U.S. Department of the Interior

U.S. Geological Survey

U.S. Geological Survey Open-File Report 2012-1231

Prepared in cooperation with the Florida Department of Agriculture and Consumer Services, and 

the Southwest Florida Water Management District

Sampling History and 2009–2010 Results for Pesticides and 

Inorganic Constituents Monitored by the Lake Wales Ridge 

Groundwater Network, Central Florida
Cover. Cover photographs of citrus orchards in the vicinity of Lake Wales Ridge Monitoring Network sites, courtesy of the Southwest 

Florida Water Management District.
Sampling History and 2009–2010 Results 

for Pesticides and Inorganic Constituents 

Monitored by the Lake Wales Ridge 

Groundwater Network, Central Florida

By A.F. Choquette, R. Scott Freiwald, and Carol L. Kraft

Prepared in cooperation with the Florida Department of Agriculture and 

Consumer Services, and the Southwest Florida Water Management District

U.S. Geological Survey Open-File Report 2012–1231

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2012

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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:

Choquette, A.F., Freiwald, R.S., and Kraft, C.L., 2012, Sampling history and 2009–2010 results for pesticides and 

inorganic constituents monitored by the Lake Wales Ridge Groundwater Network, Central Florida: U.S. Geological 

Survey Open-File Report 2012–1231, 19 p.
iii

Contents

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

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

Purpose and Scope ..............................................................................................................................2

Network Design and Methods............................................................................................................4

Water Quality Benchmarks .................................................................................................................6

Rainfall and Groundwater Levels.......................................................................................................6

Pesticides and Selected Inorganic Agrichemical Results from the Citrus Land-Use Wells, 

January 2009 through May 2010 ....................................................................................................8

Comparison of Results from Citrus and Non-Citrus Land-Use Wells, 2009........................................10

Well Construction ...............................................................................................................................10

Physical Properties, Inorganic Constituents, and Nutrients .......................................................10

Pesticides and Pesticide Degradates .............................................................................................12

Summary........................................................................................................................................................16

References Cited..........................................................................................................................................17

Figures

1. Map showing locations of the Lake Wales Ridge Monitoring Network wells, 

citrus land use, and vulnerable soils in the study area..........................................................3 

2. Chart showing long-term annual rainfall at Archbold Biological Station,1932

through 2010 ..................................................................................................................................7

3. Charts showing (A) annual rainfall departures from the long-term mean

compared to (B) water-levels records from two wells with shallow and 

deep water tables.........................................................................................................................7

4. Chart showing comparison between pesticide detection frequencies in 

samples from citrus and non-citrus land-use wells during the January 

and July 2009 sampling events.................................................................................................13

5. Image showing land use in the vicinity of the USGS Shallow Observation Well 

 on US Highway 27 (site NC1) and the Ridge WRAP P4 well (site NC2), on 

December 2, 2010........................................................................................................................14

Tables

1. Sampling history at the Lake Wales Ridge Monitoring Network wells, in Florida, 

1989 through May 2010 ..................................................................................................................

Separate Excel file

2. Description of the Lake Wales Ridge Monitoring Network wells in Florida, 

including well construction, land-use classification, and depth 

to water table ..................................................................................................................................

Separate Excel file
iv

3. Analytes evaluated and laboratory methods performed by the Southwest 

Florida Water Management District Water Quality Laboratory............................................4

4. Historical record of laboratory detection levels for pesticides and selected inorganic 

chemicals analyzed at the Florida Department of Agriculture and Consumer Services 

laboratories, April 1999 through May 2010. ................................................................................

 Separate Excel file

5. Spike recovery results for selected pesticides and pesticide degradates analyzed 

in groundwater samples, January 2009 through May 2010 ...................................................5

6. Summary of health benchmarks and agrichemical usage information for 

selected organic and inorganic chemicals detected in groundwater 

samples from Lake Wales Ridge Monitoring Network wells ..................................................

 Separate Excel file

7. Years and months during 1999–2009 when rainfall totals exceeded 10 inches at 

Archbold Biological Station near Venus, Florida.....................................................................8

8. Analyzed pesticides and pesticide degradates that were not detected in 

samples from citrus land-use wells, January 2009 through May 2010, and 

laboratory detection levels during this period.........................................................................8

9. Summary of pesticide, copper, arsenic, and nitrate detections in groundwater 

samples from citrus land-use wells, January 2009 through May 2010................................9

10. Summary of well construction and sampling depths at wells sampled in the 

land-use comparison .................................................................................................................10

11. Comparison of physical properties, nutrients, and inorganic constituent 

concentrations between groundwater samples from citrus and 

non-citrus land-use wells..........................................................................................................11

12. Comparison between detections and concentrations of pesticides, 

copper, arsenic, and nitrate in groundwater samples from citrus and 

non-citrus land-use wells..........................................................................................................15
v

Conversion Factors

Inch/Pound to SI

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square mile (mi2) 2.590 square kilometer (km2) 

SI to Inch/Pound

Multiply By To obtain

Length

micrometer (mm) 3.937008E-5 inch (in)

Volume

liter (L) 33.82 ounce, fluid (fl. oz)

liter (L) 0.2642 gallon (gal)

Mass

milligram (mg) 3.527396E-5 ounce, avoirdupois (oz.)

microgram (μg) 3.527396E-8 ounce, avoirdupois (oz)

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

°F=(1.8×°C)+32

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).

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

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

25°C).

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

or micrograms per liter (μg/L).
vi

Abbreviations and acronyms

< Less than

> Greater than

DEA deethylatrazine, or 2-chloro-4-isopropylamino-6-amino-s-triazine (CIAT)

DIA deisopropylatrazine, or chloro-6-ethylamino-4-amino-s-triazine (CEAT)

FDACS Florida Department of Agriculture and Consumer Services 

FDEP Florida Department of Environmental Protection

GC Guidance concentration

HA Health advisory

HAL Lifetime health advisory levels

HBSL Health-based screening level

HPLC High-performance liquid chromatography

LC/MS Liquid chromatography/mass spectroscopy

LWRM Lake Wales Ridge Monitoring

MCL Maximum contaminant level

MCPP Mecoprop; or 2-(4-chloro-2-methyl phenoxy) propionic acid

MDL Method detection level

μg/L microgram per liter

μS/cm microsiemens per centimeter

mg/L milligram per liter

N Nitrogen

NTU Nephelometric turbidity units

NPD Nitrogen phosphorus detector

P phosphorus

Pt-Co platinum-cobalt unit

PTFE Polytetrafluoroethylene (Teflon)

PQL Practical quantitation level

SWFWMD Southwest Florida Water Management District

SU Standard units

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey

VISA Very Intensive Study Area
vii

Acknowledgments

This report and the successful implementation of the 1999 through 2010 Lake Wales Ridge 

Monitoring Network were made possible through partnered efforts of many individuals at the 

Florida Department of Agriculture and Consumer Services (FDACS), the Southwest Florida Water 

Management District (SWFWMD), and the U.S. Geological Survey (USGS). Dennis Howard, 

Davis Daiker, and Keith Parmer, with the FDACS; Eric DeHaven and Roberta Starks, with the 

SWFWMD; and Gary Mahon, with the USGS, provided administrative support and technical 

guidance. The SWFWMD field sampling staff, under the direction of Jason Hust and Christopher 

Tomlinson, performed sample collection and land-use photo-documentation. Laboratory analyses were conducted under the supervision of Teresa Rygiel, Patricia Lucas, and Michael Page 

(FDACS Pesticides Laboratory), Mark Rials (SWFWMD Water Quality Laboratory), and Stacie 

Hammack (FDACS Food Laboratory). The authors appreciate permission granted by a number of 

land-owners for access to lands in the vicinity of monitoring wells in order to conduct sampling 

activities.
viii

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Sampling History and 2009–2010 Results for Pesticides 

and Inorganic Constituents Monitored by the Lake Wales 

Ridge Groundwater Network, Central Florida

By A.F. Choquette1, R. Scott Freiwald2 and Carol L. Kraft3

Abstract

The Lake Wales Ridge Monitoring (LWRM) Network was 

established to provide a long-term record of water quality of the 

surficial aquifer in one of the principal citrus-production areas 

of Florida. This region is underlain by sandy soils that contain 

minimal organic matter and are highly vulnerable to leaching of 

chemicals into the subsurface. This report documents the 1989 

through May 2010 sampling history of the LWRM Network and 

summarizes monitoring results for 38 Network wells that were 

sampled during the period January 2009 through May 2010. 

During 1989 through May 2010, the Network’s citrus land-use 

wells were sampled intermittently to 1999, quarterly from April 

1999 to October 2009, and thereafter quarterly to semiannually.

The water-quality summaries in this report focus on the period 

January 2009 through May 2010, during which the Network’s 

citrus land-use wells were sampled six times and the non-citrus 

land-use wells were sampled two times. Within the citrus land-use 

wells sampled, a total of 13 pesticide compounds (8 parent 

pesticides and 5 degradates) were detected of the 37 pesticide 

compounds analyzed during this period. The most frequently 

detected compounds included demethyl norflurazon (83 percent of 

wells), norflurazon (79 percent), aldicarb sulfoxide (41 percent), 

aldicarb sulfone (38 percent), imidacloprid (38 percent), and diuron 

(28 percent). Agrichemical concentrations in samples from the 

citrus land-use wells during the 2009 through May 2010 period 

exceeded Federal drinking-water standards (maximum contaminant 

levels, MCLs) in 1.5 to 24 percent of samples for aldicarb and 

its degradates (sulfone and sulfoxide), and in 68 percent of the 

samples for nitrate. Florida statutes restrict the distance of aldicarb 

applications to drinking-water wells; however, these statutes do not 

1

U.S. Geological Survey, Tallahassee, Florida

2

Florida Department of Agriculture and Consumer Services, Tallahassee, 

Florida

3

Southwest Florida Water Management District, Tampa, Florida

apply to monitoring wells. Health-screening benchmark levels that 

identify unregulated chemicals of potential concern were exceeded 

for norflurazon and diuron in 29 and 7 percent, respectively, of the 

2009–2010 samples. 

A comparison of agrichemical land-use effects on groundwater 

quality, determined on the basis of samples from LWRM Network 

wells in citrus and in non-citrus land-use areas, indicated significantly higher (p<0.05) concentrations of inorganic constituents 

in samples from citrus land-use areas compared to samples from 

non-citrus areas. These inorganic constituents include calcium, 

magnesium, chloride, sulfate, potassium, nitrate, aluminum, 

manganese, strontium, and total nitrogen, and also specific conductance, an indicator of total dissolved solutes in water. In addition 

to land use, including irrigation, site differences such as soils and 

groundwater reduction/oxidation conditions might have contributed 

to the differences in some of these constituents. Pesticide detections 

were primarily restricted to the citrus land-use wells, where 22 of 23 

wells yielded pesticide detections, with a median of four detected 

pesticide compounds per well. For the non-citrus land-use wells, 

typically surrounded by mixed land use including developed and 

undeveloped land, one of the eight sampled wells yielded pesticide 

detections consisting of norflurazon and its degradate, and the 

source(s) of these detections might have been active or recently 

active citrus orchards in the vicinity of this well. 

Results from the LWRM Network during the 1989 through 

May 2010 period have provided early warning of chemicals prone 

to leaching, guidance for developing or modifying chemical usage 

practices to minimize impacts to groundwater, and a mechanism 

for prioritizing State sampling of domestic wells to assure safe 

drinking-water supplies. Given the typically long time period 

(years to tens of years or longer) required to remove chemical 

contamination once it enters the groundwater system, groundwater 

monitoring is important to protect drinking-water sources as well as 

the numerous lakes in this region, which are closely connected with 

the surficial aquifer. Long-term monitoring of the LWRM Network 

is planned to continue providing early warning of potential for 

groundwater contamination, and to assess spatial and temporal 

trends in water quality resulting from changes in pesticide-use 

patterns and in land use. 
2 Pesticides and Inorganic Constituents Monitored by the Lake Wales Ridge Groundwater Network, Central Florida

Introduction 

The Lake Wales Ridge Monitoring Network, hereafter 

referred to as LWRM Network in this report, is located 

in a 700-square mile (mi2) upland citrus-growing area of 

central Florida, commonly referred to as “ridge citrus.” The 

LWRM Network was established to provide early warning 

of the occurrence of pesticides in the unconfined surficial 

aquifer, and to evaluate the spatial and temporal variability 

of groundwater quality, including agrichemicals and major 

water chemistry (Choquette and Sepulveda, 2000; Choquette 

and others, 2005). The Lake Wales Ridge, hereafter referred 

to as the Ridge, is one of the most extensively cultivated and 

productive citrus areas in Florida and the Nation. Citrus is 

one of the top agricultural crops in Florida, in terms of both 

acreage and revenue, and Florida provides about two-thirds of 

the Nation’s citrus production (Florida Agricultural Statistics 

Service, 2011). The study area (fig. 1) is located in Polk and 

Highlands Counties, Florida, which ranked first and second 

statewide, respectively, in total annual citrus production for 

the September 2009 through August 2010 crop year (Florida 

Agricultural Statistics Service, 2011). Citrus land use covers 

about 24 percent (107,500 acres) of the study area.

The LWRM Network represents one of the most 

intensively sampled regional-scale groundwater networks for 

seasonal and temporal variability in pesticides nationally. Due 

to a combination of soil type, geology, agricultural practices, 

and climate, the Ridge is particularly susceptible to leaching of 

agrichemicals into the subsurface (Wilson and others, 2002). 

Concentrations of nitrate and some of the detected pesticides 

in the groundwater underlying the Ridge have been among 

the highest observed among national groundwater sampling 

by the U.S. Geological Survey (USGS; Choquette and 

others, 2003). The Ridge lakes are closely connected with the 

surficial aquifer in this region. Samples from selected Ridge 

lakes (fig. 1) yielded concentrations of some pesticides that 

were among the highest observed in USGS national pesticide 

sampling in streams (Choquette and Kroening, 2009). In 

statewide sampling of about 4,000 Florida drinking-water 

wells during the 1990s, 89 percent of the 584 wells that 

yielded nitrate concentrations above the U.S. Environmental 

Protection Agency (USEPA) drinking-water standard were 

located in the ridge citrus region of central Florida (Graham 

and Alva, 1998). In response to elevated nitrate concentrations 

in groundwater in the region, best-management practices for 

fertilizer were developed for the ridge citrus region during the 

late 1990s and early 2000s (State of Florida, 2002; Parsons 

and Boman, 2006). 

Wells in the LWRM Network were first sampled for 

water quality in 1989. Historical water-quality records are 

available for 12 of the network wells that were sampled 

intermittently during the period 1989 to 1999 (table 1, Excel 

file). The data prior to April 1999 were collected as part 

of two Florida Department of Environmental Protection 

groundwater networks: (1) the Background Network, and 

(2) the citrus land-use VISA (Very Intensive Study Area) 

Network (Ouellette and others, 1998; Silvanima and others, 

[n.d.]; Florida Department of Environmental Protection, 

2012a, b), and as part of a regional USGS water-quality study 

(German, 1996). Results from the LWRM Network have been 

used by Federal and State regulatory agencies and pesticide 

manufacturers to evaluate leaching propensity of some citrus 

agrichemcials, to develop guidelines for minimizing leaching, 

and to direct sampling of domestic wells to confirm that 

drinking-water quality is meeting health guidelines. 

During recent years, several changes have occurred in 

citrus agrichemical usage and in land use near some Network 

wells. Bromacil use was prohibited in ridge citrus areas in 

1994 (State of Florida, 1994; Fishel, 2005). Aldicarb use 

has been restricted within Florida (State of Florida, 1997) 

and was recently phased out in citrus orchards nationally as 

of January 2012 (U.S. Environmental Protection Agency, 

2010). Temporal trends in groundwater quality sampled from 

Network wells have been related to herbicide usage patterns 

of bromacil and norflurazon during the 1998 to 2005 period 

(Choquette and others, 2005; Gilliom and others, 2006). 

A major recent impact on Florida’s citrus agriculture has 

been the onset and rapid spread of citrus greening disease 

(also known as “Huanglongbing” from its Asian origin) 

first detected in Florida in August 2005 (National Academy 

of Sciences, 2010). The spread of this disease, transmitted 

by an insect, the Asian citrus psyllid, has resulted in some 

major changes in pest management practices in Florida 

citrus orchards, including the increased use of insecticides 

(National Agricultural Statistics Service, 2006, 2010; Rogers 

and others, 2010, 2012; Rogers, 2011; Brlansky and others, 

2012). In addition, land-use changes that may be indicative of 

a change or reduction in agrichemical usage in the vicinity of 

some of the LWRM Network wells have included partial or 

extensive removal of orchards, subsequently either left fallow, 

replaced with new citrus replants, or converted to residential 

development.

Purpose and Scope 

The purpose of this report is to document the April 1999 

through May 2010 operation of the LWRM Network, 

including well characteristics, well-sampling records, 

sampling protocols, laboratory methods, and to summarize 

recent water-quality results for the January 2009 through 

May 2010 (2009–2010, in this report) period. The report 

includes a summary of 2009–2010 water-quality results for 

the citrus land-use wells, most of which were sampled five to 

six times during this period, and a comparison of groundwater 

quality between non-citrus and citrus land use based on 

samples collected during January and July 2009. The waterquality summaries for the LWRM Network’s citrus land-use 

wells focus on agricultural chemicals including pesticides, 

nitrate, copper, and arsenic. The water-quality data in the 

land-use comparison include physical properties, inorganic 

constituents (including nutrients), and pesticides. 
Introduction 3

HARDEE

DE SOTO

HIGHLANDS

POLK

OSCEOLA

27°15'

27°30'

27°45'

28°

28°15'

81°45' 81°30' 81°15'

Sebring

Avon

Park

Swim

Lake

Lake

Aurora

Lake

Wales

Lake

Leonore

Lake

Moody

Lake

Denton

Lake

Lynn

Lake

Annie

Lake

Placid

Haines

City

30

III-10

29

28

26

25

24

19

18

17 16

14

SWF-3

III-9

III-8

III-7

III-6

III-5

III-4

III-3

III-2

III-1

II-11

II-10

II-9

II-8

II-7

II-4

II-3

II-2

II-1

EXPLANATION

0 5 10 MILES

0 5 10 KILOMETERS

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

Location of study area in

Florida

Pabor

Lake

30

III-8

II-11

Citrus on vulnerable soil

Vulnerable soil

Citrus not on vulnerable soil

Lake Wales Ridge study area

Sampled lake and name

Lakes

Network Wells

 Citrus land use

 Phase 1–April 1999

 Phase 2–April 2000

 Phase 3–October 2000

 Non-Citrus land use

 Phase 4–January 2009

NC1

NC2

NC4

NC3

NC5

NC6

NC7

NC8

NC9

NC10

NC1

Figure 1. Locations of the Lake Wales Ridge Monitoring Network wells, citrus land use, and 

vulnerable soils in the study area (see Choquette and Kroening, 2009, for pesticide results from 

sampled lakes).
4 Pesticides and Inorganic Constituents Monitored by the Lake Wales Ridge Groundwater Network, Central Florida

Network Design and Methods 

The LWRM Network, in May 2010, consisted of 40 

wells, including 30 wells surrounded by active or, in some 

cases, recently discontinued citrus agriculture; and 10 

wells in non-citrus land use, of which 8 have been sampled 

(fig. 1; tables 1, 2, Excel files). The full history of sample collection at the LWRM Network wells from 1989 through May 2010 

is summarized in table 1. 

Establishment of the LWRM Network occurred in phases 

between April 1999 and October 2001 (fig. 1, table 1) and 

included existing, previously sampled monitoring wells and 

installation of new wells (Choquette and Sepulveda, 2000). The 

Network’s citrus land-use wells (fig. 1) were selected using a 

grid-based method restricted to areas of citrus land use on soils 

classified as vulnerable to leaching (Jackson and others, 1995; 

Obreza and Morgan, 2008). As of October 2001, the LWRM 

Network of citrus land-use wells was fully implemented and 

included a total of 31 citrus land-use wells sampled quarterly on 

an ongoing basis. Subsequently, in October 2009, the sampling 

frequency at 24 of the citrus land-use wells was reduced from 

quarterly to semiannually.

To compare potential effects of citrus agriculture on 

groundwater quality to non-citrus land use, 10 monitoring wells 

(fig. 1; table 2) in non-citrus areas were added to the LWRM 

Network in 2009. These wells were selected on the basis of 

a survey of existing surficial aquifer monitoring wells on the 

Ridge. Land use in the vicinity of these wells included scrub 

forest, rural residential and (or) commercial areas, and pasture 

land. During the 2009–2010 period, 8 of the non-citrus land-use 

wells and 23 of the citrus land-use wells were sampled twice 

(January 2009 and July 2009) to compare potential effects of 

citrus and non-citrus land use on groundwater quality. 

All citrus land-use monitoring wells were drilled and 

developed using methods and materials according to specifications for water-quality monitoring for organic compounds 

(Florida Department of Agriculture and Consumer Services, 

1995; Lapham and others, 1995). Well construction characteristics and approximate depths to water table are shown in table 2. 

At most Network wells, the screened interval is less than 10 to 

20 feet (ft) below the water table. The selection of pesticides 

analyzed in samples from LWRM Network wells focused on 

pesticides used in citrus land use (Choquette and Sepulveda, 

2000; Choquette and others, 2005); however, many of these 

pesticides also are used in urban and residential areas (Gilliom 

and others, 2006). 

Field personnel collected and processed water samples 

using standardized, parts-per-billion sampling protocols (Florida 

Department of Environmental Protection, 2002; Southwest 

Florida Water Management District, 2009) and made field 

measurements of water-quality properties (pH, dissolved 

oxygen, temperature, specific conductance) during sample 

collection. Field protocols included a minimum purge of three 

well volumes, stabilization of pH, conductance, temperature, 

and dissolved oxygen prior to sample collection, and turbidity 

of <10 nephelometric turbidity units (NTU). Quality-control 

and quality-assurance samples represented at least 25 percent 

of the collected samples. Field personnel also documented 

land-use changes near the LWRM Network’s citrus land-use 

wells using repeat photography in the immediate vicinity of 

the wells, approximately annually between October 1999 and 

January 2009.

Sample collection was performed by personnel from the 

Southwest Florida Water Management District (SWFWMD), 

with supplemental sampling during February 2010 performed 

by the Florida Department of Agriculture and Consumer 

Services (FDACS). All groundwater samples were analyzed for 

major dissolved inorganic constituents, nutrients, pesticides, and 

selected trace constituents at the FDACS and the SWFWMD 

laboratories. Analyses performed by the SWFWMD laboratory 

(table 3) included USEPA and standard methods (Southwest 

Florida Water Management District, 2011). SWFWMD field 

and laboratory data for the LWRM Network wells are available 

online at http://www18.swfwmd.state.fl.us/ResData/Search/

ExtDefault.aspx (Southwest Florida Water Management 

District, 2012).

Table 3. Analytes evaluated and analytical methods performed 

at the Southwest Florida Water Management District Water 

Quality Laboratory (Southwest Florida Water Management 

District, 2011).

[mg/L, milligram per liter; EPA, U.S. Environmental Protection Agency; 

μg/L, microgram per liter; SM, Standard Method of the Standard 

Method Organization; PCU, platinum cobalt units; SU, standard units; 

μS/cm, microsiemens per centimeter; NTU, nephelometric turbidity units]

Analyte Units Analytical method

Alkalinity (total) mg/L EPA 310.1

Aluminum (dissolved) μg/L EPA 200.7 (Al)

Ammonia (as N) (dissolved) mg/L SM18 4500-NH3 H No 

Distillation

Calcium (dissolved) mg/L EPA 200.7 (Ca)

Carbon-total organic (total) mg/L SM18 5310 B

Chloride (dissolved) mg/L EPA 300.0 (Chloride)

Color (dissolved) PCU SM18 2120 B

Fluoride (dissolved) mg/L SM18 4500-F C

Iron (dissolved) μg/L EPA 200.7 (Fe)

Magnesium (dissolved) mg/L EPA 200.7 (Mg)

Manganese (dissolved) μg/L EPA 200.7 (Mn)

Nitrate-nitrite (as N) 

(dissolved)

mg/L EPA 353.2 (Nitrate-Nitrite (N))

Nitrite (as N) (dissolved) mg/L EPA 353.2 (Nitrite (N))

Nitrogen-total (dissolved) mg/L ASTM D5176

Orthophosphate (P) 

(dissolved)

mg/L SM18 4500-P F (Orthophosphate)

pH, lab SU SM18 4500-H B

Phosphorus-total (dissolved) mg/L EPA 365.1 (Phosphorus -Total)

Potassium (dissolved) mg/L EPA 200.7 (K)

Silica (dissolved) mg/L SM18 4500-Si E

Sodium (dissolved) mg/L EPA 200.7 (Na)

Specific conductance, lab μS/cm SM18 2510 B

Strontium (dissolved) mg/L EPA 200.7 (Sr)

Sulfate (dissolved) mg/L EPA 300.0 (Sulfate)

Sulfide (total) mg/L SM18 4500-S D

Turbidity, lab NTU SM18 2130 B
Introduction 5

Pesticides and pesticide degradates were analyzed at the 

FDACS Pesticide Laboratory using gas chromatography with 

a nitrogen phosphorous detector (NPD), high-performance 

liquid chromatography (HPLC) (both ultraviolet/visible range 

and post-column derivatization), liquid chromatography/mass 

spectroscopy (LC/MS) and ion chromatography, and included 

both standard USEPA methods and FDACS custom methods for 

selected compounds (Rygiel, 2001, 2003; Brock and Rygiel, 2003; 

Page and Stepp, 2003). Pesticide samples analyzed for carbamates 

(aldicarb, aldicarb sulfoxone, aldicarb sulfoxide, methomyl, and 

oxamyl) were filtered in the laboratory using a 0.45-micrometer 

(μm) hydrophilic polytetrafluoroethylene (PTFE, or “Teflon”) 

syringe filter (Teresa Rygiel, Florida Department of Agriculture 

and Consumer Services, written commun., Februrary 24, 2009). 

Copper and arsenic were analyzed at the FDACS Food Laboratory using a modification of USEPA Method 200.8, in which 

germanium was used as the alternate internal standard in place of 

the standard yttrium specified for this method, because yttrium is 

known to be consistently present in samples from at least one of 

the Ridge monitoring wells (Stacie Hammack, Florida Department of Agriculture and Consumer Services, written commun., 

March 3, 2012). 

The 1999–2010 record of pesticide compounds and 

inorganic agrichemicals analyzed at the FDACS Pesticide 

and Food Laboratories and the associated laboratory 

detection levels are shown in table 4 (Excel file), including 

method detection levels (MDL) and practical quantitation 

levels (PQL). The MDL represents the level above which 

the presence of the compound is confirmed with 99 percent 

confidence. The PQL, typically three times the value of the 

MDL, is a more conservative determination of measured concentrations and represents the lowest level that the laboratory 

determines quantitative information can be routinely reported, 

based on specified limits of precision and accuracy during 

routine laboratory operating conditions. Detection levels and 

measured concentrations used in this report correspond to the 

MDLs. During the January 2009 through May 2010 period, 

laboratory detection levels for the pesticides and pesticide 

degradates detected in samples from LWRM Network wells 

ranged from 0.04 to 0.6 microgram per liter (μg/L), except for 

metalaxyl (1 to 2 μg/L).

Quality-control samples included blind replicates, field 

blanks, and spike samples (spiked with pesticide compounds 

of known concentration). For all laboratory analytes, about 

95 percent of replicate samples were within 10 percent 

relative difference, and 80 percent of replicate samples were 

within 1 percent relative difference. Mean percent recovery 

of detected pesticide compounds in spiked samples typically 

ranged from about 88 to 103 percent of expected concentration (table 5). The low mean recovery (53 percent) for 

deisopropylatrazine (DIA), the only pesticide compound with 

a mean percent recovery below 80 percent (table 5), indicates 

the potential for a lower likelihood of detection if sample 

concentrations were near the laboratory reporting limit, and 

possible underestimation of detected concentrations. 

Table 5. Spike recovery results for selected pesticides 

and pesticide degradates analyzed in groundwater samples, 

January 2009 through May 2010. 

[Pesticide degradates are shown in italics.]

Pesticide 

compound

Percent recovery

Mean Standard

deviation

MinimumMaximumNumber of 

samples

Aldicarb, dissolved 101.3 26.4 59 197 94

Aldicarb sulfoxide, 

dissolved

102.3 25.8 74 197 94

Aldicarb sulfone, 

dissolved

99.9 27.4 9 199 94

Bromacil 98.4 15.1 36 120 121

Deethyl atrazine 

(DEA)

80.3 26.3 60 260 58

Deisopropyl atrazine 

(DIA)

53.3 14.7 18 170 101

Demethyl 

norflurazon

98.6 14.5 38 118 121

Diuron 95.0 16.4 17 120 119

Imidacloprid 102.9 17.7 44 138 131

Metalaxyl 96.6 15.1 27 120 121

Norflurazon 96.4 14.9 28 117 121

Oxamyl, dissolved 93.8 26.0 42 200 94

Simazine 99.4 17.5 27 136 113

Thiazopyr monoacid 88.4 22.3 7.5 160 101

Several constituents were detected in blank samples, 

typically in low concentrations, during the 2009–2010 sampling 

period. Chloride was detected in 5 of 25 field blanks at concentrations of 0.08 to 0.13 milligrams per liter (mg/L). Copper was 

detected above the 6.2 μg/L reporting limit in 2 of 26 field blank 

samples (7.9 and 8.6 μg/L). Demethyl norflurazon was detected in 1 

of 26 field blanks at a concentration of 0.85 μg/L; this represents the 

only occurrence of any pesticide parent or degradate in field blanks 

from LWRM Network wells during the 1999 to 2010 period. 

Statistical comparisons between field and laboratory results, 

presented in data summaries, included the non-parametric Wilcoxon 

and Kruskal-Wallis tests to compare median values between groups 

using SAS software (SAS Institute, Inc., 2008). In these comparisons, censored values were assigned a rank commensurate with the 

detection level since, for a given analyte, the vast majority of the 

censored values had a constant detection level during the 2009–2010 

period. In addition, a common censoring level was applied in some 

comparisons among analytes with differing laboratory detection 

levels to avoid potential bias. When detection levels differ between 

analytes, the analytes having lower reporting levels are apt to be 

detected more often solely due to the higher resolution of detection.

At the time the LWRM Network was established, all of the 

citrus land-use wells were surrounded completely by extensive, 

actively farmed citrus. Changes in agrichemical sources and (or) 

agrichemical management have occurred over time near some of 

the Network’s citrus land-use wells and could potentially affect 

groundwater quality. Some of the apparent land-use and citrus-crop 

changes during 1999–2010 near several of the citrus land-use wells 

have included removal of citrus and conversion to either fallow 

land or to residential development; removal of old or diseased 

citrus and replanting with new nursery stock; and orchard decline 
6 Pesticides and Inorganic Constituents Monitored by the Lake Wales Ridge Groundwater Network, Central Florida

or abandonment. To document land-use changes over time near the 

citrus land-use wells, within an approximate radius of 0.25 mile 

(mi), repeat photographs were taken in the field approximately 

annually, in addition to periodic onsite land-use assessments. 

Water-Quality Benchmarks

Water-quality results from LWRM Network sampling were 

compared to several water-quality benchmarks, which serve as 

screening tools to protect drinking-water sources, minimize potential 

risk to human health, or to identify constituents that could warrant 

further study. These benchmarks (table 6, Excel file) included 

federally regulated enforceable primary drinking-water standards 

(USEPA maximum contaminant levels; MCLs), non-enforceable 

guidelines (USEPA lifetime health advisory levels (HALs) and 

USEPA secondary drinking-water regulations), USGS health-based 

screening levels (HBSLs), and Florida groundwater guidance 

concentrations (GCs), which include regulated standards and 

unregulated guidelines. 

The USEPA secondary drinking-water regulations for 

inorganic constituents relate to cosmetic and aesthetic effects of 

these constituents in drinking water (U.S. Environmental Protection 

Agency, 2011). The HBSLs were developed by the USGS in 

collaboration with the USEPA and others, and represent benchmark 

concentrations of contaminants in water that, if exceeded, may be of 

potential concern for human health (Toccalino, 2007). The HBSLs 

supplement established Federal drinking-water guidelines, typically 

focusing on analytes for which guidelines are not listed or have not 

yet been defined.

The Florida groundwater GCs (table 6) are used by FDACS 

and other State agencies to identify water-quality constituents in 

groundwater that might warrant further study and (or) additional 

sampling. The Florida groundwater GCs correspond to the Florida 

MCLs (State of Florida, 2012) and to the USEPA MCLs and HALs 

(U.S. Environmental Protection Agency, 2011); for pesticides for 

which MCLs or HALs have not been defined, the Florida GCs for 

groundwater correspond to Florida’s Groundwater Cleanup Target 

Levels associated with State statutes (University of Florida, 2005; 

State of Florida, 2005). The Florida groundwater GCs for parent 

pesticide compounds have been used as the GCs for their respective 

degradates and (or) parent-degradate sums.

Table 6 provides a summary of the human-health benchmarks 

for nitrate and selected pesticides and inorganic agrichemicals 

detected in groundwater samples from the LWRM Network wells, 

the USEPA classification of cancer risk, and a brief description of 

the pesticide uses. The difference between the Florida groundwater 

GC and the HBSL for norflurazon (table 6) reflects an adjustment 

incorporated into the HBSL to account for possible cancer 

risk—norflurazon has been classified as a possible carcinogen 

(U.S. Environmental Protection Agency, 2011).

Rainfall and Groundwater Levels

Annual rainfall totals during the 1999–2010 period of 

sampling of the LWRM Network included several notably wet and 

dry years compared to the 1932 through 2010 rainfall record at 

Archbold Biological Station in Venus, Fla. (fig. 2; Nancy Deyrup, 

Archbold Biological Station, written commun., May 5, 2003; 

National Oceanic and Atmospheric Administration, 2010). Archbold 

Biological Station has one of the longest climate-monitoring records 

on the Ridge. Annual rainfall from 1999 to 2009 ranged from 27 to 

72 in. (fig. 2). Convective storms as well as tropical storm systems 

and hurricanes are sources of intense rainfall in this region and can 

result in substantial spatial variability in precipitation amounts. 

Departures of annual rainfall from the long-term mean 

exceeded 30 percent during 5 years between 1999 and 2010, 

(fig. 3A), including 2002 and 2004, which were much wetter than 

average, and 2000, 2007, and 2009 which were much drier than 

average (fig. 2). During the late summer and early fall of 2004, the 

paths of three major hurricanes crossed the northern part of the study 

area (Bossak, 2004).

Some monthly totals at Archbold Biological Station during 

the 1999 through 2009 period were extremes for the 1932 through 

2009 period of record. Totals for July 2001 and August 2006 were 

the highest rainfall on record for these months; and August 2000, 

February 2001, April 2006, and November 2008 totals were the 

lowest on record for these months. During 1999 through 2009, 

monthly rainfall totals were more than 10 inches (in.) during 14 

months and exceeded 17 in. during 4 months (table 7), all of which 

occurred during the typical rainy season, June through September.

Important factors influencing groundwater levels in the 

surficial aquifer on Lake Wales Ridge are rainfall and groundwater 

pumpage (Yobbi, 1996; Spechler and Kroening, 2007). Water 

levels fluctuate seasonally in response to recharge from rainfall, 

evapotranspiration, lateral discharge of water to lakes and streams, 

and downward leakage of water to underlying aquifers. In the 

surficial aquifer underlying the Ridge, the water table is about 

10- to 90-ft higher than the potentiometric surface of the underlying 

intermediate aquifer and Upper Floridan aquifer, indicating the 

potential for downward leakage (Yobbi, 1996). Figure 3B shows a 

time-series of water levels from January 1998 through January 2010 

at two of the Network wells (in the surfical aquifer), compared to 

the 1998 through 2009 annual departures from long-term rainfall. 

Spechler and Kroening (2007) noted record or near record lows in 

2001 for groundwater levels at some wells in the surficial aquifer in 

Polk County.

Patterns of water-level responses to recharge from rainfall 

can vary with depth to water table as illustrated in figure 3B by 

comparing the water levels of the shallow well (North Lake Patrick, 

site 1, fig. 1) and the deep well (P-7 Watertank, site 30, fig. 1). 

During the 1998 through 2010 period, the shallow well (water-table 

depth about 25 to 30 ft) shows higher frequency, smaller magnitude 

water-level fluctuations compared to the lower frequency, smoother 

trend in the deeper well (water-table depth about 100 to 110 ft). 

The thicker unsaturated zone at the deeper well likely dampens the 

influence of short-term recharge events and results in a longer lag 

time in water-level response to recharge, especially following a dry 

periods. 
Introduction 7

90

80

70

60

50

40

30

20

10

0

Rainfall, in inches

Year

1932–2010 mean: 53.3 inches

Blank space = data missing

1932

1934

1936

1938

1940

1942

1944

1946

1948

1950

1952

1954

1956

1958

1960

1962

1964

1966

1968

1970

1972

1974

1976

1978

1980

1982

1984

1986

1988

1990

1992

1994

1996

1998

20002002

2004

2006

2008

2010

Figure 2. Long-term annual rainfall at Archbold Biological Station,1932 through 2010 (Nancy Deyrup, 

Archbold Biological Station, Venus, Florida, written commun., May 5, 2003; National Oceanic and 

Atmospheric Administration, 2010). Red bars indicate annual totals less than 38 inches, and dark blue 

bars indicate annual totals exceeding 65 inches.

25

20

15

10

5

0

–5

–10

–15

–20

–25

–30

Departure from 

mean rainfall, in inches

Year

+36%

–36%

–49%

–30%

+24%

%, departure expressed as percent

of the 1932–2010 annual mean

A

90

92

94

96

98

100

102

104

106

108

110

25

27

29

31

33

35

37

39

41

43

P7 Watertank well: depth to water table, in

feet below land surface

North Lake Patrick well: depth to water table, in

feet below land surface

B

North Lake Patrick well

P7 Watertank well

Gaps indicate missing data

EXPLANATION

Year

1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009

1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009

2010

Figure 3. (A) Annual rainfall departures from the long-term mean compared to (B) water-levels records from 

two wells with shallow and deep water tables. Annual rainfall is from Archbold Biological Station and the mean 

corresponds to the period 1932 through 2010 (Nancy Deyrup, Archbold Biological Station, Venus, Florida, written 

commun., May 5, 2003; National Oceanic and Atmospheric Administration, 2010). The well-screen interval is at a depth 

of 35 to 55 feet below land surface at the North Lake Patick well, and 125 to 145 feet at the P7 Watertank well.
8 Pesticides and Inorganic Constituents Monitored by the Lake Wales Ridge Groundwater Network, Central Florida

Table 7. Years and months during 1999–2009 when rainfall 

totals exceeded 10 inches at Archbold Biological Station 

near Venus, Florida (National Oceanic and Atmospheric 

Administration, 2010).

Year Month Rainfall (inches)

1999

June 10.89

August 14.31

2001 July 17.27

September 11.55

2002

June 18.66

July 11.05

August 10.80

2003 June 11.04

2004 August 17.88

September 12.54

2005 June 13.89

2006 August 20.05

2008 July 12.06

August 12.33

Pesticides and Selected Inorganic 

Agrichemical Results from the Citrus 

Land-Use Wells, January 2009 through 

May 2010

Samples from the LWRM Network’s citrus land-use wells 

during the period January 2009 through May 2010 were analyzed 

for the full suite of major and minor water chemistry and selected 

agricultural chemicals, as described under the section “Network 

Design and Methods.” The results summarized in this section 

focus on a subset of the analyzed constituents, and include parent 

and degradate pesticides, nitrate, copper, and arsenic. Three of 

the sampling events (April 2009, October 2009, and April 2010) 

included all of the citrus land-use wells. In two sampling events 

(January 2009 and July 2009), the number of citrus land-use 

wells sampled was reduced from 30 to 23 (table 1) to accomodate 

the sampling of the additional non-citrus wells for the land-use 

study. In February 2010, six citrus land-use wells were sampled to 

maintain a quarterly sampling record for these wells (table 1). 

During January 2009 through May 2010, 24 of the 37 pesticide compounds (28 parent pesticides and 9 degradates) analyzed 

in samples from citrus land-use wells were not detected (table 8). 

Water-quality results for pesticides, nitrate, arsenic, and copper 

during this period are shown in table 9. A total of 13 pesticide 

compounds (8 parent pesticides and 5 degradates) were detected at 

citrus land-use wells. Compounds most frequently detected above 

a standardized censoring level of 0.5 μg/L included demethyl 

norflurazon (83 percent of wells), norflurazon (79 percent of wells), 

aldicarb sulfoxide (41 percent of wells), aldicarb sulfone (38 

percent of wells), imidacloprid (38 percent of wells), and diuron 

(28 percent of wells). Copper and arsenic were detected in samples 

from 7 and 17 percent of the wells, respectively, although neither of 

these analytes exceeded Florida GCs for groundwater (table 9). 

Three of the pesticide compounds exceeded the USEPA 

MCL and Florida GCs for groundwater: aldicarb, aldicarb sulfone, 

aldicarb sulfoxide, as well as the sum of these compounds (table 9). 

The majority of these exceedances occurred for the aldicarb 

degradates, ranging from 18 to 24 percent of samples analyzed. 

Florida statutes that restrict the distance of aldicarb applications 

to potable wells (State of Florida, 1997) could result in lower 

concentrations of the parent compound and degradates near 

potable wells compared to monitoring wells, where the restrictions 

do not apply. In one or two of the 136 or 137 samples analyzed 

(depending on analyte), concentrations exceeded 0.5 of the Florida 

groundwater GCs for bromacil, diuron, norflurazon, and the sum 

of norflurazon plus its degradate. The HBSLs were exceeded for 

norflurazon (29 percent of samples) and for diuron (7 percent of 

samples), with 39 percent of norflurazon samples exceeding 0.5 

of the HBSL level. Nitrate exceeded the Florida groundwater GC 

(and USEPA MCL) of 10 mg/L in 68 percent of the samples, and in 

one or more samples from 21 (72 percent) of the 29 citrus land-use 

wells.

Table 8. Analyzed pesticides and pesticide degradates that 

were not detected in samples from citrus land-use wells, 

January 2009 through May 2010, and laboratory detection levels 

during this period. Analyzed sample fractions are total unless 

otherwise noted.

[Pesticide degradates are shown in italics.]

Pesticide compound Detection level

(microgram per liter)

2,4-D1 2.7, 13

Alachlor 2 

Atrazine 0.2

Bentazon 0.04

Chlorpyrifos 0.2

Clothianidin 0.04

Deethyl Atrazine (DEA) 0.4

Dicamba1 2.7,13

Dimethoate 0.2

Ethion 0.1

Fenamiphos 0.2

Fenamiphos sulfone 0.2

Fenamiphos sulfoxide 0.2

Iprodione 4

Malathion 0.2

MCPP (Mecoprop-p)1 2.7,13

Methomyl, dissolved 0.1

Metolachlor 2

Metribuzin 0.4

Oryzalin 16

Thiamethoxam 0.4

Thiazopyr 0.04

Triclopyr1 2.7,13

1

 Analyzed routinely in a subset of 10 of the network wells in citrus land 

use.
Pesticides and Selected Inorganic Agrichemical Results from the Citrus Land-Use Wells, January 2009 through May 2010 9

Table 9. Summary of pesticide, copper, arsenic, and nitrate detections in groundwater samples from citrus land-use wells, January 2009 through May 2010. Health benchmarks are defined in table 6. [GC, guidance concentration; HBSL, health-based screening level; DL, detection level; μg/L, microgram per liter; >, greater than; MCL, maximum contaminant level determined by the U.S. Environmental Protection Agency; <, less than; --, not applicable; mg/L, milligram per liter]

Detections, by well 

[N=29 wells, 1 well dry] 

Detections, by well 

above standardized DL Detections by sample

Number (and percent) of samples yielding detections above health-guidance 

benchmarks

Analyte 

(pesticide degradates 

in italics)

Florida groundwater GC/HBSL 

(μg/L)

Laboratory

 DL

(μg/L)

Number of 

wells with 

detections 

> DL

Percent of 

wells with 

detections 

> DL

Number of 

wells with 

detections 

> 0.5 μg/L

Percent of 

wells with 

detections 

> 0.5 μg/L

Total 

number of 

samples 

analyzed

Number 

of samples 

> DL

Percent 

of samples > 

DL

0.5 

Florida 

groundwater 

GC

Florida 

groundwater 

GC

0.5 

HBSL HBSL

Aldicarb, dissolved 7 (MCL1

) 0.1, 0.2 3 10.3 3 10.3 137 9 6.6 3 (2%) 2 (1.5%)

Aldicarb Sulfone, dissolved 7 (MCL1

) 0.1, 0.2 15 51.7 11 37.9 137 67 49 32(23%) 24 (18%)

Aldicarb Sulfoxide, dissolved 7 (MCL1

) 0.1, 0.2 15 51.7 12 41.4 137 62 45 34 (25%) 28 (20%)

Sum: Aldicarb + degradates, dissolved 7 (MCL1

) 0.1, 0.2 137 38 (28%) 33 (24%)

Bromacil 70 (MCL) 0.5 7 24.1 7 24.1 136 25 18 2 (1.5%) 0Diuron 14/2 0.5 8 27.6 8 27.6 136 29 21 2 (1.5%) 0 9 (7%) 9 (7%)Imidacloprid 399 0.5 11 37.9 11 37.9 137 32 23 0 0Metalaxyl 420 2 1 3.4 (1)2 (3.4)2 137 2 1.5 0 0Norflurazon 280/10 0.5 23 79.3 23 79.3 137 107 78 1 (<1%) 0 54 (39%) 40 (29%)Demethyl norflurazon 280 0.5 24 82.8 24 82.8 137 114 83 0 0Sum: 

Norflurazon+degradate

280 0.5 137 1 (<1%) 0

Oxamyl, dissolved 200 (MCL) 0.1, 0.2, 0.5 5 17.2 5 17.2 137 16 12 0 0Simazine 4 (MCL) 0.2 12 41.4 6 20.7 137 43 31 0 0Deisopropyl 

atrazine(DIA)

3 0.4 2 6.9 0 0.0 137 3 2 0 0

Deethyl atrazine (DEA) NA 0.4 0 0.0 0 0.0 137 0 0 0 0Sum (Simazine+DIA) 4 0.2, 0.4 137 0 0Thiazopyr Monoacid 56 0.04 3 10.3 1 3.4 137 14 10 0 0Arsenic, dissolved 10 (MCL) 2.8 2 6.9 1 3.4 137 4 3 0 0Copper, dissolved 1000 6.2 5 17 -- -- 137 27 18 0 0Nitrate, dissolved (as N) 

mg/L

10 mg/L (MCL) 0.02, 0.1 29 [21 

wells > 10 mg/L]

100 [72% >10 mg/L]

28 96.6 137 135 99 113 (82%) 93 (68%)

1Applies to any combination of two or more of these compounds.2 Refers to detections above 2 μg/L, which corresponds to the laboratory detection level for this compound.
10 Pesticides and Inorganic Constituents Monitored by the Lake Wales Ridge Groundwater Network, Central Florida

Comparison of Results from Citrus and 

Non-Citrus Land-Use Wells, 2009

In January through February 2009 and in July through 

August 2009, 8 of the non-citrus wells were sampled along 

with 23 of the LWRM Network citrus land-use wells. The 

following comparison between citrus and non-citrus land 

use wells includes well construction and the results of 

water-quality sampling during these two events. 

Well Construction

The well depths of the citrus and non-citrus wells 

were similar with both groups having a median of 55 ft 

(table 10). The median depth to the water table was 39.1 

ft among the citrus wells compared to 10.4 ft among the 

non-citrus wells, as measured during the January through

February 2009 sampling event (table 10). The difference 

between the groups was marginally significant (p=0.08). 

Length of screened interval ranged from 10 to 20 ft in the 

citrus land-use wells, and from 3 to 20 ft in the non-citrus 

wells (table 2). Sampling depth was measured in two ways, 

as the depth of the top and of the mid-point of the well 

screen below the water table. The range in these sampling 

depths was similar between the citrus and non-citrus well 

groups, both of which ranged from near the water table 

to about 40 ft below the water table (table 10). Sampling 

depth was denoted as the water table when the top or 

mid-point of the well screen was at or above the water 

table. The median values of the sampling depths among the 

citrus land-use wells were about 20 ft closer to the water 

table (p<0.05) compared to the medians for the non-citrus 

wells (table 10).

Table 10. Summary of well construction and sampling depths at wells sampled in the land-use 

comparison. Sampling depths and depths to water-table correspond to the Jan.–Feb. 2009 sampling event. 

Land-use 

classification

Number 

of wells

Well depth (feet) Depth to water 

table (feet)

Top-of-screen sampling 

depth, relative to water 

table (feet)

Mid-screen sampling 

depth, relative to 

water table (feet)

Median Range Median Range Median Range Median Range

Citrus 23 55 19–150 39.1 4.2–107 1 Water 

table–38 6 Water table–43

Non Citrus 8 55 23–110 10.4 5.1–82 19 5–32 25 10–42

Physical Properties, Inorganic Constituents, and 

Nutrients

A summary of the physical properties, inorganic 

constituents, and nutrients in groundwater samples, and the 

statistical comparison between water quality in the citrus 

and non-citrus land-use well groups appear in table 11. 

Comparison of the median values for each land-use 

group to the USEPA drinking-water benchmarks indicates 

nitrate (as N) and nitrate plus nitrite (as N) in samples 

from the citrus land-use wells exceeded the primary 

standard (10 mg/L). The USEPA secondary standards 

(U.S. Environmental Protection Agency, 2011), which 

relate to cosmetic or aesthetic effects of drinking water, 

include pH, aluminum, and manganese. In both the citrus 

and the non-citrus well groups the median pH was below 

the minimum secondary standard range (6.5 to 8.5 standard 

units). In non-citrus wells, median aluminum concentration 

exceeded the lower limit of the secondary standard range 

(50 to 200 μg/L); in citrus wells, median manganese 

exceeded the secondary standard (50 μg/L), and median 

aluminum exceeded the upper limit of the secondary 

standard range (table 11). The pH of groundwater in the 

surficial aquifer is naturally low and similar to that of 

rainwater, in many parts of Florida due to its low buffering 

capacity (Upchurch, 1998). Rainwater pH for sites in the 

vicinity of central Florida is typically in the range 4.6 

to 5.2 standard units (National Atmospheric Deposition 

Program, 2012).

To assess potential effects of seasonal influences on 

water quality, the median values of physical properties 

and concentrations of chemical constituents in samples 

were compared between the January through February 

2009 and the July through August 2009 sampling events 

for the citrus and non-citrus well groups (table 11). Only 

3 constituents (ammonia, fluoride, and strontium) of the 

27 physical properties and inorganic constituents showed 

significant seasonal differences (p<0.05) between the two 

sampling events. Therefore, for the land-use comparisons, 

separate analyses by sampling event were performed for 

these 3 constituents, and for all other analytes the samples 

from both events were combined (table 11). 
Comparison of Results from Citrus and Non-Citrus Land-Use Wells, 2009 11

Table 11. Comparison of physical properties, nutrients, and inorganic constituent concentrations between groundwater samples 

from citrus and non-citrus land-use wells. 

[median values in bold exceeded primary (P) or secondary (S) drinking-water regulations; values in blue indicate median values were statistically different at 

the 2-sided 0.05 p-level; 1-sided and 2-sided p-values are shown if the 2-sided p-value was between 0.05 and 0.1; citrus, citrus land-use wells; non-citrus, noncitrus land-use wells; mg/L, milligram per liter; ns, not significant; μg/L, microgram per liter; <, less than; >, greater than; Pt-Co, platinum-cobalt unit; mp, 

measuring point, approximately at land surface; ft, feet; MS, marginally significant; FDACS, Florida Department of Agriculture and Consumer Services; SU, 

standard units; μS/cm, microsiemens per centimeter; deg. C, degrees Centigrade; NTU, nephelometric turbidity units]

Analyte Units

2-sided p-value 

(1-sided/2-

sided) 

Number of 

samples 

(citrus)

Number of 

samples 

(non-citrus)

Median 

(citrus)

Median 

(non-citrus)

Difference 

between medians [citrus 

– non-citrus]

Alkalinity (total) mg/L ns 23* 15* 6.6 9.80 –3.20

Aluminum (dissolved) μg/L <0.02 46 16 507.5S 121.50S 386.00

Ammonia (as N) (dissolved) mg/L <0.001 46 16 <0.008 0.24 >–0.23

 Jan.–Feb. 2009 sampling event mg/L 0.01 23 8 0.006 0.25 –0.24

 Jul.–Aug. 2009 sampling event mg/L 0.001 23 8 <0.008 0.23 >–0.22

Calcium (dissolved) mg/L <0.001 46 16 22.8 2.67 20.14

Carbon-organic (total) mg/L <0.007 46 16 1.5 5.65 –4.15

Chloride (dissolved) mg/L <0.001 46 16 19.55 6.55 13.00

Color (dissolved) Pt-Co <0.001 46 16 0.95 12.45 –11.50

Depth to water (from mp) (Jan.–Feb. 2009) ft MS(0.04/0.08) 23 8 39.14 10.44 28.70

Dissolved oxygen (total) mg/L MS(0.04/0.09) 19* 7* 7.34 0.79 6.55

Fluoride (dissolved) mg/L <0.002 46 16 0.06 <0.02 >0.04

 Jan.–Feb. 2009 sampling event mg/L <0.015 23 8 0.06 <0.02 >0.04

 Jul.–Aug. 2009 sampling event mg/L MS (0.04/0.07) 23 8 0.04 <0.02 >0.02

Iron (dissolved) μg/L <0.001 46 16 12.85 228.5 –215.65

Magnesium (dissolved) mg/L <0.001 46 16 10.85 1.09 9.76

Manganese (dissolved) μg/L <0.004 46 16 108.5S 10.16 98.34

Nitrate (as N) (dissolved) [FDACS lab] mg/L <0.001 46 16 15.0P 0.02 14.98

Nitrate+Nitrite (as N) (dissolved) mg/L <0.001 46 16 13.4P 0.01 13.39

Nitrite (as N) (dissolved) mg/L MS (0.04/0.07) 46 16 <0.005 <0.005 non-citrus>citrus

Nitrogen- Total (dissolved) mg/L <0.001 46 16 13.8 0.61 13.19

pH SU MS (0.03/0.06) 36* 16 4.91S 5.81S –0.90

pH (Jan.–Feb. 2009 sampling event) SU MS (0.03/0.06) 23 8 4.62S 5.41S –0.79

Phosphorus-total (dissolved) mg/L ns 46 16 0.012 0.011 0.001

Potassium (dissolved) mg/L <0.001 46 16 14.9 0.83 14.07

Silica (dissolved) mg/L ns 46 16 6.8 6.45 0.35

Sodium (dissolved) mg/L ns 46 16 4.12 3.69 0.43

Specific Conductance (total) μS/cm <0.001 46 16 368.5 78.15 290.35

Strontium (dissolved) mg/L <0.004 46 16 0.11 <0.03;<0.25 >0.08

 Jan.–Feb. 2009 sampling event mg/L MS (0.04/0.07) 23 8 <0.25 <0.25 citrus>non-citrus

 Jul.–Aug. 2009 sampling event mg/L 0.01/0.02 23 8 0.14 0.02E 0.12

Sulfate (dissolved) mg/L <0.001 46 16 50.15 0.65 49.50

Sulfide (total) mg/L <0.001 46 16 <0.1 0.215 >–0.115

Temperature (total) deg.C ns 19* 7* 26.4 25.86 0.54

Turbidity (total) NTU ns 46 16 0.74 0.46 0.28

*

, indicates this group represents a subset of samples in which the analyte was not measured in all samples.

S

, indicates exceedance of U.S. Environmental Protection Agency secondary regulation for drinking water (non-enforceable).

P

, indicates exceedance of U.S. Environmental Protection Agency primary standard (MCL) for drinking water.

E, estimated
12 Pesticides and Inorganic Constituents Monitored by the Lake Wales Ridge Groundwater Network, Central Florida

A number of inorganic constituent concentrations 

differed between the citrus and non-citrus land-use areas. 

Compared to non-citrus areas, samples from the wells in 

citrus land-use yielded greater (p<0.05) concentrations 

of: aluminum, chloride, fluoride, magnesium, manganese, 

nitrate plus nitrite, total nitrogen (N), potassium, 

strontium, sulfate, and specific conductance. Elevated 

concentrations of calcium, magnesium, chloride, sulfate, 

potassium, and nitrate, and higher specific conductance 

have been documented in other studies of groundwater and 

lakes in ridge citrus compared to non-citrus areas (Stauffer, 

1991; Sacks and others, 1998; Choquette and Kroening, 

2009). These studies noted agrichemicals, including fertilizers, soil amendments, and some pesticide formulations, 

as well as irrigation water from the Upper Floridan aquifer 

as potential sources of these chemicals. 

More than 93 percent of citrus orchards in Polk and 

Highlands Counties are irrigated (Florida Agricultural 

Statistics Service, 1998), and in some areas the irrigation 

water is withdrawn from the typically more highly mineralized waters from the Upper Floridan aquifer. However, 

concentrations of potassium and sulfate in groundwater 

from the Upper Floridan aquifer in the Lake Wales Ridge 

region (Spechler and Kroening, 2007; Spechler, 2010) are 

typically lower than the concentrations observed in the 

citrus land-use wells. Also, well and casing depths of the 

permitted supply wells on the Ridge are generally shallower in Highlands County than in Polk County (Choquette 

and Sepulveda, 2000), indicating the possibility of 

proportionally fewer Upper Floridan irrigation wells in the 

southern part of the Ridge compared to the northern part. 

Additional data are needed to fully evaluate the influence 

of irrigation sources versus agrichemical applications in 

increasing the concentrations of some of these inorganic 

constituents in the surficial aquifer underlying the citrus 

orchards. A complicating factor is a process known as 

chemigation, a means of applying agrichemicals by adding 

them to the irrigation waters (Boman, 2002).

Constituents showing higher (p<0.05) concentrations 

in groundwater underlying non-citrus compared to citrus 

land-use areas included total organic carbon, color, iron, 

sulfide, and ammonia-N. Possible causes for the higher 

concentrations of these constituents could be related to 

differences in soils and geologic deposits in the vicinity of 

these wells, in addition to the typically deeper sampling 

depths (below the water table) at the non-citrus wells 

(table 10). Differences in reduction/oxidation (redox) 

conditions are influenced by and can contribute to variations in concentrations of some of these constituents by 

affecting pathways of microbial chemical breakdown 

(McMahon and Chapelle, 2008). Of the non-citrus wells, 

dissolved-oxygen concentrations were greater than 7 mg/L 

at two wells (site NC1 and site NC2) and <1 mg/L at the 

remaining six wells. Locally elevated iron concentrations 

are not uncommon in the surficial aquifer on the Ridge 

(Florida Department of Environmental Protection, 2006). 

Samples from the non-citrus land-use wells typically yielded low concentrations of nitrate (as N) and 

phosphorus (as P; table 11). Nitrate concentrations were 

<0.04 mg/L in samples from six of the eight non-citrus 

wells. Two of the wells, Ridge Wrap P-4 (site NC2) and 

the USGS Shallow Observation well (site NC1), yielded 

nitrate concentrations of 2 to 3, and 0.8 to 1.0 mg/L, 

respectively. Samples from these wells also contained 

higher dissolved-oxygen values (>7 mg/L), which can 

contribute to higher nitrate concentrations by limiting 

potential for denitrification (McMahon and Chapelle, 

2008), but citrus land use in the vicinity of these wells, as 

discussed in the section “Pesticides and Pesticide Degradates”, could also be a source of nitrate. 

Pesticides and Pesticide Degradates

Of the 30 pesticides and degradates analyzed during 

the two sampling events, the non-citrus land-use wells 

yielded only two pesticide detections in one of the eight 

wells sampled, in comparison to multiple detections in 22 

of 23 citrus land-use wells, which yielded a median of 4 

pesticide compounds per well (fig. 4). The non-citrus well 

Ridge Wrap P-4 (site NC2) yielded low concentrations of 

norflurazon (1.0 and 1.7 μg/L) and demethyl norflurazon 

(3.4 and 5.9 μg/L). Only one of the sampled citrus land-use 

wells, Muncie Road (site 18), did not yield any pesticide 

detections. This well also has been noted as yielding 

anomalously low groundwater nitrate concentrations 

compared to other Ridge citrus monitoring wells (Wheaton 

and Graham, 2000). Citrus orchards surrounding the 

Muncie Road well were removed between 2004 and 2005 

and converted to rural residential development. 

Summary statistics for pesticide, nitrate, arsenic, and 

copper concentrations in samples from the citrus and the 

non-citrus land-use wells are shown in table 12. A total 

of 12 pesticide compounds (8 parent compounds and 4 

degradates) were detected above laboratory detection 

levels in 2 to 95 percent of the samples from the citrus 

land-use wells. Those most frequently detected were 

demethyl norflurazon and norflurazon (95 and 88 percent 

of samples, respectively), aldicarb sulfone and aldicarb 

sulfoxide (53 and 47 percent of samples, respectively), 

and simazine (35 percent of samples). Table 12 also shows 

the median and (or) maximum concentrations of samples 

from the citrus wells, by analyte, that exceeded either 

the Florida groundwater GCs (aldicarb sulfone, aldicarb 

sulfoxide, and nitrate), 0.5 of the Florida groundwater GC 

(aldicarb, bromacil, and diuron), or the HBSLs (diuron and 

norflurazon).

Citrus orchards represent potential sources of elevated 

groundwater nitrate concentrations (0.8 to 3.0 mg/L) in 

samples from two LWRM Network non-citrus land-use 

wells (fig. 5, the USGS Shallow Observation well (site 

NC1) and the Ridge Wrap P-4 well (site NC2)), and 
Comparison of Results from Citrus and Non-Citrus Land-Use Wells, 2009 13

detections of norflurazon and demethyl norflurazon from 

one of the wells (Ridge Wrap P-4). Citrus orchards were 

identified on the basis of aerial images taken intermittently 

between February 1995 and December 2010 (Google 

Earth, Inc., 2011). Active (as of December 2010) or 

recently active citrus orchards (fig. 5) were located about 

0.14 and 0.22 mi south and east, respectively, of the USGS 

Shallow Observation well (site NC1) and about 0.16 to 

0.18 mi to the north, northeast, and east of the Ridge Wrap 

P-4 well (site NC2). Possible fertilizer applications in 

areas of residential or commercial development (fig. 5) 

could be additional sources of nitrate. Fertilizer typically 

is not applied in pine tree plantations in this region 

(Robin Holland, Division of Forestry, Florida Department 

of Agriculture and Consumer Services, oral commun., 

November 2, 2012). Norflurazon is not labled for use in 

Florida in areas of residential/commercial development or 

in pine plantations; however, its use is allowed in non-crop 

areas of agricultural lands, such as ungrazed fencelines, 

equipment lots, and ditchbanks (R. Scott Freiwald, Florida 

Department of Agriculture and Consumer Services, written 

commun., November 2, 2012).

10

9

8

7

6

5

4

3

2

1

0

Number of pesticides detected

Well identification number

January 2009

EXPLANATION

ND

ND

ND

ND

ND

ND

ND

ND

NC1

NC10

NC2

NC5

NC6

NC7

NC8

NC9

14

16

17

18

24

25

28

29

30

II-1

II-10

II-4

II-9

III-1

III-10

III-2

III-4

III-5

III-6

III-7

III-8

III-9

SWF-3

July 2009

Not detected

Non-citrus

land-use

Citrus land-use

ND

Figure 4. Comparison between pesticide detection frequencies in samples from citrus and non-citrus land-use 

wells during the January and July 2009 sampling events.
14 Pesticides and Inorganic Constituents Monitored by the Lake Wales Ridge Groundwater Network, Central Florida

N

Well

NC1

Well

NC2

0 0.25 MILE

0 0.2 KILOMETER

Google EarthTM and

Google MapsTM imagery

Dec. 2, 2010

Fallow/

pasture

Residential

Pine tree

plantations Residential

(Active citrus,

Dec., 2004)

Pine tree

plantation

Fallow

(Active citrus,

Mar., 2007)

Fallow

(Active citrus,

Jan., 2008)

Active citrus

Pine tree

plantation/

scrub

Pine tree

plantation/

scrub

Fallow/

pasture

Fallow/

pasture

Residential

Residential

Figure 5. Land use in the vicinity of the USGS Shallow Observation Well (site NC1) on US Highway 27 and the Ridge WRAP P4 

well (site NC2), on December 2, 2010.
Comparison of Results from Citrus and Non-Citrus Land-Use Wells, 2009 15

Table 12. Comparison between detections and concentrations of pesticides, copper, arsenic, and nitrate in groundwater samples from citrus and non-citrus land-use wells. Laboratory detection levels, Florida groundwater guidance concentrations, and health-based screening levels are in table 9.[Pesticide degradates are shown in italics; >, greater than; DL, laboratory detection level; μg/L, microgram per liter; <, less than; bold, value exceeds 0.5 of the Florida guidance concentration or exceeds the HBSL; underlined, value exceeds 0.5 of the Florida guidance concentration; blue font, value exceeds Florida groundwater guidance concentration; *, value exceeds USGS health based screening level (HBSL); +, listed values may be as much as 7 to 9 μg/L higher than actual based on observed presence in sample blanks; mg/L, milligram per liter.]

Analyte

Citrus land use (n=46 samples, 23 wells) Non-citrus land use (n=16 samples, 8 wells)

Number of 

samples 

>DL

Percent of 

samples 

>DL

Median 

concentration of 

samples 

> DL (μg/L)

 Median 

concentration 

of all

samples (μg/L)

Minimum 

concentration 

of samples 

> DL (μg/L)

Maximum 

concentration 

(μg/L)

Number 

of samples >DL

Percent of 

samples 

>DL

Maximum 

concentration

Aldicarb, dissolved 4 9 1.9 <0.1 0.79 6 0 0 <0.1, <0.2Aldicarb sulfone, dissolved 23 53 3.1 0.14 0.14 15 0 0 <0.1, <0.2Aldicarb sulfoxide, dissolved 20 47 6.7 <0.2 0.16 42 0 0 <0.1, <0.2Sum (aldicarb, aldicarb sulfoxide, 

and aldicarb sulfone)

23 53 7.4 0.22 0.19 55.8 0 0 <0.1, <0.2

Bromacil 11 26 6 <0.5 0.76 57 0 0 <0.5Diuron 10 23 1.1 <0.5 0.52 9.4* 0 0 <0.5Imidacloprid 10 23 5.35 <0.5 0.5 18 0 0 <0.5Metalaxyl 1 2 3.2 <2 3.2 3.2 0 0 <2Norflurazon 38 88 4.85 4 0.5 44* 2 13 1.7Demethyl norflurazon 41 95 8.4 8.2 0.67 52 2 13 5.9Sum (norflurazon and demethyl 

norflurazon

41 95 13.1 12.8 0.67 80 2 13 7.6

Oxamyl, dissolved 6 14 1.35 <0.1 0.87 11 0 0 <0.1, <0.2Simazine 15 35 0.33 <0.2 0.2 0.63 0 0 <0.2Deisopropyl atrazine (DIA) 1 2 0.45 <0.4 0.45 0.45 0 0 <0.4Sum (simazine and DIA) 15 35 0.33 <0.4 0.2 1.08 0 0 <0.4Arsenic, dissolved 0 0 <2.8 <2.8 ND ND 0 0 <2.8Copper , dissolved 9+ 20+ 8.9+ <6.2 6.9+ 98.6+ 0 0 <6.2Nitrate (as N), dissolved, (mg/L) 45 98 15 15 0.01 35 Median: 0.02

Minimum: <0.02, <0.1

3
16 Pesticides and Inorganic Constituents Monitored by the Lake Wales Ridge Groundwater Network, Central Florida

Summary

This report provides summaries of well characteristics, 

water-quality analyses and laboratory methods, and the 

complete sampling history from 1989 through May 2010 for 

the Lake Wales Ridge Monitoring (LWRM) Network, which 

currently includes 40 wells in the unconfined surficial aquifer 

underlying one of the principal citrus-growing regions of 

Florida. Water in the surficial aquifer flows into numerous 

groundwater seepage lakes on the Lake Wales Ridge, 

provides drinking water for rural domestic wells, and also 

ultimately recharges the underlying Upper Floridan aquifer, 

the principal municipal drinking-water supply in this region. 

During the 1999 through May 2010 period, most of the 30 

citrus land-use wells in the LWRM Network were sampled 

on a quarterly basis for water quality including major water 

chemistry, nutrients, selected inorganic constituents, and 

37 parent pesticides and degradates. The LWRM Network 

represents one of the few regional-scale, long-term (>10 years) 

monitoring programs nationally that includes quarterly sample 

collection, documenting the temporal variability of pesticides 

in groundwater. 

This report summarizes water-quality sampling results for 

January 2009 through May 2010, including 30 of the LWRM 

Network’s citrus land-use wells, first sampled between 1989 

and 2001, and eight non-citrus land-use monitoring wells, 

which were added to the Network in 2009. The report includes 

a comparison of land-use effects on groundwater quality 

underlying citrus and non-citrus areas on the basis of two 

sampling events in 2009. 

In samples from the LWRM Network’s 30 citrus land-use 

wells during 2009 through May 2010, 13 pesticide compounds 

(8 parent pesticides and 5 degradates) were detected. Those 

most frequently detected above a common 0.5-μg/L censoring 

level included: demethyl norflurazon (83 percent of wells), 

norflurazon (79 percent), aldicarb sulfoxide (41 percent), 

aldicarb sulfone (38 percent), imidacloprid (38 percent), and 

diuron (28 percent). In these samples, three of the pesticide 

compounds exceeded the USEPA MCL and Florida groundwater guidance concentrations for: aldicarb, aldicarb sulfone, 

aldicarb sulfoxide, as well as the sum of these compounds, 

with exceedances ranging from 1.5 percent (aldicarb) to 

24 percent (sum of aldicarb plus degradates) of the analyzed 

samples. The HBSLs, which are health-screening benchmarks 

for water quality, were exceeded for norflurazon (29 percent 

of samples) and for diuron (7 percent of samples). During the 

2009 through May 2010 period, nitrate exceeded the USEPA 

MCL in 68 percent of samples from the citrus land-use wells.

In the comparison of land-use effects on water quality, 

concentrations of a number of inorganic constituents were 

significantly higher (p<0.05) in groundwater samples 

(collected in January through February 2009 and in July 

through August 2009) from wells in the surficial aquifer 

surrounded by citrus land use compared to those in non-citrus 

land-use areas. Higher concentrations of calcium, magnesium, 

chloride, sulfate, potassium, nitrate (as N), total nitrogen, 

aluminum, manganese, and strontium, as well as specific 

conductance, in groundwater underlying the citrus land-use 

areas likely reflect the influence of agrichemical applications, 

as well as irrigation and site differences such as soils and 

groundwater redox (oxidation-reduction) conditions. Samples 

from the 23 citrus land-use wells yielded detections of 12 of 

30 targeted parent pesticides and degradates, and all except 

one of these wells yielded one or more pesticide detections. 

Compared to a median of four pesticides and degradates 

detected per well in areas of citrus land use, only one of the 

eight sampled wells in non-citrus land use yielded pesticide 

detections consisting of one pesticide and its degradate; citrus 

orchards in proximity to this well are likely sources of these 

detections. 

Monitoring of groundwater in the LWRM Network 

wells over time has documented the influence of land-use 

activities, as well as temporal and spatial variations in 

groundwater quality in the surficial aquifer. These variations 

likely reflect changes in agrichemical usage patterns, land use, 

establishment of regulatory restrictions on agrichemicals and 

use of new and alternative chemicals , as well as variations 

in groundwater recharge associated with climatic variability. 

Some recent changes in agrichemical usage patterns that 

have the potential to impact groundwater quality include 

the increased use of insecticides to address the spread of 

citrus greening disease, and the discontinued use in citrus 

of a widely applied nematicide and insecticide (aldicarb). 

Continued monitoring of the LWRM Network wells will 

assist in protecting drinking-water resources and in assessing 

the effects of all of these factors, as well as future changes in 

these factors, on groundwater quality in this highly vulnerable 

aquifer underlying an important citrus-production area of 

Florida. 

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Prepared by

U.S. Geological Survey

Science Publishing Network

Raleigh Publishing Service Center

3916 Sunset Ridge Road

Raleigh, NC 27607

A PDF version of this publication is available online at

http://pubs.usgs.gov/of/2012/1231/
Choquette—Results for Pesticides and Inorganic Constituents Monitored by the Lake Wales Ridge Groundwater Network, Central Florida—Open-File Report 2012–1231