment and Conservation). 2000. Letter to

Geoff Grubbs, October 5, 2000, containing comments on draft nutrient criteria

recommendations.

U.S. EPA. 2000a. Nutrient Criteria Technical Guidance Manual: Lakes and Reservoirs,

U.S. Environmental Protection Agency, Washington, DC. EPA-822-B00-001.

U.S. EPA. 2000b. Nutrient Criteria Technical Guidance Manual: Rivers and Streams,

U.S. Environmental Protection Agency, Washington, DC. EPA-822-B00-002.
27

9.0 APPENDICES

A. Descriptive Statistics Data Tables for Aggregate Ecoregion

B. Descriptive Statistics Data Tables for Level III Subecoregions within Aggregate Ecoregion 

C. Quality Control/Quality Assurance Rules
APPENDIX A

Descriptive Statistics Data Tables for Aggregate Ecoregion
 

 Aggregate Nutrient Ecoregion: VI 

1

 Lakes and Reservoirs

 Descriptive Statistics by Decade and Season

 Parameter Chla_Fluo_ug_L_Median

 SEASON N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P95

 FALL 1 15.3 15.3 15.25 . . . 15.3 15.3 15.3 15.3 15.3

 SPRING 1 20.5 20.5 20.50 . . . 20.5 20.5 20.5 20.5 20.5

 SUMMER 1 18.8 18.8 18.75 . . . 18.8 18.8 18.8 18.8 18.8

 WINTER 0 . . . . . . . . . . .

 Aggregate Nutrient Ecoregion: VI 

2

 Lakes and Reservoirs

 Descriptive Statistics by Decade and Season

 Parameter Chla_Phyto_Spec_A_ug_L_Median

 SEASON N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P95

 FALL 170 32.3 .390 152.50 29.3 2.24 91 3.62 11.3 23.5 42.7 96.3

 SPRING 136 23.5 .250 142.40 26.3 2.26 112 1.76 5.85 13.1 33.7 80.1

 SUMMER 224 32.9 .730 155.56 28.8 1.93 88 3.74 13.1 23.3 44.7 94.3

 WINTER 28 15.7 .250 68.00 15.3 2.88 97 0.89 4.01 13.1 23.0 43.8

 Aggregate Nutrient Ecoregion: VI 

3

 Lakes and Reservoirs

 Descriptive Statistics by Decade and Season

 Parameter Chla_Tric_U_ug_L_Median

 SEASON N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P95

 FALL 132 33.5 .050 133.00 30.8 2.68 92 3.15 10.9 23.1 46.7 103

 SPRING 102 23.6 .440 137.00 26.8 2.65 113 2.18 6.36 14.7 33.5 72.1

 SUMMER 164 34.5 .880 136.94 30.2 2.36 88 3.80 11.2 24.6 48.1 98.8

 WINTER 19 10.7 1.29 29.19 8.20 1.88 77 1.29 2.81 7.81 17.9 29.2

 Aggregate Nutrient Ecoregion: VI 

4

 Lakes and Reservoirs

 Descriptive Statistics by Decade and Season

 Parameter DO_mg_L_Median

 SEASON N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P95

 FALL 95 9.30 4.45 14.40 2.09 0.21 22 5.60 7.95 9.35 10.4 13.2

 SPRING 93 10.0 4.80 13.90 1.73 0.18 17 7.30 9.20 9.90 11.0 13.4

 
 

 SUMMER 138 7.86 .750 13.40 2.00 0.17 25 4.80 6.55 7.88 9.00 11.3

 WINTER 83 8.28 .450 14.30 3.87 0.43 47 0.85 5.40 9.85 11.1 13.0

 Aggregate Nutrient Ecoregion: VI 

5

 Lakes and Reservoirs

 Descriptive Statistics by Decade and Season

 Parameter NO2_NO3_mg_L_Median

 SEASON N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P95

 FALL 285 0.25 .000 9.13 0.77 0.05 306 0.00 0.01 0.03 0.12 1.55

 SPRING 267 1.46 .000 9.90 2.18 0.13 149 0.00 0.03 0.32 2.10 6.48

 SUMMER 368 0.67 .000 10.04 1.36 0.07 202 0.00 0.01 0.03 0.60 3.80

 WINTER 97 0.54 .000 8.20 1.31 0.13 242 0.00 0.07 0.18 0.34 4.45

 Aggregate Nutrient Ecoregion: VI 

6

 Lakes and Reservoirs

 Descriptive Statistics by Decade and Season

 Parameter SECCHI_m_Median

 SEASON N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P95

 FALL 323 0.91 .076 5.21 0.68 0.04 75 0.25 0.46 0.71 1.11 2.29

 SPRING 304 1.18 .076 4.88 0.92 0.05 78 0.30 0.53 0.90 1.52 3.08

 SUMMER 387 0.98 .102 4.75 0.73 0.04 74 0.28 0.51 0.79 1.19 2.48

 WINTER 116 1.29 .203 5.18 0.99 0.09 76 0.30 0.61 0.96 1.65 3.66

 Aggregate Nutrient Ecoregion: VI 

7

 Lakes and Reservoirs

 Descriptive Statistics by Decade and Season

 Parameter TKN_mg_L_Median

 SEASON N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P95

 FALL 236 1.20 .119 3.76 0.71 0.05 59 0.27 0.70 1.06 1.60 2.71

 SPRING 215 1.07 .190 3.67 0.57 0.04 54 0.30 0.68 0.99 1.30 2.08

 SUMMER 313 1.30 .025 4.95 0.70 0.04 54 0.45 0.84 1.13 1.70 2.63

 WINTER 113 1.62 .070 5.15 1.14 0.11 70 0.24 0.91 1.20 2.10 3.71

 Aggregate Nutrient Ecoregion: VI 

8

 Lakes and Reservoirs

 Descriptive Statistics by Decade and Season

 Parameter TN_mg_L_Median

 SEASON N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P95

 
 

 FALL 2 2.06 1.63 2.50 0.62 0.44 30 1.63 1.63 2.06 2.50 2.50

 SPRING 1 1.74 1.74 1.74 . . . 1.74 1.74 1.74 1.74 1.74

 SUMMER 3 1.79 1.06 2.39 0.67 0.39 38 1.06 1.06 1.92 2.39 2.39

 WINTER 3 3.09 2.44 3.62 0.60 0.35 19 2.44 2.44 3.22 3.62 3.62

 Aggregate Nutrient Ecoregion: VI 

9

 Lakes and Reservoirs

 Descriptive Statistics by Decade and Season

 Parameter TP_ug_L_Median

 SEASON N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P95

 FALL 318 115 5.00 970.00 135 7.59 118 10.0 35.0 75.0 145 323

 SPRING 291 87.2 .000 775.00 97.4 5.71 112 9.50 25.0 55.0 110 270

 SUMMER 393 129 2.50 1030.00 144 7.27 111 10.0 40.0 80.0 168 395

 WINTER 126 184 2.50 1850.00 288 25.7 157 10.0 40.0 87.5 225 725

 
APPENDIX B

Descriptive Statistics Data Tables for Level III Subecoregions within Aggregate Ecoregion
 

 Aggregate Nutrient Ecoregion: VI 1

 Lakes and Reservoirs

 Descriptive Statistics by Decade and Season

 Parameter Chla_Fluo_ug_L_Median

 Eco_

 Level_

 III SEASON N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P95

 46 FALL 0 . . . . . . . . . . .

 46 SPRING 0 . . . . . . . . . . .

 46 SUMMER 0 . . . . . . . . . . .

 46 WINTER 0 . . . . . . . . . . .

 47 FALL 1 15.3 15.3 15.25 . . . 15.3 15.3 15.3 15.3 15.3

 47 SPRING 1 20.5 20.5 20.50 . . . 20.5 20.5 20.5 20.5 20.5

 47 SUMMER 1 18.8 18.8 18.75 . . . 18.8 18.8 18.8 18.8 18.8

 47 WINTER 0 . . . . . . . . . . .

 48 FALL 0 . . . . . . . . . . .

 48 SPRING 0 . . . . . . . . . . .

 48 SUMMER 0 . . . . . . . . . . .

 48 WINTER 0 . . . . . . . . . . .

 54 FALL 0 . . . . . . . . . . .

 54 SPRING 0 . . . . . . . . . . .

 54 SUMMER 0 . . . . . . . . . . .

 54 WINTER 0 . . . . . . . . . . .

 55 FALL 0 . . . . . . . . . . .

 55 SPRING 0 . . . . . . . . . . .

 55 SUMMER 0 . . . . . . . . . . .

 55 WINTER 0 . . . . . . . . . . .

 57 FALL 0 . . . . . . . . . . .

 57 SPRING 0 . . . . . . . . . . .

 57 SUMMER 0 . . . . . . . . . . .

 Aggregate Nutrient Ecoregion: VI 2

 Lakes and Reservoirs

 Descriptive Statistics by Decade and Season

 Parameter Chla_Phyto_Spec_A_ug_L_Median

 Eco_

 Level_

 III SEASON N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P95

 46 FALL 13 33.1 4.13 141.50 38.1 10.6 115 4.13 10.0 19.4 38.7 142

 46 SPRING 9 5.31 1.98 11.00 3.50 1.17 66 1.98 3.00 3.54 8.80 11.0

 46 SUMMER 24 23.1 3.71 70.25 16.9 3.45 73 5.50 12.5 21.3 26.0 69.0

 46 WINTER 2 1.28 .995 1.57 0.41 0.29 32 1.00 1.00 1.28 1.57 1.57

 47 FALL 35 39.1 9.25 152.50 28.7 4.85 73 11.6 21.2 33.0 45.3 96.3

 47 SPRING 39 32.4 1.76 129.50 33.1 5.30 102 1.92 8.52 19.2 43.0 120

 47 SUMMER 57 34.9 2.50 129.50 29.4 3.89 84 3.50 14.8 24.1 44.2 113

 47 WINTER 9 28.5 4.00 68.00 19.3 6.45 68 4.00 14.4 27.6 32.0 68.0

 48 FALL 2 13.0 10.5 15.50 3.54 2.50 27 10.5 10.5 13.0 15.5 15.5

 48 SPRING 2 3.63 .250 7.00 4.77 3.38 132 0.25 0.25 3.63 7.00 7.00

 48 SUMMER 3 11.0 4.00 22.85 10.4 5.98 95 4.00 4.00 6.00 22.9 22.9

 
 

 48 WINTER 0 . . . . . . . . . . .

 54 FALL 114 31.3 .390 126.83 28.9 2.71 92 3.56 9.63 21.5 44.4 101

 54 SPRING 80 22.7 .410 142.40 23.4 2.62 103 1.61 6.08 15.1 33.7 70.1

 54 SUMMER 133 35.4 .730 155.56 30.4 2.64 86 3.74 12.7 25.4 49.0 95.5

 54 WINTER 16 11.2 .890 24.03 7.43 1.86 66 0.89 4.68 10.4 17.3 24.0

 55 FALL 6 15.6 1.67 31.93 10.5 4.30 67 1.67 10.8 13.2 23.0 31.9

 55 SPRING 6 10.7 4.75 26.08 8.21 3.35 76 4.75 5.65 7.03 13.9 26.1

 55 SUMMER 7 11.9 3.82 17.25 5.49 2.07 46 3.82 4.89 13.4 16.2 17.2

 55 WINTER 1 0.25 .250 0.25 . . . 0.25 0.25 0.25 0.25 0.25

 57 FALL 0 . . . . . . . . . . .

 57 SPRING 0 . . . . . . . . . . .

 57 SUMMER 0 . . . . . . . . . . .

 Aggregate Nutrient Ecoregion: VI 3

 Lakes and Reservoirs

 Descriptive Statistics by Decade and Season

 Parameter Chla_Tric_U_ug_L_Median

 Eco_

 Level_

 III SEASON N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P95

 46 FALL 1 20.4 20.4 20.40 . . . 20.4 20.4 20.4 20.4 20.4

 46 SPRING 1 5.33 5.33 5.33 . . . 5.33 5.33 5.33 5.33 5.33

 46 SUMMER 1 19.0 19.0 19.00 . . . 19.0 19.0 19.0 19.0 19.0

 46 WINTER 0 . . . . . . . . . . .

 47 FALL 11 62.5 12.0 133.00 46.8 14.1 75 12.0 20.0 43.6 112 133

 47 SPRING 13 37.8 3.00 137.00 42.1 11.7 112 3.00 9.00 22.0 43.3 137

 47 SUMMER 16 49.6 5.00 123.00 38.7 9.67 78 5.00 19.0 36.5 82.0 123

 47 WINTER 2 11.9 2.81 20.90 12.8 9.05 108 2.81 2.81 11.9 20.9 20.9

 48 FALL 0 . . . . . . . . . . .

 48 SPRING 0 . . . . . . . . . . .

 48 SUMMER 0 . . . . . . . . . . .

 48 WINTER 0 . . . . . . . . . . .

 54 FALL 114 31.6 .050 115.28 28.3 2.65 90 3.15 8.86 23.1 45.5 96.9

 54 SPRING 80 22.9 .440 136.55 24.2 2.70 106 1.97 6.41 14.7 33.1 71.4

 54 SUMMER 133 35.1 .880 136.94 29.5 2.55 84 3.46 12.9 25.1 48.1 88.8

 54 WINTER 16 11.1 1.64 29.19 7.97 1.99 72 1.64 3.90 10.2 16.9 29.2

 55 FALL 6 19.4 2.30 38.85 13.1 5.35 67 2.30 13.1 15.9 30.4 38.8

 55 SPRING 8 10.2 1.50 30.27 9.42 3.33 92 1.50 4.87 6.94 13.1 30.3

 55 SUMMER 14 12.4 1.00 27.50 7.53 2.01 61 1.00 6.02 11.5 18.8 27.5

 55 WINTER 1 1.29 1.29 1.29 . . . 1.29 1.29 1.29 1.29 1.29

 57 FALL 0 . . . . . . . . . . .

 57 SPRING 0 . . . . . . . . . . .

 57 SUMMER 0 . . . . . . . . . . .

 
 

 Aggregate Nutrient Ecoregion: VI 4

 Lakes and Reservoirs

 Descriptive Statistics by Decade and Season

 Parameter DO_mg_L_Median

 Eco_

 Level_

 III SEASON N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P95

 46 FALL 36 9.67 4.80 13.60 1.65 0.27 17 6.75 8.80 9.64 10.4 12.5

 46 SPRING 29 9.72 8.50 11.80 0.89 0.16 9 8.50 9.20 9.50 10.0 11.6

 46 SUMMER 59 7.47 .750 10.60 1.74 0.23 23 4.70 6.40 7.80 8.55 10.0

 46 WINTER 44 7.22 .700 14.30 4.12 0.62 57 0.85 3.20 8.15 10.9 12.2

 47 FALL 52 9.02 4.45 14.40 2.40 0.33 27 5.30 7.50 8.95 10.0 13.4

 47 SPRING 59 10.2 4.80 13.90 2.05 0.27 20 5.15 9.25 10.3 11.6 13.8

 47 SUMMER 73 8.30 1.58 13.40 2.12 0.25 26 5.05 7.00 8.18 9.60 12.2

 47 WINTER 32 9.40 .450 13.00 3.10 0.55 33 0.80 9.05 10.3 11.2 13.0

 48 FALL 7 9.48 7.30 10.75 1.46 0.55 15 7.30 7.60 9.80 10.7 10.8

 48 SPRING 5 9.49 8.50 10.90 0.99 0.44 10 8.50 8.70 9.30 10.1 10.9

 48 SUMMER 6 6.40 4.80 9.05 1.50 0.61 23 4.80 5.20 6.28 6.80 9.05

 48 WINTER 7 9.86 2.60 13.65 3.97 1.50 40 2.60 7.40 10.6 13.2 13.7

 54 FALL 0 . . . . . . . . . . .

 54 SPRING 0 . . . . . . . . . . .

 54 SUMMER 0 . . . . . . . . . . .

 54 WINTER 0 . . . . . . . . . . .

 55 FALL 0 . . . . . . . . . . .

 55 SPRING 0 . . . . . . . . . . .

 55 SUMMER 0 . . . . . . . . . . .

 55 WINTER 0 . . . . . . . . . . .

 57 FALL 0 . . . . . . . . . . .

 57 SPRING 0 . . . . . . . . . . .

 57 SUMMER 0 . . . . . . . . . . .

 Aggregate Nutrient Ecoregion: VI 5

 Lakes and Reservoirs

 Descriptive Statistics by Decade and Season

 Parameter NO2_NO3_mg_L_Median

 Eco_

 Level_

 III SEASON N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P95

 46 FALL 24 0.07 .002 0.31 0.07 0.01 100 0.00 0.02 0.05 0.10 0.14

 46 SPRING 19 0.10 .008 0.73 0.17 0.04 165 0.01 0.02 0.03 0.10 0.73

 46 SUMMER 58 0.12 .002 0.75 0.22 0.03 180 0.00 0.01 0.02 0.10 0.70

 46 WINTER 25 0.15 .003 0.55 0.13 0.03 84 0.01 0.09 0.11 0.23 0.41

 47 FALL 62 0.18 .000 4.00 0.60 0.08 329 0.00 0.00 0.01 0.08 0.62

 47 SPRING 67 1.02 .000 7.20 1.84 0.22 179 0.00 0.01 0.25 0.88 5.96

 47 SUMMER 79 0.68 .000 6.98 1.44 0.16 211 0.00 0.00 0.06 0.44 4.95

 47 WINTER 39 0.35 .000 4.55 0.76 0.12 218 0.00 0.04 0.12 0.25 1.47

 48 FALL 8 0.02 .003 0.05 0.02 0.01 91 0.00 0.00 0.02 0.03 0.05

 48 SPRING 3 0.02 .006 0.04 0.02 0.01 116 0.01 0.01 0.01 0.04 0.04

 
 

 48 SUMMER 8 0.02 .005 0.06 0.02 0.01 120 0.01 0.01 0.01 0.01 0.06

 48 WINTER 4 0.19 .070 0.38 0.14 0.07 74 0.07 0.09 0.15 0.28 0.38

 54 FALL 151 0.15 .003 3.70 0.43 0.04 281 0.00 0.00 0.03 0.10 1.00

 54 SPRING 130 1.57 .003 9.90 2.31 0.20 147 0.00 0.03 0.38 2.10 7.40

 54 SUMMER 167 0.62 .003 7.50 1.28 0.10 207 0.00 0.00 0.03 0.50 3.80

 54 WINTER 27 1.02 .003 8.20 1.92 0.37 189 0.03 0.10 0.27 0.90 5.18

 55 FALL 36 0.51 .010 2.46 0.67 0.11 131 0.03 0.03 0.24 0.75 2.14

 55 SPRING 38 2.28 .025 6.91 2.21 0.36 97 0.03 0.49 1.29 3.86 6.24

 55 SUMMER 47 1.27 .025 5.88 1.42 0.21 112 0.03 0.11 0.70 1.92 4.53

 55 WINTER 2 3.46 .400 6.52 4.33 3.06 125 0.40 0.40 3.46 6.52 6.52

 57 FALL 4 4.18 1.86 9.13 3.37 1.68 81 1.86 2.06 2.86 6.30 9.13

 57 SPRING 10 2.90 .025 7.98 2.75 0.87 95 0.03 0.54 2.37 4.40 7.98

 57 SUMMER 9 2.51 .025 10.04 3.11 1.04 124 0.03 0.03 2.33 2.94 10.0

 Aggregate Nutrient Ecoregion: VI 6

 Lakes and Reservoirs

 Descriptive Statistics by Decade and Season

 Parameter SECCHI_m_Median

 Eco_

 Level_

 III SEASON N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P95

 46 FALL 43 0.83 .191 2.56 0.53 0.08 64 0.27 0.45 0.75 1.12 1.83

 46 SPRING 36 1.16 .216 3.05 0.75 0.13 65 0.30 0.63 0.92 1.68 2.90

 46 SUMMER 55 0.96 .250 3.00 0.53 0.07 56 0.33 0.56 0.86 1.24 1.79

 46 WINTER 21 1.47 .203 3.66 1.05 0.23 71 0.23 0.76 1.08 2.50 3.56

 47 FALL 92 0.69 .150 3.35 0.46 0.05 66 0.20 0.36 0.62 0.90 1.50

 47 SPRING 92 1.18 .200 4.88 1.02 0.11 87 0.30 0.50 0.79 1.50 3.23

 47 SUMMER 108 0.80 .127 3.51 0.56 0.05 71 0.25 0.40 0.66 1.07 1.80

 47 WINTER 54 1.27 .203 5.18 1.02 0.14 80 0.30 0.63 1.04 1.40 3.96

 48 FALL 6 1.38 .610 2.52 0.73 0.30 53 0.61 0.84 1.18 1.97 2.52

 48 SPRING 4 2.20 .505 3.81 1.37 0.68 62 0.51 1.24 2.24 3.16 3.81

 48 SUMMER 6 1.75 .610 2.98 0.78 0.32 44 0.61 1.37 1.77 1.98 2.98

 48 WINTER 1 0.71 .711 0.71 . . . 0.71 0.71 0.71 0.71 0.71

 54 FALL 171 1.04 .076 5.21 0.79 0.06 76 0.31 0.51 0.84 1.23 2.64

 54 SPRING 161 1.19 .076 4.12 0.90 0.07 76 0.30 0.57 0.91 1.50 3.31

 54 SUMMER 197 1.09 .152 4.75 0.83 0.06 76 0.31 0.55 0.81 1.37 2.62

 54 WINTER 39 1.26 .280 3.96 0.94 0.15 74 0.31 0.51 0.91 1.74 3.70

 55 FALL 11 0.63 .305 1.05 0.29 0.09 46 0.31 0.41 0.50 0.91 1.05

 55 SPRING 11 0.80 .152 1.65 0.41 0.12 51 0.15 0.46 0.80 1.04 1.65

 55 SUMMER 20 0.63 .102 1.07 0.27 0.06 43 0.12 0.43 0.67 0.83 1.00

 55 WINTER 1 0.74 .737 0.74 . . . 0.74 0.74 0.74 0.74 0.74

 57 FALL 0 . . . . . . . . . . .

 57 SPRING 0 . . . . . . . . . . .

 57 SUMMER 1 2.60 2.60 2.60 . . . 2.60 2.60 2.60 2.60 2.60

 
 

 Aggregate Nutrient Ecoregion: VI 7

 Lakes and Reservoirs

 Descriptive Statistics by Decade and Season

 Parameter TKN_mg_L_Median

 Eco_

 Level_

 III SEASON N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P95

 46 FALL 37 1.98 .380 3.76 0.75 0.12 38 0.90 1.47 1.85 2.58 3.50

 46 SPRING 25 1.85 1.14 3.67 0.65 0.13 35 1.16 1.35 1.81 2.12 2.90

 46 SUMMER 66 1.82 .585 4.03 0.69 0.09 38 0.92 1.35 1.70 2.28 3.04

 46 WINTER 42 2.52 .800 5.15 1.11 0.17 44 1.06 1.53 2.25 3.45 4.39

 47 FALL 66 1.41 .119 3.15 0.63 0.08 45 0.74 1.00 1.25 1.77 2.84

 47 SPRING 67 1.23 .200 3.50 0.54 0.07 44 0.72 0.92 1.13 1.38 2.08

 47 SUMMER 81 1.49 .163 4.95 0.69 0.08 46 0.80 1.01 1.30 1.89 2.48

 47 WINTER 43 1.25 .138 5.00 0.84 0.13 67 0.55 0.75 1.03 1.54 2.60

 48 FALL 8 1.01 .350 1.77 0.63 0.22 63 0.35 0.40 0.97 1.60 1.77

 48 SPRING 3 1.13 .830 1.50 0.34 0.20 30 0.83 0.83 1.07 1.50 1.50

 48 SUMMER 8 1.50 .875 2.54 0.52 0.19 35 0.88 1.13 1.44 1.74 2.54

 48 WINTER 8 0.69 .070 1.76 0.66 0.23 96 0.07 0.12 0.50 1.22 1.76

 54 FALL 85 0.95 .170 2.70 0.51 0.05 53 0.24 0.60 0.90 1.15 1.70

 54 SPRING 72 0.90 .190 1.90 0.38 0.04 42 0.27 0.65 0.85 1.18 1.55

 54 SUMMER 102 1.04 .025 3.15 0.55 0.05 53 0.40 0.70 0.91 1.19 2.05

 54 WINTER 19 0.88 .200 1.70 0.44 0.10 50 0.20 0.50 0.92 1.20 1.70

 55 FALL 36 0.76 .175 2.15 0.45 0.07 59 0.25 0.48 0.65 0.88 1.75

 55 SPRING 38 0.68 .200 1.65 0.34 0.05 49 0.24 0.41 0.65 0.85 1.55

 55 SUMMER 47 0.87 .100 2.00 0.46 0.07 52 0.30 0.58 0.80 1.10 1.75

 55 WINTER 1 0.92 .920 0.92 . . . 0.92 0.92 0.92 0.92 0.92

 57 FALL 4 0.53 .300 0.90 0.26 0.13 50 0.30 0.35 0.45 0.70 0.90

 57 SPRING 10 0.66 .400 1.05 0.22 0.07 33 0.40 0.50 0.63 0.85 1.05

 57 SUMMER 9 0.87 .400 1.90 0.46 0.15 53 0.40 0.54 0.75 1.00 1.90

 Aggregate Nutrient Ecoregion: VI 8

 Lakes and Reservoirs

 Descriptive Statistics by Decade and Season

 Parameter TN_mg_L_Median

 Eco_

 Level_

 III SEASON N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P95

 46 FALL 2 2.06 1.63 2.50 0.62 0.44 30 1.63 1.63 2.06 2.50 2.50

 46 SPRING 1 1.74 1.74 1.74 . . . 1.74 1.74 1.74 1.74 1.74

 46 SUMMER 2 2.16 1.92 2.39 0.33 0.23 15 1.92 1.92 2.16 2.39 2.39

 46 WINTER 2 3.03 2.44 3.62 0.84 0.59 28 2.44 2.44 3.03 3.62 3.62

 47 FALL 0 . . . . . . . . . . .

 47 SPRING 0 . . . . . . . . . . .

 47 SUMMER 0 . . . . . . . . . . .

 47 WINTER 0 . . . . . . . . . . .

 48 FALL 0 . . . . . . . . . . .

 
 

 48 SPRING 0 . . . . . . . . . . .

 48 SUMMER 1 1.06 1.06 1.06 . . . 1.06 1.06 1.06 1.06 1.06

 48 WINTER 1 3.22 3.22 3.22 . . . 3.22 3.22 3.22 3.22 3.22

 54 FALL 0 . . . . . . . . . . .

 54 SPRING 0 . . . . . . . . . . .

 54 SUMMER 0 . . . . . . . . . . .

 54 WINTER 0 . . . . . . . . . . .

 55 FALL 0 . . . . . . . . . . .

 55 SPRING 0 . . . . . . . . . . .

 55 SUMMER 0 . . . . . . . . . . .

 55 WINTER 0 . . . . . . . . . . .

 57 FALL 0 . . . . . . . . . . .

 57 SPRING 0 . . . . . . . . . . .

 57 SUMMER 0 . . . . . . . . . . .

 Aggregate Nutrient Ecoregion: VI 9

 Lakes and Reservoirs

 Descriptive Statistics by Decade and Season

 Parameter TP_ug_L_Median

 Eco_

 Level_

 III SEASON N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P95

 46 FALL 43 192 20.0 910.00 170 25.9 89 25.0 90.0 170 235 460

 46 SPRING 32 184 10.0 500.00 125 22.2 68 20.6 90.0 150 270 410

 46 SUMMER 69 229 20.0 1030.00 196 23.6 86 30.0 80.0 180 290 670

 46 WINTER 45 331 2.50 1850.00 408 60.8 123 30.0 90.0 190 385 1000

 47 FALL 75 141 25.0 935.00 132 15.3 94 32.5 65.0 100 175 335

 47 SPRING 75 101 .000 775.00 99.5 11.5 99 25.0 50.0 80.0 110 263

 47 SUMMER 92 136 25.0 920.00 119 12.4 87 30.0 60.0 106 155 340

 47 WINTER 43 104 5.00 385.00 102 15.5 97 22.5 40.0 70.0 120 330

 48 FALL 10 96.0 5.00 280.00 89.7 28.4 93 5.00 50.0 57.5 150 280

 48 SPRING 5 93.5 40.0 160.00 48.7 21.8 52 40.0 52.5 100 115 160

 48 SUMMER 11 178 50.0 370.00 114 34.2 64 50.0 85.0 135 295 370

 48 WINTER 9 146 5.00 980.00 315 105 215 5.00 5.00 45.0 75.0 980

 54 FALL 151 85.3 5.00 740.00 105 8.51 123 10.0 25.0 60.0 110 285

 54 SPRING 131 58.0 6.50 572.50 75.7 6.62 131 9.50 20.0 30.0 70.0 200

 54 SUMMER 165 94.7 5.00 805.00 127 9.88 134 10.0 20.0 45.0 110 375

 54 WINTER 27 86.5 10.0 470.00 101 19.4 117 10.0 12.5 70.0 110 300

 55 FALL 35 77.8 5.00 545.00 99.2 16.8 128 5.75 20.0 60.0 90.0 260

 55 SPRING 38 88.6 2.50 420.00 93.6 15.2 106 2.50 30.0 57.5 110 280

 55 SUMMER 47 98.4 2.50 491.75 99.4 14.5 101 5.00 40.0 70.0 130 295

 55 WINTER 2 53.1 40.0 66.25 18.6 13.1 35 40.0 40.0 53.1 66.3 66.3

 57 FALL 4 290 10.0 970.00 460 230 158 10.0 10.0 90.0 570 970

 57 SPRING 10 50.8 8.00 140.00 42.3 13.4 83 8.00 10.0 40.0 70.0 140

 57 SUMMER 9 43.3 4.75 110.00 35.4 11.8 82 4.75 10.0 40.0 65.0 110

 
APPENDIX C

Quality Control/Quality Assurance Rules
Support for the Compilation and Analysis of National Nutrient Data

15 Nutrient Ecoregion/Waterbody Type Summary Chapters 

Prepared for:

Robert Cantilli

Environmental Protection Agency

OW/OST/HECD

Prepared by:

INDUS Corporation

1953 Gallows Road

Vienna, Virginia 22182

Contract Number: 68-C-99-226

Task Number: 04

Subtask Number: 4

August 8, 2000
ii

CONTENTS

1.0 BACKGROUND ........................................................ 1

1.1 Purpose .......................................................... 1

1.2 References ....................................................... 1

2.0 QA/QC PROCEDURES ................................................ 2

2.1 National Data Sets ............................................... 3

2.2 State Data ..................................................... 3

2.3 Laboratory Methods .............................................. 4

2.4 Waterbody Name ................................................ 4

2.5 Ecoregion Data ................................................. 5

3.0 STATISTICAL ANALYSIS REPORTS .................................... 5

3.1 Data Source Reports ............................................. 6

3.2 Remark Code Reports ............................................ 7

3.3 Median of Each Waterbody ........................................ 7

3.4 Descriptive Statistic Reports ........................................ 7

3.5 Regression Models ............................................... 8

4.0 TIME PERIOD ....................................................... 8

5.0 DATA SOURCES AND PARAMETERS FOR THE AGGREGATE NUTRIENT

ECOREGIONS ....................................................... 9

5.1 Lakes and Reservoirs ............................................ 9

5.1.1 Aggregate Nutrient Ecoregion 2 ............................... 9

5.1.2 Aggregate Nutrient Ecoregion 6 .............................. 10

5.1.3 Aggregate Nutrient Ecoregion 7 .............................. 10

5.1.4 Aggregate Nutrient Ecoregion 8 .............................. 11

5.1.5 Aggregate Nutrient Ecoregion 9 .............................. 12

5.1.6 Aggregate Nutrient Ecoregion 11 ............................. 12

5.1.7 Aggregate Nutrient Ecoregion 12 ............................. 13

5.1.8 Aggregate Nutrient Ecoregion 13 ............................. 13

5.2 Rivers and Streams .............................................. 14

5.2.1 Aggregate Nutrient Ecoregion 2 .............................. 14

5.2.2 Aggregate Nutrient Ecoregion 3 .............................. 15

5.2.3 Aggregate Nutrient Ecoregion 6 .............................. 16

5.2.4 Aggregate Nutrient Ecoregion 7 .............................. 16

5.2.5 Aggregate Nutrient Ecoregion 9 .............................. 17

5.2.6 Aggregate Nutrient Ecoregion 11 ............................. 18

5.2.7 Aggregate Nutrient Ecoregion 12 ............................. 19

5.2.8 Aggregate Nutrient Ecoregion 14 ............................. 20
iii

APPENDIX A Process Used to QA/QA the Legacy STORET Nutrient Data Set

APPENDIX B Process for Adding Aggregate Nutrient Ecoregions and Level III

Ecoregions

APPENDIX C Glossary
15 Nutrient Ecoregion/Waterbody Type Summary Chapters, Contract # 68-C-99-226, TO# 04 August 8, 2000

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1.0 BACKGROUND 

The Nutrient Criteria Program has initiated development of a national Nutrient Criteria Database

application that will be used to store and analyze nutrient data. The ultimate use of these data will

be to derive ecoregion- and waterbody-specific nutrient criteria ranges. EPA converted STOrage

and RETrieval (STORET) legacy data, National Stream Quality Accounting Network

(NASQAN) data, National Water-Quality Assessment (NAWQA) data, and other relevant

nutrient data from universities and States/Tribes into the database. The data imported into the

Nutrient Criteria Database will be used to develop national nutrient criteria ranges. 

1.1 Purpose

The purpose of this deliverable is to provide EPA with information regarding the data used to

create the statistical reports which will be used to derive ecoregion- and waterbody-specific

nutrient criteria ranges for Level III ecoregions. There are fourteen aggregate nutrient

ecoregions. Each aggregate nutrient ecoregion is divided into smaller ecoregions referred to as

Level III ecoregions. EPA will determine criteria ranges for the waterbody types and Level III

ecoregions within the following aggregate nutrient ecoregions:

• Lakes and Reservoirs

- Aggregate Nutrient ecoregions: 2, 6, 7, 8, 9, 11, 12, 13

• Rivers and Streams 

- Aggregate Nutrient ecoregions: 2, 3, 6, 7, 9, 11, 12, 14

1.2 References

This section lists documents that contain baselines, standards, guidelines, policies, and references

that apply to the data analysis. Listed editions were valid at the time of publication. All

documents are subject to revision, but these specific editions govern the concepts described in this

document. 

Nutrient Criteria Technical Guidance Manual: Lakes and Reservoirs (Draft). EPA, Office of

Water, EPA 822-D-99-001, April 1999.

Nutrient Criteria Technical Guidance Manual: Rivers and Streams (Draft). EPA, Office of

Water, EPA 822-D-99-003, September 1999.

Guidance for Data Quality Assessment: Practical Methods for Data Analysis. EPA, Office of

Research and Development, EPA QA/G-9, January 1998.
15 Nutrient Ecoregion/Waterbody Type Summary Chapters, Contract # 68-C-99-226, TO# 04 August 8, 2000

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2.0 QA/QC PROCEDURES

In order to develop nutrient criteria, EPA needed to obtain nutrient data from the states. EPA

requested nutrient data from the states and forwarded the data sets to INDUS via e-mail and/or

US mail. In addition, EPA tasked INDUS to convert data from three national data sets. EPA

provided INDUS with a Legacy STORET extraction to convert into the database. The United

States Geologic Survey (USGS) sent INDUS a CD-ROM with NASQAN data to convert. 

INDUS downloaded NAWQA files from the USGS Web site to convert the data. In total,

INDUS converted and imported the following national and state data sets into the Nutrient

Criteria Database:

• Legacy STORET

• NAWQA

• NASQAN

• Region 1

• Region 2 - Lake Champlain Monitoring Project

• Region 2 - NYSDEC Finger Lakes Monitoring Program

• Region 2 - NY Citizens Lake Assessment Program

• Region 2 - Lake Classification and Inventory Survey

• Region 2 - NYCDEP (1990-1998)

• Region 2 - NYCDEP (Storm Event data)

• Region 2 - New Jersey Nutrient Data ( Tidal Waters)

• Region 5

• Region 3 

• Region 3 - Nitrite Data

• Region 3 - Choptank River files

• Region 4 - Tennessee Valley Authority

• Region 7 - Central Plains Center for BioAssessment (CPCB)

• Region 7 - REMAP

• Region 2 - Delaware River Basin Commission (1990-1998)

• Region 3 - PA Lake Data

• Region 3 - University of Delaware

• Region 10 

• University of Auburn

As part of the conversion process, INDUS performed a number of Quality Assurance/Quality

Control (QA/QC) steps to ensure that the data was properly converted into the Nutrient Criteria

Database. Section 2 explains the steps performed by INDUS to convert the data.
15 Nutrient Ecoregion/Waterbody Type Summary Chapters, Contract # 68-C-99-226, TO# 04 August 8, 2000

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2.1 National Data Sets

INDUS converted three national data sets into the Nutrient Criteria Database: Legacy STORET

data, NASQAN data, and NAWQA data. A previous EPA contractor performed the extraction of

Legacy STORET data and documented the QA/QC procedures used on the data. This

documentation is included in Appendix A. INDUS performed minimal QA/QC on the Legacy

STORET data set because the previous contractor completed the steps outlined in Appendix A. 

INDUS and EPA also agreed to convert the NAWQA and NASQAN data sets with minimal

QA/QC on the assumption that the source agency, the USGS, QA/QC'd the data. 

For each of the three national data sets, INDUS ran queries to determine if 1) samples existed

without results and 2) if stations existed without samples. Per Task Order Project Officer

(TOPO) direction, these records were deleted from the system. For analysis purposes, EPA

determined that there was no need to keep station records with no samples and sample records

with no results. INDUS also confirmed that each data set contained no duplicate records. 

In addition, INDUS deleted all composite results from the Legacy STORET data. Per TOPO

direction, it was decided that composite sample results would not be used in the statistical

analysis.

2.2 State Data

Each state data set was delivered in a unique format. Many of the data sets were delivered to

INDUS without corresponding documentation. INDUS analyzed each state data set in order to

determine which parameters should be converted for analysis. INDUS obtained a master

parameter table from EPA and converted the parameters in the state data sets according to those

that were present in the EPA parameter table. INDUS converted all of the data elements in the

state data sets that mapped directly to the Nutrient Criteria Database; data elements that did not

map to the Nutrient Criteria Database were not converted. In some cases, state data elements

that did not directly map into the Oracle database were inserted into a comment field within the

database. Also, INDUS maintained an internal record of which state data elements were inserted

into the comment field. 

As part of the data clean-up efforts, INDUS determined whether or not there were any duplicate

records in the state data sets and deleted the duplicate records. INDUS checked the waterbody,

station, and sample entities for duplicate records. In addition, INDUS deleted station records

with no samples and sample records with no results. INDUS also deleted waterbody records that

were not associated with a station. In each case, INDUS maintained an internal record of how

many records were deleted. 

If INDUS encountered referential integrity errors, such as samples that referred to stations that

did not exist, or if INDUS was unsure of whether a record was a duplicate, INDUS contacted the

agency directly via e-mail or phone to resolve any issues that arose. INDUS saved an electronic
15 Nutrient Ecoregion/Waterbody Type Summary Chapters, Contract # 68-C-99-226, TO# 04 August 8, 2000

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copy of each e-mail correspondence with the states to ensure that a record of the decision was

maintained. INDUS also contacted each agency to determine which laboratory methods were

used for each parameter.

Finally, INDUS examined the remark codes of each result record in the state data sets. INDUS

mapped the remark codes to the STORET remark codes listed in Table 2 of Appendix A. If any

of the state result records were associated with remark codes marked as "Delete" in Table 2 of

Appendix A, the result records were not converted into the database.

2.3 Laboratory Methods 

Many of the state data sets did not contain laboratory method information. In addition, laboratory

method information was not available for the three national data sets. In order to determine

missing laboratory method information, EPA tasked another contractor to contact the data

owners to obtain the laboratory method. In some cases, the data owners responded and the

laboratory methods were added to the database. 

2.4 Waterbody Name and Class Information

A large percentage of the data did not have waterbody-specific information. The only waterbody

information contained in the three national data sets was the waterbody name, which was

embedded in the station 'location description' field. Most of the state data sets contained

waterbody name information; however, much of the data was duplicated throughout the data sets. 

Therefore, the waterbody information was cleaned manually. For the three national data sets, the

'location description' field was extracted from the station table and moved to a temporary table. 

The 'location description' field was sorted alphabetically. Unique waterbodies were grouped

together based on name similarity and whether or not the waterbodies fell within the same county,

state, and waterbody type. Finally, the 'location description' field was edited to include only

waterbody name information, not descriptive information. For example, 110 MILE CREEK AT

POMONA DAM OUTFLOW, KS PO-2 was edited to 110 MILE CREEK. Also, if 100 MILE

CREEK was listed ten times in New York, but in four different counties, four 100 MILE CREEK

waterbody records were created.

Similar steps were taken to eliminate duplicate waterbody records in the state data sets. If a

number of records had similar waterbody names and fell within the same state, county, and

waterbody type, the records were grouped to create a unique waterbody record.

Most of the waterbody data did not contain depth, surface area, and volume measurements. EPA

needed this information to classify waterbody types. EPA attempted to obtain waterbody class 
15 Nutrient Ecoregion/Waterbody Type Summary Chapters, Contract # 68-C-99-226, TO# 04 August 8, 2000

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information from the states. EPA sent waterbody files to the regional coordinators and requested

that certain class information be completed by each state. The state response was poor; therefore,

EPA was not able to perform statistical analysis for the waterbody types by class.

2.5 Ecoregion Data

Aggregate nutrient ecoregions and Level III ecoregions were added to the database using the

station latitude and longitude coordinates. If a station was lacking latitude and longitude

coordinates or county information, the data were not included in the statistical analysis. Appendix

B lists the steps taken to add the two ecoregion types (aggregate and Level III) to the Nutrient

Criteria Database. The ecoregion names were pulled from aggregate nutrient ecoregion and Level

III ecoregion Geographical Information System (GIS) coverages. In summary, the station latitude

and longitude coordinates were used to determine the ecoregion under the following

circumstances:

• The latitude and longitude coordinates fell within the county/state listed in the station

table.

• The county data was missing.

The county centroid was used to determine the ecoregions under the following circumstances:

• The latitude and longitude coordinates were missing, but the state/county information was

available.

• The latitude and longitude coordinates fell outside the county/state listed in the station

table. The county information was assumed to be correct; therefore, the county centroid

was used.

If the latitude and longitude coordinates fell outside the continental US county coverage file 

(i.e., the point fell in the ocean or Mexico/Canada), the nearest ecoregion was assigned to the

station.

3.0 STATISTICAL ANALYSIS REPORTS

Aggregate nutrient ecoregion tables were created by extracting all observations for a specific

aggregate nutrient ecoregion from the nutrient criteria database. Then, the data were reduced to

create tables containing only the yearly median values. To create these tables, the median value

for each waterbody was calculated using all observations for each waterbody by Level III

ecoregion, year, and season. Tables of decade median values were created from the yearly

median tables by calculating the median for each waterbody by Level III ecoregion by decade and

season.
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The Data Source and the Remark Code reports were created using all observations (all reported

values). All the other reports were created from either the yearly median tables or the decade

median tables. In other words, the descriptive statistics and regressions were run using the

median values for each waterbody and not the individual reported values.

Statistical analyses were performed under the assumption that this data set is a random sample. If

this assumption cannot be verified, the observations may or may not be valid. Values below the

1st and 99th percentile were removed from the Legacy STORET database prior to the creation of

the national database. Also, data were treated according the Legacy STORET remark codes in

Appendix A. 

The following contains a list of each report and the purpose for creating each report: 

• Data Source Created to provide a count of the amount of data and to identify the

source(s).

• Remark Codes Created to provide a description of the data. 

• Median of Each Waterbody by Year This was an intermediate step performed to obtain a

median value for each lake to be used in the yearly descriptive statistics reports and the

regression models. 

• Median of Each Waterbody by Decade This was an intermediate step performed to obtain

a median value for each lake to be used in the decade descriptive statistics.

• Descriptive Statistics Created to provide EPA with the desired statistics for setting criteria

levels.

• Regression Models Created to examine the relationships between biological and nutrient

variables. 

Note: Separate reports were created for each season.

3.1 Data Source Reports

Data source reports were presented in the following formats:

• The number and percentage of data from each data source were summarized in tables for

each aggregate nutrient ecoregion by season and waterbody type.

• The number and percentage of data from each data source were summarized in tables for

each Level III ecoregion by season and waterbody type.

The 'Frequency' represents the number of data values from a specific data source for each

parameter by data source. The 'Row Pct' represents the percentage of data from a specific data

source for each parameter. 
15 Nutrient Ecoregion/Waterbody Type Summary Chapters, Contract # 68-C-99-226, TO# 04 August 8, 2000

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3.2 Remark Code Reports

Remark code reports were presented in the following formats:

• The number and percentage of data associated with a particular remark code for each

parameter were summarized in tables by Level III ecoregion by decade and season.

• The number and percentage of data associated with a particular remark code for each

parameter were summarized in tables by Level III ecoregion by year and season.

The 'Frequency' represents the number of data values corresponding to the remark code in the

column. The 'Row Pct' represents the percentage of data that was associated with the remark

code in that row.

In the database, remark codes that were entered by the states were mapped to Legacy STORET

remark codes. Prior to the analysis, the data were treated according to these remark codes. For

example, if the remark code was 'K,' then the reported value was divided by two. Appendix A

contains a complete list of Legacy STORET remark codes.

Note: For the reports, a remark code of 'Z' indicates that no remark codes were recorded. It does

not correspond to Legacy STORET code 'Z.'

3.3 Median of Each Waterbody

To reduce the data and to ensure heavily sampled waterbodies or years were not over represented

in the analysis, median value tables (described above) were created. The yearly median tables and

decade median tables were delivered to the EPA in electronic format as csv (comma separated

value or comma delimited) files.

3.4 Descriptive Statistic Reports

The number of waterbodies, median, mean, minimum, maximum, 5th, 25th , 75th , 95th

percentiles, standard deviation, standard error, and coefficient of variation were calculated. The

tables (described above) containing the decade median values for each waterbody for each

parameter were used to create descriptive statistics reports for:

• Level III ecoregions by decade and season

• Aggregate nutrient ecoregions by decade and season

In addition, the tables containing the yearly median values for each waterbody for each parameter

were used to create descriptive statistics reports for:

• Level III ecoregions by year and season
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3.5 Regression Models

Simple linear regressions using the least squares method were performed to examine the

relationships between biological and nutrient variables in lakes and reservoirs, and rivers and

streams. Regressions were performed using the yearly median tables. Chlorophyll(s) in

micrograms per liter (ug/L), secchi in meters (m), dissolved oxygen in milligrams per liter (mg/L),

turbidity, and pH were the biological variables in these models. When there was little or no data

for chlorophyll, then pH or dissolved oxygen was substituted for chlorophyll. Secchi data were

used in the lake and reservoir models, and turbidity data were used in the river and stream models. 

The nutrient variables in these models include: total phosphorus in ug/L, total nitrogen in mg/L,

total kjeldahl nitrogen in mg/L, and nitrate and nitrite in mg/L. Regressions were also run for

total nitrogen and total phosphorus for ecoregions where both these variables were measured. 

Note: At the time of creation of this document only regressions for aggregate nutrient ecoregion 7

for lakes and reservoirs were delivered to the EPA. Regressions for the remaining aggregate

nutrient ecoregions will be delivered in August 2000.

4.0 TIME PERIOD

Data collected from January 1990 to December 1999 were used in the statistical analysis reports. 

To capture seasonal differences, the data were classified as follows:

• Aggregate nutrient ecoregions: 6, 7, and 8

 - Spring: April to May 

 - Summer: June to August 

 - Fall: September to October

 - Winter: November to March

• Aggregate nutrient ecoregions: 1, 2, 9, 10, 11, 12, and 13

 - Spring: March to May

 - Summer: June to August 

 - Fall: September to November

 - Winter: December to February 
15 Nutrient Ecoregion/Waterbody Type Summary Chapters, Contract # 68-C-99-226, TO# 04 August 8, 2000

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5.0 DATA SOURCES AND PARAMETERS FOR THE AGGREGATE NUTRIENT

ECOREGIONS

This section provides information for the nutrient aggregate ecoregions that were analyzed by

waterbody type. Each section lists the data sources for the aggregate nutrient ecoregion

including: 1) the data sources, 2) the parameters included in the analysis, and 3) the Level III

ecoregions within the aggregate nutrient ecoregions.

Note: For analysis purposes, the following parameters were combined to form Phosphorous,

Dissolved Inorganic (DIP):

Phosphorus, Dissolved Inorganic (DIP) 

Phosphorus, Dissolved (DP) 

Phosphorus, Dissolved Reactive (DRP)

Orthophosphate, dissolved, mg/L as P 

Orthophosphate (OPO4_PO4)

5.1 Lakes and Reservoirs

5.1.1 Aggregate Nutrient Ecoregion 2

Data Sources:

Legacy STORET

EPA Region 10

Parameter:

Chlorophyll A, Fluorometric, Corrected (ug/L)

Chlorophyll A, Phytoplankton, Spectrophotometric Acid (ug/L)

Chlorophyll A, Trichromatic, Uncorrected (ug/L)

Phosphorous, Dissolved Inorganic (DIP) (ug/L)

Dissolved Oxygen (DO) (mg/L)

Nitrite and Nitrate, (NO2+NO3) (mg/L)

Nitrogen, Total (TN) (mg/L)

Nitrogen, Total Kjeldahl (TKN) (mg/L)

Phosphorus, Total (TP) (ug/L)

Phosphorus, Total Reactive (ug/L)

SECCHI (m)

pH
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Level III ecoregions:

1, 2, 4, 5, 9, 11, 15, 16, 17, 19, 21, 23, 41, 77, 78

5.1.2 Aggregate Nutrient Ecoregion 6

Data Sources:

Legacy STORET

Parameters:

Chlorophyll A, Fluorometric, Corrected (ug/L)

Chlorophyll A, Phytoplankton, Spectrophotometric Acid (ug/L)

Chlorophyll A, Trichromatic, Uncorrected (ug/L)

Dissolved Oxygen (DO) (mg/L)

Nitrite and Nitrate, (NO2+NO3) (mg/L)

Nitrogen, Total (TN) (mg/L)

Nitrogen, Total Kjeldahl (TKN) (mg/L)

Phosphorus, Total (TP) (ug/L)

SECCHI (m)

Level III ecoregions:

46, 47, 48, 54, 55, 57

5.1.3 Aggregate Nutrient Ecoregion 7

Data Sources:

LCMPD

Legacy STORET

NYCDEP

EPA Region 1

Parameters:

Chlorophyll A, Fluorometric Corrected (ug/L)

Chlorophyll A, Phytoplankton, Spectrophotometric Acid (ug/L)

Chlorophyll A, Phytoplankton, Spectrophotometric, Uncorrected (ug/L)

Chlorophyll A, Trichromatic, Uncorrected (ug/L)

Phosphorous, Dissolved Inorganic (DIP) (ug/L)

Dissolved Oxygen (DO) (mg/L)
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Nitrite and Nitrate, (NO2+NO3) (mg/L)

Nitrogen, Total (TN) (mg/L)

Nitrogen, Total Kjeldahl (TKN) (mg/L)

Phosphorus, Orthophosphate, Total as P (ug/L)

Phosphorus, Total (TP) (ug/L)

SECCHI (m)

Level III ecoregions:

51, 52, 53, 56, 60, 61, 83

5.1.4 Aggregate Nutrient Ecoregion 8

Data Sources:

LCMPD

Legacy STORET

NYCDEP

NYCDEC

EPA Region 1

EPA Region 3

Parameters:

Chlorophyll A, Fluorometric, Corrected (ug/L)

Chlorophyll A, Phytoplankton, Spectrophotometric Acid (ug/L)

Chlorophyll A, Phytoplankton, Spectrophotometric, Uncorrected (ug/L)

Chlorophyll A, Trichromatic, Uncorrected (ug/L)

Chlorophyll B (ug/L)

Chlorophyll C (ug/L)

Phosphorous, Dissolved Inorganic (DIP) (ug/L)

Dissolved Oxygen (DO) (mg/L)

Nitrite and Nitrate, (NO2+NO3) (mg/L)

Nitrogen, Total (TN) (mg/L)

Nitrogen, Total Kjeldahl (TKN) (mg/L)

Phosphorus, Total (TP) (ug/L)

SECCHI (m)

Level III ecoregions:

49, 50, 58, 62, 82
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5.1.5 Aggregate Nutrient Ecoregion 9

Data Sources:

Auburn University

Legacy STORET

EPA Region 4

Parameters:

Chlorophyll A, Fluorometric, Corrected (ug/L)

Chlorophyll A, Pheophytin (ug/L)

Chlorophyll A, Phytoplankton, Spectrophotometric Acid (ug/L)

Chlorophyll A, Phytoplankton, Spectrophotometric, Uncorrected (ug/L)

Chlorophyll A, Trichromatic, Uncorrected (ug/L)

Phosphorous, Dissolved Inorganic (DIP) (ug/L)

Dissolved Oxygen (DO) (mg/L)

Nitrite and Nitrate, (NO2+NO3) (mg/L)

Nitrogen, Total (TN) (mg/L)

Nitrogen, Total Kjeldahl (TKN) (mg/L)

Phosphorus, Total (TP) (ug/L)

SECCHI (m)

Level III ecoregions:

29, 33, 35, 37, 40, 45, 64, 65, 71, 72, 74

5.1.6 Aggregate Nutrient Ecoregion 11

Data Sources:

Auburn University

Legacy STORET

NYSDEC

EPA Region 3

EPA Region 4

Parameters:

Chlorophyll A, Fluorometric, Corrected (ug/L)

Chlorophyll A, Pheophytin (ug/L)

Chlorophyll A, Phytoplankton, Spectrophotometric Acid (ug/L)

Chlorophyll A, Phytoplankton, Spectrophotometric, Uncorrected (ug/L)
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Chlorophyll A, Trichromatic, Uncorrected (ug/L)

Phosphorous, Dissolved Inorganic (DIP) (ug/L)

Dissolved Oxygen (DO) (mg/L)

Nitrite and Nitrate, (NO2+NO3) (mg/L)

Nitrogen, Total (TN) (mg/L)

Nitrogen, Total Kjeldahl (TKN) (mg/L)

Phosphorus, Total (TP) (ug/L)

SECCHI (m)

Level III ecoregions:

36, 38, 39, 66, 67, 68, 69, 70

5.1.7 Aggregate Nutrient Ecoregion 12

Data Sources:

Legacy STORET

Parameters:

Chlorophyll A, Phytoplankton, Spectrophotometric Acid (ug/L)

Chlorophyll A, Trichromatic, Uncorrected (ug/L)

Dissolved Oxygen (DO) (mg/L)

Nitrite and Nitrate, (NO2+NO3) (mg/L)

Nitrogen, Total (TN) (mg/L)

Nitrogen, Total Kjeldahl (TKN) (mg/L)

Phosphorus, Total (TP) (ug/L)

SECCHI (m)

Level III ecoregions:

75

5.1.8 Aggregate Nutrient Ecoregion 13

Data Sources:

Legacy STORET
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Parameters:

Chlorophyll A, Fluorometric, Corrected (ug/L)

Chlorophyll A, Phytoplankton, Spectrophotometric Acid (ug/L)

Chlorophyll A, Trichromatic, Uncorrected (ug/L)

Dissolved Oxygen (DO) (mg/L)

Nitrite and Nitrate, (NO2+NO3) (mg/L)

Nitrogen, Total (TN) (mg/L)

Nitrogen, Total Kjeldahl (TKN) (mg/L)

Phosphorus, Total (TP) (ug/L)

SECCHI (m)

Level III ecoregions:

76

5.2 Rivers and Streams

5.2.1 Aggregate Nutrient Ecoregion 2

Data Sources:

Legacy STORET

NASQAN

NAWQA

EPA Region 10

Parameters:

Chlorophyll A, Fluorometric, Corrected (ug/L)

Chlorophyll A, Phytoplankton, Spectrophotometric Acid (ug/L)

Chlorophyll A, Phytoplankton, chromotographic- fluorometric (ug/L)

Chlorophyll A, Trichromatic, Uncorrected (ug/L)

Chlorophyll B, Phytoplankton, chromotographic- fluorometric (ug/L)

Phosphorous, Dissolved Inorganic (DIP) (ug/L)

Dissolved Oxygen (DO) (mg/L)

Nitrite and Nitrate, (NO2+NO3) (mg/L)

Phosphorus, Orthophosphate, Total as P (ug/L)

Phosphorus, Total (TP) Reactive (ug/L)

Nitrogen, Total (TN) (mg/L)

Nitrogen, Total Kjeldahl (TKN) (mg/L)

Phosphorus, Total (TP) (ug/L)

Turbidity (FTU)
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Turbidity (JCU)

Turbidity (NTU)

Level III ecoregions:

1, 2, 4, 5, 8, 9, 11, 15, 16, 17, 19, 21, 23, 41, 77, 78

5.2.2 Aggregate Nutrient Ecoregion 3

Data Sources:

Legacy STORET

NASQAN

NAWQA

EPA Region 10

Parameters:

Chlorophyll A, Fluorometric, Corrected (ug/L)

Chlorophyll A, Phytoplankton, Spectrophotometric Acid (ug/L)

Chlorophyll A, Phytoplankton, chromotographic- fluorometric (ug/L)

Chlorophyll A, Trichromatic, Uncorrected (ug/L)

Chlorophyll B, Phytoplankton, chromotographic- fluorometric (ug/L)

Phosphorous, Dissolved Inorganic (DIP) (ug/L)

Dissolved Oxygen (DO) (mg/L)

Nitrite and Nitrate, (NO2+NO3) (mg/L)

Nitrogen, Total (TN) (mg/L)

Nitrogen, Total Kjeldahl (TKN) (mg/L)

Phosphorus, Total (TP) (ug/L)

Turbidity (FTU)

Turbidity (JCU)

Turbidity (NTU)

Level III ecoregions:

6, 10, 12, 13, 14, 18, 20, 22, 24, 79, 80, 81
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5.2.3 Aggregate Nutrient Ecoregion 6

Data Sources:

Legacy STORET

NASQAN

NAWQA

EPA Region 5

EPA Region 7

Parameters:

Chlorophyll A, Fluorometric, Corrected (ug/L)

Chlorophyll A, Phytoplankton, Spectrophotometric Acid (ug/L)

Chlorophyll A, Phytoplankton, chromotographic- fluorometric (ug/L)

Chlorophyll A, Trichromatic, Uncorrected (ug/L)

Chlorophyll B, Phytoplankton, chromotographic- fluorometric (ug/L)

Phosphorous, Dissolved Inorganic (DIP) (ug/L)

Dissolved Oxygen (DO) (mg/L)

Nitrite and Nitrate, (NO2+NO3) (mg/L)

Nitrogen, Total (TN) (mg/L)

Nitrogen, Total Kjeldahl (TKN) (mg/L)

Organic, Phosphorus (ug/L)

Phosphorus, Total (TP) (ug/L)

Phosphorus, Orthophosphate, Total as P (ug/L)

Turbidity (FTU)

Turbidity (JCU)

Turbidity (NTU)

Level III ecoregions:

46, 47, 48, 54, 55, 57

5.2.4 Aggregate Nutrient Ecoregion 7 

Data Sources:

LCMPD

Legacy STORET

NASQAN

NAWQA

NYCDEP
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Parameters:

Chlorophyll A, Fluorometric, Corrected (ug/L)

Chlorophyll A, Phytoplankton, Spectrophotometric Acid (ug/L)

Chlorophyll A, Phytoplankton, Spectrophotometric, Uncorrected (ug/L)

Chlorophyll A, Phytoplankton, chromotographic- fluorometric (ug/L)

Chlorophyll A, Trichromatic, Uncorrected (ug/L)

Chlorophyll B, Phytoplankton, chromotographic- fluorometric (ug/L)

Phosphorous, Dissolved Inorganic (DIP) (ug/L)

Dissolved Oxygen (DO) (mg/L)

Nitrite and Nitrate, (NO2+NO3) (mg/L)

Nitrogen, Total (TN) (mg/L)

Nitrogen, Total Kjeldahl (TKN) (mg/L)

Organic, Phosphorus (ug/L)

Phosphorus, Orthophosphate, Total as P (ug/L)

Phosphorus, Total (TP) (ug/L)

Turbidity (FTU)

Turbidity (JCU)

Turbidity (NTU)

Level III ecoregions:

51, 52, 53, 56, 60, 61, 83

5.2.5 Aggregate Nutrient Ecoregion 9

Data Sources:

Auburn University

Legacy STORET

NASQAN

NAWQA

EPA Region 3

EPA Region 5

EPA Region 7

Parameters:

Chlorophyll A, Fluorometric, Corrected (ug/L)

Chlorophyll A, Phytoplankton, chromotographic- fluorometric (ug/L)

Chlorophyll A, Phytoplankton, Spectrophotometric Acid (ug/L)

Chlorophyll A, Phytoplankton, Spectrophotometric, Uncorrected (ug/L)
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Chlorophyll A, Trichromatic, Uncorrected (ug/L)

Chlorophyll B, Phytoplankton, chromotographic- fluorometric (ug/L)

Chlorophyll B, Phytoplankton, Spectrophotometric (ug/L)

Phosphorous, Dissolved Inorganic (DIP) (ug/L)

Dissolved Oxygen (DO) (mg/L)

Organic, Phosphorus (ug/L)

Phosphorus, Orthophosphate, Total as P (ug/L)

Nitrite and Nitrate, (NO2+NO3) (mg/L)

Nitrogen, Total (TN) (mg/L)

Nitrogen, Total Kjeldahl (TKN) (mg/L)

Phosphorus, Total (TP) (ug/L)

Turbidity (FTU)

Turbidity (JCU)

Turbidity (NTU)

Level III ecoregions:

29, 33, 35, 37, 40, 45, 64, 65, 71, 72, 74

5.2.6 Aggregate Nutrient Ecoregion 11

Data Sources:

Auburn University

Legacy STORET

NASQAN

NAWQA

EPA Region 3

EPA Region 5

EPA Region 7

Parameters:

Chlorophyll A, Fluorometric, Corrected (ug/L)

Chlorophyll A, Phytoplankton, chromotographic- fluorometric (ug/L)

Chlorophyll A, Phytoplankton, Spectrophotometric Acid (ug/L)

Chlorophyll A, Phytoplankton, Spectrophotometric, Uncorrected (ug/L)

Chlorophyll A, Trichromatic, Uncorrected (ug/L)

Chlorophyll B, Phytoplankton, chromotographic- fluorometric (ug/L)

Phosphorous, Dissolved Inorganic (DIP) (ug/L)

Dissolved Oxygen (DO) (mg/L)

Organic, Phosphorus (ug/L)
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Phosphorus, Orthophosphate, Total as P (ug/L)

Nitrite and Nitrate, (NO2+NO3) (mg/L)

Nitrogen, Total (TN) (mg/L)

Nitrogen, Total Kjeldahl (TKN) (mg/L)

Phosphorus, Total (TP) (ug/L)

Turbidity (FTU)

Turbidity (JCU)

Turbidity (NTU)

Level III ecoregions:

36, 38, 39, 66, 67, 68, 69, 70

5.2.7 Aggregate Nutrient Ecoregion 12

Data Sources:

Legacy STORET

NASQAN

NAWQA

Parameters:

Chlorophyll A, Phytoplankton, Spectrophotometric Acid (ug/L)

Chlorophyll A, Phytoplankton, Spectrophotometric, Uncorrected (ug/L)

Chlorophyll A, Trichromatic, Uncorrected (ug/L)

Chlorophyll B, Phytoplankton, Spectrophotometric (ug/L)

Phosphorous, Dissolved Inorganic (DIP) (ug/L)

Dissolved Oxygen (DO) (mg/L)

Nitrite and Nitrate, (NO2+NO3) (mg/L)

Nitrogen, Total (TN) (mg/L)

Nitrogen, Total Kjeldahl (TKN) (mg/L)

Phosphorus, Orthophosphate, Total as P (ug/L)

Phosphorus, Total (TP) (ug/L)

Turbidity (FTU)

Turbidity (NTU)

Level III ecoregions:

75
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5.2.8 Aggregate Nutrient Ecoregion 14

Data Sources:

Legacy STORET

NASQAN

NAWQA

NYCDEP

EPA Region 1

EPA Region 3

Parameters:

Chlorophyll A, Fluorometric, Corrected (ug/L)

Chlorophyll A, Phytoplankton, Spectrophotometric Acid (ug/L)

Chlorophyll A, Phytoplankton, Spectrophotometric, Uncorrected (ug/L)

Chlorophyll A, Trichromatic, Uncorrected (ug/L)

Phosphorous, Dissolved Inorganic (DIP) (ug/L)

Dissolved Oxygen (DO) (mg/L)

Nitrite and Nitrate, (NO2+NO3) (mg/L)

Phosphorus, Orthophosphate, Total as P (ug/L)

Nitrogen, Total Kjeldahl (TKN) (mg/L)

Nitrogen, Total (TN) (mg/L)

Phosphorus, Total (TP) (ug/L)

Turbidity (FTU)

Turbidity (JCU)

Turbidity (NTU)

Level III ecoregions:

59, 63, 84
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APPENDIX A

Process Used to QA/QA the Legacy STORET Nutrient Data Set
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1. STORET water quality parameters and Station and Sample data items were retrieved from

USEPA's mainframe computer. Table 1 lists all retrieved parameters and data items.

TABLE 1: PARAMETERS AND DATA ITEMS RETRIEVED FROM STORET

Parameters Retrieved

(STORET Parameter Code)

Station Data Items

Included

(STORET Item Name)

Sample Data Items Included

(STORET Item Name)

TN - mg/l (600)

TKN - mg/l (625)

Total Ammonia (NH3+NH4) - mg/l (610)

Total NO2+NO3 - mg/l (630)

Total Nitrite - mg/l (615)

Total Nitrate - mg/l (620)

Organic N - mg/L (605)

TP - mg/l (665)

Chlor a - ug/L (spectrophotometric method,

32211) 

Chlor a - ug/L (fluorometric method

corrected, 32209)

Chlor a - ug/L (trichromatic method

corrected, 32210)

Secchi Transp. - inches (77)

Secchi Transp. - meters (78)

+Turbidity JCUs (70)

+Turbidity FTUs (76)

+Turbidity NTUs field (82078)

+Turbidity NTUs lab (82079)

+DO - mg/L (300)

+Water Temperature (degrees C, 10/degrees

F, 11)

Station Type (TYPE)

Agency Code (AGENCY)

Station No. (STATION)

Latitude - std. decimal degrees

(LATSTD)

Longitude - std. decimal degrees 

(LONGSTD)

Station Location (LOCNAME)

County Name (CONAME)

State Name (STNAME)

Ecoregion Name - Level III

(ECONAME)

Ecoregion Code -Level III

(ECOREG)

Station Elevation (ELEV)

Hydrologic Unit Code

(CATUNIT)

RF1 Segment and Mile

(RCHMIL)

RF1ON/OFF tag (ONOFF)

Sample Date (DATE)

Sample Time (TIME)

Sample Depth (DEPTH)

Composite Sample Code 

(SAMPMETH)

+ If data record available at a station included data only for this or other such marked parameters, data record was deleted

from data set.

The following set of retrieval rules were applied to the retrieval process:

• Data were retrieved for waterbodies specified only as 'lake', 'stream', 'reservoir', or 'estuary'

under "Station Type" parameter. Any stations specified as 'well,' 'spring,' or 'outfall' were

eliminated from the retrieved data set.

• Data were retrieved for station types described as 'ambient' (e.g., no pipe or facility

discharge data) under the "Station Type" parameter.

• Data were retrieved that were designated as 'water' samples only. This includes 'bottom'

and 'vertically integrated' water samples.
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• Data were retrieved that were designated as either 'grab' samples and 'composite' samples

(mean result only).

• No limits were specified for sample depths.

• Data were retrieved for all fifty states, Puerto Rico, and the District of Columbia.

• The time period specified for data retrieval was January 1990 to September 1998.

• No data marked as "Retired Data" (i.e., data from a generally unknown source) were

retrieved.

• Data marked as "National Urban Runoff data" (i.e., data associated with sampling

conducted after storm events to assess nonpoint source pollutants) were included in the

retrieval. Such data are part of STORET's 'Archived' data. 

• Intensive survey data (i.e., data collected as part of specific studies) were retrieved.

2. Any values falling below the 1st percentile and any values falling above the 99th percentile

were transformed into 'missing' values (i.e., values were effectively removed from the data

set, but were not permanently eliminated).

3. Based on the STORET 'Remark Code' associated with each retrieved data point, the

following rules were applied (Table 2): 

TABLE 2: STORET REMARK CODE RULES

STORET Remark Code Keep or Delete Data Point

blank - Data not remarked. Keep

A - Value reported is the mean of two or more

determinations.

Keep

B - Results based upon colony counts outside the acceptable

ranges.

Delete

C - Calculated. Value stored was not measured directly, but

was calculated from other data available.

Keep

D - Field measurement. Keep
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E - Extra sample taken in compositing process. Delete

F - In the case of species, F indicates female sex. Delete

G - Value reported is the maximum of two or more

determinations.

Delete

H - Value based on field kit determination; results may not

be accurate.

Delete

I - The value reported is less than the practical

quantification limit and greater than or equal to the method

detection limit.

Keep, but used one-half the reported value as the new value.

J - Estimated. Value shown is not a result of analytical

measurement.

Delete

K - Off-scale low. Actual value not known, but known to be

less than value shown.

Keep, but used one-half the reported value as the new value.

L - Off-scale high. Actual value not known, but known to

be greater than value shown.

Keep

M - Presence of material verified, but not quantified. 

Indicates a positive detection, at a level too low to permit

accurate quantification.

Keep, but used one half the reported value as the new value.

N - Presumptive evidence of presence of material. Delete

O - Sample for, but analysis lost. Accompanying value is

not meaningful for analysis.

Delete

P - Too numerous to count. Delete

Q - Sample held beyond normal holding time. Delete

R - Significant rain in the past 48 hours. Delete

S - Laboratory test. Keep

T - Value reported is less than the criteria of detection. Keep, but replaced reported value with 0.
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U - Material was analyzed for, but not detected. Value

stored is the limit of detection for the process in use. 

Keep, but replaced reported value with 0.

V - Indicates the analyte was detected in both the sample

and associated method blank.

Delete

W - Value observed is less than the lowest value reportable

under remark "T."

Keep, but replaced reported value with 0.

X - Value is quasi vertically-integrated sample. No data point with this remark code in data set.

Y - Laboratory analysis from unpreserved sample. Data

may not be accurate.

Delete

Z - Too many colonies were present to count. Delete

If a parameter (excluding water temperature) value was less than or equal to zero and no remark code was present, the value

was transformed into a missing value.

Rationale - Parameter concentrations should never be zero without a proper explanation. A method detection limit should

at least be listed.

4. Station records were eliminated from the data set if any of the following descriptors were

present within the "Station Type" parameter:

< MONITR - Source monitoring site, which monitors a known problem or

to detect a specific problem.

< HAZARD - Site of hazardous or toxic wastes or substances.

< ANPOOL - Anchialine pool, underground pools with subsurface

connections to watertable and ocean.

< DOWN - Downstream (i.e., within a potentially polluted area) from a

facility which has a potential to pollute.

< IMPDMT - Impoundment. Includes waste pits, treatment lagoons, and

settling and evaporation ponds.

< STMSWR - Storm water sewer.

< LNDFL - Landfill.

< CMBMI - Combined municipal and industrial facilities.

< CMBSRC - Combined source (intake and outfall).

Rationale - these descriptors potentially indicate a station location that at which an

ambient water sample would not be obtained (i.e., such sampling locations are potentially 
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biased) or the sample location is not located within one of the designated water body types (i.e,

ANPOOL). 

5. Station records were eliminated from data set if the station location did not fall within any

established cataloging unit boundaries based on their latitude and longitude.

6. Using nutrient ecoregion GIS coverage provided by USEPA, all station locations with

latitude and longitude coordinates were tagged with a nutrient ecoregion identifier

(nutrient region identifiers are values 1 - 14) and the associated nutrient ecoregion name. 

Because no nutrient ecoregions exist for Alaska, Hawaii, and Puerto Rico, stations located

in these states were tagged with "dummy" nutrient ecoregion numbers (20 = Alaska, 21 =

Hawaii, 22 = Puerto Rico).

7. Using information provided by TVA, 59 station locations that were marked as 'stream'

locations under the "Station Type" parameter were changed to 'reservoir' locations. 

8. The nutrient data retrieved from STORET were assessed for the presence of duplicate

data records. The duplicate data identification process consisted of three steps: 1)

identification of records that matched exactly in terms of each variable retrieved; 2)

identification of records that matched exactly in terms of each variable retrieved except for

their station identification numbers; and 3) identification of records that matched exactly in

terms of each variable retrieved except for their collecting agency codes. The data

duplication assessment procedures were conducted using SAS programs. 

Prior to initiating the data duplication assessment process, the STORET nutrient data set

contained:

41,210 station records

924,420 sample records

• Identification of exactly matching records

All data records were sorted to identify those records that matched exactly. For

two records to match exactly, all variables retrieved had to be the same. For

example, they had to have the same water quality parameters, parameter results

and associated remark codes, and have the same station data item and sample data

item information. Exactly matching records were considered to be exact

duplicates, and one duplicate record of each identified matching set were

eliminated from the nutrient data set. A total of 924 sample records identified as

duplicates by this process were eliminated from the data set. 

• Identification of matching records with the exception of station identification

number

All data records were sorted to identify those records that matched exactly except

for their station identification number (i.e., they had the same water quality
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parameters, parameter results and associated remark codes, and the same station

and sample data item information with the exception of station identification

number). Although the station identification numbers were different, the latitude

and longitude for the stations were the same indicating a duplication of station data

due to the existence of two station identification numbers for the same station. For

each set of matching records, one of the station identification numbers was

randomly selected and its associated data were eliminated from the data set. A

total of 686 sample records were eliminated from the data set through this process. 

• Identification of matching records with the exception of collecting agency codes

All data records were sorted to identify those records that matched exactly except

for their collecting agency codes (i.e., they had the same water quality parameters,

parameter results and associated remark codes, and the same station and sample

data item information with the exception of agency code). The presence of two

matching data records each with a different agency code attached to it suggested

that one agency had utilized data collected by the other agency and had entered the

data into STORET without realizing that it already had been placed in STORET

by the other agency. No matching records with greater than two different agency

codes were identified. For determining which record to delete from the data set,

the following rules were developed: 

< If one of the matching records had a USGS agency code, the USGS

record was retained and the other record was deleted. 

< Higher level agency monitoring program data were retained. For

example, federal program data (indicated by a "1" at the beginning

of the STORET agency code) were retained against state (indicated

by a "2") and local (indicated by values higher than 2) program

data. 

< If two matching records had the same level agency code, the record

from the agency with the greater number of overall observations

(potentially indicating the data set as the source data set) was

retained.

A total of 2,915 sample records were eliminated through this process. 

As a result of the duplicate data identification process, a total of 4,525 sample records and

36 individual station records were removed from the STORET nutrient data set. The

resulting nutrient data set contains the following:

41,174 station records

919,895 sample records
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APPENDIX B

Process for Adding Aggregate Nutrient Ecoregions and Level III Ecoregions
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Steps for assigning Level III ecoregions and aggregate nutrient ecoregion codes and names to the

Nutrient Criteria Database (performed using ESRI's ARCView v 3.2 and its GeoProcessing

Wizard). This process is performed twice; once for the Level III ecoregions and once for the

aggregate nutrient ecoregions:

- Add the station .dbf data table, with latitude and longitude data, to project by 'Add

Event Theme'

- Convert to the shapefile format

- Create 'stcojoin' field, populate the 'stcojoin' field with the following formula:

'County.LCase+State.LCase'

- Add field 'stco_flag' to the station shapefile

- Spatially join the station data with the county shapefile (cntys_jned.shp)

- Select 'stcojoin' (station shapefile) field = 'stco_join2' (county shapefile) field 

- Calculate stco_flag = 0 for selected features

- Step through all blank stco_flag records, assign the appropriate stco_flags, see list

on the following page

- Select all stco_flags = 4 or 7, switch selection

- Calculate ctyfips (station) to cntyfips (county)

- Stop editing and save edits, remove all joins

- Add in 2 new fields 'x-coord1' and 'y-coord1' into station table

- Select all stco_flags = 1, 2, and 6

- Link county coverage with station coverage

- Populate 'x-coord1' and 'y-coord1' with 'x-coord' and 'y-coord' from county

coverage

- Select all stco_flags = 1, 2, and 6, export to new .dbf file

- Add new .dbf file as event theme

- Convert to shapefile format

- Add the following fields to both tables (original station and station126 shapefiles):

'eco_omer', 'name_omer', 'dis_aggr', 'code_aggr', 'name_aggr'

- Spatially join station126 and eco-omer coverage

- Populate the 'eco_omer' field with the 'eco' value

- Repeat the previous step using the nearest method (line coverage) to determine

ecoregion assignment for the line coverage, if some records are blank

- Spatially join the ecoregion line coverage to station coverage, link the LPoly#

(from the spatially joined table) to Poly# (of the ecoregion polygon coverage)

- Populate the Eco fields with the appropriate information.

- Follow the same steps to the Rpoly#

- Remove all table joins

- Link the useco-om table with station126 table and populate 'name-omer' field

- Spatially join station aggr coverage and populate the rest of the fields. Follow the

same procedures as outlined above

- Remove all joins

- Make sure the new Eco field added into the station126 shapefile are different than
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the ones in the original station shapefile

- Join station126 and station coverage by station-id

- Populate all the Eco fields in the original station coverage

- Remove all joins

- Save table

- Make sure that all ctyfips records are populated; the county shapefile may have to

be joined to populate the records, if the stco_flag = 4

- Create 2 new fields, 'NewCounty' and 'NewState'

- Populate these new fields with a spatial join to the county coverage

- Select by feature (ecoregion shapefile) all of the records in the station shapefile

- Switch selection (to get records outside of the ecoregion shapefile)

- If any of the selected records have stco_flag = 0 (they are outside the ecoregion

shapefile boundary), calculate them to stco_flag = 3

stco_flags (state/county flags in order of importance)

0 The state and county values from the data set matched the state and county values 

from the spatial join.

(Ecoregions were assigned based on the latitude/longitude coordinates.)

1 The state and county values from the data set did not match the state and county

values from the spatial join, but the point was inside the county coverage 

boundary.

(Ecoregions were assigned based on the county centroid.)

2 The state and county values from the data set did not match the state and county 

values from the spatial join because the point was outside the county coverage 

boundary; therefore, there was nothing to compare to the point (i.e., the point 

falls in the ocean/Canada/Mexico). This occurred for some coastal samples.

(Ecoregions were assigned based on the county centroid.)

3 The state and county values from the data set matched the state and county from

the spatial join, but the point was outside the ecoregion boundary.

(Ecoregions were assigned to the closest ecoregion to the point.)

(No ecoregions were assigned to AK, HI, PR, BC, and GU.)

4 Latitude/longitude coordinates were provided, but there was no county 

 information.

(Ecoregions were assigned based on the latitude/longitude coordinates.)

5 The state and county values from the data set did not match the state and county 

 values from the spatial join due to spelling or naming convention errors.

The matches were performed manually. 

(Ecoregions were assigned based on the latitude/longitude coordinates.)

6 No latitude/longitude coordinates were provided, only state and county

information was available.

(Ecoregions were assigned based on the county centroid.)

7 No latitude/longitude coordinates were provided, only state information was 

available; therefore, no matches were possible.

(Ecoregions were not assigned. Data is not included in the analysis.)
15 Nutrient Ecoregion/Waterbody Type Summary Chapters, Contract # 68-C-99-226, TO# 04 August 8, 2000

C-1

APPENDIX C

Glossary

Coefficient of Variation- Equal to the standard deviation divided by the mean multiplied by 100.

Maximum- The highest value.

Mean- The arithmetic average.

Median- The 50th percentile or middle value. Half of the values are above the median, and half of

the values are below the median.

Minimum- The lowest value.

Standard Deviation- Equal to the square root of the variance with the variance defined as the sum

of the squared deviations divided by the sample size minus one.

Standard Error- Standard error of the mean is equal to the standard deviation divided by the

square root of the sample size.