U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2020–5108

Prepared in cooperation with the Illinois Environmental Protection Agency

Use of Real-Time Sensors to Temporally Characterize Water 

Quality in Groundwater and Surface Water in Mason County, 

Illinois, 2017–19
Front Cover: Photograph showing continuous groundwater monitoring well,

U.S. Geological Survey station 401913089534501. Photograph by William S. Morrow, 

U.S. Geological Survey.
Use of Real-Time Sensors to Temporally 

Characterize Water Quality in Groundwater 

and Surface Water in Mason County, 

Illinois, 2017–19

By Lance R. Gruhn and William S. Morrow

Prepared in cooperation with the Illinois Environmental Protection Agency

Scientific Investigations Report 2020–5108

U.S. Department of the Interior

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

DAVID BERNHARDT, Secretary

U.S. Geological Survey

James F. Reilly II, Director

U.S. Geological Survey, Reston, Virginia: 2020

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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:

Gruhn, L.R., and Morrow, W.S., 2020, Use of real-time sensors to temporally characterize water quality in 

groundwater and surface water in Mason County, Illinois, 2017–19: U.S. Geological Survey Scientific Investigations 

Report 2020–5108, 26 p., https://doi.org/10.3133/sir20205108.

ISSN 2328-0328 (online)
iii

Acknowledgments

The authors would like to thank Rick Cobb and the Illinois Environmental Protection Agency 

(IEPA) for their support of past and ongoing studies. Additionally, we would like to thank Lauren 

Lurkins and associates at the Illinois Farm Bureau for providing a stakeholder review of this 

report and coordinating meetings that helped gain valuable input from local irrigating farmers 

and the Illinois Farm Bureau. Special thanks to John Breedlove, the president of the Central 

Illinois Irrigated Growers Association, and Steve Turner for their insight into irrigated farming 

operations in the area. We also thank Joe Konczyk (IEPA) and Kevin Rogers (Illinois Department 

of Agriculture) for their assistance acquiring geospatial information, relevant reports, and other 

ancillary information relevant to the study. The authors would also like to thank the Imperial 

Valley Water Authority for furnishing precipitation and estimated irrigation use data. We would 

like to thank our colleague Jennifer Sharpe of the Central Midwest Water Science Center for 

her assistance compiling figures and graphs for this report.

v

Contents

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

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

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

Purpose and Scope ..............................................................................................................................3

Study Area..............................................................................................................................................3

Methods...........................................................................................................................................................5

Site Selection.........................................................................................................................................5

Continuous Groundwater Monitoring Instrumentation ..................................................................5

Quiver Creek Surface Water and Groundwater Characterization ................................................8

Discrete Groundwater Sample Collection and Analysis ................................................................8

Ancillary Data........................................................................................................................................8

Hydrology ......................................................................................................................................................10

Continuous Groundwater-Quality Data ....................................................................................................11

Nitrate Concentrations ......................................................................................................................11

Dissolved Oxygen................................................................................................................................13

Specific Conductance ........................................................................................................................15

Water Temperature ............................................................................................................................15

pH...........................................................................................................................................................18

Characterization of Water Quality in Quiver Creek Stream and Floodplain .......................................19

Isotopic Characterization ...........................................................................................................................22

Summary and Conclusions .........................................................................................................................24

References Cited..........................................................................................................................................25

Figures

1. Map showing nitrate concentrations greater than 10 milligrams per liter in 

Illinois Department of Agriculture monitoring wells, 2000–14 ...............................................2

2. Map showing Havana Lowlands region and the extent of the shallow, western 

part of the Mahomet aquifer in west-central Illinois ..............................................................4

3. Map showing Havana well location, Quiver Creek measurement location, 

piezometer locations, and reconnaissance locations, Mason County, Illinois ..................7

4. Graph showing monthly precipitation totals from gage nine of the Imperial 

Valley Water Authority precipitation network, March 2017–19 ..........................................10

5. Graph showing groundwater elevations plotted for paired piezometers. Quiver 

Creek surface water with piezometer 1, and the paired shallow and deep 

piezometers .................................................................................................................................11

6. Graph showing nitrate concentration and groundwater elevation in feet above 

the North American Vertical Datum of 1988, March 8, 2017, to March 31, 2019, 

U.S. Geological Survey station 401913089534501 ..................................................................12

7. Graph showing nitrate concentration, dissolved oxygen, and groundwater 

elevation in feet above the North American Vertical Datum of 1988, March 8, 2017, 

to March 31, 2019, U.S. Geological Survey station 401913089534501 .......................................14
vi

8. Graph showing nitrate concentration, dissolved oxygen, and groundwater 

elevation in feet above the North American Vertical Datum of 1988, during 

2017 peak irrigation period, May 1 to August 31, U.S. Geological Survey station 

401913089534501 ...........................................................................................................................14

9. Graph showing nitrate concentration, dissolved oxygen, and groundwater 

elevation in feet above the North American Vertical Datum of 1988, during 

2018 peak irrigation period, May 1 to August 31, U.S. Geological Survey station 

401913089534501 ...........................................................................................................................15

10. Graph showing nitrate concentration, specific conductance, and groundwater 

elevation in feet above the North American Vertical Datum of 1988, March 8, 2017, 

to March 31, 2019, U.S. Geological Survey station 401913089534501 .......................................16

11. Graph showing nitrate concentration, specific conductance, and groundwater 

elevation in feet above the North American Vertical Datum of 1988, during 

2017 peak irrigation period, May 1 to August 31, U.S. Geological Survey station 

401913089534501 ...........................................................................................................................16

12. Graph showing nitrate concentration, specific conductance, and groundwater 

elevation in feet above the North American Vertical Datum of 1988, during 

2018 peak irrigation period, May 1 to August 31, U.S. Geological Survey station 

401913089534501 ...........................................................................................................................17

13. Graph showing nitrate concentration, temperature, and groundwater elevation in 

feet above the North American Vertical Datum of 1988, March 8, 2017, to March 31, 

2019, U.S. Geological Survey station 401913089534501 .............................................................17

14. Graph showing nitrate, pH, and groundwater elevation in feet above the North 

American Vertical Datum of 1988, March 8, 2017, to March 31, 2019, U.S. Geological 

Survey station 401913089534501 .................................................................................................18

15. Graph showing continuous groundwater levels and groundwater nitrate 

concentrations at U.S. Geological Survey station 401913089534501, plotted with 

instantaneous measurements of nitrate in Quiver Creek surface water ..........................21

16. Graph showing nitrate concentrations in Quiver Creek and the shallow 

piezometers in the floodplain April 4, 2018, to December 19, 2018 .....................................21

17. Graph showing stable isotopic composition of water from Havana well and 

Quiver Creek piezometers plotted on the Global Meteoric Water Line, Mason 

County, Illinois .............................................................................................................................22

18. Graph showing nitrate isotopes for Havana well and Quiver Creek piezometers, 

Mason County, Illinois ................................................................................................................24

Tables

1. Information on data collection sites, Mason County, Illinois 2017–19 .................................6

2. Imperial Valley Water Authority’s estimated irrigation usage in gallons for 2017 

and 2018..........................................................................................................................................9

3. Summary of discrete water-quality results from the U.S. Geological Survey 

National Water Quality Laboratory analysis of nutrients .....................................................12

4. Range of instantaneous measurements of water quality and water-level 

elevations, April 11, 2017, to February 5, 2019, Mason County, Illinois ..............................20

5. Summary of results of stable isotopes of water and nitrate isotopes in 

groundwater, Mason County, Illinois .......................................................................................23
vii

Conversion Factors

U.S. customary units to International System of Units

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

International System of Units to U.S. customary units

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft)

kilometer (km) 0.6214 mile (mi)

kilometer (km) 0.5400 mile, nautical (nmi)

meter (m) 1.094 yard (yd)

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

°F = (1.8 × °C) + 32.

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

°C = (°F – 32) / 1.8.

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.
viii

Supplemental Information

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

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

at 25 °C).

Results for measurements of stable isotopes of an element (with symbol E) in water, solids, 

and dissolved constituents commonly are expressed as the relative difference in the ratio of 

the number of the less abundant isotope (iE) to the number of the more abundant isotope of a 

sample with respect to a measurement standard.

Abbreviations

< less than

HL Havana Lowlands

IEPA Illinois Environmental Protection Agency

IVWA Imperial Valley Water Authority

USGS U.S. Geological Survey
Use of Real-Time Sensors to Temporally Characterize 

Water Quality in Groundwater and Surface Water in 

Mason County, Illinois, 2017–19

By Lance R. Gruhn and William S. Morrow

Abstract

The persistence of high nitrate concentrations in shallow 

groundwater has been well documented in the shallow glacial 

aquifer of Mason County, Illinois. Nitrates in groundwater can 

be a concern when concentrations exceed 10 milligrams per 

liter in drinking water. Additionally, nitrate in groundwater 

can contribute to surface water nitrogen loads that can cause 

increased algal growth. Algal growth increases oxygen consumption causing anoxic conditions as observed in the Gulf of 

Mexico Hypoxic Zone.

From March 8, 2017, to March 31, 2019, groundwater 

level, continuous nitrate, dissolved oxygen, specific conductance, water temperature, and pH data were collected in a 

monitoring well to temporally assess changes in water quality 

using high frequency data. During this same period, instantaneous field measurements of water quality and groundwater levels were made in surface water and groundwater in 

and near Quiver Creek in the presumed groundwater flow 

path about 0.6 mile from the continuous monitoring well. 

Groundwater nitrate concentrations continuously measured 

in the aquifer during this time ranged from 14.7 to 23.2 milligrams per liter, whereas instantaneously measured nitrate concentrations in Quiver Creek ranged from 0.9 to 6.4 milligrams 

per liter. Nitrate concentrations measured by piezometer varied 

laterally and vertically in the Quiver Creek floodplain and 

beneath the stream. Irrigation and fertigation for agriculture is 

widely practiced in Mason County. This may seasonally affect 

the groundwater flow and movement as well as the persistence 

of nitrate in this area. Continuously and instantaneously measured nitrate concentrations and groundwater levels indicate 

that during the irrigation season, discharge to Quiver Creek 

from the shallow groundwater system may be limited. During 

and following periods when estimated irrigation use is highest, the low-nitrate deeper groundwater may be the dominant 

contributor to the Quiver Creek surface water, whereas during 

recharge events and when the system is not under the stress 

of irrigation, there is more contribution from the local and 

higher-nitrate shallow groundwater.

Introduction

Illinois has some of the most productive farmland in the 

United States. The use of fertilizers has increased to improve 

crop production and has resulted in an increase in the concentrations of nitrogen in many streams and aquifers (Dubrovsky 

and others, 2010). From 2000 to 2014 the Illinois Department 

of Agriculture biennially sampled their network of monitoring wells for concentrations of nitrate plus nitrite as nitrogen 

(N; hereafter referred to as “nitrate”), for a total of seven 

samples. These wells are located throughout Illinois adjacent 

to crop production and where the aquifer materials are within 

50 feet (ft) of the land surface. Through this sampling effort, 

the Illinois Department of Agriculture identified areas in the 

State where nitrate concentrations are consistently above the 

State of Illinois class I potable resource groundwater quality standard of 10 milligrams per liter (mg/L, fig. 1) (K. 

Rogers, Illinois Department of Agriculture, written commun., 2019; Illinois Pollution Control Board, 2019a). The 

national primary drinking water regulations established by the 

U.S. Environmental Protection Agency also set a maximum 

contaminant level of 10 mg/L for nitrate concentrations in 

drinking water (U.S. Environmental Protection Agency, 2019). 

Illinois has codified this maximum contaminant level in 35.Ill.

Adm.Code 611.301 (Illinois Pollution Control Board, 2019b). 

Nutrients in surface water also contribute to nutrient concentrations in the Gulf of Mexico, resulting in increased algal 

growth, which leads to increased oxygen consumption and 

creates anoxic conditions, referred to as the Gulf of Mexico 

Hypoxic Zone (U.S. Environmental Protection Agency, 2020).

A study of nitrate in the Mississippi River and several 

smaller river basins concluded that some of the highest concentrations of nitrate were observed during low-flow conditions, indicating that a large nutrient load may be transported 

to the river through groundwater or point source contribution 

(Murphy and others, 2013). During low-flow conditions, 

groundwater is presumed to be a large contributor to the 

total volume of surface water. The area in and around Mason 

County, Illinois, has several wells that have had numerous 
2 Use of Real-Time Sensors to Temporally Characterize Water Quality in Mason County, Illinois, 2017–19

Illinois River

Fox

River

Plaines

Des

River

Rock River

Spoon

River

P

River

ecatonica

Kishwaukee River

Salt

Creek

Creek

Kaskaskia River

Little Wabash River

Sangamon River

Embarras River

Big Muddy River

Cache River

La Moine River

Mackinaw River

Iroquois River

Green River

Mississippi River

LAKE

MICHIGAN

Wabash River

Ohio River

River Kankakee

Mac o upin

91° 90° 89° 88°

42°

41°

40°

39°

38°

37°

Base modified from U.S. Geological Survey 1:100,000-scale digital data, 2005 

Albers Equal-Area Conic projection 

Standard parallels 33 ̊ N. and 45 ̊ N.

Central meridian 89 ̊ W.

North American Datum of 1927

Background map image is the intellectual property of Esri and is used herein under license. 

Copyright © 2020 Esri and its licensors. 

All rights reserved. 

0 25 50 KILOMETERS

0 25 50 MILES

MASON

COUNTY

EXPLANATION

Number of times nitrate plus

nitrite was greater than

10 milligrams per liter in

Illinois Department of

Agriculture wells

! 0

! 1

$ 2 to 3

' 4 to 5

" 6 to 7

Nitrate data from K. Rogers,

Illinois Department of Agriculture,

written commun., 2019

Figure 1. Nitrate concentrations greater than 10 milligrams per liter in Illinois Department of Agriculture monitoring 

wells, 2000–14 (K. Rogers, Illinois Department of Agriculture, written commun., 2019).
Introduction 3

detections of nitrate concentrations in groundwater greater 

than 10 mg/L. These wells are grouped together in a shallow glacial aquifer, in an area known as the Havana Lowland 

(HL) region. Agriculture is the dominant land use in this 

area, and the HL region is one of the most heavily irrigated 

agricultural areas in Illinois (Bridges and Wilson, 2014). In 

November 2014, the Illinois Environmental Protection Agency 

and the Illinois Department of Agriculture announced the 

Illinois Nutrient Loss Reduction Strategy, which is an effort 

to reduce nutrient loads in surface water within the State of 

Illinois, the Mississippi River Basin, and the Gulf of Mexico 

(Illinois Department of Agriculture, 2019). Based on findings 

in Murphy and others (2013), a detailed assessment of nitrate 

concentrations in groundwater is crucial to understanding the 

potential effects groundwater may have on nitrate concentrations in surface water.

The U.S. Geological Survey (USGS), in cooperation 

with the Illinois Environmental Protection Agency (IEPA), 

completed a study using real-time sensors in Mason County, 

Illinois, to better understand the fate and transport of nitrate in 

an area with rapid drainage and intensive irrigation and fertilizer application. More specifically, how nitrate concentrations 

in groundwater may potentially affect nitrate concentrations in 

surface water in this type of setting, and the potential relations 

between nitrate and other water-quality properties and physical and atmospheric data. The results from this study can help 

inform the Illinois Nutrient Loss Reduction Strategy and other 

nutrient reduction initiatives on the fluctuations of nitrate concentrations in groundwater and its potential loading to surface 

water in an area with substantial groundwater discharge.

Purpose and Scope

The purpose of this report is to describe the findings 

from continuous data collection using real-time sensors (one 

reading every 15 minutes) in a groundwater well in a shallow glacial aquifer system in Mason County, Ill., and from 

instantaneous data (readings collected while onsite) collected 

in a nearby stream, Quiver Creek, and in the groundwater 

within the Quiver Creek floodplain, particularly to describe the 

fate and transport of nitrate as it moves through the shallow 

aquifer system, the floodplain, and potentially to surface water. 

Continuous data were analyzed with instantaneous data and 

discrete water-quality data collected by the USGS, and estimated irrigation use and precipitation data from the Imperial 

Valley Water Authority (IVWA).

Study Area

The HL region is in Mason County and the southwestern part of Tazewell County in west-central Illinois. The HL 

region is bounded to the northwest by the Illinois River, to 

the south by the Sangamon River and Salt Creek, and to the 

east by glaciated uplands (fig. 2; Walker and others, 1965; 

Sanderson and Buck, 1995). Three main physiographic 

areas within the HL region were identified by Walker and 

others (1965): (1) the floodplains of the streams and rivers, 

(2) wide sand-ridged terraces east of the Illinois River, and 

(3) the loess-covered Illinoian drift upland in southeastern 

Mason County. The principle surface-water features in the 

region include the Illinois River, Mackinaw River, Sangamon 

River, Salt Creek, Crane Creek, and Quiver Creek, which is 

also known as the Mason-Tazewell Drainage Ditch (fig. 2; 

Walker and others, 1965; Sanderson and Buck, 1995). The 

flow of Quiver Creek is primarily, if not entirely, composed 

of groundwater discharge from the portion of the Mahomet 

aquifer (Layne Hydro, 2012) that is made up of sands and 

gravels deposited in the Mahomet bedrock valley (Hackley 

and others, 2010).

The surficial geology of the HL region consists primarily of unconsolidated sand and gravel deposits of Pleistocene 

age overlying Pennsylvanian bedrock valleys (Kelly, 1997). 

These sand and gravel deposits that make up the Mahomet 

aquifer in the HL region consist of the Sankoty Sand Member 

of the Banner Formation overlain by glacial outwash deposits 

of Wisconsinan age (Kelly and Ray, 1999). With respect to 

nitrate and dissolved oxygen, there is a shallow portion (hereafter referred to as “shallow”) of this unconsolidated aquifer 

that is geochemically different than the deeper part of the 

aquifer (hereafter referred to as “deeper”). The aquifer often 

exceeds 100 ft in thickness and in some areas exceeds more 

than 200 ft (Walker and others, 1965). Sediments are hydraulically conductive and capable of allowing rapid drainage and 

infiltration. Extensive information about the geology of this 

area is discussed in Walker and others (1965).

Climate in the HL region shows typical midcontinental 

characteristics with cold winters and humid, warm summers. 

Most precipitation occurs during the warmer months with 

variability (sometimes large) in temperature and precipitation 

from year to year (Sanderson and Buck, 1995).

Agriculture is the dominant land use in the HL region 

(Bridges and Wilson, 2014). Despite typically ample precipitation and owing to the low water-retention properties of 

the sandy soil, farmers use agricultural methods that include 

extensive center pivot irrigation and the application of watersoluble fertilizer, also known as fertigation. Roadcap and 

others (2002) discuss the timing and type of fertilization, 

with anhydrous ammonium being physically added to soils 

in March, April, and June; and liquid fertilizer being added 

in May, June, and the beginning of July through fertigation. 

Type and amount of fertilizer application can be spatially and 

temporally variable dependent on precipitation, crop rotations, seasonality, and water table fluctuations. The IVWA, 

which comprises all of Mason County and four townships in 

Tazewell County, compiles data on estimated irrigation usage 

and precipitation within the HL region (Imperial Valley Water 

Authority, 2019). Estimated irrigation usage varies from year 

to year driven mostly by weather conditions, but also by crop 

types and number of acres planted. According to Bowman 

and others (1991), irrigation wells in this area normally range 

from 75 to 150 ft deep and regularly produce 1,000 gallons per 
4 Use of Real-Time Sensors to Temporally Characterize Water Quality in Mason County, Illinois, 2017–19

Sugar Creek

Middle Fork

Sugar Creek

Prairie

Creek

Central

Ditch

Red

Ditch

Copperas

Creek

Main Ditch

South Fork

Panther Creek

Fork

North

Kickapoo

Creek

Mud Creek

Clary Creek

Walnut Creek

Prairie

West Fork Sugar Creek

Lake Fork

Kickapoo

Creek

Oak

Creek

Illinois

River

Sangamon

Mackinaw

Quiver

Creek

Crane

Creek

Creek

MasonTazewell

Drainage

Ditch

Crane 

Creek

River

Salt

River

PEORIA WOODFORD

MCLEAN

TAZEWELL

FULTON

MASON

LOGAN

SCHUYLER

DE WITT

MENARD

CASS

MACON

SANGAMON

Havana

Lowlands

Research

area

Topeka

90°30' 90°20' 90°10' 90°00' 89°50' 89°40' 89°30' 89°20' 89°10'

40°50'

40°40'

40°30'

40°20'

40°10'

40°00'

0 5 10 15 20 25 MILES

0 5 10 15 20 25 KILOMETERS

Base from U.S. Geological Survey digital data, 1:100,000, and 1:2,000,000, 2005

Albers Equal-Area Conic projection 

Standard parallels 33 ̊ N. and 45 ̊ N.

Central meridian 89 ̊ W.

North American Datum of 1927 

ILLINOIS

Mahomet

aquifer

Figure 2. Havana Lowlands region and the extent of the shallow, western part of the Mahomet aquifer in west-central Illinois.
Methods 5

minute or greater. Crop production land in this area is dominated by corn and soybeans; other notable crops grown in the 

area include popcorn, pumpkins, beans, and smaller amounts 

of other specialty fruits and vegetables (U.S. Department of 

Agriculture, 2017).

Methods

Beginning March 8, 2017, the USGS in cooperation with 

the IEPA has been continuously monitoring water quality 

and water levels (one reading every 15 minutes) in a groundwater monitoring well (USGS station 401913089534501; 

hereafter referred to as the “Havana well”) in the HL region. 

Prior to the project commencing, a Quality Assurance Project 

Plan was developed with IEPA to help guide project tasks 

and ensure quality control and assurance of collected data 

(R. Cobb, Illinois Environmental Protection Agency, written 

commun., 2020). Parameters continuously monitored include 

groundwater levels, nitrate, pH, specific conductance, dissolved oxygen, and water temperature. To better understand 

water quality and hydrology, it is beneficial to collect spatially 

and temporally dense data and assess those data with physical and atmospheric data. To assess the fate and transport of 

nitrate in shallow groundwater, continuous water-quality and 

groundwater-level data were collected in the Havana well 

on the Mason State Tree Nursery. About 0.6 mile northwest 

of the Havana well, a surface-water site was established in 

Quiver Creek, and a series of piezometers was established in 

the floodplain. Instantaneous field measurements of the same 

parameters being continuously measured at the monitoring 

well were made in Quiver Creek and the associated piezometers (table 1; Gruhn and Nalley, 2020). Parameters instantaneously measured include Quiver Creek water levels, groundwater levels, nitrate, pH, specific conductance, dissolved 

oxygen, and water temperature. Data collected at sites listed 

in table 1 can be accessed through the USGS National Water 

Information System database using the USGS site number 

(U.S. Geological Survey, 2020).

These data were analyzed with local precipitation 

data and estimated irrigation usage data provided by the 

IVWA, which consists of all of Mason County, Ill., and four 

townships in Tazewell County, Ill. (Imperial Valley Water 

Authority, 2019).

Site Selection

The Mason State Tree Nursery location was chosen for 

the continuous monitoring well because of its accessibility and location within the HL region, proximity to irrigating agricultural operations, and accessible land near Quiver 

Creek. Prior to determining the location of the Havana well, 

preliminary assessment of the groundwater nitrate concentrations was conducted in four test holes at the nursery location 

using a Geoprobe. Geoprobe test holes were chosen based on 

presumed groundwater flow to Quiver Creek and proximity 

to irrigating agricultural operations. The Havana well site was 

chosen near the test hole with the highest nitrate concentration in the northeast quadrant of the Mason State Tree Nursery 

(fig. 3).

The Havana well was installed for the purpose of continuously monitoring groundwater quality and levels. The well 

was installed within 150 ft of irrigation practices and about 

0.6 mile southeast from the Quiver Creek and the piezometer 

sites (fig. 3, table 1). The well was constructed on January 11, 

2017, and the boring for the well was drilled using a 10-inch 

(in.) diameter hollow stem auger to accommodate a 4-in. well, 

which was required to deploy instrumentation down-hole. The 

well was constructed using polyvinyl chloride materials and 

was drilled to a depth of 32 ft below land surface, and a slotted 

screen was installed from 22 to 32 ft below land surface. The 

well was screened at this depth based on the findings of Kelly 

(1997), where high nitrate concentrations were measured in 

groundwater at depths between 20 and 33 ft.

A surface-water measurement site was established (USGS 

station 05568704) in Quiver Creek, also known as the MasonTazewell Drainage Ditch, for collection of instantaneous field 

measurements of water quality and water levels in the stream. 

To assess groundwater quality and hydrology beneath Quiver 

Creek and in the floodplain, a series of piezometers were 

installed, with shallow piezometers being installed on April 4, 

2018, and the deeper piezometers being installed on May 

21, 2018. The shallow piezometers were installed by hand 

and the screened interval for the piezometers on land ranged 

from 6 to 10 ft below land surface, whereas the piezometer 

below the streambed was about 3 ft below the bed. The deeper 

piezometers were installed with a truck-mounted Geoprobe 

and screened intervals were about 17–19 ft. During visits to 

the study area, the piezometers were pumped and median field 

parameters determined using a nitrate meter and multiparameter sonde.

Continuous Groundwater Monitoring 

Instrumentation

Water levels and select water-quality properties were 

collected from March 8, 2017, to March 31, 2019. Nitrate data 

were collected with a Hach nitrate sensor with 2-millimeter 

(mm) path length. Other continuous data were collected using 

an In-Situ Aquatroll 600 multiparameter sonde equipped with 

a 30-pound per square inch-pressure transducer, luminescent 

dissolved oxygen probe, pH probe, specific conductance 

probe, and water temperature thermistor. Both instruments 

were suspended within the 10-ft screened interval of the well 

casing, with the multiparameter sonde hanging below the 

nitrate sensor. Data were recorded every 15 minutes and transmitted every 6 hours through a cellular modem delivery using 

an In-Situ Tube 300S telemetry system and processed to the 

USGS National Water Information System database.
Table 1. Information on data collection sites, Mason County, Illinois 2017–19. [USGS, U.S. Geological Survey; NAVD 88, North American Vertical Datum of 1988; NA, not applicable] USGS site number Station name Field identifier Latitude Longitude Year established Land surface elevation, in feet above NAVD 88 We

be

s

401913089534501 22N7W-28.5a (Illinois Tree Nursery) Havana Well 40°19'13.44" 89°53'44.54" 2017 489.87 401935089542001 22N07W-29.1h1 (PZ–1) PZ–1 40°19'34.65" 89°54'19.85" 2018 459.74 401934089542001 22N07W-29.1h2 (PZ–2) PZ–2 40°19'34.51" 89°54'19.86" 2018 464.88 401934089542002 22N07W-29.1h5 (PZ–2B) PZ–2B 40°19'34.51" 89°54'19.86" 2018 464.80 401933089542001 22N07W-29.1h3 (PZ–3) PZ–3 40°19'33.41" 89°54'19.98" 2018 465.44 401933089542002 22N07W-29.1h6 (PZ–3B) PZ–3B 40°19'33.41" 89°54'19.98" 2018 466.31 401931089542001 22N07W-29.1h4 (PZ–4) PZ–4 40°19'30.76" 89°54'20.19" 2018 467.92 401931089542002 22N07W-29.1h7 (PZ–4B) PZ–4B 40°19'30.76" 89°54'20.19" 2018 467.96 05568704 Mason Tazewell Drainage Ditch near Topeka, Illinois Quiver Creek 40°19'35" 89°54'20" 2017 459.74 

*Measured from top of casing. **Screened interval, in feet below top of casing.
Methods 7

!

PZ–4

PZ–3

PZ–2

PZ–1

PZ–2B

PZ–3B

PZ–4B

Mason-Tazewell Drainage Ditch

Quiver Creek

Quiver Creek

Study Area

Base map data from Google, 2016

Albers Equal-Area Conic projection

Standard parallels 45°00’ N. and 33°00' N.

Central meridian −89°00’ W.

North American Datum of 1983

0 0.1 0.2 0.3 0.4 0.5 MILE

0 0.1 0.2 0.3 0.4 0.5 KILOMETER

89°55'00" 89°54'30" 89°54'00" 89°53'30"

40°19'30"

40°19'00"

ILLINOIS

Mason County

monitoring

location Presumed groundwater flow path

Shallow piezometer and identifier—Approximately 6 to 10 feet below land surface

Streambed piezometer and identifier—Approximately 3 feet below streambed

Deep piezometer and identifier—Approximately 19 feet below land surface

Imperial Valley Water Authority precipitation gage nine

Quiver Creek measurement location

PZ–4B

PZ–4

PZ–1

Havana well

!(

!

Reconnaissance location

Site type

EXPLANATION

Mason State Tree Nursery

401913089534501

!.

401913089534501

05568704

05568704

Figure 3. Study area showing Havana well location, Quiver Creek measurement location, piezometer locations, and reconnaissance 

locations, Mason County, Illinois.
8 Use of Real-Time Sensors to Temporally Characterize Water Quality in Mason County, Illinois, 2017–19

During visits to the well, which were typically made 

every 4 to 6 weeks or as needed, the nitrate sensor and 

multiparameter sonde were checked for calibration drift and 

biological and physical fouling (the formation of unwanted 

material on a surface to the detriment of function). Fouling 

checks were conducted against calibration standards because 

the monitoring environment is disturbed once the sensors are 

removed from the well, and it takes time to return to equilibrium with the aquifer, making traditional pre- and postfouling 

checks prohibitive (Gruhn and Nalley, 2020).

Quiver Creek Surface Water and Groundwater 

Characterization

Beginning in April 2017, a nitrate sensor was used to 

make instantaneous field measurements of nitrate concentration in Quiver Creek during visits to the continuous monitoring well. After a year of collecting instantaneous nitrate data, 

it was necessary to expand the data collection to include all 

the parameters being continuously monitored at the well. 

Beginning in April 2018, water-level measurements were 

made and a Eureka Manta35 multiparameter sonde was used 

to measure specific conductance, pH, dissolved oxygen, and 

temperature in the Quiver Creek surface water.

To characterize water quality and hydrology in the 

Quiver Creek floodplain and below the streambed of Quiver 

Creek, a series of piezometers were installed in spring 2018. 

One piezometer is located beneath the streambed and paired 

with the surface-water measuring location (Quiver Creek and 

PZ–1), and the other piezometer locations have paired deep 

and shallow piezometers. One pair is in the bank of Quiver 

Creek (PZ–2 and PZ–2B), one pair is about 100 ft landward 

from Quiver Creek (PZ–3 and PZ–3B), and the final pair 

is at the edge of the floodplain near the toe of the upland 

slope about 400 ft landward from Quiver Creek (PZ–4 and 

PZ–4B) (fig. 3). Beginning in spring 2018, real-time sensors 

were used to collect instantaneous field measurements of the 

same water-quality properties monitored at the well in all the 

piezometers. During visits to the continuous monitoring well, 

the piezometers were visited and pumped, and measurements 

made every 2 minutes to determine a stable median value for 

each parameter.

Discrete Groundwater Sample Collection and 

Analysis

To verify the accuracy of the nitrate measurements within 

the aquifer, four discrete water-quality samples were collected 

from the Havana well for laboratory analysis after three well 

volumes were purged and instantaneously measured water 

quality properties in the well stabilized as specified by the 

USGS National Field Manual (U.S. Geological Survey, variously dated). The well was sampled three times for nutrients, 

nitrogen isotopes, and stable isotopes of water—one prior to 

(April 11, 2017), during (June 13, 2017), and after (October 3, 

2017) intensive agricultural practices. An additional sample 

only for nitrate analysis was collected to verify accuracy of the 

sensor on April 3, 2018, following the largest observed peak 

of nitrate concentration. All samples were processed in the 

field and filtered with a 0.45-micron filter, prior to being sent 

for laboratory analysis. In addition to samples collected at the 

monitoring well, discrete isotope samples of nitrate and water 

were also collected from some of the piezometers near Quiver 

Creek on April 4 and July 18, 2018.

Filtered nutrient samples were analyzed by the USGS 

National Water Quality Laboratory in Denver, Colorado, 

using methods described by Fishman (1993) and Patton and 

Kryskalla (2003, 2011). Analytes included total nitrogen, 

ammonia, nitrite, ortho-phosphate, and nitrate plus nitrite. 

Stable isotopes of nitrogen (nitrogen 15/14 and oxygen 18/16) 

and stable isotopes of water (hydrogen-2/1 and oxygen-18/16) 

were analyzed at the USGS Reston Stable Isotope Laboratory 

using methods described by Révész and Coplen (2008a), 

Révész and Coplen (2008b), and Coplen and others (2012). 

Nitrogen isotope samples had a secondary filtration conducted 

at the USGS Reston Stable Isotope Laboratory. The results 

of laboratory analyses for nutrients, nitrogen isotopes, and 

stable isotopes are available from the USGS National Water 

Information System database (U.S. Geological Survey, 2020).

Ancillary Data

Continuous, instantaneous, and discrete data were analyzed with local estimated irrigation usage data and local precipitation data provided by the IVWA (Imperial Valley Water 

Authority, 2019). The IVWA estimated monthly irrigation 

usage data is reported in billion gallons with a lag of 1 month 

due to billing cycles, because energy consumption is used to 

estimate the amount of irrigation usage (J. Smith, Imperial 

Valley Water Authority, written commun, 2019). Irrigation 

season as used in this report is April through September based 

on the range of months during which estimated irrigation data 

are compiled by the IVWA (table 2).

The IVWA precipitation network is a spatially distributed and long-term network of rain gages in the HL region. 

Gage nine in this network is located in the Mason State Tree 

Nursery. Monthly precipitation reports, including daily rainfall 

amounts, were acquired for March 2017–19. Monthly totals 

for gage nine are summarized in figure 4.
Table 2. Imperial Valley Water Authority’s estimated irrigation usage in gallons for 2017 and 2018.

Irrigation 

season May June July August September October 

*2017 15,830,650 1,660,423,448 22,028,339,635 17,329,618,773 14,249,738,293 6,215,554,796 *2018 6,538,132,585 15,993,401,247 19,325,900,654 8,748,607,290 1,628,977,028 233,887,871 

*Monthly values as reported lag by 1 month due to billing cycle.
10 Use of Real-Time Sensors to Temporally Characterize Water Quality in Mason County, Illinois, 2017–19

Hydrology

Potentiometric surface maps prepared for the HL region 

indicate groundwater flow is generally in the northwest direction (Walker and others, 1965; Sanderson and Buck, 1995; 

Kelly and Ray, 1999). Water-level and water-quality data 

collected for this study indicate that the rate of groundwater 

flow in the shallow aquifer may be limited in that direction 

during the irrigation season. Groundwater levels collected 

at the Havana well during the period of monitoring used for 

this report ranged in elevation from 474.41 to 470.92 ft, or 

between 15 and 19 ft below land surface, on March 11, 2019, 

and August 15, 2018, respectively. Water levels consistently 

decline through most of the irrigation season and rise during 

late winter/early spring when there is minimal irrigation and 

little to no plant uptake. In 2017 water levels were stable most 

of the fall and winter; however, in 2018 water levels began 

rising in August when there was above average rainfall, resulting in much less irrigation demands. Unlike 2017, when water 

levels stabilized going into fall and winter, water levels were 

consistently rising during fall and winter in 2018. Owing to 

the rapid infiltration rates, the water table responds quickly to 

precipitation and snowmelt events. In general, during spring, 

groundwater levels increase until evapotranspiration and 

irrigation withdrawals begin causing decreases in groundwater levels.

Water-level elevations were temporally and spatially variable across the floodplain and in Quiver Creek (fig. 5). Each 

water-level measurement made in the piezometer screened 

below Quiver Creek (PZ–1) was higher than the water level 

in the stream, indicating predominantly groundwater inflow, 

or gaining conditions, into Quiver Creek. The one exception 

was when the water level in Quiver Creek was higher than the 

water level in PZ–1, which was observed following almost an 

inch of overnight rainfall in late October 2018. Also, waterlevel measurements made in the nested piezometers (PZ–2/

PZ–2B and PZ–4/PZ–4B) indicated upwelling of groundwater. Quiver Creek and PZ–1, and PZ–2 and PZ–2B, often 

displayed very similar differences in water levels, typically 

around 0.2 ft. PZ–4 and PZ–4B showed the largest upwelling 

of water, with the deeper piezometer (PZ–4B) consistently 

having water-level elevations about 0.8 ft higher than the 

water-level elevation in the shallow piezometer (PZ–4). All 

the water levels measured from greater depths were typically 

higher than those measured from shallower locations. Between 

PZ–3 and PZ–3B, the deeper piezometer (PZ–3B) typically 

had slightly higher water levels but there was more variability 

in the difference between the shallow and the deep piezometers, with water levels sometimes being nearly the same. 

PZ–3 and PZ–3B may have other factors affecting the hydrology such as soil type and geomorphology (for example, these 

paired piezometers are near a point bar on what appears to be 

a historical meander).

According to center pivot irrigation maps for Mason 

and Tazewell Counties, a total of 2,013 fields were identified 

as using center pivot irrigation, withdrawing an estimated 

volume of water exceeding 98 billion gallons (Bridges and 

Wilson, 2014); 1,553 of these systems were in Mason County, 

Ill., representing about 135,000 acres in the county (Imperial 

Valley Water Authority, 2019). The total acreage of farmland 

in Mason County is about 290,000 acres; therefore, about 

47 percent of the farmland was utilizing irrigation in 2012 

(U.S. Department of Agriculture, 2012). In 2017, the number of center pivot irrigation systems within the IVWA was 

2,237 with an estimated 61.5 billion gallons of water removed 

from the aquifer during the irrigation season (Imperial Valley 

Water Authority, 2019). In 2018, the total number of irrigation 

systems increased to 2,252, withdrawing an estimated total 

of about 52.5 billion gallons (table 2; Imperial Valley Water 

Authority, 2019).

From March to December 2017, about 33 in. of rain fell 

at gage nine, with the highest monthly rainfall occurring in 

July (6.92 in.). From May 24 to July 10, 2017, the area was in 

drought conditions, with only 0.81 in. of rain falling in about 

a 6-week period. This led to the highest monthly irrigation 

use, with about 22 billion gallons estimated to be used in 

0

1

2

3

4

5

6

7

8

Monthly precipitation, in inches

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

2017 2018 2019

Figure 4. Monthly precipitation totals from gage nine of the Imperial Valley Water Authority precipitation 

network, March 2017–19 (Imperial Valley Water Authority, 2019).
Continuous Groundwater-Quality Data 11

June 2017. During 2018, the total precipitation was 40.26 in., 

with the highest monthly total of 6.92 in. occurring in August, 

and from January through March 2019, the total precipitation 

was 9.05 in. (fig. 4).

Continuous Groundwater-Quality Data

Continuous water-quality monitoring in the Havana well 

began on March 8, 2017. Data analysis for this report includes 

data collected from March 8, 2017, to March 31, 2019. 

Parameters that were continuously monitored were groundwater levels, nitrate, dissolved oxygen, specific conductance, 

water temperature, and pH. Continuous monitoring (one 

reading every 15 minutes) allows for the collection of a larger 

dataset in comparison to periodic or event-based sampling and 

shows relations and trends that would otherwise be difficult 

to observe.

Nitrate Concentrations

Nitrate concentrations measured with the nitrate sensor 

ranged between 14.7 and 23.2 mg/L (fig. 6). Four discrete 

water-quality samples, collected after pumping the Havana 

well, verified the accuracy of the continuously measured 

nitrate concentrations (table 3). For all the discrete samples, 

the laboratory results verified the data recorded before pumping were within the accuracy of the sensor (3 percent), except 

for the April 4, 2018, sample when there was precipitation 

that may have caused some dilution of the well water during 

the discrete sampling process. There was some variability 

in nitrate concentrations during pumping, and the variability 

slightly exceeded the accuracy of the sensor one of the four 

times the well was pumped; the remaining times, variability 

was within sensor accuracy.

Generally, nitrate concentrations were stable or slowly 

declining during the fall and winter periods, with gradual 

increases through the spring and early summer, and brief 

pulses of very high nitrate during late winter/early spring. 

During the irrigation season, nitrate can fluctuate noticeably as 

a result of the complex interactions among irrigation, fertigation, plant metabolism, and precipitation. This fluctuation was 

observed more during the 2018 irrigating season. Method and 

timing of fertilizer application may be important to discerning the cause of the fluctuations; the fluctuations could be the 

result of adjacent field operations as the duration of peaks 

vary, and the rise and fall leading up to these peaks are quite 

rapid. Small spikes in nitrate, and dilution events, are seen 

during the nonirrigating season and are associated with rain 

events, often small enough that they are not evident during the 

irrigation season when more variables are affecting the subsurface hydrology and geochemistry.

462.5

463.0

463.5

464.0

464.5

463.0

463.2

463.4

463.6

463.8

464.0

464.2

464.4

464.6

462.5

463.0

463.5

464.0

464.5

465.0

465.5

Groundwater level, in feet above the North American Vertical Datum of 1988

Apr. May June July Aug. Sept. Oct. Nov. Dec. Apr. May June July Aug. Sept. Oct. Nov. Dec.

2018 2018

461.2

461.4

461.6

461.8

462.0

462.2

462.4

462.6

462.8

463.0

463.2

EXPLANATION

EXPLANATION EXPLANATION

EXPLANATION

PZ−2

PZ−2B

PZ−4

PZ−4B

Quiver Creek

PZ−1

PZ−3

PZ−3B

Figure 5. Groundwater elevations plotted for paired piezometers. Quiver Creek surface water with piezometer 1 (PZ–1), and the 

paired shallow and deep piezometers (PZ–2 and PZ–2B, PZ–3 and PZ–3B, and PZ–4 and PZ–4B).
12 Use of Real-Time Sensors to Temporally Characterize Water Quality in Mason County, Illinois, 2017–19

Nitrate concentrations were evaluated with respect to 

groundwater levels, precipitation, and estimated irrigation 

usage for the study period (fig. 6). The relation between 

groundwater levels and nitrate were temporally variable during the monitoring period and appear to be affected by the 

method of fertilizer application and the intensity of irrigation 

usage. The nitrate concentrations were largest during relatively 

short periods of time. The two periods of greatest observed 

continuous nitrate concentrations were 23.2 and 22.4 mg/L 

on March 28, 2018, and March 11, 2019, respectively. On 

March 24–25, 2018, a total of 1.90 in. of rain was recorded 

at IVWA gage nine prior to the peak on March 28, 2018. 

Similarly, on March 9, 2019, 1.75 in. of rain was recorded 

prior to the observed peak on March 11, 2019. These elevated 

nitrate concentrations may be related to physical application 

where nitrate is directly flushed into the groundwater by precipitation and (or) snowmelt. Roadcap and others (2002) discuss physical application occurring in March, April, and June. 

Besides these shorter-lived large peaks, gradual increases in 

nitrate concentrations were observed in late winter and early 

spring. Additionally, although exact days were unknown, in 

2018, the Mason State Tree Nursery physically applied nitrate 

in fields located south and southwest of the continuous monitoring well beginning in late June through August. Two of the 

fields received four applications and one field received one 

application. Immediately after fertilizer application about a 

Table 3. Summary of discrete water-quality results from the U.S. Geological Survey National Water Quality Laboratory analysis of 

nutrients.

[mg/L, milligram per liter; <, less than; —, not measured]

Field identifier Date

Alkaline 

persulfate as 

nitrogen, 

in mg/L

Ammonia, 

in mg/L

Nitrite, 

in mg/L

Orthophosphate, 

in mg/L

Nitrate plus 

nitrite, as 

nitrogen, 

in mg/L

Continuously measured 

nitrate plus nitrite, as 

nitrogen, prior to sample 

collection, in mg/L

Havana Well 4/11/2017 20.4 <0.01 0.00705 0.0352 19.9 19.6

Havana Well 6/13/2017 21.4 0.0103 0.00433 0.0266 20.1 20.7

Havana Well 10/3/2017 19.4 <0.01 0.0054 0.0244 19.7 19.4

Havana Well 4/4/2018 — — — — 20.7 21.9

474.5

474.0

473.5

473.0

472.5

472.0

471.5

471.0

470.5

16

15

14

17

18

19

20

21

22

23

24

2017 2018 2019

Nitrate plus nitrite as nitrogen, in milligrams per liter

Groundwater level, in feet above the North American

Vertical Datum of 1988 

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

Nitrate plus nitrite as nitrogen

Groundwater level

EXPLANATION

Figure 6. Nitrate concentration and groundwater elevation in feet above the North American Vertical Datum of 1988, March 8, 2017, to 

March 31, 2019, U.S. Geological Survey station 401913089534501.
Continuous Groundwater-Quality Data 13

half inch of irrigation water is broadcast over the fertilizer (G. 

Rampley, Mason State Tree Nursery, written commun., 2019). 

On June 21–22, 2018, 1.07 in. of rain was recorded at IVWA 

gage nine and coincided with nitrate concentrations beginning 

to increase. Practices at the tree nursery may play a role in the 

more variable concentrations seen at the Havana well in the 

2018 peak irrigation season; some of this variability is driven 

by precipitation events and some does not coincide with precipitation and may be related to fertigation or irrigation.

A gradual decline in water levels is observed in spring 

2017 and 2018; although this is something that would occur 

naturally through evapotranspiration, it is presumed to be 

affected by irrigation pumping as air temperatures begin to 

rise and plants require more water for growth and productivity, placing more demands on the aquifer. Through the late 

spring and early summer of 2017 and 2018, nitrate concentrations consistently increased and typically peaked several 

weeks after the start of declining water levels. Following these 

peaks, nitrate concentrations held relatively steady or slowly 

decreased through late summer, fall, and winter.

The effect of nonfertilized water infiltrating the surface 

from a ditch near the well was observed in early July 2017 as 

a result of a 5-week drought from late May to June 2017. The 

ditch was located about 40 ft from the well and nonfertilized 

irrigation water was pumped into the ditch; the response was 

a spike in water levels in the Havana well that was observed 

within hours of the start of pumping into the ditch. The flooding of the ditch occurred from July 7–9 and the noticeable 

and rapid increase in water levels at this time was a direct 

effect of the ditch flooding. The induced flooding of the ditch 

diluted nitrate concentrations appreciably for a period of about 

2 weeks. After this 2-week period, nitrate concentrations 

returned to the gradually decreasing levels observed prior to 

dilution.

Dissolved Oxygen

Dissolved oxygen was assessed in relation to nitrate 

concentrations for the study period (fig. 7). During the period 

of monitoring, dissolved oxygen ranged from 2.3 to 7.5 mg/L. 

As mentioned in McCallen and others (2008), a dissolved 

oxygen concentration of 2.0 mg/L is a common threshold for 

denitrification to occur, and below this concentration denitrification becomes more favorable. Dissolved oxygen concentrations observed at the monitoring well during the period of 

monitoring were too high to promote denitrification, which 

leads to the persistence of nitrate in the shallow groundwater. 

Dissolved oxygen concentrations were erratic while increasing during much of the irrigation season, with concentrations 

peaking in late summer or early fall and then generally declining through the fall and winter. The lowest values were during 

late spring and early summer.

Variability in dissolved oxygen concentrations seems to 

be affected by irrigation operations. Fluctuations in dissolved 

oxygen concentrations are much more noticeable beginning in 

early spring and are especially apparent when the water table 

begins to decline. Throughout the irrigation season, dissolved 

oxygen concentrations are erratic. Generally, concentrations 

begin to stabilize in late summer and early fall, then gradually decrease throughout fall and winter. It is apparent that 

the irrigation season is affecting dissolved oxygen concentrations in the aquifer, but it is uncertain what may be driving 

this relation; this may be caused by irrigation water being 

oxygenated during pumping and delivery through the centerpivot irrigation systems. In the 2017 growing season, drought 

conditions were prevalent, with only 0.81 in. of rain recorded 

at the nearby IVWA rain gage nine over about a 6-week period 

from May 24 to July 10, where the maximum irrigation usage 

was observed during June 2017. This observation was during 

a time when some of the highest estimated irrigation withdrawals occurred; based on monthly records, June 2017 had 

the maximum estimated irrigation usage. About 1.7, 22.0, 

and 17.3 billion gallons of water were used in May, June, and 

July 2017, respectively. About 16.0, 19.3, and 8.7 billion gallons of water were used in May, June, and July 2018, respectively (Imperial Valley Water Authority, 2019; table 2).

Dissolved oxygen and nitrate usually were positively 

related, but in 2017 during the irrigation season and shortly 

after the water table began to decline the inverse was observed 

(figs. 7 and 8). During the 2018 irrigation season when water 

levels began to decline, the positive relation did not change, 

which was the opposite to what was observed in 2017 (figs. 7

and 9). The collection of empirical irrigation data with better 

temporal and spatial resolution, along with additional continuous data at the well, would help cover more environmental 

extremes, which would help in understanding what may be 

causing these fluctuations.

Flooding of the ditch near the well caused the lowest 

dissolved oxygen concentrations in the record. This unfertilized irrigation water was piped to the ditch and not delivered 

through a sprinkler system, which would presumably oxygenate the water in the process of delivery. This water was 

pumped from deeper in the aquifer where water tends to be 

anoxic (Chapelle, 1993). Kelly (1997) observed a redoxcline 

at the Mason State Tree Nursery at a depth of around 40 feet, 

which was characterized by little to no nitrate and dissolved 

oxygen. A change in the color of the sand from reddish brown 

to gray around this depth indicates that iron is being reduced, 

which occurs under conditions more reducing than nitrate 

reduction reactions (Kelly, 1997). In the process of pumping and delivering, the water was probably oxygenated, but 

likely not to the extent that a sprinkler system would oxygenate the water. Additional data covering more environmental 

extremes are needed to better understand the role of irrigation 

and precipitation and how those factors may affect the relation 

between nitrate and dissolved oxygen.
14 Use of Real-Time Sensors to Temporally Characterize Water Quality in Mason County, Illinois, 2017–19

16

15

14

17

18

19

20

21

22

23

24

2017 2018 2019

Dissolved oxygen, in milligrams per liter

2.00

3.00

4.00

5.00

6.00

7.00

8.00

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

Nitrate plus nitrite as nitrogen, in milligrams per liter

474.5

474.0

473.5

473.0

472.5

472.0

471.5

471.0

470.5

Groundwater level, in feet above the North American

Vertical Datum of 1988 

Nitrate plus nitrite as nitrogen

Dissolved oxygen

Groundwater level

EXPLANATION

Figure 7. Nitrate concentration, dissolved oxygen, and groundwater elevation in feet above the North American Vertical Datum of 

1988, March 8, 2017, to March 31, 2019, U.S. Geological Survey station 401913089534501.

Dissolved oxygen, in milligrams per liter

2.00

3.00

4.00

5.00

6.00

7.00

8.00

19.0

20.0

21.0

19.5

20.5

21.5

22.0

23.0

22.5

May June July August

2017

Nitrate plus nitrite as nitrogen, in milligrams per liter

474.5

474.0

473.5

473.0

472.5

472.0

471.5

471.0

470.5

Groundwater level, in feet above the North American

Vertical Datum of 1988 

Nitrate plus nitrite as nitrogen

Dissolved oxygen

Groundwater level

EXPLANATION

Figure 8. Nitrate concentration, dissolved oxygen, and groundwater elevation in feet above the North American Vertical Datum of 

1988, during 2017 peak irrigation period, May 1 to August 31, U.S. Geological Survey station 401913089534501.
Continuous Groundwater-Quality Data 15

Specific Conductance

Specific conductance during the period of monitoring ranged from 500 to 665 microsiemens per centimeter at 

25 degrees Celsius (μS/cm at 25 °C) and was generally stable 

until irrigation began. It appears irrigation practices may 

cause considerable variability in specific conductance through 

infiltration of irrigation water from the surface to the shallow 

groundwater. This variability is most pronounced during the 

peak irrigation season, when estimated irrigation withdrawals are the highest. Although there is variability in the data, 

there is a range from about 500 to 560 μS/cm at 25 °C that 

appears to be reflective of the system when it is not affected by 

irrigation. Specific conductance often returns to this range of 

values, especially during much of the fall and winter, but also 

during the spring, and more rarely during the summer (fig. 10). 

Irrigation can occur throughout the growing season and even 

leading into fall, but September is the last month for which the 

IVWA compiles estimated irrigation usage data (table 2).

Specific conductance variation during the nongrowing 

season was minimal and generally positively related to nitrate. 

This relation changed between the 2017 and 2018 irrigation 

season. During the growing season in 2017, especially in June 

and July, a positive relation between nitrate concentrations 

and specific conductivity was observed. This positive relation 

may be related to irrigation driving constituents, including 

nitrate, down through the soils to the shallow groundwater 

(fig. 11). Specifically, when the deeper groundwater used for 

irrigation is infused with water-soluble fertilizer, the frequency and intensity of these applications may be substantial 

in driving this relation. These positive relations were mostly 

observed during a period of drought, with a total of 0.81 in. of 

rainfall occurring from May 24 to July 10, 2017. During this 

period no daily rainfall exceeded 0.1 in., except for June 20 

when 0.46 in. of rain was recorded. The opposite relation was 

observed in 2018 when there was more substantial rainfall and 

less estimated irrigation usage (fig. 12). Additional continuous 

data that cover more environmental extremes would be helpful 

in further understanding the relation between nitrate and dissolved oxygen. Improved temporal and spatial resolution on 

irrigation usage data would also be helpful to better understand how irrigation may affect relations between these data.

Water Temperature

Water temperature during the period of monitoring ranged from 12.2 to 13.9 degrees Celsius (fig. 13). 

Temperatures fluctuated annually, with the coldest groundwater temperatures recorded in the middle of summer and the 

warmest recorded in the middle of winter. Temperature data 

were also observed to be affected by precipitation, irrigation, 

16

15

17

18

19

20

21

22

Dissolved oxygen, in milligrams per liter

2.00

3.00

4.00

5.00

6.00

7.00

May June July August

2018

Nitrate plus nitrite as nitrogen, in milligrams per liter

474.5

474.0

473.5

473.0

472.5

472.0

471.5

471.0

470.5

Groundwater level, in feet above the North American

Vertical Datum of 1988 

Nitrate plus nitrite as nitrogen

Dissolved oxygen

Groundwater level

EXPLANATION

Figure 9. Nitrate concentration, dissolved oxygen, and groundwater elevation in feet above the North American Vertical Datum of 

1988, during 2018 peak irrigation period, May 1 to August 31, U.S. Geological Survey station 401913089534501.
16 Use of Real-Time Sensors to Temporally Characterize Water Quality in Mason County, Illinois, 2017–19

474.5

474.0

473.5

473.0

472.5

472.0

471.5

471.0

470.5

Groundwater level, in feet above the North American

Vertical Datum of 1988 

16

15

14

17

18

19

20

21

22

23

24

2017 2018 2019

Specific conductance,

in microsiemens per centimeter at 25 degrees Celsius

500

520

540

560

580

600

620

640

660

680

Nitrate plus nitrite as nitrogen, in milligrams per liter

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

Nitrate plus nitrite as nitrogen

Specific conductance

Groundwater level

EXPLANATION

Figure 10. Nitrate concentration, specific conductance, and groundwater elevation in feet above the North American Vertical Datum 

of 1988, March 8, 2017, to March 31, 2019, U.S. Geological Survey station 401913089534501.

19.0

20.0

21.0

19.5

20.5

21.5

22.0

23.0

22.5

May June July August

2017

500

520

540

560

580

600

620

640

660

680

Specific conductance,

in microsiemens per centimeter at 25 degrees Celsius

Nitrate plus nitrite as nitrogen, in milligrams per liter

474.5

474.0

473.5

473.0

472.5

472.0

471.5

471.0

470.5

Groundwater level, in feet above the North American

Vertical Datum of 1988 

Nitrate plus nitrite as nitrogen

Specific conductance

Groundwater level

EXPLANATION

Figure 11. Nitrate concentration, specific conductance, and groundwater elevation in feet above the North American Vertical Datum 

of 1988, during 2017 peak irrigation period, May 1 to August 31, U.S. Geological Survey station 401913089534501.
Continuous Groundwater-Quality Data 17

16

15

17

18

19

20

21

22

500

520

540

560

580

600

620

640

660

680

May June July August

2018

Specific conductance,

Nitrate plus nitrite as nitrogen, in milligrams per liter

in microsiemens per centimeter at 25 degrees Celsius

474.5

474.0

473.5

473.0

472.5

472.0

471.5

471.0

470.5

Groundwater level, in feet above the North American

Vertical Datum of 1988 

Nitrate plus nitrite as nitrogen

Specific conductance

Groundwater level

EXPLANATION

Figure 12. Nitrate concentration, specific conductance, and groundwater elevation in feet above the North American Vertical Datum 

of 1988, during 2018 peak irrigation period, May 1 to August 31, U.S. Geological Survey station 401913089534501.

16

15

14

17

18

19

20

21

22

23

24

2017 2018 2019

Temperature, in degrees Celsius

12.00

12.20

12.40

12.60

12.80

13.00

13.20

13.40

13.60

13.80

14.00

Nitrate plus nitrite as nitrogen, in milligrams per liter

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

474.5

474.0

473.5

473.0

472.5

472.0

471.5

471.0

470.5

Groundwater level, in feet above the North American

Vertical Datum of 1988 

Nitrate plus nitrite as nitrogen

Temperature

Groundwater level

EXPLANATION

Figure 13. Nitrate concentration, temperature, and groundwater elevation in feet above the North American Vertical Datum 

of 1988, March 8, 2017, to March 31, 2019, U.S. Geological Survey station 401913089534501.
18 Use of Real-Time Sensors to Temporally Characterize Water Quality in Mason County, Illinois, 2017–19

or snowmelt. The effect of precipitation and irrigation was 

especially noticeable during the growing season. During the 

2018 growing season, temperature changes coincided with 

groundwater-level declines, indicating that temperature is 

linked to irrigation intensity. A possible explanation for this 

is the presumably colder irrigation water being pulled from 

deeper within the aquifer and delivered at the surface with a 

portion of it rapidly infiltrating and returning to the shallow 

groundwater.

No general relation between nitrate and temperature was 

visually observed. Small variations in temperature are interpreted to be the result of large inputs from either precipitation, irrigation, or snowmelt. Many of the variations can be 

associated with rainfall; however, temperature spikes at other 

times in the record were not so readily defined by precipitation events, with these temperature variations most probably 

attributed to irrigation practices. Temperature variations were 

much more subdued during the fall and winter months, when 

there were less precipitation and irrigation, and became more 

dynamic in late winter driven by snowmelt, precipitation, and 

warming air temperatures.

pH

Values for pH ranged from 7.1 to 8.0 standard pH units 

during the period of monitoring (fig. 14). Although pH showed 

considerable variability during the period of record, there were 

no obvious relations with irrigation or precipitation; however, 

there does tend to be more fluctuation in pH during times 

when there is irrigation usage. The flooding of the ditch in 

July 2017 produced a noticeable decrease in pH, and similar 

decreases were also observed when groundwater levels began 

to decrease in 2017 and 2018.

Larger and smaller scale fluctuations of pH generally 

related positively with nitrate; however, at times this relation 

was negative, and at other times there did not seem to be any 

relation between the two parameters. Considering the small 

range of pH values, it does not appear pH is a substantial factor in this system especially regarding nitrate dynamics.

16

15

14

17

18

19

20

21

22

23

24

2017 2018 2019

pH, in standard units

7

7.2

7.4

7.6

7.8

8

8.2

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

Nitrate plus nitrite as nitrogen, in milligrams per liter

474.5

474.0

473.5

473.0

472.5

472.0

471.5

471.0

470.5

Groundwater level, in feet above the North American

Vertical Datum of 1988 

Nitrate plus nitrite as nitrogen

pH

Groundwater level

EXPLANATION

Figure 14. Nitrate, pH, and groundwater elevation in feet above the North American Vertical Datum of 1988, March 8, 2017, to 

March 31, 2019, U.S. Geological Survey station 401913089534501.
Characterization of Water Quality in Quiver Creek Stream and Floodplain 19

Characterization of Water Quality in 

Quiver Creek Stream and Floodplain

During visits to the Havana well, instantaneous field 

measurements of water quality and water levels were made 

in the Quiver Creek surface water. From April 11, 2017, to 

March 31, 2019, 29 instantaneous measurements of nitrate 

concentrations ranged from less than 0.1 to 6.4 mg/L. 

Beginning on April 3, 2018, instantaneous measurements of 

dissolved oxygen, specific conductance, water temperature, 

and pH were made, with instantaneous water levels started 

on April 4, 2018. During the time when these measurements 

were being made, dissolved oxygen concentrations ranged 

from 7.4 to 12.2 mg/L, specific conductance ranged from 470 

to 711 μS/cm at 25 °C, water temperature ranged from 1.7 

to 22.0 degrees Celsius, pH ranged from 6.1 to 8.1 standard 

units, and water-level elevations ranged from 461.35 to 

464.22 ft (table 4).

Nitrate concentrations in Quiver Creek were much less 

than those observed in groundwater at the Havana well. 

Concentrations in Quiver Creek were typically low during irrigating season and increased during winter, early spring, after 

precipitation events, and in general during periods of recharge 

(fig. 15). Seasonally, this is similar to what was found in 

Murphy and others (2013); however, in Layne Hydro (2012), 

the flow of Quiver Creek was documented to consist of almost 

entirely of groundwater. Data collected for this study indicate 

that under base-flow conditions the source water is the deeper 

groundwater with lower concentrations of nitrate, and during 

nonbase-flow conditions more of the shallow groundwater 

with higher concentrations of nitrate is contributing to Quiver 

Creek. In 2017, a drier year with more estimated irrigation 

usage, the low nitrate concentrations continued into winter, 

but in 2018, when there was above average rainfall and less 

estimated irrigation usage, there were higher concentrations 

of nitrate in Quiver Creek during the same time period. Some 

of the highest nitrate concentrations were measured following rain events and snowmelt; for example, on October 31, 

2018, when a nitrate concentration of 1.9 mg/L was measured, 

about 1 in. of rain was recorded at precipitation gage nine 

that evening, and the next morning the water level in Quiver 

Creek had risen about 0.5 ft and a nitrate concentration of 

5.7 mg/L was measured. Water levels in Quiver Creek and in 

PZ–1 were measured from the measuring point (top of casing) 

on PZ–1. When PZ–1 was overtopped, water levels could not 

be measured because the water was too deep to wade into. 

This was the only time that water levels in Quiver Creek were 

observed to be higher than those in the piezometer beneath the 

bed of the stream (fig. 5). During recharge events like the one 

on October 31, 2018, shallow groundwater with high nitrate 

concentration was readily entering the surface water. It is also 

important to note that this recharge event was during a period 

of little to no irrigation. This relation between groundwater 

levels, Quiver Creek water levels, and increased nitrate concentrations is difficult to understand without higher frequency 

data in the piezometers and the Quiver Creek surface water. 

Water-quality and hydrologic data indicate that during much of 

the peak irrigation season, Quiver Creek predominantly consists of the deeper low-nitrate Mahomet aquifer water, whereas 

during other periods there is more substantial input from the 

higher-nitrate shallow groundwater, especially during recharge 

periods, whether from precipitation and (or) snowmelt.

Observed ranges of nitrate, specific conductance, dissolved oxygen, temperature, pH, and water-level elevations 

for Quiver Creek and the series of piezometers are shown in 

table 4. In general, the shallow piezometers showed nitrate 

concentrations decreased towards Quiver Creek (fig. 16). 

Concentrations were consistently high at PZ–4 at the toe of 

the upland slope towards the nursery. Nitrate concentrations in 

PZ–4 ranged from 16.4 to 22.7 mg/L. PZ–3 showed considerably less nitrate than PZ–4, with nitrate concentrations 

ranging from 7.6 to 13.6 mg/L. PZ–2 was in the Quiver Creek 

streambank and concentrations ranged from less than 0.1 to 

8.3 mg/L. PZ–1, beneath the Quiver Creek streambed, never 

had a detection of nitrate consistently measuring less than 

0.1 mg/L.

The deeper piezometers (PZ–2B, PZ–3B, and PZ–4B) 

were paired with the shallow piezometers as closely as possible to assess vertical variation in groundwater levels and 

water quality. The deeper piezometers typically showed much 

less nitrate except for PZ–3B, which showed elevated nitrate 

concentrations ranging from 6.3 to 9.4 mg/L. PZ–2B, located 

deeper in the bank, did not have detectable nitrate concentrations and always measured less than 0.1 mg/L. PZ–4B also 

consistently measured less than 0.1 mg/L, except on one occasion when 0.1 mg/L was measured.

PZ–3 and PZ–3B showed little to no variation in waterlevel differences and was also the only location that consistently showed nitrate concentrations present in the shallow and 

deep piezometers, although concentrations were consistently 

lower in the deeper piezometer. This location appears to be 

near the point bar of a historical meander or former oxbow, 

presumed to be predevelopment. It is possible that this area 

readily receives greater amounts of recharge from the nearby 

and regularly disturbed soil (crop cultivation) and (or) there 

is less organic material in this area and accumulated sand 

deposits have created a preferential flow path capable of carrying nitrate deeper in the aquifer before any denitrification 

or substantial mixing can occur. The other deeper piezometers 

(PZ–1, PZ–2B, and PZ–4B) did not have detections of nitrate 

above 0.1 mg/L, which may be because of vertical hydraulic 

controls limiting the downward movement of groundwater. These wells may also experience natural denitrification, 

presumably from mixing with the deeper anoxic groundwater 

with very little to no nitrates. Roadcap and others (2002) also 

documented a combination of mixing and partial denitrification at the Mason State Tree Nursery by sampling a nest of 

multilevel sampling wells. Low levels of nitrate at depth have 

been observed numerous times in unconfined aquifers because 

conditions at depth promote denitrification (Kelly, 1997). 

Kelly (1997) identified a redoxcline and below it the redox 
Table 4. Range of instantaneous measurements of water quality and water-level elevations, April 11, 2017, to February 5, 2019, Mason County, Illinois[mg/L, milligram per liter; μS/cm, microsiemen per centimeter; °C, degrees Celsius; NAVD 88; North American Vertical Datum of 1988; <, less than] Field identifier Number of instantaneous readings Range of nitrate as nitrogen, in mg/L Range of dissolved oxygen concentrations, in mg/L Range of specific con- ductance, in μS/cm Range of water temperature, in °C Range of pH

in pH units 

Quiver Creek 28 <0.1–6.4 *7.4–12.2 *470–711 *1.7–22.0 *6.1–8.1 PZ–1 9 <0.1 <0.1–1.0 560–648 10.5–15.2 7.1–7.7 PZ–2 9 <0.1–8.3 <0.1–1.9 549–639 12.0–15.9 6.9–7.3 PZ–2B 7 <0.1 <0.1–0.1 580–648 13.3–14.4 7.2–7.7 PZ–3 9 7.6–13.6 0.2–0.5 612–728 6.9–20.4 6.4–7.3 PZ–3B 7 6.3–9.4 1.8–6.0 520–567 12.0–17.1 6.9–7.4 PZ–4 9 16.4–22.7 2.2–6.2 571–725 8.5–17.4 6.4–7.3 PZ–4B 7 <0.1–0.1 <0.1–0.6 623–636 12.4–15.1 6.5–7.2 

*16 instantaneous readings. **15 instantaneous readings.
Characterization of Water Quality in Quiver Creek Stream and Floodplain 21

conditions are more reducing, with anoxic water favoring 

denitrification. During the installation of PZ–2B, visual observation between 8 and 12 ft indicated the redoxcline defined 

at the tree nursery by Kelly (1997) was also existent in the 

floodplain but at a much shallower depth.

Instantaneous measurements of nitrate concentrations 

also varied temporally. Quiver Creek nitrate concentrations 

decreased going into summer and then were consistently low 

in late summer and into the fall, except in 2018 when there 

was above average rainfall and concentrations were higher in 

the fall and winter than in 2017. Concentrations at PZ–2 and 

PZ–3 were low in spring, increased through summer, and then 

began to decline as Quiver Creek concentrations were increasing. Concentrations at PZ–3B and PZ–4 were similar to those 

in Quiver Creek, starting with higher nitrate concentrations 

that decreased going into summer and then began increasing 

going into fall and winter. These concentrations would also 

indicate limited lateral discharge of the shallow groundwater 

system to Quiver Creek. Farther from the stream, such as the 

location of PZ–4, nitrate is similar to what is observed in the 

summer at the continuous monitoring well (Havana well). 

The slowly decreasing nitrate concentrations may result from 

the deeper irrigation water, when fertilizer is not added, being 

delivered at the surface and moving to the water table, slowly 

diluting the nitrate signal throughout the summer. If fertigation 

was occurring and nitrate was being delivered in solution, it 

would be expected to be more evenly broadcast than physical 

application, which would lead to a more subtle response in the 

data, and this is what tends to be observed in the middle of the 

summer at some locations.

15

15.5

16

16.5

17

17.5

18

18.5

19

24

23

22

21

20

19

18

17

16

15

14

2017 2018 2019

Havana well nitrate plus nitrite as nitrogen, in milligrams per liter

Quiver Creek nitrate plus nitrite as nitrogen, in milligrams per liter

Havana well water level below land surface, in feet

0

1

2

3

4

5

6

7

8

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

EXPLANATION

Havana well nitrate plus nitrite as nitrogen

Havana well water level

Quiver Creek nitrate plus nitrite as nitrogen

Figure 15. Continuous groundwater levels and groundwater nitrate concentrations at U.S. Geological Survey station 

401913089534501, plotted with instantaneous measurements of nitrate in Quiver Creek surface water.

0

5

10

15

20

25

Nitrate plus nitrite as nitrogen, 

in milligrams per liter

Apr. May June July Aug. Sept. Oct. Nov. Dec.

2018

EXPLANATION

Quiver Creek

PZ−2

PZ−3

PZ−4

Figure 16. Nitrate concentrations in Quiver Creek and the 

shallow piezometers in the floodplain (PZ–2, PZ–3, and PZ–4) 

April 4, 2018, to December 19, 2018.
22 Use of Real-Time Sensors to Temporally Characterize Water Quality in Mason County, Illinois, 2017–19

Isotopic Characterization

In 2017, three discrete isotope samples were collected at 

the Havana well and analyzed for stable isotopes of groundwater and nitrogen. These discrete samples were collected in 

2017, before, during, and after intensive agricultural operations. Additionally, in 2018, the Quiver Creek floodplain was 

sampled for these isotopes. Samples were collected from PZ–3 

in spring 2018, and from PZ–2, PZ–3, and PZ–4 on July 18, 

2018. Stable isotopes of groundwater from the Havana well 

and the piezometers plot close to one another and overlap 

on the Global Meteoric Water Line, indicating the source of 

recharge is the same for the well and the piezometers (fig. 17; 

Craig, 1961). A more refined local or regional meteoric water 

line may be beneficial in better understanding how these 

samples plot, and if evaporation of irrigation water is playing 

a role in isotopic composition of the groundwater at different 

times of the year. Results for nitrogen isotope samples had low 

isotope ratios, which is indicative of little denitrification and 

of chemical fertilizer being the source of the elevated nitrate 

levels (table 5; Kendall and others, 2008). The nitrate isotopes 

collected in the well and the piezometers plot in the typical 

range for ammonium in fertilizer (fig. 18; Kendall and others, 

2008). Although isotope samples in the floodplain indicate 

chemical fertilizer as the source of the nitrate, there were not 

enough samples to confirm this finding. Additionally, samples 

for analyses of chloride concentrations and major ions would 

be helpful in further understanding this system, especially 

regarding mixing. According to Dubrovsky and others (2010), 

as much as 100 percent denitrification has been documented in 

riparian zones; however, nitrate concentrations and associated 

isotopic values measured in the Quiver Creek floodplain may 

be the result of several mechanisms. The primary mechanism 

is believed to be the upwelling and mixing of deeper lowernitrate groundwater with the higher-nitrate shallow groundwater as evidenced by water levels measured in the piezometers 

and large differences in measured nitrate and dissolved oxygen 

concentrations between the deep and shallow piezometers. 

Anoxic conditions at depth would favor denitrification and 

deeper groundwater tends to be more reduced, potentially 

having electron donors that would favor the reduction of 

oxygen and nitrate in the shallow higher-nitrate groundwater 

(McCallum and others, 2008). Other mechanisms that may be 

involved are microbial denitrification and plant uptake.

−60

−50

−40

−30

−20

−10

−9 −7 −5 −3

delta 2-Hydrogen of water, in parts per thousand relative to

Vienna Standard Mean Ocean Water

delta 18-Oxygen of water, in parts per thousand relative to

Vienna Standard Mean Ocean Water

EXPLANATION

Global Meteoric Water Line

Havana well

Piezometers

Figure 17. Stable isotopic composition of water from Havana well 

and Quiver Creek piezometers plotted on the Global Meteoric Water 

Line (Craig, 1961), Mason County, Illinois.
Table 5. Summary of results of stable isotopes of water and nitrate isotopes in groundwater, Mason County, Illinois.[M, month; D, day; YYYY, year; mg/L, milligram per liter; H, hydrogen; O, oxygen; N, nitrogen]

Field identifier Date (M/D/YYYY) Total nitrogen, in mg/L Delta H-2/H-1 water, unfiltered per million Delta O-18/O-16, water, unfiltered per million Delta N-15/N-14, nitrate water, filtered per million 

Havana Well 4/11/2017 20.4 −39.8 −6.16 1.90 Havana Well 6/13/2017 21.4 −38.1 −6.32 3.28 Havana Well 10/3/2017 19.4 −37.1 −6.03 3.36 PZ–3 4/4/2018 9.12 −35.2 −5.71 4.62 PZ–2 7/18/2018 8.49 −41.2 −6.69 3.34 PZ–3 7/18/2018 13.26 −38.4 −6.28 5.22 PZ–4 7/18/2018 17.25 −37.7 −6.19 2.15 
24 Use of Real-Time Sensors to Temporally Characterize Water Quality in Mason County, Illinois, 2017–19

Summary and Conclusions

The persistence of high nitrate concentrations in shallow groundwater has been well documented in the glacial 

aquifer of Mason County, Illinois. Studies have indicated that 

during low-flow periods, groundwater may be a substantial 

contributor to surface-water nitrogen loads, which can affect 

the Gulf of Mexico Hypoxic Zone. Understanding the fate and 

transport of nitrate in an area with persistently high nitrates 

will help better understand the potential contributions from 

groundwater to surface water.

From March 8, 2017, to March 31, 2019, continuous 

monitoring of groundwater level, continuous nitrate, dissolved 

oxygen, specific conductance, water temperature, and pH at 

U.S. Geological Survey station 401913089534501 indicated 

there were definite relations observed at smaller and larger 

temporal scales, most notably with nitrate and water levels, 

dissolved oxygen, and specific conductance. Observations 

during smaller time scales were difficult to assess without 

additional data that cover more environmental extremes. 

Larger scale fluctuations were related to recharge events and 

irrigation. During the irrigation season, water-quality data 

were often much more variable, and this variability most often 

occurred after water levels began to decline. This is presumed 

to be from the promotion of vertical flow during irrigation 

because the deeper water delivered from the irrigation systems 

is geochemically different than that of precipitation inputs and 

shallow groundwater.

Groundwater nitrate concentrations continuously measured in the Havana well during this time ranged from 14.7 to 

23.2 milligrams per liter, whereas instantaneously measured 

nitrate concentrations in Quiver Creek ranged from 0.9 to 

6.4 milligrams per liter. During the peak irrigation season, 

nitrate concentrations in the surface water are consistently 

low; it is likely that the Quiver Creek surface water is dominated by the deeper upwelling of low nitrate, anoxic groundwater from the Mahomet aquifer, with minimal input from 

the shallow oxic, high nitrate aquifer water. During those 

periods when the system is under recharge conditions and not 

under the stress of irrigation, higher nitrate concentrations are 

observed in the Quiver Creek surface water. Data indicate that 

during the growing season, irrigation usage may be minimizing lateral groundwater flow in the shallow system, effectively 

limiting its discharge to surface water. This effect may also be 

promoting denitrification and mixing within the floodplain, 

prior to discharge events to the stream.

Manure and septic waste Ammonium Dentrification

(NH4) in

fertilizer

and rain

Soil N

NO3 fertilizer

Nitrate (NO3) in

precipitation

−10

0

10

20

30

40

50

60

70

−10 −5 0 5 10 15 20 25 30 35 40

delta 18-Oxygen in nitrate fraction, water, filtered, parts per thousand

delta 15-Nitrogen in nitrate fraction, water, filtered, parts per thousand

Havana well

Piezometers

EXPLANATION

Figure 18. Nitrate isotopes for Havana well and Quiver Creek piezometers, Mason County, Illinois. 

Boxes represent typical ranges of delta 15-nitrogen and delta 18-oxygen isotope values of nitrate from 

various sources (Kendall and others, 2008).
References Cited 25

Temporally and spatially dense irrigation data would be 

beneficial to better understand the effects of the intensity and 

timing of irrigation, and how methods of fertilizer applications 

may affect water quality and potentially, groundwater flow. 

Additional isotope data in the Quiver Creek floodplain and 

surface water at different times of the year would be helpful 

to better understand this system in relation to mixing, denitrification, and source water. Streamflow measurement data and 

better temporal resolution in the surface water (specifically, 

water-level elevation, nitrate, dissolved oxygen, and specific 

conductance data) would be particularly helpful. These data 

would allow for confident calculations of nitrate loading 

throughout the year and help understand the fate and transport 

of nitrate in groundwater and how it may temporally affect 

nitrate loads in Quiver Creek.

References Cited

Bowman, J.A., Simmons, W.F., and Kimpel, B.C., 1991, 

Irrigation in Midwest—Lessons from Illinois: Journal of 

Irrigation and Drainage Engineering, v. 117, no. 5, accessed 

March 26, 2019, at https://doi.org/10.1061/(ASCE)0733-

9437(1991)117:5(700).

Bridges, K., and Wilson, S., 2014, 2012 Center pivot irrigation in Illinois: ISWS Map Series 2014-01, 1 p., accessed 

February 27, 2020, at https://www.isws.illinois.edu/docs/

default-source/maps/2012-center-pivot-irrigation-masoncounty-illinois.pdf?sfvrsn=5b30d2c0_2.

Chapelle, F.H., 1993, Ground-water microbiology and geochemistry: New York, NY, John Wiley & Sons, Inc., 468 p.

Coplen, T.B., Qi, H., Révész, K., Casciotti, K., and Hannon, 

J.E., 2012, Determination of the δ15N and δ18O of nitrate 

in water; RSIL lab code 2900, chap. 17 of Stable isotoperatio methods, sec. C of Révész, Kinga, and Coplen, T.B. 

eds., Methods of the Reston Stable Isotope Laboratory 

(slightly revised from version 1.0 released in 2007): U.S. 

Geological Survey Techniques and Methods, book 10, 35 

p., accessed April 2020 at https://pubs.usgs.gov/tm/2006/

tm10c17/. [Supersedes version 1.0 released in 2007.]

Craig, H., 1961, Standard for reporting concentrations of 

deuterium and oxygen-18 in natural waters: Science, v. 133, 

no. 3467, p. 1833–1834. [Also available at https://doi.org/

10.1126/science.133.3467.1833.]

Dubrovsky, N.M., Burow, K.R., Clark, G.M., Gronberg, J.M., 

Hamilton, P.A., Hitt, K.J., Mueller, D.K., Munn, M.D., 

Nolan, B.T., Puckett, L.J., Rupert, M.G., Short, T.M., Spahr, 

N.E., Sprague, L.A., and Wilber, W.G., 2010, The quality of 

our Nation’s waters—Nutrients in the Nation’s streams and 

groundwater, 1992–2004: U.S. Geological Survey Circular 

1350, 174 p. [Also available at 

https://water.usgs.gov/nawqa/nutrients/pubs/circ1350.]

Fishman, M.J., ed., 1993, v. 93–125. Methods of analysis 

by the U.S. Geological Survey National Water Quality 

Laboratory—Determination of inorganic and organic 

constituents in water and fluvial sediments, U.S. Geological 

Survey Open-File Report, 217 p. [Also available at 

https://doi.org/10.3133/ofr93125.]

Gruhn, L.R., and Nalley, G.M., 2020, Continuous nitrate 

monitoring in groundwater and potential contribution to 

surface-water nitrogen loads in Mason County, Illinois: U.S. 

Geological Survey Fact Sheet 2019–3064, 4 p. [Also available at https://doi.org/10.3133/fs20193064.]

Hackley, K.C., Panno, S.V., and Anderson, T.F., 2010, 

Chemical and isotopic indicators of groundwater evolution 

in the basal sands of a buried bedrock valley in the midwestern United States—Implications for recharge, rock-water 

interactions, and mixing: Geological Society of America 

Bulletin, v. 122, p. 1047–1066, accessed October 14, 2020, 

at https://doi.org/10.1130/B26574.1.

Illinois Department of Agriculture, 2019, Illinois nutrient loss reduction strategy: accessed August 21, 2019, at 

https://www2.illinois.gov/sites/agr/Resources/NutrientLoss/

Pages/default.aspx.

Illinois Pollution Control Board, 2019a, Title 35—Procedural 

and environmental rules, section 620.410 groundwater 

quality standard for class1—Potable resource groundwater: accessed August 29, 2019, at https://pcb.illinois.gov/

documents/dsweb/Get/Document-33425/.

Illinois Pollution Control Board, 2019b, Title 35—

Procedural and environmental rules, section 611.301 

revised MCLs for inorganic chemical contaminants: 

accessed March 22, 2020, at https://pcb.illinois.gov/SLR/

IPCBandIEPAEnvironmentalRegulationsTitle35.

Imperial Valley Water Authority, 2019, Imperial Valley Water 

Authority homepage: accessed on August 21, 2019, at 

http://imperialvalleywaterauthority.org/.

Kelly, W.R., 1997, Heterogeneities in ground-water geochemistry in a sand aquifer beneath an irrigated field: Journal 

of Hydrology (Amsterdam), v. 198, no. 1–4, p. 154–176. 

[Also available at https://doi.org/10.1016/S0022-

1694(96)03316-1.]

Kelly, W.R., and Ray, C., 1999, Impact of irrigation on the 

dynamics of nitrate movement in a shallow sand aquifer: Champaign, Illinois State Water Survey, Research 

Report 128, 59 p.

Kendall, C., Elliott, E.M., and Wankel, S., 2008, Tracing 

anthropogenic inputs of nitrogen to ecosystems, chap. 12 

of Michener, R., and Lajtha, K., 2008, Stable Isotopes in 

Ecology and Environmental Science (2d ed.): Blackwell 

Publishing Ltd., p. 375–449.
26 Use of Real-Time Sensors to Temporally Characterize Water Quality in Mason County, Illinois, 2017–19

Layne Hydro, 2012, Petition to designate the Mahomet 

Aquifer as a Sole Source Aquifer: City of Champaign, 

Town of Normal, City of Urbana, University of IllinoisChampaign/Urbana, village of Savoy, City of Develan, 

Village of Mahomet, Village of Mansfield, City of Gilman.

McCallum, J.E., Ryan, M.C., Mayer, B., and Rodvang, S.J., 

2008, Missing-induced groundwater denitrification beneath 

a manured field in southern Alberta, Canada: Journal 

of Applied Geochemistry, v. 23, no. 8, p. 2146–2155, 

accessed April 2020 at https://doi.org/10.1016/

j.apgeochem.2008.03.018.

Murphy, J.C., Hirsch, R.M., and Sprague, L.A., 2013, Nitrate 

in the Mississippi River and its tributaries, 1980–2010—An 

update: U.S. Geological Survey Scientific Investigations 

Report 2013–5169, 31 p. [Also available at https://doi.org/

10.3133/sir20135169.]

Patton, C.J., and Kryskalla, J.R., 2003, Methods of analysis 

by the U.S. Geological Survey National Water Quality 

Laboratory—Evaluation of alkaline persulfate digestion 

as an alternative to Kjeldahl digestion for determination of 

total and dissolved nitrogen and phosphorus in water: U.S. 

Geological Survey Water-Resources Investigations Report 

2003–4174, 33 p. [Also available at https://doi.org/10.3133/

wri034174.]

Patton, C.J., and Kryskalla, J.R., 2011, Colorimetric determination of nitrate plus nitrite in water by enzymatic reduction, automated discrete analyzer methods: U.S. Geological 

Survey Techniques and Methods, book 5, chap. B8, 34 p. 

[Also available at https://doi.org/10.3133/tm5B8.]

Révész, K., and Coplen, T.B., 2008a, Determination of the 

δ(18O/16O) of water: RSIL lab code 489: U.S. Geological 

Survey Techniques and Methods, book 10, chap. C2, 28 p.

Révész, K., and Coplen, T.B., 2008b, Determination of the 

δ(2H/1H) of water: RSIL lab code 1574: U.S. Geological 

Survey Techniques and Methods, book 10, chap. C1, 27 p.

Roadcap, G.S., Hackley, K.C., and Hwang, H.H., 2002, 

Application of nitrogen and oxygen isotopes to identify sources of nitrate: accessed on April 22, 2020, at 

https://www.ideals.illinois.edu/bitstream/handle/2142/

55259/roadcap2002.pdf.

Sanderson, E.W., and Buck, A.G., 1995, Reconnaissance study 

of the ground-water levels in the Havana lowlands area: 

Champaign, Illinois State Water Survey Contract Report 

582, 63 p.

U.S Department of Agriculture, 2012, National 

Agricultural Statistics Service Census of Agriculture, 

Mason County, Illinois: accessed August 2020 at 

https://www.nass.usda.gov/Publications/AgCensus/2012/

index.php.

U.S. Department of Agriculture, 2017, National 

Agricultural Statistics Service Census of Agriculture, 

Mason County, Illinois: accessed April 2020 at 

https://www.agcensus.usda.gov.

U.S. Environmental Protection Agency, 2019, National 

primary drinking water regulations: accessed June 9, 2019, 

at https://www.epa.gov/ground-water-and-drinking-water/

national-primary-drinking-water-regulations.

U.S. Environmental Protection Agency, 2020, Hypoxia 101—

What is hypoxia and what causes it?: U.S. Environmental 

Protection Agency web page, accessed on February 27, 

2020, at https://www.epa.gov/ms-htf/hypoxia-101.

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 [variously paged], accessed April 2020 at 

https://pubs.usgs.gov/twri/twri9a5/.

U.S. Geological Survey, 2020, USGS water data for the 

Nation: U.S. Geological Survey National Water Information 

System database, accessed September 17, 2020, at 

https://doi.org/10.5066/F7P55KJN.

Walker, W.H., Bergstrom, R.E., and Walton, W.C., 1965, 

Preliminary report on the ground-water resources of the 

Havana region in west-central Illinois: Illinois State Water 

Survey Cooperative Ground-Water Report 3, 61 p.
 

For more information about this publication, contact:

Director, USGS Central Midwest Water Science Center

405 North Goodwin

Urbana, IL 61801

217–328–8747

For additional information, visit: 

https://usgs.gov/centers/cm-water

Publishing support provided by the

Rolla Publishing Service Center
Gruhn and Morrow—Use of Real-Time Sensors to Temporally Characterize Water Quality in Mason County, Illinois, 2017–19—Scientific Investigations Report 2020–5108

ISSN 2328-0328 (online)

https://doi.org/10.3133/sir20205108