Water Quality and Evaluation of Pesticides 

in Lakes in the Ridge Citrus Region of 

Central Florida

By Anne F. Choquette and Sharon E. Kroening

Prepared in cooperation with the

Florida Department of Environmental Protection and the

Southwest Florida Water Management District

Scientific Investigations Report 2008-5178

U.S. Department of the Interior

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

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2009

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Suggested citation:

Choquette, A.F., and Kroening, S.E., 2009, Water Quality and Evaluation of Pesticides in Lakes in the Ridge Citrus 

Region of Central Florida: U.S. Geological Survey Scientific Investigations Report 2008-5178, 55 p.
Contents

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

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

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

Influences of Agrichemicals on Lake Systems .................................................................................... 4

Acknowledgments.......................................................................................................................................5 

Description of Study Area ...................................................................................................................................5

Physical Setting, Climate, and Land Use.................................................................................................6

Agrichemical Usage....................................................................................................................................6

Hydrology of Ridge Seepage Lakes.........................................................................................................8

Selection and Description of Sampled Lakes .................................................................................................9

Methods of Study................................................................................................................................................10

Sampling and Laboratory Methods........................................................................................................10

Quality Assurance .....................................................................................................................................12

Analytical Methods ...................................................................................................................................12

Lake Water Quality..............................................................................................................................................14

Physical Properties and Major Constituents .......................................................................................14

Nutrients......................................................................................................................................................16

Pesticides and Pesticide Degradates ...................................................................................................17

Occurrence of Pesticides...............................................................................................................17

Comparisons to Benchmark Values .............................................................................................20

Areal and Seasonal Variations......................................................................................................22

Comparisons of Parent Pesticides and Degradates.................................................................24

Norflurazon and Degradate..................................................................................................24

Simazine, Atrazine, and Degradates...................................................................................24

Diuron and Degradates .........................................................................................................25

Aldicarb and Degradates ......................................................................................................25

Variations within the Lake Water Column...................................................................................28

Comparison to National Studies....................................................................................................30

Relation of Chemical Properties and Usage to Pesticide Occurrence.................................32

Discussion of Pesticide Transport and Fate ...............................................................................35

Summary...............................................................................................................................................................36

References Cited.................................................................................................................................................37 

Appendixes

1. Constituents, analytical methods, and reporting levels for water-quality samples 

 analyzed in the lake samples ....................................................................................................................43 

2. Field measurements and laboratory results for major water-quality constituents 

 and nutrients in the lake samples ............................................................................................................47 

3. Laboratory results for detected pesticides and degradates in the lake samples..........................49 

4. Summary statistics of detected pesticide and degradate concentrations by lake .......................53 

5. Summary of parent pesticides and order of degradation for triazine pesticide 

 degradates ...................................................................................................................................................55

iii
Figures

1. Map showing locations of sampled lakes, vulnerable soils, and citrus land use 

 in the Lake Wales Ridge study area, Florida .....................................................................................3

2. Photograph showing Lake Moody and nearby citrus land use..................................................... 4

3. Graph showing primary pesticides applied in Florida orange groves from 

 1995 to 2003, showing usage as the percent of total bearing acreage 

 receiving applications............................................................................................................................7

4. Graph showing statewide average application rates for selected pesticides 

 in Florida orange groves between 1997 and 2003 .......................................................................... 7

5. Generalized hydrogeologic section showing ground-water/surface-water 

 interactions in the vicinity of Ridge seepage lakes..........................................................................8 

6-8. Graphs showing:

6. Frequency of pesticides and degradates detected in samples from the 

 seven lakes in citrus areas........................................................................................................ 17

7. Total number of pesticides and degradates detected in each of the lakes 

 sampled quarterly over a year...................................................................................................19 

8. Number of citrus lakes in which targeted pesticide compounds were 

 detected in one or more quarterly samples............................................................................19 

9-11. Plots showing:

9. Median nitrate concentration related to number of pesticides detected 

 above laboratory reporting levels in each sampled lake.................................................... 20 

10. Median nitrate concentration related to total concentration of pesticides 

 and degradates in quarterly samples summed over one year in each lake....................21

11. Variations in concentrations of selected pesticides seasonally within 

 and between sampled lakes...................................................................................................... 23 

12-14. Graphs showing:

12. Number of pesticide compounds detected seasonally in each of the 

 citrus lakes..................................................................................................................................... 24 

13. Seasonal variations in concentrations of norflurazon and demethyl 

 norflurazon in samples from Lake Aurora, Swim Lake, and Lake Leonore.................... 25 

14. Seasonal variations in concentrations of simazine and its degradates 

 in samples from Lake Lynn and Swim Lake ........................................................................... 26 

15. Diagram showing generalized chemical degradation pathways and sources 

 for simazine degradates ......................................................................................................................26 

16-20. Graphs showing:

16. Seasonal variations in concentrations of diuron and its degradates in samples

 from Swim Lake and Lake Aurora............................................................................................ 27

17. Seasonal variations in concentrations of aldicarb degradates in samples 

 from Lake Leonore and Swim Lake.......................................................................................... 27 

18. Relative difference in the concentrations of norflurazon plus demethyl 

 norflurazon in water-column samples compared to the vertical distance 

 between the samples.................................................................................................................. 29

19. Ratio of demethyl norflurazon concentration to norflurazon concentration 

 in water-column samples related to lake depth at the sampled location....................... 30

20. Pesticide transport-potential indexes compared to median pesticide 

 concentrations in Swim Lake, Lake Leonore, and Lake Lynn............................................ 34

iv
Tables

1. Physical characteristics of the sampled lakes ........................................................................................9 

2. Pesticides and pesticide degradates analyzed in the lake samples ................................................ 11

3. Matrix spike recovery results for selected pesticides and pesticide degradates 

 in the lake samples......................................................................................................................................13 

4. Summary statistics for major inorganic constituent concentrations in the sampled lakes..........15 

5. Summary statistics for nutrient concentrations in the sampled lakes............................................. 16

6. Summary statistics for concentrations of pesticides and degradates detected in the 

 seven sampled citrus lakes ......................................................................................................................18 

7. Pesticide concentrations in lake samples compared to aquatic-life and 

 human-health benchmark values ............................................................................................................21 

8. Comparison between pesticide concentrations in shallow and deep samples from 

 the lake water column................................................................................................................................28 

9. Pesticide results for the sampled Ridge citrus lakes compared to U.S. Geological 

 Survey national surface-water monitoring data...................................................................................31 

10. Indexes of leaching vulnerability, pesticide usage, and transport potential for 

 selected pesticides targeted in the Ridge lake study ..........................................................................32 

Conversion Factors

Multiply By To obtain

Length

inch (in.) 2.54 centimeter

foot (ft) 0.3048 meter

mile (mi) 1.609 kilometer

Area

acre (ac) 4.047 square meter

square mile (mi2) 2.590 square kilometer

Flow rate

inch per hour (in/hr) 0.0254 meter per hour

inch per year (in/yr) 25.4 millimeter per year

Temperature

Fahrenheit (oF) oC = (oF - 32) /1.8 Celsius (oC)

Celsius (oC) oF = (1.8 x oC) + 32 Fahrenheit (oF)

Vertical coordinate information is referenced to North American Vertical Datum of 1929 (NGVD 29).

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

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius 

(μS/cm at  ̊25 C).

v
vi

Abbreviations

FDEP Florida Department of Environmental Protection

IQR interquartile range

Koc organic carbon partition coefficient

LRL laboratory reporting level

NAWQA National Water-Quality Assessment

SD standard deviation

SRL standardized reporting level

SWFWMD Southwest Florida Water Management District

TOC total organic carbon

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey

AMPA aminomethylphosphonic acid

DCPU 3,4-dichlorophyenyl urea

DCPMU 3(3,4-dichlorophenyl) methyl urea

DDA didealkylatrazine, or chlordiamino-s-triazine (CAAT)

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

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

HA hydroxyatrazine, or 2-hydroxy-4-isopropylamino-6-ethylamino-s-triazine 

2-hydroxyatrazine (OIET)

HS hydroxysimazine, or 2-hydroxy-4,6-diethylamino-s-triazine (OEET)

Other Abbreviated Units

acre/ft acre per foot

mg/L milligram per liter

μg/L microgram per liter

μS/cm microsiemens per centimeter

L liter

μm micrometer

μL microliter

μg/mL microgram per milliliter

Pt-Co platinum-cobalt unit

lb/acre pound per acre

lbs pounds
Abstract

Water chemistry, including major inorganic constituents, 

nutrients, and pesticide compounds, was compared between 

seven lakes surrounded by citrus agriculture and an undeveloped lake on the Lake Wales Ridge (herein referred to as the 

Ridge) in central Florida. The region has been recognized for 

its vulnerability to the leaching of agricultural chemicals into 

the subsurface due to factors including soils, climate, and land 

use. About 40 percent of Florida’s citrus cultivation occurs 

in “ridge citrus” areas characterized by sandy well drained 

soils, with the remainder in “flatwoods citrus” characterized 

by high water tables and poorly drained soils. The lakes on the 

Ridge are typically flow-through lakes that exchange water 

with adjacent and underlying aquifer systems. This study is 

the first to evaluate the occurrence of pesticides in lakes on 

the Ridge, and also represents one of the first monitoring 

efforts nationally to focus on regional-scale assessment of 

current-use pesticides in small- to moderate-sized lakes (5 to 

393 acres). The samples were collected between December 

2003 and September 2005. 

The lakes in citrus areas contained elevated concentrations 

of major inorganic constituents (including alkalinity, total 

dissolved solids, calcium, magnesium, sodium, potassium, 

chloride, and sulfate), total nitrogen, pH, and pesticides 

compared to the undeveloped lake. Nitrate (as N) and total 

nitrogen concentrations were typically elevated in the citrus 

lakes, with maximum values of 4.70 and 5.19 mg/L (milligrams per liter), respectively. Elevated concentrations of 

potassium, nitrate, and other inorganic constituents in the 

citrus lakes likely reflect inputs from the surficial groundwater system that originated predominantly from agricultural 

fertilizers, soil amendments, and inorganic pesticides.

A total of 20 pesticide compounds were detected in the 

lakes, of which 12 compounds exceeded the standardized 

reporting level of 0.06 μg/L (microgram per liter). Those most 

frequently detected above the 0.06-μg/L level were aldicarb 

sulfoxide, diuron, simazine degradates hydroxysimazine and 

didealkylatrazine (DDA), bromacil, norflurazon, and demethyl 

norflurazon which occurred at detection rates ranging from 

25 to 86 percent of samples, respectively. Typically, pesticide 

concentrations in the lake samples were less than 1 microgram 

per liter. The number of targeted pesticide compounds detected 

per lake in the citrus areas ranged from 9 to 14 compared to 

3 compounds detected at trace levels in the undeveloped lake. 

Consistent detections of parents and degradates in quarterly 

samples indicated the presence of pesticide compounds in 

the lakes many months or years (for example, bromacil) after 

their application, signaling the persistence of some pesticide compounds in the lakes and/or ground-water systems. 

Pesticide degradate concentrations frequently exceeded parent 

concentrations in the lakes. This study was the first in the 

Ridge citrus region to analyze for glyphosate—widely used 

in citrus—and its degradate aminomethylphosphonic acid 

(AMPA), neither of which were detected, as well as a number 

of triazine degradates, including hydroxysimazine, which 

were detected.

The lake pesticide concentrations did not exceed current 

Federal aquatic-life benchmarks, available for 10 of the 

20 detected pesticide compounds. Limited occurrences of 

bromacil, diuron, or norflurazon concentrations were within 

about 10 to 90 percent of benchmark guidelines for acute 

effects on nonvascular aquatic plants in one or two of the 

lakes. The lake pesticide concentrations for several targeted 

pesticides were relatively high compared to corresponding 

national stream-water percentiles, which is consistent with 

this region’s vulnerability for pesticide leaching into water 

resources. 

Water Quality and Evaluation of Pesticides in Lakes in the 

Ridge Citrus Region of Central Florida

By Anne F. Choquette and Sharon E. Kroening
2 Water Quality and Evaluation of Pesticides in Lakes in the Ridge Citrus Region of Central Florida

Several factors were evaluated to gain insight into the 

processes controlling pesticide transport and fate, and to assess 

their utility for estimating the relative likelihood of transport 

to the lakes for specific pesticides and for designing future 

pesticide sampling networks. These factors included variations 

in pesticide concentrations within the lake water column, 

indexes of pesticide usage estimates and chemical properties 

to identify pesticides prone to transport, comparisons between 

pesticide concentrations in the lakes and in adjacent ground 

water, and the relation between nitrate and pesticide concentrations. Further study is needed to better understand the 

role of pesticide inputs from ground-water and atmospheric 

sources, in-lake processes of pesticide breakdown, and the 

influence of the lakes on regional ground-water quality in this 

dynamic, closely linked ground-water/surface-water system.

Introduction

Lakes are the dominant surface-water feature in the 

central Florida landscape. They represent an important 

component of the region’s ecosystem, are a resource for recreation, and contribute aesthetic beauty to this region. About 

70 percent of Florida’s lakes are seepage lakes (Palmer, 1984), 

having no natural surface flow into or out of them, and are 

fed predominantly by ground-water inflow and rainfall. Due 

to its soils and climate, central Florida is also one of the most 

productive regions of the State for citrus agriculture. 

Historically, citrus has been one of the top agricultural 

crops in Florida, accounting for 68 percent of the Nation’s 

citrus production, and generating more than 1.5 billion 

dollars of revenue annually (U.S. Department of Agriculture, 

2006). During the 1990s, citrus acreage in Florida increased 

by 64 percent (Gianessi and Silvers, 2000), but experienced 

a 15-percent decline between 2004 and 2006 related to the 

intensive 2004 and 2005 hurricane seasons, the spread of 

citrus pests and diseases, and urban sprawl (U.S. Department 

of Agriculture, 2007). Citrus cultivation in Florida occurs in 

two distinct environments, including (1) the “ridge citrus,” 

which occurs on the ridges of central Florida and is characterized by well-drained sandy soils; and (2) the “flatwoods 

citrus,” which occurs predominantly in coastal and southern 

Florida and is characterized by poorly drained, high watertable, flatwoods soils (Boman, 2002). The “ridge citrus” 

comprises about 40 percent of Florida’s total citrus acreage 

(Paramasivam and Sajwan, 2001).

This study focuses on an area of central Florida recognized as susceptible to leaching of agrichemicals into the 

ground-water system. The combination of the seasonally 

high rainfall rates, highly permeable sandy soils prone to 

leaching, and the widespread use of agricultural chemicals in 

the region render the region’s lakes potentially susceptible to 

the influx of these chemicals. In 2003, the U.S. Geological 

Survey (USGS) in cooperation with the Southwest Florida 

Water Management District (SWFWMD) and the Florida 

Department of Environmental Protection (FDEP) initiated a 

study of citrus pesticides in small to moderately sized lakes 

on the Lake Wales Ridge (subsequently referred to as “the 

Ridge” in this report) in central Florida (fig. 1). Initiated as 

a companion study to a long-term ground-water monitoring 

project (Choquette and Sepulveda, 2000), this study was 

designed as a regional reconnaissance to evaluate the occurrence and variability of pesticides, pesticide degradates, and 

nitrate in lakes in citrus cultivation areas, and to compare 

water quality in the citrus lakes to that in an undeveloped lake 

in a relatively undisturbed area.

More than 200 lakes occur on the Ridge covering 

about 10 percent of its area (Barcelo and others, 1990). 

Citrus orchards (typically referred to as groves, in Florida) 

commonly are found in Ridge lake basins and often extend to 

lake shore areas (fig. 2). The Ridge lakes both receive water 

from and discharge water to the aquifers underlying the Ridge 

(Sacks and others, 1998) and, therefore, also are important for 

understanding the fate and transport of pesticides within the 

ground-water system. 

This study represents the first evaluation of the occurrence and distribution of pesticides and their degradates in 

the Ridge lakes, and is one of the first monitoring studies 

nationally to evaluate the occurrence of current-use pesticides 

in small to moderately sized lakes.1 Also, the occurrence 

of several pesticide degradates that have not been widely 

analyzed in water resources was examined. 

Purpose and Scope

This report summarizes the results of a reconnaissance 

water-quality study of eight lakes distributed across the 

Lake Wales Ridge, with an emphasis on pesticides and their 

degradates. Water chemistry was sampled seasonally in seven 

lakes surrounded predominantly by citrus groves (orchards) 

and in one lake in a relatively undisturbed area to: (1) evaluate 

the effects of citrus agriculture on water quality, and (2) assess 

potential factors influencing the occurrence of pesticides in 

the lakes. Spatial variations in water quality between the lakes 

and seasonal variability in pesticide and pesticide degradate 

concentrations were also evaluated.

A total of 36 samples was collected from the seven citrus 

lakes and one undeveloped “control” lake. Each lake was 

sampled quarterly for 1 year, and paired samples were collected 

at two depths from four of the lakes to compare pesticide 

concentrations within the lake water column. Water-quality 

analyses included pesticides, nutrients, major water chemistry, 

and field measurements of basic water chemistry (app. 1). 

The targeted pesticides included the major chemicals used 

in Florida citrus cultivation (Florida Agricultural Statistics 

1

German and Adamski (2005) also analyzed pesticides in lakes in a region 

north of the Ridge study area.
Introduction 3

HARDEE

COUNTY

DE SOTO

COUNTY

HIGHLANDS

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

27°15 ́

27°30 ́

27°45 ́

28°00 ́

28°15 ́

81°45 ́ 81°30 ́ 81°15 ́

Lake

Placid

Lake

Annie

Lake

Lynn

Sebring

Lake

Denton

Avon Park

Pabor

Lake

Lake

Moody

Lake

Leonore

Lake

Wales

Swim

Lake

Lake

Aurora

Haines

City

EXPLANATION

CITRUS ON VULNERABLE SOIL

VULNERABLE SOIL

CITRUS NOT ON VULNERABLE SOIL

LAKE

LAKE WALES RIDGE STUDY AREA

0 5 10 MILES

0 5 10 KILOMETERS

SAMPLED LAKE AND NAME

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

LOCATION

OF STUDY

AREA

Figure 1. Locations of sampled lakes, vulnerable 

soils, and citrus land use in the Lake Wales 

Ridge study area, Florida. Vulnerable soils refer 

to soils classified as susceptible to leaching of 

agrichemicals on the basis of high permeability 

and low organic-matter content (Hurt and Obreza, 

1991a,b).
4 Water Quality and Evaluation of Pesticides in Lakes in the Ridge Citrus Region of Central Florida

Service, 2000, 2004; Shahane, 2003). The pesticide concentrations were compared to aquatic-life and drinking-water benchmarks and to results of USGS surface-water sampling in other 

areas of the United States. Pesticide detections in the lakes 

were related to indexes of statewide agrichemical usage and 

pesticide properties to provide inferences regarding pesticide 

fate and transport in the region, and to assess the utility of these 

indexes for estimating the occurrence of pesticides in the lakes.

Influences of Agrichemicals on Lake Systems

Assessment of agrichemicals in Ridge lakes is important 

for a number of reasons including: the presence of elevated 

nitrate and pesticides in ground water in the surficial aquifer 

system, which feeds the lakes; the connections between the 

lakes and the underlying Upper Floridan aquifer, which is the 

primary drinking-water supply; and the potential for adverse 

effects of elevated agrichemicals on lake biota and ecosystems. Much is unknown regarding the chronic toxicity of 

pesticides and their degradates, chemical mixtures, and nitrate 

to non-target organisms. Although pesticides have been documented in ground water underlying the Ridge (Choquette and 

others, 2005), there have been no studies to date to evaluate 

the potential transport of pesticides from ground water to the 

Ridge lakes, or to assess the influences of pesticides on the 

lake ecosystems. 

The importance of pesticide and nitrate monitoring for 

protection of drinking water and ecosystems is supported by 

recent data indicating adverse effects of low concentrations of 

some pesticides on the endocrine systems of amphibians and 

other non-target organisms (Hayes and others, 2002; Gross 

and others, 2003; Barbash, 2004), and the incomplete removal 

of some pesticides from standard municipal water-filtration 

systems (Coupe and Blomquist, 2004; Carter and others, 

2007). In addition, there is indirect theoretical evidence indicating the potential for increased human toxicity of simazine 

when combined with elevated nitrate concentrations (U.S. 

Department of Health and Human Services, 2004) and the 

possibility for endocrine disruption in biota upon exposure to 

elevated nitrate concentrations (Guillette and Edwards, 2005).

Several prior studies have focused on lake water budgets, 

ground-water/surface-water interactions, and the inorganic 

chemistry of the Ridge lake systems (Katz and others, 1995; 

Lee, 1996, Sacks and others, 1998; Swancar and others, 2000). 

Compared to non-citrus areas, the lakes in citrus areas generally contain elevated concentrations of dissolved nitrate (as N) 

and other inorganic agrichemicals including sulfate, chloride, 

potassium, and magnesium (Sacks and others, 1998; Romie, 

2000; Kolasa and others, 2001). A statewide survey indicated 

that nitrate concentrations in citrus lakes on the Lake Wales 

Ridge exceeded those in 95 percent of lakes statewide (Romie, 

2000). Median values of nitrate in Ridge citrus lakes ranged 

from about 0.5 to 8.4 mg/L as N (Sacks and others, 1998; 

Figure 2. Lake Moody and nearby citrus land use. Citrus dominates the agricultural landscape of the 

Ridge, commonly found in close proximity to the lakes. Photograph courtesy of Sharon Kroening.
Description of Study Area 5

Kolasa and others, 2001); in contrast, nitrate concentrations 

were less than 0.002 mg/L in lakes located in undeveloped 

areas of the Ridge (Tihansky and Sacks, 1997).

Pesticide sampling in six golf-course ponds (Swancar, 

1996) located in central Florida areas north of the Ridge 

yielded detections of at least one pesticide in 96 percent of 

samples. Nineteen of 41 targeted pesticide compounds were 

detected, some of which were not reported as being used on 

the golf courses, and were attributed to prior land uses at the 

sites (Swancar, 1996). Atrazine, the most frequently detected 

pesticide, was reported as not used at any of the golf courses 

where it was detected. Concentrations in the pond samples 

exceeded 1 μg/L for nine pesticides including simazine, 

diuron, and atrazine. 

Results of ground-water sampling in citrus areas of the 

Ridge indicate a high potential for agrichemical transport into 

the subsurface. Concentrations of pesticides and nitrate in 

ground water underlying the Lake Wales Ridge were elevated 

compared to national USGS ground-water monitoring data, 

and exceedances of the Federal drinking-water standard 

for nitrate (10 mg/L) were common (Choquette and others, 

2005). Twelve of 29 targeted pesticides and degradates have 

been detected in ground water from the Lake Wales Ridge 

ground-water network, with as many as eight different 

pesticides or degradates found in ground water from some of 

the wells. Pesticide concentrations in ground water often show 

significant short-term variability, and concentrations of some 

pesticides appear to be increasing over time (Choquette and 

others, 2005; Gilliom and others, 2006).

The high propensity for leaching of agricultural chemicals into ground water on Lake Wales Ridge is due to a combination of factors. The sandy soils are well drained promoting 

rapid infiltration rates and contain little organic matter to sorb 

or filter pesticide compounds. Among United States agricultural categories, citrus agriculture has been ranked as having 

the highest national rate of herbicide use and the third highest 

rate of insecticide use on the basis of active ingredient applied 

annually (Brandt, 1995; Barbash and Resek, 1996). The study 

area is located in Polk and Highlands Counties, which typically rank first and third, respectively, statewide in terms of 

total annual citrus production (National Agricultural Statistics 

Service, 2005). Due to the mild climate and long growing 

season in Florida, the number and frequency of pesticide 

applications annually exceeds that in many other regions of 

the United States. Additionally, low pH has been shown to 

enhance the persistence of some pesticides in the subsurface 

(Harkin and others, 1986; Barbask and Resek, 1996), and 

may also contribute to the elevated pesticide concentrations in 

ground water on the Ridge. The pH of ground water typically 

ranges from 4 to 6 in the surficial aquifer system on the Ridge 

(Sacks and others, 1998; Choquette and others, 2003).

Chemical properties of pesticides and pesticide usage 

are commonly used to identify pesticides prone to be transported into surface- and ground-water resources, and to select 

pesticides for water-quality monitoring (Buttler and others, 

1993, 1998; Rao and Hornsby, 1993; Barbash and Resek, 

1996). However, laboratory data on pesticide properties do 

not always accurately predict pesticide behavior in field 

settings (Katz, 1993; Warner and Morrow, 2007; Krutz and 

others, 2008). The Ridge lake study provided an opportunity 

to assess the accuracy of applying published data on pesticide 

properties and usage to derive estimates of transport potential, 

by comparing these estimates of transport potential to actual 

pesticide occurrence in the Ridge lakes. 

In recent years, Federal and State laws and regulations 

have been developed to reduce leaching of agricultural chemicals into ground water and to protect drinking-water sources 

(National Association of State Departments of Agriculture 

Research Foundation, 2000; Boman, 2002). Guidelines and 

restrictions on agrichemical usage have been developed 

specifically for the sandy soils on the Ridge to minimize 

probability of leaching and protect water resources (State of 

Florida, 1995, 2002a,b; Aerts and Nesheim, 2000). In 2002, 

fertilizer best management practices were adopted specifically 

for Ridge citrus (State of Florida, 2002b) to minimize impacts 

to the ground-water system, and are being implemented by 

citrus growers on a voluntary basis. Reductions in nitrate 

concentrations in ground water have been observed in Ridge 

areas where these best management practices were implemented on a test basis (Lamb and others, 1999). 

Acknowledgments

The authors appreciate administrative and logistical 

assistance from staff within agencies that supported this 

study including Roberta Starks and Catherine Wolden of the 

Southwest Florida Water Management District, and James 

McNeill and Richard Hicks of the Florida Department of 

Environmental Protection. The Archbold Biological Research 

Station provided permission to access Lake Annie, and private 

landowners provided permission to access Swim Lake, Lake 

Leonore, and Lake Lynn. We thank Edward R. German, U.S. 

Geological Survey, for assistance with directing field sampling 

and collecting water-quality samples. 

Description of Study Area

The study area is delineated by the Lake Wales Ridge in 

Polk and Highlands Counties (fig. 1) and covers an area of 

about 700 mi2. The landscape, hydrology, and ecology of the 

Ridge are unique in a number of respects. The Ridge is the 

topographic crest and most prominent physiographic feature 

of peninsular Florida, consisting of a paleoisland remnant 

underlain by highly permeable Miocene-Holocene marine 

sands. Due in part to its geologic history, the Ridge supports 

one of the oldest ecosystems in Florida and contains one 

of the largest collections of rare and endangered species in 

the United States (Dobson and others, 1997) and one of the 

highest concentrations of endemic species in North America 

(Martin, 1998). 
6 Water Quality and Evaluation of Pesticides in Lakes in the Ridge Citrus Region of Central Florida

Physical Setting, Climate, and Land Use

The Lake Wales Ridge is the most extensive ridge in a 

complex of several parallel northwest trending ridges in the 

physiographic province delineated as the Central Lakes District 

(Brooks, 1981). Elevations on the Ridge range from about 

150 to 300 ft (NGVD of 1929). The Ridge contains numerous 

surface depressions and lakes that are typically formed by 

dissolution of underlying limestone and subsequent collapse 

of surficial sediments. Surface stream drainage is poorly 

developed or absent in most areas of the Ridge, although some 

surface-water flow occurs between lakes and on the flanks 

of the Ridge. Detailed descriptions of the study area and its 

hydrology can be found in Yobbi (1996), Sacks and others 

(1998), and Choquette and Sepulveda (2000).

Mean annual rainfall (1971-2000) in the vicinity of the Ridge 

ranges from about 48 to 51 in. and varies seasonally with about 

60 percent of the annual rainfall occurring during June through 

September (National Oceanic and Atmospheric Administration, 

2005). Rainfall amounts and intensity are highest during summer 

and early fall as a result of convective storms and hurricanes. 

Mean monthly rainfall during June through September ranges 

from about 5.5 to 8.5 in. compared with about 2 to 3 in. during 

other months. Mean monthly temperatures range from about 

16 oC in January to 28 oC in July and August.

The geologic deposits underlying the Ridge form a 

layered, wedge-shaped sequence that thickens from north 

to south (Yobbi, 1996). The hydrogeologic units include the 

surficial aquifer system (50- to 300-ft layer of unconsolidated marine and dune sands), underlain by the intermediate 

confining unit/intermediate aquifer system (0- to 500-ft thick 

unit of heterogeneous clay, sands, and limestone), in turn 

underlain by the Upper Floridan aquifer (1,200- to 1,400-ft 

thick unit of limestone and dolomite). The Upper Floridan 

aquifer is the primary source of municipal drinking-water 

supply, but the surficial aquifer system and, locally, the 

intermediate aquifer system provide drinking water for rural 

domestic wells, particularly in the southern part of the Ridge 

where the Upper Floridan aquifer is as much as 600 ft below 

land surface (Tibbals, 1990; Tihansky and others, 1996). 

Water from the surficial aquifer system generally flows to, 

or recharges, the intermediate aquifer system and Upper 

Floridan aquifer (Yobbi, 1996). The water table in the surficial 

aquifer system appears to be less affected by topography 

than by hydraulic characteristics of the underlying deposits 

(Sacks and others, 1998; Daniel Moore, Florida Department 

of Agriculture and Consumer Services, written commun., 

1998). Breaches in the underlying confining unit, seasonal 

changes in lake levels, local water-table gradients near lakes, 

and hydraulic head differences between the surficial aquifer 

system and underlying aquifer systems affect the horizontal 

and vertical ground-water flow in the surficial aquifer system.

Most of the Ridge is underlain by soils that have been 

classified as vulnerable to leaching of agrichemicals (fig. 1) 

on the basis of high permeability rates and low organicmatter content (State of Florida, 1995, 2002a; Hurt and 

Obreza, 1991a,b; Wilson and others, 2002; Brown and others, 

2003). The vulnerable soil types on the Ridge include the 

Candler, Astatula, and Paola Soil Series, which are entisols 

that exhibit minimal soil development, contain little organic 

matter, consist of about 97 to 99 percent sand-sized particles, 

and have high hydraulic conductivity, typically in the range of 

24 to 51 in/hr throughout the soil profile (U.S. Department of 

Agriculture, 1989, 1990). 

Land use on Lake Wales Ridge is predominantly 

agricultural, and citrus covers about 24 percent (787,600 

acres) of the study area. The Ridge citrus represents about 

14 percent of total citrus acreage statewide. Orange trees 

constitute about 93 percent of the citrus in Polk and Highlands 

Counties, and grapefruit represents the remainder (Shahane, 

2003). The types and amounts of pesticides used in these 

citrus classes are similar. During the past 25 years, citrus 

acreage has been decreasing in Polk County and increasing 

in Highlands County due in part to increasing urban development in Polk County and to the southerly migration of citrus 

to avoid winter freezes (Choquette and Sepulveda, 2000). 

Most of the citrus groves in Polk and Highlands Counties are 

irrigated (93 and 98 percent of groves, respectively) (Florida 

Agricultural Statistics Service, 1998). Irrigation in the Ridge 

citrus is performed by a variety of methods including drip, 

microsprinkler, and overhead sprinkler systems.

Agrichemical Usage

Statewide estimates of pesticide and fertilizer usage 

provide a general indication of the types, rates, and amounts 

of chemicals applied in Florida’s citrus agriculture. The types 

and amounts of pesticides used in individual citrus groves, 

however, can vary widely, and the active ingredients applied 

can change over time due to a number of factors including 

the registration of new products, discontinuation of products, and the rotation of chemicals to avoid development of 

resistance to the chemicals, and reduced efficacy, for targeted 

pests (Jackson and Davies, 1999; Boman, 2002). In 2001,

59.3 million pounds (active ingredient) of pesticides were 

applied in citrus regions of Florida. The primary pesticide 

compounds used in Florida orange groves between 1995 and 

2003 are shown in figure 3. Petroleum distillate (mineral oil), 

primarily used as an insecticide, was the most widely applied 

pesticide and was used in 84 to 93 percent of Florida’s 

orange groves during these years (Florida Agricultural 

Statistics Service, 2002). Excluding petroleum distillate, 

pesticide usage in orange groves in terms of pounds of active 

ingredient per year is lead by herbicides (50 percent of 

total), insecticides (29 percent), and fungicides (21 percent).

Herbicides, insecticides, and fungicides were applied in 95, 

91, and 61 percent, respectively, of Florida orange groves in 

2002-03 (Florida Agricultural Statistics Service, 2004). 

Usage of several citrus pesticides, including 1,2-dibromoethane, bromacil, fenamiphos, and aldicarb, has been 

either restricted or prohibited in the vulnerable soils of the 
Description of Study Area 7

Ridge (State of Florida, 1995, 2002a; Aerts and Nesheim, 

2000). Detections of bromacil in drinking-water wells led to 

the product being prohibited from use in most Ridge citrus 

areas in December 1994 (State of Florida, 1995), and it has 

subsequently been replaced by other herbicides including 

norflurazon and glyphosate. In Florida, aldicarb is classified as a “restricted-use pesticide” (State of Florida, 2002a), 

which indicates usage restrictions on the amounts, timing, 

and/or locations of applications. In citrus groves on sandy 

soils, aldicarb use is limited to one application per year at a 

maximum rate of 5 pounds (lbs) active ingredient per acre, 

applications must occur between January 1st and April 30th, 

and its usage is prohibited near potable wells. 

Statewide annual average application rates in Florida 

orange groves, for 1997, 1999, 2001, and 2003, for selected 

pesticides of interest in this study are shown in figure 4 

(Florida Agricultural Statistics Service, 2002, 2004). Local 

application rates on the Ridge may vary from these averages. The annual average application rate for these six 

pesticides in 2003 ranged from about 1.2 lbs/acre for bromacil 

to 3.9 lbs/acre for simazine, and increased during the 1997 

to 2003 period for bromacil, diuron, norflurazon, simazine, 

and glyphosate. The 1997 application rate for the herbicide 

simazine was higher for citrus in Florida than for any other 

crop nationally, and was twice that used in California citrus 

(Gianessi and Silvers, 2000).

0

10

20

30

40

50

60

70

80

90

100

1995

1997

1999

2001

2003

USAGE IN FLORIDA ORANGE GROVES

(PERCENT OF TOTAL ACREAGE)

EXPLANATION

Petroleum

distillate

Glyphosate

Copper hydroxide

Diuron

Simazine

S-methoprene

Sulfur

2,4-D

Abamectin

Bromacil

Norflurazon

Pyraclostrobin

Aldicarb

Paraquat

Mefenoxam

Pyridaben

Copper sulfate

Fenbutatin-oxide

Carbaryl

Imidacloprid

Basic copper sulfate

Ethion

Diflubenzuron

Chlorpyrifos

Azoxystrobin

Trifloxystrobin

Copper chloride hydrate

PESTICIDE

Figure 3. Primary pesticides applied in Florida orange groves from 1995 to 2003, showing usage as the percent of total 

bearing acreage receiving applications (Florida Agricultural Statistic Service, 2000, 2002, 2004). Pesticides are distinguished 

by colors as herbicides in green, insecticides in red, and fungicides in blue.

0

1

2

3

4

1997

1999

2001

2003

EXPLANATION

APPLICATION RATE, IN POUNDS

OF ACTIVE INGREDIENT PER

ACRE PER YEAR

Aldicarb

Bromacil

Diuron

Norflurazon

Simazine

Glyphosate

PESTICIDE

Figure 4. Statewide average application rates for 

selected pesticides in Florida orange groves between 

1997 and 2003 (Florida Agricultural Statistics Service, 

2002, 2004).
8 Water Quality and Evaluation of Pesticides in Lakes in the Ridge Citrus Region of Central Florida

Due to the long growing season and warm climate, 

pesticides are often applied multiple times per year in Florida. 

Between 1997 and 2001, the herbicides norflurazon, diuron, 

and simazine were applied in orange groves on the average 

about 1.5 times per year (Florida Agricultural Statistics 

Service, 2002). Norflurazon, diuron, and simazine are applied 

prior to weed germination (pre-emergent) during spring and/or 

fall. Diuron, simazine, bromacil, norflurazon, and aldicarb 

were estimated to have been applied on the average at 59, 31, 

27, 20, and 11 percent, respectively, of Florida orange groves 

statewide during this period. 

Citrus cultivation on the Florida ridge soils often 

requires fertilization with nitrogen, phosphorus, potassium, 

and magnesium, along with lime amendments to maintain 

a moderately acidic soil (Stauffer, 1991). Annual average 

application rates for fertilizers in Florida orange groves in 

1999 were 215 lbs/acre nitrogen, 213 lbs/acre potash, and 73 

lbs/acre phosphate applied to 94, 94, and 71 percent of orange 

groves, respectively (Florida Agricultural Statistics Service, 

2000). Nitrate fertilizers were applied an average 4.6 times 

per year in 1999, and potassium fertilizers were applied 

an average 4.3 times to orange groves in Florida (Florida 

Agricultural Statistics Service, 2000). Studies of the fate and 

transport of nitrate from citrus areas on sandy soils in central 

Florida indicate that fertilizer types (solubility), application 

rates, irrigation management, and soil organic carbon content 

can affect denitrification rates and leaching of nitrate into 

ground water (Graham and Alva, 1996; Wang and Alva, 1996; 

Paramasivam and others, 1999; Paramasivam and Sajwan, 

2001).

Hydrology of Ridge Seepage Lakes

The lakes in central Florida typically represent surface 

signatures of dissolution of limestone at depth in this karst 

region mantled by sand. A number of studies have focused 

on the geologic history, water budgets, and water quality of 

Florida’s seepage lakes (Sacks and others,1998; Lee, 2002; 

Sacks, 2002; Swancar and Lee, 2003). Regional study of 

Ridge lakes indicated that nearly 70 percent of the 27 study 

lakes derived more than 50 percent of their total annual inflow 

(including precipitation) from ground water (Sacks, 2002). 

Direct runoff from the land surface into these lakes, with the 

exception of stormwater runoff in urban areas, is considered to 

be minimal because of the high permeability of the sandy soils 

in the area. 

The lakes often exhibit complex hydraulic connections 

with the adjacent aquifer systems based on a number of factors 

including local hydrogeology, head gradients, and topography, 

as well as constructed controls on lake levels such as inter-lake 

drainage canals. Multiple episodes of subsidence and infilling 

of sediments can influence subsurface hydraulic connections (Tihansky and others, 1996). Sinkholes and subsidence 

features may also modify or breach the intermediate confining 

unit resulting in direct hydraulic connections between the 

surficial aquifer system and Upper Floridan aquifer (fig. 5). 

Lake water budgets and hydrologic models of seepage 

lakes on Florida’s central ridges have provided insight to the 

hydraulics of these lake systems. Hydraulic head is generally higher in the lakes than in the Upper Floridan aquifer, 

providing a constant potential for downward flow from the 

WATER

WATER

TABLE

Rainfall

NOT TO SCALE

Recharge

Evaporation Septic tank

Intermediate confining unit

Surficial aquifer system

Upper Floridan aquifer

Citrus

Grove Irrigation

Well

TABLE

Figure 5. Generalized 

hydrogeologic section showing 

ground-water/surface-water 

interactions in the vicinity of 

Ridge seepage lakes. Arrows 

denote direction of water 

movement (from Sacks and 

others, 1998).
Selection and Description of Sampled Lakes 9

lakes to the Upper Floridan aquifer (Sacks and others, 1998), 

the principal drinking-water supply in the region (fig. 5). 

Many Ridge lakes consist of flow-through systems that both 

receive and disperse water to the adjacent surficial aquifer 

system, and often exhibit changes or reversals in groundwater flow directions in response to transient climatic events. 

Ground-water exchange rates (inflow and outflow) can vary 

considerably between lakes and over time. 

The direction of lateral ground-water flow around lakes 

does not necessarily parallel the basin topography. For example, 

lateral ground-water outflow from one Ridge lake (Lake Starr) 

in topographically higher parts of the lake floor was attributed 

to preferential flow toward karstic features (Swancar and 

others, 2000). In the deeper parts of the lake, water typically 

flowed vertically into the Upper Floridan aquifer. Deep lakes 

tend to have higher rates of ground-water inflow and more 

delayed responses to recharge (rainfall) than lakes in shallow 

topographic basins (Sacks and others, 1998). Modeling studies 

indicate ground-water catchment areas for seepage lakes on 

the Ridge can be substantially smaller than the topographic 

drainage basins (Sacks and others, 1998; Lee, 2002). 

Ground water and atmospheric inputs are the primary 

potential sources of elevated solutes and contaminants in most 

of the Ridge lake systems. Sacks and others (1998) inferred 

that the Ridge lakes typically are well mixed with respect to 

major inorganic solutes, on the basis of little lateral or vertical 

stratification of specific conductance, and that land use can 

alter lake water quality. 

Lakes in undeveloped regions of the Ridge exhibited 

specific conductance (51 to 126 μS/cm) and pH (3.7 to 4.5) 

values that were considerably lower than lakes in residential 

and agricultural settings (Sacks and others, 1998). The Ridge 

lakes are naturally acidic due to low base cation concentrations 

and low acid neutralizing capacity in ground water and in 

rainfall (Canfield, 1983; Brenner and others, 1990; Sacks and 

others, 1998; National Atmospheric Deposition Program, 2008). 

Historical water-quality data for lakes in citrus areas of central 

Florida indicate nitrate (as N) concentrations ranging from 

about 0.5 to 21 mg/L (Sacks and others, 1998; Romie, 2000). 

Selection and Description of 

Sampled Lakes

The lakes sampled in the study included small- to 

medium-sized lakes (less than 400 acres) distributed across 

the Lake Wales Ridge from central Polk County to southern 

Highlands County (fig. 1). Criteria considered in selecting 

lakes included lake size and depth; citrus land use in the lake 

basin; elevated lake-water nitrate concentrations in historical 

sampling (Sacks and others, 1998; Romie, 2000; Kolasa 

and others, 2001); availability of information on adjacent 

ground-water flow paths from previous studies; absence of 

direct pesticide applications to the lake; and prior detections of 

pesticides in ground water within the surficial aquifer system 

in the vicinity of the lake (Choquette and others, 2005). 

The seven citrus lakes included in the study were 

located in basins dominated by citrus agriculture and sandy, 

organic-poor soils considered to be vulnerable to leaching of 

agrichemicals (State of Florida, 1995, 2002a; Hooweg and 

Hornsby, 1998; Brown and others, 2003; Hurt and Obreza, 

1991a,b). Four of the lake basins (Lakes Denton, Lynn, 

Pabor, and Leonore) included some residential development. An inactive industrial facility, which appeared to be 

a former citrus-processing plant, was located near Pabor 

Lake. The control lake (Lake Annie) was located within 

Archbold Biological Research Station, an ecological preserve 

in the southern part of the Lake Wales Ridge (fig. 1). Surface 

areas of the studied lakes ranged from 5 to about 393 acres, 

with estimated maximum depths ranging from about 20 to 

65 ft (table 1). Geometry of the sampled lakes ranged from 

small deep lakes to large shallow lakes, with surface-to-depth 

ratios ranging from 0.2 to 19.7 acres/ft. 

Four lakes with historical nitrate concentrations 

exceeding 4 mg/L were selected to increase the probability 

of sampling some lakes in which pesticides or pesticide 

degradates would be present. Elevated nitrate concentrations in the lake water may indicate higher concentrations of 

other soluble agrichemicals and higher rates of ground-water 

inflow (Sacks, 2002). Pesticides have been applied to selected 

lakes in Polk and Highlands Counties to control macrophyte 

growth (Florida Department of Environmental Protection, 

2005). The eight pesticides registered for use (in 2003) in 

Florida lake waters included copper, diquat, endothall, fluridone, glyphosate, 2,4-dicholorophenoxyacetic acid (2,4-D), 

triclopyr, and imazapyr (University of Florida, 2003). County 

personnel confirmed that the studied lakes had not previously 

received any pesticide applications.

Lake bathymetry and dominant directions of groundwater inflow have been measured in two of the study lakes—

Swim Lake and Lake Annie (Sacks and others, 1998). Both 

lakes were flow-through lakes exhibiting areas of groundwater inflow and areas of outflow. Swim Lake and Lake Annie 

Table 1. Physical characteristics of the sampled lakes.

Lake Surface area

(acres)

Approximate

maximum depth

(feet)

Lake surface/ 

depth ratio

(acres per foot)

Swim 5 30 0.2

Lynn 16 45 .4

Pabor 40 20 2.0

Denton 57 48 1.2

Annie (“control”) 92 65 1.4

Aurora 108 37 2.9

Moody 391 20 19.6

Leonore 393 20 19.7
10 Water Quality and Evaluation of Pesticides in Lakes in the Ridge Citrus Region of Central Florida

were characterized by high monthly variability in net groundwater flow compared to the other lakes examined by Sacks 

and others (1998). In Swim Lake, monthly net ground-water 

flow was usually positive (inflow greater than outflow) during 

spring and summer, and negative (outflow greater than inflow) 

during winter and fall. In contrast, net flows in Lake Annie 

were consistently positive. Such differences indicate that 

inflow and outflow rates and residence times can vary between 

lakes (Sacks and others, 1998). 

The control lake, Lake Annie, has been described as 

one of the most pristine bodies of water in southern Florida 

(Layne, 1979), although there have been some recent reductions in lake transparency and increases in dissolved nutrients 

(Swain and Gaiser, 2005). The 5.7-mi2 area of the lake 

drainage basin is dominated by native vegetation, but also 

includes some areas of livestock grazing and citrus. Thermal 

stratification, which is uncommon in Florida lakes, has been 

observed at Lake Annie between spring and fall (March 

through October), and periodically during winter (Eckblad 

and others, 1974; McDiffett, 1978). The stratification has been 

attributed to the lake’s small surface area, considerable depth, 

and protection from the mixing action of wind by a wellvegetated shoreline (McDiffett, 1978). 

Prior to and during the sampling period, annual precipitation from 2003 through 2005 ranged from 54.8 to 66 in/yr 

at Archbold Biologic Research Station, and exceeded the 

74-year, long-term mean of 53.8 in/yr (Nancy Deyrup, 

Archbold Biologic Research Station, written commun., 2003; 

National Oceanic and Atmospheric Administration, 2004, 

2005). These precipitation conditions indicate average to 

above-average annual precipitation inputs to the lakes, and the 

likelihood of similarly high ground-water inflow to the lakes 

during the sampling period on the basis of lake water-budget 

studies in the region (Sacks and others, 1998; Lee, 2002; 

Swancar and Lee, 2003).

Methods of Study 

Sampling methods adhered to stringent parts-per-billion 

protocols and laboratory analyses for all pesticides and 

pesticide degradate concentrations were performed using 

high-performance liquid chromatography/mass spectrometry. 

Sampling protocols included quality assurance sampling and 

analyses. Interpretive analyses of pesticide results included 

methods for adjusting summary statistics and the use of a 

standardized reporting level to avoid potential bias due to 

varying laboratory reporting levels. 

Sampling and Laboratory Methods

Swim Lake, Lake Lynn, Lake Aurora, and Lake Denton 

were sampled quarterly during the first year of the study 

(December 2003 to September 2004), and Lake Moody, 

Lake Leonore, Pabor Lake, and Lake Annie were sampled 

quarterly during the second year of the study (December 2004 

to September 2005). Water samples were collected near the 

deepest part of each lake using a stainless-steel Kemmerer 

sampler. Approximately 5 to 8 L of water were composited in 

a Teflon churn splitter. Samples were collected from a depth 

of 5 ft below lake surface, with the exception of a subset of 

paired samples to compare water quality at different depths in 

the lake water column. During June 2004 in four of the lakes 

(Swim, Lynn, Denton, Aurora), paired samples were collected 

near the middle of each lake at a depth about 5 ft below lake 

surface and near the lake bottom. 

Sample processing and equipment decontamination were 

performed according to USGS protocols (U.S. Geological 

Survey, 1997-2005). Samples collected for pesticide analysis 

were filtered through a 0.7-μm nominal pore size glass-microfiber filter, precombusted at 400 oC for at least 2 hours prior 

to use. Samples collected for analysis of calcium, magnesium, 

sodium, potassium, and orthophosphate concentrations were 

filtered through a 0.45-μm filter. Calcium, magnesium, potassium, and sodium samples were preserved with nitric acid to 

lower the pH to 2. Samples analyzed to determine total organic 

carbon, ammonia nitrogen, unfiltered orthophosphate, nitrite 

nitrogen, and total nitrogen concentrations were preserved 

with sulfuric acid to lower the pH to 2.

Water-quality analyses included 83 pesticides and 

pesticide degradates, major ions, nutrients, alkalinity, total 

dissolved solids, total suspended solids, turbidity, and water 

color (app. 1). The particular suites (laboratory schedules) 

of targeted pesticides analyzed in this study (table 2) include 

some pesticides not typically used in citrus agriculture because 

the suites originally were developed by the USGS for national 

pesticide analysis. Analyses of glyphosate, aminomethylphosphonic acid (AMPA), and glufosinate were not performed on 

samples collected during December 2003 and 2004, or during 

September 2004, due to cost constraints. Analyses of the diuron 

degradates 3,4-dichlorophenyl urea (DCPU) and 3(3,4-dichlorophenyl) methyl urea (DCPMU) and demethyl norflurazon 

were not performed during June and September 2005.

Appendix 1 includes documentation of laboratory 

methodology and reporting levels by constituent. Inorganic 

constituent concentrations were determined at the SWFWMD 

laboratory in Brooksville, Florida, using standard U.S. 

Environmental Protection Agency methods (Southwest Florida 

Water Management District, 2002). Pesticide concentrations 

were determined using high performance liquid chromatography and mass spectrometry at the USGS National WaterQuality and Organic Geochemistry Research Laboratories. 

Most pesticide and pesticide degradates concentrations were 

analyzed according to Furlong and others (2001). Most 

triazine pesticides and pesticide degradates and the phenylurea 

pesticides were analyzed according to Lee and others (2002b). 

Glyphosate and its degradate AMPA and glufosinate were 

analyzed according to Lee and others (2002a).
Methods of Study 11

Table 2. Pesticides and pesticide degradates analyzed in the lake samples. 

[Reporting levels and analytical methods appear in appendix 1. Pesticide degradates are shown in italics]

Pesticide or degradate name Pesticide or degradate name Pesticide or degradate name

2,4-D (2,4-Dicholorophenoxyacetic acid) Cycloate Norflurazon

2,4-D methyl ester Dacthal monoacid Demethyl norflurazon

2,4-DB Dicamba Oryzalin

3-ketocarbofuran Dichlorprop Oxamyl

Acifluorfen Dinoseb Picloram

Aldicarb Diphenamid Prometon

 Aldicarb sulfone Diuron Propazine

 Aldicarb sulfoxide DCPMU [3-(3,4-dichlorophenyl) methyl urea] Propham

Atrazine DCPU [1-(3,4-dichlorophenyl) urea] Propiconazole

Bendiocarb Fenuron Propoxur

Benomyl Flumetsulam Siduron

Bensulfuron-methyl Fluometuron Simazine

Bentazon Demethyl fluometuron Sulfometuron-methyl

Bromacil Glufosinate Tebuthiuron

Bromoxynil Glyphosate Terbacil

Carbaryl Aminomethyl phosphonic acid (AMPA) Triazine1 Degradates:

Carbofuran Imazaquin DDA (CAAT): Didealkylatrazine, 

Chlordiamino-s-triazine 

 3-Hydroxycarbofuran Imazethapyr DIA (DES, CEAT): Deisopropylatrazine; 

Deethylsimazine; 2-Chloro-6-ethylamino-4-

amino-s-triazine 

Chloramben, methyl ester Imidacloprid DEA (CIAT ): deethylatrazine; 2-Chloro-4-

isopropylamino-6-amino-s-triazine 

Chlorimuron-ethyl Linuron DEHA (HDEA; OIAT): Deethylhydroxyatrazine; 2-hydroxy-4-isopropylamino-6-

amino-s-triazine

CPMU [3-(4-chlorophenyl)-1-methyl urea] MCPA (2-methyl-4-chlorophenoxyacetic acid) DIHA (HDIA; OEAT): Deisopropylhydroxyatrazine; 2-hydroxy-6-ethylamino-4-amino-striazine 

Chlorothalonil MCPB (4-(2-methyl-4-chlorophenoxy) butyric acid HA (OIET): 2-Hydroxy-4-isopropylamino-6-

ethylamino-s-triazine (2-hydroxyatrazine)

Clopyralid Metalaxyl HS (OEET): Hydroxysimazine; 2-hydroxy-4,6-

diethylamino-s-triazine

Cyanazine Methiocarb Tribenuron-methyl

 Cyanazine amide Methomyl Triclopyr

 Cyanazine acid Metsulfuron methyl Oxamyl

 Deethylcyanazine Neburon Picloram

 Deethylcyanazine acid Nicosulfuron

 Deethylcyanazine amid

1The triazine pesticides include atrazine, simazine, cyanazine, and propazine. Alternative abbreviations and nomenclature also are shown. 

See appendix 5 for additional information on triazine parent pesticides and degradates, and degradation pathways.
12 Water Quality and Evaluation of Pesticides in Lakes in the Ridge Citrus Region of Central Florida

Quality Assurance

Quality assurance sampling included field blank samples, 

replicate samples, and matrix spike samples. Ten field blank 

samples were collected from December 2003 to September 

2005 to evaluate potential for contamination. Four of these 

samples were analyzed for the suite of constituents determined according to Furlong and others (2001). Three of the 

samples were analyzed for the suite of constituents determined 

according to Lee and others (2002b), and three samples were 

analyzed for the suite of constituents determined according 

to Lee and others (2002a). Three constituents were detected 

in the field blank samples; caffeine was detected once at a 

concentration of 0.033 μg/L, propiconazole was detected once 

at a concentration of 0.0034 μg/L, and bromacil was detected 

once at a concentration of 0.06 μg/L. 

One replicate sample was collected to evaluate the potential for variability between concurrently collected samples. 

This sample was collected from Lake Lynn in March 2004 

and analyzed for the 21 pesticides and degradates determined 

according to Lee and others (2002b). Four constituents were 

detected in the replicate and environmental sample from 

this site—bromacil, deisopropylatrazine, didealkylatrazine, 

and hydroxysimazine. The relative standard deviation for 

each of these constituents was 2.7, 0, 8.3, and 20.2 percent, 

respectively.

Three samples were spiked with 200 μL of a pesticide 

mixture to determine the recovery of these pesticides in lake 

water. The spike solution contained 2.5 μg/mL of a solution 

containing all of the pesticides analyzed [according to Furlong 

and others (2001)] in appendix 1, except for demethyl norflurazon, DCPMU, and DCPU. The lake waters used to spike 

these samples were collected from Lake Leonore and Pabor 

Lake between December 2004 and September 2005, and 

these samples were analyzed according to Furlong and others 

(2001). The minimum, maximum, and mean matrix spike 

recoveries are listed in table 3. Mean percent recovery values 

ranged from 8.7 percent for aldicarb sulfone to 93.3 percent 

for 2,4-D.

Analytical Methods

Analyses of water-quality data included descriptive 

summary statistics for both spatial (for example, by lake) and 

temporal groupings of data. To avoid bias due to censoring of 

pesticide concentrations, calculations of descriptive statistics 

for pesticides incorporated adjustments for censoring and 

standardized censoring levels. Descriptive statistics for 

censored data were determined on the basis of the log-likelihood method (Helsel and Cohn, 1988; Helsel, 2005) using 

S-Plus Software (Insightful Corporation, 2001). A minimum 

of three sample detections above laboratory reporting levels 

were required to determine summary statistics using the 

log-likelihood method (Helsel, 2005), which also incorporated 

adjustments to account for multiple censoring levels. Both 

parametric and non-parametric statistical methods were used 

to calculate summary statistics. Correlations were performed 

using the non-parametric Spearman’s rho rank correlation 

coefficient (Helsel and Hirsch, 1992) using SAS software 

(SAS Institute Inc., 2008).

Duplicate analyses performed at different laboratories 

were available for some of the targeted pesticides based on the 

methods of Furlong and others (2001) performed at the USGS 

National Laboratory, and Lee and others (2002b) performed 

at the USGS Organic Geochemistry Laboratory (apps. 1 and 3). 

Duplicate analyses were performed for atrazine, bromacil, 

diuron, chlordiamino-s-triazine (DDA), chloro-6-ethylamino4-amino-s-triazine (DIA), and 2-chloro-4-isopropylamino-6-

amino-s-triazine (DEA). For summaries in this report, results 

from one of the duplicate analyses were selected on the basis 

of method performance, as indicated by laboratory quality 

assurance data on recovery and precision, and analytical resolution (lower reporting levels), and the analyzing laboratory is 

noted for these analytes in the summary tables.

For parent-degradate pairs or groupings, consistency of 

methods between the parent and degradate analyses also was 

desirable to avoid introducing extraneous variability by using 

multiple samples, laboratories, and analytical methods. Results 

based on Lee and others (2002b) were used as the primary 

source of information for the simazine degradates (DIA, 

DDA) and bromacil. Results based on Furlong and others 

(2001) were used as the primary source of information for 

diuron, atrazine, DEA, and hydroxyatrazine.

Both unadjusted pesticide detection frequencies, 

associated with differing laboratory reporting levels, and 

standardized detection frequencies, associated with a single 

“standardized” reporting level, were used to summarize results 

for censored concentration data. The unadjusted summaries 

provided information at the highest analytical resolution 

supplied by the laboratories. Standardized pesticide detection 

frequencies were calculated using a single reporting level of 

0.06 μg/L, which was the lowest laboratory reporting level 

common to all detected compounds. The use of a standardized reporting level minimizes potential bias in comparisons 

between analytes with differing laboratory reporting levels. 

Such bias can also arise when reporting levels change over 

time, and when estimated concentrations below laboratory 

reporting levels are inconsistently reported for all of the 

samples, such as in the case of interference from the sample 

matrix or other effects. In comparisons using varying laboratory reporting levels, analytes having lower reporting levels 

are apt to be detected more often solely due to a higher resolution of detection.

The pesticide concentrations were compared to aquaticlife and drinking-water benchmark guidelines specified by 

the U.S. Environmental Protection Agency and the State of 

Florida, and to national pesticide monitoring by the USGS 

National Water-Quality Assessment (NAWQA) Program of 

surface waters that drained other land uses and crop types.
Methods of Study 13

Several indices were used to compare pesticide chemical 

properties and usage to the occurrence of pesticides in the 

lakes. The comparisons provide an indication of the utility of 

these indexes for estimating pesticide occurrence in the lakes 

on the Ridge, and possibly in other regions, and the potential 

value of such indexes in the design of monitoring networks, 

such as selecting pesticides targeted for analysis. 

The leaching vulnerability index (Schlosser and others, 

2002), the pesticide usage index, and the transport-potential 

index, were defined as:

Equation 1: Leaching vulnerability index = Koc/ t1/2, where 

Koc is the organic carbon partition coefficient, and t1/2 is 

the estimated half-life of the compound in soil; and is a 

measure of the propensity of the pesticide to leach from 

the soil into ground water.

Equation 2: Pesticide usage index = the product of the 

percent of total crop area [by crop type2] in the area of 

interest and the application rate (in AI/acre/year), where AI 

is the active ingredient of pesticide.

Equation 3: Transport-potential index = the ratio of the 

pesticide usage index to the leaching vulnerability index.

Table 3. Matrix spike recovery results for selected pesticides and pesticide degradates in the lake samples. 

[The samples were analyzed according to Furlong and others (2001). Data represent results from three samples, except as noted]

Analyte Percent recovery Analyte Percent recovery

Mean Minimum Maximum Mean Minimum Maximum

2,4-D 93.3 74.7 115.9 Diuron 76.4 50.2 121

2,4-D methyl ester 36.3 2 96.2 Fenuron 56.2 39.2 82.9

2,4-DB 52.4 33.6 77 Flumetsulam 87.5 65.4 128.4

3(4-chlorophenyl) methyl urea 37.2 22.4 55.8 Fluometuron 52.3 48.4 119.9

3-Hydroxy carbofuran 37 19.2 71.9 Imazaquin 77.2 37.4 129.1

3-Keto carbofuran 11 0 33 Imazethapyr 56.2 35.5 79.2

Acifluorfen 58.7 11.2 96.7 Imidacloprid 89.1 65 127.1

Aldicarb 19.1 13.1 25.7 Linuron 76.7 46.7 135

Aldicarb sulfone 8.7 1.9 16.9 MCPA 51.8 33.6 77

Aldicarb sulfoxide 55.5 15.9 110.2 MCPB 57.3 37.4 84.4

Bendiocarb 25.7 0 77 Metalaxyl 90.6 61.4 139.4

Benomyl 35.1 21.5 55.2 Methiocarb 53.1 16 124.7

Bensulfuron, methyl 91.8 46.7 167.3 Methomyl 42.6 41.8 102.3

Bentazon 75.2 54.2 93.2 Metsulfuron, methyl 68.8 59.8 79.8

Bromoxynil 56.2 35.5 75.6 Neburon 55.8 18.7 128

Caffeine 60.5 44.3 90.2 Nicosulfuron 76.3 69.1 102.7

Carbaryl 45.6 14.9 99.4 Norflurazon 64.2 52.1 75

Carbofuran 38.5 36.1 103.3 Oryzalin 56.4 11.2 107.8

Chloramben, methyl ester 34.2 7.5 58 Oxamyl 42.2 3.7 117.4

Chlorimuron, ethyl 33.2 4.1 70.4 Pichloram 61.5 40.9 96.8

Chlorothalonil1 43.8 38.4 49.1 Propham 75.3 48.5 124.3

Clopyralid 19.5 7.5 26 Propiconazole 55.9 9.3 104.5

Cycloate 68 20.5 133.1 Propoxur 58.8 38.7 96.3

Dacthal, monoacid 67.3 46.7 99.4 Siduron 88 50.4 150.4

Dicamba 40.2 18.7 70.4 Sulfometuron, methyl 72.1 54.7 97.6

Dichlorprop 56.2 31.8 81.1 Tebuthiuron 51.5 37 74.1

Dinoseb 87.5 22.4 145.3 Terbacil 44.8 34.4 65.5

Diphenamid 76.4 54.2 119.2 Triclopyr 56.4 43 75.9

1Spike recovery based on two samples.

2Crop type corresponded to area of orange orchards in Florida.
14 Water Quality and Evaluation of Pesticides in Lakes in the Ridge Citrus Region of Central Florida

The pesticide application estimates used to calculate 

the pesticide usage index were statewide averages for orange 

orchards in Florida, which comprise the major citrus crop in 

Polk and Highlands Counties (Florida Agricultural Statistics 

Service, 2002; Shahane, 2003). Usage in specific orchards 

may vary from these averages. 

Lake Water Quality

Lake water quality was evaluated in terms of spatial and 

seasonal variability, as well as comparison between the lakes 

in citrus areas versus the control (undeveloped) lake basin. 

Interpretive analyses of water quality focused on the occurrence of agricultural chemicals in the lakes. The water-quality 

data for agricultural chemicals were evaluated in terms of 

potential land-use impacts, aquatic life and human-health 

guidelines, pesticide degradation, concentrations relative to 

national stream samples, and factors related to the transport 

and fate of chemicals in the lakes. Field and laboratory results 

for the lake samples are listed in appendixes 2 and 3.

Physical Properties and Major Constituents

Physical properties measured in lake water included

field measurements of Secchi depth, water color, temperature, 

specific conductance, dissolved oxygen, and pH. Laboratory 

analyses included major ions, total organic carbon, total 

dissolved solids, and suspended sediment. 

Water clarity, as measured by Secchi depth, ranged 

from 10 to 21 ft in lakes Denton, Swim, Lynn, and Aurora, 

in contrast to values generally between 1.5 and 5 ft in 

lakes Moody, Pabor, Leonore, and Annie (table 4 and 

app. 2). The lakes with greater water clarity were among the 

deeper lakes sampled and also were characterized by lower 

concentrations of total organic carbon (TOC) with median 

TOC values between 4.7 and 7.2 mg/L, compared to median 

values of 13.8 to 20.5 mg/L in the four lakes with lower clarity 

(table 4). 

Water color was less than 21 Pt-Co units in samples 

from all lakes except the control lake, Lake Annie, which 

was highly colored, ranging from 100 to 175 Pt-Co 

units. The median TOC concentration in the control lake 

(18.4 mg/L) was similar to the median values in lakes Moody 

(20.5 mg/L), Leonore (13.8 mg/L), and Pabor (15.1 mg/L), 

which given their lack of color, suggests the organic matter in 

the control lake has different characteristics, such as a large 

amount of organic matter derived from lignin. Suspended 

solids concentration was less than 16 mg/L in all sampled 

lakes (table 4). The highest median suspended solids 

concentrations occurred in Lake Moody (13 mg/L), Pabor 

Lake (6.7 mg/L), and Lake Leonore (6 mg/L), which were 

the shallowest lakes in the study (depths generally less than 

20 ft). The relatively higher suspended solids concentrations 

in these three lakes may reflect increased susceptibility to 

re-suspension of bottom sediments (for example, by wind or 

precipitation) as a result of their shallow depths.

The sampled lakes typically did not exhibit strong 

thermal stratification. Median temperatures in the lakes ranged 

from 24.7 to 25.8 oC with seasonal extremes ranging from 

14 to 30 oC (table 4). The temperature difference between 

the top and bottom of the water column in the lakes was 

less than 2.0 oC in more than 50 percent of the sampling 

events. The largest temperature gradient was a difference of 

about 11 oC measured in Lake Annie (during June 2005 and 

September 2005) and in Pabor Lake (June 2005), with higher 

temperatures observed at the top of the water column.

Alkalinity and total dissolved solids, calcium, magnesium, sodium, potassium, chloride, and sulfate concentrations 

were greater in all seven sampled citrus lakes compared to the 

control lake (table 4). Elevated concentrations of inorganic 

constituents in lakes in citrus areas as compared to undeveloped areas are consistent with other central Florida results 

from Sacks and others (1998) and Stauffer (1991). Similarly, 

elevated concentrations of calcium, magnesium, potassium, 

and sulfate have also been observed in streams draining 

residential and agricultural land use compared to undeveloped 

basins in areas underlain by sand and gravel deposits in New 

Jersey (Morgan and Good, 1988). 

Increased concentrations of potassium and other 

inorganic constituents measured in lakes in this study likely 

reflect inputs to the lakes from the surficial aquifer system, 

which has been affected by the use of agricultural chemicals 

in citrus areas. Fertilizers are likely a source of increased 

potassium, sodium, chloride, and sulfate concentrations to the 

lake water because fertilizers have been applied as potassium 

nitrate, potassium chloride, or sulfate salts (Stauffer, 1991). 

Citrus pesticides that contain sulfur, sulfate, and chloride 

(fig. 3) may also contribute to elevated concentrations of 

sulfate and chloride in the lakes.

The water in Pabor Lake was more mineralized compared 

to the other sampled lakes with higher concentrations of total 

dissolved solids, alkalinity, and dissolved sodium and potassium. Sodium and potassium were the predominant cations in 

the water in Pabor Lake and comprised about 65 percent of the 

sum of cations. In contrast, sodium and potassium typically 

comprised between 20 and 46 percent of the sum of cations in 

the other sampled lakes.

The sampled lakes generally had a near neutral pH 

and dissolved oxygen concentrations greater than 5 mg/L 

(table 4). The pH in the control lake was substantially lower 

than the citrus lakes, with a median value of 5.9 compared 

to medians ranging from 7.1 to 7.7 units for the citrus lakes. 

Undisturbed lakes in this part of Florida typically are acidic 

due to low acid neutralizing capacity in the water recharging 

the lakes from the surficial aquifer system (Canfield, 

1983). The higher pH values in most lakes sampled in this 

study may result from increased concentrations of anions 

in the ground water due to the fertilization and to liming of 

citrus groves, in which calcium carbonate is added in order to 

neutralize acidic soils. 
Lake Water Quality 15

Table 4. Summary statistics for major inorganic constituent concentrations in the sampled lakes. 

[The summary includes four samples per lake which were collected quarterly over a 1-year period at a depth of 5 feet below lake surface. 

All units are in milligram per liter, except as noted; nd, not determined; μS/cm, microsiemens per centimeter; <, less than]

Constituent and units Statistic

Lake

Annie

(“control”) Aurora Denton Leonore Lynn Moody Pabor Swim

Total dissolved solids 

Minimum 42 127 149 190 190 220 303 151

Median 47 136 166 202 191 230 313 169

Maximum 50 136 171 225 199 256 316 179

Dissolved calcium 

Minimum 2.43 16.7 20.5 21.9 21.8 25.9 19.2 24.1

Median 2.59 18.1 21.9 22.8 22.3 28.1 20.75 27.8

Maximum 2.76 18.3 22.1 24.9 22.6 30.1 21.5 28.4

Dissolved magnesium 

Minimum .69 7.25 11.8 14.3 15.9 15.2 7.83 7.44

Median .75 7.62 12.4 14.85 16.1 16.7 8.2 8.45

Maximum .76 7.78 12.4 16.2 16.3 17.9 8.32 8.72

Dissolved sodium 

Minimum 2.85 5.2 4.73 6.81 6.79 7.33 42.5 4.66

Median 3.03 5.2 4.83 7.13 6.96 8.08 44.2 5.04

Maximum 3.2 5.41 4.91 8.16 7.08 8.99 45.4 5.26

Dissolved potassium 

Minimum .59 11.2 12.8 12.9 14.3 12.5 47.5 11.2

Median .69 11.7 13.2 13.25 14.4 13.3 48.8 12.3

Maximum .82 12 13.7 15 14.7 14.3 50.8 13.1

Alkalinity, as calcium carbonate

Minimum 1.85 27.8 35.22 34.6 39.5 47.88 163.43 16.04

Median 2.37 29.58 35.4 41.49 41.61 49.42 166.21 31.7

Maximum 3.69 61.21 38.37 55.59 46.42 56.73 185.83 32.79

Dissolved sulfate 

Minimum 1.11 36.03 41.4 55.6 61.8 67.22 47.88 50.73

Median 2.43 39.76 42.8 56.51 62.07 72.54 48.02 57

Maximum 2.94 38.3 44.12 59.7 64.83 84.16 50.21 57.82

Dissolved chloride 

Minimum 2.97 17.36 25.15 25.02 27.05 26 16.5 13.91

Median 5.79 18.7 26.4 26.2 27.6 27.02 16.72 15.7

Maximum 6.07 19.78 27.53 28.51 28.65 28.9 16.81 16.23

Total organic carbon 

Minimum 15.3 4.6 4.5 5 3.8 9.9 13.4 4.5

Median 18.4 7 7.2 13.8 4.7 20.5 15.1 5.9

Maximum 36.8 10 9.7 29.6 5 47 31.5 8.5

Water color, in platinum-cobalt units

Minimum 100 <5 <5 10 <5 10 15 <5

Median 138 <5 <5 15 <5 12.5 15 <5

Maximum 175 <5 <5 20 <5 15 15 <5

Secchi depth1, in feet

Minimum 2.8 12 15 1.6 9.6 1.5 2.2 12

Median ND ND ND ND ND ND ND ND

Maximum 5.0 12.5 21 7.8 18 1.6 3.6 19

Suspended solids 

Minimum 1.3 .5 .7 1.3 .9 9.9 3.7 .6

Median 1.3 1.2 .8 6 1.4 13 6.7 .9

Maximum 2 1.3 1.2 10.8 1.5 15.9 9.2 1.2

pH, field, in standard units

Minimum 5.2 6.9 7.3 6.1 6.7 7 7.3 6.4

Median 5.9 7.2 7.65 7.35 7.1 7.45 7.5 7.15

Maximum 7 7.5 7.8 7.7 8.2 7.9 7.5 7.3

Dissolved oxygen

Minimum 5.4 7.1 7.8 7.5 7.2 6.8 6.4 6.2

Median 5.95 8.3 8.7 7.8 8.1 8.35 7.05 8.4

Maximum 8.7 8.3 9.6 8.1 8.9 10.3 9.9 8.9

Specific conductance, μS/cm at 

25 degrees Celsius

Minimum 37 218 279 311 327 339 474 257

Median 37.5 222 283.5 329 329 352.5 487.5 284.5

Maximum 39 238 303 343 343 383 495 296

Temperature, in degrees Celsius

Minimum 18.6 19.4 20.4 14 20.5 15.1 17.7 19.1

Median 25.75 25.1 24.9 24.7 25 24.95 25.3 25.25

Maximum 29.2 29.5 29.2 29.5 29.7 29.1 29.4 30

1Includes two to three measurements per lake.
16 Water Quality and Evaluation of Pesticides in Lakes in the Ridge Citrus Region of Central Florida

The dissolved oxygen in the lake samples from the 

shallow zone of the water column ranged from 5.4 to 

10.3 mg/L (table 4). Dissolved oxygen concentrations generally were lower near the bottom of the sampled lakes, especially during the summer months. In Lake Lynn, the bottom 

of the water column (depths about 35 to 45 ft) was virtually 

devoid of oxygen (less than 0.1 mg/L) during most sampling 

events, compared to dissolved oxygen concentration greater 

than 10 mg/L near the bottom of the water column (about 

45 ft) at Lake Denton.

Nutrients

The nutrients included nitrate nitrogen, nitrite nitrogen, 

ammonia nitrogen, total phosphorus, and total nitrogen. 

Many of the sampled lakes were characterized by high nitrate 

concentrations and low phosphorus concentrations. Nitrate 

concentrations ranged from less than 0.001 mg/L (as N) in 

Lake Annie and Pabor Lake, to 4.70 mg/L in Swim Lake 

(table 5). Maximum nitrate concentrations were highest 

in Swim Lake (4.7 mg/L), Lake Denton (3.45), and Lake 

Leonore (3.56) and were lowest in Lake Annie (0.16) and 

Pabor Lake (0.04). Increased nitrate concentrations in citrus 

lakes on the Lake Wales Ridge have been attributed to 

fertilizer use because nitrogen isotope analyses indicate that 

nitrate in the surficial ground water typically is of inorganic or 

fertilizer origin (Tihansky and Sacks, 1997). 

Ammonia and nitrite concentrations were low in the 

study lakes. Total ammonia concentrations ranged from 

minimum values of about 0.1 mg/L in several of the lakes 

to maximum values of 0.4 to 0.5 mg/L in Lake Moody and 

Pabor Lake (table 5). Nitrite concentrations in the lake 

samples ranged from less than 0.01 to 0.05 mg/L. Total 

nitrogen concentration ranged from 0.86 to 5.19 mg/L in the 

citrus lakes compared to 0.4 to 0.68 mg/L in the control lake 

(table 5).

Although the range in total nitrogen concentrations in 

Pabor Lake was similar to that observed in the other citrus 

lakes, Pabor’s nitrate concentrations were markedly lower, 

ranging from less than 0.001 to 0.04 mg/L compared with 

0.4 to 4.7 mg/L in the other citrus lakes (table 5). The low 

nitrate concentrations in Pabor may reflect differing sources of 

nitrogen or fertilizer application practices, a lower proportion 

of ground-water inflow compared to the other study lakes, 

biogeochemical conditions more conducive to denitrification 

(such as organic carbon in water), or higher inputs of organic 

nitrogen relative to inorganic forms, and possibly a combination of these factors. The elevated total nitrogen and low 

nitrate concentrations in Pabor indicate the relative proportion of organic to total nitrogen may be higher in Pabor Lake 

compared to the other citrus lakes (table 5). 

Phosphorus generally was not detected in Lake Aurora, 

Lake Denton, and Lake Lynn. Low phosphorus concentrations 

previously have been noted in lakes in the citrus agricultural 

areas of central Florida (Stauffer, 1991), and occur despite the 

application of phosphorus fertilizers at citrus farms an average 

of 3.1 times per year at a rate of 23 lbs/acre in 1999 (Florida 

Agricultural Statistics Service, 2000). Previous studies have 

attributed the low phosphorus concentrations in the lakes in 

citrus agricultural areas of central Florida to efficient phosphorus retention in the soil (Stauffer, 1991).

Table 5. Summary statistics for nutrient concentrations in the sampled lakes. 

[The summary includes four samples per lake which were collected quarterly over a 1-year period at a depth of 5 feet below lake surface. 

All units are in milligram per liter; <, less than]

Constituent and units Statistic

Lake

Annie

(“control”) Aurora Denton Leonore Lynn Moody Pabor Swim

Total nitrate (as N)

Minimum <0.001 0.97 2.71 1.36 1.01 0.4 <0.001 2.87

Median .06 1.16 3.2 2.91 1.53 .48 .01 3.65

Maximum .16 1.49 3.45 3.56 1.81 .99 .04 4.7

Total nitrogen

Minimum .4 1.35 2.98 2.39 1.53 2.31 .86 3.33

Median .48 1.76 3.78 4.01 2.05 2.63 1.41 4.79

Maximum .68 2 3.82 4.14 2.21 2.96 1.84 5.19

Total ammonia (as N)

Minimum .014 .017 .010 .012 .016 .026 .014 <.010

Median .021 .021 .020 .026 .067 .153 .024 .025

Maximum .082 .033 .175 .161 .147 .377 .459 .042

Total phosphorus

Minimum .01 <.01-.02 <.01 .02 .01 <.01-<.02

Median .015 <.01 <.01-<.02 .015 <.01-<.02 .02 .02 <.01

Maximum .02 .011 .03 .04 .03 .01
Lake Water Quality 17

Some characteristics of inorganic water chemistry unique 

to Pabor Lake, previously discussed, may signify sources 

of nitrogen species and denitrification or redox processes 

in Pabor that differ from the other citrus lakes. Pabor Lake 

exhibited markedly higher concentrations of sodium, 

potassium, alkalinity, total dissolved solids, and specific 

conductance and slightly lower median dissolved oxygen 

concentration, and was among three lakes with elevated total 

organic carbon concentration (table 4). One potential source 

of nitrogen is wastewater (for example, septic tank leachate 

from nearby residential development), which could also 

explain the elevated sodium and potassium concentrations in 

Pabor. Another potential influence on the water chemistry in 

Pabor Lake may be the currently inactive industrial facility 

in the basin; further study would be necessary to confirm the 

source(s) of these water-quality differences between Pabor 

Lake and the other citrus lakes. 

Pesticides and Pesticide Degradates

The sampling design of the Ridge lake study provides 

baseline data documenting spatial and short-term temporal 

variability of pesticide concentrations in the lakes. Variations 

in concentration and degradate-parent ratios were examined 

within and between the lakes, and seasonally in the year 

sampled. The pesticide concentrations in lake samples were 

compared to benchmark guidelines, pesticide concentrations 

in other areas of the United States, and leaching “risk factors” 

including pesticide chemical properties and citrus pesticide 

usage data. 

Regional-scale field monitoring of pesticides in water 

resources is limited, compared to traditional inorganic waterquality monitoring, due primarily to the high costs associated 

with sampling and laboratory analyses of organic constituents 

that typically occur at “parts-per-billion” concentrations. 

However, monitoring data are critical for documenting and 

understanding the behavior of pesticides in the environment 

under a variety of application practices and variable field 

conditions (for example, climate and soil), and for providing 

realistic concentrations for use in ecotoxicity laboratory 

studies. Results of the lake pesticide study provide important 

information for the potential for leaching of the targeted 

compounds into water resources in other regions and land 

uses, protection of drinking-water resources, potential impacts 

to the Ridge lake biota and ecosystems, the utility of using 

chemical and usage data to predict risks of pesticide leaching 

into water resources, and guidance in designing future monitoring networks. 

Occurrence of Pesticides 

Of the 83 pesticide compounds evaluated in the analysis 

(table 2), 20 pesticide compounds (12 parent pesticides and 

8 degradates) were detected at or above laboratory reporting 

levels (LRLs) in the 28 samples from the 7 lakes in citrus areas 

(fig. 6; table 6). Laboratory results for the pesticide samples 

are presented in appendix 3. In addition, the atrazine degradate 

DEA was observed below the LRL in one sample (Lake Lynn), 

at an estimated concentration of 0.006 μg/L. Herbicides and 

their degradates were the class of compounds most frequently 

detected in the lake samples followed by insecticides and 

fungicides, which is consistent with their usage in Florida’s 

0

10

20

30

40

50

60

70

80

90

100

110

0.001

0.01

0.1

1.0

10

100

Standardized detection frequency

(0.06 g/L LRL)

Unadjusted detection frequency

(0.006 to 0.06 g/L LRL)

Maximum Concentration

DETECTION FREQUENCY, IN

PERCENT OF SAMPLES

CONCENTRATION, IN

MICROGRAMS PER LITER

EXPLANATION

Demethyl norflurazon

Norflurazon

Bromacil

DDA

Hydroxysimazine

Diuron

Aldicarb sulfoxide

Metalaxyl

DIA

2,4-D

Aldicarb sulfone

Simazine

DCPMU

Atrazine

Hydroxyatrazine

Tebuthiuron

Oxamyl

Imidacloprid

2,4-D ME

Imazaquin

PESTICIDE

Figure 6. Frequency of 

pesticides and degradates 

detected in samples 

from the seven lakes in 

citrus areas. Degradates 

are shown in italics. The 

laboratory reporting levels 

(LRL) for the unadjusted 

detection frequencies 

ranged from 0.006 to 0.06 

μg/L (microgram per liter). 

The summary includes 

28 samples for most 

compounds, and excludes 

the deep water-column 

samples collected in four 

of the lakes. Pesticide 

acronyms are defined in 

table 2.
18 Water Quality and Evaluation of Pesticides in Lakes in the Ridge Citrus Region of Central Florida

citrus groves (fig. 3; Florida Agricultural Statistics Service, 

2004). Based on statewide usage for all crops, 6 of the parent 

compounds detected in the Ridge lakes were ranked within the 

top 21 pesticides applied in Florida between 1999 and 2002: 

diuron, 2,4-D, simazine, aldicarb, norflurazon, and bromacil, 

with estimated agricultural usage ranging from 283,000 to 

934,000 pounds of active ingredient annually (Shahane, 2003). 

Parent pesticides and degradates were detected during 

all seasons of the year, indicating the persistence of pesticide 

compounds in the lake systems for several months or longer 

after application. The pesticide degradates (and associated 

parent compounds) detected in lake samples include: demethyl 

norflurazon (norflurazon); DDA, DIA, DEA, hydroxysimazine, and hydroxyatrazine (simazine and atrazine); aldicarb 

sulfoxide and aldicarb sulfone (aldicarb); and DCPMU 

(diuron). The degradates detected more frequently, and often at 

higher concentrations, than their respective parent compounds 

included demethyl norflurazon, DDA, hydroxysimazine, DIA, 

aldicarb sulfone, and aldicarb sulfoxide (fig. 6). The pesticide 

degradates are discussed later.

Maximum concentrations of detected compounds in the 

citrus lakes ranged from 0.006 (tebuthiuron) to 17.1 μg/L 

(demethyl norflurazon) (table 6). Summary statistics for the 

pesticide compounds in citrus lake samples showed that most 

concentrations were less than 1 μg/L; the 90th percentile concentrations were less than 1 μg/L for all detected target pesticides 

and degradates except for bromacil, norflurazon, and demethyl 

norflurazon. Variability in pesticide concentrations (table 6), as 

measured by the standard deviation (SD), ranged from 0.002 μg/L 

(hydroxyatrazine) to 4.68 μg/L (demethyl norflurazon). The interquartile range (IQR) ranged from 0.003 μg/L (hydroxyatrazine 

and atrazine) to 0.92 μg/L (demethyl norflurazon).

Table 6. Summary statistics for concentrations of pesticides and degradates detected in the seven sampled citrus lakes. 

[The summary includes quarterly samples collected 5 feet below the lake surface. Statistics were calculated using the log-probability method and included 

adjustments to account for multiple reporting levels (Helsel, 2005). Concentrations are in microgram per liter (μg/L). Pesticide degradates are italicized. 

>, greater than; ND, not determined; SD, standard deviation; CV, coefficient of variation; LRL, laboratory reporting level; SRL, standardized reporting level of 0.06 

μg/L]

Compound Mean SD CV 

(percent)

Percentiles

Maximum

Interquartile 

range 

(IQR)

IQR/

median 

(percent)

Number

 of samples

Number 

of lakes

 with

detections 

>LRL (>SRL)

 10 25 50 75 90 Total

Concentration 

> LRL (>SRL)

Herbicides and degradates

2,4-D 0.062 0.218 350 0.001 0.003 0.010 0.024 0.113 1.16 0.021 206 28 6 (4) 2 (2)

2,4-D Methyl ester ND ND ND ND ND ND ND ND .036 ND ND 28 1 (0) 1 (0)

Atrazine .006 .003 48 .003 .004 .005 .007 .011 .015 .003 53 28 10 (0) 5 (0)

 Hydroxyatrazine .008 .002 29 .005 .006 .008 .009 .012 .013 .003 40 28 8 (0) 3 (0)

Bromacil .754 1.40 186 .008 .026 .175 .723 2.78 6.29 .697 398 28 18 (17) 5 (5)

Diuron .101 .332 328 .009 .015 .040 .068 .081 1.79 .052 130 28 23 (9) 7 (4)

 DCPMU .013 .007 49 .006 .009 .012 .018 .022 .030 .010 81 22 10 (0) 6 (0)

Imazaquin ND ND ND ND ND ND ND ND .021 ND ND 28 1 (0) 1 (0)

Norflurazon .619 1.17 188 .013 .058 .180 .656 2.37 5.30 .598 333 28 28 (21) 7 (6)

 Demethyl norflurazon 2.14 4.68 219 .038 .154 .336 1.07 12.3 17.1 .917 273 22 22 (19) 7 (7)

Simazine .029 .011 40 .015 .020 .026 .040 .041 .060 .020 77 28 12 (1) 5 (1)

 DDA .085 .069 81 .020 .033 .056 .138 .187 .270 .104 188 28 13 (12) 4 (3)

 DIA .030 .037 125 .003 .006 .014 .030 .101 .140 .024 167 28 10 (4) 4 (2)

 Hydroxysimazine .127 .210 165 .004 .011 .034 .190 .464 .940 .179 530 28 10 (9) 6 (5)

 DEA ND ND ND ND ND ND ND ND .006E ND ND 28 1 (0) 1 (0)

Tebuthiuron ND ND ND ND ND ND ND ND .006 ND ND 28 2 (0) 2 (0)

Insecticides and degradates

Aldicarb sulfone .032 .078 240 <.001 .001 .004 .019 .117 .330 .018 465 28 7 ( 4 ) 2 (2)

Aldicarb sulfoxide .066 .159 241 <.001 .001 .006 .062 .216 .670 .061 978 28 8 ( 7 ) 3 (2)

Imidacloprid ND ND ND ND ND ND ND ND .016 ND ND 28 1 (0) 1 (0)

Oxamyl ND ND ND ND ND ND ND ND .036 ND ND 28 2 (0) 2 (0)

Fungicides:

Metalaxyl .037 .043 117 .005 .009 .018 .048 .112 .180 .039 212 28 14 ( 5 ) 4 (2)
Lake Water Quality 19

The total number of pesticide compounds detected (at 

or above their LRLs) per lake ranged from 9 to 14 in the 

citrus lakes (fig. 7). In the control lake, only three pesticide 

compounds were detected (tebuthiuron, diuron, and atrazine) 

at concentrations less than 0.04 μg/L. All of the citrus lakes 

yielded detections of norflurazon, demethyl norflurazon, and 

diuron at or above their LRLs, and six of the seven citrus 

lakes yielded detections of DCPMU (diuron degradate) and 

hydroxysimazine at or above their LRLs (table 6 and fig. 8). 

Of the 20 pesticide compounds detected in the citrus 

lake samples at concentrations above LRLs, 12 compounds 

were detected at or exceeding the 0.06 μg/L standardized 

reporting level (SRL) (fig. 6). The most frequently detected 

pesticide compounds at the 0.06-μg/L SRL in the citrus lakes 

were (in decreasing order) demethyl norflurazon, norflurazon, 

bromacil, DDA, hydroxysimazine, diuron, and aldicarb 

sulfoxide with respective sample detection frequencies ranging 

from 86 to 25 percent (fig. 6). Norflurazon, demethyl norflurazon, and diuron were detected above LRLs in all citrus lakes; 

hydroxysimazine, DCPMU, bromacil, simazine, and atrazine 

were detected in five or more of the seven citrus lakes.

0

2

4

6

8

10

12

14

16

NUMBER OF PESTICIDES DETECTED

Swim

Leonore

Lynn

Aurora

Moody

Denton

Pabor

Annie

(control) ND (not detected)

LAKE

Standardized detection frequency

(0.06 g/L LRL)

Unadjusted detection frequency

(0.006 to 0.06 g/L LRL)

EXPLANATION

0

1

2

3

4

5

6

7

8

DETECTION FREQUENCY,

IN NUMBER OF LAKES

Norflurazon

Bromacil

DDA

Hydroxysimazine

Diuron

Metalaxyl

DIA

2,4-D

Simazine

DCPMU

Atrazine

Hydroxyatrazine

Tebuthiuron

Oxamyl

Imidacloprid

2,4-D ME

Imazaquin

Demethyl norflurazon

Aldicarb sulfoxide

Aldicarb sulfone

PESTICIDE

Standardized detection frequency

(0.06 g/L LRL)

Unadjusted detection frequency

(0.006 to 0.06 g/L LRL)

EXPLANATION

Figure 7. Total number of pesticides and 

degradates detected in each of the lakes 

sampled quarterly over a year. The unadjusted 

detection frequency refers to laboratory 

reporting levels (LRL) that varied by pesticide 

compound from 0.006 to 0.06 μg/L (microgram 

per liter).

Figure 8. Number of citrus 

lakes in which targeted 

pesticide compounds 

were detected in one or 

more quarterly samples. 

The unadjusted detection 

frequency refers to 

laboratory reporting levels 

(LRLs) that varied by 

pesticide compound from 

0.006 to 0.06 μg/L (microgram 

per liter). Pesticide 

degradates are shown in 

italics. Pesticide acronyms 

are defined in table 2.
20 Water Quality and Evaluation of Pesticides in Lakes in the Ridge Citrus Region of Central Florida

Although DCPMU, atrazine, hydroxyatrazine, and 

tebuthiuron were detected in more than 25 percent of samples, 

these compounds were not detected at concentrations equal to 

or exceeding the 0.06-μg/L SRL. The detection frequencies 

by lake (fig. 8), which indicate detections in one or more of 

the samples collected per lake, tend to be somewhat higher 

than the sample detection frequencies (fig. 6) due to temporal 

fluctuations in concentrations and/or reporting levels. 

Some of the most commonly detected pesticides in the 

lake samples included pesticides that have not been widely 

used since 1994 (bromacil), and degradates of commonly 

used pesticides (simazine, diuron, norflurazon); by contrast, 

some commonly used pesticides and their degradates were 

not detected (glyphosate and AMPA). Bromacil continues to 

be detected in the Ridge lakes and in ground water (surficial 

aquifer system) at least 11 years after the prohibition on its 

use in most Ridge source areas, indicating bromacil’s persistence in Ridge water resources (Choquette and others, 2005; 

Gilliom and others, 2006). Bromacil was detected in five of 

the seven citrus lakes (fig. 8) and in more than 60 percent 

of lake samples (fig. 6). Bromacil has not been used in most 

Ridge citrus groves since the 1994 usage restriction (State 

of Florida, 1995), and its usage in non-citrus areas of the 

Ridge is reported to be insignificant (Danny Moore, Florida 

Department of Agriculture and Consumer Services, oral 

commun., 2003). Therefore, bromacil sources appear to be 

residues stored in soils, the surficial aquifer system, and 

perhaps lake sediments. 

Glyphosate and its primary degradate AMPA were not 

detected in any of the lake samples in spite of the fact that 

glyphosate is one of the most widely applied herbicides in 

Florida citrus (figs. 3 and 4). Its absence is possibly due to 

high sorption potential (organic carbon partition coefficient) 

and relatively short half-life in soil and in water; however, the 

lack of detections also may reflect the higher LRL for glyphosate and its degradate AMPA (0.1 μg/L) relative to the LRLs 

for the detected pesticide compounds, which ranged from 

0.006 to 0.03 μg/L. 

Tebuthiuron, detected at trace concentrations in two of 

the sampled lakes including the control lake, is typically a 

non-crop herbicide (Oregon State University, 1996b), and is 

not a major pesticide used in citrus areas (Shahane, 2003). 

Tebuthiuron was among the pesticides most frequently 

detected nationally in stream and ground-water sampling in 

urban/suburban and agricultural settings (Gilliom and others, 

2006), indicating broad usage nationally and a propensity for 

transport into water resources.

The nitrate concentration in the study lakes increased 

with the total number of pesticide compounds detected 

(fig. 9). The low nitrate concentrations in Pabor Lake (the 

outlier in fig. 9), and unusual inorganic water chemistry of 

the lake compared to the other citrus lakes, as previously 

discussed, indicate that nitrogen sources and perhaps denitrification processes in this basin differed from the other six citrus 

lakes. Similarly, total pesticide concentration (sum of quarterly 

samples for each lake) also increased with median nitrate 

concentration, except for Pabor Lake due to its low nitrate 

values (fig. 10). These relations suggest that elevated nitrate 

concentration may be an indicator of pesticide occurrence in 

the Ridge lakes, but the Pabor Lake data show that pesticides 

also may be present in lakes with low nitrate concentrations.

Comparisons to Benchmark Values

Concentrations of pesticides and degradates in the 

Ridge lake samples were compared to Federal aquatic-life 

benchmarks compiled by Gilliom and others (2006) (table 7) 

which were specified for 10 of the 20 pesticide compounds 

detected in the citrus lakes. Drinking-water criteria (U.S. 

Environmental Protection Agency, 2006), Florida guidance 

values for drinking water, and Florida’s Brownfields target 

cleanup levels for surface water (Florida Department of 

Environmental Protection, 2004) also are included in table 7 

for comparison. However, the lakes typically are not used 

directly as drinking-water sources. The benchmark comparison 

was intended to identify and prioritize potential needs for 

0.01

0.1

1.0

10

0 2 4 6 8 10 12 14 16

NITRATE CONCENTRATION,

IN MILLIGRAMS PER LITER

NUMBER OF PESTICIDES DETECTED

Lake Annie

(control)

Pabor Lake

Correlation Coefficients:

r = 0.69; p = 0.06 (Pearson)

rho = 0.66; p = 0.07 (Spearman)

Figure 9. Median nitrate concentration 

related to number of pesticides detected 

above laboratory reporting levels in each 

sampled lake. Medians are based on four 

samples from each lake.
Lake Water Quality 21

0.01

0.1

1.0

10

0.10 1.00 10.00 100.00

Pabor Lake

TOTAL PESTICIDE CONCENTRATION, IN MICROGRAMS PER LITER

NITRATE CONCENTRATION,

IN MILLIGRAMS PER LITER

Lake Annie

(control)

Figure 10. Median nitrate 

concentration related to total 

concentration of pesticides and 

degradates in quarterly samples 

summed over one year in each 

lake. Medians are based on four 

samples from each lake.

Table 7. Pesticide concentrations in lake samples compared to aquatic-life and human-health benchmark values. 

[The summary includes samples from all of the study lakes. The selected aquatic benchmark values correspond to those associated with the lowest concentrations listed in Gilliom and others (2006). A complete listing of the benchmarks is available at http://water.usgs.gov/nawqa/pnsp/. BM, benchmark; LRL, 

laboratory reporting level; ND, not detected above laboratory reporting level; NL, not listed, USEPA, U.S. Environmental Protection Agency; >, greater than; 

μg/L, microgram per liter]

Compound

Benchmarks (μg/L)

Maximum 

concentration 

in lake 

samples 

(μg/L)

Total 

number 

of lake 

samples 3

Number of 

samples 

with 

concentration 

greater 

than the 

LRL

Number of samples (and number of 

lakes) with concentrations exceeding 

specified USEPA aquatic-life criteria

USEPA 

drinking 

water

Florida 

drinking 

water

guidance

value 1

Florida 

surfacewater 

cleanup 

target 2

USEPA 

aquatic life > BM

> 0.5 

of the BM 

value to 

the BM

> 0.1 to 0.5 

of the BM 

value

 2,4-D 1,5 70 70 80 8299.2 1.16 36 6 0 0 0

Aldicarb 4,6 7, 53 67 .9 9.46 ND 36 0 ND ND ND

 Aldicarb sulfone 4,6 7, 52 67 46 10140 .33 36 7 0 0 0

 Aldicarb sulfoxide 4,6 7, 54 67 4.2 1021.5 .67 36 8 0 0 0

Atrazine 43 3 1.9 1117.5 .015 36 12 0 0 0

Bromacil 790 70 97 126.8 6.29 36 21 0 2 (1 lake) 6 (2 lakes)

Diuron 710 14 8 122.4 131.79 36 25 0 1 0

Norflurazon NL 280 NL 1213 5.30 36 28 0 0 3 (1 lake)

Oxamyl 4200 200 8.5 1090 .036 36 2 0 0 0

Simazine 44 4 7.3 1236 .06 36 12 0 0 0

1

 Values associated with ground-water cleanup targets under State of 

Florida (2004), which are used by the Florida Department of Environmental 

Protection as non-regulated guidelines for drinking water.

2 State of Florida (2004).

3 Includes the samples collected quarterly at a depth of 5 feet below lake 

surface from each of the eight study lakes, and the water-column samples 

collected near the lake bottom from four study lakes during one sampling 

event.

4

 USEPA Maximum Contaminant Level (U.S. Environmental Protection 

Agency, 2006).

5

 USEPA Maximum Contaminant Level, stayed effective date (U.S. 

Environmental Protection Agency, 2006).

6 The benchmark specifies that the sum for any combination of two or 

more of these three chemicals is not to exceed this amount, due to similar 

mode of action.

7 USEPA Lifetime Health Advisory (HAL).

8 USEPA acute vascular plants (each individual sample).

9 Chronic fish (60-day mean).

10 USEPA acute invertebrates (each individual sample).

11 USEPA chronic aquatic community (average for 60-day period).

12 USEPA acute nonvascular plants (each individual sample).

13 Duplicate analysis yielded a concentration of 3.81 μg/L (app. 3), which 

exceeds the associated USEPA aquatic benchmark for nonvascular plants.
22 Water Quality and Evaluation of Pesticides in Lakes in the Ridge Citrus Region of Central Florida

further monitoring or investigation, and was not intended as 

a risk assessment, which would include more detailed assessment of factors such as frequency, duration, and routes of 

exposure, and lethal and sublethal effects.

The aquatic-life benchmarks included acute and chronic 

effects on fish, invertebrates, nonvascular and vascular plants, 

and aquatic communities. Benchmarks have not yet been 

specified for all these aquatic-risk categories for each of the 

10 pesticide compounds in table 7. Also, these benchmarks do 

not take into consideration possible cumulative or interactive 

effects (synergistic or antagonistic) of pesticide mixtures.

Sample concentrations in the Ridge lakes were compared 

to the aquatic benchmark (table 7) that had the lowest concentration. The lowest benchmarks for the insecticides generally 

corresponded to acute effects on invertebrates, and for the 

herbicides generally corresponded to acute effects on plants or, 

in the case of atrazine, chronic effects on aquatic communities. 

None of the observed concentrations in the Ridge lakes 

exceeded the aquatic-life benchmarks, with the exception of 

one duplicate sample in which diuron exceeded the guideline 

for acute effects on nonvascular plants (sample concentration 

of 3.8 μg/L compared to guideline of 2.4 μg/L, table 7). In a 

few of the lake samples, concentrations of diuron, bromacil, 

and norflurazon approached the respective aquatic-life 

benchmarks (acute effects for nonvascular plants). Expressed 

as a percentage of the benchmark value, the maximum concentrations of diuron, bromacil, and norflurazon in lake samples 

were 75 percent, 71 percent, and 42 percent, respectively, of 

the benchmarks (table 7). 

Table 7 lists the number of sample concentrations that 

exceeded 10 percent of the benchmark concentrations for 

the respective compound detected. Diuron concentration 

exceeded 50 percent of the benchmark in one sample (Pabor 

Lake). Bromacil concentrations exceeded 50 percent of the 

benchmark in two samples (Lake Leonore), and were within 

10 to 50 percent of the benchmark in six samples (two samples 

from Lake Leonore and four samples from Lake Aurora). 

Norflurazon concentrations were within 10 to 50 percent of 

the benchmark in three samples (Swim Lake).

Some of the sample concentrations more closely 

approached the human-health benchmarks than the aquaticlife benchmarks. The maximum sampled concentrations of 

aldicarb sulfone and aldicarb sulfoxide were each 17 percent 

of the proposed Federal drinking-water maximum contaminant 

level3, and 5 to 10 percent of the current Federal and Florida 

drinking-water guidance values (table 7). Two samples from 

Swim Lake yielded concentrations of aldicarb sulfone and 

aldicarb sulfoxide (app. 3), exceeding 10 percent of the 

national guideline. The maximum concentration of diuron 

(1.79 μg/L) was 13 and 18 percent of the Federal and State 

drinking-water benchmarks, respectively. 

Areal and Seasonal Variations

More than half of the 20 detected pesticides were 

detected in at least four of the seven citrus lakes, and norflurazon, demethyl norflurazon, diuron, hydroxysimazine, and 

DCPMU were detected above their minimum LRLs in six 

or more of the citrus lakes (fig. 8 and app. 4). Inferences 

about seasonality in the lake pesticide results are exploratory 

because sampling spanned only 1 year in a small number of 

lakes. 

The amount of seasonal variation between pesticide 

concentrations in quarterly samples often exceeded one order 

of magnitude and was sometimes as high as two orders of 

magnitude (fig. 11). In the control lake, pesticide concentrations were generally near or less than reporting levels (fig. 11). 

No regionally consistent seasonal patterns were evident in 

the number of pesticides detected above the 0.06-μg/L level, 

although slightly more pesticides were detected during spring 

and/or winter than during summer or fall in lakes Swim, 

Leonore, Aurora, and Pabor (fig. 12). Norflurazon was the 

only pesticide that indicated a possible seasonal pattern 

showing lower concentrations in summer (June) compared 

to the other quarterly samples (fig. 11A). Concentrations of 

demethyl norflurazon indicated a similar, but less pronounced, 

seasonal pattern on the basis of a limited number of summer 

analyses of this compound available for three of the seven 

citrus lakes. 

 Because this study was a reconnaissance, the actual 

causes of differences in pesticide occurrence between lakes 

are unknown, but contributing factors would probably include 

pesticide usage practices in the basins; lake geometry, local 

hydrogeology (controlling lateral and vertical ground-water/

lake-water exchange), and the residence time of water in the 

lakes; and the ages, rates and amounts of ground-water flow 

into the lakes.

Lake geometry can be a general indicator of differences 

between lakes in ground-water input (Sacks and others, 1998), 

and also of the processes that may affect pesticide breakdown 

within the lake water column. For example, comparing lakes 

of similar area, shallower lakes would be expected to capture 

a larger proportion of precipitation in their source water, 

and deeper lakes would have the potential to intercept more 

ground-water inflow due to increased exposure to the surficial 

aquifer system. For lakes of similar depth, the larger lakes 

would have proportionally greater solar exposure, which 

would, in turn, increase the potential for water temperature 

changes, evaporative losses, and photodegradation of chemicals as well as mixing from wind and atmospheric delivery of 

agrichemicals.

In spite of differences in lake geometry and size, the total 

number of pesticides and degradates detected did not differ 

widely between the study lakes (fig. 7). In addition, there 

3The proposed U.S. Environmental Protection Agency maximum 

contaminant levels for aldicarb and its degradates are “under 

administrative stay” and, therefore, are not enforced. The U.S. 

Environmental Protection Agency health advisory level is used as 

Florida’s drinking-water guidance value for aldicarb and its degradates.
Lake Water Quality 23

EXPLANATION

WINTER

SPRING

SUMMER

FALL

Norflurazon

Laboratory reporting level

0.02 micrograms per liter

0.001

0.01

0.1

1.0

10 4

Simazine plus

HS, DIA, and DDA

0.001

0.01

0.1

1.0

10 2 2 1 4

Number of samples yielding non-detections

B

Bromacil

Laboratory reporting level

0.02 micrograms per liter

0.001

0.01

0.1

1.0

10

2 4 4 4

C

Diuron

Laboratory reporting level

0.015 micrograms per liter 0.001

0.01

0.1

1.0

10

2 3 2

D

Metalaxyl

Laboratory reporting level

0.02 micrograms per liter

0.001

0.01

0.1

1.0

10

1 4 4 4 4

E

2,4-D

Laboratory reporting level

0.02 micrograms per liter

0.001

0.01

0.1

1.0

10

4 1 4 1 4 4 4 4

F

CONCENTRATION, IN

MICROGRAMS PER LITER

A

CONCENTRATION, IN

MICROGRAMS PER LITER

CONCENTRATION, IN

MICROGRAMS PER LITER

CONCENTRATION, IN

MICROGRAMS PER LITER

CONCENTRATION, IN

MICROGRAMS PER LITER

CONCENTRATION, IN

MICROGRAMS PER LITER

Aurora

Denton

Leonore

Lynn

Moody

Pabor

Swim

Annie

Aurora

Denton

Leonore

Lynn

Moody

Pabor

Swim

Annie

Number of samples yielding non-detections

Number of samples yielding non-detections Number of samples yielding non-detections

Number of samples yielding non-detections Number of samples yielding non-detections

Aurora

Denton

Leonore

Lynn

Moody

Pabor

Annie

Aurora

Denton

Leonore

Lynn

Moody

Pabor

Swim

Annie

Aurora

Denton

Leonore

Lynn

Moody

Pabor

Annie

Aurora

Denton

Leonore

Lynn

Moody

Pabor

Swim

Annie

Laboratory reporting level

0.02 micrograms per liter

LAKE LAKE

Swim

Swim

Figure 11. Variations in concentrations of selected pesticides seasonally within and between 

sampled lakes. Values below the laboratory reporting level are estimated concentrations. Pesticide 

acronyms are defined in table 2.
24 Water Quality and Evaluation of Pesticides in Lakes in the Ridge Citrus Region of Central Florida

was no consistent difference in concentrations of pesticides 

and degradates in the deeper lakes (Lakes Lynn and Denton) 

compared to the shallow lakes (Lakes Leonore, Moody, 

and Pabor) (fig. 11). Swim Lake, with the lowest surface/

depth ratio of sampled lakes (table 1), yielded some of the 

highest concentrations and the highest number of pesticides 

detected; however, Lake Leonore, which was one of the shallowest lakes sampled, yielded a similar number of pesticide 

compounds and high concentrations of norflurazon, bromacil, 

and metalaxyl compared to the other lakes (figs. 11 and 12). 

The concentrations of degradates relative to parent pesticides 

generally were consistent between lakes as discussed in the 

next section. 

Comparisons of Parent Pesticides and 

Degradates 

Analysis of lake water samples (fig. 6) indicated that 

some pesticide degradates were more commonly detected 

than their respective parent pesticides. Comparison of relative 

concentrations of parent pesticides and their degradates over 

time in the lakes where both were detected provides information regarding pesticide transport and fate in the lake/groundwater systems, and on seasonal variability. 

Norflurazon and Degradate

The median concentration of demethyl norflurazon in 

samples from the citrus lakes (0.34 μg/L) was nearly twice 

that of norflurazon (0.18 μg/L) based on detections in all 

samples from the citrus lakes (table 6). On average, concentrations of demethyl norflurazon were 3.27 times those of 

norflurazon in samples; degradate to parent ratios ranged 

from 0.73 to 10.15. Of the 26 samples in which norflurazon 

and desmethyl norflurazon were analyzed and detected, 23 

of the samples (88 percent) yielded higher concentrations 

of degradate than the parent. Concentrations of demethyl 

norflurazon in samples from all the citrus lakes, except for 

Lake Leonore, were consistently higher than the parent 

compound throughout the year. This pattern is evident in 

samples from Lake Aurora and Swim Lake (fig. 13). Although 

strong seasonal patterns were not apparent in concentrations 

of each of these compounds, concentrations during summer 

(June) were typically lower than during fall, winter, and spring 

(figs. 11A and 13). In Lake Leonore, norflurazon exceeded 

demethyl norflurazon concentrations during the quarters in 

which analyses were available for both compounds (December 

and March; fig. 13C), which suggests that higher concentrations of the parent relative to the degradate were transported to 

this lake and/or less rapid rates of norflurazon breakdown in 

Lake Leonore compared to the other lakes.

Simazine, Atrazine, and Degradates

In the citrus lakes, the triazine pesticide degradates 

hydroxysimazine, DDA, and DIA were detected more 

frequently at concentrations exceeding the 0.06-μg/L SRL than 

the parent simazine (figs. 6 and 8). In samples from Lake Lynn 

and Swim Lake (fig. 14), where simazine and its degradates 

were frequently detected, concentrations of hydroxysimazine, 

DDA, and DIA consistently exceeded those of the parent simazine. Trace concentrations of atrazine (less than 0.007 μg/L) 

also were detected in each of the four samples from Lake 

Lynn (fig. 14). In most of the samples, DDA concentrations 

exceeded those of DIA, and both simazine and hydroxysimazine were detected at higher concentrations and more 

frequently in Swim Lake. Similar seasonal patterns in the 

parent-degradate concentrations were not evident between 

0

1

2

3

4

5

6

7

8

9

10

SPRING (MAR)

SUMMER (JUN)

FALL (SEP)

WINTER (DEC)

Aurora

Denton

Leonore

Lynn

Moody

Swim

Pabor

EXPLANATION

NUMBER OF PESTICIDES DETECTED

ABOVE 0.06 - MICROGRAM

PER LITER SRL

LAKE

Figure 12. Number of pesticide 

compounds detected seasonally in 

each of the citrus lakes. SRL represents 

standardized reporting level.
Lake Water Quality 25

these lakes. Hydroxysimazine was a dominant degradate in 

Swim Lake, occurring in three of four samples, but it was 

detected at relatively low concentrations in only one of the 

four samples from Lake Lynn. In both lakes, DDA concentrations exceeded those of DIA (fig. 14).

Evaluation of the triazine degradates is complicated by 

shared parent pesticides (fig. 15 and app. 5; Scribner and 

others, 1999), but several factors indicate that simazine is the 

primary source of the triazine degradates hydroxysimazine, 

DDA, and DIA in the Ridge citrus lakes. Simazine is the only 

triazine pesticide applied to Florida citrus, although several 

of its degradates also can originate from atrazine, cyanazine, 

and propazine. Neither cyanazine, banned in the United States 

since 2002, nor propazine are likely sources for these degradates on the Ridge according to their past or current labeled 

usages. Limited atrazine use may occur in non-citrus areas of 

the Ridge, such as to control weeds in residential areas. 

The pesticide results in this study are consistent with 

minimal atrazine use in the lake basins, because atrazine 

was only detected at very low concentrations (0.015 μg/L 

maximum), and concentrations of its degradates hydroxyatrazone and DEA, which are not shared with simazine, also 

were very low (less than 0.014 μg/L). In contrast, detections 

of simazine, its degradate hydroxysimazine, and its shared 

degradates with atrazine (DDA and DIA) occurred at higher 

concentrations in lake samples and were more frequently 

detected (table 6; figs. 6 and 8).

Diuron and Degradates

Diuron occurred at higher concentrations and detection 

frequencies than its degradates DCPU and DCPMU (fig. 6 

and table 6). The primary degradate, 3,4-dichloroaniline 

(Giacomazzi and Cochet, 2004), was not analyzed as part of 

this study, and reporting levels varied between these three 

compounds. Generally, the LRLs were 0.015 μg/L for diuron, 

0.06 μg/L for DCPU, and 0.03 μg/L for DCPMU. Diuron was 

detected in all of the study lakes, and DCPMU was detected 

in six of the study lakes. DCPU was not detected in any of 

the lake samples above LRLs ranging from 0.03 to 0.06 μg/L 

(app. 3). Concentrations of DCPMU were consistently lower 

than those of the parent in all samples where both were 

detected (app. 3). Concentrations of diuron in Lake Aurora 

and Swim Lake were lowest during summer sampling, and 

seasonal differences were more pronounced in Swim Lake 

than in Lake Aurora (fig. 16). DCPU was not detected in any 

lake samples; however, in these samples the DCPU reporting 

level was significantly higher than that of both diuron and 

DCPMU. 

Aldicarb and Degradates

Aldicarb was not detected in any lake samples (table 7); 

however, one or both of its degradates, aldicarb sulfone 

and aldicarb sulfoxide, were detected in three of the citrus 

lakes. The lack of aldicarb detections likely reflects its 

relatively rapid degradation rate (estimated 7- to 30-day halflife in soil; Hornsby and others, 1996; U.S. Environmental 

Protection Agency, 2007), usage restrictions on its application (one application allowed per year; State of Florida, 

2002a), and the higher laboratory reporting level for aldicarb 

(0.04 μg/L) compared to the degradates (0.0022 to 0.02 μg/L, 

app. 1). Additionally, the low recovery of aldicarb (mean 

value, 19 percent) may also have contributed to its lack of 

detection (table 3). Concentrations and detection frequencies 

of aldicarb sulfoxide, the first-order degradation product 

of aldicarb, exceeded those of aldicarb sulfone (table 6), a 

second-order aldicarb degradation product formed by the 

breakdown of aldicarb sulfoxide (Barbash and Resek, 1996). 

LAKE AURORA

0

0.1

0.2

0.3

0.4

WINTER

(DEC)

SPRING

(MAR)

SUMMER

(JUN)

FALL

(SEP)

Norflurazon

SWIM LAKE

0

4

8

12

16

20

LAKE LEONORE

0

0.2

0.4

0.6

0.8

1.0

CONCENTRATION, IN MICROGRAMS PER LITER

EXPLANATION

Not analyzed

Not analyzed

A

B

C

Demethyl norflurazon

Figure 13. Seasonal variations in concentrations of norflurazon 

and demethyl norflurazon in samples from Lake Aurora, Swim 

Lake, and Lake Leonore.
26 Water Quality and Evaluation of Pesticides in Lakes in the Ridge Citrus Region of Central Florida

0

0.1

0.2

0.3

0.4

Atrazine

Hydroxysimazine

DDA

DIA

Simazine

0

0.1

0.2

0.3

0.4

0.5

0.6

EXPLANATION

CONCENTRATION, IN MICROGRAMS PER LITER

LAKE LYNN

SWIM LAKE

WINTER

(DEC)

SPRING

(MAR)

SUMMER

(JUN)

FALL

(SEP)

ATRAZINE SIMAZINE

Deisopropylatrazine

(DIA)

Didealkylatrazine

(DDA) Deisopropylhydroxyatrazine

(DIHA)

And other

compounds

including;

Hydroxysimazine

(HS)

DEA

HDEA

HA

Figure 14. Seasonal variations 

in concentrations of simazine 

and its degradates in samples 

from Lake Lynn and Swim 

Lake. Pesticide acronyms are 

defined in table 2.

Figure 15. Generalized chemical degradation pathways and sources for simazine degradates (Scribner and 

others, 1999; M.T. Meyer, U.S. Geological Survey, written commun., 2009). Some simazine degradates may 

also originate from atrazine, cyanazine, and propazine as well as from some of their degradational products 

(appendix 1). Atrazine is not registered for use, or applied, in Florida’s citrus groves. Pesticide acronyms are 

defined in table 2.
Lake Water Quality 27

Both degradates degrade more slowly than the parent, and 

exhibit lower sorption potential. The occurrence of aldicarb 

sulfoxide as the primary degradate has been noted in other 

studies of soil water and ground water (Jones and others, 

1987; Hornsby and others, 1990; and Morgan and others, 

1993). Laboratory reporting levels for aldicarb sulfoxide 

were at times lower than those for aldicarb sulfone (app. 3), 

and average laboratory recoveries for aldicarb sulfoxide were 

considerably higher than those for aldicarb sulfone (56 and 

9 percent, respectively) (table 3).

Aldicarb sulfone was detected in two of the three lakes 

where aldicarb sulfoxide was detected. In the lakes where 

both degradates were detected, Lake Leonore and Swim 

Lake, concentrations of aldicarb sulfoxide were consistently 

higher, typically about double those of aldicarb sulfone 

during all quarters (fig. 17). Concentrations of the aldicarb 

degradates in Swim Lake were considerably higher in samples 

SWIM LAKE

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

LAKE AURORA

0

0.005

0.010

0.015

0.020

0.025

FALL

(SEP)

Diuron LRL

CONCENTRATION, IN MICROGRAMS PER LITER

WINTER

(DEC)

SPRING

(MAR)

E

SUMMER

(JUN)

ND (not detected)

E

E

DCPMU LRL

DCPU LRL

Diuron LRL

E

ND (not detected)

ND (not detected)

Diuron

DCPMU

EXPLANATION

LAKE LEONORE

0

0.05

0.10

0.15

0.20

SWIM LAKE

0

0.2

0.4

0.6

0.8

CONCENTRATION, IN MICROGRAMS PER LITER

WINTER

(DEC)

SPRING

(MAR)

SUMMER

(JUN)

FALL

(SEP)

Aldicarb sulfone

Aldicarb sulfoxide

EXPLANATION ND (not detected) ND (not detected)

Figure 16. Seasonal variations in concentrations of diuron and 

its degradates in samples from Swim Lake and Lake Aurora. 

The diuron degradate DCPU was not detected above the 0.06 

microgram per liter laboratory reporting level (LRL) in the lake 

samples. Pesticide acronyms are defined in table 2. Estimated (E) 

concentrations below laboratory reporting level.

Figure 17. Seasonal variations in 

concentrations of aldicarb degradates 

in samples from Lake Leonore and Swim 

Lake. The parent compound aldicarb 

was not detected in any samples above 

its laboratory reporting level of 0.04 

microgram per liter.
28 Water Quality and Evaluation of Pesticides in Lakes in the Ridge Citrus Region of Central Florida

collected during winter and spring than in samples collected 

during summer and fall. The high concentrations observed 

in December were somewhat unexpected given that aldicarb 

usage is restricted to the period January to April, and would 

indicate multiseasonal persistence of the degradates in ground 

water and/or the lake, assuming that application of aldicarb in 

the basin occurred within the restricted time period. 

Variations within the Lake Water Column

Several factors related to depth of sample within the lake 

water column could influence pesticide concentrations and 

degradation rates, such as concentration differences within 

source waters (rainfall and ground water), physicochemical 

characteristics of the water (temperature, light penetration, and 

dissolved oxygen concentrations), biologic communities and 

population densities (to degrade chemicals), and proximity to 

the sediment-water interface (influencing suspended-sediment 

concentrations, availability of organic matter and biota for 

sorption and chemical breakdown). Because many Ridge lakes 

exhibit high clarity (University of Florida, 2005b; University 

of South Florida, 2008), there is potential for chemical 

breakdown by photolysis in the shallow zone of the lake water 

column. Although the number of water-column comparisons 

in the study were limited, these results may be useful for 

directing future research and sampling designs, especially 

given the sparse information on variability of pesticide 

concentrations within lake environments.

Comparisons between shallow and deep water-column 

samples were possible only when at least one of the sample 

concentrations exceeded LRLs. In the 8 depth samples 

collected (4 sample pairs), a total of 14 pesticide compounds 

were detected one or more times above LRLs (table 8), 

including 6 parent pesticides and 9 degradates. Results 

included 1 to 4 sample pairs per compound, and a total of 

29 sample pairs. Pesticide concentrations in deep samples 

exceeded those in shallow samples for 19 of the 29 sample 

pairs (table 8). 

Table 8. Comparison between pesticide concentrations in shallow and deep samples from the lake 

water column.

[Paired samples were collected from four lakes at depths of about 5 feet below the lake surface and near the lake bottom. The comparison 

is limited to pairs that yielded detectable concentrations in the shallow and/or deep sample. μg/L, microgram per liter; >, greater than; 

<, less than; =, equal to]

Analyte

Number of 

sample pairs 

with one 

or more 

detections

Number of sample pairs

Concentration 

ratios 

(deep /shallow), 

in μg/L

Concentration 

difference 

(deep minus 

shallow), 

in μg/L

Concentration 

in deep sample 

> shallow 

sample

Concentration 

in deep sample 

= shallow 

sample

Concentration 

in deep sample 

< shallow 

sample

Parent compounds

2,4-D 2 2 0 0 1>1.27 to 2.10 1>0.024 to .029

Bromacil 3 3 0 0 1>1.14 to 2.5 1>.005 to .12

Diuron 2 1 0 1 .88 to 1.14 -.002 to .003

Metalaxyl 2 1 0 1 .97 to 1.13 -.002 to .001

Norflurazon 4 4 0 0 1.72 to 16.05 .005 to .330

Simazine 1 0 1 0 1 0

Total 14 11 1 2

Degradates

Aldicarb sulfone 1 1 0 0 1.39 .007

Aldicarb sulfoxide 1 1 0 0 1.49 .036

DDA 3 1 0 2 .56 to 1.27 -.11 to .04

DIA 2 0 0 2 .50 to .91 -.05 to -.01

DCPMU 1 0 0 1 .91 -.001

Demethyl norflurazon 4 4 0 0 1.30 to 9.04 .02 to .66

Hydroxysimazine 1 0 1 0 1 0

Hydroxyatrazine 2 1 0 1 .85 to 2.00 -.002 to .007

Total 15 8 1 6

Overall total 29 19 2 8

1

 The low value in the range is a minimum estimate, because concentration in the shallow sample was below the laboratory 

reporting level.
Lake Water Quality 29

Except for norflurazon and its degradate, there was 

insufficient evidence to suggest pronounced differences in pesticide concentrations within the lake water column. The observed 

differences in concentrations between the water-column 

samples were generally less than 0.1 μg/L (table 8). Although 

bromacil and 2,4-D concentrations were consistently greater 

in the deep samples, the concentration differences were small, 

and the limited number of detections (2 to 3 sample pairs) was 

insufficient to evaluate the significance of these differences. 

The results for norflurazon and its degradate, given that 

photolysis is an important mechanism for their chemical 

breakdown, indicate the possibility that concentrations and 

perhaps degradation rates can vary with depth in the water 

column. Secchi depths were 12.5, 18, 19, and 21 ft for 

Lake Aurora, Lake Lynn, Swim Lake, and Lake Denton, 

respectively, at the time that the water-column samples were 

collected (app. 2), which are comparable to Secchi-depth 

values measured in these lakes in other studies (Sacks and 

others, 1998; University of Florida, 2005b; University of 

South Florida, 2008). These Secchi depths extend into the 

top one-third to one-half of the respective maximum depths 

of these lakes, and indicate the high potential for sunlight 

exposure at the 5-ft depth where the shallow samples were 

collected. 

In the four sampled lakes, water temperature was 0.3 to 

5.8 oC cooler in deep samples compared to shallow samples. 

Between deep and shallow samples, values were comparable for specific conductance, pH, and dissolved oxygen 

concentrations (within 16 μS/cm2, 0.5 units, and 3 mg/L, 

respectively) with the exception of Lake Lynn, which had very 

low dissolved oxygen in the deep (0.8 mg/L) versus shallow 

(7.8 mg/L) zones sampled in the water column.

The degradation rates of both norflurazon and demethyl 

norflurazon appeared to decrease with depth in the lake water 

column on the basis of concentration comparisons in and 

between the lakes. Both norflurazon and demethyl norflurazon 

concentrations were greater in the deep samples compared 

to shallow samples in all four lakes sampled (app. 3). 

Concentrations of norflurazon ranged from 1.7 to 16 times 

higher in deep samples compared to shallow samples, and 

concentrations of demethyl norflurazon ranged from 1.3 to 

9 times higher in the deep versus shallow samples (table 8). 

Differences in concentrations between shallow and deep 

samples ranged from 0.005 to 0.33 μg/L for norflurazon and 

from 0.02 to 0.66 μg/L for demethyl norflurazon. 

The relative difference in total norflurazon plus demethyl 

norflurazon concentrations between deep and shallow 

samples, calculated as the ratio of the difference (concentration in deep minus shallow samples) to the mean concentration 

of the two depth samples, increased with the vertical distance 

between sampling points (fig. 18). Further, the degradate/

parent ratios for norflurazon were greater in shallow compared 

to deep samples in three of the four study lakes, indicating 

degradation had proceeded further in shallow than in deep 

samples (fig. 19). Also, the magnitude of differences between 

degradate/parent ratios in shallow samples compared to deep 

samples increased with the depth interval between samples 

(lake depth) (fig. 19), consistent with decreasing degradation rates with depth in the water column. These inferences 

regarding degradation are based on the assumption that the 

lakes are well mixed with respect to source waters including 

ground-water inflows, which presumably contain a higher 

proportion of parent compared to degradate concentrations. The lack of observable differences in the norflurazon 

0

10

20

30

40

50

0 10 20 30 40 50

VERTICAL DISTANCE BETWEEN WATER-COLUMN

SAMPLES, IN FEET

RELATIVE DIFFERENCE

IN CONCENTRATION

BETWEEN 

WATER-COLUMN

SAMPLES, IN PERCENT

Lake Lynn

Lake Denton

Lake Aurora Swim Lake

Figure 18. Relative difference in the concentrations of 

norflurazon plus demethyl norflurazon in water-column 

samples compared to the vertical distance between the 

samples. Relative difference is concentration in deep sample 

minus shallow sample/mean concentration of paired samples 

for each lake.
30 Water Quality and Evaluation of Pesticides in Lakes in the Ridge Citrus Region of Central Florida

degradate/parent ratios in the Lake Denton samples may be 

associated with difficulty in quantification of the very low 

concentrations of norflurazon and demethyl norflurazon 

(0.004 to 0.03 μg/L) in this lake, one to two orders of magnitude lower than concentrations in the other sampled lakes, and 

near their laboratory reporting levels. 

Due to the limited number of samples, additional 

sampling is needed to confirm the inference of more rapid 

degradation of norflurazon and its degradate in shallow depths 

of the Ridge citrus lakes. 

Comparison to National Studies

Several of the pesticides and degradates targeted in 

the Ridge lake study have been analyzed by the USGSNAWQA Program in streams and ground water throughout 

the United States. Although lakes were not sampled in the 

NAWQA Program, many conditions affecting pesticide 

transport and fate in streams also occur in lake environments, and differ from ground-water environments. Some of 

these conditions include potential for chemical breakdown 

through exposure to sunlight and aquatic biota, availability of 

organic matter and sediment on which pesticides may sorb, 

exposure to air for volatilization, and ground-water sources 

of inflow. The NAWQA pesticide data consisted of more 

than 1,468 samples collected from about 80 sampling sites 

on streams draining agricultural lands (Gilliom and others, 

2006), in addition to more than 1,400 pesticide samples 

collected from streams draining non-agricultural land uses 

such as urban, mixed (combination of urban, agricultural, 

and/or undeveloped), and undeveloped. The NAWQA data are 

referred to as “national” data in the remainder of the report.

Concentrations of many of the pesticides detected in 

the citrus lakes were higher than those observed in streams 

nationally, and included pesticides associated with agricultural 

use (norflurazon and aldicarb degradates), as well as some 

pesticides associated nationally with urban land use (diuron, 

simazine, 2,4-D, bromacil, and tebuthiuron) (Gilliom and 

others, 2006). The 90th percentile (Q90) concentrations for 

Ridge lake samples exceeded the national Q90 for agricultural 

land use for 6 of the 11 pesticides analyzed in both studies 

(table 9). Norflurazon and bromacil, the two most frequently 

detected compounds in Ridge citrus lakes, were detected in 

less than 5 percent of national samples in any land-use class. 

Norflurazon, bromacil, aldicarb sulfoxide, and aldicarb 

sulfone were detected at higher concentrations (maximum 

and Q90) in the Ridge lakes than in the national samples from 

agricultural lands, and the Ridge Q90 concentrations of diuron 

and 2,4-D exceeded the national Q90 values for agricultural 

areas (table 9). The elevated concentrations of pesticides in 

Ridge surface waters relative to national values for agricultural 

land use are consistent with comparisons of Ridge groundwater pesticide concentrations, with national ground-water 

values (Choquette and others, 2003) highlighting the vulnerability of the Ridge to transport of agricultural chemicals into 

ground and surface waters. 

SHALLOW SAMPLE

DEEP SAMPLE

EXPLANATION

MEAN CONCENTRATION OF NF PLUS

DMNF, IN MICROGRAMS PER LITER:

0

2

4

6

8

10

12

0 5 10 15 20 25 30 35 40 45 50

LAKE DEPTH, IN FEET, AT SAMPLED LOCATION

0.32 0.48 0.029

RATIO OF DMNF/NF

Lake

Lynn

Lake

Denton

Lake

Aurora

Swim

Lake

2.91

Figure 19. Ratio of demethyl norflurazon (DMNF) concentration to norflurazon (NF) 

concentration in water-column samples related to lake depth at the sampled location.
Lake Water Quality 31

The maximum concentrations of bromacil (6.29 μg/L) 

and norflurazon (5.30 μg/L) in the Ridge lakes were higher 

than the national maximums (1.9 and 1.24 μg/L, respectively) 

in streams draining agricultural lands (table 9). The Q90 

concentration of diuron in the Ridge lakes (0.081 μg/L) was 

twice the national Q90 (0.040 μg/L). The Ridge Q90 concentration for 2,4-D (0.113 μg/L) was similar to the national Q90 

for agricultural basins (0.110 μg/L). In contrast, the Ridge 

Q90 for simazine (0.041) was about half of the national Q90 

(0.074 μg/L). Atrazine concentrations in the Ridge lakes 

(maximum = 0.015 μg/L) were considerably lower than 

national concentrations in streams draining agricultural land 

use (maximum = 201 μg/L, Q90 = 0.80 μg/L), and reflect the 

lack of atrazine usage in Florida citrus (Florida Agricultural 

Statistics Service, 2000; 2002). 

The pesticides diuron, simazine, 2,4-D, atrazine, 

bromacil, and tebuthiuron were detected nationally in urban 

and mixed land uses at similar or higher frequencies as 

in agricultural areas (table 9). Notably, national detection 

frequencies in urban areas for simazine (65 percent) and 

diuron (22 percent) were higher than in agricultural areas 

(58 and 13 percent, respectively). Simazine and atrazine were 

also detected nationally in mixed land use (63 and 85 percent 

of samples, respectively) and in undeveloped areas (24 and 

54 percent of samples). National detection frequencies for 

2,4-D were similar between agricultural and urban areas 

(13 percent of samples) (table 9). 

The relatively frequent detection of tebuthiuron and 

atrazine (and its degradate DEA) in undeveloped areas 

nationally and their detection in the undeveloped control lake 

on the Ridge (app. 3) suggest non-agricultural usage and/or 

atmospheric transport as possible sources of low-level concentrations of these compounds in the Ridge lakes. Tebuthiuron, 

a broad spectrum herbicide typically used in non-cropland 

areas to control weeds, was detected at trace concentrations in 

all samples from the Ridge control lake and in 21 percent of 

national samples from undeveloped areas (table 9). Atrazine 

was also detected in two of four samples from the control lake 

on the Ridge and was detected nationally in urban, mixed, 

and undeveloped land use (71, 85, and 54 percent of samples, 

Table 9. Pesticide results for the sampled Ridge citrus lakes compared to U.S. Geological Survey national surface-water monitoring data. 

[The comparison includes target pesticides common to both the Ridge and the U.S. Geological Survey National Water-Quality Assessment (USGS NAWQA) 

Program surface-water monitoring networks. The NAWQA data summaries are from Gilliom and others (2006). All concentrations are in micrograms per liter; 

ND, not determined; LTRL, long-term laboratory reporting level]

Compound 

Lake Wales Ridge 

Citrus Lakes1

USGS NAWQA surface-water sampling, 1992-2001

Agricultural land use Detection frequency2

 in other land-use 

classes (percent of samples)

Detection 

frequency2

(percent of 

samples)

90th 

percentile 

concentration

Maximum

concentration

Detection 

frequency2

(percent of 

samples)

90th 

percentile 

concentration

Maximum 

concentration

No. of sites 

(samples) 

NAWQA 

maximum 

LTRL

Urban3 Mixed4 Undeveloped5

Norflurazon 100 2.37 5.30 4.2 <0.021 1.24 62 (1,468) 0.021 0 0.06 1.8

Diuron 82 .081 1.79 13 .040 14 62 (1,469) .06 22 8.4 4.2

Bromacil 64 2.78 6.29 1.68 <.040 1.9 62 (1,470) .04 1.6 .03 .85

Simazine 43 .041 .060 58 .074 5.76 83 (2,009) .006 65 63 24

Atrazine 36 .011 .015 90 .800 201 83 (2,009) .004 71 85 54

Aldicarb sulfoxide 29 .216 .67 .13 <.140 1.91 62 (1,470) .14 0 .05 0

Aldicarb sulfone 25 .117 .33 0 <.100 <.100 62 (1,469) .1 0 0 0

2,4-D 21 .113 1.16 13 .110 15 62 (1,465) .08 13 6.4 .38

Oxamyl 7 ND .036 .57 <.080 .16 62 (1,469) .08 0 0 0

Tebuthiuron 7 ND .006 21 .013 6.33 83 (1,983) .008 30 29 21

DEA 4 ND .006 79 .128 3.03 83 (2,000) .003 42 72 47

1 Includes 7 sites (lakes) and 28 samples.

2 Detections above laboratory reporting levels.

3 Includes 19 to 30 sites and 523 to 812 samples, depending on compound.

4 Includes 36 to 65 sites and 798 to 1385 samples, depending on compound.

5 Includes 5 to 8 sites and 100 to 144 samples, depending on compound.
32 Water Quality and Evaluation of Pesticides in Lakes in the Ridge Citrus Region of Central Florida

respectively). Atrazine and DEA also were detected at trace 

levels in a nearly pristine lake in Orange County, Florida 

(German and Adamski, 2005). Atmospheric transport of 

atrazine has been documented in a number of studies (Richards 

and others, 1987; Goolsby and others, 1994; Muir and others, 

2004; Scribner and others, 2005), and estimated atmospheric 

emissions of atrazine from agricultural source areas near the 

Ridge are identified in Cohen and others (1997). Atmospheric 

transport of atrazine in rainfall over large distances and its 

deposition in lakes located hundreds of kilometers from atrazine source areas has been documented in areas of the northern 

United States and in Canada (Stamer and others, 1998; Muir 

and others, 2004).

Relation of Chemical Properties and Usage to 

Pesticide Occurrence

Estimates of pesticide chemical properties and usage 

were compiled for selected target pesticides on the basis of 

published data and were used to define indices related to pesticide transport into water resources (table 10). The indexes of 

leaching, pesticide usage, and pesticide transport are defined 

in the “Analytical Methods” section.

Lower values of the leaching vulnerability index 

(Schlosser and others, 2002) indicate increased propensity for 

the pesticide to leach from soils into ground water. The leaching 

Table 10. Indexes of leaching vulnerability, pesticide usage, and transport potential for selected pesticides targeted in the Ridge 

lake study.

[The indexes are shown in shaded column. Pesticides are ranked according to the leaching “vulnerability index” (Schlosser and others, 2002). Lower values 

are indicative of increased potential to leach based on these properties. Sources for pesticide properties were obtained from Hornsby and others (1996), except 

where noted. AI, active ingredient; I, insecticide; H, herbicide; N, nematicide; F, fungicide; d, degradate; Ac, acaricide; mL/g, milliliter per gram; acre/yr, acre 

per year; ND not determined; NA, not available; lb, pounds; mg/L, milligram per liter]

Pesticide 

(or degradate)

Soil organic 

carbon 

partition 

coefficient, 

Koc

(mL/g)

Soil 

half-life, 

t1/2

(days)

Leaching 

vulnerability 

index1

(Koc/ t1/2)

Solubility in 

water2

(mg/L)

Pesticide 

class

Percent 

of Florida 

orange 

orchards3

Annual 

application 

rates in Florida

 orange 

orchards3 

(lb AI/acre/yr)

Pesticide 

usage index

 for Florida

orange 

orchards4

Ranking by 

usage for 

major crops

 in Florida 

(lb AI/

crop year)5

Transportpotential 

index (usage 

index/

vulnerability 

index)

Tebuthiuron 80 360 0.22 2,500 H NA NA NA NA ND

Aldicarb sulfone 10 20 .50 10,000 d 614 62.78 38.9 615 78

Bromacil7 32 60 .53 700 H 24 1.61 38.6 21 72

Metalaxyl 50 70 .71 8,400 F NA ND NA 69 ND

Aldicarb 30 30 1.00 6,000 I, Ac, N 14 2.78 38.9 15 39

Bentazon 834 820 1.70 82,300,000 H NA NA NA 47 ND

2,4-D 920 910 2.00 9890 H 3 .31 .9 10 .47

Simazine 130 60 2.17 6.2 H 32 2.47 79.0 12 36

Methomyl 72 30 2.40 58,000 I NA NA NA 29 ND

Imidacloprid 10203 1050 4.06 10510 I NA NA NA 76 ND

Diuron 480 90 5.33 42 H 57 2.18 124.3 10 23

Oxamyl 25 4 6.25 282,000 I, Ac, N NA NA NA 81 ND

Norflurazon 700 30 23.33 28 H 17 1.91 32.5 18 1.4

Benomyl 1,900 67 28.36 2 F <1 .86 <.86 54 .03

Oryzalin 600 20 30.00 2.5 H NA NA NA 114 ND

Glyphosate 1124,000 1147 510.64 11900,000 H 88 2.52 221.8 17 .43

1 Schlosser and others (2002).

2 Within range of 20 to 25 degrees Celsius.

3

 Percent by area, Florida Agricultural Statistics Service (2002); Shahane (2003).

4 The product of usage area (percent of total crop area) and application rate (AI/acre/yr).

5 Includes pesticides used for 32 major crop types in Florida, 2001 (Shahane, 2003).

6

 Parent pesticide, aldicarb.

7 Bromacil has been prohibited from use in Ridge citrus orchards located on sandy well drained soils since December 1994.

8 Chemical properties associated with formulation bentazon sodium salt.

9 Chemical properties associated with formulation 2,4-D acid.

10 Source: Oregon State University ExToxNet (Extension Toxicology Network), available online at http://extoxnet.orst.edu/ 

11 Chemical properties associated with formulation glyphosate isopropylamine salt.
Lake Water Quality 33

vulnerability index ranged from 0.22 (tebuthiuron) to 510 

(glyphosate) for the pesticides listed in table 10, and values 

were less than about 5 for most of the pesticides detected in 

the Ridge lakes. Published values of aldicarb soil half-life 

varied from 7 to 30 days (Hornsby and others, 1996; U.S. 

Environmental Protection Agency, 2007). The lack of aldicarb 

detections in the Ridge ground water (Choquette and others, 

2005) and lake samples, in light of concurrent detections of 

its degradates, suggests that aldicarb breakdown rates in the 

Ridge region may correspond to the lower (more rapid) values 

in this range. 

Pesticide usage data were available for eight of the parent 

compounds listed in table 10 (Shahane, 2003). The pesticide 

usage index (table 10) was indicative of the areal extent of 

usage and application rate. Highest rankings of the pesticide 

usage index were associated with glyphosate (222), diuron 

(124), and simazine (79), followed by aldicarb (39), bromacil 

(39), and norflurazon (33) (table 10). The bromacil value 

reflects average usage in citrus statewide; however, in Ridge 

citrus, the usage index for bromacil should be near zero 

because its use is prohibited in nearly all of the Ridge citrus 

areas (State of Florida, 1995; Choquette and Sepulveda, 

2000). The transport potential index is determined by leaching 

potential and pesticide usage, and higher values indicate 

increased potential of a pesticide to be transported by way of 

ground water to the lakes (table 10). 

Most of the pesticides that were detected in two or 

more of the lakes, including norflurazon, diuron, simazine, 

bromacil, metalaxyl, aldicarb degradates, 2,4-D, oxamyl 

(fig. 8), exhibited either a leaching index less than about 

5, or a moderate to high usage index greater than about 30 

(table 10). The high leaching index (23.33) for norflurazon, 

relative to the other compounds, did not suggest a high 

propensity for leaching of this compound, contrary to the high 

frequency of norflurazon detections in the lake samples, and 

in ground-water sampling (Choquette and others, 2005). Also, 

the lower relative solubility of simazine (6.2 mg/L) and norflurazon (28 mg/L) compared to the other detected compounds 

(42 to 282,000 mg/L) does not appear to be a limiting factor in 

their transport to the Ridge lakes. 

The transport-potential index provided a generally 

accurate estimate of occurrence (or absence) of the pesticides 

glyphosate, atrazine, benomyl, alicarb sulfoxide, and diuron 

in the lakes on the basis of standardized detection frequencies (figs. 6 and 8). However, the transport-potential index 

underestimated the occurrence of bromacil, norflurazon, and 

2,4-D, and overestimated the occurrence of aldicarb, aldicarb 

sulfone, and simazine in the lake samples. Bromacil detections in the lake samples indicate that bromacil persists much 

longer in the environment than expected on the basis of its 

chemical properties, based on the assumption that it has not 

been used in most Ridge citrus areas since 1994 (discussed in 

the “Agrichemical Usage” section). The transport-potential 

index for norflurazon significantly underestimated its occurrence in the Ridge lakes relative to other pesticides given 

that it was the most frequently occurring pesticide and was 

detected in all of the lake samples (fig. 8). This result suggests 

that norflurazon’s propensity to leach was underestimated 

by the chemical properties in the index, and/or the statewide 

usage averages were lower than its actual usage on the Ridge. 

Comparing median pesticide concentrations in Swim Lake and 

Lakes Lynn and Leonore with the transport-potential index for 

selected pesticides (fig. 20) indicates patterns similar to the 

regional results, and highlights the strengths and limitations of 

using this approach for estimating the occurrence of pesticide 

compounds in the lakes. 

The transport-potential and the leaching vulnerability 

indexes indicated that aldicarb sulfone should be detected 

at about twice the rate of aldicarb sulfoxide; however, the 

standardized detection rate of aldicarb sulfoxide (above the 

0.06 μg/L level) was 25 percent compared to 10 percent for 

aldicarb sulfone. The lower laboratory recoveries for aldicarb sulfone and aldicarb (table 3), as previously discussed, 

may account for their lower than expected frequency of 

detection. The lack of aldicarb detections in any of the lake 

samples and in few Ridge ground-water samples (Choquette 

and others, 2005) indicates that it degrades rapidly in the 

Ridge environment.

The lack of detection of the target pesticide glyphosate 

and its primary degradate AMPA in lake samples, despite the 

widespread application of glyphosate in citrus (table 10 and 

fig. 4), is consistent with its chemical properties. Glyphosate, 

moderately persistent with an average soil half life of 47 days 

and highly soluble, has a strong tendency to sorb to soils, 

as indicated by its high organic-carbon partition coefficient 

(Koc), even those with low organic and low clay content 

(Oregon State University, 1996a). 

Some factors that may limit the utility of chemical 

properties to predict propensity for pesticide transport into 

ground water and surface water include preferential transport 

of water into and through the subsurface (resulting in more 

rapid transport rates than expected based solely on soil or 

lithologic properties), and variations in pesticide behavior due 

to differences in chemical, physical, and biological conditions 

between controlled laboratory environments and the field 

(Barbash and Resek, 1996). Recent study of pesticide degradation indicates higher than expected rates of degradation as well 

as changes in degradation pathways in soils adapted to the 

application of the pesticide (Krutz and others, 2008). The limitations of the transport potential index for estimating relative 

pesticide occurrence in the lake samples also could be due to 

differences between pesticide usage in Ridge groves compared 

to statewide averages for citrus. The low organic content in the 

sandy soils and geologic deposits underlying the Ridge may 

limit the utility of Koc (organic carbon partition coefficient) 

and, hence, the leaching vulnerability index, for identifying 

compounds having a propensity to leach into ground water and 

lakes in this region. 
34 Water Quality and Evaluation of Pesticides in Lakes in the Ridge Citrus Region of Central Florida

0.1

1

10

100

0.01

0.1

1

10

100

SWIM LAKE

0.1

1

10

100

0.01

0.1

1

10

LAKE LEONORE

0.1

1

10

100

0.01

0.1

1

LAKE LYNN

TRANSPORT-POTENTIAL INDEX TRANSPORT-POTENTIAL INDEX TRANSPORT-POTENTIAL INDEX

MEDIAN CONCENTRATION,

IN MICROGRAMS PER LITER

MEDIAN CONCENTRATION,

IN MICROGRAMS PER LITER

MEDIAN CONCENTRATION,

IN MICROGRAMS PER LITER

Glyphosate

2,4-D

Norflurazon

Diuron

Simazine

Aldicarb

Bromacil

Aldicarb

sulfone

Aldicarb +

degradates

Norflurazon +

degradate

Simazine +

degradates

EXPLANATION

PESTICIDE

ND ND

ND

ND

ND

ND ND

ND

ND

ND ND ND ND

TRANSPORT- POTENTIAL INDEX--Corresponds to parent pesticides

Median concentration (based on four samples)

ND Not Detected

Transport-potential index corresponds to average usage in Florida

orange groves. The index would be zero for most Ridge citrus where

usage has been prohibited since 1994

Figure 20. Pesticide transport-potential indexes compared to median pesticide concentrations in 

Swim Lake, Lake Leonore, and Lake Lynn. Higher values of the transport-potential index indicate 

higher likelihood of transport into ground and surface water. The application of bromacil has been 

prohibited in most Ridge citrus orchards since 1994. The median concentrations correspond to 

four samples collected over a year at each lake.
Lake Water Quality 35

Discussion of Pesticide Transport and Fate

The Ridge lake study, although intended as a reconnaissance, also provides some insight into the transport and 

fate of pesticides and their degradates in the surficial aquifer 

system and citrus lakes. Land use, soils, and climate were 

similar among the citrus study basins and, therefore, these 

factors are not expected to significantly contribute to spatial 

differences observed in pesticide occurrence between the 

study lakes. Given the limited availability of field data on 

pesticides in surface water and ground water, the transport and 

fate processes indicated by the Ridge results may be useful for 

future studies in the Ridge or in other field settings.

The high incidence of norflurazon and demethyl 

norflurazon detections and typically higher concentrations 

of demethyl norflurazon in more than 90 percent of the lake 

samples that yielded norflurazon detections (fig. 6 and app. 4) 

are consistent with ground-water sources contributing to lake 

inflow and with the in-lake processes influencing the breakdown of norflurazon. Photodegradation has been identified 

as the probable primary route of degradation for norflurazon 

(U.S. Environmental Protection Agency, 1996). 

Norflurazon degradation can occur after its application 

at land surface in soils and as it moves through the unsaturated zone and into the aquifer, and also while ground water 

moves through the subsurface and into the lakes. Norflurazon 

has been described as a relatively mobile compound that is 

expected to be persistent in surface waters due to its resistance 

to abiotic hydrolysis, low volatilization potential, and relatively low susceptibility to degradation under aerobic conditions, and is even more persistent in anaerobic environments 

(U.S. Environmental Protection Agency, 1996). Although less 

is known about the properties of norflurazon’s primary degradate, demethyl norflurazon, it too is persistent in aerobic and 

anaerobic conditions and has been reported to be more persistent than norflurazon (U.S. Environmental Protection Agency, 

1996). Both norflurazon and demethyl norflurazon exhibit low 

soil/water partitioning, indicating that their mass in surface 

waters would likely be dissolved in the water column rather 

than adsorbed to suspended and bottom sediment. 

Norflurazon in water irradiated in sunlight has an 

estimated half-life of 2 to 3 days (compared to 130 days and 

6-8 months in non-irradiated soils and sediment, respectively) 

(U.S. Environmental Protection Agency, 1996). The clarity 

of lakes in the Lake Wales Ridge region would provide an 

environment for photolysis. The Secchi depths of lakes on 

the Ridge typically range from about 3.5 to 13 ft, averaging 

between about 6 and 9 ft (Romie, 2000), and (as previously 

described) has been measured at 15 to 21 ft in some of the 

sampled lakes (Sacks and others, 1998; University of Florida, 

2005b; University of South Florida, 2008). 

The possibility of photodegradation in shallow zones of 

the lake water column more directly exposed to sunlight was 

supported by consistently lower concentrations of norflurazon 

and its degradate in shallow versus deep zones, and larger 

concentrations of the degradate than the parent in shallow 

versus deep zones. Low dissolved oxygen concentrations 

(Lake Lynn only) and cooler water temperatures in deeper 

zones of the lake water column also could contribute to slower 

degradation rates at depth. The typically higher concentrations 

of norflurazon and demethyl norflurazon in ground water 

compared to lakes sampled in Ridge citrus areas (Choquette 

and Kroening, 2006) may in part reflect photodegradation of 

these compounds after they enter the lakes.

Widespread detections of atrazine and bromacil in the 

lakes (fig. 8) were not anticipated on the basis of minimal 

usage in Ridge citrus. Atrazine has no historic use in Florida 

citrus, but was detected at trace concentrations (less than 

0.016 μg/L) in five of the citrus lakes and in the control lake 

(app. 3), which may be indicative of atmospheric transport 

of atrazine into Ridge lakes or applications in non-citrus 

source areas. 

The occurrence of bromacil in the lakes, at relatively 

high concentrations compared to other compounds (table 6), 

was consistent with recent bromacil detections in Ridge 

ground water (Choquette and others, 2005) and, as previously 

discussed, may originate from residual bromacil because its 

use has been prohibited in most Ridge citrus since 1994. In 

ground water, detections of bromacil have decreased slightly 

with time, whereas detections of one of its replacements, 

norflurazon, are increasing (Choquette and others, 2005; 

Gilliom and others, 2006). Given the significant ground-water 

inflow to the Ridge lakes, this result suggests the possibility of 

such temporal trends in bromacil and norflurazon in the lakes. 

Neither glyphosate, which is widely used in Florida 

citrus orchards (fig. 3), nor its degradate AMPA were 

detected above the LRL (0.1 μg/L) in any of the lake samples 

(app. 3). The lack of detections of glyphosate and its degradate 

AMPA, despite widespread usage, is likely due to its relatively 

short half-life in water of 7 to 14 days (Giesy and others, 

2000) and high potential to sorb to phosphate binding sites, 

iron and aluminum oxides, clay, and organic matter (Sprankle, 

1975; Hance, 1976; and Hornsby and others, 1996). The presence of glyphosate in ground water on the Ridge has not been 

evaluated. In contrast, Battaglin and others (2005) documented 

moderate to high detection rates of glyphosate and AMPA in 

streams of the midwestern United States.

The variation in pesticide compounds detected among 

the citrus lakes suggests that differences in usage practices 

and amounts of ground-water inflow contribute to pesticide 

occurrence in the study lakes. Positive correlations have been 

observed between the amount of ground-water inflow into 

Ridge lakes and nitrate concentration indicating the importance of ground-water transport of agricultural chemicals into 

the lakes (Kolasa and others, 2001). The seasonal variations in 

pesticide concentrations in the lakes may reflect local differences in the timing, types, and rates of pesticide applications; 

differences in ground-water flow path lengths; ages of ground 

water flowing into the lakes; and the residence time of water 

in the lakes. 
36 Water Quality and Evaluation of Pesticides in Lakes in the Ridge Citrus Region of Central Florida

Comparison of the lake pesticide results to Ridge 

ground-water sampling indicated that median concentrations 

of most pesticides and degradates were lower in lakes than in 

ground water (Choquette and Kroening, 2006). This pattern 

is likely due to increased opportunity for biogeochemical 

degradation (including photolysis), sorption, and dilution of 

pesticides both within the lakes and during the transport of 

ground water to the lakes. The ratios of degradate-to-parent 

pesticide concentrations were typically higher in lakes 

than in ground water, which is consistent with degradation 

occurring as ground water moves through the subsurface 

into the lakes and with chemical breakdown within the lake 

systems. The detections of parent pesticides in the lakes 

may reflect relatively rapid ground-water transit times, slow 

chemical breakdown and/or sorption and subsequent desorption of pesticides within the lakes, and possible atmospheric 

deposition of pesticides. 

Summary

This report presents the first assessment of pesticides in 

lakes on the Lake Wales Ridge, central Florida, and is one of 

the first pesticide studies in the Nation focused on small- to 

moderate-sized lakes. The sampled lakes included seven lake 

basins characterized predominately by citrus agriculture and 

one undeveloped (“control”) lake basin, which ranged in 

area from 5 to 393 acres with maximum depths ranging from 

20 to 65 ft. Quarterly water-quality samples were collected 

over a 1-year period from each of the lakes, along with a 

single sampling event to compare water quality in shallow 

and deep zones in four of the lakes. Water-quality data 

included field properties, and laboratory analyses of major 

inorganic constituents, nutrients, and 83 selected pesticides 

and degradates.

Compared to the control lake, the citrus lakes typically 

exhibited elevated pH, and elevated concentrations of 

dissolved solids, major inorganic constituents (including 

calcium, magnesium, sodium, potassium, chloride, and 

sulfate), nitrate, and pesticide compounds. The elevated 

concentrations of the major inorganic constituents and 

pesticides in the citrus basins likely reflect inputs from citrus 

agrichemcials that have leached into the surficial aquifer 

system which feeds the lakes. Atmospheric transport may 

also contribute these constituents to the lakes, but it is likely 

a less important source of these constituents than ground 

water. The median pH of the citrus lakes ranged from 7.1 to 

7.7, in contrast to 5.9 in the control lake. These differences 

in pH likely reflect the increased concentrations of anions 

in the ground water due to fertilization and liming in citrus 

groves, the acidic pH of rainfall in Florida, and the low acidneutralizing capacity of the surficial aquifer system. 

Median nitrate (as N) concentrations in the citrus lakes 

generally ranged from 0.6 to 3.7 mg/L, and median total 

nitrogen in the citrus lakes ranged from 1.41 to 4.79 mg/L 

compared to 0.48 mg/L in the control lake. One of the citrus 

lakes, Pabor Lake, exhibited low nitrate concentrations 

(less than 0.06 mg/L); however, total nitrogen in the lake 

was similar to the other citrus lakes, indicating that the 

nitrogen in Pabor Lake may consist predominantly of organic 

nitrogen. 

A total of 20 pesticide compounds (12 parent compounds 

and 8 degradates) were detected above laboratory reporting 

levels in the study lakes. Herbicides and their degradates accounted for most (75 percent) of these pesticide 

compounds, followed by insecticides and one fungicide. The number of pesticide compounds detected in each of 

the citrus lakes ranged from 9 to 14. Trace concentrations of 

three pesticides were detected in the control lake. 

Of the 20 pesticides detected above LRLs, 12 were 

detected at concentrations equal to or exceeding the 0.06-μg/L 

SRL. The compounds with the highest standardized detection frequencies included, in decreasing order of detection, 

demethyl norflurazon, norflurazon, bromacil, simazine 

degradates (DDA and hydroxysimazine), diuron, and aldicarb 

sulfoxide, which occurred in 86 to 25 percent of the samples 

from the citrus lakes. Other compounds detected in less 

than 20 percent of samples at or above the 0.06-μg/L level 

included metalaxyl, DIA, aldicarb sulfone, and simazine. 

This study was one of the first to evaluate the occurrence of 

hydroxysimazine in surface waters of the United States.

Pesticide concentrations in the lakes did not exceed any 

currently specified (2007) Federal aquatic-life benchmarks, 

which were available for 10 of the 20 detected pesticides, 

with the exception of one duplicate sample in which diuron 

slightly exceeded a guideline for aquatic plants. The concentrations of diuron, bromacil, and norflurazon in a few 

samples approached the benchmarks for acute effects on 

nonvascular plants. None of the lake samples yielded pesticide concentrations that exceeded State or Federal humanhealth guidance values for drinking water. Potential impacts 

of agrichemical mixtures on aquatic life cannot be assessed 

until criteria for such mixtures are developed. 

Maximum pesticide concentrations were less than 

1 μg/L for 15 of the 20 detected compounds, and ranged, 

by compound, from 0.006 to 17.1 μg/L. Summary statistics 

for concentrations of the pesticide compounds detected in 

the citrus lakes were calculated using the log-likelihood 

method. The coefficient of variation (standard deviation/

mean) of concentrations, by compound, ranged from 29 to 

363 percent (median 133 percent); the ratio of the interquartile range to the median, a nonparametric measure of variability, ranged from 41 to 813 percent (median 205 percent).

Atrazine, which is not a citrus pesticide, was detected 

in six of the eight study lakes, including the control lake, at 

trace concentrations (less than 0.016 μg/L); its degradates 

hydroxyatrazine and DEA also were detected at low concentrations in some samples. Atmospheric transport may be a 

source of atrazine in the study lakes, but atrazine may also 

originate from usage in non-citrus source areas in the basins. 
References Cited 37

In the citrus lakes, pesticide degradates were detected 

more frequently and at higher concentrations than the parent 

concentrations for most parent-degradate combinations 

analyzed. Norflurazon and its degradate demethyl norflurazon were detected in all of the lake samples in which both 

compounds were analyzed, and 88 percent of these samples 

yielded higher concentrations of the degradate than the 

parent. The simazine degradates hydroxysimazine, DDA, 

and DIA were detected more frequently than simazine in 

the lake samples above the 0.06-μg/L level. The degradates 

aldicarb sulfone and aldicarb sulfoxide were detected in 25 

and 29 percent of the samples, respectively, but aldicarb was 

not detected in any of the samples. Laboratory recoveries 

differed between these compounds and mean recoveries 

in matrix spikes were low (8.7 to 55 percent) compared 

to other targeted analytes. Diuron, in contrast to other 

parent-degradate results, occurred at higher concentrations 

and detection frequencies than its degradates DCPMU and 

DCPU, although its primary degradate, 3,4-dichloroaniline, 

was not analyzed as part of this study.

Preliminary analysis indicated that norflurazon and 

demethyl norflurazon degradation rates may be higher in 

shallow zones and decrease with depth in the lake water 

column, presumably due to increased sunlight penetration 

near the lake surface, but additional sampling is needed to 

confirm these results. These data are consistent with prior 

studies indicating photodegradation is an important process 

for breakdown of norflurazon and its degradate.

Concentrations of pesticides detected in the Ridge 

study were compared to those detected in national (USGS 

NAWQA) monitoring at 2,868 sites on streams. The 

90th percentile concentrations for Ridge lake samples 

exceeded or were similar to the national 90th percentiles for 

agricultural land use for 6 of the 11 target pesticides analyzed 

in both the lake study and national monitoring, indicating 

that the concentrations in the Ridge lakes are relatively high 

in a national context. Some of the pesticides detected in the 

Ridge lakes occurred more frequently in “urban” or “mixed” 

land-use settings nationally than in agricultural settings, 

including diuron, simazine, 2,4-D, and tebuthiuron.

The lake pesticide detections were compared to an index 

of pesticide leachability, and other indexes representing 

pesticide usage and transport potential. The transport-potential index underestimated the relative occurrence of bromacil, 

norflurazon, and 2,4-D, and overestimated the occurrence 

of aldicarb, aldicarb sulfone, and simazine in the lake 

samples. The discrepancies between expected and observed 

detection frequencies indicate that field monitoring is critical 

for assessing agrichemical transport into water resources and 

to verify the accuracy of such estimates in specific regions. 

For example, bromacil detections in the lakes and in ground 

water on the Ridge more than 10 years since the prohibition 

of its use in most Ridge citrus suggest that bromacil persists 

much longer in this environment than expected on the basis 

of its chemical properties. 

Agrichemical usage practices implemented to protect 

water resources on the Ridge include the prohibition on 

bromacil usage, restrictions on aldicarb applications, and 

establishment of voluntary fertilizer “best management 

practices.” These changes are anticipated to result in reductions in nitrate and pesticide concentrations in ground water 

over time, which may, in turn, result in future reductions of 

their concentrations in the Ridge lakes. 

This study indicates that parent pesticides as well as 

their degradates persist in the Ridge lakes, despite contributions via relatively slow ground-water transport and opportunity for chemical breakdown, dilution, and sorption in the 

lake environments. The typically lower pesticide concentrations in Ridge lakes compared to ground water suggest that 

in-lake processes may be important for reducing agrichemical concentrations in lake waters discharging to the surficial 

aquifer system and Upper Floridan aquifer. Considerations 

for future studies include periodic monitoring to evaluate 

temporal trends of pesticides in the lakes, bioassessment of 

aquatic communities in the lakes to evaluate potential effects 

of pesticides and pesticide mixtures, and measurement of 

atmospheric contributions of agrichemicals to the lakes. 

Local-scale ground-water flow-path studies could provide 

information on the processes and hydrogeologic factors 

controlling the transport and fate of pesticides in the lakes 

and in adjacent aquifers, and help to determine the influence of the lakes on regional water quality in this dynamic, 

closely linked ground-water/surface-water system.

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Appendix 1 43

Appendix 1. Constituents, analytical methods, and reporting levels for water-quality samples analyzed in the lake samples.

[Units are in micrograms per liter, except as noted. IC, ion chromatography; ICP-AES, inductively coupled plasma—atomic emission spectroscopy; HPLC-MS, 

high performance liquid chromatography—mass spectrometry; SPE, solid phase extraction; USEPA, U.S. Environmental Protection Agency]

Constituent Reporting level Analysis method Reference

Filtered calcium 0.25 mg/L ICP-AES USEPA (2001) (Method 200.7)

Filtered magnesium .25 mg/L ICP-AES USEPA (2001) (Method 200.7)

Filtered sodium .25 mg/L ICP-AES USEPA (2001) (Method 200.7)

Filtered potassium .25 mg/L ICP-AES USEPA (2001) (Method 200.7)

Filtered chloride .25 mg/L IC USEPA (1993a) (Method 300.0)

Filtered sulfate .48 mg/L IC USEPA (1993a) (Method 300.0)

Filtered fluoride .08 mg/L Ion selective electrode Greenberg (1992) (Method 4500F-C)

Unfiltered ammonia nitrogen .02 mg/L as nitrogen Spectroscopy Greenberg (1992) (Method 4500NH3-H)

Unfiltered nitrite plus nitrate nitrogen .01 mg/L as nitrogen Spectroscopy USEPA (1993b) (Method 353.2)

Unfiltered nitrogen .1 mg/L as nitrogen Spectroscopy USEPA (1993b) (Method 353.2)

Filtered orthophosphate .0125 mg/L Spectroscopy Greenberg (1992) (Method 4500-P)

Unfiltered phosphate .0125 mg/L Spectroscopy USEPA (1993c) (Method 365.1)

Unfiltered organic carbon 1.0 mg/L High temperature combustion Greenberg (1992) (Method 5310B)

Unfiltered alkalinity Titration USEPA (1978) (Method 310.1)

Turbidity Nephelometry Greenberg (1992) (Method 2130B)

Total suspended solids Gravimetric Greenberg (1992) (Method 2540D)

Total dissolved solids Gravimetric Greenberg (1992) (Method 2540C)

2,4-D (2,4-Dicholorophenoxyacetic acid) .02181, .02182, .0382 HPLC-MS/SPE Furlong and others (2001)

2,4-D ME (2,4-D methyl ester) .00861, .00862, .0162 HPLC-MS/SPE Furlong and others (2001)

2,4-DB .0161, .0162, 0.0202 HPLC-MS/SPE Furlong and others (2001)

3-ketocarbofuran 1.51, .0142, .023 HPLC-MS/SPE Furlong and others (2001)

Acifluorfen .00661, .00662, .0283 HPLC-MS/SPE Furlong and others (2001)

Aldicarb .041, .042, .043 HPLC-MS/SPE Furlong and others (2001)

 Aldicarb sulfone .021, .022, .0183 HPLC-MS/SPE Furlong and others (2001)

 Aldicarb sulfoxide .00821, .00822, .00223 HPLC-MS/SPE Furlong and others (2001)

Atrazine .0091

, 0.0092, 0.0083 HPLC-MS/SPE Furlong and others (2001)

.025 HPLC/MS Lee and others (2002b)

Bendiocarb .02521, .02522, .0203 HPLC-MS/SPE Furlong and others (2001)

Benomyl .00381, .00382, .00223 HPLC-MS/SPE Furlong and others (2001)

Bensulfuron-methyl .01581, .01582, .0183 HPLC-MS/SPE Furlong and others (2001)

Bentazon .0111, .0112, .0123 HPLC-MS/SPE Furlong and others (2001)

Bromacil .0331, .0332, .0183 HPLC-MS/SPE Furlong and others (2001)

.025 HPLC-MS Lee and others (2002b)

Bromoxynil .0171

, .0172, .0283 HPLC-MS/SPE Furlong and others (2001)

Caffeine .00961, .00962, .0183 HPLC-MS/SPE Furlong and others (2001)

Carbaryl .02841, .02842, .0183 HPLC-MS/SPE Furlong and others (2001)

Carbofuran .00561, .00562, .0163 HPLC-MS/SPE Furlong and others (2001)

 3-Hydroxycarbofuran .00581, .00582, .0083 HPLC-MS/SPE Furlong and others (2001)

Chloramben, methyl ester .0181, .0182, .0243 HPLC-MS/SPE Furlong and others (2001)

Chlorimuron-ethyl .00961, .00962, .0323 HPLC-MS/SPE Furlong and others (2001)

Chlorothalonil .0351, .0352, .0353 HPLC-MS/SPE Furlong and others (2001)
44 Water Quality and Evaluation of Pesticides in Lakes in the Ridge Citrus Region of Central Florida

Constituent Reporting level Analysis method Reference

Clopyralid .01381, .01382, .0243 HPLC-MS/SPE Furlong and others (2001)

CPMU (3-(4-chlorophenyl)-1-methyl urea) .02421, .02422, .0363 HPLC-MS/SPE Furlong and others (2001)

Cyanazine .025 HPLC-MS Lee and others (2002b)

Cyanazine amide .025 HPLC-MS Lee and others (2002b)

Cyanazine acid .025 HPLC-MS Lee and others (2002b)

Deethylcyanazine .025 HPLC-MS Lee and others (2002b)

Deethylcyanazine acid .025 HPLC-MS Lee and others (2002b)

Deethylcyanazine amid .025 HPLC-MS Lee and others (2002b)

Cycloate .0131, .0132, .0143 HPLC-MS/SPE Furlong and others (2001)

Dacthal monoacid .01161, .01162, .0283 HPLC-MS/SPE Furlong and others (2001)

Dicamba .01281, .01282, .0363 HPLC-MS/SPE Furlong and others (2001)

Dichlorprop .01381, .01382, .0283 HPLC-MS/SPE Furlong and others (2001)

Dinoseb .0121, .0122, .0383 HPLC-MS/SPE Furlong and others (2001)

Diphenamid .02641, .02642, .0103 HPLC-MS/SPE Furlong and others (2001)

Diuron .0151, .0152, .0153 HPLC-MS/SPE Furlong and others (2001)

.20 HPLC-MS Lee and others (2002b)

 DCPMU: 3(3,4-Dichlorophenyl) methyl urea .02426, .0367 HPLC-MS/SPE Furlong and others (2001)

 DCPU: 3,4-Dichlorophenylurea .06 HPLC-MS/SPE Furlong and others (2001)

Fenuron .03161, .03162, .0193 HPLC-MS/SPE Furlong and others (2001)

Flumetsulam .0111, .0112, .0403 HPLC-MS/SPE Furlong and others (2001)

Fluometuron .0311, .0312, .0163 HPLC-MS/SPE Furlong and others (2001)

.20 HPLC-MS Lee and others (2002b)

Demethylfluometuron .20 HPLC-MS Lee and others (2002b)

Glufosinate .10 HPLC-MS Lee and others (2002a)

Glyphosate .10 HPLC-MS Lee and others (2002a)

 Aminomethylphosphonic acid (AMPA) .10 HPLC-MS Lee and others (2002a)

Imazaquin .0161, .0162, .0363 HPLC-MS/SPE Furlong and others (2001)

Imazethapyr .0171, .0172, .0383 HPLC-MS/SPE Furlong and others (2001)

Imidacloprid .00681, .00682, .0203 HPLC-MS/SPE Furlong and others (2001)

Linuron .01441, .01442, .0143 HPLC-MS/SPE Furlong and others (2001)

.20 HPLC-MS Lee and others (2002b)

MCPA .01621, .01622, .0303 HPLC-MS/SPE Furlong and others (2001)

MCPB .0151, .0152, .0103 HPLC-MS/SPE Furlong and others (2001)

Metalaxyl .021, .022, .0123 HPLC-MS/SPE Furlong and others (2001)

Methiocarb .0081, .0082, .0103 HPLC-MS/SPE Furlong and others (2001)

Methomyl .00441, .00442, .0203 HPLC-MS/SPE Furlong and others (2001)

Metsulfuron methyl .0251, .0252, .0253 HPLC-MS/SPE Furlong and others (2001)

Neburon .0121, .0122, .0123 HPLC-MS/SPE Furlong and others (2001)

Nicosulfuron .0131, .0132, .043 HPLC-MS/SPE Furlong and others (2001)

Norflurazon .0161, .0162, .0203 HPLC-MS/SPE Furlong and others (2001)

 De(s)methyl norflurazon .03 HPLC-MS/SPE Furlong and others (2001)

Oryzalin .01761, .01762, .0123 HPLC-MS/SPE Furlong and others (2001)

Appendix 1. Constituents, analytical methods, and reporting levels for water-quality samples analyzed in the lake samples.—Continued

[Units are in micrograms per liter, except as noted. IC, ion chromatography; ICP-AES, inductively coupled plasma—atomic emission spectroscopy; HPLC-MS, 

high performance liquid chromatography—mass spectrometry; SPE, solid phase extraction; USEPA, U.S. Environmental Protection Agency]
Appendix 1 45

Constituent Reporting level Analysis method Reference

Oxamyl .01221, .01222, .0303 HPLC-MS/SPE Furlong and others (2001)

Picloram .01981, .01982, .0323 HPLC-MS/SPE Furlong and others (2001)

Prometon .025 HPLC-MS Lee and others (2002b)

Propazine .025 HPLC-MS Lee and others (2002b)

Propham .00961, .00962, .0303 HPLC-MS/SPE Furlong and others (2001)

Propiconazole .0211, .0212, .0103 HPLC-MS/SPE Furlong and others (2001)

Propoxur .0081, .0082, .0083 HPLC-MS/SPE Furlong and others (2001)

Siduron .01681, .01682, .0203 HPLC-MS/SPE Furlong and others (2001)

Simazine .025 HPLC-MS Lee and others (2002b)

Sulfometuron-methyl .00881, .00882, .0383 HPLC-MS/SPE Furlong and others (2001)

Tebuthiuron .00621, .00622, .0263 HPLC-MS/SPE Furlong and others (2001)

Terbacil .00981, .00982, .0163 HPLC-MS/SPE Furlong and others (2001)

Triazine Degradates5:

 DDA (CAAT): 

 Didealkylatrazine; Chlordiamino-s-triazine; 

 Deethyldeisopropylatrazine

.011, .0442, .043

.025

HPLC-MS/SPE

HPLC-MS

Furlong and others (2001)

Lee and others (2002b)

 DIA (CEAT; DES): 

 Deisopropylatrazine; 2-Chloro-6-ethyl-

 amino-4-amino-s-triazine; Deethylsimazine

.0441, .012, .083

.025

HPLC-MS/SPE

HPLC-MS

Furlong and others (2001)

Lee and other (2002b)

 DEA (CIAT): 

 Deethylatrazine; 2-Chloro-4-isopropyl-

 amino-6-amino-s-triazine

.02821, .02822, .0282

.025

HPLC-MS/SPE

HPLC-MS

Furlong and others (2001)

Lee and others (2002b)

 DEHA (HDEA): 

 Deethylhydroxyatrazine 

.025 HPLC-MS Lee and others (2002b)

 DIHA (OEAT; HDIA): 

 Deisopropylhydroxyatrazine; 2-hydroxy-

 6-ethylamino-4-amino-s-triazine

.025 HPLC-MS Lee and others (2002b)

 HA (OIET): 

 2-hydroxyatrazine; 2-Hydroxy-4-isopropyl-

 amino-6-ethylamino-s-triazine

.0081, .0082, .0323

.025

HPLC-MS/SPE

HPLC-MS

Furlong and others (2001)

Lee and others (2002b)

 HS (OEET): 

 Hydroxysimazine; 2-hydroxy-4,6-diethyl-

 amino-s-triazine

.025 HPLC-MS Lee and others (2002b)

Tribenuron-methyl .0881, .0884 HPLC-MS/SPE Furlong and others (2001)

Triclopyr .02241, .02242, .0263 HPLC-MS/SPE Furlong and others (2001)

1Reporting level from December 1, 2004 – June 30, 2004.

2Reporting level from July 1, 2004 – September 30, 2004.

3Reporting level from October 1, 2004 – September 30, 2005.

4Reporting level from July 1, 2004 – August 3, 2005.

5

Depending on the degradate, possible parent compounds may include atrazine, simazine, propazine, cyanazine, or cyanazine degradates. Triazine degradation pathways and details regarding analytical methods are described in Scribner and others (1999), and in appendix 5. Simazine degradation is depicted in 

figure 15.

6Reporting level from December 1, 2003 – September 30, 2004.

7Reporting level from October 1, 2004 – September 30, 2005.

Appendix 1. Constituents, analytical methods, and reporting levels for water-quality samples analyzed in the lake samples.—Continued

[Units are in micrograms per liter, except as noted. IC, ion chromatography; ICP-AES, inductively coupled plasma—atomic emission spectroscopy; HPLC-MS, 

high performance liquid chromatography—mass spectrometry; SPE, solid phase extraction; USEPA, U.S. Environmental Protection Agency]
46 Water Quality and Evaluation of Pesticides in Lakes in the Ridge Citrus Region of Central Florida
Appendix 2 47

Appendix 2. Field measurements and laboratory results for major water-quality constituents and nutrients in the lake samples. 

[Includes samples collected at a depth of 5 feet below lake surface. All values in milligrams per liter, except as noted; oC, degrees Celsius; NA, not analyzed; E, estimated value; NTU, nephelometric turbidity units; 

μs/cm, microsiemens per centimeter; < , less than]

Station identifier Station name Date Time

Color 

(platinumcobalt units)

pH, field 

(standard 

units)

Temperature, 

field (oC)

Specific 

conductance, 

field (μS/cm)

Dissolved 

oxygen, field 

Secchi depth 

(feet)

Total 

alkalinity1 ,

as CaCO3

Suspended 

sediment 

Total 

organic 

carbon 

Dissolved 

calcium 

02266923 Swim Lake near Alcoma 12/09/03 1120 NA 7.2 19 283 8.9 NA NA NA NA NA

02266923 Swim Lake near Alcoma 03/23/04 1050 < 5 6.4 22 286 8.7 NA 31.7 .60 8.5 28.4

02266923 Swim Lake near Alcoma 06/08/04 1130 < 5 7.3 30 296 8.1 19 32.79 .87 4.5 27.8

02266923 Swim Lake near Alcoma 09/15/04 1005 < 5 7.1 28 257 6.2 12 16.04 1.23 5.9 24.1

02270655 Lake Lynn near De Soto City 12/08/03 1155 NA 7.5 21 327 8.9 NA NA NA NA NA

02270655 Lake Lynn near De Soto City 03/22/04 1050 < 5 6.7 20.5 328 8.4 NA 39.5 .88 3.8 22.6

02270655 Lake Lynn near De Soto City 06/07/04 1010 < 5 6.7 29.5 343 7.8 18 41.61 1.52 5 22.3

02270655 Lake Lynn near De Soto City 09/14/04 1035 < 5 8.2 29 330 7.2 9.6 46.42 1.40 4.7 21.8

02270700 Lake Annie near Lake Placid 12/20/04 1015 175 7 18.5 38 6.3 2.8 2.64 1.32 15.3 2.74

02270700 Lake Annie near Lake Placid 03/23/05 1150 125 5.2 22.5 37 8.7 NA 1.85 1.29 20.7 2.43

02270700 Lake Annie near Lake Placid 06/22/05 1300 100 5.8 29 39 5.6 5.0 2.09 1.38 16 2.43

02270700 Lake Annie near Lake Placid 09/06/05 1400 150 6 29 37 5.4 3.2 3.69 2.00 36.8 2.76

273323081290800 Lake Denton near Avon Park 12/08/03 1505 NA 7.7 20.5 284 9.4 NA NA NA NA NA

273323081290800 Lake Denton near Avon Park 03/22/04 1440 < 5 7.6 21 283 9.6 NA 35.4 .66 9.7 22.1

273323081290800 Lake Denton near Avon Park 06/07/04 1350 < 5 7.8 29 303 8 21 35.22 .84 4.5 21.9

273323081290800 Lake Denton near Avon Park 09/14/04 1355 < 5 7.7 26 287 11.2 15 38.37 1.21 7.2 20.5

273850081310800 Pabor Lake near Avon Park 12/20/04 1420 15 7.5 17.5 490 7.3 2.3 185.83 4.54 13.4 21.5

273850081310800 Pabor Lake near Avon Park 03/23/05 0815 15 7.3 21.5 474 9.9 NA 164.83 9.00 31.5 20.9

273850081310800 Pabor Lake near Avon Park 06/22/05 0850 15 7.5 29 495 6.4 2.2 167.58 9.21 14.9 20.6

273850081310800 Pabor Lake near Avon Park 09/07/05 1300 15 NA 29.5 485 6.8 3.6 163.43 3.73 15.3 19.2

274648081315100 Lake Moody near Frostproof 12/21/04 0920 15 7.9 15 383 8.8 1.5 56.73 13.75 27.9 30.1

274648081315100 Lake Moody near Frostproof 03/22/05 1310 10 7.6 21 350 10.3 NA 48.35 12.18 9.9 28.2

274648081315100 Lake Moody near Frostproof 06/21/05 1400 10 7.3 29 355 7.9 1.6 50.49 15.94 13.1 28

274648081315100 Lake Moody near Frostproof 09/06/05 0930 15 7 29 339 6.8 1.5 47.88 9.88 47 25.9

274744081304200 Lake Leonore near Frostproof 12/21/04 1200 20 7.7 14 343 8.1 1.6 55.59 10.84 21.1 24.9

274744081304200 Lake Leonore near Frostproof 03/22/05 0930 10 7.2 21 340 7.6 NA 37.08 1.31 29.6 23.5

274744081304200 Lake Leonore near Frostproof 06/21/05 0900 10 6.1 29.5 318 7.5 7.8 34.6 1.32 5 21.9

274744081304200 Lake Leonore near Frostproof 09/07/05 0850 20 7.5 28.5 311 8 1.8 45.89 10.60 6.5 22.1

275256081275900 Lake Aurora at Hesperides 12/09/03 0900 NA 6.9 19.5 220 8.3 NA NA NA NA NA

275256081275900 Lake Aurora at Hesperides 03/23/04 0805 < 5 7.4 21.5 224 8.3 NA 27.8 < .50 10 18.3

275256081275900 Lake Aurora at Hesperides 06/08/04 0815 < 5 7.1 29.5 238 8.3 12.5 29.58 1.28 4.6 18.1

275256081275900 Lake Aurora at Hesperides 09/15/04 0810 < 5 7.5 28.5 218 7.1 12 61.21 1.20 7 16.7
48 Water Quality and Evaluation of Pesticides in Lakes in the Ridge Citrus Region of Central Florida

Station identifier Dissolved 

magnesium 

Dissolved

sodium 

Dissolved 

potassium

Dissolved 

calcium

Dissolved 

sulfate Fluoride 

Total 

dissolved 

solids 

Turbidity 

(NTUs)

pH lab 

(standard 

units)

Nutrients

Total nitrogen1, 

as N 

Ammonia1, 

as N 

Nitrite1, 

as N 

Nitrite plus 

nitrate1, 

as N 

Nitrate1,2, 

as N 

Total 

phosphorus, 

as P 

Dissolved 

orthophosphate, 

as P 

02266923 NA NA NA NA NA NA NA NA 7.21 NA < 0.01 < 0.01 2.9 2.89 < 0.02 NA

02266923 8.72 5.04 13.1 15.7 57 .090 179 .46 7.14 4.79 .02 .029 4.44 4.41 .011 < .01

02266923 8.45 5.26 12.3 16.23 57.82 .074 169 .114 7.46 5.19 .03 .049 4.75 4.70 < .01 < .01

02266923 7.44 4.66 11.2 13.91 50.73 .066 151 .9 6.82 3.33 .04 .028 2.90 2.87 < .01 < .01

02270655 NA NA NA NA NA NA NA NA 7.45 NA .11 .010 1.40 1.39 < .02 NA

02270655 16.3 6.79 14.7 27.6 61.8 .040 199 .61 7.97 2.21 .02 .015 1.82 1.81 < .01 < .01

02270655 16.1 7.08 14.4 28.65 64.83 .031 191 .339 8.27 2.05 .02 .019 1.69 1.67 < .01 < .01

02270655 15.9 6.96 14.3 27.05 62.07 .039 190 .02 7.28 1.53 .15 .015 1.03 1.01 < .01 < .01

02270700 .74 3.01 .66 2.97 1.11 .031 48 .09 5.51 .68 .08 .005 .12 .11 .014 .01

02270700 .75 3.05 .72 5.93 2.63 .036 42 4.21 5.2 .54 .01 .007 .16 .15 .017 < .01

02270700 .76 3.2 .82 6.07 2.94 .02 46 .145 5.8 .42 .02 < .005 .01 E .004 .011 < .01

02270700 .69 2.85 .59 5.64 2.22 .0208 50 .08 NA .40 .02 .007 .01 .001 .015 < .01

273323081290800 NA NA NA NA NA NA NA NA 7.67 NA .01 .020 3.00 2.98 < .02 NA

273323081290800 12.4 4.83 13.7 26.4 42.8 .27 166 .43 7.31 3.82 .18 .028 3.45 3.42 < .01 < .01

273323081290800 12.4 4.91 13.2 27.53 44.12 .258 171 .111 7.89 3.78 .03 .034 3.48 3.45 < .01 < .01

273323081290800 11.8 4.73 12.8 25.15 41.4 .239 149 .02 7.36 2.98 .01 .023 2.73 2.71 < .01 < .01

273850081310800 8.32 45.4 47.6 16.63 50.21 .142 316 5.98 7.63 1.84 .46 .009 .05 .04 .02 < .01

273850081310800 8.18 44.1 47.5 16.8 48.13 .107 313 8.7 NA 1.47 .01 < .005 .03 E .025 .032 < .01

273850081310800 8.21 44.3 50 16.5 47.88 .098 312 6.99 NA 1.36 .03 < .005 .01 E .001 .015 < .01

273850081310800 7.83 42.5 50.8 16.81 47.91 .095 303 3.72 NA .86 .02 < .005 .00 E .001 .016 < .01

274648081315100 17.9 8.99 14.3 28.9 84.16 .196 256 11.3 7.17 2.96 .38 .010 .41 .40 .020 < .01

274648081315100 16.9 8.27 13.4 27.38 73.07 .180 232 14 NA 2.88 .03 .007 1.00 .99 .037 < .01

274648081315100 16.5 7.88 13.2 26.65 72 .174 220 17.8 NA 2.37 .03 .007 .42 .41 .021 < .01

274648081315100 15.2 7.33 12.5 26 67.22 .151 227 13.9 NA 2.31 .28 .006 .56 .55 .017 < .01

274744081304200 16.2 8.16 15 28.51 57.25 .101 225 11.2 7.21 3.98 .16 .009 2.29 2.28 .018 < .01

274744081304200 15.2 7.39 13.3 26.77 59.7 .101 204 2.82 NA 4.14 .01 .021 3.58 3.56 .015 < .01

274744081304200 14.5 6.86 12.9 25.02 55.77 .092 190 2.25 NA 4.03 .04 .014 3.56 3.55 < .01 < .01

274744081304200 14.3 6.81 13.2 25.63 55.6 .111 199 12.7 NA 2.39 .02 .009 1.38 1.37 .025 < .01

275256081275900 NA NA NA NA NA NA NA NA NA NA .02 < .010 .98 E .97 < .02 NA

275256081275900 7.62 5.2 12 18.7 38.3 .060 136 .57 7.02 2.00 .02 .011 1.50 1.49 .011 < .01

275256081275900 7.78 5.41 11.7 19.78 39.76 .053 136 .112 6.65 1.76 .02 .015 1.35 1.34 < 0.1 < .01

275256081275900 7.25 5.2 11.2 17.36 36.03 .051 127 .2 6.92 1.35 .03 .006 .9874 .98 < 0.1 < .01

1 Unfiltered.

2 Calculated as [(nitrite plus nitrate) – nitrite].

Appendix 2. Field measurements and laboratory results for major water-quality constituents and nutrients in the lake samples.—Continued

[Includes samples collected at a depth of 5 feet below lake surface. All values in milligrams per liter, except as noted; oC, degrees Celsius; NA, not analyzed; E, estimated value; NTU, nephelometric turbidity units; 

μs/cm, microsiemens per centimeter; < , less than]
Appendix 3 49

Appendix 3. Laboratory results for detected pesticides and degradates in the lake samples. 

[All values in micrograms per liter. Shaded columns indicate duplicate analyses conducted at separate laboratories that typically were not included in summaries in the report. Full chemical names for abbreviated 

pesticide compounds are listed in table 2. Degradates are shown in italics. NL, USGS National Water Quality Laboratory; KS, USGS Organic Geochemistry Research Lab; NA, not analyzed; E, estimated value; 

L., Lake; ft, feet; < less than]

Station identifier Station name Short name Date Time Depth1 2,4-D 2,4-D Methyl Ester Aldicarb sulfone Aldicarb sulfoxide Atrazine-NL Atrazine-KS

02266923 Swim Lake near Alcoma Swim L. 12/09/03 1120 Top < 0.0218 <0.0086 E 0.26 E 0.67 E 0.004 0.025

02266923 Swim Lake near Alcoma Swim L. 03/23/04 1050 Top < .0218 < .0086 E .33 E .54 < .009 < .025

02266923 Swim Lake near Alcoma Swim L. 06/08/04 1125 Bottom (19 ft) < .0218 < .0086 E .025 E .11 < .009 < .025

02266923 Swim Lake near Alcoma Swim L. 06/08/04 1130 Top < .0218 < .0086 E .018 E .074 < .009 < .025

02266924 Swim Lake near Alcoma Swim L. 09/15/04 1005 Top < .0218 < .0086 E .101 E .18 < .009 < .025

02270655 Lake Lynn near De Soto City L. Lynn 12/08/03 1155 Top E .0113 < .0086 < .02 E .026 E .0063 < .025

02270655 Lake Lynn near De Soto City L. Lynn 03/22/04 1050 Top < .0218 < .0086 < .02 < .0082 E .0067 < .025

02270655 Lake Lynn near De Soto City L. Lynn 06/07/04 1000 Bottom (37 ft) .138 < .0086 < .02 < .0082 < .009 < .025

02270655 Lake Lynn near De Soto City L. Lynn 06/07/04 1010 Top .109 < .0086 < .02 < .0082 E .0056 < .025

02270656 Lake Lynn near De Soto City L. Lynn 09/14/04 1035 Top .023 < .0086 < .02 < .0082 E .0046 < .025

02270700 Lake Annie near Lake Placid L. Annie 12/20/04 1015 Top < .038 < .016 < .018 < .022 < .008 < .025

02270700 Lake Annie near Lake Placid L. Annie 03/23/05 1150 Top < .04 < .016 < .02 < .022 E .007 < .025

02270700 Lake Annie near Lake Placid L. Annie 06/22/05 1300 Top < .04 < .016 < .02 < .022 < .008 < .025

02270700 Lake Annie near Lake Placid L. Annie 09/06/05 1400 Top < .04 < .016 < .02 < .022 E .005 < .025

273323081290800 Lake Denton near Avon Park L. Denton 12/08/03 1505 Top .144 < .0086 < .02 < .0082 < .009 < .025

273323081290800 Lake Denton near Avon Park L. Denton 03/22/04 1440 Top 1.16 .0359 < .02 < .0082 < .009 < .025

273323081290800 Lake Denton near Avon Park L. Denton 06/07/04 1345 Bottom (45 ft) .0458 < .0086 < .02 < .0082 < .009 < .025

273323081290800 Lake Denton near Avon Park L. Denton 06/07/04 1350 Top < .0218 < .0086 < .02 < .0082 < .009 < .025

273323081290800 Lake Denton near Avon Park L. Denton 09/14/04 1355 Top .0621 < .0086 < .02 < .0082 < .009 < .025

273850081310800 Pabor Lake near Avon Park Pabor L. 12/20/04 1420 Top < .038 < .016 < .018 < .022 < .008 < .025

273850081310800 Pabor Lake near Avon Park Pabor L. 03/23/05 815 Top < .04 < .016 < .02 < .022 E .015 < .025

273850081310800 Pabor Lake near Avon Park Pabor L. 06/22/05 850 Top < .04 < .016 < .02 < .022 .011 < .025

273850081310800 Pabor Lake near Avon Park Pabor L. 09/07/05 1300 Top < .04 < .016 < .02 < .022 E .013 < .025

274648081315100 Lake Moody near Frostproof L. Moody 12/21/04 920 Top < .038 < .016 < .018 < .022 < .008 < .025

274648081315100 Lake Moody near Frostproof L. Moody 03/22/05 1310 Top < .04 < .016 < .02 < .022 < .008 < .025

274648081315100 Lake Moody near Frostproof L. Moody 06/21/05 1400 Top < .04 < .016 < .02 < .022 < .008 < .025

274648081315100 Lake Moody near Frostproof L. Moody 09/06/05 930 Top < .04 < .016 < .02 < .022 E .004 < .025

274744081304200 Lake Leonore near Frostproof L. Leonore 12/21/04 1200 Top < .038 < .016 .028 .075 < .008 < .025

274744081304200 Lake Leonore near Frostproof L. Leonore 03/22/05 930 Top < .04 < .016 .06 .102 < .008 < .025

274744081304200 Lake Leonore near Frostproof L. Leonore 06/21/05 900 Top < .04 < .016 E .02 .079 < .008 < .025

274744081304200 Lake Leonore near Frostproof L. Leonore 09/07/05 850 Top < .04 < .016 < .02 < .022 < .008 < .025

275256081275900 Lake Aurora at Hesperides L. Aurora 12/09/03 900 Top < .0218 < .0086 < .02 < .0082 E .0038 < .025

275256081275900 Lake Aurora at Hesperides L. Aurora 03/23/04 805 Top < .0218 < .0086 < .02 < .0082 < .009 < .025

275256081275900 Lake Aurora at Hesperides L. Aurora 06/08/04 810 Bottom (23 ft) < .0218 < .0086 < .02 < .0082 < .009 < .025

275256081275900 Lake Aurora at Hesperides L. Aurora 06/08/04 815 Top < .0218 < .0086 < .02 < .0082 < .009 < .025

275256081275900 Lake Aurora at Hesperides L. Aurora 09/15/04 810 Top < .0218 < .0086 < .02 < .0082 < .009 < .025

1Top samples were collected about 5 feet below the lake surface. Bottom samples were collected near the lake bottom at depth shown.
50 Water Quality and Evaluation of Pesticides in Lakes in the Ridge Citrus Region of Central Florida

Appendix 3. Laboratory results for detected pesticides and degradates in the lake samples—Continued. 

[All values in micrograms per liter. Shaded columns indicate duplicate analyses conducted at separate laboratories that typically were not included in summaries in the report. Full chemical names for abbreviated 

pesticide compounds are listed in table 2. Degradates are shown in italics. NL, USGS National Water Quality Laboratory; KS, USGS Organic Geochemistry Research Lab; NA, not analyzed; E, estimated value; 

L., Lake; ft, feet; < less than]

Short name Bromacil-NL Bromacil-KS CPMU Diuron-NL Diuron-KS DCPU DCPMU Imazquin Imidacloprid Metalaxyl Norflurazon Demethyl-norflurazon Oxamyl

Swim L. E 0.062 < .025 < 0.02 0.07 < 0.2 < 0.06 E 0.0215 < 0.016 < 0.0068 < 0.02 E 5.3 E 17.1 < 0.012

Swim L. E .042 < .025 < .02 .057 < .2 < .06 .0301 < .016 < .0068 .0564 E 3.3 E 14.97 < .012

Swim L. E .018 < .025 < .02 .023 < .2 < .06 E .008 < .016 < .0068 E .0102 E .79 2.58 < .012

Swim L. E .018 < .025 < .02 .02 < .2 < .06 E .0088 < .016 < .0068 E .009 E .46 1.99 < .012

Swim L. < .033 < .025 < .02 .046 < .2 < .06 < .03 < .016 .0162 .03 E 2.27 E 5.99 < .012

L. Lynn E .31 .45 < .02 < .131 < .2 < .03 < .06 < .016 < .0068 < .02 E .27 .858 < .012

L. Lynn E .21 .26 < .02 < .015 < .2 < .06 < .03 < .016 < .0068 < .02 E .074 .629 < .012

L. Lynn E .15 .2 < .02 < .015 < .2 < .06 < .03 < .016 < .0068 < .02 E .13 .743 < .012

L. Lynn E .046 .08 < .02 < .015 < .2 < .06 < .03 < .016 < .0068 < .02 E .008 .082 < .012

L. Lynn E .066 .06 < .02 E .009 < .2 < .06 < .03 < .016 < .0068 < .02 E .081 .262 < .012

L. Annie < .018 < .025 < .04 .015 < .2 < .06 < .03 < .036 < .02 < .012 < .02 < .03 < .03

L. Annie < .02 < .05 < .04 < .01 < .2 < .06 < .03 < .04 < .02 < .01 < .02 < .03 < .03

L. Annie < .02 < .025 < .04 < .01 < .2 NA NA < .04 < .02 < .01 < .02 NA < .03

L. Annie < .02 < .025 < .04 .02 < .2 NA NA < .04 < .02 < .01 < .02 NA < .03

L. Denton E .091 .09 < .02 .015 < .2 < .06 E .004 < .016 < .0068 < .02 E .052 .038 < .012

L. Denton E .034 .04 < .02 < .015 < .2 < .06 < .03 < .016 < .0068 < .02 E .014 .038 < .012

L. Denton E .026 .03 < .02 < .015 < .2 < .06 < .03 < .016 < .0068 < .02 E .009 .031 < .012

L. Denton E .017 < .025 < .02 < .015 < .2 < .06 < .03 < .016 < .0068 < .02 E .004 .014 < .012

L. Denton E .037 < .025 < .02 E .011 < .2 < .06 < .03 < .016 < .0068 < .02 E .047 .078 < .012

Pabor L. < .018 < .025 < .04 .075 < .2 < .06 E .021 E .021 < .02 < .012 .582 E 1.16 < .03

Pabor L. < .02 < .05 < .04 .07 < .2 < .06 E .018 < .04 < .02 < .01 .31 1.04 < .03

Pabor L. < .02 < .025 < .04 .05 < .2 NA NA < .04 < .02 < .01 .15 NA < .03

Pabor L. < .02 < .025 < .04 1.79 3.81 NA NA < .04 < .02 < .01 .83 NA < .03

L. Moody .247 .39 < .04 .041 < .2 < .06 < .03 < .036 < .02 .042 .179 .230 E .036

L. Moody .34 .51 < .04 .04 < .2 < .06 E .0094 < .04 < .02 .03 .18 .302 < .03

L. Moody .38 .52 < .04 .07 < .2 NA NA < .04 < .02 .03 .05 NA < .03

L. Moody .71 .61 < .04 .08 < .2 NA NA < .04 < .02 .03 .21 NA < .03

L. Leonore 4.87 3.53 < .04 .06 < .2 < .06 < .033 < .036 < .02 .18 .771 .606 < .03

L. Leonore < .02 6.29 < .04 .04 < .2 < .06 E .016 < .04 < .02 .13 .68 .575 < .03

L. Leonore E 3.35 2.48 < .04 .09 < .2 NA NA < .04 < .02 .11 .68 NA < .03

L. Leonore E 3.86 2.7 < .04 E .06 < .2 NA NA < .04 < .02 E .09 .37 NA < .03

L. Aurora E .52 .76 < .02 .021 < .2 < .03 < .03 < .016 < .0068 < .02 E .17 .315 < .012

L. Aurora E .71 .88 < .02 .018 < .2 < .06 E .011 < .016 < .0068 .0665 E .16 .358 E .009

L. Aurora E .67 .95 < .02 E .014 < .2 < .06 < .03 < .016 < .0068 .0484 E .092 .287 < .012

L. Aurora E .84 .83 < .02 .016 < .2 < .06 E .0077 < .016 < .0068 .0499 E .043 .208 < .012

L. Aurora E .366 .43 < .02 .021 < .2 < .06 < .03 < .016 < .0068 .04 E .088 .178 < .012
Appendix 3 51

Appendix 3. Laboratory results for detected pesticides and degradates in the lake samples.—Continued 

[All values in micrograms per liter. Shaded columns indicate duplicate analyses conducted at separate laboratories that typically were not included in summaries in the report. Full chemical names for abbreviated 

pesticide compounds are listed in table 2. Degradates are shown in italics. NL, USGS National Water Quality Laboratory; KS, USGS Organic Geochemistry Research Lab; NA, not analyzed; E, estimated value; 

L., Lake; ft, feet; < less than]

Short name Simazine-KS Tebuthiuron Hydroxyatrazine-NL 

(OIET)

Hydroxyatrazine-KS 

(OIET) DEA-NL (CIAT) DEHA-KS (HDEA) 

HydroxysimazineKS (HS) DDA-KS 

(CAAT) 

DDA-NL 

(CAAT) DIA-KS (CEAT) DIA-NL (CEAT) 

DIHA-KS 

(HDIA) 

Swim L. 0.06 <0.0062 <0.008 < 0.025 < 0.028 < 0.025 < 0.025 0.13 < 0.01 0.14 E 0.071 < 0.025

Swim L. .04 <.0062 <.008 < .025 < .028 < .025 .27 .14 E .02 .1 E .051 < .025

Swim L. .04 <.0062 <.008 < .025 < .028 < .025 .11 .24 E .034 .1 E .061 < .025

Swim L. .04 <.0062 <.008 < .025 < .028 < .025 .11 .27 E .03 .11 E .069 < .025

Swim L. < .025 <.0062 <.008 < .025 < .0282 < .025 .13 .18 E .043 .05 E .022 < .025

L. Lynn .03 <.0062 E .01 < .025 < .028 < .025 < .025 .14 E .009 .05 E .026 < .025

L. Lynn < .025 <.0062 E .0093 < .025 < .028 < .025 .04 .09 E .034 .03 E .017 < .025

L. Lynn < .025 <.0062 E .013 < .025 < .028 < .025 < .025 .14 E .014 .05 E .016 < .025

L. Lynn < .025 <.0062 E .0065 < .025 E .0056 < .025 < .025 .25 E .019 .1 E .028 < .025

L. Lynn < .025 <.0062 E .012 < .025 < .0282 < .025 < .025 .14 E .043 .03 E .017 < .025

L. Annie < .025 .03 <.032 < .025 < .028 < .025 < .025 < .025 < .04 < .025 < .08 < .025

L. Annie < .025 .039 <.032 < .025 < .03 < .025 < .05 < .025 < .04 < .025 < .08 < .025

L. Annie < .025 .027 <.032 < .025 < .03 < .025 < .025 < .025 < .04 < .025 < .08 < .025

L. Annie < .025 E .023 <.032 < .025 < .03 < .025 < .025 < .025 < .04 < .025 < .08 < .025

L. Denton < .025 <.0062 <.008 < .025 < .028 < .025 < .025 .12 < .01 .03 E .017 < .025

L. Denton < .025 <.0062 <.008 < .025 < .028 < .025 < .025 .07 E .01 < .025 E .008 < .025

L. Denton < .025 <.0062 <.008 < .025 < .028 < .025 < .025 .19 E .02 < .025 E .014 < .025

L. Denton < .025 <.0062 <.008 < .025 < .028 < .025 < .025 .15 E .022 < .025 E .011 < .025

L. Denton < .025 <.0062 <.008 < .025 < .0282 < .025 < .025 .09 E .044 < .025 E .007 < .025

Pabor L. .04 <.026 <.032 < .025 < .028 < .025 < .025 < .025 < .04 < .025 <.08 < .025

Pabor L. .04 <.026 <.032 < .025 < .03 < .025 .5 < .025 < .04 < .025 E .02 < .025

Pabor L. .03 <.026 <.032 < .025 < .03 < .025 < .025 < .025 < .04 .03 E .02 < .025

Pabor L. < .025 <.026 <.032 < .025 < .03 < .025 < .025 < .025 < .04 < .025 NA < .025

L. Moody < .025 <.026 E .013 < .025 < .028 < .025 < .025 < .025 < .04 < .025 < .08 < .025

L. Moody .03 <.026 E .009 < .025 < .03 < .025 < .05 < .025 < .04 < .025 < .08 < .025

L. Moody .025 <.026 <.032 < .025 < .03 < .025 .94 < .025 < .04 < .025 < .08 < .025

L. Moody .025 <.026 <.032 < .025 < .03 < .025 < .025 < .025 < .04 < .025 < .08 < .025

L. Leonore .03 E .006 <.032 < .025 < .028 < .025 < .025 < .025 < .04 < .025 < .08 < .025

L. Leonore .05 <.026 <.032 < .025 < .03 < .025 .26 < .025 < .04 < .025 E .01 < .025

L. Leonore .04 <.026 <.032 < .025 < .03 < .025 < .025 .05 < .04 < .025 E .01 < .025

L. Leonore .04 <.026 <.032 < .025 < .03 < .025 .46 < .025 < .04 < .025 E .01 < .025

L. Aurora < .025 E .0038 <.008 < .025 < .028 < .025 < .025 < .025 < .01 < .025 < .044 < .025

L. Aurora < .025 <.0062 E .009 < .025 < .028 < .025 .26 < .025 < .01 < .025 < .044 < .025

L. Aurora < .025 <.0062 E .011 < .025 < .028 < .025 < .025 < .025 < .04 < .025 < .01 < .025

L. Aurora < .025 <.0062 E .013 < .025 < .028 < .025 < .025 < .025 < .04 < .025 < .01 < .025

L. Aurora < .025 <.0062 <.01 < .025 < .0282 < .025 .21 < .025 < .04 < .025 E .004 < .025
52 Water Quality and Evaluation of Pesticides in Lakes in the Ridge Citrus Region of Central Florida
Appendix 4 53

Appendix 4. Summary statistics of detected pesticide and degradate concentrations by lake.

[The samples were collected at a depth of 5 feet below the lake surface. Four samples were analyzed per lake for all compounds, 

except where noted. NL, U.S. Geological Survey National Water-Quality Laboratory; LRL, laboratory reporting level; ND, not 

determined; KS, U.S. Geological Survey Organic Geochemistry Laboratory]

Compound

Summary of samples yielding detectable concentrations (micrograms per liter)

No. of samples 

with detections

Sum 

of detected 

concentrations

Median 

detected 

concentration

Minimum 

detected 

concentration

Maximum 

detected 

concentration

Lake Annie (“control”)

Atrazine-NL 2 0.012 ND 0.005 0.007

Diuron-NL 2 .035 ND .015 .020

Tebuthiuron 4 .119 .029 .023 .039

Lake Aurora2

Atrazine-NL 1 .004 ND ND ND

Bromacil-KS 5 3.85 .83 .43 .95

Bromacil-NL 5 3.11 .670 .366 .840

DIA-NL 1 .004 ND ND ND

DCPMU-NL 2 .019 ND .008 .011

Diuron-NL 5 .090 .018 .014 .021

Demethyl norflurazon 5 1.35 .287 .178 .358

Hydroxysimazine-KS 2 .470 ND .210 .260

Metalaxyl 4 .205 .049 .04 .067

Norflurazon 5 .553 .092 .043 .170

HA-NL 3 .033 .011 .009 .013

Oxamyl 1 .009 ND ND ND

Tebuthiuron 1 .004 ND ND ND

Lake Denton2

2,4-D 4 1.41 .103 .046 1.16

2,4-D Methyl ester 1 .036 ND ND ND

Bromacil-NL 5 .205 .034 .017 .091

Bromacil-KS 3 .16 .04 .03 .09

DDA-KS 5 .62 .12 .07 .19

DIA-NL 5 .057 .011 .007 .017

DCPMU-NL 1 .004 ND ND ND

Diuron-NL 2 .026 ND .011 .015

Demethyl norflurazon 5 .198 .038 .014 .078

Norflurazon 5 .126 .014 .004 .052

Lake Leonore

Aldicarb sulfone 3 .108 .028 .02 .06

Aldicarb sulfoxide 3 .256 .079 .075 .102

Bromacil-NL 3 12.1 3.86 3.35 4.87

Bromacil-KS 4 15.0 3.12 2.48 6.29

DDA-KS 1 .05 ND ND ND

DIA-NL 3 .03 .01 .01 .01

DCPMU-NL 11 .016 ND ND ND

Diuron-NL 4 .25 .06 .04 .09

Demethyl norflurazon 12 1.18 ND .575 .606

Hydroxysimazine-KS 2 .72 ND .26 .46

Metalaxyl 4 .51 .12 .09 .18

Norflurazon 4 2.50 .68 .37 .771

Simazine-KS 4 .16 .04 .03 .05

Tebuthiuron 1 .006 ND ND ND

Lake Lynn2

2,4-D 4 .281 .066 .011 .138

Aldicarb sulfoxide 1 .026 ND ND ND

Atrazine-NL 4 .023 .006 .005 .007

Bromacil-NL 5 .782 .15 .046 .31

Bromacil-KS 5 1.05 .20 .06 .45

DDA-KS 5 .76 .14 .09 .25

DIA-NL 5 .104 .017 .016 .028
54 Water Quality and Evaluation of Pesticides in Lakes in the Ridge Citrus Region of Central Florida

Compound

Summary of samples yielding detectable concentrations (micrograms per liter)

No. of samples 

with detections

Sum 

of detected 

concentrations

Median 

detected 

concentration

Minimum 

detected 

concentration

Maximum 

detected 

concentration

DEA-NL 1 .006 ND ND ND

Diuron-NL 1 .009 ND ND ND

Demethyl norflurazon 5 2.57 .629 .082 .858

Hydroxysimazine-KS 1 .04 ND ND ND

Norflurazon 5 .563 .081 .008 .270

HA-NL 5 .050 .01 .007 .013

Simazine-KS 1 .03 ND ND ND

Lake Moody

Atrazine-NL 1 .004 ND ND ND

Bromacil-NL 4 1.68 .36 .247 .71

Bromacil-KS 4 2.03 .515 .39 .61

DCPMU-NL 11 .009 ND ND ND

Diuron-NL 4 .231 .056 .04 .08

Demethyl norflurazon 12 .532 ND .23 .302

Hydroxysimazine-KS 1 .94 ND ND ND

Metalaxyl 4 .132 .03 .03 .042

Norflurazon 4 .619 .180 .05 .210

HA-NL 2 .022 ND .009 .013

Oxamyl 1 .036 ND ND ND

Simazine-KS 1 .03 ND ND ND

Parbor Lake

Atrazine-NL 3 .039 .013 .011 .015

DIA-NL 2 .04 ND .020 .020

DCPMU-NL 12 .039 ND .018 .021

Diuron-NL 4 1.99 .073 .05 1.79

Demethyl norflurazon 12 2.20 ND 1.04 1.16

Hydroxysimazine-KS 1 .50 ND ND ND

Imazaquin 1 .021 ND ND ND

Norflurazon 4 1.87 .446 .15 .830

Simazine-KS 3 .11 .04 .03 .04

Swim Lake2

Aldicarb sulfone 5 .734 .101 .018 .33

Aldicarb sulfoxide 5 1.57 .180 .074 .67

Atrazine-NL 1 .004 ND ND ND

Bromacil-NL 4 .14 .03 .018 .062

DDA-KS 5 .96 .18 .13 .27

DIA-NL 5 .274 .061 .022 .071

DCPMU-NL 4 .068 .015 .008 .03

Diuron-NL 5 .216 .046 .020 .07

Demethyl norflurazon 5 42.6 5.99 1.99 17.1

Hydroxysimazine-KS 4 .62 .12 .11 .27

Imidacloprid 1 .016 ND ND ND

Metalaxyl 4 .106 .020 .009 .056

Norflurazon 5 12.1 2.27 .46 5.30

Simazine-KS 4 .18 .04 .04 .06

1A total of two samples were analyzed for this compound.

2

A total of five samples were analyzed for each compound, including two samples, collected in the shallow and in the deep 

zone of the lake water column, during one sampling event.

Appendix 4. Summary statistics of detected pesticide and degradate concentrations by lake.—Continued

[The samples were collected at a depth of 5 feet below the lake surface. Four samples were analyzed per lake for all compounds, 

except where noted. NL, U.S. Geological Survey National Water-Quality Laboratory; LRL, laboratory reporting level; ND, not 

determined; KS, U.S. Geological Survey Organic Geochemistry Laboratory]
Appendix 5 55

Appendix 5. Summary of parent pesticides and order of degradation for triazine pesticide degradates.

[Degradation pathways from Scribner and others (1999); see fig. 15 for generalized schematic of pathways]

Triazine pesticide degradate

[abbreviations, chemical name, alternative name(s)] Parent pesticide compound Order of degradation

Hydroxyatrazine (HA, OIET): 

2-Hydroxy-4-isopropylamino-6-ethylamino-s-triazine

Atrazine 1st

Deethylatrazine (DEA, CIAT): 

2-Chloro-4-isopropylamino-6-amino-s-triazine

Atrazine, propazine 1st

Deisopropylatrazine (DIA, DES, CEAT): 

Deethylsimazine; 2-Chloro-6-ethylamino-4-amino-s-triazine

Atrazine, simazine, cyanazine (and several 

cyanazine degradates)

1st

Hydroxysimazine (HS, OEET): 

2-hydroxy-4,6-diethylamino-s-triazine 

Simazine 1st

Didealkylatrazine (DDA, CAAT ): 

Deethyldeisopropylatrazine; Chlordiamino-s-triazine

DIA, DEA, and the cyanazine degradates 

DCAC and CAC [indirectly, atrazine, 

simazine, propazine, and cyanazine]

2nd to 4th

Deethylhydroxyatrazine (DEHA, HDEA, OIAT): 

2-hydroxy-4-isopropylamino-6-amino-s-triazine 

DEA, HA 

[indirectly, atrazine, propazine]

2nd

Deisopropylhydroxyatrazine (DIHA, HDIA, OEAT): 

2-hydroxy-6-ethylamino-4-amino-s-triaxine

DIA, HA, HS 

[indirectly, atrazine, simazine, and 

cyanazine]

2nd