Trends in Nutrients and Suspended Solids at the 

Fall Line of Five Tributaries to the Chesapeake Bay 

in Virginia, July 1988 through June 1995

By C.F. Bell, D.L. Belval, and J.P. Campbell

U.S. GEOLOGICAL SURVEY 

Water-Resources Investigations Report 96-4191

Prepared in cooperation with the

VIRGINIA DEPARTMENT OF ENVIRONMENTAL QUALITY, 

CHESAPEAKE BAY AND COASTAL PROGRAMS

Richmond, Virginia 

1996
U.S. DEPARTMENT OF THE 

BRUCE BABBITT, Secre

U.S. GEOLOGICAL SURV 

Gordon P. Eaton, Directo

sITERIOR 

ary

Y

For additional information write to:

District Chief

U.S. Geological Survey

3600 West Broad Street, Room 606

Richmond, VA 23230-4916

Copies <

U.S. Ge 

Informa 

Box 25 

Denver,

f this report can be purchased from:

logical Survey 

on Services 

86 

O 80225-0046
CONTENTS

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

Introduction........................................................................................................................................................................... 2

Purpose and scope ....................................................................................................................................................... 2

Previous investigations................................................................................................................................................ 4

Description of study area............................................................................................................................................. 4

Acknowledgments....................................................................................................................................................... 6

Methods of study ................................................................................................................................................................... 6

Trend estimation............................................................................................._^ 6

Multivariate log-linear regression ..................................................................................................................... 7

Flow-adjusted seasonal Kendall test.................................................................................................................. 7

Streamflow partitioning............................................................................................................................................... 8

Quality assurance and quality control......................................................................................................................... 8

Trends in nutrients and suspended solids .............................................................................................................................. 8

James River.................................................................................................................................................................. 12

Rappahannock River.................................................................................................................................................... 12

Appomattox River....................................................................................................................................................... 13

Pamunkey River.......................................................................................................................................................... 13

Mattaponi River........................................................................................................................................................... 14

Conclusions ........................................................................................................................................................................... 26

References cited..................................................................................................................................................................... 26

Appendixes............................................................................................................................................................................ 29

A. Quality-assurance and quality-control procedures and results...................................................................................... 30

Table A-l. Results of variance computations for duplicate quality-assurance samples analyzed by the

Virginia Division of Consolidated Laboratory Services ................................................................................. 31

A-2. Results of the Wilcoxon signed-rank test comparing constituent concentration analyzed by the 

Virginia Division of Consolidated Laboratory Services with constituent concentration analyzed

by the National Water-Quality Laboratory...................................................................................................... 33

A-3. Summary of the number of field blanks with concentrations greater than the minimum reporting

limit for samples analyzed by the Virginia Division of Consolidated Laboratory Services ........................... 35

B. Summary statistics of constituent concentration and load............................................................................................. 35

Table B-l. Summary statistics for selected water-quality constituents at the Fall Line monitoring stations.................... 36

B-2. Summary of load statistics for the Fall Line monitoring stations.................................................................... 37

FIGURES

1-3. Maps showing:

1. Chesapeake Bay drainage area........................................................................................................................... 3

2. Locations of Fall Line monitoring stations........................................................................................................ 5

3. Significant trends in total nitrogen and total phosphorus for the study period.................................................. 11

4-14. Time-series plots showing relation of selected constituent concentration, and flow-adjusted residuals of the 

concentration/discharge regression for:

4. Total phosphorus at the James River monitoring station................................................................................... 15

5. Total nitrogen at the Rappahannock River monitoring station .......................................................................... 16

6. Total Kjeldahl nitrogen at the Rappahannock River monitoring station ........................................................... 17

7. Dissolved nitrite-plus-nitrate at the Rappahannock River monitoring station................................................... 18

8. Total phosphorus at the Rappahannock River monitoring station..................................................................... 19

Contents III
9. Total suspended solids at the Rappahannock River monitoring station............................................................ 20

10. Total Kjeldahl nitrogen at the Appomattox River monitoring station............................................................... 21

11. Total phosphorus at the Appomattox River monitoring station ........................................................................ 22

12. Total phosphorus at the Pamunkey River monitoring staticn............................................................................ 23

13. Total Kjeldahl nitrogen at the Mattaponi River monitoring

14. Dissolved nitrite-plus-nitrate at the Mattaponi River moni

TABLES

1. Location, drainage area, upstream land use, and major upstream w< 

Fall Line monitoring stations ............................................................

2. Flow-adjusted trends in concentration and load of selected constitu

CONVERSION FACTORS, VERTICAL DATUM, AND ABBREVIATED WATER-QUALITY UNITS

Multiply By

Length

foot (ft) 

mile (mi)

0.3048 

1.609

Area

square mile (mi2) 2.590

Flow

million gallons per day (Mgal/d) 0.04381

Mass

pound, avoirdupois (Ib) 0.4536

To obtain

meter 

kilometer

square kilometer

cubic meter per second

kilogram (kg)

station.................................................................. 24

oring station......................................................... 25

stewater discharge at the

:nts at the Fall Line monitoring stations............... 9

Vertical datum: In this report, "sea level" refers to the National Geodeti 

from a general adjustment of the first-order level nets of the United States and Canada

: Vertical Datum of 1929 a geodetic datum derived 

, formerly called Sea Level Datum of 1929.

Water-quality units: Chemical concentration is reported in milligrams peri

Milligrams per liter is a unit expressing the concentration of chemical constituents in 

per unit volume (liter) of water. One thousand micrograms per liter is equival 

less than 7,000 mg/L, the numerical value is the same as for concentrations in part 

of water is reported in microsiemens per centimeter at 25 degrees Celsius (uS, 

converted to degrees Fahrenheit (°F) by the following equation:

°F=1.8X(°C)

iter (mg/L) or micrograms per liter (|J.g/L).

solution as weight (milligrams) of solute 

to one milligram per liter. For concentrations 

:s per million. Specific electrical conductance 

'cm). Temperature in degree Celsius (°C) can be

IV Contents
Trends in Nutrients and Suspended Solids at the Fall 

Line of Five Tributaries to the Chesapeake Bay in 

Virginia, July 1988 through June 1995

By Clifton F. Bell, Donna L. Belval, and Jean P. Campbell

Abstract

Water-quality samples were collected at the 

Fall Line of five tributaries to the Chesapeake Bay 

in Virginia during a 6- to 7-year period. The 

water-quality data were used to estimate loads of 

nutrients and suspended solids from these tributaries to the non-tidal part of Chesapeake Bay 

Basin and to identify trends in water quality. 

Knowledge of trends in water quality is required 

to assess the effectiveness of nutrient management strategies in the five basins.

Multivariate log-linear regression and the 

seasonal Kendall test were used to estimate flowadjusted trends in constituent concentration and 

load. Results of multivariate log-linear regression 

indicated a greater number of statistically significant trends than the seasonal Kendall test; however, when both methods indicated a significant 

trend, both agreed on the direction of the trend. 

Interpretation of the trend estimates for this report 

was based on results of the parametric regression 

method.

No significant trends in total nitrogen 

concentration were detected at the James River 

monitoring station from July 1988 through June 

1995, though total Kjeldahl nitrogen concentration decreased slightly in base-flow samples. 

Total phosphorus concentration decreased about 

29 percent at this station during the sampling 

period. Most of the decrease can be attributed to 

reductions in point-source phosphorus loads in 

1988 and 1989, especially the phosphate detergent ban of 1988. No significant trends in total 

suspended solids were observed at the James

River monitoring station, and no trends in runoffderived constituents were interpreted for this 

river.

Significant decreases were detected in 

concentrations of total nitrogen, total Kjeldahl 

nitrogen, dissolved nitrite-plus-nitrate nitrogen, 

and total suspended solids at the Rappahannock 

River monitoring station between July 1988 and 

June 1995. A similar downward trend in total 

phosphorus concentration was significant at the 

90-percent confidence level, but not the 95- 

percent confidence level. These decreases can be 

attributed primarily to reductions in nonpoint 

nutrient and sediment loads, and may have been 

partially caused by implementation of best 

management practices on agricultural and 

silvicultural land.

Flow-adjusted trends observed at the 

Appomattox, Pamunkey, and Mattaponi monitoring stations were more difficult to explain than 

those at the James and Rappahannock stations. 

Total Kjeldahl nitrogen and total phosphorus 

increased 16 and 23 percent, respectively, at the 

Appomattox River monitoring station from July 

1989 through June 1995. Total phosphorus 

concentration increased about 46 percent at the 

Pamunkey River monitoring station between July 

1989 and June 1995. At the Mattaponi River 

monitoring station, decreases in dissolved nitriteplus-nitrate nitrogen were offset by increases in 

total Kjeldahl nitrogen, resulting in no net change 

in total nitrogen concentration from October 1989 

through June 1995.

Abstract 1
INTRODUCTION

The Chesapeake Bay is the largest and most 

productive estuary in the United States, extending 

nearly 200 mi from the mouth of the Susquehanna 

River in Maryland to where it discharges to the 

Atlantic Ocean along the southeastern coast of 

Virginia (fig. 1). The basin includes approximately 

64,000 mi2 of Delaware, Maryland, New York, 

Pennsylvania, Virginia, West Virginia, and the 

District of Columbia and has a population of more 

than 15 million people. In addition to providing 

habitats for coastal and marine wildlife, Chesapeake 

Bay is an important economic and recreational 

resource, providing thousands of jobs in commercial 

fishing, sport fishing, shipping, and other industries.

Intense agriculture, silviculture, and development in the Chesapeake Bay Basin have degraded 

the water quality of the Bay, and threatens its 

economic, recreational, and ecological value. The 

Chesapeake Bay Program, a partnership of government agencies and Bay-related organizations formed 

in 1983 to direct restoration efforts, has targeted three 

critical issues for intense investigation: (1) nutrient 

enrichment, (2) toxic substances, and (3) the decline of 

submerged aquatic vegetation. Nutrient enrichment 

has been identified as the primary cause of the decline 

in water quality of the Bay (Jordan and others, 1992). 

High concentrations of nitrogen and phosphorus cause 

excessive growth of algae, which deprives submerged 

aquatic vegetation of sunlight and, during decay, consumes dissolved oxygen. This in turn has a detrimental 

effect on fish, crabs, molluscs, and other species.

In order to address the problem of nutrient 

enrichment, the States of Maryland, Pennsylvania, 

Virginia, and the District of Columbia signed the 

Chesapeake Bay Agreement in 1987. This agreement 

commits Federal, State, and other agencies to work 

toward a 40 percent reduction in controllable nitrogen 

and phosphorus inputs to Chesapeake Bay by the year 

2000, based on 1985 levels. The achievement of this 

goal will greatly restrict algal blooms in the Bay and 

will result in higher oxygen levels throughout the year.

Freshwater flow to Chesapeake Bay from its 

major tributaries is the single largest source of nutrients to the Bay. In 1992, The Chesapeake Bay Executive Council approved amendments to the 1987 

agreement that committed member States to reduce 

nutrients inputs in the major river basins that discharge 

to the Bay. In response to the amendments, the 

member States are currently developing or

implementing "tributary strategies," which are 

docurients that outline plans for meeting nutrient

reduc

In order to assess the effectiveness of tributary 

strategies and determine the effects of nutrients and 

sediment on water quality and living resources, loads 

and temporal trends of water-quality constituents at

theFc 

ically

Load

reduc

ion goals for the 10 largest tributaries to the Bay.

11 Line are quantified. The Fall Line is geographdefined as the boundary between the Piedmont

and Coastal Plain Physiographic Provinces, and generally represents the upstream limit of tides (fig. 1).

estimates at the Fall Line are particularly important for use as input to computer models of the Bay, 

and trends in water-quality at the Fall Line are a key 

indicator of the effectiveness of tributary strategies at

ng nutrient loads entering the estuary from the

non-tidal part of its tributary basins.

In 1984, the U.S. Geological Survey (USGS), in 

cooperation with the Virginia Department of Environmental Quality (DEQ), began collecting water-quality 

samples at sites near the Fall Line on the James, 

Rappahannock, Pamunkey, and Mattaponi Rivers on a 

twicejper-month basis, with the purpose of quantifying loads of selected water-quality constituents 

contributed to the tidal part of the Bay Basin. In 1988, 

the monitoring program was expanded to include 

stormflow sampling at the James and Rappahannock 

Rivers; and in 1989, stormflow sampling was initiated 

at the Appomattox, Pamunkey, and Mattaponi Rivers. 

The Maryland District of the USGS, in cooperation 

with t le Maryland Department of Natural Resources, 

has operated a similar monitoring program for the 

Susquehanna, Potomac, Patuxent, and Choptank 

River$ since 1984. This information is now being used 

to assess the response of water quality to the implementation of nutrient reduction strategies.

Purpose and Scope

The purpose of this report is to: (1) Present flowadjusted trends in nutrients and suspended solids for 

five rr ajor tributaries to Chesapeake Bay in Virginia 

near the Fall Line; (2) discuss possible factors that 

affect trends in water quality at these rivers, and (3) 

assess quality assurance/quality control results.

Water-quality samples were collected at the 

James and Rappahannock Rivers from July 1, 1988 

through June 30, 1995; at the Appomattox and Pamunkey Rivers from July 1, 1989 through June 30, 1995; 

and at the Mattaponi River from September 1, 1989

2 Trends in Nutrients and Suspended Solids at the Fall Line of Five Tributaries to the Chesapeake Bay in Virginia, 7/88 6/95
50 100 KILOMETERS

Figure 1. Chesapeake Bay drainage area.

Introduction 3
through June 30, 1995. The samples were collected on 

a twice-per-month scheduled basis, most often during 

base-flow conditions, and also during high-flow events 

to cover a range in discharge conditions. Monthly 

loads and flow-adjusted trends of selected constituents 

were estimated by use of a seven-parameter log-linearregression model (Cohn and others, 1992). Flowadjusted trends also were estimated by use of the 

seasonal Kendall test.

Statistical summaries of concentrations, loads, 

and flow-adjusted trends of selected constituents are 

presented in tabular format. Time-series plots of constituent concentrations that exhibit significant trends 

are presented, as well as time-series plots of flowadjusted residuals of the concentration/discharge 

regression, which are more useful for displaying flowadjusted trends than time-series plots of the concentration data. Quality-assurance/quality-control procedures and results from July 1988 to June 1995 are 

presented in appendix A.

Previous Investigations

Several previous investigations provide information on water-quality monitoring and load estimation within the Chesapeake Bay Basin. Lang and 

Grason (1980) provided water-quality monitoring data 

for the Susquehanna, Potomac, and James Rivers. 

Lang (1982) computed loads of nutrients and metals 

from these rivers by use of a bivariate linearregression equation method. Cohn and others (1992) 

used the minimum variance unbiased estimator of 

Bradu and Mundlak (1970) to estimate nutrient loads 

from the Susquehanna, Potomac, Patuxent, and 

Choptank Rivers near the Fall Line in Maryland. This 

method also was used to estimate constituent 

concentrations and loads in tributaries to the 

Chesapeake Bay from Pennsylvania (Ott and others, 

1990; Fishel and others, 1991), from Maryland 

(Maryland Department of the Environment, 1993, 

1994, 1995), and from Virginia (Belval and others, 

1994; Belval and others, 1995).

All concentration data and load estimates used 

in the preparation of this report are presented in two 

previous reports (Belval and others, 1995; Belval and 

Campbell, 1996). Regression results used in this report 

to estimate trends are presented in Belval and 

Campbell (1996).

Description of Study Area

The contributing drainage areas discussed in this 

report collectively represent about 17 percent of the 

total Chesapeake Bay drainage area, and about 46 

percent of the Virginia part of Chesapeake Bay Basin; 

the reriainder of the Virginia part of Chesapeake Bay 

Basin consists mostly of the Virginia Coastal Plain 

Physic graphic Province and the Virginia part of the 

Potomac River Basin. The locations of the five monitoring stations are shown in figure 2, and the station 

coordinates, drainage areas, land-use data, and upstream wastewater discharge for the five basins are 

listed in table 1.

The James River monitoring station is located at 

Carter sville and receives drainage from about 60 

percent of the James River Basin. The river is the third 

largest source of freshwater to Chesapeake Bay, after 

the Susquehanna and Potomac Rivers. The James 

River 3asin extends from the eastern part of West 

Virginia through four physiographic provinces (1) 

Valley and Ridge, (2) Blue Ridge, (3) Piedmont, and 

(4) Coastal Plain (fig. 1). The monitoring station is 

about 40-mi upstream of the Fall Line, but was selected because of the well-documented long-term discharge record, and because no major streams contribute to the discharge of the river between this station 

and the Fall Line at Richmond. Of the five basins discussed in this report, the James River Basin has the 

seconc 1 highest percentage forested land upstream 

from tie monitoring station (72 percent), the lowest 

percentage agricultural land (23 percent), and the 

greatest volume of wastewater discharge (table 1).

The Rappahannock River monitoring station is 

located upstream of Fredericksburg, Va. The area of 

the drainage basin upstream from the monitoring 

station is approximately 1,596 mi2 , which is about 

56 percent of the Rappahannock River Basin. The 

river f ows from the eastern edge of the Blue Ridge 

Physiographic Province through the Piedmont and 

Coastal Plain Physiographic Provinces to the Chesapeake Bay, and it is the second largest contributing 

stream to the Chesapeake Bay in Virginia. The high 

relief in the basin produces rapid or "flashy" streamflow peaks during storm events. The Rappahannock

River

4 Trends in Nutrients and Suspended Solids at the Fall Line of Five Tributaries to the Chesapeake Bay in Virginia, 7/88 6/95

:an carry large loads of suspended solids and

other constituents relative to the size of the basin 

(Belval and others, 1995). This basin has the highest 

percentage of agricultural land use upstream from the 

monitoring station (44 percent) of the five basins 

discussed in this report.
EXPLANATION

 DRAINAGE BASIN BOUNDARY 

A FALL LINE MONITORING STATION WASHINGTON, D. C.

MATTAPONI RIVER STATION

PAMUNKEY RIVER STATION 

76°

TENNESSEE NORTH CAROLINA

Figure 2. Locations of Fall Line monitoring stations in the James, Rappahannock, Appomattox, Pamunkey, and 

Mattaponi River Basins, Virginia.

Table 1. Location, drainage area, upstream land use, and major upstream wastewater discharge at the Fall Line 

monitoring stations in Virginia

[mi2 , square mile; Mgal/d, million gallons per day; land-use data from Langland and others, 1995; land use as percentage of total land-surface area in 

each drainage basin upstream from monitoring station; USGS, U.S. Geological Survey; VDEQ, Virginia Department of Environmental Quality]

Upstream land use

USGS 

station 

number

02035000

01668000

02041650

01673000

VDEQ 

station 

number

TF5.1

TF3.1

TF5.4A

TF4.1

Station , ... . Latitude 

name

James River at Cartersville 37°40' 15"

Rappahannock River near 38°19'20"

Fredericksburg

Appomattox River at 37° 1 3'28"

Matoaca

Pamunkey River near 37°46'03"

Longitude

78°05'10"

77°31'05"

77°28'32"

77°19'57"

Drainage 

area 

(mi2)

6,257

1,596

1,344

1,081

Developed 

(percent)

4

2

2

2

Agricultural 

(percent)

23

44

34

35

Forested1 

(percent

72

50

62

59

Major

upstream 

wastewater 

discharge 

(Mgal/d)

89.4

4.7

1.1

5.0

Hanover

01674500 TF4.3 Mattaponi River near 37°53'02" 77°09'55" 

Beulahville

601 28 69

Includes wetlands.

Introduction 5
The Appomattox River Basin lies within the 

James River Basin but was monitored separately 

because the Appomattox River enters the James River 

below the Fall Line and is not included as a source to 

the James River monitoring station. The Appomattox 

River Basin begins in the Piedmont Physiographic 

Province, flows through a small part of the Coastal 

Plain Physiographic Province, and enters the James 

River estuary near Hopewell, Va. Discharge in the 

Appomattox River is affected by a dam 2.8-mi upstream of the monitoring station at Matoaca. This 

control acts to delay water-level rise during storm 

events, so that the water level is very slow to rise and 

fall in comparison with the other monitoring stations. 

Biogeochemical processes in Lake Chesdin, the lake 

formed by the dam, may strongly affect water quality 

at the Appomattox River monitoring station (Belval 

and others, 1995).

The York River Basin constitutes about 6.5 

percent of the Virginia part of Chesapeake Bay Basin. 

The basin consists of the Pamunkey River, the Mattaponi River, and the coastal area below the confluence 

of these two rivers. Agriculture is an important component of the economy of the York River Basin, and 

the area is primarily rural. Although the Pamunkey 

and Mattaponi Rivers are often presented collectively 

as the York River Basin, each river has unique 

drainage basin, discharge, and water-quality characteristics and were monitored separately for this study.

The Pamunkey River Basin begins in the lower, 

eastern part of the Piedmont Physiographic Province 

where the relief is relatively low and extends into the 

Coastal Plain Physiographic Province. The drainage 

basin above the monitoring station represents about 40 

percent of the York River Basin, and about 4 percent 

of the Virginia part of Chesapeake Bay Basin. The 

basin is of low relief and relatively wide, and tends to 

produce stormflow that is slow to peak and recede. 

Some regulation of the Pamunkey River occurs from 

the Lake Anna Dam, approximately 60-mi upstream 

of the monitoring station. The basin contains expanses 

of forested wetlands and marshes.

The total area of the Mattaponi River Basin 

above the monitoring station is approximately 601 

mi2, which is about 23 percent of the entire York River 

Basin, and 2 percent of the Virginia part of Chesapeake Bay Basin. Like the Pamunkey River, the 

Mattaponi River Basin has a relatively low relief and 

expanses of wetland areas. Stormflows are even 

slower to peak and recede than at the Pamunkey River.

Acknowledgments

The authors wish to thank Frederick Hoffman 

and IV ark Bushing of the Virginia DEQ, Chesapeake 

Bay and Coastal Programs, for their assistance and 

guidance of the program. The majority of analyses for 

this project were performed by the Nutrients Laboratory and Non-metals Laboratory of the Virginia Division of Consolidated Laboratory Services (VDCLS), 

Bureau of Chemistry. Norma Roadcap of the Nutrients 

Laboratory provided assistance with water-quality 

data management, and some field-data collection was 

perfoimed by the Piedmont Field Office of the 

Virginia DEQ. We thank all persons involved for their 

high-( uality work.

METHODS LT|H OF STUDY

Methods of field-data collection and load estimation are described by Belval and others (1995). The 

following discussion pertains to methods used to (1) 

estimate temporal trends in water-quality data at the 

Fall Line; (2) perform streamflow partitioning for use 

in interpreting trends; and (3) present qualityassurance and quality-control results.

Trend Estimation

Trends in constituent concentration were 

estimated by two methods: (1) multivariate log-linear 

regression, and (2) the flow-adjusted seasonal Kendall 

test. The log-linear regression method is parametric 

and requires that certain assumptions regarding the fit

of the water-quality data to the model are met, whereas

the seasonal Kendall test is nonparametric and requires that the data are independent and are from the 

same statistical distribution (Schertz and others, 

1991). Though the seasonal Kendall test requires 

fewer assumptions, parametric methods generally 

have more power to test hypotheses in situations 

where^ their use is appropriate. For this study, trends 

were interpreted primarily by use of the results of the 

log-linear regression. Other investigations of waterquality trends in the tidal part of Chesapeake Bay

Basin

6 Trends in Nutrients and Suspended Solids at the Fall Line of Five Tributaries to the Chesapeake Bay in Virginia, 7/88 6/95

have employed the seasonal Kendall test;

therefore, results from the seasonal Kendall test are 

presented in this report for purposes of comparison.
Multivariate Log-Linear Regression

Trends in water-quality data were estimated by 

use of the same multivariate log-linear regression 

model that was used to estimate daily concentration 

and load for the data-collection period (Cohn and 

others, 1992; Belval and others, 1995). The regression 

model has the following form:

ln(C) = Pn + P, ln| * 

Q

+ (3 3 [T- T] + (3 4 [T- T] " + (3 5 sin (2nT) 

+ p6 cos(27iF) +8,

where

C = the constituent concentration (in mg/L), 

Q = the instantaneous discharge (in ft3/s), 

T = time (in years),

(3 = coefficient of the regression model, 

8 = model error, and 

Q and T = centering variables.

Equation (1) estimates daily concentration (or 

load) as a function of discharge, time, and seasonality. 

The minimum variance unbiased estimator (MVUE) 

of Bradu and Mundlak (1970) was employed to correct for the bias introduced by retransformation of 

concentration estimates from logarithmic space to real 

space. The adjusted maximum likelihood estimator 

(AMLE) (Cohn, 1988) was used to assign concentration values to censored data, which are data 

reported as less than a certain value, called the 

reporting limit.

Temporal trends in concentration are estimated 

by use of the coefficient of the time parameter ((33 ) in 

the regression model, which represents the slope of the 

log-linear trend in concentration, independent of discharge and seasonality. The magnitude of the trend is 

calculated as follows:

%AC = lOOj/3 '-!}, (2)

where %AC is the percent change in concentration 

during any time t, with t expressed in years. The statistical significance of the trend may be determined from 

the probability value (p-value) of (33 , which is the estimated probability that random variation in concentration alone could produce the observed trend. For this 

report, a trend is considered significant if the p-value 

for [33 is less than, or equal to, 0.05, which corresponds 

to a 95-percent confidence level. Some trends with 

p-values less than, or equal to, 0.10 are also considered significant.

Use of regression for trend analysis requires 

certain assumptions about the results of the regression 

model; specifically, that the model form is correct and 

that the regression residuals are independent, normally 

distributed, and homoscedastic (Helsel and Hirsch, 

1992). Model results and residual plots were examined 

for each regression output before trends were calculated. Water-quality data and regression results from 

the Fall Line monitoring stations met the requirements 

for use of this method with the possible exception of 

model form; misspecification of model form could 

bias the magnitude of predicted trends. Model misspecification, however, is unlikely to affect the 

direction of the predicted trend. Trends were not 

calculated for dissolved orthophosphorus and ammonia because of the high percentage of censored data.

Trends estimated by the regression model are 

inherently flow-adjusted because multivariate regression separates variability in concentration that is due 

to variability in flow from that due to variability in 

time. If this is not done, trends in discharge can mask 

trends in concentration that are independent of discharge. Flow-adjusted trends in concentration calculated by this method are identical to flow-adjusted 

trends in load.

Flow-Adjusted Seasonal Kendall Test

The flow-adjusted seasonal Kendall test is 

described by Hirsch and others (1982), and was performed on Fall Line water-quality data by use of the 

program ESTREND, which is documented by Schertz 

and others (1991). This method is a variant of the 

Mann-Kendall test, which tests the hypothesis that the 

sum of the signs of the differences between temporally successive data is significantly different from 

zero. Flow-adjustment was accomplished by performing the seasonal Kendall test on the residuals 

from a locally weighted scatterplot smoothing 

(LOWESS) line fitted to the concentration/discharge 

relation. Seasonal adjustment was accomplished by 

restricting possible comparisons to residuals of waterMethods of Study 7
quality data collected during the same month, and then 

summing the test statistics for all 12 months. All 

censored data were set to half the reporting limit for 

the seasonal Kendall test.

Trends detected by the seasonal Kendall test 

were considered significant if they had a p-value 

less than, or equal to, 0.05, which corresponds to a 

95-percent confidence level. If significant, the magnitude of a trend was calculated as the median slope 

of all possible pairwise comparisons (Schertz and 

others, 1991).

Streamflow Partitioning

Each water-quality sample was labeled as a 

"base-flow" or "stormflow" sample to assist in the 

interpretation of the water-quality data and trend estimates. Daily base flow and stormflow were estimated 

at each station for the entire sampling period by use of 

the streamflow-partitioning program PART, which is 

described by Rutledge (1993). Water-quality samples 

were then labeled as base-flow samples if, on the day 

on which they were collected, base flow constituted at 

least 70 percent of the total streamflow. Otherwise, the 

sample was labeled as a stormflow sample.

Quality Assurance and Quality Control

A quality-assurance and quality-control (QA/ 

QC) plan was developed for the Fall Line monitoring 

program to ensure the quality of data collected and 

analyzed for this study; copies of the QA/QC plan are 

on file at the Virginia District office of the USGS in 

Richmond, Va. The QA/QC plan includes the 

following: (1) A field component to ensure that waterquality samples were representative of river conditions; and (2) a laboratory component to assess the 

variance, accuracy, and bias of analytical results. A 

comprehensive discussion of quality-assurance results 

from July 1988 through June 1995 is presented in 

appendix A.

TRENDS IN NUTRIENTS AND 

SUSPENDED SOLIDS

Trend analysis for concentration and load was 

conducted on data collected during 1988-95 for the 

James and Rappahannock River monitoring stations,

and ofl 1989-95 data for the Appomattox, Pamunkey, 

and IV attaponi River monitoring stations. Constituents 

for which trends were estimated include total nitrogen, 

total Kjeldahl nitrogen, dissolved nitrite-plus-nitrate 

nitrogen, total phosphorus, and total suspended solids. 

Flow-adjusted trends detected by multivariate regression and the seasonal Kendall test are presented in 

table 2, and flow-adjusted trends detected by regression are illustrated in figure 3. For comparison of 

water-quality characteristics between the five river 

basins, summary statistics of constituent load and 

concentration are presented in appendix B.

Time-series plots of concentrations with the 

corresponding LOWESS line are presented for those 

constituents that exhibited a significant trend in concentration and load. The LOWESS line is useful for 

displaying trends that might not be obvious from the 

raw w^ater-quality data; however, these plots are not 

flow adjusted and the computations used to generate 

the LOWESS line are independent of those used to 

estimate the trends presented in table 2. Where 

appropriate, the time-series plots of base-flow and 

stormiow samples are displayed separately.

Also displayed are time-series plots of flowadjusted residuals of the concentration/discharge 

regression, which are more useful for examining flowadjustjed trends than time-series plots of the concentratioiii data. The time-series residual plots are prepared by performing a least-squares regression of the 

natural logarithm of discharge against time, regressing 

the naltural logarithm of constituent concentration 

against the residuals of the first regression, and then 

plotting the residuals of the second regression against 

time with a LOWESS line. Trends in flow-adjusted 

concentration/discharge residuals are similar, but not

identi zai, to trends calculated by the multivariate

regres

The regression method was more likely to detect 

a significant trend in concentration and load than the 

seasonal Kendall test. Of the nine trends detected by

regres

(table

8 Trends in Nutrients and Suspended Solids at the Fall Line of Five Tributaries to the Chesapeake Bay in Virginia, 7/88 6/95

sion model.

sion and considered significant at the 95-percent

confidence level, only three also were considered 

significant at this level by the seasonal Kendall test

2). The two methods agreed on the direction of

the three trend results. Interpretation of trends in this 

section is based mostly on the results of the regression 

method.
Table 2. Flow-adjusted trends in concentration and load of selected constituents at the Fall Line monitoring stations 

in Virginia

[Trends expressed as percent change in constituent concentration during the sampling period; trends in shaded areas are considered significant 

at the 95-percent confidence level corresponding to ap-value less than 0.05]

James River at Cartersville, Va.

Water-quality constituent

Total nitrogen

Total Kjeldahl nitrogen

Dissolved nitrite-plus-nitrate nitrogen

Total phosphorus

Total suspended solids

Rappahannock River near Fredericksburg,

Water-quality constituent

Total nitrogen

Total Kjeldahl nitrogen

Dissolved nitrite-plus-nitrate nitrogen

Total phosphorus

Total suspended solids

Appomattox River at Matoaca, Va.

Water-quality constituent

Total nitrogen

Total Kjeldahl nitrogen

Dissolved nitrite-plus-nitrate nitrogen

Total phosphorus

Total suspended solids

Trend from 

regression

-9

-18

-6

-29

-12

Va.

Trend from 

regression

-27

-29

-29

-24

-48

Trend from 

regression

+6

+16

-19

+23

+18

Trend period:

p-value from 

regression

0.196

.075

.430

.003

.425

Trend period:

p-value from 

regression

0.000

.003

.031

.069

.004

Trend period:

p-value from 

regression

0.337

.022

.075

.025

.163

July 1988 through June 1995

Trend from 

seasonal 

Kendall test

-11

-6

-9

-13

0

p-value from 

seasonal 

Kendall test

0.340

.447

.395

.561

.964

July 1988 through June 1995

Trend from 

seasonal 

Kendall test

-20

-21

-16

-24

-16

p-value from 

seasonal 

Kendall test

0.261

.037

.182

.070

.043

July 1989 through June 1995

Trend from 

seasonal 

Kendall test

+15

+9

+30

+32

-22

p-value from 

seasonal 

Kendall test

0.093

.212

.303

.255

.357

Trends in Nutrients and Suspended Solids 9
Table 2. Flow-adjusted trends in concentration and load of selected 

in Virginia Continued

[Trends expressed as percent change in constituent concentration during 

at the 95-percent confidence level corresponding to a p-value less than 0.05]

constituents at the Fall Line monitoring stations

the sampling period; trends in shaded areas are considered significant

Pamunkey River near Hanover, Va.

Water-quality constituent

Total nitrogen

Total Kjeldahl nitrogen

Dissolved nitrite-plus-nitrate nitrogen

Total phosphorus

Total suspended solids

Mattaponi River near Beulahville, Va.

Water-quality constituent

Total nitrogen

Total Kjeldahl nitrogen

Dissolved nitrite-plus-nitrate nitrogen

Total phosphorus

Total suspended solids

Trend period:

Trend from 

regression

+5

+4

+9

446

+37

Tr

Trend from 

regression

+3

+11

-33

p-value from 

regression

0.401

.600

.269

.000

.075

July 1989 through June 1995

Trend from 

seasonal 

Kendall test

-5

-7

+26

+31

-40

p-value from 

seasonal 

Kendall test

0.871

.704

.175

.082

.704

end period: October 1989 through June 19951

p-value from 

regression

0.533

.081

.000

+12 .165

-11 .501

Trend from 

seasonal 

Kendall test

-5

-4

-55

-6

-37

p-value from 

seasonal 

Kendall test

0.819

.481

.000

.871

.093

sampling began at the Mattaponi River in September 1989, discharge data was not available until October 1989.

10 Trends in Nutrients and Suspended Solids at the Fall Line of Five Tributaries to the Chesapeake Bay in Virginia, 7/88 6/95
79° 78° 77° 76° 79° 78°

39°38°

37°

Q. tt) 5' z

0)

3 Q.w

TOTAL NITROGEN

.WASHINGTON, D.C. 

Rappahannock River Station 

James River Station

Mattaponi River Station 

Pamunkey River Station 

.Appomattox River Station

NORTH CAROLINA

0 10 20 30 40 MILES

39°38°

37°

77"__________76°________ 

TOTAL PHOSPHORUS

WASHINGTON, D.C. 

 Rappahannock River Station 

James River Station

Mattaponi River Station 

Pamunkey River Station Appomattox River Station

NORTH CAROLINA

0 20 40 KILOMETERS

0 10 20 30 40 MILES

I I ' I i 1 | ' '

0 20 40 KILOMETERS EXPLANATION 

TRENDS IN CONCENTRATION ARE SHOWN IN PERCENT INCREASE OR DECREASE PER YEAR

INCREASE

DECREASE 

AREA OF CIRCLE INDICATES RELATIVE MAGNITUDE OF TREND

Figure 3. Significant trends in total nitrogen and total phosphorus for the period of study.
James River

No significant flow-adjusted trend in total 

nitrogen concentration was detected at the James 

River monitoring station (table 2). The regression 

model did detect a slight decrease in total nitrogen 

concentration in base flow caused by a 37 percent 

decrease in total Kjeldahl nitrogen (TKN) concentration in base flow; the 18 percent decrease in overall 

TKN concentration is considered significant at the 90- 

percent confidence level but not at the 95-percent 

confidence level. This decrease was not large enough 

to cause an overall trend in total nitrogen concentration. The cause of the decrease in TKN concentration 

in base flow is unclear; estimates of total nitrogen 

loads from point sources delivered to the Fall Line on 

the James River indicate an increase of approximately 

20 percent during 1989-94 (Virginia Department of 

Environmental Quality, written commun., 1996). The 

decrease in base-flow TKN concentration might be 

caused by decreased nitrogen concentration in groundwater discharge, which in turn might be related to 

reductions in total nitrogen load contributed by 

agriculture or silviculture to the James River.

A significant downward flow-adjusted trend in 

total phosphorus concentration ( 29 percent) was 

detected by the regression model at the James River 

monitoring station for the period July 1988 through 

June 1995 (table 2). A time-series plot of total phosphorus concentration in base flow and stormflow 

(fig. 4A) shows little trend in stormflow concentration, 

but shows a marked decrease in total phosphorus 

concentration in base flow during late 1988-91, and 

then a leveling-off, or slight increase, for the remainder of the sampling period. Similarly, a time-series 

plot of flow-adjusted residuals from the concentration/ 

discharge regression (fig. 46) shows a dramatic 

decrease during 1988-90, and then a leveling-off. No 

significant trends were detected by limiting the model 

input data to stormflow samples, or by limiting the 

model input data to samples collected from January 1, 

1990 through June 30, 1995.

The large flow-adjusted trend in total phosphorus concentration for the sampling period appears 

to be mostly owing to a dramatic decrease in dissolved 

orthophosphorus concentration in base flow during 

1988-90. The most important cause for this decrease 

is the Statewide phosphate detergent ban that went 

into effect in 1988. About 90 Mgal/d of treated municipal and industrial wastewater enters the James River 

above the monitoring station (table 1), and pointsource total phosphorus load to the James River 

decreased more than 30 percent during 1985-89 

(Virginia Department of Environmental Quality, 

written commun., 1996). Point-source total phosphorusj loads, however, increased during 1990-94, 

with the 1994 level approaching the 1985 level 

(Virginia Department of Environmental Quality, 

written1 commun., 1994). Therefore, increases in pointsource | load may be partially erasing the improvements 

caused! by the detergent ban.

The lack of trends in runoff-associated constituents, such as total suspended solids and nutrients in 

stormflow, suggests that the contribution of nutrients 

and sediment from nonpoint sources, such as agricultural! and forested land, did not significantly change 

during the sampling period. The James River Basin 

has thej lowest percentage of agricultural land use 

upstream from the monitoring station (about 23 

percenf:) of the five basins (table 1); thus, the effect of 

best-mjanagement practices (BMP's) may be masked 

by contributions from other nonpoint sources, or offset 

by incieasing development. In addition, BMP's that 

reduce nitrogen concentration in ground water may 

take m my years to affect water quality in the James 

River due to low rates of ground-water flow.

Rappahannock River

The regression model detected significant downward flbw-adjusted trends in concentrations of total 

nitrogen (-27 percent), TKN (-29 percent), dissolved 

nitrite-plus-nitrate nitrogen ( 29 percent), and total 

suspended solids (-48 percent) at the Rappahannock 

River rhonitoring station (table 2) from July 1988 

througm June 1995. The seasonal Kendall test detected 

a significant decrease in TKN concentration and load

for this]

phorus

same period. The regression model also detected u-24 percent flow-adjusted trend in total phosconcentration, which was significant at the

j,

90-percent confidence level but not at the 95-percent 

confidence level (p»-value = 0.07).

lime-series plots of constituent concentrations 

and flo|v-adjusted residuals of the concentration/ 

discharge relations clearly illustrate the trends in total 

nitroge|n (fig. 5), total Kjeldahl nitrogen (fig. 6), dissolved hitrite-plus-nitrate nitrogen (fig. 7), total phosphorus |(fig. 8), and total suspended solids (fig. 9) for 

the sampling period. Flow-adjusted trend in dissolved 

nitrite-plus-nitrate nitrogen concentration was upward 

during the first 3 years of the sampling period, then

12 Trends in Nutrients and Suspended Solids at the Fall Line of Five Tributaries to the Chesapeake Bay in Virginia, 7/88 6/95
became downward, resulting in an overall decrease for 

this constituent.

The Rappahannock River Basin has the largest 

percentage of agricultural land use upstream from the 

monitoring station (about 44 percent) of the five basins 

(table 1), and nonpoint sources contribute the majority 

of nutrient loads from this basin (Belval and others, 

1995). Decreases in nutrients probably were caused by 

reduced runoff of particulate and dissolved nutrients 

from nonpoint sources, such as agricultural and 

silvicultural land. The large flow-adjusted decrease in 

total suspended solids concentration suggests that 

rates of soil erosion decreased during the sampling 

period, possibly resulting from the implementation of 

BMP's.

The Virginia Department of Conservation and 

Recreation estimates that between 1985 and 1993 

there was a reduction in nitrogen (-11.2 percent) and 

in phosphorus (-17.5 percent) from nonpoint sources 

(Virginia Department of Conservation and Recreation, 

written commun., 1996) in the entire Rappahannock 

River Basin due to the implementation of different 

BMP's, including conservation tillage, nutrient management, highly credible land retirement, and forest 

harvesting practices. Similar nutrient reductions, 

however, have been predicted for other basins in 

Virginia where no dramatic improvement in water 

quality has been observed. It is unknown which basin 

characteristics (for example, slope, land use, or 

percentage of land affected by BMP's) might be 

responsible for the unique response of the Rappahannock River to BMP's, if these are a cause of the 

observed trends. Despite the reductions in nutrient and 

sediment concentrations in the Rappahannock River 

Basin, this basin still has the highest yield of total 

nitrogen, total phosphorus, and total suspended solids 

of the five Virginia river basins (Belval and others, 

1995).

Appomattox River

The regression model detected significant upward flow-adjusted trends in concentration and load of 

TKN (+16 percent) and total phosphorus (+23 percent) 

from July 1989 through June 1995 at the Appomattox 

River monitoring station (table 2). Dissolved nitriteplus-nitrate nitrogen showed a decrease (-19 percent) 

that was significant at the 90-percent confidence level 

but not the 95-percent confidence level (table 2). The 

model detected no significant trend in total suspended

solids, and the seasonal Kendall test detected no significant trends in any constituent.

Time-series plots of TKN concentration (fig. 10) 

and total phosphorus concentration (fig. 11) show a 

similar pattern. Constituent concentrations increased 

during the first part of the sampling period, and then 

decreased toward the end of the sampling period. For 

example, total phosphorus concentration increased 

during 1989-93, with the highest concentration in 

1993, and then decreased during 1993-95 (fig. IIA). 

Similarly, a time-series plot of the flow-adjusted 

residuals of the concentration/discharge regression 

(fig. 1 IB) shows that flow-adjusted total phosphorus 

concentration peaked by 1993 and actually decreased 

during 1993-95.

The causes of trends in TKN and total phosphorus in the Appomattox River are unclear. The only 

major point source upstream of the monitoring station 

is a treatment plant at Farmville which releases about 

1.1 Mgal/d municipal wastewater (table 1); this plant 

probably does not have a large effect on nutrient concentrations at the Fall Line monitoring station. Biogeochemical processes in Lake Chesdin, on the other 

hand, may strongly affect water quality at the monitoring station. The marginally significant decrease in 

dissolved nitrite-plus-nitrate nitrogen for the sampling 

period suggests that increased uptake of this constituent by phytoplankton could be responsible for the 

increase in TKN, with no net effect on total nitrogen 

concentration. The similarity in temporal pattern of 

TKN and total phosphorus suggests that trends in both 

constituents might have been caused by a trend in 

some suspended material, even though no significant 

trend in suspended solids was detected.

Pamunkey River

The regression model detected no trends in 

nitrogen species at the Pamunkey River monitoring 

station from July 1989 through June 1995. Total phosphorus concentration, however, increased about 46 

percent (table 2). This trend is more pronounced in 

base-flow samples than in stormflow samples, as revealed in time-series plots of both base-flow and 

stormflow total phosphorus concentration (fig. 12A). 

A similar flow adjusted trend in total suspended solids 

(+37 percent) was considered significant at the 90- but 

not the 95-percent confidence level. Visual inspection 

of dissolved orthophosphate concentration suggests 

that increases in this constituent are at least partly

Trends in Nutrients and Suspended Solids 13
responsible for the increase in total phosphorus. A 

time-series plot of flow-adjusted residuals of the 

total phosphorus concentration/discharge regression 

(fig. 12B) shows that the flow-adjusted total phosphorus concentration increased during 1989-92, then 

leveled off.

Trends in concentration of nutrients in base flow 

usually are caused by point source or ground-water 

contributions to the river. Estimates of point-source 

loads delivered to the Fall Line in the York River 

Basin (most of which come from the Pamunkey River 

Basin) suggest that point-source phosphorus load did 

not increase during 1989-94 (Virginia Department of 

Environmental Quality, written commun., 1996). The 

increase of total phosphorus in base flow, therefore, 

may be caused by undocumented point sources, or by 

nonpoint sources such as agriculture or increased land 

development. Increased development in the basin, 

such as that in Hanover County, around Lake Anna, 

and along the 1-95 corridor, may be contributing additional phosphorus to the river, and may also be causing 

the marginally significant trend in total suspended 

solids concentration.

Mattaponi River

The regression model detected an upward flowadjusted trend in TKN (+11 percent, significant at the 

90- but not the 95-percent confidence level) from

October 1989 through June 1995 at the Mattaponi 

River monitoring station (table 2). Conversely, both 

the regression model and the seasonal Kendall test 

detected downward flow-adjusted trends in dissolved 

nitrite^plus-nitrate nitrogen concentration (-33 and 

-55 pejrcent, respectively). Trends in these two 

nitrogen species seemed to offset each other, so that 

there vl'as no significant change in the concentration 

and load of total nitrogen. No significant flow-adjusted

trends

in base

were detected in total phosphorus or total

suspended solids concentrations and loads at this site. 

'. 'he upward trend in TKN is more pronounced

flow than in stormflow (fig. 13^4). Similarly,

the downward flow-adjusted trend in dissolved nitriteplus-njtrate nitrogen concentration is more pronounced in base-flow samples than in stormflow 

sample s, as illustrated by a time-series residual plot 

(fig. l*B) though the decrease is observed in both 

kinds (J>f samples. Only about 0.1 Mgal/d wastewater 

discharges to the Mattaponi River upstream of the 

monitojring station (table 1); therefore, trends in pointsource| contributions probably did not cause the trends 

in nitrogen species. Concentrations of both TKN and 

dissolved nitrite-plus-nitrate nitrogen show significant 

seasonal variations at the Mattaponi River monitoring 

stationj (Belval and others, 1995). The trends in both 

nitrogen species may have been caused by increased 

uptake! of dissolved inorganic nitrogen by phytoplanktim (especially algae), which converts nitrate to 

organi: nitrogen forms.

14 Trends in Nutrients and Suspended Solids at the Fall Line of Five Tributaries to the Chesapeake Bay in Virginia, 7/88 6/95
10

<

DC

zS

LU H 

§*

o£

W rn

OR RA

0. 

W

G

si

, z

0.5

0.2

0.1

0.05

0.02

0.01

(A)

EXPLANATION

STORMFLOW

BASE FLOW

LOWESS LINE STORMFLOW

LOWESS LINE BASE FLOW

1988 1989 1990 1991 1992 1993 1994 1995

LU 

DC

-1

-2

(B)

-

1988 1989 1990 1991 1992 1993 1994 1995

Figure 4. Relation of total phosphorus concentration in base flow and in stormflow (A), and flow-adjusted residuals of the 

total phosphorus concentration/discharge regression (B) to time at the James River monitoring station, Cartersville, 

Virginia.

Trends in Nutrients and Suspended Solids 15
o

< CC 

CC LU

LU f,-

|l

LU ££ 

O rn

O

LIJ

CC

1988 1989 1990 1991

Figure 5. Relation of total nitrogen concentration (A), and flow-adjustei 

discharge regression (B) to time at the Rappahannock River monitorini

1992 1993 1994 1995

j residuals of the total nitrogen concentration/ 

3 station near Fredericksburg, Virginia.

16 Trends in Nutrients and Suspended Solids at the Fall Line of Five Trjbutaries to the Chesapeake Bay in Virginia, 7/88 6/95

I
DC 

LU

	DC

Z LU

LU Q-

(D W

O S

f 2

z cs

I

LU

DC

LU 

O

O 

O

10

5

2

1

0.5

0.2

0.1

0.05

0.02

0.01

: (A)

 

1988 1989 1990 1991 1992 1993 1994 1995

C/3 

<

Q

C/3 

LU 

DC

-1

-2 l i i

1988 1989 1990 1991 1992 1993 1994 1995

Figure 6. Relation of total Kjeldahl nitrogen concentration (A), and flow-adjusted residuals of the total Kjeldahl nitrogen 

concentration/discharge regression (B) to time at the Rappahannock River monitoring station near Fredericksburg, 

Virginia.

Trends in Nutrients and Suspended Solids 17
2 n- LU B-

§5CC -1 

I- CC

1.1

1.0 

0.8

CO

<

£C

CO 0.6

UJ "~

b z 

Eg 0.4

o,

to 

<

Q 

to

LU 

CC

-2

LOWESS LINE

1988 1989 1990 1991 1992 1993 1994 1995

1988 1989 1990 1991

Figure 7. Relation of dissolved nitrite-plus-nitrate nitrogen concentration 

nitrite-plus-nitrate nitrogen concentration/discharge regression 

near Fredericksburg, Virginia.

(A), and flow-adjusted residuals of the dissolved 

(B) to 1 me at the Rappahannock River monitoring station

18 Trends in Nutrients and Suspended Solids at the Fall Line of Five Tri butaries to the Chesapeake Bay in Virginia, 7/88 6/95

1992 1993 1994 1995
CC CC 

I- HI Z Ip§1II

U) 

o

10

5

2

1

0.5

0.2

0.1

0.05

0.02

0.01

0.005

0.002

0.001

(A)

1988 1989 1990 1991 1992 1993 1994 1995

CO 

<

Q

CO 

LU 

CC

-1

-2

re;

 .

. %

1988 1989 1990 1991 1992 1993 1994 1995

Figure 8. Relation of total phosphorus concentration (A), and flow-adjusted residuals of the total phosphorus 

concentration/discharge regression (B) to time at the Rappahannock River monitoring station near Fredericksburg, 

Virginia.

Trends in Nutrients and Suspended Solids 19
o

£

EC

LLJ 

> Q.

Wi

LU '

Q. ;

UJ 

EC

0.5 1988 1995

-2

(B)

1988 1989 1990 1991

Figure 9. Relation of total suspended solids concentration (A), and floi v 

concentration/discharge regression (B) to time at the Rappahannock F 

Virginia.

20 Trends in Nutrients and Suspended Solids at the Fall Line of Five Tri

1992 1993 1994 1995

'-adjusted residuals of the total suspended solids 

iver monitoring station near Fredericksburg,

Outaries to the Chesapeake Bay in Virginia, 7/88 6/95
£

	a: 

z LLJ

o w

O 5

a: <

i- tr

z o

0.7

0-5

0.4

0.3

LU

? o.2

g 0.1 

o

0 0.07

0.05

(A)

LOWESSLINE

1989 1990 1991 1992 1993 1994 1995

LUtr

-1

-2

(B)

! t * * 

1989 1990 1991 1992 1993 1994 1995

Figure 10. Relation of total Kjeldahl nitrogen concentration (A), and flow-adjusted residuals of the total Kjeldahl nitrogen 

concentration/discharge regression (B) to time at the Appomattox River monitoring station, Matoaca, Virginia.

Trends in Nutrients and Suspended Solids 21
o

LU 

CC !" "^ / "."" : V ." . " "**." «

-1

-2

1989 1990 1991 1992

Figure 11. Relation of total phosphorus concentration (A), eind flow-ad 

concentration/discharge regression (B) to time at the Appomattox Rive

22 Trends in Nutrients and Suspended Solids at the Fall Line of Five Tributaries to the Chesapeake Bay in Virginia, 7/88 6/95

1993 1994 1995

usted residuals of the total phosphorus 

monitoring station, Matoaca, Virginia.
o-in . 1

zee 8"

ccQf

x<

CL_I

1

0.7

0.5

0.4

0.3

0.2

0.1

0.07

0.05

0.04

0.03 

0.02

n m

I I I I

f (A)

- .

.

.

. * * .

 . . . 

 . . . . .. tm .

l ill

EXPLANATION

. STORMFLOW

. BASE FLOW

 LOWESS UNE BASE FLOW

 "

:

 . 

M* ... .

~~ « -

« . ... -

-

I

1989 1990 1991 1992 1993 1994 1995

U)

g

en LU 

cc

(B)

-1

-2

"

1989 1990 1991 1992 1993 1994 1995

Figure 12. Relation of total phosphorus concentration in base flow and in stormflow (A), and flow-adjusted residuals of the 

total phosphorus concentration/discharge regression (B) to time at the Pamunkey River monitoring station near Hanover, 

Virginia.

Trends In Nutrients and Suspended Solids 23
1.3

z"

Q 1

< 0.9

CC

^ 0.8

LU

O CC 0.7

Z LU

8^ 0.6

z cc

LU LU

O 0. 0.5

O W

F 5

1- <

Z CC 0.4

_i 0

3^ 0.3

I Z

LU ~

2 0.25

O 0.2

i i i

: (A)

:

- 

-

 

 

(

EXPLANATION

 STORMFLOW

 BASE FLOW

'* LOWESS LINE STORMFLOW

~

*

_

 ^^M ^B ^B 

I I

- .... ..- . ~

I ...

I I I

1989 1990 1991 1992

II

1.0

0.5

3

<

:D

9 o

w

LU

CC

-0.5

-1.0

I I I

(B)

 . 

 *.

-t . 

 4fc** 

 LOWESS LINE BASE FLOW 

_

^^** -

-

^^

 

-

_ _ ^^

 » 

- __ 

W 

.

-

..M. .... -

-

J

I I I

1993 1994 1995

i i i

. * f "

.

« * .y % * *i«»»»»w » 

 _^ _ i / . A *

-v% .** %» " *

*

i i i

A ^BA _ _ ^ A

V '

1989 1990 1991 1992

Figure 13. Relation of total Kjeldahl nitrogen concentration in base flow

the total Kjeldahl nitrogen concentration/discharge regression (B) to tirr

Beulahville, Virginia.

* v»

" ^

 .

I I I

1993 1994 1995

and in stormflow (A), and flow-adjusted residuals of

e at the Mattaponi River monitoring station near

24 Trends in Nutrients and Suspended Solids at the Fall Line of Five Tributaries to the Chesapeake Bay in Virginia, 7/88 6/95
LU CC

8£

if

LU 7^

Z CD 3^

LLJ b z

pc o

liIB

CO O 

Q O

0.7

0.5 

0.4

0.3 

0.2

0.1 

0.07

0.05

0.04

0.03

0.02

(A)

EXPLANATION

STORMFLOW

BASE FLOW

LOWESS LINE STORMFLOW

LOWESS LINE BASE FLOW

1989 1990 1991 1992 1993 1994 1995

Q

W

LLJ

-2

(B)

 v-x »:.-. * * ' * ' , 

^4^1_4,^iTA^l_±iij^L

r «:;., >> /..: ; > ;«

1989 1990 1991 1992 1993 1994 1995

Figure 14. Relation of dissolved nitrite-plus-nitrate nitrogen concentration in base flow and in stormflow (A), and flowadjusted residuals of the dissolved nitrite-plus-nitrate nitrogen concentration/discharge regression (B) to time at the 

Mattaponi River monitoring station near Beulahville, Virginia.

Trends in Nutrients and Suspended Solids 25
CONCLUSIONS

The five rivers discussed in this report showed 

markedly different flow-adjusted trends in nutrients 

and suspended solids at the Fall Line, between the late 

1980's and 1995, that correspond to different basin 

characteristics. The James River is the only river with 

a large number of point-source nutrient sources upstream of the Fall Line, and this river responded measurably to reductions in point-source phosphorus loads. 

The phosphate detergent ban of 1988 probably is 

responsible for much of the 29 percent decrease in 

total phosphorus in the James River. Point-source 

phosphorus loads, however, are once again increasing 

in this basin, and further reductions in total phosphorus may be dependent on additional nutrient control 

strategies. No trends in runoff-derived constituents are 

interpreted for the James River Basin, because the 

positive effects of BMP's are being (1) masked by 

contributions from other sources, (2) delayed by 

ground-water residence time, or (3) offset by increased 

development or agriculture.

In contrast to the James River, water quality of 

the Rappahannock River is dominated by nonpoint 

sources, such as agriculture and silviculture. The 

consistent decrease of nitrogen, phosphorus, and 

suspended solids for the sampling period makes the 

Rappahannock River the "most-improved" of 

Virginia's monitored tributaries to the non-tidal part of 

Chesapeake Bay Basin. Management practices may 

have contributed to the observed decreases; however, 

the Rappahannock River still has the highest yields of 

nitrogen, phosphorus, and suspended solids of the five 

rivers.

Trends in water quality of the Appomattox, 

Pamunkey, and Mattaponi Rivers were less consistent 

than trends in the James and Rappahannock Rivers. 

Total phosphorus concentration increased in both the 

Appomattox and Pamunkey Rivers, whereas trends in 

total nitrogen concentration were not observed in the 

Appomattox and Mattaponi Rivers. In general, none of 

these three rivers displayed consistent trends in 

nutrients or suspended solids for the entire sampling 

period.

Continued monitoring of nutrient concentrations 

in streams at the Fall Line will improve understanding 

of the effects of changes in land use, management 

practices, and point sources on overall water quality. 

The effects of ground-water residence time on nitrogen concentrations and loads are particularly interesting to monitor; specifically, whether base-flow

nitrate concentrations decrease in response to 

reductions in agricultural nitrogen applications. Trend 

analysis of water-quality data from the James and 

Rappahannock River monitoring stations suggests that 

point-source reductions and BMP's can improve water 

quality on a basin-wide scale. Future trends in 

nutrient^ and suspended solids, however, will depend 

on the complex interaction of these efforts and the 

effects of increased population, land development, and 

agriculture.

REFERENCES CITED

I

Belval, I}.L., and Campbell, J.P., 1996, Water-quality data 

and estimated loads of selected constituents in five tributaries to the Chesapeake Bay at the Fall Line, Virginia, 

July 1993 through June 1995: U. S. Geological Survey 

Opdn-File Report 96-220, 79 p.

Belval, E .L., Campbell, J.P., Phillips, S.W., and Bell, C.F.,

199 \ Water-quality characteristics of five tributaries to 

the Chesapeake Bay at the Fall Line, Virginia, July 

198 3 through June 1993: U.S. Geological Survey 

Waller-Resources Investigations Report 95^1258, 71 p.

Belval, Ejl.L., Woodside, M.D., and Campbell, J.P, 1994, 

Relation of stream quality to streamflow, and estimated 

loaqs of selected water-quality constituents in the 

Jarrtes and Rappahannock Rivers near the Fall Line of 

Virginia, July 1988 through 1990: U.S. Geological Survey Water-Resources Investigations Report 94-4042, 

85 p;.

Bradu, Dan, and Mundlak, Yair, 1970, Estimation in lognormal linear models: Journal of the American Statistical 

Association, v. 65, no. 329, p. 198-211.

Cohn, T.K., 1988, Adjusted maximum likelihood estimation 

of the moments of lognormal populations from type 1 

censored samples: U. S. Geological Survey Open-File 

Report 88-350, 34 p.

Cohn, T.K., Caulder. D.L., Gilroy, E.J., Zynjuk, L.D., and 

Summers, R.M., 1992, The validity of a simple statistical model for estimating fluvial constituent loads: An 

empirical study involving nutrient loads entering Chesapefrke Bay: Water Resources Research, v. 28, no. 9, 

p. 2,353-2,363.

Fishel, .K., Langland, M.J., and Truhlar, M.V., 1991,

Hydrology and hypothetical effects of reducing nutrient 

app lications on water quality in the Bald Eagle Creek 

Headwaters, southeastern Pennsylvania prior to implementation of agricultural best-management practices: 

U.S|. Geological Survey Water-Resources Investigation's Report 91^006, 59 p. 

IHelsel, l|>.R., and Hirsch, R.M., 1992, Statistical methods in 

water resources: New York, Elsevier, 522 p.

26 Trends in Nuirients and Suspended Solids at the Fall Line of Five Tributaries to the Chesapeake Bay in Virginia, 7/88 6/95
Hirsch, R.M., Slack, J.R., and Smith, R.A., 1982, Techniques of trend analysis for monthly water-quality data: 

Water Resources Research, v. 18, no. 1, p. 107-121.

Horowitz, A.J., Demas, C.R., Fitzgerald, K.K., Miller, T.L., 

and Rickert, D.A., 1994, U.S. Geological Survey protocol for the collection and processing of surface-water 

samples for the subsequent determination of inorganic 

constituents in filtered water: U.S. Geological Survey 

Open-File Report 94-539, 57 p.

Jordan, Steve; Stenger, Cynthia; Olson, Marcia; Batiuk, 

Richard; and Mountford, Kent, 1992, Chesapeake Bay 

dissolved oxygen goal for restoration of living resource 

habitats: Chesapeake Bay Program Technical Report 

Series 88/93, 81 p.

Lang, D.J., 1982, Water quality of the three major tributaries 

to the Chesapeake Bay, the Susquehanna, Potomac, and 

James Rivers, January 1979 April 1981: U.S. Geological Survey Water-Resources Investigations Report 

82-32, 63 p.

Lang, D.J., and Grason, David, 1980, Water-quality monitoring of three major tributaries to the Chesapeake 

Bay Interim Data Report: U.S. Geological Survey 

Water-Resources Investigation 80-78, 66 p.

Langland, M.J., Lietman, PL., and Hoffman, Scott, 1995, 

Synthesis of nutrient and sediment data for watersheds 

within the Chesapeake Bay drainage basin: U.S. Geological Survey Water-Resources Investigations Report 

95-4233, 121 p.

Maryland Department of the Environment, 1993, Chesapeake Bay Water-Quality Monitoring, River Input 

Monitoring Component, Level I Data Summary 

Report 1991: Watershed Management Program,

Chesapeake Bay and Watershed Management Administration Technical Report 93-014, 31 p.

Maryland Department of the Environment, 1994, Chesapeake Bay Water-Quality Monitoring, River Input 

Monitoring Component, Level 1 Data Summary 

Report 1992: Watershed Management Program, 

Chesapeake Bay and Watershed Management Administration Technical Report 94-012, 91 p.

Maryland Department of the Environment, 1995, Chesapeake Bay Water-Quality Monitoring, River Input 

Monitoring Component, Level 1 Data Summary 

Report 1993: Watershed Management Program, 

Chesapeake Bay and Watershed Management Administration Technical Report 95-002, 56 p.

Ott, A.N., Takita, C.S., Edwards, R.E., and Bollinger, S.W, 

1990, Interim report nitrogen and phosphorus loads 

of the Susquehanna River Basin, 1985-87: Susquehanna River Basin Commission Publication No. 127, 

60 p.

Rutledge, A.T., 1993, Computer programs for describing the 

recession of ground-water discharge and for estimating 

mean ground-water recharge and discharge from 

streamflow records: U.S. Geological Survey WaterResources Investigations Report 93-4121,45 p.

Schertz, T.L., Alexander, R.B., and One, D.J., 1991, The 

computer program Estimate Trend (ESTREND), a system for the detection of trends in water-quality data: 

U.S. Geological Survey Water-Resources Investigations Report 91-4040, 63 p.

References Cited 27

APPENDIXES
APPENDIX A. QUALITY-ASSURANCE 

AND QUALITY-CONTROL PROCEDURES 

AND RESULTS

A quality-assurance/quality-control (QA/QC) 

plan was developed for the Fall Line monitoring 

program to ensure the quality of data collected and 

analyzed for this study. The QA/QC plan includes the 

following: (1) A field component to ensure that water 

quality samples were representative of river conditions; and (2) a laboratory component to assess 

variance and bias of analytical results, as well as 

verification that clean techniques were used to collect 

and analyze samples. Much of the discussion of QA/ 

QC procedures presented in this appendix has been 

taken from the U.S. Geological Survey WaterResources Investigations Report 95-4258; however, 

all QA/QC results have been updated to include 

analysis of data collected through June 1995.

General results. Results of the field 

component of the QA/QC plan indicate that representative samples were collected, samples were 

collected for a range in discharge conditions, and 

proper sampling techniques and equipment were used 

by field personnel. Field blank results show a need to 

identify a source of low-level contamination for dissolved ammonia nitrogen, total phosphorus, and 

dissolved orthophosphorus.

Results of the laboratory component follow. 

Duplicate data showed good agreement between 

samples, therefore no qualification of constituent concentration or load was necessary. Total variance for 

duplicate samples depends on the concentration range 

of the constituent of interest, whereas measurement 

variance depends on the number of ties between 

sample pairs and the range of constituent concentration. A value of 10 percent, or less for percent of 

total variance, caused by measurement variance is 

acceptable for these data. The percent of total variance 

from measurement variance for seven of the constituents sampled ranged from 0.9 to 7.8 percent. Dissolved ammonia nitrogen showed 25-percent total 

variance from measurement variance. Concentrations 

for dissolved ammonia nitrogen showed a small range 

in total variability because more than half of the data 

was censored and measurement variability was limited 

by 62-percent ties between sample pairs. Loads were 

not calculated for dissolved ammonia nitrogen 

because of the large percentage of censored data.

Results of the Wilcoxon signed-rank test on 

laboratory-split samples between Virginia Division of

Consolid; 

U.S. Geo

ted Laboratory Services (VDCLS) and the 

ogical Survey (USGS) National WaterQuality Laboratory (NWQL) in Arvada, Colo. showed 

a statistically significant difference between the laboratories for dissolved nitrite-plus-nitrate nitrogen, 

dissolved ammonia nitrogen, total phosphorus, total 

organic carbon, and dissolved silica. Investigation

revealed

storm samples was underestimated by VDCLS during

the perioi 

causing t 

estimatec

remaining four constituents had no environmental

significan

Procedures. The field component of the QA/

QC plan 

collectior 

1. Water 

appro 

were

hat concentration of total organic carbon in

August 1988 through February 1992, 

tal organic carbon load to also be underDifferences between laboratories for the

;e with respect to concentration or load.

onsists of documentation of field conditions, 

procedures, and equipment as follows: 

quality samples were collected according to 

'ed USGS guidelines to ensure that samples 

epresentative of the river cross section

(Horowitz and others, 1994). These guidelines 

ensured the collection of a representative, 

composite sample from the horizontal and vertical 

cross section of the river.

2 Sampling criteria based on discharge characteristics were documented to ensure that waterquality samples were collected over a range in 

discharge conditions. In addition, detailed documentation of field procedures ensured consistency 

of procedures between field personnel. 

3. Propejr use of sampling equipment and samplecollecjtion techniques by field personnel was 

verified through in-house testing of procedures and 

through comparisons of field- and laboratoryanalysed results.

The laboratory component of the QA/QC plan 

consisted of the collection of duplicate, laboratorysplit, and field blank samples as follows:

1. Dupl

analytical results. Duplicate samples were prepared

by w

2. Labo

30 Trends in Nutrients and Suspended Solids at the Fall Line of Five Tributa

cate samples document the variance of the

thdrawing two subsamples of the full-sample

volurie collected, with both samples being analyzed :>y VDCLS. The second subsample was 

disguised as an environmental sample by labeling 

it wit i a different time from the first subsample. 

Approximately ten percent of the samples collected at each site were duplicate samples.

atory-split samples document bias in the

analytical results. Laboratory-split samples were 

collected in a similar manner to duplicate samples;

ies to the Chesapeake Bay in Virginia, 7/88 6/95
however, one subsample was analyzed by VDCLS 

and the other subsample was analyzed by NWQL 

to assess the comparability of results between the 

two laboratories. Approximately ten percent of the 

samples collected at each site were laboratory-split 

samples.

3. Field blank samples document the ability of field 

personnel to collect a clean sample and/or the 

laboratory to perform analytical procedures without contaminating the sample. A field blank is 

prepared by passing carbon-free deionized water 

through all equipment used to collect and process a 

sample at the field site. A portion of the blank 

sample water also passes from the churn through 

the capsule filter for analysis of dissolved constituents. Approximately two percent of the samples 

collected at each site during the study period were 

field blanks.

Quality-assurance samples were collected at all 

five rivers throughout the period of study. All data 

were reviewed for transcription errors and corrected. 

Concentrations below the minimum reporting limit 

("censored") were considered equal to the minimum 

reporting limit for all QA/QC calculations.

Duplicate samples. The duplicate samples, 

which provide an indication of variability or spread in 

the data, were compared by determining total variance, measurement variance, and the percentage of the

total variance from measurement variance. Total variance is the variability within the regularly scheduled 

samples. Total variance equals the sum of the variability in the measurement process and naturally occurring 

environmental variability. Measurement variance is a 

measure of the variability between the regularly scheduled sample and a duplicate sample. The source of 

measurement variability can be either field-collection 

techniques or laboratory analytical procedures and 

should ideally be ten percent or less of the total variability. Transformations are often applied to waterquality data to make these data more symmetric. Logtransformation was determined to be inappropriate for 

these data because of a large percentage of ties, or zero 

differences, between the regular and duplicate 

samples.

Eight water-quality constituents were included 

in the analysis of duplicate samples for this study. 

Results of variance calculations on raw concentration 

data for duplicate samples are presented in table A-l.

Total suspended solids has the highest total 

variance. The naturally occurring variability between 

base-flow and stormflow samples results in concentration ranges of more than several orders of magnitude for this constituent. Constituents with about half 

of the concentrations in the range of 4.0 to 11.0 mg/L, 

such as total organic carbon and dissolved silica, show 

larger total variability than nitrogen and phosphorus

Table A-1. Results of variance computations for duplicate quality-assurance samples analyzed by the Virginia 

Division of Consolidated Laboratory Services

[(mg/L) , milligrams per liter squared]

Constituent

Suspended solids, total

Nitrite-plus-nitrate nitrogen, dissolved

Ammonia nitrogen, dissolved

Kjeldahl nitrogen, total

Phosphorus, total

Orthophosphorus, dissolved

Organic carbon, total

Silica, dissolved

Total 

variance 

(mg/L)2

17,211

.0469

.0008

.2163

.0486

.0031

16.48

10.54

Measurement 

variance 

(mg/L)2

317

.0018

.0002

.0049

.0014

.00003

1.285

.0979

Percent of

total

variance 

from 

measurement

variance

1.8

3.8

25

2.3

2.9

1.0

7.8

.9

Percent 

ties

31

47

62

59

51

73

15

64

Percent 

censored 

data

12

5.7

33

.7

2.9

19

0

0

Number 

of sample 

pairs

135

139

138

138

138

138

119

137

Appendix A. QA/QC Procedures and Results 31
species, which generally have concentrations below 

1.0 mg/L. Dissolved ammonia nitrogen showed the 

smallest total variability because of the high percentage of censored data for this constituent (51 percent of 

the regular sample concentrations were less than, or 

equal to, the minimum reporting limit).

Measurement variance followed a pattern 

similar to total variance, where larger variances 

occurred with high concentration ranges. Tied values, 

or zero difference between the regular and duplicate 

sample, showed no variability between the sample 

pairs. Dissolved orthophosphorus had the highest 

percentage of tied values between regular and duplicate samples and, therefore, had the lowest measurement variance. The low concentration range for dissolved orthophosphorus (50 percent of the values were 

from 0.01 to 0.03 mg/L) caused a high percentage of 

tied values and thus limited the measurement variability. The limiting effect of ties on variability had the 

greatest impact on nutrient species because of the low 

concentration ranges of these constituents compared to 

total suspended solids and total organic carbon. 

Measurement variance for dissolved silica was small 

because 64 percent of the 137-sample pairs were ties. 

Total organic carbon, with only 15-percent ties, has 

the best estimate of measurement variance of the eight 

constituents.

The percentage of total variance from measurement variance as shown in table A-l is not affected by 

the concentration range of the constituent of interest. 

The higher the percentage of measurement variance, 

the greater the effect of laboratory analytical technique 

on total variance. The high percentage for dissolved 

ammonia nitrogen, however, is due to the low total 

variance and measurement variance for this constituent. A value of 10 percent, or less, for percent of total 

variance from measurement variance is acceptable for 

these data.

Sample water for both regular and duplicate 

samples was collected according to USGS guidelines 

and stored in one churn prior to processing; therefore, 

the duplicates represent variability in the analysis of 

identical samples by VDCLS. Concurrent replicate 

sampling, which quantifies variance in the field 

sampling process, involves the use of two churns to 

collect two samples simultaneously. Two verticals are 

collected at each cross section and are stored separately in two churns. Differences between concurrent 

replicates could be caused by field collection techniques, short-term variability of in-stream concentrations, and (or) differences in laboratory precision.

Low-level analyses by the VDCLS began 

January 1, 1994, for the following constituents: 

dissolved nitrite-plus-nitrate nitrogen, dissolved 

ammonia nitrogen, and dissolved orthophosphorus. 

Although the minimum reporting limit was lowered to 

parts per billion for these constituents, 86 percent of 

the dup icate data reflects the higher reporting limits. 

As the percentage of duplicate data analyzed under the 

new, lower reporting limit increases, measurement 

variancfe will also increase because the number of tied 

values will decrease. Total variance will also increase

owing

j aboratory-split samples. A nonparametric

test, th 

analyz 

referen

laboratory bias. A nonparametric test was chosen

becaus

o the lower minimum value.

Wilcoxon signed-rank test, was used to 

the data for laboratory-split and standard- 

:e samples, which provided an indication of

it computes statistics using the median value

of a population. The median value, as opposed to the 

mean value in parametric tests, was used because it is 

unaffected by outliers that are common in waterquality data and by data censoring less than 50 

percent. The Wilcoxon signed-rank test has more 

power to detect differences between two groups of 

paired lata in most situations than other nonparametric tests, such as the sign test, and does not require 

norma ity of the data.

The null hypothesis associated with the 

Wilcoion signed-rank test indicates that for a given 

constituent the median of the differences between 

concer tration reported by NWQL and the concentration reported by the VDCLS is equal to zero. 

Probability (p) is the significance level reached by the 

test, orj the probability of incorrectly rejecting the null 

hypothesis when it is true. If p>0.05 then no statistical 

difference between laboratories was inferred. If 

/?<0.0:5 then a significant bias by one or both laboratories was indicated. Results of the Wilcoxon signedrank test for laboratory-split samples, including a twosided probability value for raw concentrations; the

were] 

nitrog

32 Trends in Nutrients and Suspended Solids at the Fall Line of Five Trib

medial of the differences between the laboratories

(also r

used

uentf

table i

:f erred to as median bias), and the highest value 

s the minimum reporting limit for each constitom August 1988 through June 1995 are listed in

,-2.

statistically significant differences (p>0.05) 

ound for total suspended solids, total Kjeldahl 

:n, or dissolved orthophosphorus. A statistically

taries to the Chesapeake Bay in Virginia, 7/88 6/95
Table A-2. Results of the Wilcoxon signed-rank test comparing constituent concentration analyzed by the 

Virginia Division of Consolidated Laboratory Services with constituent concentration analyzed by the National 

Water-Quality Laboratory

[Two-sided probability values less than, or equal to, 0.05 show a statistical difference between laboratories]

_ ... . Two-sided Constituent . ..... . probability value

Suspended solids, total

Nitrite-plus-nitrate nitrogen, dissolved

Ammonia nitrogen, dissolved

Kjeldahl nitrogen, total

Phosphorus, total

Orthophosphorus, dissolved

Organic carbon, total

Silica, dissolved

0.196

.035

.000

.421

.000

.299

.000

.000

Median of Highest 

differences reporting limit

0 3.0

.002 .10

-.013 .04

0 .20

-.01 .01

0 .01

.4 1.0

0 .10

Number of 

sample pairs

217

228

229

230

231

231

192

229

significant difference (/?<0.05) was observed in the 

median of the differences between the two laboratories 

for the following constituents: dissolved nitrite-plusnitrate nitrogen, dissolved ammonia nitrogen, total 

phosphorus, total organic carbon, and dissolved silica. 

These statistics do not specify the source of the difference, but only that a difference exists in the analytical 

method, preservation method, holding time, and (or) 

the environment of each laboratory.

Preservation method and holding time differ 

between laboratories for nutrient samples, and therefore results for dissolved nitrite-plus-nitrate nitrogen, 

dissolved ammonia nitrogen and total phosphorus may 

have been affected by these differences. Nutrient 

samples sent to VDCLS were preserved by chilling at 

4°C and were usually analyzed within a 24-hour 

period. Samples sent to NWQL were preserved with 

mercuric chloride, chilled in a dark bottle, and shipped 

to the laboratory in Arvada, Colo. These nutrient 

samples were usually analyzed within 1 week. The 

NWQL discontinued use of mercuric chloride as a 

preservative for nutrient samples on October 1, 1994 

because a NWQL comparison of sample preservation 

techniques determined that chilling nutrient samples 

gave results that were comparable to those preserved 

with mercuric chloride or sulfuric acid. The NWQL 

also was concerned about potential environmental 

contamination from the storage, use, and disposal of 

mercuric chloride. Nutrient samples sent to NWQL 

were preserved by chilling to 4°C after the October 

1994 date.

Dissolved nitrite-plus-nitrate nitrogen showed a 

statistically significant difference between laboratories 

(/?=0.035). These data were checked for symmetry 

and were found to be random around zero difference. 

The reporting limit for this analysis was lowered to 

parts per billion by VDCLS on January 1, 1994; however, a higher reporting limit provided by the NWQL, 

0.10 mg/L, was the limiting factor for this constituent. 

The median of the differences in concentrations 

between laboratories (0.002 mg/L) is much lower than 

the highest minimum reporting limit throughout the 

study period (0.10 mg/L), indicating this bias has no 

significance with respect to concentrations or loads.

The difference in concentrations reported by 

VDCLS and NWQL for dissolved ammonia nitrogen 

showed a statistically significant difference from zero 

(p=0.000). The minimum reporting limit for dissolved 

ammonia nitrogen at VDCLS was 0.04 mg/L from 

August 1988 through December 1993 and was 

lowered by the laboratory to 0.004 mg/L on January 1, 

1994, whereas the minimum reporting limit for dissolved ammonia nitrogen was 0.01 mg/L at NWQL 

throughout the entire period. The VDCLS results for 

dissolved ammonia nitrogen before January 1994 

contained a large percentage of censored data that 

caused difficulties in data interpretation. Results of the 

Wilcoxon signed-rank test on 40 cases available from 

January 1, 1994 through June 30, 1995 showed no significant difference between the laboratories (p=0. 127). 

The median of the differences (in absolute value) 

between the two laboratories was lowered by an order 

of magnitude, from 0.013 mg/L for the entire data set

Appendix A. QA/QC Procedures and Results 33
to 0.002 mg/L after January 1994, by lowering the 

minimum reporting limit to parts per billion. Analyses 

for dissolved ammonia nitrogen have been acceptable 

throughout the period of study. Data after January 

1994 show a significant lowering of the median bias 

for this constituent.

A statistically significant difference was shown 

between the laboratories for total phosphorus (p= 

0.000). The median bias for total phosphorus, 0.01 

mg/L, is the same as the minimum reporting limit for 

both laboratories and, therefore, has no environmental 

significance.

Total organic carbon showed a statistically 

significant difference (p=0.000) between the two 

laboratories. From August 1988 through February 

1992, VDCLS used a technique that did not allow 

mixing a sample before withdrawing an aliquot for 

analysis. Therefore, concentrations and loads for 

storm samples were underestimated for total organic 

carbon during this period. Data for total organic 

carbon were divided into two groups based on the 

technique change at the VDCLS on March 1, 1992. 

The 103 cases available from August 1988 through 

February 1992 showed a significant difference 

between laboratories (p=0.000). The 89 cases 

available from March 1992 to June 1995 showed no 

statistical difference between laboratories (p =0.906) 

and the median bias was zero for these data. The 

estimates for total organic carbon loads from August 

1988 through February 1992 are lower for stormflow 

samples than if analytical methods appropriate for 

large sediment concentrations were used. Samples 

after March 1, 1992, have no consistent bias present; 

therefore, the analyzed concentrations better represent 

in-stream conditions.

The Wilcoxon signed-rank test showed a 

significant difference between the laboratories for 

dissolved silica. The median of the differences 

between the laboratories was zero, which indicates 

that a difference does not exist, however, the p-value 

was 0.000. The Wilcoxon signed-rank test is an 

evaluation of the hypothesis that the median of the 

population for the difference between two variables is 

equal to zero, based on the proportion of the differences above and below zero. Of the 229 cases 

available for analysis, 45 percent showed differences 

greater than zero, whereas only 24 percent showed 

differences less than zero, indicating these data are not 

symmetric around zero. A positive bias at NWQL for 

dissolved silica has been documented by the USGS

Branch of Quality Assurance in Golden, Colo. This

bias caused the statistical test to fail for dissolved 

silica. r 'he largest variability between laboratories 

occurred when the concentration of dissolved silica 

was greater than 10 mg/L; however, there is no 

statistically significant difference for dissolved silica

based

the laboratories for this constituent.

7ield blank samples. Field blanks are

proces

blank

n the median bias of zero difference between

ed under actual field conditions and, therefore,

are subject to the same potential contamination 

sources during sample collection, field processing, 

preservation, transportation, and laboratory handling 

as environmental samples. Contamination found in a 

field b ank could also be the result of improper 

cleaning of sampling equipment by field personnel 

prior to sample collection. District-prepared deionized 

water (carbon-free) was used as a blank solution for 

most o ? the field blank samples. Laboratory blanks 

containing deionized water were sent to NWQL for 

analys LS and showed no contamination of the constituents of interest for this project. Collection of field

amples by USGS personnel began in

Septerhber 1991. Field blanks were submitted to 

VDCIJS for analysis simultaneously with 

environmental samples.

K summary of the number of samples showing 

concentrations greater than the minimum reporting 

limit is shown in table A-3. The following constituents are listed in table A-3 twice because their minimum reporting limits were lowered from parts per 

millio:i to parts per billion on January 1, 1994: 

dissolved nitrite-plus-nitrate nitrogen, dissolved 

ammonia nitrogen, and dissolved orthophosphorus.

^Jo field blanks had concentrations of total 

suspended solids or total Kjeldahl nitrogen that were 

greater than the minimum reporting limits (no "hits"). 

Total organic carbon and dissolved silica each had one 

hit at concentrations much higher than the reporting 

limit; these hits may have been caused by a mix up 

with an environmental sample. Blanks analyzed for 

total phosphorus showed seven hits in the range 0.02 

to 0.04 mg/L. Environmental samples with concentratioitis of total phosphorus at or below 0.04 mg/L 

should be interpreted with caution.

The VDCLS lowered the minimum reporting 

limit :'or dissolved nitrite-plus-nitrate nitrogen, 

disso! ved ammonia nitrogen, and dissolved orthophosphorus on January 1, 1994. Field blanks for these 

constituents were grouped into two categories based

34 Trends in Nutrients and Suspended Solids at the Fall Line of Five Tributaries to the Chesapeake Bay in Virginia, 7/88 6/95
Table A-3. Summary of the number of field blanks with concentrations greater than the 

minimum reporting limit for samples analyzed by the Virginia Division of Consolidated 

Laboratory Services

[mg/L, milligrams per liter]

Number of 

samples greater 

Constituent than the 

minimum 

reporting limit

Suspended solids, total

Nitrite-plus-nitrate nitrogen, dissolved

Nitrite-plus-nitrate nitrogen, dissolved

Ammonia nitrogen, dissolved

Ammonia nitrogen, dissolved

Kjeldahl nitrogen, total

Phosphorus, total

Orthophosphorus, dissolved

Orthophosphorus, dissolved

Organic carbon, total

Silica, dissolved

0

0

2

0

12

0

7

0

10

1

1

Number of 

samples

27

11

16

11

16

27

27

11

16

11

27

Minimum 

reporting limit 

(mg/L)

3.0

.04

'.004

.04

J .004

.10

.01

.01

J .002

1.0

.10

'Minimum reporting limit lowered to parts per billion on January 1, 1994.

on whether they were analyzed before or after the 

reporting limit was lowered. There were no hits before 

January 1994 for any of the three constituents. 

Dissolved nitrite-plus-nitrate nitrogen had two hits 

after January 1994. Dissolved ammonia nitrogen 

showed 12 hits in the range of 0.005 to 0.015 mg/L. 

Therefore, environmental samples with dissolved 

ammonia nitrogen concentrations at or below 0.015 

mg/L should be interpreted with caution. Dissolved 

Orthophosphorus showed 10 hits in the range of 0.003 

to 0.006 mg/L. Environmental samples with concentrations of dissolved Orthophosphorus below 0.007 

mg/L should also be interpreted with caution. The 

low-level contamination of dissolved ammonia nitrogen and dissolved Orthophosphorus in field blanks was 

below the minimum reporting limit in use for each 

constituent before January 1994.

Field blank results show a need to pinpoint the 

source of contamination for dissolved ammonia 

nitrogen, total phosphorus, and dissolved Orthophosphorus. Preparing a blank before each step in the 

sample collection process as well as a laboratory blank 

will help to identify possible sources of this contamination.

Although low-level concentrations for dissolved 

ammonia nitrogen, total phosphorus, and dissolved 

Orthophosphorus should be interpreted with caution, 

load estimates for these constituents were not 

markedly affected by contamination. Concentrations 

of constituents derived from contamination were not 

high enough to significantly raise the concentrations in 

environmental samples that are used as input to the 

load estimation model.

APPENDIX B. SUMMARY STATISTICS 

OF CONSTITUENT CONCENTRATION 

AND LOAD

Presented in this appendix are summary 

statistics for constituent concentrations (table B-l) 

and loads (table B-2) for all five rivers for the 

sampling period. Summary statistics are useful for 

comparing the water-quality characteristics of the five 

river basins. A discussion of the factors affecting 

water quality in these basins is provided in Belval and 

others (1995).

Appendix B. Summary Statistics of Constituent Concentration and Load 35
Table B-1. Summary statistics for selected water-quality constituents at the Fall Line monitoring stations in Virginia during the 

[mg/L, milligrams per liter; uS/cm, microsiemens per centimeter; <, less than]

Is inNutrients and

wc

W

1a

(D

a

w

o_

a

(0

SU

(D 

Tl

A

Statistic

Oxygen, 

dissolved 

(mg/L, 

as O2)

Specific 

conductance 

(uS/cm)

PH

Nitrogen, 

total 

(mg/L, 

asN)

Total Nitrogen, 

Kjeldahl ammonia, Nitrogen, 

nitrogen dissolved organic1 

(mg/L, (mg/L, (mg/L) 

as N) as N)

.... Phos- Nitrogen, . 

u** urt phorus, NOo + NO-?, . 

.. 2 . ,3' total 

dissolved

(mg/L, as N) * ~1 '

Phosphorus, 

ortho, 

dissolved 

(mg/L, as P

James River monitoring station (July 1988 June 1995)

Minimum

Median

Maximum

5.2

10.

16.9

60

125

352

6.1

7.3

8.9

0.191

.73

3.3

<0. 1 <.004 0.046

.4 <.04 .36

3.0 .54 2.86

0.029 0.03

.29 .14

1.05 1.4

<.01

.04

.51

Rappahannock River monitoring station (July 1988 June 1995)

Minimum

Median

Maximum

6.0

9.8

15.0

40

77

159

5.8

7.1

8.4

.14

1.

3.41

<. 1 .004 .06

.4 .04 .34

2.8 .27 2.63

<.004 <.01

.57 .08

1 .03 1 .5

.002

.01

.06

Ti Appomattox River monitoring station (July 1989 June 1995)

(D

H

C

ST

5'

(0

o

(D

0

(D 

(0

CO

Q>

7*

(D

DO

CO

5'

^

(Q5'

0)'

Minimum

Median

Maximum

4.3

10.7

14.8

38

81

135

6.1

7.1

8.1

.27

.59

2.91

<.l .005 .01

.4 .04 .36

2.8 .22 2.72

.02 <.01

.16 .05

.54 .20

.004

.01

.04

Pamunkey River monitoring station (July 1989 June 1995)

Minimum

Median

Maximum

5.4

9.7

14.1

41

80

263

5.5

6.9

8.5

.34

.68

2.23

.2 <.004 .13

.4 .04 .38

2.0 .17 1.92

.04 .02

.23 .07

.61 .50

.006

.013

.10

Mattaponi River monitoring station (September 1989 June 1995)

Minimum

Median

Maximum

4.8

9.7

14.2

33

53

95

5.3

6.6

8.1

.21

.59

1.19

.2 <.004 .06

.4 <.04 .38

.9 .14 .86

.025 <.01

.132 .06

.87 .23

.004

.01

.10

'Computed as difference between concentration of total Kjeldahl nitrogen and concentration of dissolved ammonia nitrogen.
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Appendix B. Summary Statistics of Constituent Concentration and Load 37