U.S. DEPARTMENT OF THE INTERIOR

U.S. GEOLOGICAL SURVEY

Prepared in cooperation with the

PUERTO RICO DEPARTMENT OF NATURAL AND 

ENVIRONMENTAL RESOURCES 

SCIENTIFIC INVESTIGATIONS MAP 3092

Groundwater-Quality Survey of the South Coast Aquifer 

of Puerto Rico, April 2 through May 30, 2007

by José M. Rodríguez and Fernando Gómez-Gómez

GROUNDWATER-QUALITY SURVEY OF THE SOUTH COAST AQUIFER OF PUERTO RICO, APRIL 2 THROUGH MAY 30, 2007

By José M. Rodríguez and Fernando Gómez-Gómez

2009

Calcium-bicarbonate

EXPLANATION

Sodium-bicarbonate

Calcium-chloride Bedrock

Area of potential change from calcium-bicarbonate 

to calcium-chloride or sodium-chloride water types

Sodium-chloride

Guayama

Salinas

Santa Isabel

Ponce

Arroyo

53

52

1

54

3

3

2

14

12

10

15

52

53

1

R

15P

ío

Coamo

Río Bucaná

Río

Río

Canal

de Juana

Díaz

Río

Río

Río

Río

í R o

Río

Jacaguas

Cañas

Jueyes

Nigua

Nigua

Patillas

Seco

Descalabrado

Flood channel

Río

Canal

de

Patillas

Guamaní

Lago

Patillas

Lago

Melania

Lago

Coamo

18°00'00"

17°58'00"

17°56'00"

61

60 59

58 57

56 55

54

53

52 51

50

49

48

47

46

45

43

42

41

40

39 38

37

36

35

34

33

32

31

30

29

28

27

26

25

24

23

22

21

20

19

18

17

16

15

14

13

11 12

10 9

7 8

4 5

3 2

R oí

Portuguez

Highway

Primary Road

54

3

44

Caribbean Sea

Geology modified from U.S. Geological Survey OFR 98-38. (Bawiec, 2001)

Roads from the P.R. Department of Transportation.

Lambert conformal conic projection. North American Datum 1983.

0 2.5 5 10 Kilometers

0 2.5 5 10 Miles

53

52

1

54

3

3

2

14

12

10

15

52

53

1

R

15P

ío

Coamo

Río Bucaná

Río

Río

Canal

de Juana

Díaz

Río

Río

Río

Río

í R o

Río

Jacaguas

Cañas

Jueyes

Nigua

Nigua

Patillas

Seco

Descalabrado

Flood channel

Río

Canal

de

Patillas

Guamaní

Lago

Patillas

Lago

Melania

Lago

Coamo

Guayama

Salinas

Santa Isabel

Ponce

Arroyo

500

1,000

400

400

500

800

500

500

800

400

400

750

800

450

450

1,000

400

500

800

1,000

GROUNDWATER DATA CONTROL POINT - Number in left is well number 

in map and number in right is dissolved solids concentration in milligrams per liter

EXPLANATION

LINE OF EQUAL DISSOLVED SOLIDS CONCENTRATION - Dashed line indicates 

where approximately located. Contour interval variable

SURFACE-WATER QUALITY DATA CONTROL POINT - Top number indicates 

specific conductance in microsiemens per centimeter at 25 degrees Celsius and

bottom number is estimated dissolved solids concentration in milligrams per liter

calculated by multiplying 0.6 times specific conductance value

61/446

59/267 60/403

58/345

57/744

56/484 55/393

52/598 54/758

53/757

51/582

50/2,590

49/3,810

48/614

47/698

46/747

45/1,880

44/530

43/810

42/527

41/502

40/537

39/19,900

39/1,900

38/4,260

37/936

36/545

35/492

34/541

33/637

32/454

31/464

30/713

29/1,650

28/887

27/582

26/7,130

25/703

24/585

23/739

22/473

21/452

20/564

19/677

18/595

17/703

16/1,110

15/465

14/441

13/472

12/462 11/431

10/472 9/330

8/411

6/492

7/385

4/573 5/942

3/1,550

2/1,630

1/583

1012

(2002)

676

(2002)

379 (2002) 940 (2002)

5860 (2002)

324 (2002)

607

364

935

571

672

403

607

364

690

414

672

402

677

406

18°00'00"

17°58'00"

17°56'00"

Bedrock

R oí

Portugués

Highway

Primary Road

54

3 Caribbean Sea

Figure 3. Dissolved solids distribution in the South Coast aquifer, April 2 through May 30, 2007.

Table 1. Physical and chemical characteristics in groundwater samples collected from selected wells in the Ponce to Arroyo area, Puerto Rico.

[NAD83, North American Datum 1983; concentrations are given in milligrams per liter, unless otherwise noted; μS/cm, microsiemens per centimeter at 25 degrees Celsius; μg/L, micrograms per liter; CaCO3; Calcium 

Carbonate; HCO3, Bicarbonate; Ca, Calcium; Mg, Magnesium; K, Potassium; Na, Sodium; Cl, Chloride; F, Fluoride; SiO2, Silica; SO4, Sulfate; B, Boron; Fe, Iron; Br, Bromide; <, less than the analytical detection limit; na, 

data not available] [Map number refers to well locations shown on figure 2]

Map 

number 

(fig.2)

USGS site 

identification

Latitude 

Longitude 

(NAD83)

Date of 

sample 

collection

Depth 

of well 

(feet) pH

Specific 

conductance 

(μS/cm)

Temperature 

(Degrees 

C)

Acid 

neutralization 

capacity, 

(mg/L as 

CaCO 3 )

Bicarbonate 

(mg/L as 

HCO 3 )

Dissolved constituents

Cl/Br 

(molar 

basis)

Sodium

adsorption

rate

(SAR)

na

1.0

9.0

na

na

0.8

2.0

na

0.8

na

0.9

na

na

na

3.4

6.1

1.3

1.6

1.7

1.4

na

na

na

1.2

na

na

na

na

2.5

2.9

3.1

Ca Mg K Na Cl F SiO 2 SO 4 

B 

(μg/L) 

Fe 

(μg/L) 

Mn 

(μg/L) Br

 Dissolved 

solids, 

sum of 

constituents 

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

175952066362200

175839066361500

175835066353900

175948066351500

175947066343200

180009066320100

175921066322000

175923066314300

180027066313200

180031066301800

180001066311200

175958066295200

180029066281700

175952066272800

175931066264300

180007066245500

175921066252700

175848066253100

175839066253900

175758066242700

175844066241500

175916066235600

175748066234200

175825066233900

175734066233300

175734066233301

1758090662302

175728066225800

175835066223600

175848066213300

175917066205400

175945 

663621

175832 

663614

175828 

663538

175941 

663514

175940 

663431

180001 

663200

175914 

663219

175915 

663142

180020 

663131

180024 

663017

175954 

663111

175951 

662951

180023 

662816

175945 

662727

175924 

662642

180002 

662456

175914 

662512

175842 

662529

175830 

662535

175752 

662428

175837 

662414

175908 

662355

175741 

662341

175818 

662338

175728 

662332

175728 

662332

175804 

662302

175721 

662256

175828 

662235

175841 

662131

175911 

662052

4/30/2007

5/30/2007

5/30/2007

4/19/2007

4/30/2007

4/19/2007

4/19/2007

5/2/2007

4/19/2007

5/2/2007

4/17/2007

4/18/2007

4/17/2007

4/13/2007

4/13/2007

4/13/2007

4/11/2007

4/11/2007

4/11/2007

4/12/2007

4/5/2007

4/11/2007

4/18/2007

4/11/2007

5/18/2007

5/18/2007

4/5/2007

4/18/2007

4/12/2007

4/12/2007

4/12/2007

na

na

192

na

256

na

64

120

148

165

na

na

na

na

na

na

na

na

160

193

na

na

na

216

70

250

na

40

na

na

157

7.0

7.4

7.7

6.9

7.2

7.1

7.1

7.3

7.2

7.2

7.2

7.3

6.8

7.7

7.6

7.3

7.2

7.1

7.0

7.0

7.1

7.2

7.2

7.2

7.1

7.1

7.0

7.4

7.0

7.1

7.2

960

3,070

3,040

980

1,620

864

632

706

546

764

693

765

772

778

765

1,740

1,130

1,070

1,100

929

753

770

1,360

928

1,020

12,000

933

1,580

2,950

1,140

741

28.6

30.2

27.3

28.1

27.2

28

27.8

29.1

28

28.3

27.8

27.7

27.7

26.5

27.5

28.2

27.5

27.5

27.5

27.8

27.5

27.8

27.7

27.5

28.8

28.1

27.6

29.4

27.7

27.1

27.6

283

116

107

236

211

244

253

249

199

318

253

251

291

227

287

383

247

273

271

289

263

263

193

273

349

101

329

295

333

419

301

345

141

130

288

257

297

308

303

242

387

308

306

355

275

349

466

301

333

330

352

320

320

235

333

425

123

401

356

406

510

367

115

404

104

126

115

110

54.0

87.8

70.7

76.3

83.1

82.2

61.3

38.0

34.2

72.8

108

90.7

86.6

78.6

82.1

87.1

124

93.4

79.7

1310

70.1

103

270

88.4

44.7

21.9

57.5

26.5

25.1

31.6

22.5

14.2

15.5

10.9

20.3

20.2

28.9

26.8

29.2

22.6

38.1

38.5

39.9

41.8

37.5

24.4

24.1

48.7

32.6

55.2

430

25.5

48.4

88.4

27.7

18.8

1.01

2.87

21

0.67

1.72

0.88

0.82

1.14

1.21

0.72

1.26

0.8

0.7

0.41

0.98

2.12

1.04

0.78

0.91

1.14

1.01

1.6

0.89

0.84

7.61

10

0.96

1.49

2.29

1.61

0.67

57.0

77.9

398

36.1

164

35.3

65.4

34.9

26.9

62.4

36.8

37.9

73.0

81.3

103

260

60.1

71.0

76.3

59.2

37.3

41.3

62.3

52.5

87.3

622

96.5

153

187

120

97.3

61.5

879

847

110

289

67.7

20.5

33.9

26.0

24.6

27.8

43.8

43.2

86.8

45.8

174

95.6

85.0

98.7

69.4

45.6

46.7

266

54.5

93.0

4,220

50.4

276

730

54.1

32.0

0.13

<0.10

0.11

0.10

0.16

0.12

0.21

0.15

0.11

0.16

0.12

0.20

0.43

0.33

0.24

0.65

0.18

0.15

0.16

0.18

0.16

0.12

0.15

0.20

0.26

0.10

0.19

0.15

0.14

0.15

0.53

37.3

27.7

30.3

34.5

38.2

33.9

34.0

35.5

28.4

39.6

34.3

37.3

35.8

38.5

32.4

39.8

38.4

39.8

39.8

42.2

36.6

36.5

41.3

41.4

40.7

26.4

44.6

48.3

41.1

42.9

40.1

110

106

57.3

94.5

170

68.4

38.3

48.7

33.2

50.8

49.6

74.8

38.1

26.7

34.9

239

112

97.3

109

58.1

39.6

48.7

42.0

55.8

80.1

257

61.8

73.3

85.3

87.2

29.4

90.8

27.6

76.7

51.2

79.2

52.2

89.4

67.7

51

171

71.4

56.4

210

156

198

560

143

169

176

136

87.1

103

115

137

197

231

225

325

340

429

243

<6

<6

18

<6

7

<6

<6

25

<6

15

<6

<6

<6

<6

<6

<6

<6

<6

<6

<6

<6

<6

6

<6

402

1,930

<6

<6

<6

<6

<6

1.1

0.8

2.7

<0.2

0.8

0.2

0.9

3.1

<0.2

<0.2

0.5

3.7

<0.2

<0.2

<0.2

<0.2

<0.2

<0.2

<0.2

0.6

<0.2

<0.2

4.1

<0.2

187

1,500

<0.2

0.4

<0.2

<0.2

<0.2

0.20

3.07

3.02

0.36

0.89

0.22

0.08

0.13

0.08

0.09

0.13

0.19

0.14

0.28

0.18

2.08

2.49

1.72

1.12

0.47

0.21

0.20

1.15

1.93

0.45

15.1

0.30

1.11

2.91

0.28

0.12

583

1,630

1,550

573

942

492

385

411

330

472

431

462

472

441

465

1,110

703

595

677

564

452

473

739

585

703

7,130

582

887

1,650

713

464

693

645

632

689

732

694

578

588

732

616

482

520

695

699

573

189

87

111

199

333

489

526

521

64

466

630

379

560

565

435

601

Table 1. Physical and chemical characteristics in groundwater samples collected from selected wells in the Ponce to Arroyo area, Puerto Rico—Continued

[NAD83, North American Datum 1983; concentrations are given in milligrams per liter, unless otherwise noted; μS/cm, microsiemens per centimeter at 25 degrees Celsius; μg/L, micrograms per liter; CaCO3; Calcium 

Carbonate; HCO3, Bicarbonate; Ca, Calcium; Mg, Magnesium; K, Potassium; Na, Sodium; Cl, Chloride; F, Fluoride; SiO2, Silica; SO4, Sulfate; B, Boron; Fe, Iron; Br, Bromide; <, less than the analytical detection limit; na, 

data not available] [Map number refers to well locations shown on figure 2]

Map 

number 

(fig.2)

USGS site 

identification

Latitude 

Longitude 

(NAD83)

Date of 

sample 

collection

Depth 

of well 

(feet) pH

Specific 

conductance 

(μS/cm)

Temperature 

(Degrees 

C)

Acid 

neutralization 

capacity, 

(mg/L as 

CaCO 3 )

Bicarbonate 

(mg/L as 

HCO 3 )

Dissolved constituents

Cl/Br 

(molar 

basis)

Sodium

adsorption

rate

(SAR)

Ca Mg K Na Cl F SiO 2 SO 4 

B 

(μg/L) 

Fe 

(μg/L) 

Mn 

(μg/L) Br

 Dissolved 

solids, 

sum of 

constituents 

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52

53

54

55

56

57

58

59

60

61

175944066204200

175853066182800

175826066180600

175851066174600

175823066164600

175708066163001

175708066163000

175708066163002

175748066160400

175810066152700

175858066151600

175735066151800

175721066151400

175721066151401

175804066150700

175854066144500

175916066144000

175711066143600

175711066143601

175828066142200

175822066134800

175812066133200

175824066130900

175755066105000

175747066101000

175728066100900

175719066085500

175832066031300

175840066024100

175832066022000

175937 

662040

175846 

661827

175819 

661805

175843 

661743

175816 

661645

175701 

661629

175701 

661629

175701 

661629

175741 

661605

175803 

661526

175851 

661515

175728 

661517

175714 

661513

175714 

661513

175757 

661506

175847 

661444

175908 

661435

175704 

661435

175704 

661435

175821 

661421

175815 

661348

175805 

661331

175816 

661309

175748 

661049

175740 

661009

175721 

661008

175713 

660901

175827 

660312

175833 

660240

175823 

660216

4/12/2007

4/18/2007

4/4/2007

4/4/2007

4/17/2007

5/22/2007

5/22/2007

5/22/2007

4/4/2007

4/18/2007

4/3/2007

5/17/2007

5/16/2007

5/16/2007

4/4/2007

4/2/2007

4/2/2007

5/16/2007

5/17/2007

4/2/2007

4/2/2007

4/18/2007

4/4/2007

4/3/2007

4/3/2007

5/2/2007

5/17/2007

4/3/2007

4/3/2007

4/3/2007

na

na

na

120

na

50

115

220

162

120

na

82

87

20

150

na

300

74

17

na

118

150

na

85

na

na

99

120

na

na

7.1

7.1

7.0

7.1

6.9

7.0

6.7

6.6

7.3

7.0

7.1

7.0

7.2

7.0

7.1

7.1

6.9

6.8

7.5

7.0

7.1

7.1

7.5

6.8

6.8

7.5

7.3

6.3

6.6

6.9

777

1,040

906

819

884

1,740

7,740

31,000

874

810

869

1,340

870

3,560

1,150

1,140

1,110

6,060

4,320

977

993

1,290

1,250

673

825

1,270

572

449

669

783

28.2

28.3

29

28.6

27.7

27.2

27.7

28.2

27.4

28

28.3

27.5

27.3

27.5

27.8

28.8

27.6

28.1

28.5

28.4

27.9

27.8

29.3

27.9

27.7

28.7

27.8

28.2

28.5

27.5

279

317

303

277

293

254

203

179

315

261

251

431

297

275

309

341

311

379

419

309

315

379

451

269

336

497

205

123

215

201

340

386

369

336

357

310

248

218

384

318

306

525

362

335

377

416

379

462

509

377

384

462

548

328

410

604

250

150

262

245

66.7

83.6

94.3

87.5

91.9

141

578

1,970

79.7

80.3

85.3

110

90.8

216

85.4

96.2

88.2

409

63.8

85.0

89.0

86.3

23.6

65.9

47.8

57.1

48.3

36.0

63.2

63.8

24.8

30.7

26.2

24.1

28.6

65.7

251

988

26.9

26.6

28.8

58.4

31.2

126

22.8

25.3

16.9

195

43.0

30.2

27.9

30.3

7.5

21.8

22.0

59.6

21.8

9.6

23.7

26.4

0.97

1.55

1.07

1.04

1.37

2.32

4.75

15.4

0.93

0.83

0.96

0.5

1.06

1.12

1.23

0.96

1.07

2.5

6.24

0.97

1.11

1.08

0.51

0.92

0.75

0.53

0.31

0.54

0.45

0.46

62.4

95.4

59.8

50.8

57.6

102

609

4,180

68.2

53.4

48.8

98.3

54.8

222

129

106

117

700

794

74.1

84.0

148

246

45.6

98.7

141

44.7

42.0

47.3

54.1

47.5

75.2

57.5

49.0

51.7

345

2,390

11,000

45.3

47.3

62.9

117

61.9

852

96.5

105

107

1,510

1070

80.5

86.3

130

71.5

38.2

46.9

105

39.4

40.2

56.1

81.9

0.43

0.42

0.31

0.29

0.33

0.22

0.30

0.12

0.38

0.34

0.28

0.39

0.29

0.97

0.27

0.48

0.63

0.47

0.42

0.32

0.34

0.43

1.81

0.32

0.60

0.65

0.51

0.27

0.27

0.28

37.4

36.8

39.3

38.0

38.4

41.8

38.8

30.6

40.7

39.1

37.7

39.7

39.2

37.0

39.4

38.9

33.5

39.2

38.2

38.4

38.4

38.8

35.8

35.6

36.1

37.5

39.1

30.9

37.3

37.0

39.2

83.0

73.9

68.9

65.8

65.9

246

1,520

50.8

54.1

71.6

118

51.5

169

96.9

59.5

63.4

699

158

44.7

45.2

84.8

87.5

16.5

23.0

39.2

24.5

30.7

42.3

57.2

179

276

184

161

153

184

178

392

177

125

109

227

121

375

286

264

314

838

566

192

219

395

711

114

203

421

116

99.2

86.2

100

<6

<6

<6

<6

<6

156

6

103

<6

3

<6

<6

16

<6

<6

<6

<6

<6

8.4

<6

<6

8

16

<6

<6

80

<6

<6

<6

<6

<0.2

0.5

<0.2

<0.2

<0.2

21.3

0.41

25.1

<0.2

0.2

0.6

4.9

<0.2

694

<0.2

0.2

1.7

18.2

242

<0.2

<0.2

0.2

0.3

<0.2

<0.2

158

0.7

<0.2

0.3

<0.2

0.17

0.35

0.22

0.19

0.20

1.27

8.53

39.9

0.23

0.22

0.27

0.44

0.24

2.86

0.42

0.40

0.38

4.76

3.75

0.36

0.32

0.48

0.28

0.14

0.16

0.30

0.13

0.14

0.19

0.29

454

637

541

492

545

936

4,260

19,900

537

502

527

810

530

1,880

747

698

614

3,810

2,590

582

598

757

758

393

484

744

345

267

403

446

630

484

589

581

583

612

631

621

444

485

525

599

258

298

518

592

635

715

643

504

608

610

576

615

661

789

683

647

665

637

na

na

na

na

1.3

na

na

na

1.7

na

1.2

na

na

na

3.2

na

na

na

na

na

na

3.5

na

na

na

na

na

na

na

na

SULFATE PLUS CHLORIDE

CALCIUM PLUS MAG

CARBONATE PLUS BICARBO

SODIUM PLUS POTASSIUM

CaCl2

NaCl

NaHCO3

Ca(HCO3)

2

CALCIUM CHLORIDE

MAGNESIUM

SULFATE

PERCENT

25

75 75

50

75 50

75 25

50

2 75

75 50 25 25 50 75

75

50

25 75

50

25 50

25

50

25

50

75

25

SULFATE PLUS CHLORIDE

CALCIUM PLUS MAGNESIUM

CARBONATE PLUS BICARBONATE

SODIUM PLUS POTASSIUM

CaCl2

NaCl

NaHCO3

Ca(HCO3)

2

CALCIUM CHLORIDE

MAGNESIUM

SULFATE

PERCENT

25

75 75

50

75 50

75 25

50

25 75

75 50 25 25 50 75

75

50

25 75

50

25 50

25

50

25

50

75

25

SULFATE PLUS CHLORIDE

CALCIUM PLUS MAGNESIUM

CARBONATE PLUS BICARBONATE

SODIUM PLUS POTASSIUM

CaCl2

NaCl

NaHCO3

Ca(HCO3)

2

CALCIUM CHLORIDE

MAGNESIUM

SULFATE

PERCENT

25

75 75

50

75 50

75 25

50

25 75

75 50 25 25 50 75

75

50

25 75

50

25 50

25

50

25

50

75

25

SULFATE PLUS CHLORIDE

CALCIUM PLUS MAGNESIUM

CARBONATE PLUS BICARBONATE

SODIUM PLUS POTASSIUM

CaCl2

NaCl

NaHCO3

Ca(HCO3)

2

CALCIUM CHLORIDE

MAGNESIUM

SULFATE

PERCENT

25

75 75

50

75 50

75 25

50

25 75

75 50 25 25 50 75

75

50

25 75

50

25 50

25

50

25

50

75

25

SULFATE PLUS CHLORIDE

CALCIUM PLUS MAGNESIUM

CARBONATE PLUS BICARBONATE

SODIUM PLUS POTASSIUM

CaCl2

NaCl

NaHCO3

Ca(HCO3)

2

CALCIUM CHLORIDE

MAGNESIUM

SULFATE

PERCENT

25

75 75

50

75 50

75 25

50

25 75

75 50 25 25 50 75

75

50

25 75

50

25 50

25

50

25

50

75

25

15

10

13

14

7

12

9 11

8

14

15 13

8

9

8 11

12

12

13 15

9

11

7, 10

Figure 4B. Piper diagram showing main groundwater constituents and historical data in the Río Jacaguas to 

the Río Descalabrado area, Puerto Rico.

10

7

EXPLANATION

Sample collected during 1960-1965

and well number

Sample collected during 1986-1987

and well number

Sample collected during 2007

and well number

Direction of change of water type

7

5

5

14

1

54

46

40

51

46

53

54

51 53 52 52

20

19

32

21

28

30

27

16

28

20 19

32

27

19 19

28

28

30

20

20

16

1632

32

21 27

56

57

55

55

55

55

55

61

58

59

59

60

57 58

56

59

61 60

55 55

59 61

60

56

58

57

Figure 4E. Piper diagram showing main groundwater constituents and historical data east of the Río Seco area,

Puerto Rico.

26

29 23

28

16

16

31

32

27

30

17

19

18

20

24 25

22

21

25

16 31

19 28

20

18

29 30

23

17,24

22

21

26

26 29

23 28

16

17

20

25

19

18

24

32 21

22 27

31

Figure 4C. Piper diagram showing main groundwater constituents and historical data in the Río Descalabrado

to the Río Jueyes area, Puerto Rico.

38

39

37 49

50

45

54

53 48

46

36 42

41

44

35

34 33

43

51

47

52

54

50

49

39

53

48 46

45

43

3833

47

52

40, 51

35 34

36

42, 44

41

37

50

49

39

38

37 45

46

48, 53

47

42, 43

34 33

40

54

44 51, 52

4136

35

Figure 4D. Piper diagram showing main groundwater constituents and historical data in the Río Jueyes to 

the Río Seco area, Puerto Rico.

2

4 5

3 6

6

1

3

5 5

6

6 2

4

1 2

3

4 5

1

6 6

Figure 4A. Piper diagram showing main groundwater constituents and historical data in the area west of 

the Río Jacaguas, Puerto Rico.

NESIUM

NATE

5

414

10

0.1 1 10 100 1,000

100

1,000

10,000

CHLORIDE, IN MILLIMOLES PER LITER

CHLORIDE/BROMIDE, IN MILLIMOLES PER LITER

Seawater 

38 26 39

49

50

2

29 37

5

23 28

53

24

17

18

19 16

45

45

44

56

6

Freshwater/seawater

mixing line 

10

9

11

55

20

43

60

3

48

47

4 57

52

14 61

7

1

58

13 34

59

31

8

54

21

32

15

42

12 41

40

22 27

30

35

36

46

25

51 33

Figure 6. Chloride-to-bromide ratio as a function of chloride in groundwater from wells in the South Coast 

aquifer, April 2 through May 30, 2007.

EXPLANATION

Data point and well number

Highway

Primary Road

0 2.5 5 10 Kilometers

0 2.5 5 10 Miles

Generalized Geology

EXPLANATION

Guayama

Salinas

Santa Isabel

Ponce

Arroyo

Geology modified from U.S. Geological Survey OFR 98-38. (Bawiec, 2001) 

Roads from the P.R. Department of Transportation.

Lambert conformal conic projection. North American Datum 1983.

53

52

1

54

3

3

2

14

12

10

15

52

53

1

R

15P

ío

Coamo

Río Bucaná

R oí

Portugués

Río

Río

Canal

de Juana

Díaz

Río

Río

Río

Río

í R o

Río

Jacaguas

Cañas

Jueyes

Nigua

Nigua

Patillas

Seco

Descalabrado

Flood channel

Río

Canal

de

Patillas

Guamaní

Lago

Patillas

Lago

Melania

Lago

Coamo

61

60 59

57 58

56

55

54

53

52 51

49, 50

48

47

46

44, 45

43

42

41

40

37, 38, 39

36

35

34

33

32

31

30

29

28

27

25, 26

24

23

22

21

20

19

18

17

16

15

14

13

11 12

10 9

8

6

7

4 5

3 2

1

18°00'00"

17°58'00"

17°56'00"

Quaternary

Undifferentiated surficial deposits (fan delta and alluvial deposits)

Beach deposits

Swamp deposits

Undifferentiated volcanics

Cretaceous

Juana Díaz Formation

Ponce Limestone

Volcanics

Tertiary

Middle

Early

54

3 Caribbean Sea

Figure 2. Sampled well locations and generalized geology in the Ponce to Arroyo area, Puerto Rico.

66°35'00" 66°30'00" 66°25'00" 66°20'00" 66°15'00" 66°10'00" 66°05'00"

Figure 8. Classification of groundwater using dissolved solids concentration in samples from wells in the 

South Coast aquifer, April 2 through May 30, 2007.

100,000

100,000

10,000

10,000

1,000

1,000

100

100

SPECIFIC CONDUCTANCE, IN MICROMHOS PER CENTIMETER AT 25°C

DISSOLVED SOLIDS, IN MILLIGRAMS PER LITER

Seawater

Very Saline

Moderately Saline

Slightly Saline

Fresh

Type of wells

EXPLANATION

Domestic

Monitoring

Industrial

Irrigation

Public supply

Figure 7. Classification of the groundwater collected from irrigation wells in the South 

Coast aquifer, April 2 through May 30, 2007 (irrigation water classification diagram modified

from Richards, 1954).

C1-S4

C1-S3

C1-S2

C1-S1

C2-S4

C2-S3

C2-S2

C2-S1

C3-S4

C3-S3

C3-S2

C3-S1

C4-S4

C4-S3

C4-S2

C4-S1

EXPLANATION

2 Data point and well number

7

3

6 2

15

11

9

20

24

29

17,18,19

30

31

16

40

46

36,42

53

LOW MEDIUM HIGH VERY HIGH

LOW MEDIUM HIGH VERY HIGH

CLASS

SALINITY HAZARD

CONDUCTIVITY-MICROMHOS/CM, AT 25o C

SODIUM (ALKALI) HAZARD

SODIUM - ADSORPTION-RATIO (SAR)

22

24

26

28

30

20

18

16

14

12

10

8

6

4

2

1 2 3 4

100 2 3 4 5 6 7 8 9 1,000 2 3 4 5,000

100 250 750 2250

1 2 3 4

0

0 2.5 5 10 Kilometers

0 2.5 5 10 Miles

66°35'00" 66°30'00" 66°25'00" 66°20'00" 66°15'00" 66°10'00" 66°05'00"

Geology modified from U.S. Geological Survey OFR 98-38. (Bawiec, 2001)

Roads from the P.R. Department of Transportation.

Lambert conformal conic projection. North American Datum 1983.

66°35'00" 66°30'00" 66°25'00" 66°20'00" 66°15'00" 66°10'00" 66°05'00"

Ponce

Juana 

Díaz Santa 

Isabel

CORDILLERA CENTRAL

Guayama

Patillas

Arroyo

Fan-delta and 

alluvial deposits

EXPLANATION

Salinas

Survey area

Figure 1. Location of survey area.

0 10 Kilometers

0 10 Miles

Caribbean Sea

Atlantic Ocean

Sierra de Cayey

Figure 5. Hydrochemical facies in the South Coast aquifer, April 2 through May 30, 2007.

Introduction

 The increased potential for variability of groundwater quality in the South Coast aquifer of Puerto Rico due to 

saline water encroachment from the Caribbean Sea and from deep parts of the aquifer has become a major concern of water 

planners and managers. In an effort to determine the extent and sources of this encroachment, the U.S. Geological Survey 

(USGS) and the Puerto Rico Department of Natural and Environmental Resources conducted a synoptic 

groundwater-quality survey from April 2 through May 30, 2007, for the South Coast aquifer between Ponce and Arroyo (fig. 

1). Groundwater resources in this aquifer extend 150 square miles in south-central Puerto Rico and provide an estimated 44.2 

million gallons per day (Mgal/d) or about 61 percent of the total water needs. This amount includes: 15.3 Mgal/d for 

irrigation, 27.4 Mgal/d for public supply, and 1.5 Mgal/d for industrial and other uses (W.L. Molina-Rivera, U.S. Geological 

Survey, written commun., 2007). Since 1980 when most of the south coastal plain was intensively cultivated for sugarcane, 

total groundwater withdrawals have declined about 32 Mgal/d with the greatest decline occurring in irrigation (37.2 Mgal/d) 

and the greatest increase occurring in public supply (5.5 Mgal/d). Although withdrawals have declined substantially, a major 

concern is that aquifer recharge provided by irrigation return flow from surface-water irrigation canals has essentially 

dropped to zero because of the large-scale implementation of groundwater drip irrigation systems. 

Purpose and Scope

 

 The purpose of this report is to present the assessment of groundwater-quality data obtained during the synoptic 

survey conducted April 2 through May 30, 2007, that can be used by water-resource managers and planners to gain a better 

understanding of aquifer conditions. The data consist of in-situ measurements at active wells or piezometers for specific 

conductance, pH, temperature and acid neutralizing capacity (ANC, formerly referred to as alkalinity, ANC is now used 

exclusively for filtered water samples) and water sample collection and preparation for laboratory analyses of common 

dissolved constituents (ANC, calcium, magnesium, sodium, potassium, sulfate, fluoride, silica) and trace constituents 

(boron, iron, and manganese). These data were used to define the regional distribution of dissolved solids concentration and 

major hydrochemical facies in the aquifer. The data were also compared to historical data collected at several sites in the 

study area.

Hydrogeologic Setting

 The survey area in Puerto Rico is between Ponce and Arroyo and is bound to the north by the foothills of the 

Cordillera Central and the Sierra de Cayey Mountains, to the south by the Caribbean Sea, to the west by the Río Portugués, 

and to the east by the Río Patillas (fig. 1 and 2). The major geologic units in the survey area are also presented on figure 2. 

Groundwater occurs primarily in Quaternary surficial deposits that include fan delta and alluvial sediments. These deposits 

are typically less than 100 feet thick in areas east of the Río Jueyes, but could be as much as 1,000 feet thick near the coast 

in areas to the west of the Río Jueyes (Renken and others, 2002). The surficial deposits between the Río Portugués at Ponce 

and the Río Coamo are underlain by carbonate units of Tertiary age with permeable limestone units that are hydraulically 

connected with the surficial deposits, thus both hydrogeologic units are considered as one aquifer unit in this area. The basal 

part of the aquifer in the Río Portugués to Río Coamo area consists of claystone strata of the underlying Juana Díaz 

Formation (fig. 2). Volcanic rocks of Tertiary and Cretaceous age form the base of the aquifer east of Santa Isabel.

Methods and Techniques

 Groundwater samples were collected once from 50 active wells that include: 23 wells for irrigation use, 19 wells 

for public-supply use, 5 wells for industrial use, and 3 wells for domestic use. In addition, 11 piezometers were pumped 

using a 0.75 horsepower submersible pump to collect samples representative of groundwater at the respective well-screen 

interval open to the aquifer. Water samples were collected during April and May 2007, a period of stable hydrologic 

conditions at the end of the relatively dry season along the south coast. Field measurements were obtained and water samples 

for laboratory analyses were collected and preserved according to USGS protocols (U.S. Geological Survey, variously 

dated). Water samples were analyzed at the USGS National Water Quality Laboratory in Denver, Colorado. The analytical 

results were used to develop a map showing the distribution of dissolved solids in the aquifer consisting of the sum of the 

concentration of cations (calcium, magnesium, sodium, and potassium), anions (sulfate and fluoride), silica, and carbonate. 

Specific conductance measurements and their conversion to dissolved solids concentration values at selected surface-water 

sites are included in figure 3 to present the effects of streams on dissolved solids concentrations in the aquifer.

Hydrochemical facies were used to delineate aquifer areas as to the prevailing water type using the trilinear diagram method 

(Piper, 1944). Predominant end-member hydrochemical facies in the survey area are: sodium bicarbonate [NaHCO3], 

sodium chloride [NaCl], calcium bicarbonate [Ca(HCO3)2], and calcium chloride [CaCl2]. The NaHCO3 results from the 

weathering of plagioclase minerals present in the volcanic rocks along the northern perimeter of most of the coastal plain and 

in areas where the volcanic rocks form the base of the aquifer. The NaCl results from seawater encroachment along the coast 

and Ca(HCO3)2 from freshwater infiltration through soils and surficial deposits. The CaCl2 results from groundwater in the 

limestone rocks that lie along the perimeter of the aquifer or at depth, especially in the western half of the coastal plain. This 

residual sodium chloride in the rock matrix and from aquifer areas contains a mix of freshwater and seawater. Hydrochemical 

facies for inland parts of the survey area were delineated using water-quality information from USGS databases. Because the 

effect of saline intrusion is nonexistent in the inland parts of the survey area, hydrochemical facies are expected to be 

influenced by the composition of the aquifer matrix.

In addition to the use of major ions to define the hydrochemical facies, the chloride-to-bromide mass ratio was used as an 

indicator of freshwater and seawater mixing. The chloride-to-bromide mass ratio has been successfully used to study the 

origin of saline groundwater and brines from relatively deep geologic formations and to characterize atmospheric 

precipitation, shallow groundwater, and domestic sewage (Davis and others, 1998; Panno and others, 2006).

Dissolved Solids and Hydrochemical Facies

 Groundwater sample collection sites are shown on figure 2. Numbers identifying sample collection sites in figure 

2 correspond to well map numbers shown in table 1, which presents the analytical results of the collected samples. Figure 3 

shows the distribution of dissolved solids in the South Coast aquifer for conditions during April–May 2007. Six values from 

samples collected during 2002 in the Ponce area (Rodríguez and others, 2005) were added to figure 3 to delineate the 

dissolved solids concentration contours. Dissolved solids concentrations are lowest in the area east of the Río Jueyes in the 

eastern part of the coastal plain. In this part, dense and relatively impermeable volcanic rocks border the aquifer along the 

north or form its base. Low dissolved solids concentrations occur near stream courses and are indicative of streamflow 

infiltration to the aquifer. To the west of the Río Jueyes, limestone rocks border the aquifer along the north and extend beneath 

the surficial deposits. The limestone is moderately permeable and, unlike the volcanic rocks, is subject to dissolution by 

groundwater flow that increases the concentration of calcium and bicarbonate. In areas where the limestone is constituted by 

the mudstone unit of the Juana Díaz Formation, the limestone can also contribute sodium, chloride, and sulfate from halite and 

anhydrite that are present in small quantities in the rock matrix (Glover, 1971). Throughout the entire aquifer, dissolved solids 

concentrations increase toward the coast. The sources of these increases may be natural causes such as upwelling of 

mineralized deep groundwater to coastal discharge areas, evapotranspiration of groundwater, infiltration of seawater during 

high sea level stages, and the effects of sea aerosols near the coast. Artificial causes that could contribute to an increase in 

dissolved solids concentrations are upconing of more mineralized groundwater by overexploitation of the aquifer, dewatering 

works, and inland movement of the freshwater-saltwater interface from a reduction of groundwater flow toward the coast. The 

analytical results from sampled wells and piezometers given in table 1 were used to prepare the trilinear diagrams presented 

in figures 4A through 4F. The areal distribution of hydrochemical facies in the aquifer are shown in figure 5. 

 A more thorough discussion of the areal variation of dissolved solids concentrations and hydrochemical facies is 

presented by subareas due to the heterogeneous nature of aquifer hydrologic and hydrogeologic conditions. The subareas with 

relatively similar aquifer hydrologic and hydrogeologic conditions are west of the Río Jacaguas, the Río Jacaguas to the Río 

Descalabrado, the Río Descalabrado to the Río Jueyes, the Río Jueyes to the Río Seco, and east of the Río Seco. Historical 

data from different wells collected during 1960 through 1965 and from 1986 through 1987 were used to compare changes in 

hydrochemical facies.

West of the Río Jacaguas

 Dissolved solids concentrations in this area ranged from 324 to 5,860 mg/L (fig. 3). Dissolved solids in well 5 had 

increased by 338 mg/L from 1965 to 2007, and the chemical characteristics indicate a trend of increasing sodium and chloride 

concentrations (fig. 4A). Water-quality data from well 6 showed a trend towards the calcium-chloride water type (fig. 4A) and 

a decrease in dissolved solids concentration of 61 mg/L from 1986 (Gómez-Gómez, 1991) to 2007. The principal 

hydrochemical facies west of the Río Jacaguas is the calcium-chloride water type (figs. 4A and 5). This water type is 

associated with the mixture of saline and freshwater, and with geochemical facies of the Juana Díaz Formation (Rodríguez and 

others, 2005). Results from water samples collected at well 3 indicate a water type of sodium-chloride (fig. 4A), which is an 

indicator of saline water intrusion (Rodríguez and others, 2005).

The Río Jacaguas to the Río Descalabrado

 Dissolved solids concentrations ranged from 330 to 472 mg/L (fig. 3). The lower dissolved solid concentrations to 

the east of the Río Jacaguas are indicative of streamflow infiltration into the aquifer. Historical groundwater-quality data 

(Gómez-Gómez, 1991) indicated no change in water type (calcium-bicarbonate) in well 14 (fig. 4B) and a dissolved solids 

concentration decrease of 22 mg/L from 1986 to 2007. The principal hydrochemical facies is calcium-bicarbonate (figs. 4B

and 5) in the Río Jacaguas to the Río Descalabrado area.

The Río Descalabrado to the Río Jueyes

 Historical data from well 16 indicated a change from sodium-bicarbonate to sodium-chloride water type (fig. 4C) and 

an increase of 586 mg/L in dissolved solids concentration from 1986 to 2007 (Gómez-Gómez, 1991). This increase was 

probably caused by the upconing of groundwater from deeper parts of the aquifer. The historic trend of chemical 

characteristics in well 28 indicates a decrease of 144 mg/L in dissolved solids concentration and a change from 

calcium-bicarbonate to calcium-chloride water type from 1986 to 2007 (fig. 4C). Analytical results from well 19 indicated a 

trend of potential change from calcium-bicarbonate to calcium-chloride water type (figs. 4C and 5). The principal 

hydrochemical facies in this area is calcium-bicarbonate based on analytical results from the wells east of Río Coamo. These 

results indicate that the aquifer is affected by this stream (figs. 4C and 5). Sodium-chloride water type is present in well 16, 

which is located near an outcrop of the Ponce Limestone and the Juana Díaz Formation. The Juana Díaz Formation can 

contain evaporates and other residues of marine origin (Giusti, 1971), thus causing the chemical characteristics measured in 

well 16. Well 17 plots on the boundary between calcium-bicarbonate and calcium-chloride water types. The calcium-chloride 

water type was present in wells 23, 28, and 29. 

The Río Jueyes to the Río Seco

 Dissolved solids concentrations in this area ranged from 492 to 19,900 mg/L (fig. 3). Wells 37 (936 mg/L), 38 

(4,260 mg/L), and 39 (19,900 mg/L) were drilled to 50, 115, and 225 feet, respectively (table 1). These results are evidence 

of the mixing of freshwater and saline water related to depth in the aquifer. Dissolved solids concentrations increased in wells 

43 (270 mg/L), 51 (70 mg/L), 52 (151 mg/L), and 53 (130 mg/L) from 1986 to 2007 (Gómez-Gómez, 1991). The area where 

these wells are located is classified as having the potential of changing its water type from calcium-bicarbonate to 

calcium-chloride or sodium-chloride (fig. 5). The dissolved solids 800-contour-line delineated inland near wells 43 and 49 

coincides with a buried bedrock-high inferred by Renken and others (2002). In the Río Jueyes to the Río Seco area, the 

principal water type is calcium-bicarbonate (figs. 4D and 5). Calcium-chloride water type was detected in wells 37, 38, 45, 

and 49 located near the coastline in the Salinas area (fig. 5). Well 39 is on the boundary between the calcium-chloride and 

sodium-chloride types and well 50 indicates a sodium-chloride water type. Well 54 showed sodium-bicarbonate water type 

that is associated with inter-fan areas. Lower dissolved solids concentrations distribution east of the Río Nigua indicates that 

the aquifer is affected by streamflow infiltration. 

East of the Río Seco

 Dissolved solids concentrations in this area ranged from 267 to 744 mg/L (fig. 3). Dissolved solids concentration 

in well 55 increased from 320 to 393 mg/L from 1986 to 2007. The principal water type in this area is the 

calcium-bicarbonate (figs. 4E and 5). Well 56 plots on the boundary of calcium-sodium bicarbonate type water (fig. 4E). 

Chloride-to-Bromide Ratio in Groundwater

 Figure 6 shows the chloride-to-bromide ratio on a mole per mole basis in relation to chloride showing the theoretical 

freshwater-saltwater mixing line. The molar chloride-to-bromide ratio of seawater is approximately 640, based on chloride 

and bromide concentrations of 19,000 and 67 mg/L, respectively (Hem, 1989), which is similar to low chloride freshwater 

from wells located near the streams flowing through the coastal plain. Groundwater mixing between these endpoint sources 

will yield a proportional value to each that plots along the freshwater-seawater mixing line (Land and others, 2004). 

Groundwater with higher chloride concentration (above 250 mg/L) from wells 2, 3, 5, 23, 26, 28, 29, 37 to 39, 45, 49, and 50 

reflects the mixing with seawater. Wells 16 to 19 and 24 in the Santa Isabel area and wells 44 and 45 in the Salinas area 

showed a low chloride-to-bromide ratio (less than 300) indicative of bromide enrichment relative to chloride. Bromide 

enrichment is associated with brine sources (Morell and others, 1986) and the use of pesticides and their application to 

agricultural fields (Flury and Papritz, 1993; Vengosh and Pankatov, 1998; Custodio and Alcalá-García, 2003). 

Classification of Groundwater Quality

 Groundwater samples collected from irrigation wells were classified (fig.7) using the U.S. Department of 

Agriculture (USDA) irrigation water classification diagram (Richards, 1954). Water is classified as being a low, medium, 

high, or extremely high sodium and salinity hazard, using the sodium adsorption ratio (SAR) and the specific conductance. 

Most of the samples from wells used for irrigation were in the classification of low sodium hazard and medium to high 

salinity hazard. High salinity hazard water can be used for irrigation only in soils with moderate to good permeability. Wells 

2, 3, and 29 were classified as very high salinity hazard water. Water in this classification is generally undesirable for 

irrigation and is only suitable for occasional use on soils of good or high permeability (Richards, 1954). During the sample 

collection period, users of groundwater from well 29 attributed the adverse effects to their crops on the quality of the 

irrigation water.

All groundwater samples were classified into four categories based on the dissolved solids concentrations and the specific 

conductance: fresh, slightly saline, moderately saline, and very saline (fig. 8; Robinove and others, 1958; Díaz, 1974). All 

samples from public-supply, domestic, and industrial wells were in the freshwater category. Seven samples from 

public-supply wells had dissolved solids concentrations above 500 mg/L, which is the drinking-water secondary maximum 

contaminant level (SMCL) for dissolved solids (U.S. Environmental Protection Agency, 2003). Most samples from irrigation 

wells were classified as freshwater; however, four wells plotted in the slightly saline category (wells 2, 3, 16, and 29). 

Samples from piezometers near the coast in Santa Isabel and Salinas (wells 38, 39, 45, 49, and 50) were in the slightly (wells 

45 and 50), moderately saline (26, 38, and 49), and very saline (well 39) water categories.

Conclusions

A synoptic groundwater-quality survey was conducted by the U.S. Geological Survey in the South Coast aquifer 

from Ponce to Arroyo in south-central Puerto Rico from April 2 through May 30, 2007. The data obtained consisted of in-situ 

measurements at active wells or piezometers for physical properties and water sample collection for laboratory analyses of 

common dissolved constituents and trace constituents. A total of 61 water samples were collected. These data were used to 

define the regional distribution of dissolved solids concentration and major hydrochemical facies in the aquifer system. 

Dissolved solids concentrations ranged from 324 to about 19,900 mg/L. In general, dissolved solids concentrations below 500 

mg/L were detected near streams and concentrations above 800 mg/L matched with areas of potential mixing of freshwater 

and seawater. The principal hydrochemical facies in the study area is calcium-bicarbonate, which is associated with the 

freshwater infiltration through soils and surficial deposits. Calcium-chloride water type, which is associated with the mixture 

of saline and freshwater, has been detected in the Ponce, Santa Isabel, and Salinas areas. Analyses of samples collected at a 

well in the Ponce area indicate that the water is a sodium-chloride type associated with the intrusion of saline water. 

Sodium-bicarbonate water, which is associated with the weathering of minerals present in volcanic rocks, was detected in the 

inter-alluvial-fan areas near Santa Isabel and Salinas. Hydrochemical facies delineated in this survey are similar to those 

defined by a USGS study conducted in 1986. Compared to the 1986 conditions, changes were observed in the Ponce and 

Santa Isabel area, where sodium-chloride and calcium-chloride facies were detected, respectively. The chloride-to-bromide 

ratio in groundwater samples is evidence that mixing of freshwater and seawater is occurring in areas near the coast in Ponce, 

Santa Isabel, and Salinas. Some wells in the Santa Isabel and Salinas areas showed a low chloride-to-bromide ratio, which is 

indicative of bromide enrichment relative to chloride.

 Samples from irrigation wells were classified into categories of low to high sodium hazard and categories of medium 

to very high salinity hazard using the USDA irrigation water classification. Collected samples were classified based on the 

dissolved solids concentrations; all samples from public-supply, domestic, and industrial wells were in the freshwater 

classification. Samples collected from USGS piezometers were classified in the fresh, slightly saline, moderately saline, and 

very saline water categories.

 Initiating a salinity monitoring program would help resource managers evaluate further increases in dissolved solids 

concentrations near the coast of Ponce, Santa Isabel, and Salinas. A monitoring program could include existing piezometers 

and new piezometers drilled in areas where potential mixing of fresh and seawater was detected. Periodic sampling of the 

piezometers for dissolved solids could be used to determine the existence of any increased trends. 

 Base flow needs to be maintained in streams that traverse the coastal plain because of the effect that base flow has in 

maintaining low dissolved solids concentrations in the aquifer. Reduction of base flow resulting from surface-water diversions 

from these streams could contribute to the encroachment of saline water in the aquifer. The relation of surface water and 

groundwater needs to be evaluated to determine the amount of recharge to the aquifer from these streams.

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