Prepared in cooperation with the Puerto Rico Department of Natural and 

Environmental Resources

Evaluation of Groundwater Quality and Selected Hydrologic 

Conditions in the South Coast Aquifer, Santa Isabel Area, 

Puerto Rico, 2008–09

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2012–5254
Cover. View from north to south of the South Coastal Plain at the Santa Isabel area, Puerto Rico. Photograph by José M. Rodríguez, U.S. Geological Survey.
Evaluation of Groundwater Quality and 

Selected Hydrologic Conditions in the 

South Coast Aquifer, Santa Isabel Area, 

Puerto Rico, 2008–09

By José M. Rodríguez 

Prepared in cooperation with the 

Puerto Rico Department of Natural and Environmental Resources 

Scientific Investigations Report 2012–5254

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2013

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Suggested citation:

Rodríguez, J.M., 2013, Evaluation of groundwater quality and selected hydrologic conditions in the 

South Coast aquifer, Santa Isabel area, Puerto Rico, 2008–09: U.S. Geological Survey Scientific Investigations 

Report 2012–5254, 36 p.
iii

Acknowledgments

The author wishes to thank the farm owners in the Santa Isabel area that permitted 

access to their properties during the data-collection phase of the study and provided valuable 

data related to water use and fertilizer application rates on their farms. 

The author also wishes to thank USGS employees Senén Guzmán-Ríos, Kathleen 

McCarthy, and Michael Deacon for providing the internal reviews of this manuscript and Francisco Maldonado for the preparation of the illustrations.

Río Coamo at Paso Seco area, Santa Isabel, Puerto Rico. Photograph by José M. Rodríguez, USGS.
iv

Row crops at the Santa Isabel area, Puerto Rico. Photograph by José M. Rodríguez, 

U.S. Geological Survey.

Irrigation pond at the Santa Isabel area, Puerto Rico. Photograph by José M. Rodríguez, 

U.S. Geological Survey.
v

Contents

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

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

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

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

Description of the Study Area ............................................................................................................2

Climate ..........................................................................................................................................2

Physiographic Features..............................................................................................................2

Land Use .......................................................................................................................................5

Methods of Investigation..............................................................................................................................5

Hydrologic Setting .........................................................................................................................................9

Groundwater Conditions...............................................................................................................................9

Water Quality................................................................................................................................................12

Specific Conductance and Dissolved Solids .................................................................................12

Pesticides.............................................................................................................................................16

Nitrate ..................................................................................................................................................19

Sources of Nitrate .....................................................................................................................19

Nitrogen and Oxygen Isotopic Characterization of Nitrate Sources ................................23

Potential Nitrate Load to the Aquifer......................................................................................27

Summary and Conclusions.........................................................................................................................31

References Cited..........................................................................................................................................32

Appendix........................................................................................................................................................36
vi

Figures

1. Map showing location of the study area in relation to the South Coast aquifer 

 of Puerto Rico................................................................................................................................3

2. Map showing location of data collection sites in the Santa Isabel area, 

Puerto Rico, 2008–09 ....................................................................................................................4

3. Graph showing distribution of monthly total and historical monthly mean 

total rainfall at the Santa Isabel 2ENE National Weather Service station, 

Santa Isabel, Puerto Rico............................................................................................................5

4. Map showing general land use in the Santa Isabel area, Puerto Rico, 

2008–09 ...........................................................................................................................................6

5. Map showing surficial geology in the Santa Isabel area, Puerto Rico .............................10

6. Map showing water-table configuration in the South Coast aquifer, Santa Isabel

 area, Puerto Rico, March 31-April 3, 2008 ..............................................................................11

7. Graph showing water levels at site 44, observation well Alomar 1, Santa Isabel, 

Puerto Rico ..................................................................................................................................12

8. Map showing specific conductance values in groundwater and surface water, 

Santa Isabel area, Puerto Rico, May 2008-April 2009...........................................................15

9. Graph showing frequency of detection of pesticide compounds in groundwater 

samples collected from selected wells in the Santa Isabel area, Puerto Rico, 

May 2008 ......................................................................................................................................16

10. Map showing deethylatrazine-to-atrazine ratios in groundwater samples collected 

from selected wells in the Santa Isabel area, Puerto Rico, May 2008 ..............................18

11. Graph showing sum of detected concentrations of pesticide compounds in 

groundwater samples collected from selected wells in the Santa Isabel area, 

Puerto Rico, May 2008 ...............................................................................................................20

12. Map showing nitrate concentrations in groundwater and surface water in the 

Santa Isabel area, Puerto Rico, during May 2008 to April 2009 and February to 

October, 1967 ...............................................................................................................................21

13. Graph showing relation of delta nitrogen-15 and delta oxygen-18 of nitrate in 

groundwater and surface water in the Santa Isabel area, Puerto Rico, 

May 2008-April 2009 ...................................................................................................................24

14. Map showing delta nitrogen-15 and delta oxygen-18 of nitrate in groundwater 

and surface water in the Santa Isabel area, Puerto Rico, May 2008- April 2009.............25

15. Graph showing relation of the inverse of nitrate concentration and delta 

nitrogen-15 of nitrate in groundwater in the central section of the study area, 

Santa Isabel area, Puerto Rico, May 2008-April 2009...........................................................26

16. Graph showing relation of the natural logarithm of nitrate concentration and delta 

nitrogen-15 of nitrate in groundwater in the central section of the study area, 

Santa Isabel area, Puerto Rico, May 2008- April 2009..........................................................26
vii

Tables

1. Information for selected wells in the Santa Isabel area, Puerto Rico.................................7

2. Nutrient concentrations and delta nitrogen-15 and delta oxygen-18 of nitrate

in groundwater samples collected from selected wells in the Santa Isabel area, 

Puerto Rico, 2008-09...................................................................................................................13

3. Nutrient concentrations, delta nitrogen-15, and delta oxygen-18 of nitrate in 

surface-water samples collected from selected streams in the Santa Isabel area, 

Puerto Rico, 2008-09...................................................................................................................14

4. Pesticide compounds detected in groundwater samples collected from selected 

wells in the Santa Isabel area, Puerto Rico, 2008 .................................................................17

5. Groundwater ubiquity score index for pesticide compounds detected in 

groundwater samples collected from selected wells in the Santa Isabel area, 

Puerto Rico, 2008 ........................................................................................................................20

6. Potential nitrogen load estimates from fertilizers applied to crops and other 

sources in the study area, Santa Isabel, Puerto Rico, 2008 ................................................27

7. Estimated nitrogen load from irrigation groundwater in the central and western 

sections of the study area, Santa Isabel, Puerto Rico, 2008 ...............................................29

8. Potential nitrogen load estimates from fertilizers applied to crops and other 

sources in the central and western sections of the study area, Santa Isabel, 

Puerto Rico, 2008 ........................................................................................................................30
viii

Conversion Factors and Abbreviations

SI to Inch/Pound

Multiply By To obtain

Length

millimeter (mm) 0.039 inch (in.)

meter (m) 3.281 foot (ft)

kilometer (km) 0.621 mile (mi)

Area

hectare (ha) 2.471 acre

square kilometer (km2

) 247.1 acre

hectare (ha) 0.004 square mile (mi2

)

square kilometer (km2

) 0.386 square mile (mi2)

Flow rate

cubic meter per year (m3

/yr) 0.001 acre-foot per year (acre-ft/yr)

cubic meter per day (m3

/d) 35.31 cubic foot per day (ft3/d)

liter per second (L/s) 15.85 gallon per minute (gal/min)

cubic meter per day (m3

/d) 264.2 gallon per day (gal/d)

millimeter per year (mm/yr) 0.039 inch per year (in/yr)

Mass

gram (g) 0.002 pound avoirdupois (lb)

kilogram (kg) 2.205 pound avoirdupois (lb)

kilogram per year (kg/yr) 2.205 pound per year (lb/yr)

Hydraulic conductivity

meter per day (m/d) 3.281 foot per day (ft/d)

Flow rate

liter per second (L/s) 15.85 gallon per min (gal/min)

Recharge rate

meter per year (m/yr) 3.281 foot per year (ft/yr)

Production rate

gram per day (g/d) 0.035 ounce, avoirdupois, per day (oz/d)

Application rate

kilograms per year (kg/yr) 0.892 pounds per acre per year (lb/yr)

Application rate per area

kilograms per hectare per year 

(kg/ha)/yr

0.892 pounds per acre per year (lb/acre)/yr
ix

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

°F=(1.8×°C)+32

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

°C=(°F-32)/1.8

Vertical coordinate information is referenced to local mean sea level.

Horizontal coordinate information is referenced to the Puerto Rico Datum, 1940 adjustment.

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

*Transmissivity: The standard unit for transmissivity is cubic foot per day per square foot 

times foot of aquifer thickness [(ft3/d)/ft2]ft. In this report, the mathematically reduced 

form, foot squared per day (ft2/d), is used for convenience.

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

(μS/cm at 25°C).

Concentrations of chemical constituents in water are given either in milligrams per liter 

(mg/L) or micrograms per liter (μg/L).

Abbreviations

DAR Deethylatrazine to atrazine ratio

DEA Deethylatrazine

EPTC S-ethyl-dipropyl-thiocarbamate

GUS Groundwater ubiquity score

GPS Global positioning system

MCL Maximum contaminant level

MDL Method detection level

MRL Method reporting level

PRASA Puerto Rico Aqueduct and Sewer Authority

PRDNER Puerto Rico Department of Natural and Environmental Resources

USGS U.S. Geological Survey
x
Evaluation of Groundwater Quality and Selected 

Hydrologic Conditions in the South Coast Aquifer, Santa 

Isabel Area, Puerto Rico, 2008–09

By José M. Rodríguez

Abstract 

An evaluation to define groundwater-quality 

conditions, with emphasis on the distribution of nitrate 

concentration, to identify potential sources leading to 

elevated nitrate concentrations and to estimate the nitrate 

loads from major sources, was conducted by the U.S. 

Geological Survey in the South Coast aquifer, Santa 

Isabel area, between January 2008 and May 2009. The 

study area covers approximately 5,800 hectares of the 

south coastal plain of Puerto Rico and 470 hectares in 

the foothills within the municipality of Santa Isabel. 

Agriculture is the principal land use in the area and 

includes the cultivation of diverse truck-farm crops, 

bioengineered crops for seed production, and until 2007, 

commercial poultry farms. Nitrate concentrations in 

groundwater near the center of the South Coast aquifer, 

Santa Isabel area, have increased from a range of 0.9 to 

5.9 milligrams per liter as nitrogen in 1967 to a range 

of 1.3 to 23.6 milligrams per liter as nitrogen in 2008. 

Analyses for the stable isotope ratios of nitrogen-15/

nitrogen-14 and oxygen-18/oxygen-16 in nitrate were 

used to infer the source of nitrate in the aquifer and 

in the foothills north of the coastal plain. Results 

obtained from the samples analyses of stable isotope 

ratios indicate that the high nitrate concentrations in the 

western and central parts of the study area were in the 

range typically associated with soil nitrogen. 

Potential residual nitrogen loads from areas under 

cultivation were estimated for the principal crops in the 

area. The residual load estimates ranged from 

25 kilograms of nitrogen per hectare per year in tomato 

crops to 228 kilograms of nitrogen per hectare per year 

in banana crops. The potential nitrogen loads from 

two communities without sanitary sewer systems were 

estimated to be approximately 101 and 195 kilograms of 

nitrogen per hectare per year, respectively. 

Groundwater withdrawals in the South Coast 

aquifer, Santa Isabel area were estimated to be 

approximately 64,400 cubic meters per day, of which 

52 percent were for agriculture, and 48 percent were 

for public-water supply. Groundwater flow in the study 

area was primarily southward from the north towards 

two cones of depression present within the central and 

southwestern parts of the study area. The presence of 

those cones of depression may contribute to an increase 

in nitrate concentrations within a total area of about 

1,300 hectares owing to the recirculation of groundwater 

with elevated nitrate concentrations that is used to 

irrigate cultivated lands.

Introduction

The source of drinking water in the Santa Isabel 

and Coamo areas of Puerto Rico (Molina and GómezGómez, 2008) is the South Coast aquifer (hereafter 

referred to as the aquifer), which supplies about 

30,700 cubic meters per day (m3

/d) to Puerto Rico 

Aqueduct and Sewer Authority (PRASA) public-supply 

wells. In addition, approximately 45 wells provide an 

estimated 33,700 m3

/d of groundwater to irrigate crops 

in the area. In 1967, baseline nitrate concentrations 

in groundwater throughout most of the aquifer were 

generally less than 6 milligrams per liter (mg/L) as 

nitrogen in collected water samples (U.S. Geological 

Survey, 2012). In 2007, elevated nitrate concentrations 

were detected in the aquifer, near Santa Isabel and the 

foothills north of the coastal plain at Santa Isabel as part 

of a regional groundwater-quality assessment conducted 

by the U.S. Geological Survey (USGS) during 2007 

(Rodríguez and Gómez-Gómez, 2008). The increase 

in nitrate concentrations has been of concern to local 

government agencies because of its potential effect on 

public supply. 
2 Evaluation of Groundwater Quality and Selected Hydrologic Conditions in the South Coast Aquifer, Santa Isabel Area.

To address public-supply concerns, the U.S. 

Geological Survey (USGS), in cooperation with the 

Puerto Rico Department of Natural and Environmental 

Resources (PRDNER), evaluated groundwater quality in 

the aquifer near the Santa Isabel area between January 

2008 and May 2009. The objectives of the study were 

to (1) define the groundwater-quality conditions of the 

aquifer, with emphasis on the distribution of nitrate 

concentrations; (2) identify potential sources leading to 

elevated nitrate concentrations; (3) estimate the nitrate 

loads from major sources identified; and 

(4) estimate the groundwater withdrawals by principaluse categories in the area. Results of this study will be 

used by Commonwealth of Puerto Rico and Federal 

agencies in developing strategies that can result in 

containment of high nitrate groundwater to minimize 

degradation of fresh groundwater in the aquifer. 

Purpose and Scope

The purpose of this report is to evaluate the 

groundwater-quality and selected hydrologic conditions 

in the aquifer near the Santa Isabel area and the Gabia 

area in the foothills during 2008-09 using data obtained 

from 74 groundwater wells and 5 surface-water sites. 

Water-quality data collected during the study included 

analyses for nutrients, pesticides, and the stable isotope 

ratios oxygen-18/16 and nitrogen-15/14 in nitrate. As 

part of the study, data were also obtained to define 

groundwater withdrawals by major use categories, 

delineate the water table, determine land use patterns, 

and estimate potential nitrogen loads to the aquifer from 

fertilizer application, disposal of animal wastes, and 

domestic septic wastes from unsewered communities.

Description of the Study Area

The study area is located along the south coastal 

plain of Puerto Rico (fig. 1). The study area includes 

approximately 5,800 hectares (ha) that overlie the 

aquifer and approximately 470 ha in the foothills to the 

north of the coastal plain that overly volcanic rocks. The 

aquifer in the Santa Isabel area is composed of fan-delta 

and alluvial deposits. The study area is bordered by the 

Río Descalabrado to the west, the Río Jueyes 

(an intermittent stream) to the east, the hydrologic divide 

formed by the foothills to the north, and the Caribbean 

Sea to the south (figs. 1 and 2).

The study area is located within the municipality 

of Santa Isabel, which has historically been one of 

the most intensively used agricultural areas along the 

south coastal plain (fig. 1). Sugarcane cultivation was 

the principal land use at the Santa Isabel area until 

the 1970s, when sugarcane was gradually replaced by 

vegetable, fruit, row crops, and seed production crops. 

This transition resulted in a parallel shift from furrow 

irrigation to micro-drip irrigation. By the early 1990s, 

all of the cultivated areas were using micro-drip 

irrigation.

Climate 

The south coastal plain is warmer and drier than the 

other parts of Puerto Rico because of the rain-shadow 

effect the east-west trending Cordillera Central mountain 

range has on the prevailing northeast trade winds. The 

mean annual rainfall in the coastal plain near the study 

area from 1955 to 2006 was 809 millimeters (mm) 

(Santa Isabel 2 ENE National Weather Service station 

668940, figs. 1 and 2), as compared to 2,114 mm at 

652 meters (m) altitude in the Cordillera Central 

(Aceituna National Weather Service station 660040, 

fig. 1), and 1,346 mm on the northern coast in San Juan 

(National Weather Service station 668812, fig. 1). The 

monthly mean rainfall at the Santa Isabel 2 ENE weather 

station ranged from 15.5 to 179 mm during its 

1955-2006 period of record (fig. 3). 

A relatively dry season lasts from December to 

April, with February being the driest month, and a 

relatively wet season lasts from August to November, 

with October being the wettest month (fig. 3). The study 

was conducted during a period in which 11 of the 17 

months of the study received below-average rainfall. The 

mean annual temperature along the south coastal plain 

is approximately 3 to 5°C higher than in the mountains. 

Monthly mean maximum temperatures at the Aguirre 

Central weather station, about 19 kilometers (km) east of 

the urban center of Santa Isabel, ranged from 29.7°C in 

February to 32.2°C in August (from 1955 to 2006).

Physiographic Features

 The south coastal plain is characterized by a 

parched vegetative cover, except in areas under irrigation 

or near intermittently flowing streams. Altitudes in the 

coastal plain range from 0 to approximately 40 m above 

mean sea level. The east-central part of the coastal plain 

at Santa Isabel is separated from the Caribbean Sea by a 

zone of mangroves, swampland, and tidal flats. The hills 

to the north also form a hydrologic barrier that partially 

surrounds the Gabia area basin. The maximum altitude 

of the hills bordering the Gabia basin is about 200 m 

above mean sea level. The altitude of the basin floor in 

the Gabia area ranges from approximately 55 m above 

mean sea level in the inland part of the coastal plain to 

140 m above mean sea level in the northern part.
Introduction 3 Figure 1. Location of the study area in relation to the South Coast aquifer of Puerto Rico.
4 Evaluation of Groundwater Quality and Selected Hydrologic Conditions in the South Coast Aquifer, Santa Isabel Area.

Figure 2. Location of data collection sites in the Santa Isabel area, Puerto Rico, 2008–09.
Methods of Investigation 5

field visits and interviews with farmers in the study area. 

Information about fertilizer application rates and crop 

types were obtained by interviewing farmers and during 

field reconnaissance.

Groundwater levels were measured in 47 wells 

from March 31 to April 3, 2008, and used to prepare a 

water-table map of the aquifer within the study area. 

The map was used to delineate the general direction of 

groundwater flow and evaluate the sources of nitrate 

affecting the sampled wells.

Groundwater samples were collected from 

28 wells, and surface-water samples were collected 

from 3 streamflow measurement sites in May 2008. In 

addition, 2 wells and 3 surface-water sites were sampled 

on April 16 and April 17, 2009. The collected samples 

were analyzed for nutrients (organic nitrogen, ammonia, 

nitrate, nitrite, and phosphorus), and stable isotopes 

ratios of oxygen-18/16 (18O/16O) and nitrogen-15/14 

(

15N/14N) in nitrate; samples from 10 selected wells 

were also analyzed for pesticides. Temperature, specific 

conductance, and pH were determined in the field at 

all sites sampled. Groundwater samples were collected 

from active production wells as near as possible to 

the pump discharge location and before water entry 

into the storage tanks. Surface-water samples were 

collected near the middle of stream cross sections. Field 

measurements and samples for laboratory analyses 

were collected, processed, and preserved according to 

procedures described by the National Field Manual for 

Land Use 

Land use in the study area during 2007 included 

agriculture, urban, and industrial uses (fig. 4). Active 

agricultural land use occupied approximately 

56 percent of the study area, urban use occupied about 

10 percent, and industrial use occupied less than 

1 percent. The remaining 34 percent of the study area 

consisted of uncultivated land—primarily grassland, 

scrub, and shrub. Major crops grown within the coastal 

plain include corn, tomato, bananas, mangos, and 

vegetables. Tomato, corn, and vegetable farms occupy 

approximately 16, 18, and 22 percent, respectively, of 

the actively used agricultural land.

In the Gabia area, which comprises approximately 

470 ha in the foothills north of the coastal plain, about 

56 percent is dedicated to poultry farms. Forty poultry 

farms were in operation until 2007 and occupied 

approximately 267 ha.

Methods of Investigation

As part of this study, the status of selected wells 

within the study area was revised to update the USGS 

Ground-Water Site Inventory database (fig. 2, 

table 1). The location of each well was confirmed 

using a global positioning system (GPS) unit, and 

groundwater-withdrawal rates were obtained from 

periodic water-meter readings. General land-use data 

were obtained from 2007 aerial photographs and from 

Figure 3. Distribution of monthly total (2008–09) and historical monthly mean total (1955-2009) rainfall at the 

Santa Isabel 2ENE National Weather Service station (668940), Santa Isabel, Puerto Rico.
6 Evaluation of Groundwater Quality and Selected Hydrologic Conditions in the South Coast Aquifer, Santa Isabel Area.

Figure 4. General land use in the Santa Isabel area, Puerto Rico, 2008–09.
Methods of Investigation 7

Table 1. Information for selected wells in the Santa Isabel area, Puerto Rico.—Continued

[nd, no data; E, estimated value]

Number on 

figure 2

Depth of well, 

in meters 

below land 

surface

Screened interval, in 

meters below land 

surface

Groundwater withdrawals 

Cubic meters 

per day

 Million gallons 

per day

1 64.0 0-64.0 nd nd

2 nd nd 1,060 0.28

3 103 nd 0 0.00

4 57.0 0-57.0 0 0.00

5 nd nd 1,703 0.45

6 39.9 4.9-20.7, 29.2-39.9 0 0.00

7 72.2 0-9.1, 12.8-72.2 833 0.22

8 72.8 0-72.8 492 0.13

9 62.8 7.3-62.8 76 0.02

10 nd nd 492 0.13 E

11 24.4 nd 0 0.00

12 48.8 12.2-48.8 871 0.23

13 nd nd 530 0.14

14 67.7 7.3-67.4 1,249 0.33

15 nd nd 909 0.24

16 67.0 0-67.0 681 0.18

17 57.3 4.9-57.3 606 0.16

18 nd nd 909 0.24

19 nd nd 416 0.11 E

20 73.1 0-73.1 1,249 0.33

21 nd nd 454 0.12 E

22 nd nd 0 0.00

23 51.8 0-51.8 1,401 0.37 E

24 61.0 59.4-61.0 0 0.00

25 nd nd 1,590 0.42

26 nd nd 1,022 0.27

27 76.2 nd 1,401 0.37 E

28 nd nd 1,287 0.34

29 nd nd 4,618 1.22

30 nd nd 76 0.02

31 nd nd 76 0.02

32 nd nd 76 0.02

33 33.5 0-33.5 946 0.25

34 61.6 nd 0 0.00

35 nd nd 4,315 1.14

36 nd nd 1,438 0.38 E

37 54.9 nd 6,170 1.63

38 59.4 0-59.4 492 0.13

39 65.8 0-6508 530 0.14

40 46.3 6.1-46.3 0 0.00

41 nd nd 946 0.25
8 Evaluation of Groundwater Quality and Selected Hydrologic Conditions in the South Coast Aquifer, Santa Isabel Area.

Table 1. Information for selected wells in the Santa Isabel area, Puerto Rico.—Continued

[nd, no data; E, estimated value]

Number on 

figure 2

Depth of well, 

in meters 

below land 

surface

Screened interval, in 

meters below land 

surface

Groundwater withdrawals 

Cubic meters 

per day

 Million gallons 

per day

42 21.3 nd 0 0.00

43 nd nd 0 0.00

44 34.1 nd 0 0.00

45 57.9 0-57.9 1,211 0.32

46 61.0 nd 5,754 1.52

47 61.0 nd 3,028 0.80

48 nd nd 757 0.20

49 nd nd 379 0.10

50 63.1 12.2-63.1 492 0.13

51 32.9 0-32.9 1,173 0.31 E

52 nd nd 1,817 0.48 E

53 nd nd 1,249 0.33

54 nd nd 1,022 0.27

55 nd nd 719 0.19

56 nd nd 0 0.00

57 nd nd 1,022 0.27

58 nd nd 0 0.00

59 nd nd 795 0.21

60 nd nd 1,173 0.31 E

61 56.4 21.6-56.4 114 0.03

62 nd nd 0 0.00

63 nd nd 568 0.15 E

64 64.0 nd 946 0.25 E

65 nd nd 0 0.00

66 nd nd 795 0.21 E

67 nd nd 0 0.00

68 nd nd 0 0.00

69 47.5 0-47.5 681 0.18 E

70 nd nd 416 0.11

71 nd nd 530 0.14 E

72 35.0 16.8-35.0 0 0.00

73 nd nd 757 0.20

74 45.7 nd 0 0.00

Total groundwater withdrawal 64,311 16.99
Groundwater Conditions 9

the Collection of Water-Quality Data (U.S. Geological 

Survey, variously dated). Samples for nutrients and 

pesticides were analyzed at the USGS National Water 

Quality Laboratory in Denver, Colorado.

Analyses of oxygen and nitrogen isotopes in nitrate 

were performed in the USGS Stable Isotope Laboratory 

in Reston, Virginia. Samples collected for this analysis 

were passed through a 0.20-micrometer filter into 

125-milliliter (mL) plastic bottles and chilled at 4°C. 

Results for oxygen isotopes are reported as delta units 

(δ) per mil relative to the Vienna Standard Mean Ocean 

Water reference water and normalized on a scale such 

that Standard Antarctic Precipitation reference water is 

-55.5 per mil (Coplen, 1988, 1994). 

Water samples for nitrogen isotopes were analyzed 

by bacterial conversion of nitrate to nitrous oxide and 

subsequent measurement on a continuous flow isotope 

ratio mass spectrometer (Sigman and others, 2001; 

Casciotti and others, 2002; Revesz and Casciotti, 2003). 

Nitrogen isotope ratios are reported as delta units (δ) in 

per mil relative to N2

 in air (Mariotti, 1983).

The concentrations of 18O, and 15N in water are 

defined by the expression:

 δ (‰) = [(R sample / R standard)-1] × 1,000 (1)

where 

 δ (‰) is delta units in per mil, and 

R corresponds to the ratios of 18O/16O or 15N/14N.

Hydrologic Setting

The study area lies within the areal boundaries of 

the South Coast aquifer and extends from the coast to 

the bedrock hills north of the coastal plain. Fan delta and 

alluvial deposits constitute the aquifer in the Santa Isabel 

area (fig. 5); these deposits exceed 300 m in thickness 

along the coast and thin to approximately 18 m near the 

bedrock hills (Renken and others, 2002). 

The estimated hydraulic conductivity of the fan 

delta deposits in the Santa Isabel area ranges from 

less than 1 meter per day (m/d) to approximately 80 

m/d (Bennett, 1976; Renken and others, 2002); values 

are greatest in the thickest part of the aquifer (Renken 

and others, 2002). The depth to water during 2008 

was between 3 and 35 m below land surface, which 

represents a range in groundwater altitude from -3.5 to 

39.2 m relative to mean sea level. Yields from wells used 

for agriculture and public supply in the study area ranged 

from 22 to 126 liters per second (L/s).

Swamp and beach deposits compose the 

southeastern boundary of the coastal plain in the 

study area and a bedrock hill composed of the Ponce 

Limestone crops out in the northwestern part of the study 

area (fig. 5). Swamp deposits consist of unconsolidated 

clay, silt, and organic material (Glover and others, 

1977) and beach deposits consist of unconsolidated 

sand, gravel, and cobbles (Glover, 1961). The bedrock 

outcrop was described by Glover and Mattson (1973) as 

consisting of limestone and sandstone formations.

The hills near the Gabia area consist primarily of 

Tertiary and Cretaceous volcanic rocks (Glover and 

Mattson, 1973). Yields from wells in the Gabia area 

range from 1.3 to 2.5 L/s.

Groundwater Conditions

Groundwater in the study area generally flows 

southward toward two cones of depression, as 

determined from the configuration of the water table in 

the area. Contours of equal groundwater altitude were 

delineated using water-level measurements obtained 

between March 31 and April 3, 2008 (fig. 6). The cones 

of depression are located in the northern central and the 

southwestern parts of the study area and were created 

by local groundwater withdrawals from irrigation and 

public-supply wells. Measured water levels were below 

mean sea level over most of the Santa Isabel area. A 

V-shaped water-table contour in the southeastern part of 

the study area indicates a potential zone of freshwaterseawater mixing. 

The water-table configuration in the aquifer 

determined from the data collected during March-April 

2008 was compared with a configuration determined 

from data collected in March 1986 (Rodríguez-Del Río 

and Gómez-Gómez, 1990). The comparison of these 

water-table configurations indicates that the groundwater 

levels in the aquifer decreased by an average of about 

4 m between March 1986 and March-April 2008. 

The historical data obtained from observation well 

Alomar 1 (number 44 in figs. 2 and 7), which is located 

in the central part of the study area, indicates a declining 

water-level trend from 1986 to 1994, with a total decline 

of about 8 m. Data collected from the same well during 

1993-2008 indicate groundwater-level fluctuations were 

near or below mean sea level. 

A study to evaluate the effects of aquifer 

development and changes in irrigation practices on 

groundwater availability from 1987 to 2003 in the 

Santa Isabel area was conducted by Kuniansky 

and others (2003). This study, which included the 

development of digital groundwater flow model, 

indicated that the main effects of the changes in 

agricultural practices were reductions in recharge to the 

aquifer and in total irrigation withdrawals. The study 

also determined that groundwater withdrawals for public 

supply wells steadily increased from 7,580 m3

/d in 1973 
10 Evaluation of Groundwater Quality and Selected Hydrologic Conditions in the South Coast Aquifer, Santa Isabel Area.

Figure 5. Surficial geology in the Santa Isabel area, Puerto Rico.
Groundwater Conditions 11

Figure 6. Water-table configuration in the South Coast aquifer, Santa Isabel area, Puerto Rico, March 31-

April 3, 2008.
12 Evaluation of Groundwater Quality and Selected Hydrologic Conditions in the South Coast Aquifer, Santa Isabel Area.

Figure 7. Water levels at site 44, observation well Alomar 1 (175829066232200), Santa Isabel, Puerto Rico.

to about 26,530 m3/d in 2003. In addition, the reduction 

of recharge lowered the water table below mean sea 

level over most of the Santa Isabel area; this process 

reversed the natural groundwater gradient along the 

coast and caused saltwater intrusion into the aquifer. 

Total groundwater withdrawals from the aquifer in 

the coastal plain section of the study area were estimated 

during 2008 to be about 64,300 m3

/d (table 1), consisting 

of 32,700 m3

/d for irrigation and 31,600 m3/d for public 

supply. In the Gabia area, groundwater withdrawals 

during 2008 were negligible because poultry farms had 

ceased operations by that time.

Water Quality

Water samples were collected within the study area 

during May 2008 and April 2009. Data were collected 

from a total of 30 groundwater wells and 5 surface-water 

sites (tables 2 and 3). Data were collected in April 2009 

to include 2 wells and 2 surface-water sites that were not 

sampled during May 2008. All samples were analyzed 

for nutrients and stable isotopes of nitrogen and oxygen 

in nitrate. Samples from 10 wells were analyzed for 

pesticides. Field measurements of specific conductance 

were used to estimate dissolved-solids concentrations.

Specific Conductance and Dissolved Solids

Specific conductance data collected during the 

May 2008 and April 2009 are shown on figure 8. As ion 

concentrations increase, the conductance of a solution 

increases; therefore, the conductance measurement 

provides an indication of the ion concentration 

(Hem, 1989). This relation between ion concentration 

and specific conductance permits dissolved-solids 

concentrations to be estimated using specific 

conductance determinations. Specific conductance and 

dissolved-solids concentration are linearly related by the 

equation

KA=S (2) 

where 

K is the specific conductance, 

A is the slope of the linear relation, and 

S is the approximate total dissolved-solids 

concentration (Hem, 1985). 

Specific conductance and dissolved-solids data 

collected for the South Coast aquifer by Díaz (1979) 

and Rodríguez and Gómez-Gómez (2008) indicate 

that the constant A is about 0.60 for the range of 

conductance data shown on figure 8. Groundwater 

having a specific conductance of less than 850 

microsiemens per centimeter (μS/cm) at 25°C, which 

represents a dissolved-solids concentration of less than 

500 mg/L, is preferred for public water supply. The 

U.S. Environmental Protection Agency has established 

a drinking water secondary maximum contaminant 

level (MCL) for dissolved solids of 500 mg/L (U.S. 

Environmental Protection Agency, 2009). 

Irrigation water with specific conductance in the 

range of 750 to 2,250 μS/cm is considered a high salinity 

hazard for crops, according to the U.S. Department of 

Agriculture irrigation water classification (Richards, 

1954). Irrigation water with specific conductance above 

2,250 μS/cm is considered a very high salinity hazard 
Water Quality 13

Table 2. Nutrient concentrations and delta nitrogen-15 and delta oxygen-18 of nitrate in groundwater samples collected from selected 

wells in the Santa Isabel area, Puerto Rico, 2008-09.

[Nutrient concentrations are given in milligrams per liter, unless otherwise noted; μS/cm, microsiemens per centimeter at 25 degrees Celsius; <, less than the analytical detection limit; δ15N, ratio of nitrogen-15/ nitrogen-14; δ18O, ratio of oxygen-18/ oxygen-16]

Number on 

figure 2 

Date of 

sample 

collection

pH

Specific 

Conductance 

(μS/cm)

Temperature 

(degrees 

Celsius)

Nitrate 

plus 

nitrite

Nitrite Nitrate

Ammonia plus 

organic 

nitrogen

Ammonia

Orthophosphate

Phosphorus

δ15N in 

nitrate 

(per 

mil)

δ18O in 

nitrate 

(per mil)

8 4/16/2009 7.6 1,110 27.8 14.9 <0.002 14.9 <0.1 <0.02 0.044 0.031 7.4 5.91

9 5/6/2008 7.5 1,230 28.5 19.02 0.002 19.02 0.152 <0.02 0.054 0.045 5.56 7.47

10 5/7/2008 7.2 1,250 27.2 12.87 0.002 12.87 0.155 <0.02 0.036 0.026 6.22 5.52

13 5/6/2008 7.4 1,030 27.6 16.04 <0.002 16.04 0.149 <0.02 0.030 0.029 6.36 7.03

15 5/6/2008 7.6 1,140 27.4 20.91 0.002 20.91 0.224 0.023 0.031 0.038 5.82 6.03

18 5/6/2008 7.2 1,860 28.0 11.44 <0.002 11.44 0.144 <0.02 0.050 0.042 7.60 7.43

19 5/7/2008 7.1 1,180 27.7 18.49 <0.002 18.49 0.165 <0.02 0.033 0.026 5.71 5.15

21 5/7/2008 7.5 1,150 27.3 12.44 <0.002 12.44 0.099 <0.02 0.038 0.028 6.08 4.74

23 5/7/2008 7.2 725 27.5 8.93 0.011 8.92 0.249 0.114 0.038 0.031 4.89 5.07

26 5/5/2008 7.3 787 27.9 6.91 <0.002 6.91 <0.14 <0.02 0.033 0.032 6.01 6.69

28 5/6/2008 7.6 759 27.5 4.92 <0.002 4.92 0.101 <0.02 0.034 0.024 7.39 4.63

31 5/9/2008 7.7 640 28.3 1.30 <0.002 1.30 0.082 <0.02 0.034 0.021 14.55 8.25

33 5/9/2008 7.0 1,070 27.9 2.37 <0.002 2.37 0.120 <0.02 0.037 0.032 9.76 4.87

35 5/8/2008 7.5 770 27.9 5.38 <0.002 5.38 0.299 <0.02 0.036 0.028 10.30 5.52

39 5/6/2008 7.5 1,010 27.7 23.63 <0.002 23.63 0.121 <0.02 0.035 0.028 6.71 7.34

41 5/5/2008 7.4 1,120 27.9 10.76 0.005 10.76 0.085 <0.02 0.032 0.036 9.17 8.92

47 5/8/2008 7.2 803 28.1 2.34 <0.002 2.34 0.128 <0.02 0.038 0.042 9.82 6.40

49 5/5/2008 7.1 808 27.9 2.99 <0.002 2.99 0.140 <0.02 0.046 0.043 8.65 6.14

51 5/8/2008 7.6 1,200 27.2 11.46 <0.002 11.46 0.099 <0.02 0.052 0.042 6.15 3.80

52 5/8/2008 7.4 1,020 27.4 14.22 <0.002 14.22 0.098 <0.02 0.031 0.027 5.26 4.52

54 5/5/2008 7.0 1,140 27.7 19.91 <0.002 19.91 0.082 <0.02 0.035 <0.04 5.93 4.75

55 5/5/2008 7.3 940 27.6 6.32 <0.002 6.32 0.127 <0.02 0.032 0.030 na na

57 5/6/2008 7.1 3,480 27.8 7.08 <0.002 7.08 0.140 <0.02 0.037 0.035 6.87 5.06

63 5/8/2008 7.1 1,090 27.2 10.20 <0.002 10.20 0.145 <0.02 0.045 0.038 5.79 3.78

64 5/7/2008 7.7 1,140 27.1 7.89 <0.002 7.89 0.128 <0.02 0.048 0.042 6.88 5.23

66 5/7/2008 7.0 1,140 27.2 7.53 <0.002 7.53 0.142 <0.02 0.040 0.031 7.30 6.56

69 5/7/2008 7.6 768 27.6 3.39 <0.002 3.39 0.110 <0.02 0.033 0.022 10.90 5.59

71 5/8/2008 7.4 1,060 27.9 10.40 <0.002 10.40 0.116 <0.02 0.030 0.020 11.50 4.95

73 5/8/2008 7.4 1,440 28.1 15.17 <0.002 15.17 0.309 <0.02 0.038 0.028 9.65 6.45

74 4/16/2009 7.1 967 27.7 5.49 <0.002 5.49 E0.06 <0.02 0.046 E0.038 11.58 6.90
14 Evaluation of Groundwater Quality and Selected Hydrologic Conditions in the South Coast Aquifer, Santa Isabel Area.

Table 3. Nutrient concentrations, delta nitrogen-15, and delta oxygen-18 of nitrate in surface-water samples collected from 

selected streams in the Santa Isabel area, Puerto Rico, 2008-09.

[Nutrient concentrations are given in milligrams per liter, unless otherwise noted; μS/cm, microsiemens per centimeter at 25 degrees Celsius; <, less 

than the analytical detection limit; δ15N, ratio of nitrogen-15/ nitrogen-14; δ18O, ratio of oxygen-18/ oxygen-16]

Number in 

map

USGS site 

identifier

Date of sample 

collection

Instantaneous 

discharge (cubic 

feet per second)

pH

Specific 

conductance 

(μS/cm)

Temperature 

(degrees 

Celsius)

Nitrate 

plus nitrite, 

dissolved

1 50106830 5/15/2008 6.1 8.3 690 28.2 0.52

4/17/2009 12 8.2 635 30.3 1.00

2 50106910 5/15/2008 2.1 8.2 607 33.6 0.42

3 50111330 5/15/2008 11 8.0 249 29.6 0.20

4 50108200 4/17/2009 1.2 8.0 845 26.5 1.99

5 50108375 4/17/2009 1.0 8.2 813 29.3 1.23

Table 3. Nutrient concentrations, delta nitrogen-15, and delta oxygen-18 of nitrate in surface-water samples collected from 

selected streams in the Santa Isabel area, Puerto Rico, 2008-09.—Continued

[Nutrient concentrations are given in milligrams per liter, unless otherwise noted; μS/cm, microsiemens per centimeter at 25 degrees Celsius; <, less 

than the analytical detection limit; δ15N, ratio of nitrogen-15/ nitrogen-14; δ18O, ratio of oxygen-18/ oxygen-16]

Number 

in map

Nitrite, 

dissolved

Nitrate, 

dissolved

Ammonia plus 

organic nitrogen, 

dissolved

Ammonia, 

dissolved

Orthophosphate, 

dissolved

Phosphorus, 

total

δ15N in 

nitrate 

(Per mil)

δ18O in 

nitrate 

(Per mil)

1 0.007 0.51 0.400 <0.02 0.109 0.111 17.60 9.43

0.007 0.99 0.400 <0.02 0.107 0.126 15.28 8.18

2 0.042 0.38 0.700 0.105 0.110 0.151 13.96 -2.40

3 0.009 0.19 0.300 0.036 0.037 0.101 6.56 -0.68

4 0.008 1.98 0.300 <0.02 0.071 0.086 15.71 9.02

5 0.012 1.22 0.700 <0.02 0.058 0.075 16.43 12.32
Water Quality 15

Figure 8. Specific conductance values in groundwater and surface water, Santa Isabel area, Puerto Rico, 

May 2008- April 2009.
16 Evaluation of Groundwater Quality and Selected Hydrologic Conditions in the South Coast Aquifer, Santa Isabel Area.

Pesticides

Groundwater samples collected at 10 wells 

(table 4) in the study area were analyzed for 83 organic 

compounds used as pesticides (appendix 1). Analytical 

results indicated that 13 of these 83 organic compounds 

were detected in the samples. The detected organic 

compounds included 8 herbicides, 3 fungicides, and 

2 insecticides. Pesticide concentrations include 

detections above and below the method reporting level 

(MRL) (fig. 9). The reported concentrations that are 

below the MRL, but are above the method detection 

level (MDL), are presented in table 4 as estimated values 

(“E” remark code). 

The compounds with a frequency of detection 

above 50 percent were deethylatrazine (DEA), atrazine, 

simazine, and metolachlor (fig. 9). Atrazine, simazine, 

and metolachlor are herbicides, and deethylatrazine 

(DEA) is a metabolite and an environmental degradation 

product of atrazine and other triazine herbicides. DEA 

concentrations exceeded the parent compound (atrazine) 

concentration in 9 of the 10 sampled wells. The DEA 

to atrazine ratio (DAR) can be used as an indicator 

of atrazine residence time in soil during transport to 

groundwater (Thurman and Fallon, 1996; Iker and 

others, 2010). A high DAR value may indicate a long 

residence time for atrazine in the soil column, where it 

is exposed to conditions and processes that contribute to 

DEA formation (fig. 10). 

for crops. East of Río Coamo, groundwater specific 

conductance generally increases in the southeast 

direction from less than 700 to more than 

3,000 μS/cm. The maximum specific conductance 

values, which coincide with the inverted v-shaped 

-2 m contour on the water-table surface map (fig. 6), 

may be the result of lateral seawater intrusion. Samples 

collected in 2007 (Rodríguez, 2009) at well 57, which 

is located near the inverted v-shaped contour, indicated 

that the chemical characteristics of the groundwater 

were of the calcium-chloride type. This water type is 

associated with a mixture of freshwater and saltwater. 

Specific conductance values less than 800 μS/cm 

were detected parallel to and east of the Río Coamo, 

which coincides with zones of elevated hydraulic 

conductivity in the study area delineated by Renken 

and others (2002) and areas of streamflow infiltration 

from the Río Coamo (Kuniansky and others, 2003). 

The specific conductance values in the area west of 

Rio Coamo ranged from 1,030 to 1,860 μS/cm. The 

highest value (1,860 μS/cm) was detected in a well 

located near the outcrop of Ponce Limestone shown 

in figure 5. A comparison of the dissolved-solids 

estimates distribution in 2008 with dissolved-solids 

concentrations measured in 1986 (Gómez-Gómez, 

1991) indicates the same general pattern of dissolvedsolids concentrations in the southern coastal plain, 

except its southeastern part where the maximum 

concentrations were 740 mg/L in 1986 and increased to 

2,090 mg/L in 2008.

Figure 9. Frequency of detection of pesticide compounds in groundwater samples collected from selected wells in the 

Santa Isabel area, Puerto Rico, May 2008.
Water Quality 17

Table 4. Pesticide compounds detected in groundwater samples collected from selected wells in the Santa Isabel area, Puerto Rico, 2008.

[Pesticide concentrations are given in micrograms per liter, unless otherwise noted; E, value below the method reporting level but above the method detection level, <, less than the method reporting level]

Number on 

figure 2

Date of 

sample 

collection

Simazine Atrazine

Deethylatrazine 

(DEA)

Dieldrin Meto- lachlor Meta- laxyl Ala- chlor S-ethyl-di- propylthio- carbamate Endo- sulfan sulfate Metri- buzin cis-Propi- conazole trans-Propi- conazole Pro metryn

13 5/6/2008 E0.006 0.014 0.082 <0.009 0.011 <0.007 E0.005 <0.002 <0.022 <0.012 <0.006 <0.02 <0.006

15 5/6/2008 E0.006 0.011 0.056 <0.009 <0.010 0.006 <0.006 <0.002 E0.008 E0.006 <0.006 <0.02 E0.004

18 5/6/2008 <0.006 <0.007 E0.011 <0.009 0.011 0.008 <0.006 0.168 <0.022 <0.012 <0.006 <0.02 <0.006

23 5/7/2008 E0.005 E0.005 E0.013 <0.009 0.737 <0.007 0.034 <0.002 <0.022 <0.012 0.008 E0.022 <0.006

26 5/5/2008 <0.006 E0.007 0.017 <0.009 <0.010 <0.007 <0.006 <0.002 <0.022 <0.012 <0.006 <0.02 <.000639 5/6/2008 0.007 <0.007 E0.015 E0.0007 <0.010 <0.007 <0.006 0.004 E0.006 E0.009 <0.006 <0.02 <0.006

41 5/5/2008 E0.006 0.027 0.017 <0.009 0.498 <0.007 <0.006 <0.002 <0.022 <0.012 E0.002 E0.007 <0.006

52 5/8/2008 0.008 0.009 0.029 <0.009 <0.010 <0.007 <0.006 0.005 <0.022 <0.012 <0.006 <0.02 <0.006

63 5/8/2008 0.007 E0.005 0.021 <0.009 E0.006 <0.007 <0.006 <0.002 <0.022 <0.012 <0.006 <0.02 <0.006

71 5/8/2008 E0.006 E0.006 0.016 <0.009 E0.006 <0.007 <0.006 <0.002 <0.022 <0.012 <0.006 <0.02 <0.006
18 Evaluation of Groundwater Quality and Selected Hydrologic Conditions in the South Coast Aquifer, Santa Isabel Area.

Figure 10. Deethylatrazine-to-atrazine ratios (DAR) in groundwater samples collected from selected wells in the 

Santa Isabel area, Puerto Rico, May 2008.
Water Quality 19

The sum of the concentrations of the pesticide 

compounds detected in the sampled wells ranged from 

0.024 to 0.82 microgram per liter (μg/L) (fig. 11). The 

MCL for drinking water for atrazine and simazine are 

3 and 4 μg/L, respectively (U.S. Environmental 

Protection Agency, 2009).

The Groundwater Ubiquity Score (GUS) index 

(Gustafson, 1989) can be used to rank pesticides 

according to their potential to leach into groundwater. 

The GUS was calculated for 11 of the 13 detected 

compounds (table 5). The GUS is an index based on the 

graphical examination of a plot formed by two pesticide 

properties: half-life in soil (t1/2) and partition coefficient 

between soil organic carbon and water (Koc). The GUS 

is determined using the following equation:

GUS = log10 (t1/2)×[4-log10(Koc)]. (3)

Pesticides that have a GUS value higher than 2.8 

are compounds classified as “leachers,” with a high risk 

for contaminating groundwater. Pesticides with GUS 

values between 1.8 and 2.8 are “marginal compounds,” 

and compounds with a GUS value lower than 1.8 are 

classified as “non-leachers.” Atrazine, metolachlor, 

and simazine, which had a detection frequency above 

50 percent during this study, are compounds classified 

as “leachers.” Metalaxyl and metribuzin are also 

compounds classified a “leachers”, but both compounds 

had a lower detection frequency during this study. 

Alachlor, prometryn, and propiconazole are classified as 

“marginal leachers.” Dieldrin, endosulfan, and s-ethtyldipropyl-thiocarbamate (EPTC) are classified as “nonleachers.”

According to interviews with farm managers, 

the most commonly used pesticides for crops in the 

study area during 2008 were endosulfan, atrazine, 

and metolachlor. Endosulfan was used for vegetables, 

tomatoes, and corn crops. Atrazine was used for 

vegetables, corn crops, and probably was applied to land 

in the past during sugarcane production. Metolachlor 

was used for vegetables, soy, and corn crops. 

Nitrate

Analyses of groundwater samples collected 

during May 2008 and April 2009 indicated nitrate 

concentrations ranged from 1.3 to 23.6 mg/L nitratenitrogen (NO3

-N) in wells located in the coastal plain, 

and from 5.5 to 15.2 mg/L NO3

-N in wells 73 and 

74 located in the Gabia area (table 2 and fig. 12A). A 

comparison of nitrate concentrations determined during 

the study with those obtained during 1967 (National 

Water Information System) (fig. 12B) indicated that 

nitrate concentrations increased by a factor of about 7 

within some areas of the aquifer in the coastal plain. In 

1967, NO3

-N concentrations within the greater part of 

the aquifer averaged 3.1 mg/L, and ranged from 0.9 to 

5.9 mg/L NO3

-N in a group of 20 samples 

(U.S. Geological Survey, 2012). 

Concentrations of NO3

-N in surface water ranged 

from 0.19 to 1.98 mg/L (table 3 and fig 12A) during 

May 2008 and April 2009. No data for nitrate 

concentrations in surface water in 1967 were available 

for comparison. Measured nitrate concentrations were 

greater in Río Descalabrado than in Río Coamo. 

The NO3

-N concentration pattern shown in figure 

12A indicates that the movement of groundwater with 

low (less than 5 mg/L) concentrations is associated 

with surface-water infiltration from Río Coamo toward 

the center of the study area. Two areas having elevated 

NO3

-N concentrations greater than 10 mg/L (fig. 12A) 

were detected during the study, one in the south-central 

part of the study area and the other west of Río Coamo. 

The areas of elevated NO3

-N concentrations partially 

overlap the two cones of depression (fig. 6) created by 

groundwater pumping centers. NO3

-N concentrations 

above the MCL of 10 mg/L for drinking water (U.S. 

Environmental Protection Agency, 2009) were detected 

in 15 wells in the coastal plain and 1 well in the foothills. 

All of the wells exceeding the nitrate MCL were used for 

agriculture. 

Sources of Nitrate

Nitrate is one of the primary forms of dissolved 

nitrogen in ambient water and is produced by the rapid 

oxidation of its reduced or organic forms. Nitrate is 

generally the dominant form in waters containing 

dissolved oxygen, is highly soluble, and is readily 

transported in groundwater. Nitrate in groundwater 

can originate from organic nitrogen complexes, such 

as animal manure or septic waste discharges, that are 

converted to (1) ammonia nitrogen through the processes 

of ammonification (the decomposition of organic matter 

into ammonium ions), and (2) nitrate by nitrification 

(the oxidation of ammonium to nitrate). Nitrification is 

an aerobic process that can cause an accumulation of 

nitrate in groundwater (Chapelle, 1993). Denitrification 

is the process by which nitrate is reduced to nitrogen gas 

and is the prevalent process by which nitrate is lost from 

groundwater. 

The intensive use of fertilizers in the cultivation 

of corn, vegetables, tomatoes, bananas, and other crops 

suggests that farmland cultivation has the greatest 

potential for increasing nitrate concentrations in the 

aquifer in the study area. The other potential sources of 

nitrate in the study area are animal farms, leakage from 

sewer mains, and leakage from septic tanks in unsewered 

communities.
20 Evaluation of Groundwater Quality and Selected Hydrologic Conditions in the South Coast Aquifer, Santa Isabel Area.

Figure 11. Sum of detected concentrations of pesticide compounds in groundwater samples collected from selected wells 

in the Santa Isabel area, Puerto Rico, May 2008.

Table 5. Groundwater ubiquity score (GUS) index (Gustafson, 1989) for pesticide compounds detected in groundwater samples 

collected from selected wells in the Santa Isabel area, Puerto Rico, 2008.

[Koc, partition coefficient between soil organic water and water; GUS>2.3, leacher; GUS 1.8-2.8, marginal leacher; GUS <1.8, non-leacher]

Compound Half life in soil1 (days) Koc1 Groundwater Ubiquity 

Score (GUS) Index Leaching potential

Alachor 15 170 2.08 Marginal leacher

Atrazine 60 100 3.56 Leacher

Dieldrin 1,000 12,000 -0.24 Non-leacher

Endosulfan 50 12,400 -0.16 Non-leacher

EPTC 6 200 1.32 Non-leacher

Metalaxil 70 50 4.25 Leacher

Metolachor 90 200 3.32 Leacher

Metribuzin 40 60 3.56 Leacher

Prometryn 60 400 2.49 Marginal leacher

Propiconazole 110 650 2.42 Marginal leacher

Simazine 60 130 3.35 Leacher

1

 Oregon State University Pesticides Properties Database
Water Quality 21

Figure 12A. Nitrate concentrations in groundwater and surface water in the Santa Isabel area, Puerto Rico, 

during (A) May 2008 to April 2009 and (B) February to October, 1967.
22 Evaluation of Groundwater Quality and Selected Hydrologic Conditions in the South Coast Aquifer, Santa Isabel Area.

Figure 12B. Nitrate concentrations in groundwater and surface water in the Santa Isabel area, Puerto Rico, 

during (A) May 2008 to April 2009 and (B) February to October, 1967.
Water Quality 23

Fertilizers containing nitrogen used in the 

croplands within the study area include urea, CO(NH2

)2, 

ammonium sulfate, (NH4

)2SO4, and ammonium nitrate, 

NH4

NO3. Urea is mostly used in the study area for 

banana, plantain, corn, vegetable, mango, and papaya 

crops. 

Septic-tank leachate is a probable source of nitrate 

in groundwater in shallow aquifers. Septic tanks remove 

most settleable solids and floatable material and function 

as an anaerobic bioreactor. Septic tank densities in a 

community, however, may exceed the capacity of even 

suitable soils to assimilate wastewater flows and retain 

and transform their contaminants. In such cases, septic 

tanks represent a source of nitrate to the aquifer.

Nitrogen and Oxygen Isotopic Characterization 

of Nitrate Sources

Groundwater samples collected in the study area 

were analyzed for delta nitrogen-15 (δ15N-NO3

) and 

delta oxygen-18 (δ18O-NO3

) in nitrate (table 2). The 

combined use of δ15N- NO3

 and δ18O-NO3 provides a 

tool for distinguishing between the following major 

nitrate sources: atmospherically deposited nitrate, 

nitrate in synthetic fertilizers, nitrate derived from 

soil nitrification, and nitrate in manure and sewage 

(Mayer, 2005). Typical δ15N-NO3

 and δ18O-NO3 values 

in groundwater associated with various sources are 

presented in figure 13 (Clark and Fritz, 1997; Kendall, 

1998; and Mayer, 2005). 

Groundwater samples collected during 2008 and 

2009 from wells within the coastal plain had NO3

-N 

concentrations between 1.3 and 23.6 mg/L (table 2) 

and δ15N-NO3

 values from +4.9 to +14.6‰ (fig. 14). 

Groundwater samples collected from wells within the 

cultivated farmland had δ15N-NO3

 values from +4.9 

to +11.5‰ and δ18O-NO3

 values from +3.5 to +8.9. 

Samples from wells in the eastern and west-central 

parts of the intensively cultivated sections of the study 

area had δ15N-NO3

 values from +5.3‰ to +7.6‰ 

and δ18O-NO3

 values from +3.8‰ to +7.5‰, which 

are associated with soil nitrogen according to Clark 

and Fritz (1997) and Mayer (2005) (figs. 13 and 14). 

Considering the intensive use of synthetic fertilizers 

and the high nitrate concentrations detected in the areas 

located east and west of Río Coamo, it is likely that soil 

nitrate produced by ammonia volatilization of nitrogen 

fertilizer, which is isotopically more positive, percolates 

to the groundwater system. 

Soil nitrogen exists in inorganic and organic forms, 

with organic forms being predominant. Biological 

processes in the soil convert organic nitrogen present 

in plant residues and other organic matter to inorganic 

forms. Farms in the eastern and central parts of the study 

area are intensively cultivated for tomato, corn, and 

vegetables. Nitrate loss from mineralized crop residues 

may also increase nitrate concentrations in groundwater 

in the study area.

Values of δ15N-NO3

 and δ18O-NO3 in the northcentral part of the study area and near the margins of the 

intensively cultivated farmland (fig. 14, table 2) exceed 

+9‰ and +6‰, respectively. These values coincide with 

the low NO3

-N and low dissolved-solids concentrations 

discussed previously, and with areas of streamflow 

infiltration from Río Coamo into the aquifer (Kuniansky 

and others, 2003). Two surface-water samples collected 

from Río Coamo had NO3-N concentrations of 0.51 and 

0.38 mg/L and δ15N-NO3

 values of +17.6 and +14.0‰. 

The relation between δ15N-NO3

 and δ18O-NO3 (fig. 13) 

suggests that nitrate concentrations in the north-central 

part of the study area (fig. 2, wells 31, 33, 35, 47, and 

49), southeast of the urban center of Santa Isabel 

(fig. 2, well 41) and eastward near the Peñuelas 

community (fig. 2, wells 69 and 71) are associated with 

organic waste that is possibly related to leakage from 

septic systems or sewer mains.

Biogeochemical reactions such as denitrification can 

alter the isotopic composition of nitrogen. Denitrification 

causes the δ15N of the residual nitrate to increase as 

nitrate concentration decreases. The δ18O-NO3

 value 

is useful in determining whether denitrification is 

occurring in the system. The ratio of enrichment of 

oxygen relative to nitrogen of δ18O-NO3

 and δ15N-NO3

is approximately 1:2 when denitrification is occurring 

(Kendall, 1998). General groundwater flow in the study 

area is southward; therefore, the increasing nitrate 

concentrations in that direction cannot be explained by 

denitrification because this process along the flowpath 

would result in decreasing nitrate concentrations 

and increases in δ15N-NO3

 and δ18O-NO3 values. A 

mixing process therefore seems more probable than 

denitrification in the north-central part of the study area.

The mixture of two end-members that have 

different isotope ratios and different concentrations of 

a given element form hyperbolas when the isotopes 

ratios are plotted against concentration (Kendall, 1998). 

Mixing hyperbolas can be transformed into straight 

lines by plotting isotopes ratios against the inverse of 

the concentration. The relation between δ15N-NO3

 and 

the inverse of NO3

 concentration in samples collected 

from wells in the central part of the study area (fig. 15) 

indicates a mixing of surface water from Río Coamo 

(low nitrate concentration and high δ15N-NO3

), with 

groundwater having a greater nitrate concentration and 

lower δ15N-NO3

. To verify if the data fit a fractionation 

process instead of a mixing process, the data were 

also plotted as δ15N-NO3

 against the natural logarithm 

of NO3

-N concentration (fig. 16). The denitrification 

process yields an exponential relation that will produce 

a straight line when delta 15N-NO3

 is plotted against the 
24 Evaluation of Groundwater Quality and Selected Hydrologic Conditions in the South Coast Aquifer, Santa Isabel Area.

Figure 13. Relation of delta nitrogen-15 (15N) and delta oxygen-18 (18O) of nitrate in groundwater and surface water in the 

Santa Isabel area, Puerto Rico, May 2008-April 2009.
Water Quality 25

Figure 14. Delta nitrogen-15 (15N) and delta oxygen-18 (18O) of nitrate in groundwater and surface water in the 

Santa Isabel area, Puerto Rico, May 2008- April 2009.
26 Evaluation of Groundwater Quality and Selected Hydrologic Conditions in the South Coast Aquifer, Santa Isabel Area.

Figure 16. Relation of the natural logarithm of nitrate concentration and delta nitrogen-15 (15N) of nitrate 

in groundwater in the central section of the study area, Santa Isabel area, Puerto Rico, 

May 2008- April 2009.

Figure 15. Relation of the inverse of nitrate concentration and delta nitrogen-15 (15N) of nitrate in 

groundwater in the central section of the study area, Santa Isabel area, Puerto Rico, May 2008- 

April 2009.
Water Quality 27

natural logarithm of NO3-N concentration 

(Kendall, 1998).

The δ15N-NO3

 and δ18O-NO3 signatures for the 

groundwater samples (wells 28, 31, 35, 47, and 49) and 

surface-water samples from Rio Coamo (sites 1 and 4) 

collected in the north-central part of the of the study area 

plot according to the general groundwater flow direction, 

but inverse to the theoretical denitrification trend 

(fig. 13), which also suggests that a surface-water and 

groundwater mixing process is occurring in the area.

The relation between δ15N-NO3

 and δ18O-NO3 in 

the study area (fig. 13) indicates that the groundwater 

samples collected from wells 73 and 74 within the Gabia 

area had values in the range associated with organicwaste nitrate sources. 

The four surface-water samples collected from Río 

Coamo and Río Descalabrado showed a relation between 

δ15N-NO3

 and δ18O-NO3 in the range associated to 

manure and septic waste nitrate sources (fig. 13). Water 

samples collected at stations Río Coamo near Paso Seco 

(site 2) and Canal de Juana Díaz at Paso Seco 

(site 3) had distinctive δ18O-NO3

 signatures and ammonia 

concentrations (table 3). Site 2 is located downstream 

from Lago Coamo, and site 3 is located in an irrigation 

canal that conveys surface water from Lago Guayabal 

(fig. 1). The distinctive ammonia concentrations may be 

caused by the decay of organic matter present in these 

reservoirs, and the distinctive δ18O-NO3

 signatures may 

be associated with the nitrification of ammonia.

Potential Nitrate Load to the Aquifer

Fertilizer applied to cropland within the study area 

represents a major source of nitrate to the aquifer. The 

potential nitrogen load from agricultural land under 

cultivation in the study area is variable and depends 

on many factors, including the type of crop under 

cultivation and the fertilizer management practices in use 

(table 6). The fertilizer application rates were estimated 

on the basis of data obtained from interviews with 

farmers in the study area. Only a fraction of the nitrogen 

contained in fertilizer used in cropland remains available 

to migrate to the subsurface, because most is either 

incorporated into the vegetation or mineralized within 

the soil. 

The potential amounts of nitrogen from fertilizer 

used on crops that can be leached to the aquifer were 

estimated by subtracting the nitrogen uptake of the 

different crops in the study area from the amount 

of nitrogen contained in fertilizers applied to the 

crops (table 6). As an example, the reported nitrogen 

application for corn crops is estimated to range from 

approximately 225 to 240 kilograms N per hectare-year 

(kg-N/ha-yr). The uptake of nitrogen is estimated to 

be approximately 174 kg-N/ha-yr (Natural Resources 

Conservation Service, 2001) of the applied nitrogen. 

The amount of nitrogen available for volatilization, 

mineralization, and potential migration to the aquifer 

ranges from 51 to 66 kg-N/ha-yr. The residual amount 

of nitrogen that has the potential to affect the aquifer 

is estimated to range from about 15,200 to 19,700 

kilograms N per year (kg-N/yr) for the 298 ha of corn 

cropland reported in 2008. The estimates obtained for 

Table 6. Potential nitrogen load estimates from fertilizers applied to crops and other sources in the study area, Santa Isabel, 

Puerto Rico, 2008.

[kg N/ha-yr, kilogram of nitrogen per hectare per year; N, nitrogen; ha, hectare; kg N/yr, kilogram of nitrogen per year; na, not applicable]

Crop or source Fertilizer application 

rate, as kg N/ha-yr

Crop nitrogen uptake, as kg N/ha-yrPotential residual 

N, as kg N/ha-yr Cultivated area, ha Total residual N, in kg N/yr

Bananas 154-340 112a 42-228 192 8,064 - 43,778

Corn 225-240 174 51-66 298 15,198 - 19,668

Mango 78 42a 36 379 13,644

Soybeans 225 219b 6 122 732

Tomato 107 82a 25 273 6,825

Vegetables and 

papaya 74-156 18-64a 88 796 70,048

Un-sewered 

communities n/a n/a n/a n/a 9,400

a

 Natural Resources Conservation Service, 2001

b

 Salvagiotti and others, 2008
28 Evaluation of Groundwater Quality and Selected Hydrologic Conditions in the South Coast Aquifer, Santa Isabel Area.

the other crops, using the same method, are presented in 

table 6.

Poultry farms in the Gabia area represent the other 

potential agricultural source of nitrate in the study area 

and ceased operation in 2007. Interviewed farm owners 

stated that partial removal of the poultry litter was 

conducted approximately every 3 months, and that the 

removed litter was transported out of the Gabia area and 

processed for use as cattle food. Because the poultry 

litter was not disposed, spread, or integrated into the 

soil of the farms in the Gabia area, it is assumed that the 

nitrogen load from this source is negligible. 

The potential nitrate load to the aquifer from 

septic systems in the Peñuelas and Las Ollas areas, 

two communities without sanitary sewer systems, 

was estimated by multiplying the population of each 

community in 2000 by the per-capita total nitrogen 

excreted by humans, which averages 17 grams per day 

(g/d) (Kaplan, 1987). Based on 381 housing units and an 

average of 3 persons per household within the Peñuelas 

community (U.S. Census Bureau, 2002), the estimated 

nitrogen load for the community is 7,090 kg-N/yr. An 

approximate NO3

-N load of 3,545 kg-N/yr is estimated 

if it is assumed that only 50 percent of the excreted 

nitrogenous compounds will result in nitrate. The 

estimated total area contained within the alluvial fan 

in the Peñuelas area with unsewered housing is 

35 ha; therefore, a potential NO3

-N load of about 

101 kg-N/ha-yr is estimated for the Peñuelas community. 

The potential nitrate load to the aquifer from septic 

tanks of the Las Ollas community was estimated based 

on 651 housing units and 3 persons per household within 

the community (U.S. Census, 2000), the estimated 

nitrogen load for the Las Ollas community is 

12,120 kg-N/yr. Assuming that only 50 percent of the 

excreted nitrogenous compounds will result in nitrate, an 

approximate NO3

-N load of 6,060 kg-N/yr is estimated. 

The estimated total area of Las Ollas with unsewered 

housing is 31 ha; therefore, a potential NO3

-N load of 

about 195 kg-N/ha-yr is estimated for the Las Ollas 

community. 

 Another source of nitrogen related to the 

agricultural activities in the study area is the 

recirculation of groundwater (for irrigation) that has 

an elevated NO3

-N concentration. For the purpose of 

discussion in this report the western sector of the study 

area is limited to the west by the Río Descalabrado 

and to the east by the Río Coamo. The central sector is 

limited to the west by the Río Coamo and to the east by 

the Río Cayures. In the central sector of the study area, 

groundwater NO3

-N concentrations ranged from 1.3 to 

23.6 mg/L (fig.12A). The estimated nitrogen load from 

groundwater in this area was calculated using the sum 

of the load from groundwater for each well, which is 

determined by multiplying the NO3

-N concentration for 

the sample collected in kilogram per cubic meter and 

the total annual groundwater withdrawal from the well 

in cubic meters per year (table 7). For wells used for 

irrigation, but not sampled during the study, the NO3

-N 

concentration was estimated using figure 12A. For 

example, for well 39 the NO3

-N concentration of 

23.6 mg/L (equivalent to 0.0236 kilogram per cubic 

meter (kg/m3

)) was multiplied by the estimated 

groundwater withdrawal for that well during 2008 of 

193,435 m3

/yr, which resulted in an estimated load of 

4,565 kg-N/yr (table 7). The load of NO3

-N from the 

irrigation water to the farms in the central sector of the 

study area was estimated as approximately 

60,900 kg-N/yr. In the western sector of the study area, 

groundwater NO3

-N concentrations ranged from 

5.0 to 20.9 mg/L (fig. 12A). The load of NO3

-N from 

the irrigation water to the farms in the western sector 

was estimated as approximately 56,000 kg-N/yr. The 

net effect of the nitrogen load from the recirculation of 

groundwater is assumed to be zero, however, because the 

load is removed from the aquifer and then added back.

The principal potential nitrogen load from sources 

in the study area expressed on a unit area basis and by 

total land-use area is summarized in table 6. Principal 

nitrogen sources were fertilizer used at vegetable and 

papaya farms (70,048 kg-N/yr), corn farms (15,198 

to 19,668 kg-N/yr), banana farms (8,064 to 

43,778 kg-N/yr), and mango farms (13,644 kg-N/yr). 

Generalized estimates of the expected nitrate 

concentration in groundwater contained within the 

western and central sections of the study area were 

obtained by dividing the potential nitrate load for each 

area (table 8) by the estimated rainfall recharge rate of 

0.10 meter per year (m/yr) and surface area of about 

16.8 and 30.8 square kilometers (km2

), respectively. The 

rainfall recharge value of 0.10 m/yr, which represents 

10 percent of the annual rainfall, was assumed by Giusti 

(1971) for the aquifer (fig. 1). Kuniansky and others 

(2003) used values of rainfall recharge between 4 and 

12 percent of the annual rainfall for that same area. Only 

rainfall recharge is used in the estimate of the expected 

nitrate concentration because the irrigation return flow 

from micro-drip irrigation is assumed to be negligible 

(Kuniansky and others, 2003). The rainfall recharge for 

the western and central sections were estimated to be 

1.7×106

 m3/yr (16.8 km2 x 0.10 m/yr) and 

3.1×106

 m3/yr (30.8 km2 x 0.10 m/yr), respectively. 

The expected NO3

-N concentration in groundwater in the 

central section of the study area would be as high as 32 

mg/L (99,600 kg N/yr divided by 3.1×106

 m3/yr). 

The NO3

-N measured concentrations in this area ranged 

from 4 to 74 percent of the expected concentration. 

In the western section of the study area, the expected 

NO3

-N concentration in groundwater would be as high 

as 27 mg/L (45,300 kg N/yr divided by 1.7×106

 m3/yr). 

The measured NO3

-N concentrations in this area ranged 

from 42 to 77 percent of the expected concentration.
Water Quality 29

Table 7. Estimated nitrogen load from irrigation groundwater in the central and western sections of the study area, 

Santa Isabel, Puerto Rico, 2008.

Number on figure 2 Nitrate as nitrogen, 

milligrams per liter

Average groundwater 

withdrawal, cubic 

meters per day

Total annual groundwater withdrawal, 

cubic meters per year 

Estimated nitrogen 

load, kilograms per 

year

Central Section

26 6.9 1,022 373,030 2,574

36 10.0 E 1,438 524,870 5,249

39 23.6 530 193,435 4,565

41 10.8 946 345,290 3,729

45 10.0 E 1,211 442,015 4,420

48 2.0 E 757 276,305 553

49 3.0 379 138,335 415

50 5.0 E 492 179,618 898

51 11.5 1,173 428,145 4,924

52 14.2 1,817 663,205 9,418

53 21 .0 E 1,249 455,885 9,574

54 19.9 1,022 373,030 7,423

59 10.0 E 795 290,175 2,902

60 10.0 E 1,173 428,145 4,281

Total 60,924

Western Section

7 5.0 E 833 304,045 1,520

8 14.9 492 179,580 2,676

10 12.9 492 179,580 2,317

12 13.0 E 871 317,915 4,069

13 16 530 193,450 3,095

14 20.0 E 1,249 455,885 9,118

15 20.9 909 331,785 6,934

16 15.0 E 681 248,565 3,728

17 19.0 E 606 221,190 4,203

18 11.4 909 331,785 3,782

19 18.5 416 151,840 2,809

20 19.5 1,249 455,885 8,890

21 12.4 454 165,801 2,056

33 2.4 946 345,290 829

Total 5,6026
30 Evaluation of Groundwater Quality and Selected Hydrologic Conditions in the South Coast Aquifer, Santa Isabel Area.

Table 8. Potential nitrogen load estimates from fertilizers applied to crops and other sources in the central and western sections of 

the study area, Santa Isabel, Puerto Rico, 2008.

[kg N/ha-yr, kilogram of nitrogen per hectare per year; N, nitrogen; ha, hectare; kg N/yr, kilogram of nitrogen per year]

Crop or source Fertilizer application 

rate, kg N/ha-yr

Crop nitrogen uptake, kg N/ha-yrPotential residual N, 

kg N/ha-yr Cultivated area, ha Total residual N, kg N/yr

Central Section

Bananas 154-340 112a 42-228 40 1,680-9,120

Corn 225-240 174a 51-66 298 151,98-19,668

Mango 78 42a 36 301 10,836

Soybeans 225 219b 6 0 0

Tomato 107 82a 25 49.3 1,232

Vegetables and 

papaya 74-156 18-64a 88 639 35,784-58,788

Unsewered communities na na na na 0

Total 64,700-99,600

Western Section

Bananas 154-340 112a 42-228 59 2,478-13,452

Corn 225-240 174a 51-66 0 0

Mango 78 42a 36 78 2,808

Soybeans 225 219b 6 68 408

Tomato 107 82a 25 224 5,600

Vegetables and 

papaya 74-156 18-64a 88 157 8,792-14,444

Unsewered communities na na na na 6,060

Total 28,600-45,300

a

Natural Resources Conservation Service, 2001

b

Salvagiotti and others, 2008

The estimate of the expected concentration assumes 

that the entire estimated potential nitrogen load enters 

the aquifer, that there is no NO3

-N accumulation in the 

unsaturated zone, and that groundwater flow is steady 

state. The unsaturated zone in the area of higher NO3

-N 

concentration ranged from 11 to 35 m below land surface 

during 2008, which indicates that part of the calculated 

load could be in transient storage and some may have 

been released from storage. Most of the sampled wells 

are screened over long intervals (ranging from 32.9 to 

72.8 m), which means that collected samples represent a 

mixture of water recharged over a number of years; this 

fact represents a limitation to the methodology used in 

this study.

The expected NO3

-N concentration in groundwater 

calculated using the current fertilizer application rates 

was compared to the expected NO3

-N concentration 

during the period when sugarcane was the principal crop 

in the area. Sugarcane cultivation was the principal landuse activity in the area from the early 1900s until the 

1970s. The fertilizer application rate to sugarcane crops 

ranged from 135 to 208 kg N/ha-yr (Natural Resources 

Conservation Service, 2001). The N uptake of sugarcane 

crops is estimated to range from 86 to 117 kg N/ha-yr 

(Natural Resources Conservation Service, 2001). 

Using the average values of the fertilizer application 

rate and the N uptake, the amount of N available for 

volatilization, mineralization, and potential migration to 
Summary and Conclusions 31

the aquifer would be about 70 kg/ha-yr during the period 

when sugarcane was the principal crop in the area.

The expected NO3

-N concentration in groundwater 

within the same area used in the previous calculation 

under present land-use conditions but, for the period 

when sugarcane was the principal crop in the area, was 

estimated using a recharge rate from rainfall infiltration 

and furrow-irrigation return flows of 0.36 m/yr 

(Fernando Gómez-Gómez, U.S. Geological Survey, oral 

commun., 2003) and a potential N load of 70 kg/ha-yr. 

If the cultivated land area is assumed to equal the total 

area used in the previous calculations in this section, 

the potential N available to move into the aquifer would 

be 333,200 kg/yr (4,760 ha×70 kg/ha-yr). The expected 

NO3-N concentration in the aquifer would be about 

19 mg/L. Concentrations of NO3

-N from samples 

collected in wells during 1967 indicate concentrations 

ranged from 0.9 to 5.9 mg/L (fig. 12B) (median of 

3.1 mg/L, n = 20). This result indicates that measured 

NO3

-N concentrations in the aquifer range from 5 to 31 

percent of the expected concentrations calculated from 

the potential N load. Consequently, a large portion of the 

N fertilizer load may have been lost to volatilization or 

soil mineralization, or was in transient storage. 

Given the uncertainty of the methodology used, 

a comparison of the relation between the potential 

nitrogen loads to the aquifer with the groundwater 

NO3

-N concentrations—for the historical and present 

land-use conditions—indicates that as much 77 percent 

of the estimated fertilizer residual load can reach the 

aquifer within cropland areas under the existing land-use 

conditions and fertilizer application rates in the study 

area. 

Summary and Conclusions

Between January 2008 and May 2009, the U.S. 

Geological Survey conducted a study to evaluate the 

groundwater-quality and selected hydrologic conditions 

in the South Coast aquifer, Santa Isabel area and in the 

Gabia area, a semi-enclosed basin in the foothills north 

of the coastal plain. Agriculture is the principal land use 

within the study area, which includes truck-farm crops, 

bioengineered crops for seed production, and confined 

animal feeding operations. The study area includes 

part of the South Coast aquifer and bedrock hills north 

of the coastal plain. The principal groundwater flow 

zone within the study area is within the fan–delta and 

alluvial deposits. These deposits in the study area 

have a thickness of as much as 300 meters (m) along 

the coast. The hills north of the coastal plain consist 

of volcanic rocks. Hydraulic conductivity in the Río 

Coamo fan ranges from less than 1 meter per day (m/d) 

to approximately 80 m/d, with the depth to groundwater 

during 2008 ranging from 3 to 35 m below land surface.

A water-table map prepared during the study 

indicates that the general groundwater flow direction 

is southward and also indicates the presence of two 

cones of depression, one in the north central part of the 

study area and another in the southwestern part of the 

study area. The water-table altitude during 2008 was 

approximately 4 m lower, on average, than it was in 

1986 during a similar study. Groundwater withdrawals 

from the aquifer were estimated to be approximately 

64,400 cubic meters per day (m3

/d) in 2008. In 2008, 

51 percent of total groundwater withdrawals in the 

Santa Isabel area were for agriculture and 49 percent 

were for public water supply. 

Estimated dissolved-solids concentrations for 

groundwater in the study area, determined from specific 

conductance data, ranged from 384 to 2,090 milligrams 

per liter (mg/L) in wells in the coastal plain. Estimated 

dissolved-solids concentrations below 500 mg/L were 

detected parallel to Río Coamo.

Groundwater samples collected at 10 wells in the 

study area were analyzed for 83 organic compounds used 

as pesticides. Analytical results indicated that 13 of these 

organic compounds were detected in the samples. The 

detected organic compounds included 8 herbicides, 

3 fungicides, and 2 insecticides.

Nitrate concentrations above the U.S. 

Environmental Protection Agency maximum 

contaminant level (MCL) of 10 mg/L for drinking water 

were detected in 15 wells sampled in the coastal plain 

and 1 well in the foothills. All wells exceeding the nitrate 

MCL are used for agricultural purposes. 

Sources of nitrate in the study area are associated 

with soil nitrate and fertilizers used in the cultivated 

farmlands and organic waste from leakage of sewer 

mains or septic systems. Groundwater samples analyzed 

for the stable isotope ratios of δ15N and δ18O in nitrate 

suggest that nitrate concentrations are associated with 

leakage from sewer mains or septic systems in northcentral, south-central, and eastern parts of the study area. 

Values of δ15N and δ18O in samples from wells located 

in the western and central parts of the study area were 

in the range typically associated with soil nitrogen. 

Potential nitrogen loads from areas under cultivation 

were estimated for the principal crops in the area. 

Principal potential nitrogen loads were associated to 

fertilizer use at vegetable and papaya farms

(70,048 kg-N/ha-yr), corn farms (15,198 to 

19,668 kg-N/yr), banana farms (8,064 to 

43,778 kg-N/yr), and mango farms (13,644 kg- N/yr).

Nitrate concentrations above the maximum 

contaminant level (MCL) indicate the movement of 

nitrogen to the soil and aquifer. An increase in fertilizer 

application would be expected to increase nitrogen 

movement to the soil and the aquifer and a decrease 
32 Evaluation of Groundwater Quality and Selected Hydrologic Conditions in the South Coast Aquifer, Santa Isabel Area.

would be expected to have the opposite effect. Fertilizer 

management in farms in the study area should account 

for the amount of NO3

-N present in groundwater used 

for crop irrigation, which in the central and western 

sectors of the study area, ranged from 1.3 to 23.6 mg/L 

and from 11.4 to 20.9 mg/L, respectively, in 2008. The 

nitrogen content of crop residues may also be evaluated 

to determine if it is a substantial source that can increase 

the nitrate concentrations in the aquifer.

Increased use of surface water from the Canal de 

Juana Díaz for the irrigation of crops in the Santa Isabel 

area would reduce the need for groundwater withdrawals 

from the aquifer. The reduction of groundwater 

withdrawals would be expected to increase groundwater 

levels in the aquifer, maintain the groundwater flow 

towards the coast, and decrease the potential of inland 

movement of seawater. In addition, a decrease in the use 

of groundwater would potentially reduce the nitrogen 

load to the aquifer as surface water with lower nitrogen 

concentration is applied to the crops.

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Appendix
36 Evaluation of Groundwater Quality and Selected Hydrologic Conditions in the South Coast Aquifer, Santa Isabel Area.

Appendix. Pesticides analyzed from groundwater samples collected in the 

Santa Isabel area, Puerto Rico, 2008.

1-naphthol fenamiphos sulfone 

cis-permethrin fenamiphos sulfoxide 

2,6-diethylaniline fipronil 

2-chloro-2,6-diethylacetanilide fipronil sulfide 

2-chloro-4-isopropylamino-6-amino-s-triazine (ciat) fipronil sulfone

2-ethyl-6-methylaniline fonofos 

3,4-dichloroaniline hexazinone 

3,5-dichloroaniline iprodione 

4-chloro-2-methylphenol isofenphos 

acetochlor lambda-cyhalothrin 

alachlor malaoxon 

alpha-endosulfan malathion 

atrazine metalaxyl 

azinphos-methyl methidathion 

azinphos-methyl-oxon metolachlor 

benfluralin metribuzin

carbaryl molinate 

carbofuran myclobutanil 

chlorpyrifos oxyfluorfen 

cis-propiconazole paraoxon-methyl 

cyanazine parathion-methyl

cyfluthrin pendimethalin 

cypermethrin phorate 

dacthal phosmet 

desulfinylfipronil phosmet-oxon 

desulfinylfipronil amide prometon 

diazinon prometryn 

diazoxon pronamide 

dichlorvos propanil 

dicrotophos propargite 

dieldrin simazine 

dimethoate tebuconazole 

disulfoton tebuthiuron 

disulfoton sulfone tefluthrin 

endosulfan sulfate terbufos 

eptc terbuthylazine 

ethion thiobencarb 

ethion monoxon trans-propiconazole 

ethoprophos tribufos 

fenamiphos trifluralin 
Prepared by the Raleigh Publishing Service Center

For additional information contact: 

 Jose M. Rodriguez

 Caribbean Water Science Center

 GSA Center

 651 Federal Drive, Suite 400-15

 Guaynabo, PR 00965

 (787) 749-7400

or visit the Caribbean Water Science Center at:

 http://pr.water.usgs.gov/
José M. Rodríguez—Evaluation of Groundwater Quality and Selected Hydrologic Conditions in the South Coast Aquifer, Santa Isabel Area—SIR 2012-5254