Prepared in cooperation with the Delaware Department of Natural Resources and 

Environmental Control and the Delaware Geological Survey

Monitoring the Water-Quality Response of Agricultural 

Conservation Practices in the Bucks Branch Watershed, 

Sussex County, Delaware, 2014–16

Scientific Investigations Report 2018–5020

U.S. Department of the Interior

U.S. Geological Survey
Cover. The Bucks Branch watershed is in a relatively flat, predominantly agricultural setting with with many fields that are irrigated. 

Photograph by John Clune, U.S. Geological Survey.
Monitoring the Water-Quality Response of 

Agricultural Conservation Practices in the 

Bucks Branch Watershed, Sussex County, 

Delaware, 2014–16

By Judith M. Denver, Alexander M. Soroka, Betzaida Reyes, Todd R. Lester, 

Deborah A. Bringman, and Michael S. Brownley

Prepared in cooperation with the Delaware Department of Natural Resources 

and Environmental Control and the Delaware Geological Survey

Scientific Investigations Report 2018–5020

U.S. Department of the Interior

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

RYAN K. ZINKE, Secretary

U.S. Geological Survey

James F. Reilly II, Director

U.S. Geological Survey, Reston, Virginia: 2018

For more information on the USGS—the Federal source for science about the Earth, its natural and living 

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For an overview of USGS information products, including maps, imagery, and publications, 

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U.S. Government.

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

as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.

Suggested citation:

Denver, J.M., Soroka, A.M., Reyes, B., Lester, T.R., Bringman, D.A., and Brownley, M.S., 2018, Monitoring the 

water-quality response of agricultural conservation practices in the Bucks Branch watershed, Sussex County, Delaware, 2014–16: U.S. Geological Survey Scientific Investigations Report 2018–5020, 43 p., https://doi.org/10.3133/

sir20185020.

ISSN 2328-031X (print)

ISSN 2328-0328 (online)
iii

Acknowledgments

The Delaware Department of Natural Resources and Environmental Control (DNREC) funded this 

project under a cooperative agreement between the Delaware Geological Survey (DGS) and the 

U.S. Geological Survey (USGS). The authors extend sincere appreciation to the landowner who 

provided access for sampling and made this project possible. John Schneider, Hassan Mirsajadi, 

Marcia Fox, Kathy Knowles, Ben Pressley, John Cargill, and many other DNREC employees are 

thanked for their assistance and in-kind support during the project, especially for Geoprobe 

data collection and laboratory analysis. The University of Delaware Cooperative Extension, 

including Amy Shober and James Adkins and many others at the Carvel Research Center, also 

are thanked for their support and sharing of information, including soil moisture and irrigation 

data, and analysis of plant material. Clinton Church and Sarah Fishel of the U.S. Department of 

Agriculture, Agricultural Research Service (ARS) in University Park, Pennsylvania are thanked for 

soil analysis.

v

Contents

Acknowledgments........................................................................................................................................iii

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

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

Purpose and Scope ..............................................................................................................................4

Description of Study Area ...................................................................................................................4

Crop Production Practices at the Bucks Branch Study Site, 2015 Growing Season ................5

Network Design, Sampling, and Analytical Methods..............................................................................6

Initial Groundwater Investigation ......................................................................................................6

Groundwater Flowpath Networks......................................................................................................6

Groundwater Sampling Methods and Analysis...............................................................................7

Soil Water...............................................................................................................................................9

Surface Water.......................................................................................................................................9

Discrete Sample Quality Assurance................................................................................................10

Real-Time Water Quality....................................................................................................................10

Soil Sampling.......................................................................................................................................10

Cover-Crop Soil and Plant Data........................................................................................................10

Data from Other Sources...................................................................................................................10

Data Availability ..................................................................................................................................11

The Water-Quality Response of Agricultural Conservation Practices at the 

Bucks Branch Study Site ..............................................................................................................11

Hydrogeology.......................................................................................................................................11

Aquifer Recharge.......................................................................................................................13

Apparent Age of Groundwater.........................................................................................................14

Soil Chemistry......................................................................................................................................14

Water Chemistry in Groundwater and Soils...................................................................................15

Characterization of Groundwater Quality Using Temporary Wells ...................................15

Transport of Water and Chemicals from Soil to Groundwater...........................................17

Transport of Nutrients through Groundwater to the Stream..............................................22

Isotopes of Nitrate and Denitrification in Groundwater.............................................22

Surface-Water Flow and Chemistry ................................................................................................26

Streamflow..................................................................................................................................27

Water Quality..............................................................................................................................27

Seasonal Trends in Surface-Water Quality..................................................................27

Results from Continuous Monitoring....................................................................27

Monthly and Storm Data.........................................................................................28

Effect of Groundwater on Surface Water in the Bucks Branch Watershed ...................31

Estimated Nitrate Loss to Bucks Branch Through Denitrification ...........................31

Comparison of the Intensive Surface-Water Site to Watershedwide SurfaceWater Quality in Bucks Branch..................................................................................33

Effects of Cover Crops on Soil and Water ......................................................................................34

Summary and Conclusions.........................................................................................................................38

References Cited..........................................................................................................................................40
vi

Figures

1. Map showing location of the study area, and land use and surface-water sites in

the Bucks Branch watershed, Sussex County, Delaware.....................................................3

2. Map showing location of temporary well points and water-table elevation in

February 2014 at the Bucks Branch study site, Sussex County, Delaware.........................6

3. Map showing location of groundwater flowpath networks, Bucks Branch study

site, Sussex County, Delaware...................................................................................................8

4. Cross sections showing groundwater flowpaths, the apparent age of groundwater,

and the hydraulic-head gradients in flowpath wells beneath the A, dryland and

B, irrigated fields Bucks branch study site, Sussex County, Delaware, April 2016.........12

5. Graphs showing continuous monitoring of water-table and stream elevation on

the irrigated and dryland fields; and rainfall in Bridgeville, Delaware showing

A, the effects of irrigation during the growing season, and B, groundwater

recharge during the winter, Bucks Branch Study site, Sussex County, Delaware .........13

6. Graphs showing comparison of soil moisture in the A, irrigated and B, dryland

fields, Bucks Branch study site, Sussex County, Delaware, June and July 2015............14

7. Graph showing comparison of silica concentrations to the apparent age of

groundwater from flowpath wells, Bucks Branch study site, Sussex County,

Delaware......................................................................................................................................14

8. Graphs showing total Kjeldahl Nitrogen values in soil for A, the mean concentrations in samples near all wells shown for different sampling times on the

dryland and irrigated fields, and B, for the three main sampling locations on the

dryland and irrigated fields in the flowpath networks, Bucks Branch study site,

Sussex County, Delaware..........................................................................................................16

9. Boxplots showing summary of selected water-quality constituents from the

temporary wells installed on the irrigated and dryland fields at the Bucks Branch

study site: A, boxplots of selected constituents, and B, selected constituents

compared to increasing age as indicated by increasing concentrations of silica .........18

10. Graphs showing samples of lysimeters and shallow groundwater showing trends

over time in A, specific conductance, B, nitrate, and C, orthophosphate from the

dryland and irrigated study fields, Bucks Branch study site, Sussex County,

Delaware, 2014–15......................................................................................................................20

11. Graphs showing comparison of concentrations of A, nitrate and B, orthophosphate to depth below land surface in lysimeters and wells from the Bucks Branch

study site, Sussex County, Delaware ......................................................................................21

12. Graphs showing the relation between specific conductance and pH, nitrate,

orthophosphate, sulfate, calcium, magnesium, potassium, sodium, and chloride

in A, lysimeter samples, and B, groundwater samples from the dryland and

irrigated fields, Bucks Branch study site, Sussex County, Delaware................................23

13. Cross sections showing median concentrations of nitrate in groundwater beneath

A, the dryland and B, the irrigated fields of the Bucks Branch study site, Sussex

County, Delaware........................................................................................................................24

14. Boxplot showing concentration of nitrate as nitrogen in relation to the estimated

apparent age of groundwater under the irrigated and dryland fields at the Bucks

Branch study site, Sussex County, Delaware........................................................................26

15. Graph showing ratios of the nitrogen (delta15N) and oxygen (delta18O) isotopes

of nitrate in groundwater from the Bucks Branch study site, Sussex County,

Delaware......................................................................................................................................26

16. Graph showing streamflow at the gaged site SW2 during the study period,

Bucks Branch study site, Sussex County, Delaware............................................................27
vii

17. Boxplots showing concentrations of A, phosphorus and B, nitrogen in surface 

water, site SW2, Bucks Branch study site, Sussex County, Delaware..............................28

18. Graphs showing continuous monitoring data at site SW2 for A, specific 

conductance, B, dissolved oxygen, C, pH, and D, temperature, site SW2, 

Bucks Branch study site, Sussex County, Delaware, February 2014 through 

September 2015...........................................................................................................................29

19. Graphs showing concentrations of selected constituents over time in base flow 

and stormflow, site SW2, Bucks Branch study site, Sussex County, Delaware ..............30

20. Graph showing differences in the concentrations of nitrate in base flow at similar 

flow rates during the fall through winter and spring through summer, site SW2, 

Bucks Branch study site, Sussex County, Delaware............................................................31

21. Graphs showing trends in A, the ratio of the nitrogen (delta15N) and oxygen 

(delta18O) isotopes of nitrate, and B, delta15N and nitrate in water by season and 

flow conditions, site SW2, Bucks Branch study site, Sussex County, Delaware ............32

22. Graph showing spring base-flow synoptic sampling showing relation of streamflow 

to nitrate concentrations in surface water collected around the Bucks Branch 

watershed, Sussex County, Delaware, 2014–16....................................................................34

23. Graphs showing nutrient data by month for the watershed outlet at SW1, 

Bucks Branch, Sussex County, Delaware, 2003–12..............................................................35

24. Photographs showing alternating rows of rye and radish at the study site with 

a radish shown on the right to demonstrate the possible size of a radish tuber in 

the irrigated field, Bucks Branch study site, Sussex County, Delaware...........................35

25. Graph showing nitrate as nitrogen concentrations for shallow and medium depth 

wells and soil-water lysimeters over time from irrigated and dryland fields at the 

Bucks Branch study site, Sussex County, Delaware............................................................37

26. Graphs showing soil-water samples of A, nitrate as nitrogen and B, dissolved 

organic nitrogen from irrigated and dryland fields at the Bucks Branch study site, 

Sussex County, Delaware..........................................................................................................38

Tables

1. Data for temporary wells installed and sampled at the Bucks Branch study site, 

Sussex County, Delaware............................................................................................................7

2. Mean Total Kjeldahl Nitrogen values for soils collected at various depths for the 

dryland and irrigated fields for all sampling locations at the Bucks Branch study 

site, Sussex County, Delaware.................................................................................................15

3. Summary statistics for water samples from January and February 2014 in 

temporary wells installed on the study fields in the Bucks Branch study site, 

Sussex County, Delaware..........................................................................................................17

4. Multivariate correlations of groundwater samples from the dryland and irrigated 

fields, Bucks Branch study site, Sussex County, Delaware................................................19

5. Correlation between specific conductance, and nutrients and major ions in 

samples from lysimeters, Bucks Branch study site, Sussex County, Delaware..............22

6. Summary statistics for samples from flowpath wells, instream piezometers, and 

the intensive stream site (SW2), Bucks Branch study site, Sussex County, Delaware..25

7. Mixing model input data and model output used for estimating the loss of nitrate 

from groundwater discharging to surface water at site SW2, Bucks Branch, 

Sussex County Delaware ..........................................................................................................33
viii

8. Cover crop biomass nutrient content prior to radish die-off from freezing and 

following radish die-off in the irrigated field at the Bucks Branch study site, 

Sussex County, Delaware..........................................................................................................36

9. Cover crop biomass nutrient content prior to radish die-off from freezing and 

following radish die-off in the dryland field at the Bucks Branch study site, 

Sussex County, Delaware..........................................................................................................36

10. Soil fertility results for the top foot of soil from the dryland and irrigated fields 

associated with the Bucks Branch study site, Sussex County, Delaware .......................37

Conversion Factors

U.S. customary units to International System of Units

Multiply By To obtain

Length

inch of water column 0.024 centibars (cb)

inch (in.) 2.54 centimeter (cm)

inch (in.) 0.3048 meter (m)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

acre 0.4047 hectare (ha)

square mile (mi2

) 2.590 square kilometer (km2)

Flow rate

cubic foot per second (ft3

/s) 0.02832 cubic meter per second (m3/s)

Mass

ounce, avoirdupois (oz) 28.35 gram (g)

pound, avoirdupois (lb) 0.4536 kilogram (kg)

ton, short (2,000 lb) 0.9072 metric ton (t)

ton, long (2,240 lb) 1.016 metric ton (t)

Hydraulic gradient

foot per mile (ft/mi) 0.1894 meter/kilogram (m/kg)

Transmissivity

foot squared per day (ft2

/d) 0.09290 meter squared per day (m2/d)

Application rate

pounds per acre per year (lb/acre/yr) 1.121 kilogram per hectare per year (kg/ha/yr)

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

 °C = (°F – 32) / 1.8
ix

Datum

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

(NAVD 88).

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

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

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (μS/cm at 

25 °C).

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

or micrograms per liter (μg/L).

Abbreviations

DEOS Delaware Environmental Observing System

DGS Delaware Geological Survey

DNREC Department of Natural Resources and Environmental Control

EPA U.S. Environmental Protection Agency

UD University of Delaware

USDA ARS U.S. Department of Agriculture Agricultural Research Service

USGS U.S. Geological Survey

Monitoring the Water-Quality Response of Agricultural 

Conservation Practices in the Bucks Branch Watershed, 

Sussex County, Delaware, 2014–16

By Judith M. Denver, Alexander M. Soroka, Betzaida Reyes, Todd R. Lester, Deborah A. Bringman, and 

Michael S. Brownley

Abstract

The purpose of this study was to evaluate the effects of 

irrigation and cover crops as conservation practices on water 

quality in groundwater and streams. Bucks Branch, a stream 

in the Nanticoke River watershed in southwestern Delaware, 

was identified as having one of the highest concentrations 

of nitrate in all surface-water sites sampled by the Delaware 

Department of Natural Resources and Environmental Control 

(DNREC). The study site is on two adjacent fields bordering 

Bucks Branch, one that has used irrigation since 2000 and 

one with dryland farming; both under conservation tillage and 

long-term rotation of corn, soybean, and small grain crops. A 

streamgage was installed near the study site fields to measure 

streamflow and water quality. The study area is typical of 

farming practices and environmental conditions throughout 

much of the intensively farmed agricultural land of the Coastal 

Plain of Delaware and surrounding parts of Maryland. Monitoring was conducted from January 2014 through June 2016. 

Corn was grown on both fields during the two growing seasons of the study period, and cover crops were planted before 

or shortly after harvest on both fields. During the second year 

of data collection, the effects of radish and rye grass cover 

crops on nutrient transport were studied.

The combined results from data collected for this study 

show that water and nitrate moved below the root zone year 

round when soil moisture was high, especially after significant 

rainfall and frequently after irrigation. Soil water sampled 2 to 

3 weeks after nutrients were applied had nitrate concentrations greater than 50 milligrams per liter as nitrogen (mg/L as 

N) and may be a significant source of nitrate to groundwater. 

Whereas recharge containing elevated nitrate concentrations 

also occurred under the dryland field, it was less frequent and 

of lower concentration than recharge under the irrigated field.

Nitrate was present in all groundwater samples from 

these sites. Groundwater estimated to have recharged 

within 10 years or less had higher median concentrations of 

nitrate than in older water samples. The oldest groundwater 

encountered was over 30 years old, and had traveled along 

the longest, deep flowpaths from upland fields to the stream. 

The median nitrate concentration was 18 mg/L as N in 

younger water (less than 10 years old) beneath the irrigated 

field, compared to about 10 mg/L as N in younger water 

beneath the dryland field. Samples from the shallow upland 

wells in both study fields showed little, if any, evidence of 

denitrification. Several samples from deeper wells and from 

wells near forested riparian zone wetlands that border both 

fields did show partial denitrification.

A mixing model estimated that between 12 and 

22 percent of the nitrate discharging to the stream was 

lost through uptake and denitrification upstream of the 

streamgage on Bucks Branch. Continuous data collected at 

this site and evidence of denitrification in the surface-water 

samples showed a greater potential for loss of nitrate during 

the warmer months than the colder months. This pattern 

was similar to that seen below the streamgage at the most 

downstream site in the watershed.

A mixed cover crop of radishes and rye was planted prior 

to removal (radishes) and just after harvest (rye) of the corn 

crop on the irrigated field. Rye grass was planted shortly after 

crop harvest on the dryland field. Cover crop biomass samples 

collected while radishes were growing and after they were 

killed by freezing temperatures indicates that the early planted 

radish crop effectively scavenged available nitrogen from 

the soil. Whereas radish biomass initially held more nitrogen 

than rye, at 55 to 8 pounds per acre, respectively, leaching of 

inorganic nitrate following radish die-off was minimal. Soilwater nitrate concentrations during the cover-crop growing 

period were lower than during the growing season prior to 

planting of the cover crop. There also was an increase in soil 

fertility and dissolved organic nitrate in samples of soil water 

that was likely related to increased soil microbial metabolism. 

Results indicate that cover crops stored plant nutrients 

over the winter and did not increase shallow groundwater 

concentrations of nitrate.

Although conservation practices such as cover crops 

and nutrient management have been applied to these fields, 

there was still significant leaching of nitrate to groundwater, 

especially under the irrigated field. This will likely continue to 

be a challenge in this area and other parts of the Coastal Plain 
2 Monitoring Water Quality in the Bucks Branch Watershed, Sussex County, Delaware

where soil moisture capacity is relatively low and managing irrigation around rainfall is difficult. Cover crops, when 

planted in standing corn, are one practice that can effectively 

pull nitrate from below the root zone to the top layer of soil, 

thus limiting the amount of potential nitrate leaching to 

groundwater. Irrigation management that would lower average 

soil moisture conditions during the growing season also could 

potentially limit nitrogen transport.

Introduction

The Delaware Department of Natural Resources and 

Environmental Control (DNREC) has collected water-quality 

data throughout the Nanticoke River watershed in Delaware to 

better understand the distribution of elevated nutrient concentrations in groundwater and surface water and to identify 

areas where management efforts should be targeted. These 

data identified Bucks Branch as having one of the highest 

concentrations of nitrate in surface water of all sites sampled 

by DNREC (fig. 1). This stream receives excessive amounts 

of nutrients and sediment from both agricultural and residential sources and is on Delaware’s 303(d) list of impaired 

water bodies (Delaware Department of Natural Resources and 

Environmental Control, 2017). The State of Delaware’s Total 

Maximum Daily Load (TMDL) developed for this watershed 

requires a 30-percent and 50-percent reduction in nitrogen and 

phosphorus loads, respectively. 

In a recent study, the U.S. Geological Survey (USGS) 

and DNREC provided initial information about the transport 

and fate of nitrate within the Bucks Branch watershed (Clune 

and Denver, 2012). Concentrations of nitrate exceeded the 

U.S. Environmental Protection Agency (EPA) drinking water 

standard (10 milligrams per liter as nitrogen, or mg/L as 

N) in 60 percent of groundwater samples and 42 percent of 

surface-water samples. Geochemical analyses indicated that 

agriculture is the predominant source of nitrate contamination in Bucks Branch. The median apparent age of sampled 

groundwater was 18 years, and the estimated average residence time of groundwater contributing to the streamflow for 

the entire Bucks Branch watershed at the outlet was approximately 19 years. This study provided a basic understanding of 

water-quality conditions from past management practices and 

a general understanding of the residence time of water in this 

system. Resource managers would like to better evaluate how 

current conservation practices, such as irrigation and cover 

crops, are affecting shallow groundwater and how long it will 

take to see these potential effects in streams. 

Agronomic studies in corn production areas have long 

shown that irrigation significantly raises crop yields, despite 

variability in annual weather conditions (Al-Kaisi and Yin, 

2003; Eck, 1984; Olson and others, 1964). A proposed benefit 

of irrigation is its ability to raise the nitrogen consumption of 

irrigated corn (Al-Kaisi and Yin, 2003; Bushong and others, 

2014; Kim and others, 2008). A more efficient use of nitrogen 

Growth of aquatic vegetation in the Bucks Branch stream channel, March 2014. 

Photograph by Mark Nardi, U.S. Geological Survey

by plants under irrigated land is hypothesized to lower the 

amount of residual nitrogen available in soil that would 

eventually leach to groundwater. The use of irrigation on 

cropland has expanded to cover over 25 percent of all cropland in Delaware due to its yield-stabilization potential and 

economic benefit to farmers (University of Delaware, 2017). 

The implementation of irrigation and statewide yield stability 

has been encouraged by the Delaware Economic Development Office through the Delaware Rural Irrigation Program 

(DRIP), which provides small interest free loans to farmers 

for up to 7 years (Delaware Department of Agriculture, 2017). 

The rate of irrigation, weather conditions, and crop growth are 

deciding factors in determining whether irrigation assists in 

either nitrogen leaching or crop plant consumption (Agostini 

and others, 2010; Eck, 1984; Randall and Mulla, 2001; Zhang 

and others, 2005). Previous studies in the Mid-Atlantic region 

indicate that irrigation could increase groundwater nitrate 

concentrations in the eastern United States (Ritter and others, 

1993; Ritter and others, 1991).

Cover crops are considered one of the most important 

agricultural conservation practices due to their ability to prevent topsoil erosion, increase soil organic matter, retain nutrients, and alleviate soil compaction. A large body of research 

indicates that cover crops reduce nutrient losses from agricultural lands (Dean and Weil, 2009; Jackson and others, 1993; 

Kristensen and Thorup-Kristensen, 2004; Staver and Brinsfield, 1998; Weinert and others, 2002). Cover crops potentially lower nitrate concentrations in groundwater by holding 

nitrogen in the soil zone for use by the subsequent crops. The 

variety of planted crop is important as studies comparing rye 

and radish crops indicated that radishes could be more effective than rye at scavenging deep soil nitrogen (Kristensen and 

Thorup-Kristensen, 2004), but could leach higher nitrate concentrations in sandy soils of the Coastal Plain following winter 

die-off of the cover crop (Dean and Weil, 2009).
Introduction 3

0.5 1 MILE

0.5 1 KILOMETER

0

0

75°41' 75°40' 75°39' 75°38' 75°37'

38°43'

38°42'

38°41'

National land cover data modified from 2007 Delaware Land Use and

Land Cover; 2000 hydrography modified from U.S. Geological Survey

National Hydrography Dataset (NHD) digital data, 1999; Albers

Equal-Area Conic projection

0 20 40 60 80 100

Land use, in percent

EXPLANATION

Hay/pasture

Concentrated animal 

 feeding operations

Cropland

Forest

Residential

Water

Wetlands

Orchards

Surface-water

 sampling site

 and identifier

Bucks Branch

watershed

(7.02 square miles)

Delaware River

C&D CANAL

NEW

CASTLE

KENT

SUSSEX

Wilmington

Bucks Branch

watershed

Dover

DELAWARE

DELAWARE

MARYLAND

MARYLAND

DELAWARE

BAY

ATLANTIC OCEAN

Indian River Bay

Rehoboth Bay

N PENNSYLVANIA

NEW JERSEY

EXPLANATION

Study Area

FALL 

LINE

Coastal Plain 

of Delaware

Bridgeville

Gilbert

Trivitts

Ditch

Freidel

Prong

Bucks

Branch

SW7

SW4

SW10

SW6

SW9

SW8

SW5

SW3

SW2

SW1

SW1

Streamgage

Figure 1. Location of the study area, and land use and surface-water sites in the Bucks Branch watershed, Sussex County, 

Delaware.
4 Monitoring Water Quality in the Bucks Branch Watershed, Sussex County, Delaware

Purpose and Scope

The purpose of this report was to evaluate the effects of 

current conservation practices on water quality in the Bucks 

Branch watershed. Specific conservation practices that were 

studied include the effects of irrigation on nutrient transport 

to groundwater compared to nutrient transport under dryland 

farming, and the effect of cover crops on nutrient transport. 

Surface-water quality and the transport of nutrients to surface 

water also were studied as part of this project.

Study networks on two adjacent fields, one with irrigation and one with dryland farming (both under conservation 

tillage), and a streamgage were established for monitoring of 

soil, shallow groundwater, and surface water. Monitoring was 

conducted from January 2014 through June 2016. Corn was 

grown on both fields during the two growing seasons of the 

study period and cover crops were planted at or near the end of 

each growing season on both fields. The potential differences 

between a cover crop mix (radish and rye) and a monocrop 

(rye) on the- soil and soil-water chemistry were studied during 

the second year of data collection. 

Results from data collected during this study are 

described. Data collection included groundwater levels, both 

continuous and discrete, and samples for chemical analysis; 

samples of unsaturated zone soil water and soils, surface-water 

flow, and water quality. Continuous discharge and continuous 

and discrete water quality were monitored at a streamflowgaging station near the groundwater study site to relate 

surface-water chemistry to groundwater inputs of nitrogen and 

instream processing of nitrogen and phosphorus. The spatial 

and temporal distribution of nutrients in groundwater and 

surface water were determined using these data. Data collected 

by DNREC at the outlet to the larger Bucks Branch watershed 

and synoptic streamflow data that were collected at multiple 

locations during spring base-flow conditions around the watershed were compared to results from the intensive study site. 

Description of Study Area

Bucks Branch is a 7.02-square-mile (mi2) watershed that 

flows into the Nanticoke River, which drains into the Chesapeake Bay (fig. 1). The area is underlain by sediments of the 

Beaverdam Formation, which consists mainly of fluvial and 

estuarine deposits of sand, gravel, silt, and clays, and contains 

a surficial unconfined aquifer (Andres, 1994). Soils are predominantly well-drained sandy loam (Soil Survey Staff, Natural Resources Conservation Service, 2016). The unconfined 

surficial aquifer is between 60 and 100 feet (ft) thick and the 

water table is 5 to 10 ft below land surface (Andres and Klingbeil, 2006). The topography in the Bucks Branch watershed 

is relatively flat, with elevations that range from 30 to 50 ft 

above sea level. In this area of the Coastal Plain, groundwater 

provides the largest contribution of freshwater flow in streams, 

and is the predominant influence on surface-water quality and 

the major source of nitrate (Lindsey and others, 2003; Shedlock and others, 1999; Spahr and others, 2010).

Previous studies have documented elevated levels of 

nitrate in groundwater and surface water of the Delmarva 

Peninsula (Andres, 1991; Denver, 1989; Denver and others, 2004; Hamilton and others, 1993) and within the Bucks 

Branch watershed (Clune and Denver, 2012). The application 

of fertilizer and manure from poultry has been cited as a major 

source of contamination in groundwater, with local contributions from septic systems (Ator and Denver, 2015; Denver, 

1989; Ritter and Chirnside, 1982). The median concentration 

of nitrate in groundwater-dominated stream base flow on the 

Delmarva Peninsula was around 3 mg/L as N for samples 

collected in 2000 (Denver and others, 2004). In 1995, concentrations of nitrate in Bucks Branch were about 6 mg/L as 

N (USGS streamflow station 01487150, Bucks Branch near 

Atlanta, Delaware). Concentrations greater than 10 mg/L as 

N have been measured more recently in Bucks Branch (Clune 

and Denver, 2012).

Land use in the Bucks Branch watershed is predominantly agricultural, consisting mainly of cropland (90.1 percent) and concentrated animal feeding operations (0.8 percent), along with wetlands (4.1 percent), residential dwellings 

(2.6 percent), forest (1.9 percent) and other settings (0.5 percent) (fig. 1; Clune and Denver, 2012). Most wetlands are in 

a forested area along the main stem of Bucks Branch where 

the soils are poorly drained and less suited for agriculture 

(fig. 1). In addition, wetlands are present in the riparian zone 

along Bucks Branch and its main tributary, Freidel Prong. The 

watershed has been extensively channelized to improve the 

drainage of local uplands, which has created steep banks along 

most streambeds that often disconnect the stream from the 

local riparian wetlands, except during periods of high water 

table and high base flow in the stream. Ditches and tile drains 

added to drain agricultural fields extend from the stream into 

fields throughout the watershed. 

Agriculture has primarily included poultry production 

and the cultivation of corn, soybeans, and small grains during 

the past 50 years. The vast majority of nutrient inputs to the 

land surface in this part of Sussex County, Delaware, are from 

manure and fertilizer. Irrigation is practiced on 35 percent of 

cropland in the Bucks Branch watershed; in Sussex County, 

over 45 percent of annually harvested cropland is irrigated 

(U.S. Department of Agriculture, National Agricultural Statistics Service, 2012).

Prior to and during the study period, the Bucks Branch 

study site had a nutrient management plan developed by the 

Sussex Conservation District based on general nutrient management recommendations made by the University of Delaware (UD) Cooperative Extension (University of Delaware, 

College of Agriculture and Natural Resources, 2017) (see 

text inset). Conservation tillage has been practiced for over 

15 years on both fields. Other conservation practices utilized 

include nutrient management planning to enhance crop production and increase nutrient uptake, irrigation management, 

grassed buffers, and the planting of cover crops to retain soil 
Introduction 5

Crop Production Practices at the Bucks Branch Study Site, 2015 Growing Season

Late February–Early March Soil fertility tests done by crop consultant

March 20 Cover crops terminated with herbicide prior to corn planting

April 2 Herbicide applied for weed control by GPS guided sprayer

April 24 Poultry litter applied and incorporated, 2 tons/acre

Poultry litter was applied using calibrated and GPS guided equipment. Nitrogen applications of poultry litter estimated to account 

for 60 pounds of available nitrogen for crops in the first year.

May 7–15 Corn planted with starter fertilizer

May 8 Herbicide applied for weed control by GPS guided sprayer

June 11 First irrigation event on corn crop

The corn crop was irrigated 10 times over the 2015 growing season with a total of about 5.6 inches of water applied.

June 7–15 Majority of nitrogen fertilizer applied to corn

 165 pounds/acre to irrigated land

 100 pounds/acre to dryland

June 1 Herbicide applied for weed control by GPS guided sprayer

July 8 Aerial fungicide application

August 17 Cover crops of radishes planted in the standing corn crop using an experimental air-seeder and then irrigated to water cover crop seeds

Mid-late September Corn harvested and rye cover crop planted on both fields

 Irrigated yield: 237.8 bushels/acre

 Dryland yield: 164 bushels/acre

After corn harvest Corn residue cut and incorporated using turbo-tiller

Conservation practices used at Bucks Branch site to reduce erosion and promote soil health: 

Residue and Tillage Management—Strip Till, Cover Crops, Conservation Crop Rotation, Nutrient 

Management, and Integrated Pest Management
6 Monitoring Water Quality in the Bucks Branch Watershed, Sussex County, Delaware

nutrients after the growing season and enhance soil fertility. 

Fields included in this study have been used to grow corn, 

soybeans, and small grains. During the study period in 2014 

and 2015, corn was grown on both fields.

Network Design, Sampling, and 

Analytical Methods

Sampling at the Bucks Branch study site began in January 2014 and continued through June 2016. Data collection 

included groundwater levels and samples; samples of unsaturated zone soil water and soils; surface-water flow and water 

quality; and the collection of agronomic data on crop and 

cover-crop production. The UD Cooperative Extension collected additional data on soil moisture in both fields over the 

two growing seasons during which sampling took place. The 

farmer provided crop yield and irrigation data for 2015. Corn, 

plant, and grain samples were collected and analyzed for total 

nitrogen by the UD Cooperative Extension staff. Rainfall data 

were obtained from a site near Bridgeville, Delaware, approximately 3.5 mi north of the study site that is operated by the 

Delaware Environmental Observing System (DEOS) (University of Delaware, 2017).

Initial Groundwater Investigation

Groundwater-flow direction and water quality were 

initially characterized in January and February 2014 using 

temporary well points installed with the DNREC Division 

of Waste and Hazardous Substances Geoprobe. To collect 

samples and water-level data, a well point with a 4-ft screen 

(referred to as a temporary well) was driven into the surficial 

aquifer at 19 locations around the study site (fig. 2). After the 

water table in the temporary wells stabilized and prior to sampling, depth to water was measured with an electric tape gage. 

Single temporary wells were installed near the water table at 

11 locations; multiple wells were installed from near the water 

table at 15- to 20-ft intervals to depths of about 45 ft at 7 locations (fig. 2; table 1). Thirty-two samples were collected and 

analyzed for field parameters, major ions, and nutrients. 

Each temporary well was surveyed to determine its location and the elevation of the land surface. A general watertable map for the study site was derived using water levels 

from the shallowest temporary well at each site, along with 

water levels in adjacent ditches and determination of the elevation of adjacent streams acquired using a lidar dataset. This 

map was used to guide installation of flowpath network wells.

Groundwater Flowpath Networks

Based on flowpaths identified during temporary well 

mapping, networks of shallow permanent wells (9 to 25 ft 

deep) were installed along a groundwater flowpath on both 

fields in April 2014 and augmented with deeper wells (30 to 

75 ft deep) in July 2015 (fig. 3). The transects consisted of 

clusters of multiple wells at different depths installed across 

each field with additional shallow wells placed east and west 

between well clusters (fig. 3). USGS drillers from Reston, 

35

34

33

32

31

30

DR-07

IR-06 IR-05 (3)

IR-01

IR-02

IR-08

IR-03 (3)

IR-04 

IR-07

IR-09 (3) IR-10

DR-08 DR-01 (3)

DR-02

DR-03 (3)

DR-06 DR-04 (3) DR-05

IR-11 (2)

Dryland

Field

Irrigated

Field

Meadow

EXPLANATION

IR-11 (2) Location of temporary well and site number—

(2) Indicates number of depths drilled at site,

if greater than 1

30 Water-table elevation, in feet—February 2014

Figure 2. Location of temporary well points and water-table elevation in February 2014 at the Bucks Branch study site, Sussex 

County, Delaware.
Network Design, Sampling, and Analytical Methods 7

Table 1. Data for temporary wells installed and sampled at the 

Bucks Branch study site, Sussex County, Delaware.

[ID, identification; depth, well depth; ft, feet; location of temporary wells 

shown on figure 2.]

Well ID Date Depth 

(ft)

Water level below 

land surface

(ft)

Dryland wells

DR01-1 2/27/2014 12.0 6.49

DR01-2 2/27/2014 22.1 7.51

DR01-3 2/27/2014 42.0 10.65

DR02-1 2/27/2014 16.0 7.05

DR03-1 2/26/2014 16.7 9.44

DR03-2 2/26/2014 25.2 9.8

DR03-3 2/26/2014 36.7 10.0

DR04-1 2/11/2014 12.8 5.3

DR04-2 2/11/2014 25.6 5.2

DR04-3 2/11/2014 45.1 5.8

DR05-1 2/11/2014 9.5 4.64

DR06-1 2/11/2014 13.1 5.73

Meadow wells

DR07-1 2/11/2014 13.8 8.3

DR08-1 2/27/2014 15.9 7.65

Irrigated wells

IR01-1 1/27/2014 14.0 6.49

IR02-1 1/30/2014 14.0 5.99

IR03-1 2/12/2014 8.9 3.15

IR03-2 2/12/2014 24.8 3.2

IR03-3 2/12/2014 40.8 3.75

IR04-1 1/30/2014 13.5 6.5

IR05-1 1/27/2014 17.2 8.37

IR05-2 1/27/2014 27.7 8.6

IR05-3 1/27/2014 44.0 9.36

IR06-1 2/26/2014 16.6 5.6

IR07-1 2/26/2014 13.1 6.42

IR08-1 2/12/2014 13.0 4.0

IR09-1 1/30/2014 18.6 4.96

IR09-2 1/30/2014 27.6 5.05

IR09-3 1/30/2014 37.5 6.16

Streambank wells

IR10-1 2/6/2014 11.4 2.62

IR11-1 2/6/2014 11.3 2.42

IR11-2 2/6/2014 21.3 5.47

Virginia installed the wells using a hollow stem auger. The 

wells were constructed using 2-inch (in.) polyvinyl chloride 

(PVC) pipe with 3-ft PVC screens on the bottom of each well. 

The well network was sampled in the spring before fertilizer 

and manure application, and in the fall after crop removal 

during the study period. Wells were located directly in the 

fields and were constructed so that they could be buried during 

planting and harvest and brought back up for sampling. Temporarily burying the wells allowed the farmer to plant, fertilize, and harvest crops directly over the wells without leaving a 

barren area that could affect shallow water quality and reduce 

cropped acreage. Groundwater also was collected once along a 

transect across the streambed near the downstream end of the 

irrigated field flowpath using a stainless steel piezometer that 

was inserted into the streambed (fig. 3).

Water-level sensors for continuous measurement of 

groundwater levels were installed in three wells on the irrigated field, one well on the dryland field, and one well on the 

streambank downgradient of the irrigated field (fig. 3). Waterlevel equipment (if present) was removed when wells were 

buried and reinstalled after planting or harvest. All continuous and discrete groundwater-level data were collected and 

reviewed according to standard USGS procedures as outlined 

in Cunningham and Schalk (2011).

Groundwater Sampling Methods and Analysis

Groundwater samples collected from the flowpath 

network wells were analyzed for nutrients, major ions, the 

isotope composition of nitrate, and apparent age. Groundwater 

samples were collected prior to fertilizer and manure application in the spring, after crop removal in the fall of 2014–15, 

and in the spring of 2016. All samples were collected following protocols outlined in the USGS National Field Manual 

(U.S. Geological Survey, variously dated). Either a submersible gear-driven suction pump or a peristaltic pump was used 

to collect samples depending on water levels and the waterlevel recovery rate in the wells. Wells were purged until field 

parameters (pH, specific conductance, water temperature, 

and dissolved oxygen) stabilized and the final readings were 

recorded. All water-quality data were collected and reviewed 

according to standard USGS procedures (U.S. Geological 

Survey, variously dated). Water samples for major ions and 

nutrients were analyzed at the DNREC Environmental Laboratory in Dover, Delaware. The DNREC laboratory participates 

in the USGS blind sample program, in which a comparison 

of analytical results between this lab and others across the 

Nation is performed. Results from the DNREC laboratory are 

approved by this program and a previous USGS Laboratory 

Evaluation Program that was used for the first study at this site 

(Clune and Denver, 2012).

The samples for major ions and nutrient chemistry were 

filtered onsite through a disposable filter with 0.45-micrometer (μm) pore size and delivered to the DNREC Environmental Laboratory for analysis. Samples were analyzed for 
8 Monitoring Water Quality in the Bucks Branch Watershed, Sussex County, Delaware

EXPLANATION

DR-1, IR-2 

Irrigated Field 

Dryland Field 

Bucks Branch 

Irrigation Well

W, L

W, L

W, L

W P

W, L

L

L

L

W

P

Location of continuous water-level data collection

Location of lysimeter nest

Location of streambed piezometer transect

A A’

Indicates location of wells and well clusters 

Location of cross sections shown in Figures 4 and 13

Figure 3. Location of groundwater flowpath networks, Bucks Branch study site, Sussex County, Delaware.

major ions by either the inductively coupled plasma method 

with atomic emission spectrometry, ion chromatography, or 

automated colorimetry. Nutrient analyses were performed 

by automated colorimetry (with persulfate digestion when 

required) with uncertainties of less than 10 percent (less than 

20 percent for silica) (Ben Pressly, DNREC, written commun., 

2012). Surface-water and soil-water samples for nutrients and 

major ions were analyzed in the DNREC laboratory using the 

same methods. 

Samples collected for analysis of nitrogen isotopes were 

filtered onsite, first with a 0.45-μm pore size filter and then 

through a nitrate-free 0.2-μm filter. Samples were preserved 

with a potassium chloride tablet until shipment to the USGS 

Stable Isotope Laboratory in Reston, Virginia, where they 

were analyzed using the bacterial reduction method (Böhlke, 

2002; Casciotti and others, 2002; Sigman and others, 2001).

Samples for determination of the apparent age of 

groundwater were collected once in each of the medium to 

deep wells. Samples from shallow wells screened near the 

surface of the aquifer could not be used for the determination 

of groundwater age because of the potential for mixing of 

groundwater with the atmosphere during sampling. Samples 

for age dating were processed, preserved, and shipped to the 

USGS Chlorofluorocarbon Laboratory in Reston, Virginia, and 

the apparent age of groundwater was determined from concentrations of dissolved atmospheric gases and gaseous sulfur 

hexafluoride (SF6

) using methods described in Busenburg and 

Plummer (2000). Dissolved gases (including nitrogen, argon, 

oxygen, carbon dioxide, and methane) are used to determine 

the recharge temperature of groundwater, which is used with 

SF6 to determine groundwater age. SF6

 is soluble in groundwater, and the increasing rate of anthropogenic concentrations 
Network Design, Sampling, and Analytical Methods 9

of SF6

 has been well established for the atmosphere over time. 

Detectable amounts of SF6

 are often present in younger waters 

(approximately 1970s), such as those in the surficial aquifer of 

Bucks Branch, and can be used for age dating. The apparent 

age of groundwater reflects atmospheric concentrations of SF6 

at the time the water reached the water table. The nitrogen and 

argon ratio in dissolved-gas samples associated with age dating also was used to determine if denitrification had affected 

nitrate concentrations in groundwater as described in Böhlke 

and Denver (1995). When denitrification occurs, nitrogen 

gas is released and the ratio of dissolved argon to nitrogen 

in groundwater changes, indicating an excess in nitrogen 

gas compared to the natural ratio of nitrogen to argon gas in 

the sample. 

Soil Water

Soil water was sampled using a network of suction 

lysimeters. Six sets of paired shallow (1-ft) and deep (3-ft) 

lysimeters were installed adjacent to well clusters in the 

flowpath network in each study field (fig. 3). Lysimeters were 

removed during planting and harvest and then reinstalled in 

each field. Samples from the shallow lysimeters represent 

water chemistry in the root zone, where it is plant-available, 

whereas samples collected from the deeper lysimeters 

represent water that has flowed to the base of the active plant 

root zone. Soil-water samples were collected year round after 

precipitation events greater than 0.75 in. over the study period 

to track the movement of water and chemicals through the 

soil zone. 

Lysimeters, purchased commercially from Soil Moisture, 

Inc., were installed following the manufacturer’s directions. 

Each lysimeter consists of 1.9-in.-diameter PVC tube, with 

a porous ceramic cup on the bottom sealed on the top by a 

rubber stopper. A 2-in. bucket auger was used to excavate 

holes to the appropriate depth for each lysimeter. Soils were 

removed into a bucket and mixed with deionized water to create slurry with the approximate consistency of cement mortar. 

A small volume of the slurry was poured down the hole and 

the lysimeter was pushed into the slurry leaving a 2-in. section 

of lysimeter pipe protruding from the soil surface. A native 

soil slurry insured good contact between the soil surface and 

porous ceramic cup. The remaining soil was then back-filled 

into the hole, and firmly tamped down. The stopper assembly 

was protected from weather and animals by placing plastic 

bottles over the above-ground part of the lysimeter.

Prior to expected precipitation events greater than 

0.75 in., a pistol-style hand pump was used to create a vacuum 

inside the samplers. The vacuum in the lysimeter pulled soil 

water through the porous ceramic cup where it was extracted 

for chemical analysis 1 day after the precipitation event. 

Sample recovery from lysimeters was quite variable—ranging from no sample volume to the maximum volumes of 

949 milliliters (mL) and 1,180 mL in 1- and 3-ft-deep lysimeters, respectively. Due to variable sample volumes, analytes 

were prioritized in the following order: specific conductance, 

nutrients, major ions, isotopes, and alkalinity. Laboratories and 

procedures used for sample analyses were the same as those 

used for groundwater-sample analysis.

Lysimeters were sampled after precipitation events 

through the duration of the study period, for a total of 

19 sampling events and 110 total samples. Adequate sample 

volume was acquired to measure specific conductance in a 

total of 108 samples, nitrate and phosphorus in 89 samples, 

and major ions in 39 samples. Other analytes were measured 

less frequently.

An additional 15 lysimeters were installed near the existing lysimeters on the irrigated field to sample soil water during 

the cover-crop growing season of 2015. Additional lysimeter nests were sampled at 1-, 1.5-, and 3-ft depths, and split 

between rye and radish/rye cover-crop rows in the irrigated 

field. The irrigated field had two additional nests with 1- and 

3-ft depth lysimeters plus six 1.5-ft lysimeters were added to 

existing nests. The dryland field, which was planted with a rye 

cover crop, received one additional lysimeter nest and three 

1.5-ft lysimeters were added to existing nests. The difference 

between fields was due to the larger size of the irrigated field 

and the variations in cover-crop planting. During the covercrop season, the irrigated field yielded 22 samples whereas the 

dryland field yielded only 2 samples.

Surface Water

A USGS streamflow station, 01487150, Bucks Branch 

near Atlanta, Delaware (referred to as site SW2), was installed 

to monitor flow and water quality. Streamgaging began in 

February 2014 and lasted through June 2016 (fig. 1). During 

the period of record, gage height was collected by use of a 

pressure transducer; discharge measurements were collected 

at base flow and during storm events. Gage heights were then 

used to develop a rating curve from which discharge could be 

predicted for all streamflow levels. Additional base-flow and 

stormflow discharge measurements were collected regularly 

to verify rating accuracy. All continuous stage and (computed) 

discharge data were collected and reviewed according to standard USGS procedures as outlined in Sauer and Turnipseed 

(2010).

Surface-water samples were collected upstream of the 

gage using a depth-integrated sampler and then composited in 

a churn following USGS protocols for equal-width-increment 

sampling (U.S. Geological Survey, variously dated). Field 

measurements of surface-water flow, pH, specific conductance, 

dissolved oxygen, and temperature were collected at the time 

of sampling. Monthly samples of base flow and periodic 

samples of storms were collected for nutrients, major ions, 

and nitrogen isotopes at the gaged site. Samples for major ions 

and nutrients were analyzed at the DNREC Environmental 

Laboratory in Dover, Delaware, as described above for 

groundwater samples. Additional analyses for total nitrogen 

and total phosphorus were included in surface-water sampling. 
10 Monitoring Water Quality in the Bucks Branch Watershed, Sussex County, Delaware

A total of 48 samples were collected at site SW2 for base 

flow and storms. Streamflow was measured and samples 

were collected synoptically at base flow at an additional 

nine surface-water sites located around the Bucks Branch 

watershed in the spring of 2014–15 and late winter of 2016 

(fig. 1). 

Discrete Sample Quality Assurance

The quality assurance of sampling included both replicates and blank samples for major ions and nutrients in 

groundwater and surface water. There was no significant variability or bias apparent in the sampling or laboratory techniques for these chemicals, with the exception of the last blank 

sample collected, which had a chloride detection of 34 mg/L. 

No other chemicals were detected above reporting levels in 

that blank or in previous ones. Water samples collected on the 

same day as this blank had chloride concentrations lower than 

the blank and charge balances within 10 percent for positive 

and negative ions, so the data were deemed good to use with 

the measured chloride analyses.

After receiving field data and results from laboratories, 

graphical and statistical tests were used to describe the data 

and identify important patterns in soil-water chemistry; 

groundwater chemistry, recharge, and transport; and surfacewater flow and water chemistry. Data testing included relative 

abundance patterns, correlation matrices, linear correlations, 

and comparison of means. All data were reviewed and 

analyzed according to the Quality Assurance Project Plan 

submitted to DNREC and EPA. 

Real-Time Water Quality

Water quality in the stream was monitored for temperature, specific conductance, pH, and dissolved oxygen using 

a multi-parameter sonde starting in December 2014 and 

ending in August 2015. The monitor was co-located with 

the streamgaging equipment and deployed vertically near 

the center of flow. Continuous real-time data were collected 

every 15 minutes and transmitted every hour to the USGS 

National Water Information System website (data are accessible via NWIS for USGS site 01487150 at: https://waterdata.

usgs.gov/de/nwis/). The monitor was checked for fouling and 

calibration regularly; at least one or two times a month, and 

if necessary, appropriate corrections were made to the data. 

The maintenance of water-quality monitors and data collection 

followed guidance in Wagner and others (2006). Low stream 

levels in Bucks Branch during some summer months prevented the collection of continuous water-quality data because 

the sensors had to be removed until water level increased 

enough to keep them covered. All continuous water-quality 

data were collected and reviewed according to standard USGS 

procedures as outlined in Wagner and others (2006).

Soil Sampling

Soil samples were collected before and after the main 

growing season on both irrigated and dryland fields of the 

intensive study site to a depth of 36 in. and subsequently 

analyzed for Total Kjeldahl Nitrogen (TKN). Values for TKN 

include the nitrogen held in organic and ammonium forms, 

and do not directly measure nitrate, however, TKN values 

are a good indicator of nutrient and organic matter in the 

soil profile. In fall 2014, one core was taken within a 20-ft 

radius of each well. Sampling methods were expanded to 

ensure a more representative soil sample so that a composite 

sample was made by taking three cores within a 50-ft radius 

of each well. Push probes were used to sample the top 2 in. 

of soil after scraping plant residue from the surface. A bucket 

auger was used to obtain a composite sample of the remaining profile, which was split into four vertical segments: 2–6, 

6–12, 12–24, and 24–36 in. Soil samples were air-dried within 

4 hours of collection then ground and passed through a 2-millimeter sieve. Samples were submitted to the U.S. Department 

of Agriculture (U.S. Department of Agriculture Agricultural 

Research Service) (USDA ARS) Laboratory at University 

Park, Pennsylvania, for TKN analysis.

Cover-Crop Soil and Plant Data

Additional soil and plant samples were collected to 

study the effects of cover crops on soil chemistry and water 

quality. Cover-crop plants and soil samples were collected 

along an upland-to-lowland transect across the irrigated and 

dryland fields before and after radishes died in the winter. A 

2.7-square-foot (ft2

) quadrat was thrown by hand 16 times in 

the larger irrigated field and 8 times in the smaller dryland 

field. All above ground plant material and radish tubers 

within the quadrat were removed and five 1-ft soil cores were 

collected with a push probe. Soil samples were air-dried 

within 4 hours of collection. Radish tubers were separated 

from foliage and freeze-dried separately to avoid rotting. 

Radish and rye greens were air-dried in ovens. All material 

was ground and sieved before analysis at the UD Soil Test 

Laboratory for nutrients and major ions.

Data from Other Sources

Data on soil moisture, irrigation timing and amounts, 

and local precipitation were acquired for the study site. Soil 

moisture data were collected by UD using matric potential 

sensors placed at 6-, 12- and 18-in. depths in both the dryland 

and irrigated fields. Data, which were provided by UD who 

maintained the equipment, were transmitted at 5-minute 

intervals by cell tower to the Irrometer Company web site 

for real-time access. Records of applied irrigation water were 

collected from the farmer. Rainfall and other meteorological 

parameters were downloaded from the DEOS station 
The Water-Quality Response of Agricultural Conservation Practices at the Bucks Branch Study Site 11

maintained at Bridgeville, Delaware (University of Delaware, 

2017).

Data Availability

Groundwater, soil water, soil TKN, and plant data are 

available at [https://doi.org/10.5066/F77H1HK5 (Soroka 

and Denver, 2018)]. These data are not identified by specific 

location in order to maintain the privacy of the landowner. 

Continuous and discrete surface-water quality and flow data 

are available on the USGS National Water Information System 

(NWIS; U.S. Geological Survey National Water Information System, 2017) for the gaged site, USGS 01487150, and 

the synoptic surface-water sites (USGS 384112075370701; 

USGS 384132075380301; USGS 384211075383301; USGS 

384132075383901; USGS 384203075395601; USGS 

384236075395601; USGS 01487148; USGS 01487155; 

USGS 01487160; and USGS 384139075392001).

The Water-Quality Response of 

Agricultural Conservation Practices at 

the Bucks Branch Study Site

The intensive study site consists of two adjacent fields 

with long-term rotational cropping of corn, soybeans, and 

small grains. These are the most common crops in Sussex 

County, Delaware and most of the Delmarva Peninsula, and 

are largely produced to supply feed for the local poultry 

industry. Soil types and aquifer characteristics are the same 

in both fields. Irrigation began on the larger field in the study 

site (41.5 acres) in about 2000; the smaller field (9 acres) 

has not been irrigated. Both fields have been farmed using 

conservation tillage practices for over 15 years, which means 

that the soils are largely undisturbed between crops. Cover 

crops have been used on the dryland field after corn crops 

during several previous years and on the irrigated field for 

only 2 years after the corn crops during the study period. 

Hydrogeology

The Bucks Branch study site is underlain by an 80- to 

90-ft-thick surficial aquifer (fig. 4). Sediments consist mainly 

of orange to tan colored, fine to coarse sand and gravel with 

discontinuous layers of silt and clay. The land surface is 

relatively flat with only about a 5–10 ft change in elevation 

between the uplands to the streambank. The streambed in 

Bucks Branch is about 4 ft lower than the adjacent streambank because of channelization to lower the water table and 

improve land drainage in the watershed. Rainfall at a nearby 

gage in Seaford, Delaware was 38.8 in. in 2014 and 43.1 in. 

in 2015, which is considered within the normal range for this 

region (Delaware Environmental Observing System, 2017). 

The average annual recharge to the surficial aquifer in this area 

is about 14 inches per year (Sanford and others, 2012).

Groundwater flow is from the uplands towards the stream 

in both fields (fig. 4). Water levels were deepest beneath the 

uplands and ranged from about 8 to 12 ft below land surface 

during the study period. The depth to water decreased downgradient toward the stream, where water levels ranged from 

about 4 ft to 8 ft below land surface beneath the edge of the 

dryland field, and less than 3 ft to about 6 ft below land surface beneath the edge of the irrigated field (fig. 4). 

The vertical hydraulic gradient between the shallow 

and deeper wells was downward in upgradient parts of the 

flowpaths and upward near the stream (fig. 4). There was a 

strong upward hydraulic gradient near and beneath the stream 

indicating that Bucks Branch receives groundwater discharge 

from the entire thickness of the aquifer. The upward gradient 

was especially evident at site IR4, adjacent to the stream 

where water levels measured in the deepest well were almost 

1.3 ft shallower than in the two shallowest wells (fig. 4). An 

upward water-level gradient also was observed in a transect 

of temporary piezometers inserted into the streambed near site 

IR4 (see photograph). Water levels in the piezometers ranged 

from 15 to 18 in. above the streambed, and were also 4 to 9 in. 

above the stream surface.

Continuous water levels collected in four wells located 

in the flowpath of the irrigated field and one well in the center 

of the dryland field (fig. 3) show the effect of irrigation well 

pumping on water levels during the growing season (fig. 5A), 

and the effect of recharge to the water table during the winter 

(fig. 5B). The water-table elevation was similar in wells IR-1M 

near the irrigation well, and DR-2S on the adjacent dryland 

field, when the irrigation well was not functioning during the 

growing season or during the colder months (fig. 5). 

During irrigation pumping, the water level-gradient was 

toward the irrigated well on the irrigated field; water levels 

rapidly recovered to near previous levels when the pumping 

Water level in streambed piezometer shows upward gradient of groundwater 

discharge to surface water in Bucks Branch. Photograph by Alexander Soroka, 

U.S. Geological Survey
12 Monitoring Water Quality in the Bucks Branch Watershed, Sussex County, Delaware

Surficial Aquifer

Elevation, in feet

Distance, in feet

Confining Unit

Stream

Irrigation well Observation well

B. Bucks Branch Irrigated Flowpath

1

20

40

0

-20

-40

-60

0 500 1,000 1,500

6 2 7

8 3

Water Table 3/17-19/2015 4

Water Table 11/25/2014

NORTH SOUTH

B B'

Farm Field Wetland

Lysimeters

11.3

33.3

26.8

9.8

24.8

8.8

15.8

33.8

10

20

30

32.5

32.35

31.94

31.79

31.39

31.13

31.43

31.76

30.69

31.01

31.98

?

Discontinuous

Clay Layer

?

27.3

30.69

32.44

Surficial Aquifer

Elevation, in feet

Distance, in feet

Confining Unit

A. Bucks Branch Dryland Flowpath

Stream

20

40

0

-20

-40

-60

0 500 1,000

NORTH SOUTH

1 5 2 4

6 3 7

Water Table 3/17-19/2015

Water Table 11/25/2014

A A'

Farm Field

Lysimeters

Wetland Field

9.3

36.3

16.3

12.3

19.8

19.3

6.8

20

10

30

? ?

Discontinuous

Clay Layer

33.89

33.78

33.94

32.20

33.61 33.77

32.50

33.21

32.95

10

Direction of groundwater flow

Apparent age, in years

12.3

33.77

Apparent age, in years

Elevation of hydraulic head, in feet

EXPLANATION

S

D

DD

M

1

Indicates location of single wells

and well clusters—Number on top

corresponds to location on figure 2.

Screened intervals of wells indicated

by short black bars in column. 

Individual wells are identified by

their relative depths as “S” for

shallow, “M” for median, “D” for 

deep, and “DD” for the deepest

well in a well cluster.

Well labels in text and tables

identified by “DR” for dryland, and 

“IR” for irrigated. 

Figure 4. Groundwater flowpaths, the apparent age of groundwater, and the hydraulic-head gradients 

in flowpath wells beneath the A, dryland and B, irrigated fields Bucks branch study site, Sussex County, 

Delaware, April 2016.
The Water-Quality Response of Agricultural Conservation Practices at the Bucks Branch Study Site 13

B. December 1, 2015–February 1, 2016

Date

A. June 1, 2015–August 1, 2015

23

26

29

32

35

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Water-level elevation, in feet above

North American Vertical Datum of 1988 

Rainfall, inches

06/01/2015

06/11/2015

06/21/2015

07/01/2015

07/11/2015

07/21/2015

07/31/2015

12/01/2015

12/11/2015

12/21/2015

12/31/2015

01/10/2016

01/20/2016

01/30/2016

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Rainfall, inches

23

26

29

32

35

Water-level elevation, in feet above

North American Vertical Datum of 1988 

Date

EXPLANATION

IR-1M

IR-2S

DR-2S

IR-4S

IR-3S

Well numbers Rainfall data—Collected in Bridgeville, Delaware

by the Delaware Environmental Observing System,

University of Delaware (2017)

Bucks Branch streamgage SW2

Figure 5. Continuous monitoring of water-table and stream elevation on the irrigated and dryland fields; and rainfall in Bridgeville, 

Delaware showing A, the effects of irrigation during the growing season, and B, groundwater recharge during the winter, Bucks Branch 

Study site, Sussex County, Delaware.

stops (fig. 5A). Drawdown during irrigation near the irrigation 

well at IR-1M was about 7 ft. It was generally less than 1 ft 

at IR-3S, located about 1,000 ft away from the irrigation well 

towards the stream, and barely noticeable at IR-4S, adjacent to 

the stream (fig. 5A). Water levels collected at DR-2S, beneath 

the dryland field over 1,600 ft away from the irrigation well, 

also appear to be slightly affected by irrigation pumping, 

from both the adjacent study field and irrigation on the field 

immediately upgradient from the dryland field (fig. 5A). Bucks 

Branch did not show the effects of local irrigation pumping at 

the streamgage.

Aquifer Recharge

Most recharge to the water table occurs during colder 

months when evapotranspiration is least. During this time, 

continuous water-level records showed similar patterns in 

recharge occurrence after rainfall under both fields (fig. 5B). 

Recharge to the water table also was observed during the 

growing season in all wells at the study site. This pattern was 

especially evident in the continuous water-level record at 

IR-3S and IR-4S, where water levels were the closest to land 

surface of all monitored sites (fig. 5A). The water table ranged 

from 2.6 to 6.3 ft below land surface at IR-3S, and from 1.3 to 

4.3 ft below land surface at IR-4S. The shallow water levels 

resulted in the potential for rapid transport of water through 

the soil zone during irrigation or precipitation events (fig. 5A). 

Increases in the water table during the growing season did 

occur, but were less frequent and showed a smaller increase 

in the water-table elevation at sites IR-1M, IR-2S, and DR-2S 

where the water table is deeper than at the sites with a shallower water table. 

Soil moisture probes installed on both fields during the 

growing season to help guide irrigation timing were used 

for this study to compare amounts of plant-available water 

under irrigated and dryland conditions (fig. 6). These data 

also served as an indicator of potential soil conditions where 

recharge to the water table can occur. As soil moisture is 

depleted, soil tension increases (measured by centibars) and 

soils become dryer. A soil tension of 30 centibars in the sandy 

soils such as those in the Bucks Branch fields is considered 

to be a critical level for plant-available moisture and the level 

recommended for irrigation (James Adkins, University of 

Delaware Cooperative Extension, written commun., 2016) 

(fig. 6). Recharge to the water table occurred when soil 

moisture was high (soil tension was low) on both fields, but 

rainfall that occurred during or soon after irrigation increased 

the ability of water to move downward and recharge the 

aquifer. The difference in soil moisture between irrigated and 

dryland farming was especially evident in soil moisture values 
14 Monitoring Water Quality in the Bucks Branch Watershed, Sussex County, Delaware

0

20

40

60

80

Date

Soil tension, in centibars

A. Irrigated field

06/01/201506/11/201506/21/201507/01/201507/11/201507/21/201507/31/2015

0

2

8

6

4

Rainfall, in inches

B. Dryland field

0

20

40

60

80

Soil tension, in centibars

0

2

8

6

4

Rainfall, in inches

Date

06/01/201506/11/201506/21/201507/01/201507/11/201507/21/201507/31/2015

6 inches below surface

12 inches below surface

18 inches below surface

Rain (Delaware Environmental 

Observing System), Bridgeville

EXPLANATION

30 centibars

30 centibars

Depth of soil-moisture probe and 

rainfall amounts

Figure 6. Comparison of soil moisture in the A, irrigated and 

B, dryland fields, Bucks Branch study site, Sussex County, 

Delaware, June and July 2015.

recorded in the second half of July 2015 (fig. 6), when soil 

tension exceeded 30 centibars on the dryland field (fig. 5B) 

but soil moisture was maintained at a higher level under the 

irrigated field where irrigation was frequent (fig. 5A).

Apparent Age of Groundwater

The apparent age of groundwater at the study site ranged 

from about 6 years to over 37 years in samples from medium 

and deeper wells (fig. 4). Age dates could not be obtained from 

the shallowest wells because of the potential for introduction of air into the samples during pumping if the water table 

was drawn down to below the top of the well screens. Silica, 

which dissolves from aquifer material over time, correlated 

significantly with the apparent age of groundwater (fig. 7). 

This correlation was used to estimate the relative apparent age 

of groundwater from the shallow wells where age dating could 

not be determined directly. Although there is some variability 

in water with younger apparent ages, silica concentrations of 

less than 12 mg/L were estimated to represent water with an 

apparent age of less than 10 years. 

Soil Chemistry

TKN is a measure of the total nitrogen in organic and 

ammonia forms that are available for plant growth in soils 

(U.S. Environmental Protection Agency, 1993). To determine 

concentrations and movement of the available nitrogen, TKN 

was measured before the growing season prior to nitrogen 

application and after crops were removed in fall 2014–15, 

and in spring 2016. The dryland and irrigated fields had 

similar mean concentrations of TKN, responses to seasonal 

Apparent age, years

0 10 15 20 25 30 35 40

Silica, milligrams per liter

0

5

10

15

20

25

30

5

R2=0.67

Figure 7. Comparison of silica concentrations to the apparent 

age of groundwater from flowpath wells, Bucks Branch study site, 

Sussex County, Delaware.
The Water-Quality Response of Agricultural Conservation Practices at the Bucks Branch Study Site 15

changes, and TKN distribution in the soil profile and across 

the fields (table 2, fig. 8). The highest TKN concentrations 

were observed closest to ground surface and correspond to the 

historical plow layer and greatest biomass density (table 2). 

The top foot of soil on both fields held 1,900–2,000 pounds 

(lbs) of TKN per acre, several times what was observed in the 

2- and 3-ft depths. TKN will be most effectively utilized by 

plant roots at shallow depths in the soil. Nitrogen, mostly in 

the form of nitrate, is available to leach to deeper depths in the 

soil and aquifer at the 3-ft depth, as it has moved below the 

zone of most plant uptake.

Most leaching of nitrogen would be expected over the 

non-growing season when plant uptake is lowest and recharge 

is greatest. This would result in lower measured TKN during spring sampling times than during fall sampling times. 

Although some leaching was indicated between spring and 

fall sampling times on the irrigated field, in particular, where 

spring concentrations across the soil profile were lower than 

the preceding fall, this trend was not statistically significant 

(p > 0.05) (fig. 8A). TKN was higher in samples collected 

in 2015 than in 2014 in both fields (fig. 8A). The addition of 

manure to the fields for 2 years in a row may have contributed 

to the higher concentrations of TKN in 2015.

On both fields, upland sampling locations (field position 1 on fig. 8B; sites IR-1 and DR-1 on fig. 3) had higher 

TKN through the soil profile compared to lowland (field 

position 3 on fig. 8B; sites IR-3 and DR-3 on fig. 3) sampling 

locations. Lowland parts of the field were close to the forest 

edge, had the shallowest water table, and generally had wetter 

soils than upland areas. Greater wetness in the lowlands can 

result in greater potential nitrate loss through denitrification, 

mineralization, and leaching than in the dryer uplands of the 

fields and may account for this difference. 

Water Chemistry in Groundwater and Soils

Samples from the temporary wells screened from near 

the water table to about 40 ft below land surface were initially 

Table 2. Mean Total Kjeldahl Nitrogen values for soils collected 

at various depths for the dryland and irrigated fields for all 

sampling locations at the Bucks Branch study site, Sussex 

County, Delaware.

[in., inches; mg/kg, milligrams per kilogram of dry soil; lb/acre, pounds per 

acre]

Depth, 

in.

Dryland Irrigation

mg/kg lb/acre mg/kg lb/acre

0–2 999 602 1,049 632

2–6 609 734 553 666

6–12 397 718 364 658

12–24 186 672 197 712

24–36 129 466 118 427

used to characterize groundwater quality at the Bucks Branch 

study site. Flowpath network wells and lysimeters were used 

to analyze the transport of nutrients, in particular nitrate, 

through soil and the surficial aquifer to the stream, and 

changes in groundwater quality over time. 

Characterization of Groundwater Quality Using 

Temporary Wells

Groundwater beneath both fields at the Bucks Branch 

intensive study site was oxic, with dissolved oxygen concentrations between 3.4 and 10 mg/L, and acidic, with pH ranging 

from 4.4 to 5.9 (table 3). Concentrations of nitrate and specific 

conductance were above those typically found under natural 

conditions (0.4 mg/L as N for nitrate, and less than 60 microsiemens per centimeter at 25 degrees Celsius (°C) for specific 

conductance; Denver and others, 2010) in all samples from 

both fields (table 3). 

The median concentration of nitrate was 14.1 mg/L as 

N beneath the irrigated site and 12.4 mg/L as N beneath the 

dryland site in water from the temporary wells (fig. 9, table 3). 

Although the medians were similar, the range of nitrate concentrations was greater at the irrigated site (fig. 9A). Median 

specific conductance, a measure of the total ionic strength 

of water, and concentrations of calcium and chloride also 

were higher and had greater ranges at the irrigated site than 

at the dryland site (fig. 9A, table 3). These constituents were 

noticeably higher in younger water beneath the irrigated field 

compared to the dryland field, using increasing silica as an 

indicator of increases in the apparent age of water (fig. 9B). 

Chemicals commonly associated with agricultural inputs; 

including calcium, magnesium, potassium, and chloride that 

are associated with the application of lime and potash to 

fields are positively correlated with specific conductance in 

groundwater from both study fields (table 4). This pattern 

is common on the Delmarva Peninsula, where natural water 

is very dilute and chemicals from fertilizer, manure, and 

lime applied to fields typically dominate water chemistry in 

agricultural areas (Denver, 1986; Hamilton and others, 1993). 

There are differences in the concentrations of some chemicals 

and the strength of correlation coefficients for these chemicals 

between the study fields, however. In addition to nitrate and 

specific conductance, chloride and calcium also had higher 

median concentrations beneath the irrigated field whereas 

the medians for potassium and sulfate were higher beneath 

the dryland field than the irrigated field (fig. 9A). It is not 

immediately obvious why these differences occur, but they 

may be related to differences in agricultural practices on these 

fields related to manure application, cover crops, and irrigation 

over time.

Agricultural effects on water quality seem to be 

greater in younger waters, rather than older waters, beneath 

both fields as seen in higher concentrations of many ions 

associated with agricultural chemical applications and specific 

conductance compared to the concentration of silica (fig. 9B). 
16 Monitoring Water Quality in the Bucks Branch Watershed, Sussex County, Delaware

Dryland Irrigated

Field

Total Kjeldahl Nitrogen, milligrams per kilogram

0 500 1,000 1,500

30

18

9

4

1

0 500 1,000 1,500

Midpoint of soil sampling interval, inches

Season

Fall14

Fall15

Spring15

Spring16

EXPLANATION

A

Dryland Irrigated

Field

Total Kjeldahl Nitrogen, milligrams per kilogram

0 500 1,000 1,500

Midpoint of soil sampling interval, inches

30

18

9

4

1

0 500 1,000 1,500

Field position

1

2

3

EXPLANATION

B

Figure 8. Total Kjeldahl Nitrogen values in soil for A, the mean concentrations in samples near all wells shown for different sampling 

times on the dryland and irrigated fields, and B, for the three main sampling locations on the dryland and irrigated fields in the 

flowpath networks, Bucks Branch study site, Sussex County, Delaware. [Well site locations shown on figure 3; field positions 1, 2, and 

3 indicate locations from the field uplands by well site 1 to the field lowlands at well site 3 near the edge of both fields.]
The Water-Quality Response of Agricultural Conservation Practices at the Bucks Branch Study Site 17

Table 3. Summary statistics for water samples from January and February 2014 in temporary wells 

installed on the study fields in the Bucks Branch study site, Sussex County, Delaware.

[n, number of samples; ft, feet; °C, degrees Celsius; μS/cm, microsiemens per centimeter at 25 °C; mg/L, milligrams 

per liter; min, minimum; max, maximum]

Constituent Dryland field (n=12) Irrigated field (n=15)

Median (min–max) Median (min–max)

Physical properties

Depth (ft) 22.1 (9.5–45.1) 17.2 (8.9–44)

Water temperature (°C) 12 (7.4–14.1) 11.8 (8.6–13.6)

Specific conductance (μS/cm at 25 °C) 181 (100–281) 221 (113–324)

Dissolved oxygen (mg/L) 7 (3.4–10.2) 7.9 (4.6–9.2)

pH (standard units) 5.2 (4.4–5.9) 5.1 (4.7–5.9)

Nutrients (mg/L)

Nitrate plus nitrite as nitrogen 12.4 (7.6–18.4) 14.1 (8.8–22.5)

Ammonia 0.07 (0.02–0.14) 0.06 (0.04–0.30)

Orthophosphate 0.009 (0.004–0.012) 0.008 (0.006–0.013)

Major ions or elements (mg/L)

Calcium 10.8 (5.1–20.6) 14.3 (6.9–25.3)

Magnesium 5.2 (3.8–9.2) 5.4 (2.5–9.9)

Sodium 2.3 (1.6–6.3) 4.8 (2.9–7.6)

Potassium 8.9 (2.5–23.4) 4.6 (1.9–12.2)

Bicarbonate 3.1 (0.9–41) 3.7 (0.9–14.5)

Chloride 7 (5–14) 13 (6–21)

Sulfate 16.9 (1.5–29) 11.2 (0.8–16.5)

Silica 11.2 (3.5–17) 10.7 (6.7–19.9)

Concentrations of silica below approximately 12 mg/L can 

be related to an apparent age of groundwater less than about 

10 years. The greatest influence of agricultural chemicals at 

the Bucks Branch study site is seen in younger water beneath 

the irrigated field. Silica has a strong negative correlation 

with specific conductance, nitrate, calcium, potassium, and 

chloride beneath the irrigated field, indicating less agricultural 

influence in older groundwater (table 4). Results from the 

dryland field are similar, although concentrations of potassium 

are higher in younger water beneath the dryland field than 

beneath the irrigated field (fig. 9B). Overall, differences 

in groundwater chemistry indicate a greater degree of 

agricultural influence in water quality beneath the irrigated 

field, particularly in younger water, than the dryland field. 

Concentrations of ammonia and orthophosphate were 

very low beneath both fields (table 3). Orthophosphate has a 

strong positive correlation with silica and a negative correlation with specific conductance; ammonia does not show 

a strong correlation with either compound (table 4). Both of 

these chemicals are not typically found at high concentrations 

in oxic groundwater (Denver and others, 2004). Phosphorus 

sorbs readily to particles in the soil and aquifer materials. 

Ammonia also is sorbed onto clay minerals and is less mobile 

than nitrate in groundwater. In addition, dissolved ammonia is 

readily converted to nitrate through the nitrification process in 

well-drained soils such as those found at this study site.

Transport of Water and Chemicals from Soil to 

Groundwater

Samples of water from the soil zone are useful for 

understanding nutrient transport to groundwater. Much of 

the nitrogen applied in ammonia and organic forms to welldrained sandy soils, such as those at the Bucks Branch study 

site, is converted to nitrate in the soil zone. Nitrate that is not 

used by plants is soluble and highly mobile, and can be carried 

by infiltrating water to the groundwater system. Although most 

phosphorus is sorbed in the soil zone, transport of dissolved 

phosphorus to groundwater, represented by orthophosphate, 

also can occur under anoxic conditions or when sorption sites 

on sediment particles are limited. 

Specific conductance and concentrations of nitrate 

varied seasonally and with depth over time in the soil zone 

in samples from lysimeters from both study fields (fig. 10). 

The highest values were measured during the growing season 

shortly after fertilizer was applied to the fields. Concentrations 
18 Monitoring Water Quality in the Bucks Branch Watershed, Sussex County, Delaware Specific conductance, in microsiemens per centimeter at 25 degrees Celsius Nitrate as N, in milligrams per liter Chloride, in milligrams per liter Calcium, in milligrams per liter Potassium, in milligrams per liter Sulfate, in milligrams per liter

Land use

Dryland

Irrigated

Silica, milligrams per liter

Outlier

Maximum

75th percentile

Median

25th percentile

Minimum

EXPLANATION

Dryland Irrigated B. Geoprobe groundwater

A. Land use

100

200

300

10

20

30

0

10

20

0

10

20

0

10

20

0

10

20

30

0 5 10 15 20

100

200

300

10

20

30

0

10

20

0

10

20

0

10

20

0

10

20

30

Figure 9. Summary of selected water-quality constituents from the temporary wells installed on the irrigated and dryland 

fields at the Bucks Branch study site: A, boxplots of selected constituents, and B, selected constituents compared to 

increasing age as indicated by increasing concentrations of silica.
The Water-Quality Response of Agricultural Conservation Practices at the Bucks Branch Study Site 19

Table 4. Multivariate correlations of groundwater samples from 

the dryland and irrigated fields, Bucks Branch study site, Sussex 

County, Delaware.

[0.99, highlighted values indicate correlation coefficient is considered 

significant; positive coefficients are shown in black and negative coefficients 

are shown in red; --, not considered]

Multivariate 

correlations

Correlation coefficient

Dryland Irrigated

Constituent Silica

Specific 

conductanceSilica

Specific 

conductance

Dissolved oxygen 0.34 -0.29 -0.41 0.00

pH 0.07 0.48 0.59 -0.56

Specific conductance -0.64 -- -0.65 --

Orthophosphate 0.99 -0.57 0.95 -0.57

Ammonia 0.4 0.16 -0.29 0.28

Nitrate -0.3 0.69 -0.66 0.86

Calcium -0.61 0.93 -0.65 0.9

Magnesium -0.24 0.74 -0.12 0.46

Sodium 0.65 0.07 0.05 0.31

Potassium -0.76 0.63 -0.56 0.51

Bicarbonate -0.35 0.64 0.8 -0.58

Chloride -0.11 0.24 -0.64 0.82

Sulfate -0.17 -0.23 -0.45 0.66

Silica -- -0.64 -- -0.65

declined over time and were relatively low in soil water collected during the winter and early spring. Overall, 30 samples 

were collected from the dryland field lysimeters and 55 samples were collected from the irrigated field lysimeters, not 

including sampling for the cover-crop project at the end of the 

2015 growing season. Sample recovery was greater beneath 

the irrigated field than the dryland field during the growing 

season, probably due to greater soil moisture and water movement as a result of irrigation (see fig. 6).

The highest nitrate concentrations in soil water beneath 

both fields were in samples from the 1-ft lysimeters (fig. 10B). 

Higher concentrations were measured in the deeper 3-ft 

lysimeters later in the growing season, with concentrations 

sometimes higher at 3 ft, than at 1 ft (figs. 10A and B). Once 

water reaches the 3-ft depth, it is essentially at the limit of 

the root zone and most nitrate in soil water is no longer taken 

up by plants and can be transported to groundwater. Nitrate 

concentrations in shallow groundwater (from wells less than 

or equal to 14 ft below land surface) were similar to those 

in the 3-ft lysimeters during times when both were sampled 

(fig. 10B). Concentrations of nitrate and specific conductance 

were noticeably higher in the irrigated field soil-water samples 

than in the dryland field samples (figs. 10A and B). 

Orthophosphate was frequently detected in soil-water 

samples but concentrations were much lower in shallow 

groundwater (fig. 10C). Phosphorus readily sorbs onto soil 

and sediment particles where soils are well-drained, as they 

are in the Bucks Branch study site fields. Orthophosphate was 

detected more frequently in samples from the dryland field 

than in samples from the irrigated field, especially at the 3-ft 

depth. Analysis of soils in these fields by UD found that soils 

beneath the dryland field had much higher concentrations 

of soil test phosphorus than soils from the irrigated field 

(Amy Shober, University of Delaware, written commun., 

2016). Proximity to former poultry houses and past manure 

application, prior to implementation of nutrient management 

plans, may have led to higher phosphorus in the dryland field 

water samples.

As water moves from the soil zone into groundwater, 

the range in concentrations of nitrate and orthophosphate is 

dampened (fig. 11). Some of this pattern is related to the difference in the sampling interval between lysimeters and wells. 

The permeable tip of the lysimeters is only 2 in. long, so the 

sample represents only a small interval of soil water whereas 

the well screens pull water from a 3-ft interval of the aquifer, 

which integrates a larger vertical interval with potentially 

variable water quality within the aquifer. Concentrations 

of nitrate, referenced to 10 mg/L as N on figure 11, are less 

variable as groundwater moves into the aquifer, compared to 

water samples from the soil zone. Orthophosphate concentrations, referenced to 0.2 mg/L, behave differently than nitrate 

as water moves from the soil to groundwater and are generally lower in groundwater than soil water because of the high 

degree of phosphorus sorption to soil and aquifer materials. As 

a result, concentrations of dissolved phosphorus in groundwater are typically well below the level of concern for surface 

water in Delaware (0.2 mg/L; Delaware Department of Natural Resources and Environmental Control, 2017), especially at 

deeper depths in the aquifer, making it unlikely that substantial 

phosphorus is contributed to surface water from upgradient 

groundwater sources (fig. 11B).

Specific conductance of soil water was strongly 

correlated with nitrate and other major ions associated with 

inputs of fertilizer and manure in soil-water samples from both 

study fields (table 5, fig. 12A). Nitrate, calcium, magnesium, 

and chloride had the strongest correlations (table 5). Trends 

in these chemicals were similar between the two study fields, 

although cation samples were analyzed less frequently because 

of variable amounts of water recovered, especially from 

samples collected during the growing season when plants are 

actively taking water from the soil zone (fig. 12A). Increasing 

concentrations of these chemicals with increasing specific 

conductance indicate a greater effect of agricultural chemicals 

on water quality. Differences in soil-water chemistry between 

the dryland and irrigated study fields are seen in the trends 

of orthophosphate, sulfate, and potassium. As mentioned 

previously, higher orthophosphate concentrations in the 

dryland samples than in the irrigated samples are probably 

related to previous manure application. It is not clear why 

trends in sulfate, sodium, and potassium are different between 

the two fields.
20 Monitoring Water Quality in the Bucks Branch Watershed, Sussex County, Delaware

A

B

C

Well depth 

1-foot lysimeter

3-foot lysimeter

Shallow wells

Dryland Irrigated

Land use

Date

04/01/201407/01/2014

10/01/2014

01/01/2015

04/01/2015

07/01/2015

10/01/2015

01/01/201604/01/2016

Specific conductance, 

in microsiemens per centimeter at

25 degrees Celsius

0

500

1,000

1,500

2,000

Nitrate as N, milligrams per liter

-50

0

50

100

150

200

Orthophosphate, milligrams per liter

0.0

0.5

1.0

1.5

04/01/2014

07/01/201410/01/201401/01/2015

04/01/2015

07/01/2015

10/01/201501/01/2016

04/01/2016

EXPLANATION

Figure 10. Samples of lysimeters and shallow groundwater showing trends over time in A, specific conductance, B, nitrate, and 

C, orthophosphate from the dryland and irrigated study fields, Bucks Branch study site, Sussex County, Delaware, 2014–15. [Gray bars 

indicate corn growing season.]
The Water-Quality Response of Agricultural Conservation Practices at the Bucks Branch Study Site 21

Land use

Dryland

Irrigated

Depth, in feet

0 10 20 30 40 50 60 70 80

Nitrate as N, milligrams per liter

0.1

0.2

0.3

0.4

0.5

0.6

0.7

1

2

3

4

5

6

7

10

20

30

40

50

60

80

100

200

Orthophosphate, milligrams per liter

0.003

0.004

0.005

0.006

0.008

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.1

0.2

0.3

0.4

0.5

0.6

0.7

1

EXPLANATION

10 milligrams per liter as N

0.2 milligrams per liter

A

B

Figure 11. Comparison of concentrations of A, nitrate and B, orthophosphate to depth below land surface in lysimeters and wells 

from the Bucks Branch study site, Sussex County, Delaware.
22 Monitoring Water Quality in the Bucks Branch Watershed, Sussex County, Delaware

Table 5. Correlation between specific conductance, and 

nutrients and major ions in samples from lysimeters, Bucks 

Branch study site, Sussex County, Delaware.

[Correlation value of 0 indicates no relation, value of 1 (black) or -1 (red) 

indicates strong relation; -, negative; bold indicates strong correlation; black 

indicates positive correlation, red indicates negative correlation]

Parameter Dryland Irrigated

Correlation value

pH 0.01 -0.45

Phosphate as phosphorus 0.23 -0.18

Nitrate and nitrite as nitrogen 0.97 0.95

Ammonia 0.19 0.19

Calcium 0.97 0.96

Magnesium 0.97 0.97

Sodium 0.81 0.72

Potassium 0.55 0.51

Chloride 0.9 0.86

Sulfate 0.63 0.42

Transport of Nutrients through Groundwater to 

the Stream

Factors affecting the transport of nutrients from groundwater to streams include the lag time between nutrients reaching the water table and their transport through groundwater, 

and also the potential for retention or loss in the soil zone or 

along flowpaths. The main processes that affect nutrient transport are the formation and loss of nitrate through nitrification 

and denitrification, and sorption or desorption of phosphorus 

onto sediment particles under oxic conditions (when dissolved 

oxygen is present) and anoxic conditions (when dissolved 

oxygen is absent). 

Under oxic conditions, typical in the well-drained soils 

over much of the study site, nitrate is readily formed in the 

soil zone from ammonium that has been applied in commercial fertilizer formulations or after mineralization of organic 

nitrogen in manure. As long as dissolved oxygen is present, 

nitrate can move with water through the aquifer to the stream. 

If nitrate in soil or groundwater encounters anoxic conditions, 

nitrate can be lost from water through denitrification. This 

process often occurs in near-stream areas with poorly drained 

soils, such as riparian wetlands. In contrast, phosphorus is 

strongly sorbed onto soil particles in oxic environments and 

not readily transported through the soil zone to groundwater. 

As a result, there is very little transport of dissolved phosphorus in groundwater to streams, and most transport is overland. 

Once in streams, this sorbed phosphorus may be desorbed 

in streambed sediments, where anoxic conditions develop as 

vegetation degrades.

Concentrations of nitrate are widely variable but nitrate 

is present at all depths of the unconfined groundwater system 

beneath the irrigated and dryland fields (fig. 13, table 6). The 

maximum apparent age of groundwater measured in wells 

on the downgradient end of the flowpaths was from 33 to 

36 years, which is well within the date range of higher nutrient 

application rates to agricultural land in this area (fig. 4; Ator 

and Denver, 2015). In most water samples with estimated 

apparent ages of less than 10 years, concentrations of nitrate 

were higher in shallow younger water than deeper older water, 

and higher beneath the irrigated field than beneath the dryland 

field (figs. 12B and 14). Irrigation was reported to have begun 

15 years prior to the study period. The highest concentrations 

of nitrate measured in groundwater were from wells IR-3S 

and IR-6S (table 6). These wells also had samples with the 

greatest range in concentrations, from less than 10 to over 

40 mg/L as N. These patterns are probably related to a shallow 

water table and resulting fast recharge to groundwater near 

these wells, and differences in the chemistry of infiltrating 

groundwater over time.

Isotopes of Nitrate and Denitrification in Groundwater

The isotopic signature of nitrate can be affected by the 

mixing of water from different sources that have different 

isotopic signatures (Böhlke, 2002). Manure has been applied 

to the study fields along with commercial fertilizer for many 

years. Isotopes of nitrogen in nitrate generally have lower per 

mil values in commercial fertilizer and higher per mil values 

in manure (fig. 15). Most of the water samples have delta15N 

values between 3 and 9 per mil, indicating a mixture of nitrogen from fertilizer and manure (fig. 15). The exceptions to this 

pattern are in samples from three wells in the dryland field 

(DR-1M, DR-1D, and DR-3D) that have values of delta15N 

that resemble the values from fertilizer. Groundwater from 

these wells was likely recharged beneath the adjacent upgradient field that has not received manure as there is a discontinuous clay layer above well DR-1M that limits downward 

infiltration to the water table at that location (fig. 13A). Water 

in the deepest wells in the dryland flowpath site, DR-1DD and 

DR-3DD, also is likely to have originated in a recharge area 

upgradient of the study field (fig. 15).

Although most of the nitrogen isotopes of nitrate are 

similar in range between the dryland and irrigated fields, the 

oxygen isotopes of nitrate, delta18O, are typically lower (less 

than 0 per mil) in water from the dryland field than from the 

irrigated field (fig. 15). Water samples from the irrigated field 

likely exhibit characteristics indicating mixing of water with 

a similar isotopic signature in nitrate as the dryland field, and 

nitrate from irrigation water that has a heavier delta18O, but a 

similar range of delta15N (fig. 15).

The presence of dissolved oxygen in all groundwater 

samples from the Bucks Branch study site indicates that 

denitrification is not a major process affecting nitrate 

concentrations in most groundwater samples (table 6). 

Denitrification typically occurs in poorly drained, organicrich soils or in older water where dissolved oxygen has been 

depleted. When denitrification occurs, the lighter isotopes 
The Water-Quality Response of Agricultural Conservation Practices at the Bucks Branch Study Site 23

0 100 200 300 400 500 600 700

4.0

4.5

5.0

5.5

6.0

6.5

0

10

20

30

40

-0.2

0.0

0.2

0.4

0.6

-10

0

10

20

30

40

0

10

20

30

40

50

0

5

10

15

-5

0

5

10

15

20

0

5

10

15

-10

0

10

20

30

40

7.0

EXPLANATION

Specific conductance, in microsiemens per centimeter at 25 degrees Celsius

0 500 1,000 1,500 2,000 2,500

pH

Nitrate as N,

in milligrams

per liter

Orthophosphate,

in milligrams

per liter

Sulfate,

in milligrams

per liter

Calcium,

in milligrams

per liter

Magnesium,

in milligrams

per liter

Sodium,

in milligrams

per liter

Chloride,

in milligrams

per liter

Potassium,

in milligrams

per liter

Land use

Dryland

Irrigated

A. Lysimeter samples B. Groundwater samples

4.0

4.5

5.5

6.0

6.5

-50

0

50

100

150

200

-0.5

0.0

0.5

1.0

1.5

-50

0

50

100

150

200

-20

0

20

40

60

80

-5

0

10

15

20

0

5

15

20

25

-5

10

25

0

50

100

150

Figure 12. The relation between specific conductance and pH, nitrate, orthophosphate, sulfate, calcium, magnesium, potassium, sodium, 

and chloride in A, lysimeter samples, and B, groundwater samples from the dryland and irrigated fields, Bucks Branch study site, Sussex 

County, Delaware.
24 Monitoring Water Quality in the Bucks Branch Watershed, Sussex County, Delaware

Surficial Aquifer

Elevation, in feet

Distance, in feet

Confining Unit

Stream

Irrigation well Observation well

B. Bucks Branch Irrigated Flowpath

1

20

40

0

-20

-40

-60

0 500 1,000 1,500

6 2 7

8 3

Water Table 3/17-19/2015 4

Water Table 11/25/2014

NORTH SOUTH

B B'

Farm Field Wetland

Lysimeters

?

19.5

12.7

16.7

23.8

15.6

16

25 10.5

25.9

11.9

4

13.1

7.7

8.8

15.4

17.8

Discontinuous Clay Layer ?

Surficial Aquifer

Elevation, in feet

Distance, in feet

Confining Unit

A. Bucks Branch Dryland Flowpath

Stream

20

40

0

-20

-40

-60

0 500 1,000

NORTH SOUTH

1 5 2 4

6 3 7 Water Table

3/17-19/2015

Water Table 11/25/2014

A A'

Farm Field

Lysimeters

Wetland Field

? ?

5.2

9.9

11.7

19

9

10.3

22.7

3.2

5.1

8.7

7.4 10

14.2

Discontinuous Clay Layer

DR-1M

S

D

DD

M

1

Indicates location of single wells

and well clusters—Number on top

corresponds to location on figure 2.

Screened intervals of wells indicated

by short black bars in column. 

Individual wells are identified by

their relative depths as “S” for

shallow, “M” for median, “D” for 

deep, and “DD” for the deepest

well in a well cluster.

Well labels in text and tables

identified by “DR”for dryland, and 

“IR” for irrigated. 

EXPLANATION

Number indicates median nitrate

concentration in groundwater—

bold indicates partial denitrification of nitrate. 

11.7, 22.7

Figure 13. Median concentrations of nitrate in groundwater beneath A, the dryland and B, the irrigated fields of the Bucks 

Branch study site, Sussex County, Delaware.
The Water-Quality Response of Agricultural Conservation Practices at the Bucks Branch Study Site 25

Table 6. Summary statistics for samples from flowpath wells, instream piezometers, and the intensive stream site (SW2), Bucks 

Branch study site, Sussex County, Delaware.

[n, number of samples; ft, feet; SC, specific conductance; μS/cm, microsiemens per centimeter at 25 degrees Celsius; °C, degrees Celsius; DO, dissolved 

oxygen; mg/L, milligrams per liter; NO3+NO2-N, Nitrate plus nitrite as nitrogen; SiO2, silica; bold, mean; min, minimum; max, maximum; DR, Dryland field; 

IR, irrigated field; -- no data. Sampling sites shown on figure 3.]

Site n Depth

(ft)

SC (μS/cm, at 25°C) DO (mg/L) NO3+NO2-N (mg/L) SiO2 (mg/L)

Mean (min–max) Mean (min–max) Mean (min–max) Mean (min–max)

Dryland wells

DR-1M 6 24 126 (99–195) 7.1 (5.7–8.1) 5.2 (1–7.5) 17.2 (5.8–20.2)

DR-2M 4 24 168 (165–170) 8.5 (6.4–10.1) 9 (8.8–9.2) 12 (11.6–12.6)

DR-1D 2 39 165 (161–169) 8.9 (8.7–9.2) 11.7 (10.1–13.3) 17.2 (16.6–17.7)

DR-1DD 2 69.5 227 (226–227) 6.4 (6.3–6.5) 19 (18.6–19.4) 17 (16.9–17)

DR-2S 6 14 242 (170–374) 9.4 (7.6–10.5) 9.9 (5.9–22.1) 7.9 (6.3–9.2)

DR-3M 4 19 168 (164–171) 5.8 (5.1–6.7) 10.3 (9.7–11.4) 10.1 (9.4–11.1)

DR-3D 2 38 259 (239–278) 4.2 (4–4.5) 22.7 (20.2–25.2) 17.1 (16.9–17.2)

DR-3DD 2 74 114 (109–118) 2.2 (2.1–2.2) 5.1 (4.8–5.5) 26.6 (26.5–26.7)

DR-3S 6 9 207 (187–277) 6.9 (5.2–7.8) 13.2 (8.7–17.1) 8.8 (6.8–9.9)

DR-4S 4 14 195 (127–378) 9.4 (7.8–11.2) 8.7 (2.3–25.9) 5.7 (5–6.1)

DR-5S 4 14 168 (165–172) 9.4 (7.8–10.4) 7.4 (6.8–7.9) 12.5 (11.4–13.4)

DR-6S 5 14 210 (201–225) 7.9 (5.5–9.2) 10 (7.6–11.6) 7.7 (4.8–9.3)

DR-7S 4 9 235 (178–370) 7.7 (6.7–9.2) 14.2 (11–21.2) 7.3 (6.2–8.3)

Irrigated wells

IR-1S 4 14 296 (276–309) 8.8 (7.6–10) 19.5 (16.1–22.2) 8.2 (7.7–9.1)

IR-1M 7 24 179 (169–223) 8.2 (5.6–9) 12.7 (10.7–14.2) 17.3 (9.6–20.5)

IR-1D 2 40 205 (172–238) 8.5 (8.5–8.6) 16.7 (12–21.3) 19.7 (19.2–20.2)

IR-2S 6 11 239 (209–284) 8 (7–9.7) 15.6 (11.8–20.8) 8.2 (4.8–19.5)

IR-2M 3 21 229 (196–276) 8 (7.7–8.3) 16 (11.7–22.3) 7.4 (6.2–8.2)

IR-3S 5 9 451 (239–630) 5.2 (3.7–8) 25.9 (9.1–39.7) 7 (3.8–9.4)

IR-3M 4 19 152 (147–157) 6.4 (6.2–6.5) 11.9 (10.8–12.4) 16.4 (15.1–19.7)

IR-3D 2 40 92 (91–93) 1.9 (1.8–1.9) 4 (3.7–4.4) 22.3 (22–22.5)

IR-3DD 2 74 183 (182–184) 7.1 (7–7.3) 13.1 (12.1–14.1) 19.5 (19.5–19.5)

IR-6S 5 11 320 (175–607) 9.7 (8.7–10.7) 23.8 (7.7–45.6) 3.9 (2.6–6.4)

IR-7S 4 9 351 (294–400) 7.8 (6.7–8.5) 25 (19.9–32.4) 9.8 (7.5–11.6)

IR-8S 4 9 219 (172–258) 9.3 (8.1–10.3) 10.5 (6–15.2) 5.1 (3.4–6.1)

Streambank wells

IR-4S 4 7 130 (76–155) 3.5 (1.6–5.6) 7.7 (1.8–12) 16.5 (10.7–18.9)

IR-4M 5 17 141 (130–151) 3.9 (3.1–5) 8.8 (7.4–9.7) 21.4 (19.6–22.3)

IR-4D 3 38 269 (230–301) 3.8 (2.6–4.7) 15.4 (11.3–18.6) 22.6 (22.2–23)

IR-4DD 3 70 228 (221–233) 6.1 (5.6–6.4) 17.8 (16.9–19.3) 21.1 (20.8–21.3)

Instream piezometers

Piezometers 4 1 199 (186–209) 7.1 (5.7–8.1) 15.5 (13.2–17) 20 (19.1–20.7)

Streamgage

SW2 48 -- 177 (99–211) 9.8 (5.6–13.8) 9 (3.8–14.7) 19.9 (9.4–25.8)
26 Monitoring Water Quality in the Bucks Branch Watershed, Sussex County, Delaware

Estimated apparent age

<10 years >10 Years 0

10

20

30

40

50

Nitrate as nitrogen, in milligrams per liter

Land use

Dryland

Irrigated

Outlier

Maximum

75th percentile

Median

25th percentile

Minimum

EXPLANATION

Figure 14. Concentration of nitrate as nitrogen in relation to the 

estimated apparent age of groundwater under the irrigated and 

dryland fields at the Bucks Branch study site, Sussex County, 

Delaware.

Groundwater

Land use

Dryland

Irrigated

Streambed

delta18O, per mil

-2.5 0.0 2.5 5.0 7.5 10.0

delta15N, per mil

0

5

10

15

EXPLANATION

Irrigation test well

1DD

3DD

4D

4D 3S 3S

4S

4S

3S

4S

1M

1D

1M

1M 1M

1M

3D

Manure, septic Fertilizer and

natural soil

denitrification

Figure 15. Ratios of the nitrogen (delta15N) and oxygen (delta18O) 

isotopes of nitrate in groundwater from the Bucks Branch study 

site, Sussex County, Delaware. [Well numbers shown represent 

water samples that exhibit characteristics of denitrification. 

Specific well locations shown on figure 3.]

of nitrogen (delta14N) and oxygen (delta17N) in nitrate are 

preferentially removed by bacterial reactions enriching 

water in the heavier isotopes, delta15N and delta18O, in the 

remaining nitrate (Böhlke, 2002). A trend of increasing 

delta15N and delta18O away from the main clusters of samples 

from the irrigated and dryland fields is indicative of partial 

denitrification in some water samples (fig. 15). Although some 

degree of denitrification is indicated in samples from certain 

wells, nitrate is present in all samples and the water was not 

totally denitrified, in which case no dissolved oxygen or nitrate 

would be present (fig. 13). 

Dissolved gases (including nitrogen, argon, oxygen, 

carbon dioxide, and methane) that were measured during age 

dating indicate that water samples from several wells near the 

downgradient edge of both fields and on the streambank show 

chemical evidence of denitrification in the form of excess 

nitrogen gas (IR-4D, DR-1DD, DR-3D, DR-3DD; fig. 13). 

The remainder of the wells sampled for age dating did not 

show evidence of denitrification in the dissolved-gas analyses. 

Denitrification also is indicated by the ratios of delta15N 

and delta18O in some shallow wells that did not have dissolved-gas analyses, most notably IR-4S and IR-3S, which are 

in a poorly drained part of the field near the riparian wetland 

(fig. 13). These wells were sampled multiple times over the 

study period. Water samples from these wells had similar isotopic ratios, but different isotopic values and a wide range in 

concentrations of nitrate in different samples (fig. 15, table 6). 

Conditions promoting denitrification are greater during periods 

of high water table when soil is saturated than when the water 

table is low and below the shallow wetland sediments, leading 

to varying degrees of nitrate loss over time. These wells and 

the other wells with water chemistry indicative of denitrification had dissolved oxygen concentrations that were generally 

lower (below 5 mg/L) in some samples than those measured in 

the other wells at these sites (table 6).

A transect of four piezometers installed in the streambed 

near the irrigated field had similar isotopic ratios in nitrate as 

upgradient oxic groundwater (fig. 15). Dissolved oxygen was 

greater than 5 mg/L and nitrate ranged from 15–17 mg/L in the 

water samples, indicating nitrate in water discharging through 

the streambed was relatively unaltered by denitrification at this 

location (table 6). The apparent age of the instream piezometer 

samples was about 25 years, based on concentrations of dissolved silica (see fig. 7). This age is somewhat older than the 

estimated average age for surface water of 19 years in Bucks 

Branch at its downstream outlet (SW1, fig. 1), but surface 

water is a mixture of groundwater from flowpaths with many 

different ages (Clune and Denver, 2012). 

Surface-Water Flow and Chemistry

The intensive surface-water site, where most stream data 

were collected for this project, was located on Bucks Branch 

at site SW2, upstream from the main watershed outlet at site 

SW1 (fig. 1). The watershed above SW2 drains about 3 mi2

 in 
The Water-Quality Response of Agricultural Conservation Practices at the Bucks Branch Study Site 27

the headwaters of the Bucks Branch watershed. Nitrate data 

from this site also were compared to nitrate data from samples 

collected at multiple sites around the watershed during spring 

base flow, and samples collected at the stream outlet collected 

by DNREC at SW1 (fig. 1). Base flow represents flow in the 

stream that has not been affected by runoff from recent storms.

Streamflow

Flow measured at the streamgage during base flow at 

Bucks Branch ranged from a high of about 10 cubic feet per 

second (ft3

/s) during the winter and early spring to about 

0.50 ft3/s in the late summer; the highest stormflow during 

the study period was about 34 ft3

/s (fig. 16). Some stormflow 

discharges during periods of low base flow in the summer 

were actually lower than base-flow discharge during the colder 

months. Flow measurements could not be made at some times 

during the summer when flow was very low and impeded by 

rooted aquatic vegetation.

Direct runoff is relatively limited in this stream because 

of the flat topography of the surrounding uplands and the 

low topographic gradient, as well as the extensive riparian 

wetlands on both sides of the stream and an earthen ridge 

on one side that was created during channelization (fig. 3). 

Although runoff does affect streamflow in larger storms, about 

93 percent of the flow in Bucks Branch at SW2 is estimated 

to be from groundwater discharge based on an estimate of 

base flow using the methods developed by Eckhardt (2005) 

(Jeff Raffensperger, USGS, written commun., 2017). Groundwater discharge from the entire thickness of the surficial 

aquifer provides flow to Bucks Branch. Inflowing groundwater 

was indicated by a strong upward hydraulic gradient in wells 

adjacent to Bucks Branch (fig. 4) and by instream piezometer measurements of hydraulic heads higher than the stream 

surface (fig. 3).

05/2014 09/2014 01/2015 05/2015 09/2015 01/2016 05/2016

0

5

10

15

20

25

30

35

Bucks Branch discharge, in cubic feet per second

Date

Figure 16. Streamflow at the gaged site SW2 during the study 

period, Bucks Branch study site, Sussex County, Delaware.

Water Quality

Nutrient concentrations in Bucks Branch result in large 

growths of aquatic plants and algae in the stream. The median 

concentration for total phosphorus, 0.025 mg/L, measured 

during the study period at SW2, was lower than the State of 

Delaware target thresholds for phosphorus in fresh waters 

of 0.2 mg/L (Delaware Department of Natural Resources 

and Environmental Control, 2017). Total phosphorus 

concentrations exceeded the threshold in some high flows 

but were lower than the standard in about 90 percent of the 

samples collected for this study (fig. 17A). During base flow, 

when dissolved phosphorus is the major phosphorus species, 

the threshold was not exceeded. The median concentrations 

of total nitrogen (9.0 mg/L) at SW2 during the study period 

were significantly above the State of Delaware target 

threshold for nitrogen in fresh surface waters of 3.0 mg/L, 

and among the highest measured in streams by DNREC 

(fig. 17B). The majority of this nitrogen is nitrate discharging 

from groundwater. 

Seasonal Trends in Surface-Water Quality

Seasonal trends in water quality were seen in both continuous and discrete samples at SW2. These trends are related to 

changes in the relative proportions of groundwater and runoff, 

in-stream biological activity, and the water-table elevation that 

affects the interception and denitrification of groundwater in 

near-stream areas.

Results from Continuous Monitoring

Specific conductance generally decreases with increasing 

flow as runoff dilutes concentrations of ions in inflowing 

groundwater (figs. 16 and 18A). During some storms that 

occur during the beginning of the growing season, higher 

concentrations of particulate nitrogen and phosphorus and 

suspended sediment appear to cause an initial increase in 

specific conductance, followed by a decrease in specific 

conductance, as is typically seen in other storms throughout 

the year (fig. 18A). An increase in the potential for sediment 

transport when soils are being prepared for planting is the 

likely cause of this pattern. 

Dissolved oxygen concentrations in surface water were 

consistently above State of Delaware standards of a minimum 

daily average of 5.5 mg/L, or an instantaneous concentration 

of 4 mg/L in both continuous and discrete samples (fig. 18B; 

table 6) (Delaware Department of Natural Resources and 

Environmental Control, 2017). The continuous inflow of oxic 

groundwater helps to maintain dissolved oxygen concentrations in Bucks Branch in spite of the production and degradation of aquatic vegetation. Dissolved oxygen has daily diurnal 

variation throughout the year related to stream-community 

respiration. As temperature and light increased in the spring, 

this daily variation exceeded 8 mg/L (fig. 18B). The largest 

variations occurred coincident with blooms of floating aquatic 

plants such as duckweed and filamentous algae. Rooted 
28 Monitoring Water Quality in the Bucks Branch Watershed, Sussex County, Delaware

0.001

0.002

0.003

0.005

0.01

0.02

0.03

0.05

0.1

0.2

0.3

0.5

0.8

0.001

0.002

0.003

0.004

0.006

0.01

0.02

0.03

0.04

0.06

0.1

0

5

10

15

0.0

5.0

10.0

15.0

Total phosphorus,

in milligrams per liter as phosphorus

Total nitrogen,

in milligrams per liter

Orthophosphate,

in milligrams per liter as phosphorus

Nitrate as N,

in milligrams per liter

Outlier

Maximum

75th percentile

Median

25th percentile

Minimum

EXPLANATION

0.2 milligrams per liter

3.0 milligrams per liter

A B

Figure 17. Concentrations of A, phosphorus and B, nitrogen in surface water, site SW2, Bucks Branch study site, Sussex 

County, Delaware.

vegetation was observed to dominate instream plants over the 

remainder of the year, and the diurnal fluctuation was lower. 

Water temperature and pH also followed seasonal patterns (figs. 18C and D). Water temperature ranged from near 

freezing to almost 25 °C during the period with continuous 

monitoring. Although water temperature was closely related to 

air temperature, because of continuous groundwater discharge 

with a mean temperature of about 15 °C, stream water rarely 

froze. The pH of water ranged between 6 and 7 and fluctuated 

daily. The largest fluctuations were during the time of greatest instream respiration and diurnal fluctuations in dissolved 

oxygen. Otherwise, pH increased with temperature during the 

summer months.

Monthly and Storm Data

The major-ion chemistry of surface water in Bucks 

Branch closely resembles that of groundwater, as would be 

expected in a stream with over 90 percent of its flow from 

groundwater discharge. Specific conductance, pH, and the 

major elements contributed by groundwater in predominantly 

agricultural areas, including nitrate, calcium, and magnesium 

are higher during base flow than stormflow, and highest when 

base flow is lowest and there is little discharge from nearstream wetlands (fig. 19). Concentrations of silica, which is 

derived from the dissolution of silicate minerals in soil and 

aquifer sediments, also are inversely related to flow (fig. 19). 
The Water-Quality Response of Agricultural Conservation Practices at the Bucks Branch Study Site 29 27.525 22.520 17.515 12.510 7.5

5

2.5

0

-2.5

C. pH

Mar. Apr. May June July Aug. Sep. Oct. Nov. Dec. Jan. Feb. Mar. Apr. May June July Aug.

2014 2015

7.2

7

6.86.66.46.2

6

5.85.6

1816141210

8642

Dissolved oxygen, in milligrams per liter

Mar. Apr. May June July Aug. Sep. Oct. Nov. Dec. Jan. Feb. Mar. Apr. May June July Aug.

2014 2015

Mar. Apr. May June July Aug. Sep. Oct. Nov. Dec. Jan. Feb. Mar. Apr. May June July Aug.

2014 2015

400350300250200150100

50

0 Mar. Apr. May June July Aug. Sep. Oct. Nov. Dec. Jan. Feb. Mar. Apr. May June July Aug.

2014 2015

Specific conductance, in microsiemens per centimeter

at 25 degrees Celsius

A. Specific conductance B. Dissolved oxygen

D. Temperature

Degrees Celsius

Figure 18. Continuous monitoring data at site SW2 for 

A, specific conductance, 

B, dissolved oxygen, 

C, pH, and 

D, temperature, site SW2, Bucks Branch study site, Sussex 

County, Delaware, February 2014 through September 2015. [Data from USGS National Water Information System (2017). Gaps in data occurred when instruments were removed from the stream during very low flow.]
30 Monitoring Water Quality in the Bucks Branch Watershed, Sussex County, Delaware

Base flow

Stormflow

0

15

30

0

10

20

75

150

225

0.0

5.0

10.0

0.01

0.1

5

10

15

0.0

2.0

4.0

2

4

6

2

6

10

Base flow Stormflow

Discharge,

in cubic feet

per second

Silica,

in milligrams

per liter

Specific

conductance,

in

microsiemens

per centimeter

at 25 degrees

 Celsius

Nitrate as N,

in milligrams

per liter

Orthophosphate,

in milligrams

per liter

as

phosphorus

Calcium,

in milligrams

per liter

Magnesium,

in milligrams

per liter

Potassium,

in milligrams

per liter

Sulfate,

in milligrams

per liter

12.5

20

01/01/2014

05/01/2014

09/01/2014

01/01/2015

05/01/2015

09/01/2015

01/01/2016

05/01/2016

Date

Outlier

Maximum

75th percentile

Median

25th percentile

Minimum

EXPLANATION

Figure 19. Concentrations of selected constituents over time in base flow and stormflow, site SW2, Bucks Branch study site, Sussex 

County, Delaware.
The Water-Quality Response of Agricultural Conservation Practices at the Bucks Branch Study Site 31

In addition, silica was highest during the lowest base flows in 

the late summer, indicating a higher proportion of older water 

entering the stream from agricultural uplands when the water 

table was low and shallow near-stream wetland sediments 

were dry. Specific conductance, nitrate, calcium, and magnesium were also highest when base flow was lowest. When 

base flow was higher in the winter and spring, the near-stream 

wetland contribution to streamflow increased and diluted the 

effects of agricultural chemicals. Sulfate concentrations were 

higher when base flow was higher, corresponding to periods 

when groundwater was discharging from the riparian wetlands (fig. 19).

Concentrations of nitrate in stream water at the same 

flow rate are lower during the spring and summer months 

when plant uptake and respiration are greatest than in the 

colder fall and winter months (fig. 20). Diurnal samples for 

nitrate in surface water collected for 2 days when fluctuations 

of dissolved oxygen were greatest in April 2015 showed a 

decrease of about 1 mg/L as N from the dissolved oxygen low 

prior to sunrise, and the high at about noon at site SW2. Total 

phosphorus is sometimes higher during stormflow, although 

the low potential for overland transport at this site limits 

phosphorus movement. Potassium, which is associated with 

agricultural inputs, also increases during stormflow in Bucks 

Branch (fig. 19). This increase is likely related to the release 

and resuspension of potassium, which is a major component of 

plants, from degradation of instream plant material (Brady and 

Weil, 2008), or potassium carried in runoff from local fields. 

The loss of nitrate through denitrifcation is indicated by 

an increasing trend in the ratio of nitrate and oxygen isotopes 

of nitrate in surface water (fig. 21A). Surface-water samples 

Season

Fall_Winter

Spring_Summer

Discharge, cubic feet per second

Nitrate as N, in milligrams per liter

EXPLANATION

0.0 2.5 5.0 7.5 10.0 12.5

2

4

6

8

10

12

14

16

Figure 20. Differences in the concentrations of nitrate in base 

flow at similar flow rates during the fall through winter and spring 

through summer, site SW2, Bucks Branch study site, Sussex 

County, Delaware.

had a relatively consistent ratio in the nitrogen and oxygen isotopes of nitrate; some samples in the spring and summer were 

more enriched (higher per mil values) and some samples from 

the fall and winter showed lower enrichment (fig. 21A). If the 

sources of nitrate to surface water remain the same, which is 

assumed in Bucks Branch, samples with higher per mil values 

indicate a greater degree of denitrification. The potential for 

greater denitrification is supported by a decrease in nitrate 

concentration with increasing isotope values in the spring and 

summer (fig. 21B). Nitrate concentration is diluted during 

stormflow compared to base flow in the fall and winter. Nitrate 

concentrations are highest in the stream during fall and winter 

base flow (figs. 20 and 21).

Effect of Groundwater on Surface Water in the 

Bucks Branch Watershed

Agricultural land, primarily cultivated land, but also 

land including concentrated animal feeding operations, covers 

about 67 percent of the land surface in the watershed above 

site SW2 and, as such, is the major source of nitrate to the 

stream (fig. 1). Most of this nitrate comes from discharging 

groundwater. Other land uses, including combined forest 

and wetland (about 29 percent), and residential land (about 

5 percent) also contribute groundwater to the stream and 

affect overall stream-water quality. Nitrate can be lost from 

discharging groundwater that flows through poorly drained 

sediments with anoxic conditions in wetlands associated with 

the forested riparian zone. 

Estimated Nitrate Loss to Bucks Branch Through 

Denitrification

The total amount of nitrate potentially contributed to 

the stream from groundwater (totalN) and the loss of nitrate 

through denitrification in near-stream areas (Nloss) was estimated during periods of higher base flow using a simple mixing model. The model estimates potential nitrate loadings from 

different proportions of three major land uses in the watershed, 

and is compared to the median instream nitrate concentrations 

of 9 mg/L as N to determine the potential loss of nitrate from 

discharging groundwater (table 7; eq. 1). 

The part of the watershed covered by each land-use group 

was delineated based on the National Land Cover (NLCD) 

2011 dataset (Homer and others, 2015). Concentrations of 

nitrate discharging from agricultural areas were estimated 

based on the range of concentrations measured for this study 

in the instream piezometers (15–17 mg/L as N). A constant 

concentration of 0.4 mg/L as N was applied to the forested/

wetland area based on previous research in natural parts 

of the local area (Hamilton and others, 1993). A constant 

concentration of 2.7 m/L as N was applied to the residential 

area, based on results of recent sampling in a residential 

area of Sussex County, Delaware (Kasper, 2014). Nitrate 

concentrations for each group were multiplied by the 

proportion of each land use and added together to estimate the 
32 Monitoring Water Quality in the Bucks Branch Watershed, Sussex County, Delaware

Fall_Winter

Spring_Summer

Routine sample

Stormflow

delta18O, per mil

3.0 3.5 4.0 4.5 5.0 5.5

delta15N, per mil

5

6

7

8

9

10

11

Nitrate as N, in milligrams per liter

2.5 5.0 7.5 10.0 12.5

EXPLANATION

A B

Figure 21. Trends in A, the ratio of the nitrogen (delta15N) and oxygen (delta18O) isotopes of nitrate, and B, delta15N and nitrate in 

water by season and flow conditions, site SW2, Bucks Branch study site, Sussex County, Delaware.
The Water-Quality Response of Agricultural Conservation Practices at the Bucks Branch Study Site 33

Table 7. Mixing model input data and model output used for estimating the loss of nitrate from groundwater discharging to surface 

water at site SW2, Bucks Branch, Sussex County Delaware.

[%, percent; mg/L as N; milligrams per liter as nitrogen]

Model input data:

Land-use class Units C1, Wetland + Forest C2, Agricultural C3, Residential C4, Surface water

Area square miles 0.935 2.178 0.161 3.273

Land-Use Proportion 0.290 0.660 0.050 1

Land-Use Percent of Area % 28.566 66.527 4.907 100

Nitrate Minimum mg/L as N 15

Nitrate Median mg/L as N 0.4 16 2.7

Nitrate Maximum mg/L as N 17

Nitrate Median Measured mg/L as N 9

Model output:

Units Minimum Median Maximum

totalN: Groundwater nitrate input mg/L as N 10.2 10.9 11.6

Nloss: Loss of nitrate from groundwater mg/L as N 1.2 1.9 2.6

Percent loss of nitrate % 12 17 22

potential total nitrate concentration in inflowing groundwater 

throughout the watershed prior to the potential loss in nearstream anoxic discharge areas (eq. 1). The modeled total 

estimated concentrations of nitrate in discharging groundwater 

ranged from 10.2 to 11.6 mg/L as N (table 7). The percent 

loss of nitrate was estimated by subtracting the median stream 

nitrate concentration of 9 mg/L as N from the total estimated 

nitrate concentrations from the three models with differences 

in agricultural nitrate inputs in discharging groundwater 

(eq. 1). The percent difference between the loss of nitrate 

and total estimated nitrate concentrations in discharging 

groundwater was then calculated (eq. 2; table 7). 

Nloss = {[(C1 * median NO3

-N of C1) +

(C2 * median, maximum, or minimum NO3

-N of C2) +

(C3 * median NO3

-N of C3)] – C4} (1)

Percent loss of nitrate = (Nloss/totalN)* 100 (2)

where

Nloss is the estimated loss of nitrate (NO3-N) in 

groundwater discharging to surface water;

totalN is modeled NO3-N input from groundwater to 

surface water;

C1 is percent of wetland and forest land;

C2 is the percent of agricultural land;

C3 is the percent of residential land; and

C4 is the NO3-N measured in surface water.

The estimated total loss of nitrate ranged between 

12 and 22 percent of potential groundwater inputs (table 7). 

This loss probably occurred primarily in shallow parts of the 

aquifer near the stream, and affects younger water flowing 

along shallow flowpaths, although some loss could occur in 

deeper parts of the flow system where denitrification also has 

been observed (figs. 4 and 13). When the water table is low 

and wetland sediments are dry, more water that has not been 

affected by loss through denitrification in shallow near-stream 

sediments can potentially flow into the stream, as indicated 

by the higher nitrate concentrations during lower base flows 

shown in figure 19. 

Comparison of the Intensive Surface-Water Site 

to Watershedwide Surface-Water Quality in 

Bucks Branch

Ten sites were sampled synoptically three times during 

high base flow in the spring to examine the distribution of 

nitrate in surface water around the Bucks Branch watershed 

(fig. 1). The watershed has two main branches that provide 

over two-thirds of the flow to the most downstream site at 

SW1: Bucks Branch at SW2, which was measured intensively 

for this study, and Freidel Prong (site SW3) (fig. 22). Most 

of the other sites sampled around the watershed are upstream 

of sites SW2 or SW3, with the exception of SW10, Gilbert 

Trivitts Ditch in the eastern part of the watershed (fig. 1). 

Flow at SW3 is much less than at SW2 in the main stem of the 

watershed, but concentrations of nitrate were higher, probably because this tributary has less riparian buffer than SW2 

(fig. 22). The tributary at SW10 is a relatively shallow ditch 

with low flow and has little influence on stream quality.
34 Monitoring Water Quality in the Bucks Branch Watershed, Sussex County, Delaware

ID

SW1

SW2

SW3

SW4

SW5

SW6

SW7

SW8

SW9

SW10

Streamflow, cubic feet per second

0 10 15

Nitrate as N, milligrams per liter

0

5

10

15

5

Figure 22. Spring base-flow 

synoptic sampling showing 

relation of streamflow to nitrate 

concentrations in surface water 

collected around the Bucks 

Branch watershed, Sussex County, 

Delaware, 2014–16. [Synoptic 

sampling sites shown on figure 1.]

DNREC has collected data samples from the outlet of 

Bucks Branch at site SW1 since 2003 (fig. 1; Delaware Water 

Quality Portal, [2017]). The median concentration of nitrate 

for multiple samples from SW1 plotted by month is very close 

to 9 mg/L as N, which is the median measured for this study at 

SW2 (fig. 23). Changes in water quality during flow and season at SW2 are probably representative of processes throughout the Bucks Branch watershed. Concentrations of dissolved 

phosphorus and nitrate follow similar trends as shown in 

figure 19 at SW2. The downward trend in nitrate and upward 

trend in phosphorus in the summer months is consistent with 

the likelihood of nutrient processing in the stream during that 

time (fig. 23). The application of manure and ammoniumbased fertilizers is the likely cause of the earlier increased 

ammonia concentrations in the spring.

Effects of Cover Crops on Soil and Water

The effects of cover crops on soil and water quality were 

studied following the 2015 growing season. Cover crops are 

utilized by the Bucks Branch farmer to raise soil fertility and 

decrease soil compaction. In 2015, the producer was enrolled 

in a pilot air-seeding program with the Sussex Conservation 

District, which helps the farmer plant cover crops. The airseeding program utilizes specialized equipment, owned by the 

conservation district, and allows for planting cover crops into 

standing corn in late summer. Cover crops planted in late summer instead of fall have more time to grow and develop root 

systems and utilize soil nutrients. In 2015 at the Bucks Branch 

study site, radishes and rye grass were planted on the irrigated 

field whereas only rye was planted on the dryland field. Radishes were air-seeded on the irrigated field in mid-August prior 

to crop removal. A clogged planting nozzle led to only strips 

of radish plants on the field. After crop removal on both fields, 

the producer planted rye over the entire area of the fields, 

including the parts of the irrigated field with radish plants. As 

a result, the irrigated field had strips of radish and rye plants, 

and strips with only rye (fig. 24). 

Overall plant production can be compared by the amount 

of plant tissue, or biomass, produced. When dried for chemical 

analysis, biomass is known as dry matter (DM) and is directly 

related to overall plant productivity from rye and radish 

plants. The difference between living tissue and DM is the 

weight of water held within plant vascular systems. Nutrient 

balances of fields are attained by multiplying DM by nutrient 

concentration. At the December sampling time, radish above 

and below ground DM was estimated at 1,600 pounds per 

acre (lb/acre) whereas rows only covered with rye produced 

660 lb/acre DM. The mass of radish tubers produced more 

DM (1,000 lb/acre) than radish shoots (leaves) (600 lb/acre). 

Radish shoots, however, had a higher nutrient uptake due to 

greater nutrient concentrations per pound of DM (table 8). 

Samples of plant tissue were collected, weighed, and 

processed for analysis of nutrients and ions important to plant 

growth on December, 17, 2015 (table 8). Nutrient uptake 

by radishes was typically higher than uptake by rye or other 

green material inter-seeded with radishes on the irrigated field. 

Radish growth was intensive and produced large tubers with 

several in excess of 5 lbs (fig. 24). Nitrogen concentrations in 

the radish tubers and shoots of radish were within, but lower 

than, nitrogen content ranges reported by other researchers 

(Dean and Weil, 2009; Lainé and others, 1993). Samples of 

biomass in the radish crop contained less nitrogen compared 

to other studies on radishes planted earlier in the Mid-Atlantic 

Coastal Plain (Ray Weil, University of Maryland, written commun., 2017). Despite their large size, lower nitrogen yields 
The Water-Quality Response of Agricultural Conservation Practices at the Bucks Branch Study Site 35

Sample month

Jan. Feb. Mar. Apr. May June July Aug. Sep. Oct. Nov. Dec.

Phosphate as P,

milligrams per liter

0

0.2

0.4

0.6

Ammonia as N,

milligrams per liter

0.00

0.10

0.20

Nitrate and Nitrite as N, 

milligrams per liter

0.0

5.0

10.0

Figure 23. Nutrient data by month for the watershed outlet at SW1, Bucks Branch, Sussex County, Delaware, 2003–12. 

[Location of site SW1 shown on figure 1.]

Figure 24. Alternating rows of rye and radish at the study site with a radish shown on the right to demonstrate 

the possible size of a radish tuber in the irrigated field, Bucks Branch study site, Sussex County, Delaware. 

Photographs by Alexander Soroka, U.S. Geological Survey
36 Monitoring Water Quality in the Bucks Branch Watershed, Sussex County, Delaware

Table 8. Cover crop biomass nutrient content prior to radish die-off from freezing (pre-kill; December 17, 2015) and following radish 

die-off (post-kill; April 11, 2016) in the irrigated field at the Bucks Branch study site, Sussex County, Delaware.

[Total N, total amount of nitrogen; Total C, total amount of carbon; P, phosphorus; K, potassium; Ca, calcium; Mg, magnesium; S, sulfur; Zn, zinc; Fe, iron; 

B, boron; lb/acre, pounds of nutrient estimated in plant biomass per acre of plant]

Plant tissue

Total N Total C P K Ca Mg S Zn Fe B

lb/acre

Radish Shoots (pre-kill) 35 470.6 6 42 33.9 4.7 9.4 0.03 0.2 0.04

Radish Tuber (pre-kill) 12.3 362.5 4.8 35.6 5.9 2.7 6.7 0.04 0.05 0.02

Rye (pre-kill) 8.3 137.5 1.5 7.9 1.7 0.8 0.7 0.01 0.09 <0.1

Other green (pre-kill) 2.5 42.8 0.5 2.7 1 0.3 0.2 <0.1 0.03 <0.1

Rye (post-kill) 9.5 276.5 1.9 12.3 1.8 0.9 0.8 0.02 0.06 <0.1

Field difference (pre-kill–post-kill) -48.6 -737.1 -10.9 -75.9 -40.7 -7.7 -16.3 -0.07 -0.31 -0.06

could be due to sub-optimal nitrogen levels for plant growth in 

the soil. Evidence of low soil nitrogen fertility was provided 

by discoloration of some radish leaves, known as chlorosis, 

which is indicated by an overall pale yellowish-green color 

with yellowing near the plant veins (Brady and Weil, 2008). 

Radishes perished following several consecutive days 

of freezing weather in mid-January 2016. When the second 

biomass sampling occurred on April 11, 2016, the entire field 

was covered with rye grass and radish plots were undiscernible. Post-kill values shown in table 8 were considered to be 

an average of the two (as sampling rate of radish or rye prior 

to winter kill was nearly a 1:1 ratio). These data indicate that a 

loss of 48.6 lb/acre of nitrogen in biomass of the radishes was 

not absorbed by the rye crop (table 8). 

Nutrient uptake by rye plants on the dryland field continued through winter to the time of sampling, which generally 

resulted in higher nitrogen held in biomass after the second 

sampling date than after the first (table 9). Biomass increased 

by more than 300 percent from 460 lb/acre DM in the first 

sample (December 17, 2015) to 1,700 lb/acre DM in the second sample (April 11, 2016). Over the course of the winter, the 

dryland field rye crop held a total of 28.4 lb/acre of nitrogen 

within its biomass.

No significant difference was observed in soil chemistry 

between areas dominated by radish or rye on the irrigated 

field prior to radish die-off (all t-test parameters had p > 0.31; 

p < 0.05 is significant). Significant differences in soil 

chemistry were apparent between the December and April 

sampling dates on both fields (table 10). The differences in 

soil chemistry were observed following radish die-off (on the 

irrigated field), increasing average temperatures associated 

with spring, and the subsequent soil thaw. Changes in soil 

chemistry could be due to soil microbial metabolism, which 

released nutrients stored as organic matter (OM). The release 

of nutrients from OM by microbes in warming periods 

of spring is well documented (Brady and Weil, 2008). As 

this study did not sample fields devoid of cover crops, it is 

uncertain if cover crops aided in raising soil fertility. 

Samples of soil water collected during the cover crop 

experiment generally had lower concentrations of nitrate 

compared to shallow groundwater samples collected during the main growing season (figs. 10 and 25). Differences 

between rye- and radish-dominated parts of the field were not 

apparent in soil water (fig. 25). Chemical constituents of soil 

water under the dryland field were poorly characterized due to 

only two samples being obtained (fig. 26). An increase in soil 

dissolved organic nitrogen (DON) was apparent during soilwater sampling of the irrigated field from the initial samples 

collected in February and early April (fig. 26B). Higher 

DON is associated with a release of nitrogen from microbial 

Table 9. Cover crop biomass nutrient content prior to radish die-off from freezing (pre-kill, December 17, 2015) and following radish 

die-off (post-kill, April 11, 2016) in the dryland field at the Bucks Branch study site, Sussex County, Delaware.

[lb/acre, pounds of nutrient estimated in plant biomass per acre of plant; Total N, total amount of nitrogen; Total C, total amount of carbon; P, phosphorus; 

K, potassium; Ca, calcium; Mg, magnesium; S, sulfur; Zn, zinc; Fe, iron; B, boron]

Plant tissue Total N Total C P K Ca Mg S Zn Fe B

lb/acre

Rye (pre-kill) 16.6 191 2.6 13.7 1.6 1 1 0.02 0.17 0

Rye (post-kill) 28.4 741.3 6 41.2 3.9 2.2 2.3 0.04 0.08 0.01

Field difference 11.8 550.3 3.5 27.5 2.3 1.2 1.2 0.02 -0.09 0.01
The Water-Quality Response of Agricultural Conservation Practices at the Bucks Branch Study Site 37

Table 10. Soil fertility results for the top foot of soil from the dryland and irrigated fields associated with the Bucks Branch study 

site, Sussex County, Delaware.

[One-way analysis of variance tested the difference between December and April soil fertility; light shading corresponds to significance at p ≤ 0.05; darker 

shading refers to significance at p ≤ 0.001; n, number of samples; mg/kg, milligrams per kilogram of soil; bold, mean; min, minimum; max, maximum]

Parameter Dryland field Irrigated field

12/7/2015; n=4 4/22/2016; n=8 12/7/2015; n=16 4/22/2016; n=15

Percent: Mean (min–max)

Total nitrogen 0.08 (0.07–0.1) 0.09 (0.07–0.11) 0.07 (0.06–0.1) 0.08 (0.07–0.11)

Total carbon 0.86 (0.67–1.1) 0.91 (0.66–1.1) 0.77 (0.63–1.1) 0.86 (0.67–1.2)

mg/kg: Mean (min–max)

Ammonium as nitrogen 2.2 (2.1–2.4) 4.6 (3.7–5.6) 1.7 (1.4–2.2) 4.5 (3.1–7.6)

Nitrate as nitrogen 1.8 (1.7–2.1) 2.4 (1.8–2.9) 1.2 (0.2–2.6) 2.9 (1.8–4.5)

Phosphorus 422 (269–558) 508 (340–692) 153 (60.3–228) 162 (75.7–236)

Potassium 129 (101–153) 161 (127–230) 70.2 (52.1–112) 87.2 (48.6–155)

Calcium 630 (485–742) 695 (552–1,131) 334 (206–457) 392 (248–501)

Magnesium 128 (98.1–146) 125 (96.9–150) 75.2 (49.3–91.5) 85 (54–122)

Manganese 17.8 (13.5–20.5) 23.9 (20.6–28.8) 11 (5.7–18.2) 13.7 (8.5–22.2)

Zinc 11.4 (8.5–14.2) 11.7 (10.1–17) 4.8 (3–6.7) 5.8 (3.5–7.1)

Copper 7.8 (7–8.5) 8.5 (7.7–10) 3.7 (2.4–5.3) 4.3 (3–5.6)

Iron 126 (91.2–155) 162 (127–194) 101 (72.8–142) 117 (91.3–163)

Boron 0.86 (0.65–1) 1.1 (0.93–1.6) 0.59 (0.43–0.79) 0.73 (0.61–0.98)

Sulfur 8.6 (6.9–10.5) 10.2 (8.9–11.8) 8.2 (6.2–10.2) 10.2 (7.3–13.4)

Aluminum 688 (541–817) 936 (760–1,155) 572 (446–753) 674 (580–854)

Date

11/01/201512/01/201501/01/201602/01/201603/01/201604/01/201605/01/2016

Nitrate as N, in milligrams per liter

0

10

20

30

40

50

Figure 25. Nitrate as nitrogen 

concentrations for shallow and 

medium depth wells (red) and soilwater lysimeters (blue) over time 

from irrigated (closed circle) and 

dryland (open circle) fields at the 

Bucks Branch study site, Sussex 

County, Delaware.
38 Monitoring Water Quality in the Bucks Branch Watershed, Sussex County, Delaware

Date

03/01/2016 04/01/2016 05/01/2016

Dissolved organic nitrogen,

in milligrams per liter

0.0

0.5

1.0

1.5

2.0

2.5

Nitrate as N,

in milligrams per liter

-2

0

2

4

6

8

A

B

Figure 26. Soil-water samples of 

A, nitrate as nitrogen and B, dissolved 

organic nitrogen from irrigated (red) 

and dryland (blue) fields at the Bucks 

Branch study site, Sussex County, 

Delaware.

breakdown of OM, but could also come from previous poultry 

manure applications (Singh and others, 2013). DON concentrations were similar to nitrate concentrations in soil water 

during the cover crop season (fig. 26).

Evidence indicates that the radish crop effectively 

scavenged available N from the soil and that leaching of 

inorganic nitrate during this time was minimal. An increase in 

soil fertility and dissolved organic nitrate in springtime was 

likely related to soil microbial metabolism. Soil-water nitrate 

concentrations during the cover-crop growing period were 

lower than those during the corn growing season, as shown 

in figure 10. Results indicate that cover crops had a positive 

impact in storing plant nutrients over the winter, and did not 

increase shallow groundwater concentrations of nitrate.

Summary and Conclusions

Bucks Branch in the Nanticoke River watershed in southwestern Delaware was identified as having one of the highest 

concentrations of nitrate in surface water of all stream sites 

sampled by the Delaware Department of Natural Resources 

and Environmental Control. The purpose of this study was to 

evaluate the effects of current conservation practices on water 

quality in the Bucks Branch watershed in Sussex County, 

Delaware. Conservation practices during the study included 

the analyzing the effects of irrigation on nutrient transport to 

groundwater compared to nutrient transport under dryland 

farming, and the effect of cover crops on nutrient transport. 

Surface-water quality and the transport of nutrients to surface 

water also were studied as part of this project.

The study site was located on two adjacent fields, one 

that has been irrigated since 2000, and one with dryland farming. Both were under conservation tillage and long-term rotation of corn, soybean, and small grain crops. Corn was grown 

on both fields during the two growing seasons of the study 

period, and cover crops were planted before or closely after 

harvest on both fields. The effects of cover crops on nutrient 

transport were studied in 2015 after the growing season. The 

study area represents farming and environmental conditions 

throughout intensively farmed agricultural land in the Coastal 

Plain of Delaware and the surrounding part of Maryland on the 

Delmarva Peninsula. The fields on the farm in the study area 

have a nutrient management plan developed by the Sussex 

Conservation District, which was based on nutrient management recommendations from the University of Delaware 

Cooperative Extension. Staff from the Cooperative Extension 

worked with the U.S. Geological Survey on this project to better understand nutrient uptake by crops to determine nutrient 

use efficiencies under irrigated and dryland crop production. 

The Bucks Branch watershed overlies a thick surficial 

aquifer with a shallow water table overlain by predominantly 

well-drained sandy soils. Rainfall frequency and intensity over 

the study period was average, with no major storm events. 

Most recharge to the water table occurred during the colder 

months when evapotranspiration is low, but groundwater 

recharge also occurred after significant rainfall during the 

growing season. Water levels indicated that recharge to the 

groundwater system was more likely during the growing season under the irrigated field than under the dryland field. 
Summary and Conclusions 39

When rainfall occurred during the growing season, 

samples of soil water showed that water and chemicals were 

transported below the root zone (about 3 feet [ft] below land 

surface) towards the groundwater system under both fields. 

Movement of water and nitrate to groundwater was enhanced 

by greater soil moisture conditions beneath the irrigated field. 

Soil-water concentrations of nitrate were highest beneath 

both fields during the growing season, and were approximately 125 milligrams per liter as nitrogen (mg/L as N) and 

about 240 mg/L as N at the 1-ft depth, and about 20 mg/L 

as N and over 100 mg/L as N at the 3-ft depth under the 

dryland and irrigated fields, respectively. Concentrations of 

nitrate in wells screened near the water table were similar 

to the median of those in samples from the 3-ft lysimeters. 

Although orthophosphate was measured in some soil-water 

samples, the median concentration was less than 0.01 mg/L in 

groundwater samples. 

Groundwater beneath both fields is dominated by agricultural chemicals used for the production of corn, soybeans, and 

small grains, including manure, inorganic nitrogen fertilizer, 

lime, and potassium chloride. Overall, the irrigated site had 

higher median specific conductance and concentrations of 

major ions including nitrate, calcium, magnesium, and chloride. Nitrate was present in all groundwater samples, ranging 

from 1 to 46 mg/L as N, with a median of about 9 mg/L as 

N beneath the dryland field, and 14 mg/L as N beneath the 

irrigated field. 

The measured apparent age of groundwater ranged 

from less than 10 to about 37 years. Groundwater estimated 

to have recharged within 10 years or less had higher median 

concentrations of nitrate beneath both fields, but the median 

concentration was highest in young water beneath the irrigated field—about 18 mg/L as N compared to about 10 mg/L 

beneath the dryland field. Water beneath the irrigated field that 

recharged the aquifer prior to irrigation (less than 15 years) 

had lower concentrations of nitrate than younger water. 

Beneath the dryland field, the water quality in deeper parts of 

the aquifer was likely affected by recharge from an upgradient 

field that was not typically used for corn and soybean production. The longest flowpaths to reach the stream are under 

the uplands of the fields, where water will travel for several 

decades along deep flowpaths to the stream. 

Over 90 percent of flow in Bucks Branch is from groundwater and the chemistry of surface water resembles that of 

groundwater affected by agricultural land use. The median 

nitrate concentration in Bucks Branch at streamgage site SW2 

was 9 mg/L. Trends related to flow and season in water quality 

were seen in both discrete and continuous samples at SW2. 

Concentrations of dissolved nitrogen species that are contributed from groundwater generally decrease with increasing 

flow during storms, as do other dissolved constituents such 

as silica, especially during the colder months. Total phosphorus, ammonia, and potassium concentrations are higher 

during storms than during base flow, although concentrations 

of dissolved phosphorus are similarly low during base flow 

and stormflow. Concentrations of nitrate during base flow are 

higher in stream water during the colder months than during 

the warmer months with the same flow rate, likely because of 

decreases in nitrate caused by instream biological processing 

during the warmer months.

Continuous specific conductance, dissolved oxygen, pH, 

and temperature data also showed trends. Specific conductance 

generally decreased with increasing flow, with the exception 

of some storms that occurred during the beginning of the 

growing season when fields were being worked and the 

greater potential for sediment transport also affected specific 

conductance. Dissolved oxygen has daily diurnal variation 

throughout the year related to community respiration, with a 

maximum daily variation exceeding 8 mg/L as temperature 

and light increases in the spring and the stream is filled 

with extensive blooms of floating aquatic plants. Nitrate 

fluctuated about 1 mg/L as N daily during the spring period 

of high community respiration. Rooted vegetation dominates 

instream plants over the remainder of the year, and the diurnal 

fluctuation is usually between 2 to 4 mg/L. Although water 

temperature is closely related to air temperature, because of 

continuous groundwater discharge with a mean temperature of 

about 15 degrees Celsius, the stream water rarely freezes. The 

pH of water ranged between 6 and 7 and also fluctuated daily. 

Although nitrate is relatively high in stream water, an 

increasing trend in the ratio of nitrogen and oxygen isotopes of 

nitrate in surface water and in some near-stream groundwater 

samples indicated that loss of nitrate through denitrification 

has affected the chemistry of stream water. Continuous data 

collected at this site and evidence of denitrification in the 

surface-water samples showed a greater potential for loss of 

nitrate through uptake by aquatic plants and denitrification 

during the warmer months than during the colder months. 

This pattern was similar to that seen at site SW1, the most 

downstream site in the watershed. Between 12 to 22 percent of 

nitrate in groundwater was estimated to be lost through denitrification prior to discharge into Bucks Branch using a mixing 

model. Most of the loss probably occurred where groundwater 

with nitrate came into contact with anoxic riparian wetland 

sediments, although some loss also occurred in deeper groundwater samples prior to stream discharge. 

Cover-crop biomass samples of radishes and rye grass 

collected before and after winter die-off of radishes indicates 

that the early planted radish crop effectively scavenged available nitrogen from the soil. Whereas radishes initially held 

more nitrogen than rye, at 55 to 8 pounds per acre, respectively, leaching of inorganic nitrate during this time was 

minimal. Soil-water nitrate concentrations during the covercrop growing period were lower than those during the growing season. An increase in soil fertility and dissolved organic 

nitrate observed in the springtime was likely related to soil 

microbial metabolism. Results indicate that cover crops had a 

positive impact in storing plant nutrients over winter and did 

not increase shallow groundwater concentrations of nitrate.

The combined results from data collected for this study 

showed that water and nutrients are carried below the root 

zone and through the soil zone in infiltrating water year round 
40 Monitoring Water Quality in the Bucks Branch Watershed, Sussex County, Delaware

when soil moisture is high after significant rainfall and after 

irrigation. Water infiltrating below the soil zone shortly after 

nutrients were applied had the highest nitrate concentrations, 

often greater than 50 mg/L, and may be a significant source 

of nitrate to groundwater. Once nitrate enters the groundwater 

system beneath these fields, there is a high likelihood of it 

reaching the stream. 

Although conservation practices have been applied to the 

fields, there is still significant leaching of nitrate to groundwater. This is especially true under the irrigated field, where 

nitrate concentrations are higher in younger water recharged 

since irrigation began on this field. Continued research on 

irrigation and nutrient management practices that reduce 

leaching, especially in well-drained upland areas, will be necessary to improve the precision and nutrient uptake efficiency 

of these practices in the future. Cover crops, when planted in 

standing corn, are one such practice that can effectively pull 

nitrate from below the root zone to the top layer of soil, thus 

limiting the amount of potential nitrate leaching to groundwater. Irrigation management that would lower average soil 

moisture conditions also could potentially limit nitrogen 

transport. Limiting the leaching of nitrate will continue to be a 

challenge in sandy soils of the eastern part of the Mid-Atlantic 

Coastal Plain, where soil moisture capacity is relatively low 

and managing irrigation around rainfall is difficult.

References Cited

Agostini, F., Tei, F., Silgram, M., Farneselli, M., Benincasa, 

P., and Aller, M.F., 2010, Decreasing nitrate leaching in vegetable crops with better N management, in Lichtfouse, E., 

ed.,: Genetic Engineering, Biofertilisation, Soil Quality and 

Organic Farming, v. 14, p. 147–200, accessed December 14, 

2017, at https://doi.org/10.1007/978-90-481-8741-6_6.

Al-Kaisi, Mahdi, and Yin, Xinhua, 2003, Effects of nitrogen 

rate, irrigation rate, and plant population on corn yield 

and water use efficiency: Agronomy Journal, v. 95, no. 6, 

p. 1475–1482, accessed October 24, 2017, at https://doi.

org/10.2134/agronj2003.1475.

Andres, A.S., 1991, Results of the coastal Sussex County, 

Delaware ground-water quality survey: Delaware Geological Survey Report of Investigations No. 49, 28 p., accessed 

December 14, 2017, at http://udspace.udel.edu/bitstream/

handle/19716/3164/RI49.pdf?sequence=1andisAllowed=y.

Andres, A.S., 1994, Geohydrology of the Seaford area, 

Delaware: Delaware Geological Survey Hydrologic Map 

No. 9, 1:24,000, accessed December 14, 2017, at 

http://www.dgs.udel.edu/publications/hm9-geohydrologyseaford-area-delaware.

Andres, A.S., and Klingbeil, A.D., 2006, Thickness and 

transmissivity of the unconfined aquifer of Eastern Sussex 

County, Delaware: Delaware Geological Survey Report of 

Investigations No. 70, 25 p., accessed December 14, 2017, 

at https://www.dgs.udel.edu/publications/ri70-thicknessand-transmissivity-unconfined-aquifer-eastern-sussexcounty-delaware.

Ator, S.W., and Denver, J.M., 2015, Understanding nutrients 

in the Chesapeake Bay watershed and implications for management and restoration—The Eastern Shore: U.S. Geological Survey Circular 1406, 72 p., accessed December 14, 

2017, at https://dx.doi.org/10.3133/cir1406.

Böhlke, J.K., 2002, Groundwater recharge and agricultural contamination: Hydrogeology Journal, v. 10, no. 1, 

p. 153–179, accessed December 14, 2017, at https://doi.

org/10.1007/s10040-001-0183-3.

Böhlke, J.K., and Denver, J.M., 1995, Combined use of 

groundwater dating, chemical and isotopic analyses to 

resolve the history and fate of nitrate contamination in two 

agricultural watersheds, Atlantic Coastal Plain, Maryland: 

Water Resources Research, v. 31, no. 9, p. 2319–2339, 

DOI: 10.1029/95WR01584, accessed December 14, 2017, 

at http://onlinelibrary.wiley.com/doi/10.1029/95WR01584/

abstract.

Brady, N.C., and Weil, R.R., 2008, The nature and properties 

of soils (14th ed.): New York, Macmillan Publishing Co., 

965 p.

Busenberg, Eurybiades, and Plummer, L.N., 1992, Use of 

Chlorofluorocarbons (CC13F and CCl2F2) as hydrologic tracers and age-dating tools: The alluvium and terrace system of 

central Oklahoma: Water Resources Research, v. 28, no. 9, 

p. 2257–2283, accessed December 14, 2017, at 

http://onlinelibrary.wiley.com/doi/10.1029/92WR01263/pdf.

Busenberg, Eurybiades, and Plummer, L.N., 2000, Dating 

young ground water with sulfur hexafluoride: Natural 

and anthropogenic sources of sulfur hexafluoride: Water 

Resources Research, v. 36, no. 10, p. 3011–3030, accessed 

June 10, 2018, at https://doi.org/10.1029/2000WR900151.

Bushong, J.T., Miller, E.C., Mullock, J.L., Arnall, D.B., and 

Raun, W.R., 2014, Effect of irrigation and preplant nitrogen fertilizer source on maize in the Southern Great Plains: 

International Journal of Agronomy, v. 2014, 10 p., accessed 

October 24, 2017, at https://doi.org/10.1155/2014/247835.

Casciotti, K.L., Sigman, D.M., Hastings, M.G., Böhlke, 

J.K., and Hilkert, A., 2002, Measurement of the oxygen 

isotopic composition of nitrate in seawater and freshwater 

using the dentirifier method: Analytical Chemistry, v. 74, 

no. 19, p. 4905–4912, accessed October 24, 2017, at 

https://doi.org/10.1021/ac020113w.
References Cited 41

Clune, J.W., and Denver, J.M., 2012, Residence time, chemical and isotopic analysis of nitrate in the groundwater 

and surface water of a small agricultural watershed in the 

Coastal Plain, Bucks Branch, Sussex County, Delaware: 

U.S. Geological Survey Scientific Investigations Report 

2012–5235, 15 p., accessed December 14, 2017, at 

https://pubs.usgs.gov/sir/2012/5235/.

Cunningham, W.L., and Schalk, C.W., comps., 2011, 

Groundwater technical procedures of the U.S. Geological 

Survey: U.S. Geological Survey Techniques and Methods 1–A1, 151 p., accessed December 14, 2017, at 

https://pubs.usgs.gov/tm/1a1/ [online only].

Dean, J.E., and Weil, R.R., 2009, Brassica cover crops for 

nitrogen retention in the Mid-Atlantic Coastal Plain: Journal 

of Environmental Quality, v. 38, no. 2, p. 520–528, accessed 

October 24, 2017, at https://doi.org/10.2134/jeq2008.0066.

Delaware Department of Agriculture, 2017, Grants and Loans, 

accessed on December 27, 2017, at https://dda.delaware.

gov/financial_assistance.shtml.

Delaware Department of Natural Resources and Environmental Control, 2017, The State of Delaware 2016 combined 

watershed assessment report (305(b) and Determination of 

the Clean Water Act Section 303(b) list of waters needing TMDLs, accessed on December 27, 2017, at 

http://regulations.delaware.gov/AdminCode/

title7/7000/7400/7406.shtml#TopOfPage.

Delaware Water Quality Portal, 2017, Historical data for 

304381: Delaware Water Quality Portal, accessed January 8, 

2018, at http://demac.udel.edu/waterquality/wq_layout.

php?stationid=21DELAWQ_WQX-304381.

Denver, J.M., 1986, Hydrogeology and geochemistry of the 

unconfined aquifer, West-Central and Southwestern Delaware: Delaware Geological Survey Report of Investigations No. 41, 100 p., accessed December 14, 2017, at 

http://www.dgs.udel.edu/sites/default/files/publications/

rfi41.pdf.

Denver, J.M., 1989, Effects of agricultural practices and 

septic-system effluent on the quality of water in the unconfined aquifer in parts of eastern Sussex County, Delaware: 

Delaware Geological Survey Report of Investigations 

No. 45, 66 p., accessed December 14, 2017, at 

http://www.dgs.udel.edu/sites/default/files/publications/

ri45.pdf.

Denver, J.M., Ator, S.W., Debrewer, L.M., Ferrari, M.J., Barbaro, J.R., Hancock, T.C., Brayton, M.J., and Nardi, M.R., 

2004, Water quality in the Delmarva Peninsula, Delaware, 

Maryland, and Virginia, 1999–2001: U.S. Geological Survey Circular 1228, 40 p., accessed December 17, 2017, at 

https://pubs.usgs.gov/circ/2004/1228/.

Denver, J.M., Tesoriero, A.J., and Barbaro, J.R., 2010, Trends 

and transformation of nutrients and pesticides in a Coastal 

Plain aquifer system, United States: Journal of Environmental Quality, v. 39, no. 1, p. 154–167, accessed June 18, 2018, 

at https://doi.org/10.2134/jeq2009.010.

Eck, H., 1984, Irrigated corn yield response to nitrogen and 

water: Agronomy Journal, v. 76, accessed October 24, 

2017, at https://pubag.nal.usda.gov/pubag/downloadPDF.

xhtml?id=502&content=PDF.

Eckhardt, K., 2005, How to construct recursive digital filters 

for baseflow separation: Hydrological Processes, v. 19, 

no. 2, p. 507–515, doi:10.1002/hyp.5675, accessed January 

14, 2014, at http://dx.doi.org/10.1002/hyp.5675.

Hamilton, P.A., Denver, J.M., Phillips, P.J., and Shedlock, 

R.J., 1993, Water-quality assessment of the Delmarva 

Peninsula, Delaware, Maryland, and Virginia—Effects of 

agricultural activities on, and the distribution of, nitrate and 

other inorganic constituents in the surficial aquifer: U.S. 

Geological Survey Open-File Report 93–40, 95 p., accessed 

December 14, 2017, at https://pubs.usgs.gov/of/1993/0040/

report.pdf.

Homer, C.G., Dewitz, J.A., Yang, L., Jin, S., Danielson, P., 

Xian, G., Coulston, J., Herold, N.D., Wickham, J.D., and 

Megown, K., 2015, Completion of the 2011 National Land 

Cover Database for the conterminous United States-Representing a decade of land cover change information. Photogrammetric Engineering and Remote Sensing, v. 81, no. 5, 

p. 345–354, accessed January 8, 2018, at https://www.mrlc.

gov/nlcd2011.php.

Jackson, L.E., Wyland, L.J., and Stivers, L.J., 1993, Winter 

cover crops to minimize nitrate losses in intensive lettuce production: Journal of Agricultural Science, v. 121, 

p. 55–62, accessed October 24, 2017, at http://ucanr.edu/

sites/Jackson_Lab/files/155592.pdf.

Kasper, J.W., 2014, Delaware’s 2014 305(b) GroundwaterQuality Assessment Based on Public-Well Data: Results 

of Sampling, 2012–13, accessed October 24, 2014, at 

http://www.dnrec.delaware.gov/swc/wa/Documents/WAS/

dave%27s%20docs/305b_GW_2014_FINAL.PDF.

Kim, K.I., Clay, D.E., Carlson, C.G., Clay, S.A., and Trooien, 

T., 2008, Do synergistic relationships between nitrogen and 

water influence the ability of corn to use nitrogen derived 

from fertilizer and soil?: Agronomy Journal, v. 100, 

no. 3, p. 551–556, accessed October 24, 2017, at 

https://doi.org/10.2134/agronj2007.0064.

Kristensen, H.L., and Thorup-Kristensen, K., 2004, Root 

growth and nitrate uptake of three different catch crops in 

deep soil layers: Soil Science Society of America Journal, 

v. 68, no. 2, p. 529–537, accessed October 24, 2017, at 

https://doi.org/10.2136/sssaj2004.5290.
42 Monitoring Water Quality in the Bucks Branch Watershed, Sussex County, Delaware

Lainé, P., Ourry, A., Macduff, J., Boucaud, J., and Salette, J., 

1993, Kinetic parameters of nitrate uptake by different catch 

crop species: effects of low temperatures or previous nitrate 

starvation: Physiologia Plantarum, v. 88, no. 1, p. 85–92, 

accessed December 14, 2017, at http://onlinelibrary.wiley.

com/doi/10.1111/j.1399-3054.1993.tb01764.x/abstract.

Lindsey, B.D., Phillips, S.W., Donnelly, C.A., Speiran, G.K., 

Plummer, L.N., Böhlke, J.K., Focazio, M.J., Burton, M.C., 

and Busenberg, Eurybiades, 2003, Residence times and 

nitrate transport in ground water discharging to streams in 

the Chesapeake Bay watershed: U.S. Geological Survey 

Water-Resources Investigations Report 03–4035, 201 p., 

accessed December 14, 2017, at https://pa.water.usgs.gov/

reports/wrir03-4035.pdf.

Olson, R.A., Dreier, A.F., Thompson, C., Frank, K., and 

Grabouski, P., 1964, Using fertilizer nitrogen effectively on 

grain crops: Bulletin of the Agricultural Experiment Station 

of Nebraska, Station Bulletin SB 479, accessed October 24, 

2017, at http://digitalcommons.unl.edu/agronomyfacpub/553/.

Randall, G.W., and Mulla, D.J., 2001, Nitrate nitrogen in 

surface waters as influenced by climatic conditions and agricultural practices: Journal of Environmental Quality, v. 30, 

no. 2, p. 337–344, accessed October 24, 2017, at 

https://doi.org/10.2134/jeq2001.302337x.

Ritter, W.F., and Chirnside, A.E.M., 1982, Groundwater 

quality in selected areas of Kent and Sussex Counties, 

Delaware: Dover, Delaware. Technical Report, Delaware 

Department of Natural Resources and Environmental Control, 229 p.

Ritter, W.F., Scarborough, R.W., and Chirnside, A.E.M., 1991, 

Nitrate leaching under irrigation on Coastal Plain soil: 

Journal of Irrigation and Drainage Engineering, v. 117, 

no. 4, p. 490–502, accessed October 24, 2017, at https://doi.

org/10.1061/(ASCE)0733-9437(1991)117:4(490).

Ritter, W.F., Scarborough, R.W., and Chirnside, A.E.M., 1993, 

Nitrate leaching under irrigated corn: Journal of Irrigation and Drainage Engineering, v. 119, no. 3, p. 544–553, 

accessed October 24, 2017, at http://ascelibrary.org/doi/

abs/10.1061/(ASCE)0733-9437(1993)119:3(544).

Sanford, W.E., Pope, J.P., Selnick, D.L., and Strumvoll, R.F., 

2012, Simulation of groundwater flow in the shallow aquifer system of the Delmarva Peninsula, Maryland and Delaware: U.S. Geological Survey Open-File Report 2012–1140, 

58 p., accessed December 14, 2017, at http://pubs.usgs.gov/

of/2012/1140 [online only].

Sauer, V.B., and Turnipseed, D.P., 2010, Stage measurement 

at gaging stations: U.S. Geological Survey Techniques and 

Methods, book 3, chap. A7, 45 p., accessed October 24, 

2017, at http://pubs.usgs.gov/tm/tm3-a7/.

Shedlock, R.J., Denver, J.M., Hayes, M.A., Hamilton, P.A., 

Koterba, M.T., Bachman, L.J., Phillips, P.J., and Banks, 

W.S., 1999, Water-quality assessment of the Delmarva 

Peninsula, Delaware, Maryland, and Virginia; results of 

investigations, 1987–91: U.S. Geological Survey WaterSupply Paper 2355–A, accessed December 14, 2017, at 

https://pubs.er.usgs.gov/publication/wsp2355A.

Sigman, D.M., Casciotti, K.L., Andreani, M., Barford, C., 

Galanter, M., and Böhlke, J.K., 2001, A bacterial method 

for the nitrogen isotopic analysis of nitrate in seawater and 

freshwater: Analytical Chemistry, v. 73, no. 17, p. 4145–

4153, accessed October 24, 2017, at https://doi.org/10.1021/

ac010088e.

Singh, Shatrugan, Inamdar, Sheeram, and Scott, Durelle, 2013, 

Comparison of two PARAFAC models of dissolved organic 

matter fluorescence for a Mid-Atlantic forested watershed 

in the USA: Journal of Ecosystems, v. 2013, 16 p., accessed 

October 24, 2017, at https://doi.org/10.1155/2013/532424.

Soil Survey Staff, Natural Resources Conservation Service, 

U.S. Department of Agriculture, 2016, Web Soil Survey, 

accessed December 14, 2017, at https://websoilsurvey.

sc.egov.usda.gov/app/Help/Citation.htm.

Soroka, A.M., and Denver, J.M., 2018, Chemistry analysis 

results for water, soil-pore water, soil and plant material 

collected from two agricultural sites in the Nanticoke and 

Chester River watersheds on the Delmarva Peninsula from 

2013 to 2016: U.S. Geological Survey data release, 

https://doi.org/10.5066/F77H1HK5.

Spahr, N.E., Dubrovsky, N.M., Gronberg, J.M., Franke, O.L., 

and Wolock, D.M., 2010, Nitrate loads and concentrations 

in surface-water base flow and shallow groundwater for 

selected basins in the United States, water years 1990–2006: 

U.S. Geological Survey Scientific Investigations Report 

2010–5098, [variously paged], accessed December 14, 

2017, at https://pubs.usgs.gov/sir/2010/5098/pdf/SIR10-

5098.pdf.

Staver, K.W., and Brinsfield, R.B., 1998, Using cereal grain 

winter cover crops to reduce groundwater nitrate contaminaniton in the mid-atlantic Coastal Plain: Journal of Soil 

and Water Conservation, v. 53, no. 3, p. 230–240, accessed 

December 14, 2017, at http://www.jswconline.org/content/53/3/230.abstract?ijkey=0fe17afb893faf25e3bc525798

735d68cab77394&keytype2=tf_ipsecsha.

University of Delaware, 2017, Delaware Environmental 

Observing System, accessed January 2, 2018, at 

http://www.deos.udel.edu.

University of Delaware, College of Agriculture and Natural 

Resources, 2017, Extension, accessed on December 27, 

2017, at http://extension.udel.edu/ag/.
References Cited 43

U.S. Department of Agriculture, National Agricultural Statistics Service, 2012, 2012 Census of Agriculture-County 

data, Delaware irrigation: 2012 and 2007, 18(D), accessed 

December 10, 2016, at https://www.agcensus.usda.gov/Publications/2012/Full_Report/Volume_1,_Chapter_2_County_

Level/Delaware/st10_2_010_010.pdf.

U.S. Environmental Protection Agency, 1993, Method 351.2, 

Revision 2.0: Determination of Total Kjeldahl Nitrogen by 

semi-automated colorimetry, accessed August 29, 2015, at 

https://www.epa.gov/sites/production/files/2015-08/documents/method_351-2_1993.pdf.

U.S. Geological Survey, variously dated, National field manual for the collection of water-quality data: U.S. Geological Survey Techniques of Water-Resources Investigations, 

book 9, chaps. A1–A10, accessed December 14, 2017, at 

http://pubs.water.usgs.gov/twri9A. 

U.S. Geological Survey, National Water Information System 

(NWIS) data (USGS Water Data for the Nation), accessed 

October 24, 2017, at https://waterdata.usgs.gov/nwis.

Wagner, R.J., Boulger, R.W., Jr., Oblinger, C.J., and Smith, B., 

2006, Guidelines and standard procedures for continuous 

water-quality monitors: Station operation, record computation, and data reporting: U.S. Geological Survey Techniques 

and Methods, 1–D3, [variously paged], accessed December 

10, 2016, at https://pubs.usgs.gov/tm/2006/tm1D3/pdf/

TM1D3.pdf.

Weinert, T.L., Pan, W.L., Moneymaker, M.R., Santo, G.S., and 

Stevens, R.G., 2002, Nitrogen recycling by nonleguminous 

winter cover crops to reduce leaching in potato rotations: 

Agronomy Journal, v. 94, no. 2, p. 365–372, accessed 

December 14, 2017, at https://dl.sciencesocieties.org/publications/aj/abstracts/94/2/365.

Zhang, Y.-M., Hu, C.-S., Zhang, J.-B., Chen, D.-L., and 

Li, X.-X., 2005, Nitrate leaching in an irrigated wheatmaize rotation field in the north China plain: Pedosphere, 

v. 15, no. 2, p. 196–203, accessed December 14, 2017, at 

http://pedosphere.issas.ac.cn/trqen/ch/reader/view_abstract.

aspx?file_no=20050207&flag=1.

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