United States

Environmental Protection

Agency

Office of Water

4304

EPA 822-B-00-017

December 2000

Ambient Water Quality

Criteria Recommendations

Information Supporting the Development

of State and Tribal Nutrient Criteria

Rivers and Streams in 

Nutrient Ecoregion VI
EPA 822-B-00-017

AMBIENT WATER QUALITY CRITERIA RECOMMENDATIONS

INFORMATION SUPPORTING THE DEVELOPMENT OF STATE AND TRIBAL

NUTRIENT CRITERIA

FOR

RIVERS AND STREAMS IN NUTRIENT ECOREGION VI

Corn Belt And Northern Great Plains

including all or parts of the States of:

South Dakota, North Dakota, Nebraska, 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 Rivers and Streams 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 rivers and streams in Nutrient Ecoregion VI (Corn Belt

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

Streams Nutrient Criteria Technical Guidance Manual (U.S. EPA, 2000b). 

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, NASQAN, NAWQA, EPA Region 5, and EPA

Region 7 were compiled and analyzed. 

! Reference sites/reference conditions in Nutrient Ecoregion VI. 

Reference conditions presented are based on 25th percentiles of all nutrient data including a

comparison of reference condition for the aggregate ecoregion versus the subecoregions. 

States and Tribes are urged to determine their own reference sites for rivers and streams

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. 

! 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.

! Downstream effects of criteria.
vi

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

downstream water quality and uses.

In addition the following QA/QC procedures were followed on data collection and

analysis: all data were reviewed for duplications. All data are from ambient waters 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.

A summary of reference conditions for the Aggregate nutrient ecoregion, as well as the

subecoregions (level III ecoregions) within Aggregate Nutrient Ecoregion VI, for TP, TN, water

column chlorophyll a, and turbidity is presented below. 

BASED ON 25th PERCENTILES ONLY

Nutrient Parameters Aggregate Nutrient Ecoregion VI

Reference Conditions

Total phosphorus (μg/L) 76.25

Total nitrogen (mg/L) 2.18

Chlorophyll a (μg/L) (Fluorometric method) 2.7

Turbidity (FTU) 6.36

For subecoregions 46, 47, 48, 54, 55, and 57 the ranges of nutrient parameter criteria are:

BASED ON 25th PERCENTILE ONLY

Nutrient Parameters Range of Level III Subecoregions

Reference Conditions

Total phosphorus (μg/L) 62.5-118.13

Total nitrogen (mg/L) 1.16-3.26

Chlorophyll a (μg/L) (Fluorometric method) 2-6.62

Turbidity (FTU) 4.33-9.21
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; (513) 489-8190 or toll free (800) 490-9198. 

Please refer to EPA document number EPA-822-B-00-017.
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; the Tribes within Ecoregion

VI; 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 ...... 8

Figure 3 Sampling locations within each level III ecoregion .......... 12

Figure 4a Illustration of data reduction process for stream data ......... 22

Figure 4b Illustration of reference condition calculation ............... 23

Tables

Table 1 Rivers and Streams records for Aggregate Ecoregion VI– Corn

Belt and Northern Great Plains ......................... 13

Table 2 Reference conditions for Aggregate Ecoregion VI streams .... 15

Table 3a-f Reference conditions for level III ecoregion streams ......... 16
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 Northern 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 Rivers and Streams 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 Rivers/Streams within Aggregate Ecoregion VI ................ 10

4.5 Statistical Analysis of Data ......................................... 10

4.6 Classification of River/Stream Type .................................. 20

4.7. Summary of Data Reduction Methods ................................. 20

5.0 Reference Sites and Conditions in Aggregate Ecoregion VI ...................... 21

6.0 Models Used to Predict or Verify Response Parameters ......................... 21

7.0 Framework for Refining Recommended Nutrient Criteria for Rivers and Streams in 

 Aggregate Ecoregion VI ................................................. 21

7.1 Example Worksheet for Developing Aggregate Ecoregion and Subecoregion Nutrient 

 Criteria ........................................................ 24

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

 Level III Subecoregions ............................................ 25

7.3 Setting Seasonal Criteria ........................................... 26

7.4 When Data/Reference Conditions Are Lacking .......................... 27
xi

7.5 Site-Specific Criteria Development ................................... 27

8.0 Literature Cited ....................................................... 27

9.0 Appendices ........................................................... 28
1

1.0 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
2

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

approaches described in the waterbody guidance manuals and this document. 

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
3

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

elements except modeling.

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
4

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

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.
5

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 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 sitespecific 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
6

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

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 region’s 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 Ecoregion VI

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

corn production dominates land usage (Figure 1). The eastern portion of North and South 
7

Figure 1. Aggregate Ecoregion VI.

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

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

northwestern Ohio, and very small segments of Wisconsin and Michigan are included in this

ecoregion.

3.3 Level III Ecoregions Within Aggregate Ecoregion VI

There are six Level III ecoregions contained within Aggregate Ecoregion VI (Figure 2). 

The following provides brief descriptions of the climate, vegetative cover, topography, and other

ecological information pertaining to these subecoregions.

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.
8

Figure 2. Ecoregion VI with level III ecoregions shown.

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

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
9

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 elmash 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.

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 RIVERS AND STREAMS 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 3 of the

Rivers and Streams Nutrient Criteria Technical Guidance Manual [U.S. EPA, 2000b] for a

complete discussion on choosing causal and response variables.) 
10

4.1 Data Sources

Data sets from Legacy STORET, NASQAN, NAWQA, and EPA Regions 5 and 7 were

used to assess nutrient conditions from1990 to1999. 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 Ecoregion VI for TP, TN, Chlorophyll a and Turbidity

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) were the data used in the 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. South Dakota, North Dakota and

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

4.4 Data for all rivers and streams within Aggregate Ecoregion VI 

Figure 3 shows the location of the sampling stations within each subecoregion. Table 1

presents all data records for all parameters for Aggregate Ecoregion VI and subecoregions within

the Aggregate Ecoregion. 

4.5 Statistical Analysis of Data

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

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

upper 25th percentile (75th percentile) of a reference population of streams. 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 streams are not identified, the second method is to

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

percentile of the entire population was chosen by EPA to represent a surrogate for an actual
11

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], 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 75th percentile compare well

based on user perceptions of water resources (NYSDEC, 2000).
12

Figure 3. Map of sampling locations within each level III ecoregion.
13

 Table 1. Rivers and Streams 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 Streams 1295 113 292 62 341 350 155

# of Stream Stations 3077 183 563 114 707 1143 367

Key Nutrient Parameters

(listed below)

 - # of records for Turbidity

(all methods)

21,009 254 5432 310 13,891 316 806

- # of records for Chlorophyll

a (all methods) + Periphyton

3546 41 1241 0 900 559 805

- # of records for Total

Kjeldahl Nitrogen (TKN)

60,074 1678 5528 1478 10,922 16,057 24,411

- # of records for Nitrate +

Nitrite (NO2 + NO3)

46,106 2746 6632 1252 18,213 10,363 6900

- # of records for Total

Nitrogen (TN)

6654 76 847 129 1016 48 4538

- # of records for Total

Phosphorus (TP)

80,368 3359 9630 1903 20,638 19,214 25,624

Total # of records for key

nutrient parameters 

217,754 8154 29,309 5072 65,578 46,557 63,084
14

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 stream stations refers to the total number of river and

stream stations within the aggregate or subecoregion from which

nutrient data were collected. Since streams and rivers can cross

ecoregional boundaries, it is important to note that only those

portions of a river or stream (and data associated with those stations)

that exist within the ecoregion are included within this table.

Tables 2 and 3a-f present the potential reference conditions for both the aggregate

ecoregion and the subecoregions using both methods. However, the reference stream column is

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

information on reference streams. Appendix A and B provide complete presentations of all

descriptive statistics for both the aggregate ecoregion and the level III subecoregion.
15

Table 2. Reference conditions for aggregate ecoregion VI streams.

 Parameter

No. of

Streams 

 N ++

Reported values 25th Percentiles based

on all seasons data for

the Decade

Reference Streams **

Min Max P25-all seasons+ P75 - all seasons

TKN (mg/L) 628 0.025 4.495 .591

NO2 + NO3 (mg/L) 717 0.01 10.65 .633

TN (mg/L) - calculated NA .035 15.15 1.22

TN (mg/L) - reported 77 .885 10.06 2.18

TP (ug/L) 815 5 2225 76.25

Turbidity (NTU) 35 4 140 9.89

Turbidity (FTU) 239 0.95 178 6.36

Turbidity (JCU) 58 4.23 116.5 10.4

Chlorophyll a (ug/L) -F 63 0.25 47.6 2.7

Chlorophyll a (ug/L) -S 45 1.14 93.69 7.33

Chlorophyll a (ug/L) -T 21 2.62 44.05 6.83

Periphyton Chl a (mg/m2) ---- ----- ---- -----

 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 

Tables 3a-f present potential reference conditions for rivers and streams in the Level III

subecoregions within the Aggregate Ecoregion. Note that the footnotes for Table 2 apply to

Tables 3a-l.
16

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

 Parameter

No. of

Streams 

 N ++

Reported values 25th Percentiles based

on all seasons data for

the Decade

Reference Streams **

Min Max P25-all seasons+ P75 - all seasons

TKN (mg/L) 60 0.25 2.99 0.935

NO2 + NO3 (mg/L) 62 0.014 6.1 0.074

TN (mg/L) - calculated NA 0.264 9.09 1.01

TN (mg/L) - reported 4 1.33 2.71 1.4

TP (ug/L) 72 35 1233.75 102.38

Turbidity (NTU) ---- ----- ----- ------

Turbidity (FTU) 9 1.55 32.5 4.33

Turbidity (JCU) ---- ---- ---- -----

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

Chlorophyll a (ug/L) -S 4 10.41 18.1 14zz

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

Periphyton Chl a (mg/m2)

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

 Parameter

No. of

Streams 

 N ++

Reported values 25th Percentiles based

on all seasons data for

the Decade

Reference Streams **

Min Max P25-all seasons+ P75 - all seasons

TKN (mg/L) 136 0.57 4.42 0.65

NO2 + NO3 (mg/L) 141 0.083 9.6 1.965

TN (mg/L) - calculated NA 0.653 14.02 2.615

TN (mg/L) - reported 32 1.65 10.06 3.26

TP (ug/L) 187 11.25 1720 118.13

Turbidity (NTU) 32 4 160 15

Turbidity (FTU) 74 0.975 178 7.69

Turbidity (JCU) 56 4.23 116.5 10.15

Chlorophyll a (ug/L) -F 24 1.8 45.2 4.4

Chlorophyll a (ug/L) -S 25 3.76 90.6 7.85

Chlorophyll a (ug/L) -T 3 9.38 31.0 9.38

Periphyton Chl a (mg/m2) ---- ----- ----- -----
17

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

 Parameter

No. of

Streams 

 N ++

Reported values 25th Percentiles based

on all seasons data for

the Decade

Reference Streams **

Min Max P25-all seasons+ P75 - all seasons

TKN (mg/L) 36 0.103 1.729 0.816

NO2 + NO3 (mg/L) 24 0.009 1.054 0.034

TN (mg/L) - calculated NA .112 2.78 0.85

TN (mg/L) - reported 4 1.05 1.7 1.158

TP (ug/L) 38 26.25 453.75 87.5

Turbidity (NTU) 1 z 5.18 5.18 5.18

Turbidity (FTU) 12 3.8 44.75 6.14

Turbidity (JCU) ---- ------ ---- -----

Chlorophyll a (ug/L) -F — ---- — ----

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

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

Periphyton Chl a (mg/m2) ---- ----- ---- -----

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

 Parameter

No. of

Streams 

 N ++

Reported values 25th Percentiles based

on all seasons data for

the Decade

Reference Streams **

Min Max P25-all seasons+ P75 - all seasons

TKN (mg/L) 113 0.138 3.33 0.663

NO2 + NO3 (mg/L) 185 0.115 10.65 1.798

TN (mg/L) - calculated NA 0.253 13.98 2.461

TN (mg/L) - reported 11 1.44 7.35 2.95

TP (ug/L) 203 10 1471.25 72.5

Turbidity (NTU) 1 z 14 14 14

Turbidity (FTU) 104 2.3 51.75 6.04

Turbidity (JCU) 1 z 31.6 31.6 31.6

Chlorophyll a (ug/L) -F 15 1.3 39.85 2

Chlorophyll a (ug/L) -S 8 1.35 27.15 7.01

Chlorophyll a (ug/L) -T 10 2.26 38.46 3.18

Periphyton Chl a (mg/m2) ----- ------ ----- ------
18

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

 Parameter

No. of

Streams 

 N ++

Reported values 25th Percentiles based

on all seasons data for

the Decade

Reference Streams **

Min Max P25-all seasons+ P75 - all seasons

TKN (mg/L) 198 0.05 3.5 0.4

NO2 + NO3 (mg/L) 219 0.025 8.13 1.60

TN (mg/L) - calculated NA 0.075 11.63 2

TN (mg/L) - reported 2 z 3.63 3.78 3.63

TP (ug/L) 225 10 1820 62.5

Turbidity (NTU) 1 z 10.4 10.4 10.4 zz

Turbidity (FTU) 12 3.3 50.65 9.21

Turbidity (JCU) 1 z 28 28 28 zz

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

Chlorophyll a (ug/L) -S 8 4.32 19.24 6.62

Chlorophyll a (ug/L) -T 8 6.67 22.72 7.99

Periphyton Chl a (mg/m2) ---- ------ ---- ----- -----

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

 Parameter

No. of

Streams 

 N ++

Reported values 25th Percentiles based

on all seasons data for

the Decade

Reference Streams **

Min Max P25-all seasons+ P75 - all seasons

TKN (mg/L) 85 0.25 3.15 0.65

NO2 + NO3 (mg/L) 86 0.153 8.818 0.897

TN (mg/L) - calculated NA 0.4 11.97 1.55

TN (mg/L) - reported 24 1.02 5.81 1.91

TP (ug/L) 90 10 1225 70

Turbidity (NTU) — ---- ---- ----

Turbidity (FTU) 28 2.5 54.5 4.68

Turbidity (JCU) 1 z 40 40 40

Chlorophyll a (ug/L) -F 24 0.63 41.88 3.19

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

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

Periphyton Chl a (mg/m2)
19

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

1. Number of Streams in Table 2 refers to the largest number of streams and rivers for which

data existed for a given season within an aggregate nutrient ecoregion.

2. Number of Streams in Table 3 refers to the number of streams and rivers 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 stream for the decade were

reduced to one median for that stream. This prevents over-representation 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

streams/season. However, this table provides 25th percentiles that were derived with less than 4

streams/season in order to retain all information for all seasons. In calculating the 25th

percentile for a season with less than 4 stream medians, the statistical program automatically

used the minimum value within the less-than-4 population. If less than 4 streams were used in

developing a seasonal quartile and or all-seasons median, the entry is flagged (zz).

Preferred Data Choices and Recommendations When Data Are 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

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

N/A is placed in this box. 
20

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 River/Stream Type

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

the data analysis. There was no readily available 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 stream within

ecoregion III, for which data existed. For example, if one stream had 300 observations for

phosphorus over the decade or one year’s time, one median resulted. Each median from each 

stream 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).
21

5.0 REFERENCE SITES AND CONDITIONS IN 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 streams. 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 RIVERS AND STREAMS 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. 
22

Figure 4a. Illustration of data reduction process for stream data.
23

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

Chl a

Turbidity

} 25%

} 25%

Figure 4b. Illustration of reference condition calculation.
24

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
25

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 &

Northern Great Plains

Proposed Criterion

Total Phosphorus (μg/L)

Total Nitrogen (mg/L)

Chlorophyll a (μg/L or mg/m2)

Turbidity (NTU or other units)

Other (Index; other parameter such as DO)

C Literature sources

C Historical data and trends

C Reference condition
26

C Models

C RTAG expert review and consensus

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)

Turbidity (NTU or other units)

Other (Index; other parameter such as DO)

7.3 Setting Seasonal Criteria

The recommendations 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
27

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.

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 Rivers and Streams Nutrient

Criteria Technical Guidance Manual (U.S. EPA, 2000b). The Technical Guidance Manual

presents sections on each of the following factors to consider in setting criteria:

- refinements to ecoregions (Section 2.3)

- classification of waterbodies (Chapter 2)

- setting seasonal criteria to reflect major seasonal climate differences and accounting for 

significant or cyclical precipitation events (high flow/low flow conditions) (Chapter 4)

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.
28

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

 Rivers and Streams

 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 44 12.2 .400 91.00 19.3 2.91 159 1.00 2.19 5.13 11.6 39.5

 SPRING 36 12.2 1.00 50.00 12.4 2.07 102 1.80 4.00 7.35 15.1 40.9

 SUMMER 63 12.5 .250 45.20 12.0 1.51 96 1.10 3.20 8.19 18.5 38.8

 WINTER 27 3.43 .250 12.00 2.62 0.50 77 0.80 1.90 3.00 4.00 10.0

 Aggregate Nutrient Ecoregion: VI 2

 Rivers and Streams

 Descriptive Statistics by Decade and Season

 Parameter Chla_Phyto_C_F_ug_L_Med

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

 FALL 1 11.5 11.5 11.50 . . . 11.5 11.5 11.5 11.5 11.5

 SPRING 1 23.0 23.0 23.00 . . . 23.0 23.0 23.0 23.0 23.0

 SUMMER 3 25.0 3.05 68.00 37.3 21.5 149 3.05 3.05 3.90 68.0 68.0

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

 Aggregate Nutrient Ecoregion: VI 3

 Rivers and Streams

 Descriptive Statistics by Decade and Season

 Parameter Chla_Phyto_S_A_ug_L_Med

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

 FALL 37 39.8 1.79 144.18 42.0 6.90 105 5.05 8.68 16.0 59.1 138

 SPRING 22 14.5 .500 43.20 9.55 2.04 66 5.60 8.08 12.0 18.0 26.0

 SUMMER 45 23.7 1.78 148.00 29.5 4.40 124 2.48 6.58 17.2 24.6 104

 WINTER 14 3.97 .320 10.59 3.47 0.93 87 0.32 0.80 3.48 5.72 10.6

 Aggregate Nutrient Ecoregion: VI 4

 Rivers and Streams

 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 18 21.9 3.14 153.15 33.7 7.95 154 3.14 9.28 14.5 18.0 153

 SPRING 16 13.8 2.78 31.00 8.46 2.12 61 2.78 8.01 11.2 19.8 31.0

 SUMMER 21 18.6 2.05 57.10 16.6 3.63 89 2.49 5.64 13.4 24.2 54.0

 WINTER 11 6.68 2.46 13.63 3.49 1.05 52 2.46 3.87 5.46 8.73 13.6

 
 

 Aggregate Nutrient Ecoregion: VI 5

 Rivers and Streams

 Descriptive Statistics by Decade and Season

 Parameter Chlb_Phyto_C_F_ug_L_Med

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

 FALL 1 1.85 1.85 1.85 . . . 1.85 1.85 1.85 1.85 1.85

 SPRING 1 2.00 2.00 2.00 . . . 2.00 2.00 2.00 2.00 2.00

 SUMMER 3 1.25 .200 3.10 1.61 0.93 129 0.20 0.20 0.45 3.10 3.10

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

 Aggregate Nutrient Ecoregion: VI 6

 Rivers and Streams

 Descriptive Statistics by Decade and Season

 Parameter DIP_ug_L_Median

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

 FALL 107 119 5.00 820.00 150 14.5 126 9.21 30.0 68.0 143 395

 SPRING 86 91.2 5.00 845.00 125 13.5 137 8.50 20.0 48.5 115 270

 SUMMER 109 114 3.07 765.00 122 11.7 107 18.9 40.0 77.8 142 318

 WINTER 90 127 7.50 790.00 147 15.5 116 10.8 40.0 80.0 165 380

 Aggregate Nutrient Ecoregion: VI 7

 Rivers and Streams

 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 329 9.24 1.45 14.60 1.89 0.10 20 6.00 8.35 9.30 10.2 12.3

 SPRING 353 10.0 3.80 14.50 1.40 0.07 14 7.75 9.30 10.0 10.9 12.4

 SUMMER 378 7.95 1.35 14.10 1.49 0.08 19 5.50 7.20 7.90 8.60 10.6

 WINTER 330 11.4 .900 14.50 1.63 0.09 14 8.50 10.8 11.7 12.4 13.3

 Aggregate Nutrient Ecoregion: VI 8

 Rivers and Streams

 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 589 1.80 .003 8.80 1.96 0.08 109 0.03 0.30 1.10 2.58 6.20

 SPRING 428 3.61 .008 14.00 3.10 0.15 86 0.03 0.66 3.10 6.08 9.15

 SUMMER 717 2.83 .000 10.70 2.63 0.10 93 0.03 0.61 2.08 4.35 8.60

 WINTER 416 3.56 .018 10.60 2.63 0.13 74 0.14 1.14 3.26 5.40 8.25

 
 

 Aggregate Nutrient Ecoregion: VI 9

 Rivers and Streams

 Descriptive Statistics by Decade and Season

 Parameter Org_P_ug_L_Median

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

 FALL 19 41.6 1.77 131.98 31.2 7.16 75 1.77 16.8 36.1 55.5 132

 SPRING 0 . . . . . . . . . . .

 SUMMER 21 73.6 18.8 249.79 52.2 11.4 71 21.6 38.3 70.0 90.1 137

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

 Aggregate Nutrient Ecoregion: VI 10

 Rivers and Streams

 Descriptive Statistics by Decade and Season

 Parameter Orthophosphate_T_as_P_ug_L_Med

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

 FALL 25 128 5.00 460.00 136 27.2 107 10.0 50.0 70.0 140 430

 SPRING 26 139 5.00 495.00 119 23.3 86 12.5 50.0 95.0 200 370

 SUMMER 32 163 12.5 932.50 187 33.0 115 15.0 53.8 90.0 195 570

 WINTER 30 201 10.0 1300.00 257 46.9 127 20.0 62.5 120 235 650

 Aggregate Nutrient Ecoregion: VI 11

 Rivers and Streams

 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 536 0.87 .025 4.55 0.56 0.02 64 0.20 0.50 0.74 1.13 1.80

 SPRING 396 1.06 .025 4.40 0.53 0.03 50 0.33 0.73 1.00 1.32 1.90

 SUMMER 628 0.97 .025 5.10 0.61 0.02 63 0.20 0.56 0.86 1.30 2.10

 WINTER 377 1.05 .088 4.44 0.68 0.04 65 0.33 0.63 0.88 1.30 2.40

 Aggregate Nutrient Ecoregion: VI 12

 Rivers and Streams

 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 65 3.15 .220 11.08 2.46 0.31 78 0.51 1.25 2.50 4.40 8.14

 SPRING 48 4.04 1.00 9.90 2.38 0.34 59 1.30 2.20 3.59 4.99 8.65

 SUMMER 77 3.99 .890 10.21 2.46 0.28 62 1.20 2.15 3.50 4.99 9.44

 WINTER 57 3.71 .880 8.55 1.90 0.25 51 1.20 2.30 3.12 4.80 7.05

 
 

 Aggregate Nutrient Ecoregion: VI 13

 Rivers and Streams

 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 665 232 2.50 2430.00 277 10.8 120 20.0 75.0 150 270 763

 SPRING 531 199 5.00 2105.00 223 9.67 112 30.0 77.5 140 245 540

 SUMMER 815 253 10.0 2200.00 286 10.0 113 25.0 90.0 175 298 830

 WINTER 501 208 5.00 2250.00 247 11.0 119 30.0 70.0 130 259 625

 Aggregate Nutrient Ecoregion: VI 14

 Rivers and Streams

 Descriptive Statistics by Decade and Season

 Parameter Turb_FTU_Median

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

 FALL 212 15.0 .900 102.70 14.4 0.99 96 2.40 5.83 11.0 19.0 46.0

 SPRING 199 15.8 1.00 170.00 16.6 1.17 105 3.40 6.90 11.0 20.0 39.0

 SUMMER 239 19.1 .500 186.00 21.0 1.36 110 2.60 7.20 13.0 21.0 60.0

 WINTER 198 14.0 1.05 190.00 21.9 1.56 157 2.50 4.40 7.59 15.0 44.5

 Aggregate Nutrient Ecoregion: VI 15

 Rivers and Streams

 Descriptive Statistics by Decade and Season

 Parameter Turb_JCU_Median

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

 FALL 43 23.9 4.80 92.00 19.5 2.98 82 5.60 9.80 18.0 30.3 66.0

 SPRING 20 39.4 4.35 141.00 35.9 8.04 91 9.08 19.5 26.8 36.1 131

 SUMMER 58 34.4 4.10 190.00 38.1 5.00 111 6.90 11.0 21.5 40.0 138

 WINTER 23 15.9 .700 44.50 12.4 2.59 78 3.00 7.00 11.0 28.0 37.0

 Aggregate Nutrient Ecoregion: VI 16

 Rivers and Streams

 Descriptive Statistics by Decade and Season

 Parameter Turb_NTU_Median

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

 FALL 26 38.0 4.00 160.00 35.9 7.04 94 5.00 12.0 27.0 51.0 104

 SPRING 4 38.4 4.45 120.00 54.7 27.3 142 4.45 7.78 14.6 69.0 120

 SUMMER 35 44.9 4.00 230.00 49.1 8.30 109 5.00 13.0 30.0 49.0 140

 WINTER 1 3.15 3.15 3.15 . . . 3.15 3.15 3.15 3.15 3.15

 
APPENDIX B

Descriptive Statistics Data Tables for Level III Subecoregions within Aggregate Ecoregion
 

 Aggregate Nutrient Ecoregion: VI 1

 Rivers and Streams

 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 22 10.5 .400 91.00 19.3 4.11 183 0.60 2.50 5.85 8.50 32.0

 47 SPRING 3 28.4 24.3 30.75 3.62 2.09 13 24.3 24.3 30.3 30.8 30.8

 47 SUMMER 24 13.7 1.80 45.20 11.9 2.43 87 2.40 4.40 10.3 19.9 39.5

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

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

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

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

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

 54 FALL 2 47.6 10.7 84.50 52.2 36.9 110 10.7 10.7 47.6 84.5 84.5

 54 SPRING 7 10.8 1.80 40.90 14.2 5.35 131 1.80 2.50 4.00 15.9 40.9

 54 SUMMER 15 6.56 .500 38.80 10.0 2.58 153 0.50 1.50 3.05 6.90 38.8

 54 WINTER 7 1.70 .800 3.20 0.85 0.32 50 0.80 0.95 1.50 2.35 3.20

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

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

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

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

 57 FALL 20 10.4 1.00 39.50 12.2 2.73 117 1.00 2.13 4.25 16.0 38.3

 57 SPRING 26 10.6 1.00 50.00 11.5 2.26 108 2.00 4.00 6.75 11.0 38.0

 57 SUMMER 24 15.1 .250 44.25 12.4 2.54 82 1.00 5.50 11.3 23.5 37.0

 57 WINTER 20 4.03 .250 12.00 2.77 0.62 69 0.63 2.38 3.75 4.75 11.0

 Aggregate Nutrient Ecoregion: VI 2

 Rivers and Streams

 Descriptive Statistics by Decade and Season

 Parameter Chla_Phyto_C_F_ug_L_Med

 Eco_

 Level_

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

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

 46 SPRING 1 23.0 23.0 23.00 . . . 23.0 23.0 23.0 23.0 23.0

 46 SUMMER 3 25.0 3.05 68.00 37.3 21.5 149 3.05 3.05 3.90 68.0 68.0

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

 47 FALL 0 . . . . . . . . . . .

 47 SPRING 0 . . . . . . . . . . .

 47 SUMMER 0 . . . . . . . . . . .

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

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

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

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

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

 54 FALL 1 11.5 11.5 11.50 . . . 11.5 11.5 11.5 11.5 11.5

 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 . . . . . . . . . . .

 57 WINTER 0 . . . . . . . . . . .

 Aggregate Nutrient Ecoregion: VI 3

 Rivers and Streams

 Descriptive Statistics by Decade and Season

 Parameter Chla_Phyto_S_A_ug_L_Med

 Eco_

 Level_

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

 46 FALL 2 11.0 11.0 11.00 0.00 0.00 0 11.0 11.0 11.0 11.0 11.0

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

 46 SUMMER 4 18.4 9.82 25.20 7.96 3.98 43 9.82 11.7 19.4 25.2 25.2

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

 47 FALL 20 56.3 5.05 138.00 42.7 9.54 76 5.57 8.68 51.4 76.9 130

 47 SPRING 9 17.7 5.60 43.20 12.2 4.08 69 5.60 7.02 13.3 25.8 43.2

 47 SUMMER 25 31.5 2.48 148.00 37.0 7.40 118 2.48 11.5 20.8 27.7 115

 47 WINTER 5 0.89 .320 2.20 0.77 0.35 87 0.32 0.32 0.80 0.80 2.20

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

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

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

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

 54 FALL 8 32.2 1.79 144.18 46.4 16.4 144 1.79 11.0 16.5 28.2 144

 54 SPRING 5 12.9 .500 23.14 8.57 3.83 66 0.50 9.79 13.4 17.9 23.1

 54 SUMMER 8 14.2 1.78 31.15 11.3 4.01 80 1.78 4.23 12.1 23.9 31.2

 54 WINTER 2 2.49 .925 4.05 2.21 1.56 89 0.93 0.93 2.49 4.05 4.05

 55 FALL 7 9.84 6.99 13.25 2.28 0.86 23 6.99 7.87 9.31 12.1 13.3

 55 SPRING 8 11.9 5.71 25.22 6.17 2.18 52 5.71 8.25 10.5 13.4 25.2

 55 SUMMER 8 11.7 2.27 37.34 11.3 4.00 97 2.27 5.37 7.31 14.3 37.3

 55 WINTER 7 6.59 2.92 10.59 2.85 1.08 43 2.92 4.17 5.72 9.93 10.6

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

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

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

 57 WINTER 0 . . . . . . . . . . .

 Aggregate Nutrient Ecoregion: VI 4

 Rivers and Streams

 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 0 . . . . . . . . . . .

 
 

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

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

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

 47 FALL 1 3.63 3.63 3.63 . . . 3.63 3.63 3.63 3.63 3.63

 47 SPRING 1 31.0 31.0 31.00 . . . 31.0 31.0 31.0 31.0 31.0

 47 SUMMER 3 29.5 9.38 54.00 22.6 13.1 77 9.38 9.38 25.0 54.0 54.0

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

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

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

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

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

 54 FALL 10 30.1 3.14 153.15 44.3 14.0 147 3.14 11.0 17.8 24.3 153

 54 SPRING 7 10.1 2.78 19.82 7.12 2.69 71 2.78 3.42 8.00 19.7 19.8

 54 SUMMER 10 18.1 2.05 57.10 17.6 5.57 97 2.05 2.54 15.1 24.2 57.1

 54 WINTER 4 3.62 2.46 4.74 0.95 0.48 26 2.46 2.93 3.63 4.31 4.74

 55 FALL 7 12.8 8.11 16.18 3.06 1.16 24 8.11 9.28 13.8 15.7 16.2

 55 SPRING 8 15.0 8.02 29.26 7.24 2.56 48 8.02 10.3 12.0 18.8 29.3

 55 SUMMER 8 15.1 3.54 45.01 13.4 4.74 89 3.54 6.69 11.8 17.6 45.0

 55 WINTER 7 8.43 5.32 13.63 3.16 1.19 37 5.32 5.46 7.26 11.7 13.6

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

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

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

 57 WINTER 0 . . . . . . . . . . .

 Aggregate Nutrient Ecoregion: VI 5

 Rivers and Streams

 Descriptive Statistics by Decade and Season

 Parameter Chlb_Phyto_C_F_ug_L_Med

 Eco_

 Level_

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

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

 46 SPRING 1 2.00 2.00 2.00 . . . 2.00 2.00 2.00 2.00 2.00

 46 SUMMER 3 1.25 .200 3.10 1.61 0.93 129 0.20 0.20 0.45 3.10 3.10

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

 47 FALL 0 . . . . . . . . . . .

 47 SPRING 0 . . . . . . . . . . .

 47 SUMMER 0 . . . . . . . . . . .

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

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

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

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

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

 54 FALL 1 1.85 1.85 1.85 . . . 1.85 1.85 1.85 1.85 1.85

 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 . . . . . . . . . . .

 57 WINTER 0 . . . . . . . . . . .

 
 

 Aggregate Nutrient Ecoregion: VI 6

 Rivers and Streams

 Descriptive Statistics by Decade and Season

 Parameter DIP_ug_L_Median

 Eco_

 Level_

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

 46 FALL 5 173 72.5 297.50 84.7 37.9 49 72.5 140 145 208 298

 46 SPRING 6 252 75.0 845.00 295 120 117 75.0 105 134 219 845

 46 SUMMER 5 275 80.0 765.00 290 130 106 80.0 90.0 123 318 765

 46 WINTER 5 327 158 722.50 231 103 71 158 183 245 328 723

 47 FALL 35 114 9.21 455.00 115 19.4 101 10.4 30.0 73.1 150 373

 47 SPRING 15 133 10.0 305.00 85.2 22.0 64 10.0 72.5 120 200 305

 47 SUMMER 37 125 3.07 362.50 90.4 14.9 73 8.77 69.8 108 158 335

 47 WINTER 17 186 20.0 585.00 133 32.3 72 20.0 90.0 155 213 585

 48 FALL 14 100 5.00 190.00 68.5 18.3 68 5.00 45.0 119 165 190

 48 SPRING 14 79.6 10.0 192.50 57.6 15.4 72 10.0 12.5 82.5 115 193

 48 SUMMER 14 122 20.0 240.00 65.0 17.4 53 20.0 77.5 120 170 240

 48 WINTER 14 129 8.75 347.50 99.5 26.6 77 8.75 82.5 88.8 200 348

 54 FALL 32 153 5.00 820.00 220 38.9 144 10.0 37.5 69.0 120 810

 54 SPRING 32 80.1 5.00 650.00 124 21.9 155 10.0 25.0 37.8 66.5 320

 54 SUMMER 32 110 5.00 650.00 146 25.8 133 20.5 38.8 56.0 119 565

 54 WINTER 32 108 10.0 790.00 161 28.5 149 13.0 40.0 50.0 105 540

 55 FALL 9 113 15.0 410.00 157 52.3 139 15.0 23.8 38.8 72.5 410

 55 SPRING 9 45.8 5.00 165.00 52.8 17.6 115 5.00 19.0 20.0 35.0 165

 55 SUMMER 9 81.4 20.0 252.50 82.2 27.4 101 20.0 22.5 41.5 80.0 253

 55 WINTER 10 89.4 7.50 350.00 107 33.9 120 7.50 20.0 36.5 138 350

 57 FALL 12 48.4 8.75 125.00 36.7 10.6 76 8.75 18.0 41.9 71.3 125

 57 SPRING 10 24.8 6.00 70.00 20.7 6.54 84 6.00 8.50 17.5 35.0 70.0

 57 SUMMER 12 40.0 7.50 100.00 28.2 8.14 71 7.50 22.3 35.0 47.4 100

 57 WINTER 12 40.0 10.0 90.00 27.0 7.78 67 10.0 15.0 35.0 62.5 90.0

 Aggregate Nutrient Ecoregion: VI 7

 Rivers and Streams

 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 43 10.1 2.73 13.40 2.38 0.36 24 4.60 9.10 10.3 12.0 13.0

 46 SPRING 57 10.1 6.15 13.00 1.41 0.19 14 7.20 9.20 9.95 11.1 12.8

 46 SUMMER 49 7.53 1.35 13.10 1.97 0.28 26 4.63 6.43 7.45 8.40 11.1

 46 WINTER 53 11.3 .900 14.30 2.33 0.32 21 7.98 10.2 11.7 13.0 13.7

 47 FALL 106 9.73 1.45 14.60 1.88 0.18 19 7.30 8.70 9.79 11.0 13.1

 47 SPRING 100 10.3 6.50 14.20 1.20 0.12 12 8.20 9.50 10.2 11.1 12.0

 47 SUMMER 135 8.30 2.25 14.10 1.51 0.13 18 6.60 7.60 8.10 8.78 11.2

 47 WINTER 94 11.7 4.60 14.50 1.57 0.16 13 8.60 11.3 12.1 12.6 13.4

 48 FALL 29 9.47 6.45 13.10 1.31 0.24 14 7.30 8.65 9.30 10.1 11.4

 48 SPRING 36 10.7 8.20 13.10 1.21 0.20 11 8.65 9.94 10.7 11.1 13.0

 48 SUMMER 35 7.49 5.50 9.95 1.06 0.18 14 6.00 6.80 7.25 8.20 9.95

 48 WINTER 34 10.8 6.00 13.23 1.79 0.31 17 7.00 9.50 11.3 12.0 12.9

 54 FALL 97 8.18 1.75 11.53 1.67 0.17 20 4.68 7.50 8.40 9.35 10.6

 
 

 54 SPRING 94 9.27 5.15 13.00 1.28 0.13 14 6.65 8.75 9.40 10.0 10.9

 54 SUMMER 99 7.46 4.10 11.30 1.13 0.11 15 5.30 6.80 7.50 8.10 9.10

 54 WINTER 94 11.2 5.33 13.30 1.29 0.13 12 8.15 10.9 11.4 11.9 12.6

 55 FALL 32 9.58 6.33 13.05 1.30 0.23 14 7.40 9.05 9.56 10.1 12.5

 55 SPRING 37 10.5 8.58 14.50 1.20 0.20 11 8.90 9.70 10.3 11.0 13.4

 55 SUMMER 38 8.78 6.40 11.50 1.25 0.20 14 6.40 7.90 8.83 9.45 11.1

 55 WINTER 33 11.4 8.40 13.70 0.99 0.17 9 10.3 10.8 11.4 12.0 13.2

 57 FALL 22 9.07 5.80 11.30 1.17 0.25 13 7.50 8.50 9.08 9.88 10.9

 57 SPRING 29 10.2 3.80 13.20 1.85 0.34 18 7.43 9.40 10.2 11.2 12.9

 57 SUMMER 22 8.23 5.50 10.90 1.38 0.29 17 5.90 7.60 8.14 9.20 10.3

 57 WINTER 22 11.9 8.80 13.90 1.38 0.29 12 8.90 11.8 12.2 12.8 13.5

 Aggregate Nutrient Ecoregion: VI 8

 Rivers and Streams

 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 46 0.47 .020 3.30 0.68 0.10 143 0.03 0.10 0.20 0.57 1.80

 46 SPRING 72 0.74 .008 9.41 1.47 0.17 200 0.01 0.03 0.11 0.60 3.40

 46 SUMMER 62 0.79 .010 6.20 1.31 0.17 166 0.01 0.05 0.38 0.80 2.60

 46 WINTER 60 0.94 .018 6.00 1.22 0.16 129 0.03 0.20 0.53 1.36 3.83

 47 FALL 116 2.88 .003 8.80 2.41 0.22 84 0.05 0.83 2.38 4.68 7.70

 47 SPRING 80 4.69 .025 10.10 2.95 0.33 63 0.52 1.77 5.00 7.00 9.28

 47 SUMMER 141 4.81 .140 10.20 3.02 0.25 63 0.50 2.17 4.66 7.40 9.80

 47 WINTER 87 4.02 .200 9.10 2.21 0.24 55 0.80 2.30 3.90 5.80 7.45

 48 FALL 18 0.06 .005 0.26 0.07 0.02 117 0.01 0.02 0.03 0.05 0.26

 48 SPRING 26 0.20 .010 1.15 0.25 0.05 121 0.01 0.03 0.09 0.30 0.50

 48 SUMMER 24 0.20 .007 1.15 0.30 0.06 151 0.01 0.04 0.08 0.29 1.00

 48 WINTER 20 0.28 .030 0.96 0.27 0.06 95 0.04 0.08 0.21 0.43 0.94

 54 FALL 152 1.81 .003 8.70 1.99 0.16 110 0.01 0.23 1.21 2.45 6.40

 54 SPRING 125 5.30 .100 14.00 3.22 0.29 61 0.60 2.33 5.20 7.80 9.65

 54 SUMMER 185 3.68 .003 10.70 2.77 0.20 75 0.10 1.27 3.73 5.30 8.90

 54 WINTER 136 4.87 .020 10.60 2.77 0.24 57 0.30 2.60 4.90 7.23 9.01

 55 FALL 168 1.71 .025 7.94 1.62 0.12 95 0.05 0.48 1.20 2.46 4.72

 55 SPRING 70 3.26 .025 6.91 1.53 0.18 47 0.50 2.35 3.24 4.23 5.79

 55 SUMMER 219 2.14 .025 9.02 1.61 0.11 75 0.16 0.86 1.80 3.08 5.22

 55 WINTER 68 3.49 .590 8.33 1.47 0.18 42 0.96 2.60 3.54 4.27 5.41

 57 FALL 89 1.62 .025 8.15 1.74 0.18 107 0.03 0.44 0.96 2.38 5.59

 57 SPRING 55 4.02 .310 10.50 2.88 0.39 72 0.50 1.56 3.60 6.00 9.68

 57 SUMMER 86 1.68 .000 7.05 1.68 0.18 100 0.03 0.32 1.03 2.83 4.82

 57 WINTER 45 3.79 .280 9.49 2.66 0.40 70 0.51 1.35 3.25 5.70 8.80

 Aggregate Nutrient Ecoregion: VI 9

 Rivers and Streams

 Descriptive Statistics by Decade and Season

 Parameter Org_P_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 19 41.6 1.77 131.98 31.2 7.16 75 1.77 16.8 36.1 55.5 132

 47 SPRING 0 . . . . . . . . . . .

 47 SUMMER 21 73.6 18.8 249.79 52.2 11.4 71 21.6 38.3 70.0 90.1 137

 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 . . . . . . . . . . .

 57 WINTER 0 . . . . . . . . . . .

 Aggregate Nutrient Ecoregion: VI 10

 Rivers and Streams

 Descriptive Statistics by Decade and Season

 Parameter Orthophosphate_T_as_P_ug_L_Med

 Eco_

 Level_

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

 46 FALL 3 161 62.8 240.00 90.2 52.0 56 62.8 62.8 180 240 240

 46 SPRING 3 115 5.00 240.00 118 68.3 103 5.00 5.00 100 240 240

 46 SUMMER 4 363 70.0 932.50 402 201 111 70.0 80.0 224 645 933

 46 WINTER 4 416 62.5 1300.00 592 296 142 62.5 86.3 150 745 1300

 47 FALL 7 161 5.00 430.00 184 69.4 114 5.00 30.0 80.0 420 430

 47 SPRING 8 190 80.0 495.00 139 49.2 73 80.0 90.0 148 235 495

 47 SUMMER 11 184 40.0 570.00 145 43.7 79 40.0 90.0 145 230 570

 47 WINTER 9 278 110 650.00 205 68.2 74 110 130 165 430 650

 48 FALL 4 77.5 10.0 140.00 58.5 29.3 76 10.0 30.0 80.0 125 140

 48 SPRING 4 109 12.5 192.50 94.6 47.3 87 12.5 27.5 115 190 193

 48 SUMMER 4 113 47.5 185.00 65.0 32.5 57 47.5 58.8 110 168 185

 48 WINTER 4 144 20.0 315.00 138 68.9 96 20.0 32.5 120 255 315

 54 FALL 3 150 40.0 310.00 142 81.9 95 40.0 40.0 100 310 310

 54 SPRING 5 107 35.0 267.50 94.0 42.0 88 35.0 45.0 85.0 100 268

 54 SUMMER 5 146 43.8 380.00 138 61.8 95 43.8 62.5 82.5 160 380

 54 WINTER 5 172 75.0 255.00 80.6 36.0 47 75.0 100 195 235 255

 55 FALL 2 270 80.0 460.00 269 190 100 80.0 80.0 270 460 460

 55 SPRING 2 129 27.5 230.00 143 101 111 27.5 27.5 129 230 230

 55 SUMMER 2 166 62.5 270.00 147 104 88 62.5 62.5 166 270 270

 55 WINTER 2 119 67.5 170.00 72.5 51.3 61 67.5 67.5 119 170 170

 57 FALL 6 46.9 20.0 70.00 18.5 7.55 39 20.0 30.0 51.9 57.5 70.0

 57 SPRING 4 131 50.0 370.00 159 79.6 121 50.0 50.0 52.5 213 370

 57 SUMMER 6 35.4 12.5 60.00 18.9 7.70 53 12.5 15.0 37.5 50.0 60.0

 57 WINTER 6 34.6 10.0 75.00 21.9 8.95 63 10.0 22.5 32.5 35.0 75.0

 
 

 Aggregate Nutrient Ecoregion: VI 11

 Rivers and Streams

 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 45 1.17 .190 4.55 0.76 0.11 64 0.30 0.69 1.10 1.45 2.30

 46 SPRING 65 1.14 .255 2.30 0.45 0.06 39 0.46 0.90 1.14 1.35 1.91

 46 SUMMER 60 1.38 .240 2.68 0.52 0.07 37 0.49 1.09 1.47 1.70 2.21

 46 WINTER 58 1.34 .314 3.30 0.60 0.08 45 0.48 0.97 1.40 1.60 2.55

 47 FALL 118 0.91 .050 2.78 0.50 0.05 55 0.22 0.60 0.79 1.23 1.70

 47 SPRING 100 1.18 .050 4.40 0.64 0.06 54 0.42 0.80 1.00 1.53 2.29

 47 SUMMER 136 1.13 .063 4.50 0.68 0.06 60 0.40 0.65 1.00 1.52 2.38

 47 WINTER 102 1.19 .088 4.44 0.91 0.09 77 0.38 0.65 0.90 1.30 3.01

 48 FALL 30 0.87 .025 1.48 0.34 0.06 39 0.25 0.70 0.96 1.10 1.40

 48 SPRING 35 1.15 .480 1.71 0.30 0.05 26 0.70 0.99 1.11 1.40 1.70

 48 SUMMER 36 1.00 .025 1.75 0.43 0.07 43 0.03 0.83 1.05 1.28 1.65

 48 WINTER 31 1.13 .181 2.10 0.47 0.09 42 0.60 0.80 1.00 1.45 2.00

 54 FALL 109 0.94 .025 4.20 0.58 0.06 62 0.29 0.55 0.81 1.25 1.80

 54 SPRING 70 1.12 .200 3.55 0.57 0.07 51 0.50 0.75 1.00 1.35 2.30

 54 SUMMER 113 1.02 .075 3.10 0.51 0.05 50 0.29 0.68 0.90 1.34 1.98

 54 WINTER 75 1.02 .200 3.10 0.59 0.07 57 0.40 0.65 0.81 1.30 2.60

 55 FALL 150 0.68 .050 3.40 0.53 0.04 78 0.13 0.35 0.53 0.90 1.70

 55 SPRING 69 0.70 .025 2.00 0.42 0.05 59 0.25 0.40 0.60 0.95 1.45

 55 SUMMER 198 0.71 .050 4.05 0.56 0.04 80 0.08 0.40 0.57 0.90 2.10

 55 WINTER 63 0.68 .100 3.60 0.50 0.06 74 0.24 0.40 0.56 0.80 1.40

 57 FALL 84 0.89 .175 3.80 0.51 0.06 57 0.35 0.60 0.76 1.04 1.70

 57 SPRING 57 1.03 .325 2.50 0.39 0.05 38 0.55 0.80 0.90 1.21 1.80

 57 SUMMER 85 0.96 .125 5.10 0.61 0.07 64 0.38 0.60 0.86 1.10 1.68

 57 WINTER 48 0.93 .380 1.90 0.36 0.05 39 0.49 0.70 0.80 1.09 1.63

 Aggregate Nutrient Ecoregion: VI 12

 Rivers and Streams

 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 4 2.28 1.25 4.60 1.57 0.79 69 1.25 1.33 1.63 3.23 4.60

 46 SPRING 4 1.73 1.00 2.30 0.63 0.31 36 1.00 1.20 1.80 2.25 2.30

 46 SUMMER 4 1.97 1.40 2.73 0.61 0.31 31 1.40 1.48 1.88 2.46 2.73

 46 WINTER 4 2.38 1.83 2.70 0.42 0.21 17 1.83 2.06 2.50 2.70 2.70

 47 FALL 30 4.01 .220 11.08 2.92 0.53 73 0.57 1.68 3.55 6.61 8.85

 47 SPRING 10 5.54 2.40 9.90 2.87 0.91 52 2.40 3.50 3.84 8.30 9.90

 47 SUMMER 32 5.41 .890 10.21 2.82 0.50 52 1.78 3.02 4.85 7.82 10.2

 47 WINTER 11 5.14 2.70 8.55 1.81 0.54 35 2.70 3.85 4.80 6.28 8.55

 48 FALL 4 0.92 .650 1.20 0.24 0.12 26 0.65 0.73 0.91 1.11 1.20

 48 SPRING 4 1.49 1.10 2.00 0.39 0.19 26 1.10 1.20 1.43 1.78 2.00

 48 SUMMER 4 1.25 1.00 1.40 0.18 0.09 14 1.00 1.12 1.29 1.38 1.40

 48 WINTER 4 1.48 1.20 2.15 0.46 0.23 31 1.20 1.20 1.28 1.75 2.15

 54 FALL 11 3.87 .940 7.02 1.86 0.56 48 0.94 2.48 3.37 5.12 7.02

 54 SPRING 11 4.86 1.60 8.65 2.26 0.68 46 1.60 2.70 4.90 5.67 8.65

 
 

 54 SUMMER 11 4.60 1.30 7.65 1.78 0.54 39 1.30 3.26 4.95 5.80 7.65

 54 WINTER 11 5.00 1.58 7.05 1.91 0.57 38 1.58 3.20 6.00 6.65 7.05

 55 FALL 2 3.58 3.55 3.60 0.04 0.03 1 3.55 3.55 3.58 3.60 3.60

 55 SPRING 2 3.80 3.70 3.90 0.14 0.10 4 3.70 3.70 3.80 3.90 3.90

 55 SUMMER 2 3.58 3.50 3.65 0.11 0.08 3 3.50 3.50 3.58 3.65 3.65

 55 WINTER 2 3.95 3.80 4.10 0.21 0.15 5 3.80 3.80 3.95 4.10 4.10

 57 FALL 14 1.55 .340 3.40 0.90 0.24 58 0.34 0.97 1.53 2.04 3.40

 57 SPRING 17 3.78 1.57 8.50 1.96 0.47 52 1.57 2.20 3.10 4.91 8.50

 57 SUMMER 24 2.67 1.16 4.65 1.02 0.21 38 1.20 1.63 2.75 3.35 4.39

 57 WINTER 25 3.06 .880 6.98 1.51 0.30 49 1.05 2.22 2.75 3.79 5.90

 Aggregate Nutrient Ecoregion: VI 13

 Rivers and Streams

 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 57 255 30.0 1497.50 245 32.4 96 45.0 100 185 320 680

 46 SPRING 76 222 40.0 790.00 145 16.6 65 60.0 105 195 303 520

 46 SUMMER 72 402 50.0 1277.50 292 34.4 73 80.0 210 314 448 1030

 46 WINTER 65 284 27.5 1190.00 247 30.7 87 37.5 100 240 370 770

 47 FALL 153 222 12.5 900.00 175 14.1 79 25.0 100 195 293 590

 47 SPRING 134 286 10.0 2105.00 328 28.3 115 45.0 130 205 300 785

 47 SUMMER 187 275 25.0 1335.00 217 15.8 79 25.0 130 240 359 695

 47 WINTER 123 272 5.00 2250.00 295 26.6 108 30.0 106 213 310 705

 48 FALL 33 165 2.50 325.00 92.5 16.1 56 30.0 90.0 160 240 325

 48 SPRING 39 170 22.5 417.50 95.2 15.2 56 25.0 85.0 160 250 363

 48 SUMMER 38 284 50.0 2200.00 337 54.7 118 50.0 170 230 285 570

 48 WINTER 36 174 30.0 490.00 131 21.9 76 32.5 47.8 145 258 428

 54 FALL 162 278 10.0 2000.00 342 26.8 123 40.0 80.0 160 275 960

 54 SPRING 137 185 10.0 1307.50 198 16.9 107 25.0 60.0 118 240 620

 54 SUMMER 203 195 10.0 1217.50 194 13.6 99 32.5 75.0 140 230 670

 54 WINTER 146 203 5.00 1635.00 272 22.5 134 25.0 70.0 110 220 920

 55 FALL 171 225 10.0 2430.00 324 24.8 144 10.0 60.0 105 270 845

 55 SPRING 82 125 5.00 1180.00 151 16.7 121 10.0 60.0 90.0 145 240

 55 SUMMER 225 253 10.0 2120.00 365 24.3 144 10.0 70.0 120 225 1010

 55 WINTER 75 145 10.0 1520.00 186 21.5 128 25.0 65.0 100 170 415

 57 FALL 89 186 10.0 1790.00 248 26.3 133 35.0 70.0 115 195 685

 57 SPRING 63 134 10.0 855.00 133 16.7 99 40.0 70.0 93.0 140 355

 57 SUMMER 90 203 10.0 1595.00 295 31.1 145 10.0 70.0 115 190 940

 57 WINTER 56 99.2 10.0 420.00 68.2 9.12 69 30.0 60.0 80.0 110 221

 Aggregate Nutrient Ecoregion: VI 14

 Rivers and Streams

 Descriptive Statistics by Decade and Season

 Parameter Turb_FTU_Median

 Eco_

 Level_

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

 46 FALL 6 19.2 7.35 48.50 15.5 6.34 81 7.35 8.50 15.1 20.5 48.5

 
 

 46 SPRING 6 9.21 1.00 20.00 8.70 3.55 94 1.00 1.98 6.14 20.0 20.0

 46 SUMMER 9 15.5 1.10 45.00 17.4 5.79 112 1.10 4.50 6.73 17.0 45.0

 46 WINTER 6 8.94 2.00 18.00 6.49 2.65 73 2.00 4.15 7.25 15.0 18.0

 47 FALL 69 16.2 .900 58.00 12.0 1.44 74 1.75 8.00 13.7 21.0 44.0

 47 SPRING 69 21.8 2.85 170.00 23.8 2.86 109 3.40 7.38 15.6 30.0 41.0

 47 SUMMER 74 31.9 .500 186.00 31.1 3.62 98 2.60 11.5 20.3 45.5 96.0

 47 WINTER 63 18.4 1.05 190.00 29.1 3.66 158 1.45 4.00 9.25 17.7 64.7

 48 FALL 8 13.8 5.95 40.00 11.0 3.89 79 5.95 7.38 11.5 13.5 40.0

 48 SPRING 10 14.4 3.75 49.50 14.3 4.54 99 3.75 6.30 8.95 12.5 49.5

 48 SUMMER 12 19.5 3.85 58.00 15.4 4.44 79 3.85 5.98 17.5 27.0 58.0

 48 WINTER 10 5.48 2.90 13.00 2.97 0.94 54 2.90 3.70 4.50 6.55 13.0

 54 FALL 94 14.3 1.30 102.70 16.4 1.69 115 2.20 5.30 9.53 15.0 53.1

 54 SPRING 68 12.0 3.20 47.50 7.85 0.95 66 3.65 6.79 9.25 16.0 24.0

 54 SUMMER 104 12.9 2.10 56.00 8.53 0.84 66 2.60 7.20 11.5 17.0 23.7

 54 WINTER 77 8.80 2.50 44.50 6.67 0.76 76 3.50 4.80 7.00 10.0 23.0

 55 FALL 13 16.7 1.40 48.00 13.9 3.86 84 1.40 9.00 11.3 22.4 48.0

 55 SPRING 17 17.0 4.30 62.50 14.5 3.51 85 4.30 9.60 11.0 15.0 62.5

 55 SUMMER 12 19.4 6.80 53.30 13.8 3.97 71 6.80 9.43 14.7 28.5 53.3

 55 WINTER 22 18.0 2.30 48.00 12.1 2.58 67 3.00 6.55 19.1 25.4 33.0

 57 FALL 22 12.3 3.20 67.00 13.6 2.91 111 3.70 4.70 7.93 15.6 23.0

 57 SPRING 29 11.5 2.10 42.00 8.30 1.54 72 2.60 6.40 9.30 14.0 25.0

 57 SUMMER 28 9.31 2.00 25.00 5.97 1.13 64 2.20 4.65 7.35 14.5 18.5

 57 WINTER 20 21.3 2.90 140.00 39.6 8.84 185 3.30 4.40 7.50 11.0 135

 Aggregate Nutrient Ecoregion: VI 15

 Rivers and Streams

 Descriptive Statistics by Decade and Season

 Parameter Turb_JCU_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 42 23.7 4.80 92.00 19.7 3.04 83 5.60 9.80 17.5 29.0 66.0

 47 SPRING 20 39.4 4.35 141.00 35.9 8.04 91 9.08 19.5 26.8 36.1 131

 47 SUMMER 56 34.2 4.10 190.00 38.8 5.18 113 6.90 10.5 20.0 31.5 138

 47 WINTER 21 14.6 .700 44.50 12.2 2.66 84 3.00 7.00 9.20 17.8 37.0

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

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

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

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

 54 FALL 1 35.9 35.9 35.85 . . . 35.9 35.9 35.9 35.9 35.9

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

 54 SUMMER 1 40.7 40.7 40.70 . . . 40.7 40.7 40.7 40.7 40.7

 54 WINTER 1 31.6 31.6 31.60 . . . 31.6 31.6 31.6 31.6 31.6

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

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

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

 55 WINTER 1 28.0 28.0 28.00 . . . 28.0 28.0 28.0 28.0 28.0

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

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

 57 SUMMER 1 40.0 40.0 40.00 . . . 40.0 40.0 40.0 40.0 40.0

 57 WINTER 0 . . . . . . . . . . .

 
 

 Aggregate Nutrient Ecoregion: VI 16

 Rivers and Streams

 Descriptive Statistics by Decade and Season

 Parameter Turb_NTU_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 24 40.0 4.00 160.00 36.7 7.48 92 5.00 13.5 29.5 52.0 104

 47 SPRING 1 120 120 120.00 . . . 120 120 120 120 120

 47 SUMMER 32 48.2 4.00 230.00 50.2 8.87 104 5.00 15.0 40.0 54.5 140

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

 48 FALL 1 8.45 8.45 8.45 . . . 8.45 8.45 8.45 8.45 8.45

 48 SPRING 1 4.45 4.45 4.45 . . . 4.45 4.45 4.45 4.45 4.45

 48 SUMMER 1 5.90 5.90 5.90 . . . 5.90 5.90 5.90 5.90 5.90

 48 WINTER 1 3.15 3.15 3.15 . . . 3.15 3.15 3.15 3.15 3.15

 54 FALL 1 19.0 19.0 19.00 . . . 19.0 19.0 19.0 19.0 19.0

 54 SPRING 1 11.1 11.1 11.10 . . . 11.1 11.1 11.1 11.1 11.1

 54 SUMMER 1 14.0 14.0 14.00 . . . 14.0 14.0 14.0 14.0 14.0

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

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

 55 SPRING 1 18.0 18.0 18.00 . . . 18.0 18.0 18.0 18.0 18.0

 55 SUMMER 1 10.4 10.4 10.40 . . . 10.4 10.4 10.4 10.4 10.4

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

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

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

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

 57 WINTER 0 . . . . . . . . . . .

 
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.
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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.
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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
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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 
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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. 
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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 
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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.)
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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.