U.S. Department of the Interior

Storage Capacity and Water Quality of 

Lake Ngardok, Babeldaob Island, 

Republic of Palau, 1996–98

U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 99-4118

Prepared in cooperation with the

REPUBLIC OF PALAU

PHILIPPINE SEA

PACIFIC

OCEAN

Lake

Ngardok

BABELDAOB
Storage Capacity and Water Quality of

Lake Ngardok, Babeldaob Island, Republic of

Palau, 1996–98

By Chiu Wang Yeung and Michael F. Wong

U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 99-4118

Prepared in cooperation with the

REPUBLIC OF PALAU

Honolulu, Hawaii

1999
U.S. DEPARTMENT OF THE INTERIOR

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

The use of firm, trade, and brand names in this report is for identification purposes

only and does not constitute endorsement by the U.S. Geological Survey.

For additional information write to: Copies of this report can be purchased

from:

District Chief U.S. Geological Survey

U.S. Geological Survey Branch of Information Services

677 Ala Moana Blvd., Suite 415 Box 25286

Honolulu, HI 96813 Denver, CO 80225-0286

U. S. DEPARTMENT OF THE INTERIO

R

MARCH 3, 1849
Contents iii

CONTENTS

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

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

Purpose and Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Previous Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

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

Data-Collection Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Storage Capacity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Water Quality. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

References Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

FIGURES

1–4. Maps showing:

1. The island of Babeldaob, Palau Islands, Western Pacific Ocean. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2. Cross sections and water-quality sampling sites, Lake Ngardok,

Palau, 1996. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

3. Bathymetry of Lake Ngardok, Palau, 1996 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

4. Bed elevation at cross sections, Lake Ngardok, Palau, 1996. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

5. Elevation/surface-area and elevation/capacity curves for Lake Ngardok, Palau, 1996 . . . . . . . . . . . . . . . . . . . 8

6. Rainfall and lake elevation at Lake Ngardok, Palau, from July 1996 to July 1997 . . . . . . . . . . . . . . . . . . . . . . 12

TABLES

1. Daily mean lake elevation at Lake Ngardok near Melekeok, Palau . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2. Elevation/surface-area and elevation/storage-capacity for Lake Ngardok near Melekeok,

Palau, 1996 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3. Daily total rainfall, in inches, from Ngardok rain gage near Melekeok, Palau. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

4. Temperature, dissolved oxygen, and specific conductance at selected cross sections,

Lake Ngardok, Palau, 1996. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

5. Water-quality results at outflow station on April 3, 1996 and November 10, 1997,

Lake Ngardok, Palau. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

6. Pesticides, herbicides, and other volatile organic constituents found to be less than

detection limits in water samples from outflow station on April 3, 1996 and

November 10, 1997, Lake Ngardok, Palau. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

7. Data from AECOS Inc. station 20, 1.4 miles downstream of Lake Ngardok

outflow, Palau . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
iv Storage Capacity and Water Quality of Lake Ngardok, Babeldaob Island, Republic of Palau, 1996–98

CONVERSION FACTORS

Water temperature is given in degree Celsius (°C), which can be converted to degrees Fahrenheit (°F) by the following

equation:

Temp.°F = (1.8 temp. × °C) + 32.

Air temperature is given by degrees Fahrenheit (°F), which can be converted to degrees Celsius (°C)

by the following equation:

Temp.°C = (temp. °F – 32) / 1.8.

Chemical concentrations are given only in metric units. These are milligrams per liter (mg/L), micrograms per liter (μg/L), or

picograms per liter (pg/L). Milligrams per liter is a unit expressing the solute mass per unit volume (liter) of water.

One thousand micrograms per liter is equivalent to 1 milligram per liter. One million picograms per liter is equivalent

to 1 microgram per liter. Specific conductance is given in microsiemens per centimeter (μS/cm) at 25°C. Turbidity

is given in NTU, nephelometric turbidity units.

Multiply By To obtain

acre 4,047 square meter

acre-foot (acre-ft) 1,233 cubic meter

foot (ft) 0.3048 meter

inch (in.) 2.54 centimeter

square mile (mi2) 2.590 square kilometer
Introduction 1

Abstract

A bathymetric survey conducted during March

and April, 1996, determined the total storage

capacity Lake Ngardok to be between 90 and 168

acre-feet. Elevation-surface area and elevationcapacity curves summarizing the current relations

among elevation, surface area, and storage capacity

were created from the bathymetric map. Rainfall

and lake-elevation data collected from April 1996

to March 1998 indicated that lake levels correlated

to rainfall values with lake elevation rising rapidly

in response to heavy rainfall and then returning to

normal levels within a few days. Mean lake elevation for the 22 month period of data was 59.5 feet

which gives a mean storage capacity of 107 acrefeet and a mean surface area of 24.1 acre. A floating mat of reeds, which covered 58 percent of the

lake surface area at the time of the bathymetric survey, makes true storage capacity difficult to estimate.

Water-quality sampling during April 1996 and

November 1997 indicated that no U.S. Environmental Protection Agency primary drinking-water

standards were violated for analyzed organic and

inorganic compounds and radionuclides. With suitable biological treatment, the lake water could be

used for drinking-water purposes. Temperature and

dissolved oxygen measurements indicated that

Lake Ngardok is stratified. Given that air temperature on Palau exhibits little seasonal variation, it is

likely that this pattern of stratification is persistent.

As a result, complete mixing of the lake is probably

rare. Near anaerobic conditions exist at the lake

bottom. Low dissolved oxygen (3.2 milligrams per

liter) measured at the outflow indicated that water

flowing past the outflow was from the deep oxygen-depleted depths of the lake.

INTRODUCTION

To alleviate overcrowding in Koror, the current

capital of the Republic of Palau, the Palau government

passed a resolution to relocate its capital to Melekeok

on Babeldaob Island, the largest of the Palau Islands.

Melekeok is on the east coast of Babeldaob and a

nearby freshwater body, Lake Ngardok, was proposed

as the major source of domestic water supply for the

new capital and nearby villages. To evaluate the storage

capacity and water quality of Lake Ngardok, and to

learn more about freshwater lakes in tropical environments, the Republic of Palau entered into a cooperative

agreement with the U.S. Geological Survey (USGS).

Purpose and Scope

This report describes storage capacity and water

quality of Lake Ngardok. A bathymetric survey of Lake

Ngardok was done to estimate storage capacity in relation to lake water-surface elevation. Samples were collected in late March and early April 1996, and

November 1997, from the lake outflow for water-quality analyses. At selected lake cross sections, samples

were collected at depth for analysis of water-quality

properties and constituents.

Storage Capacity and Water Quality of Lake Ngardok,

Babeldaob Island, Republic of Palau, 1996–98

By Chiu Wang Yeung and Michael F. Wong
2 Storage Capacity and Water Quality of Lake Ngardok, Babeldaob Island, Republic of Palau, 1996–98

Previous Studies

No previous studies on the storage capacity and

water quality of Lake Ngardok exist, although various

estimates have been made. The U.S. Army Chief of

Engineers (1956) estimated Lake Ngardok to be 2,400

ft long, 600 ft wide at the widest part, as much as 9 ft in

depth, and with a capacity of 46 acre-ft. Van der Brug

(1984) used these estimates in his water-resource investigation of the Palau Island. AECOS Inc. (1997) estimated the lake area to be 16 acres and the capacity to be

46 acre-ft also. The AECOS Inc. (1997) study also

included water-quality data for a sampling point 1.4 mi

downstream from the Lake Ngardok outlet.

Acknowledgments

Special acknowledgment is made to Patrick Colin

of the Coral Reef Research Foundation for providing

the aerial photographs of Lake Ngardok.

DESCRIPTION OF STUDY AREA

The Republic of Palau consists of a chain of

islands in the western Pacific Ocean, east of the Philippine Sea, between latitude 6°53′ and 8°12′ north and

longitude 134°07′ and 134°39′ east (fig. 1). Babeldaob

Island is near the northern end of the island chain and,

2 with an area of 153 mi , is the largest of the Palau

Islands. Koror, the current administrative, commercial,

and population center of Palau, lies directly south of

Babeldaob Island, from which it is separated by a narrow channel (fig. 1).

Lake Ngardok is east of Rael Kedam, the central

ridge of Babeldaob, less than 2 mi northwest of Melekeok. The lake is part of the headwaters of the North

Fork of Ngerdorch River (fig. 1). Ngerdorch River, with

2 a drainage area of almost 18 mi , is the largest perennial

stream in Palau (van der Brug, 1984).

The reference marker at USGS lake-stage gage

16891425 (fig. 2) , near the outlet of Lake Ngardok, was

estimated to be at an elevation of 70 ft above mean sea

level from the 1:25,000-scale USGS Ngermetengel

topographic map (1983). The drainage area upstream of

2 the outlet is 1.66 mi and the highest elevation in the

drainage area is 590 ft, near Mount Ometochel (fig. 1).

The main channel length from the headwaters to the

outlet is about 2.22 mi with an average slope of 0.014

ft/ft.

Climate.--The Palau Islands have a uniform tropical warm and moist climate throughout the year. Seasonal changes are minimal. At the long-term weather

station on Koror Island, the mean temperature is 81.7°F

(National Oceanic and Atmospheric Administration,

1995). Mean temperature for the coolest months (January and February) is 81.0°F and for the warmest months

(April, May, and November) is 82.2°F (National Oceanic and Atmospheric Administration, 1995). Average

annual rainfall is 148 in. (National Oceanic and Atmospheric Administration, 1995). Rainfall is variable from

month to month and is usually heaviest in June and July

(National Oceanic and Atmospheric Administration,

1995).

Rainfall data for Babeldaob are available for short

periods from scattered locations. Van der Brug (1984)

determined that climatic descriptions for Koror are representative of conditions at Babeldaob. Comparison of

the available rainfall records for Babeldaob with those

for Koror showed no significant difference in rainfall

totals. Nance (1986) estimated that rainfall across

Babeldaob was between 90 and 110 percent of the rainfall at Koror and that rainfall in the region of Lake Ngardok is 90 percent that of Koror.

Geology.--Babeldaob is a volcanic island formed

primarily by the accumulation of volcanic breccia and

interbedded tuff which form three geologic formations:

the Babelthuap Formation, the Aimeliik Formation, and

the Ngeremlengui Formation (U.S. Army Chief of

Engineers, 1956). The general features of the island

include rolling uplands, rugged areas of sharp ridges

and cliffs, flat alluvial areas, and irregular coastlines

bounded by dense mangrove swamps (van der Brug,

1984).

About 76 percent of the total drainage of Lake

Ngardok is underlain by the Aimeliik Formation. The

remainder of the drainage area is underlain by the

Babelthuap Formation (20 percent) and alluvium (4 percent). Percentages were determined by geographic

information systems (GIS) analysis of a 1:62,500-scale

geologic map (U.S. Army Chief of Engineers, 1956).

The Aimeliik Formation is Eocene in age (38 to 58 million years old) and consists of andesitic-basaltic volcanic breccias and tuff. The upper layers are deeply

weathered to depths up to 10 ft, with weathered breccia

covering 58 percent and weathered tuff covering

18 percent of the drainage basin (U.S. Army Chief of

Engineers, 1956). The Babelthuap Formation is also
Q

Mount

Ometochel

Rael

Kadem

134 ̊25' 134 ̊35' 134 ̊40' 134 ̊30'

7 ̊40'

7 ̊45'

7 ̊35'

7 ̊30'

7 ̊25'

7 ̊20'

PHILIPPINE

SEA

PACIFIC

OCEAN

Ngerchelong

Peninsula

Ngerchelong Pt

Ngeremlengui Pt

0 2 4 MILES

0 2 4 KILOMETERS

Ngemai

Bay

Lake

Ngardok

Ngerubesang

Oikull

Ngermetengel

A

A

A

North Fork Ngerdorch River

Figure 1. The island of Babeldaob, Palau Islands, western Pacific Ocean.

BABELDAOB

Koror

Melekeok

DRAINAGE DIVIDE

RAIN GAGE

EXPLANATION

1

1

120 ̊ 140 ̊ 160 ̊E 160 ̊W 180 ̊

0 ̊

20 ̊

S

20 ̊

N

40 ̊

Philippines

Australia

Japan

Palau Islands

Hawaii

Wake

Guam

Samoa

LOCATION MAP

PACIFIC OCEAN

0 1,000 MI

0 1,000 KM

Tokyo

Honolulu

Manila

Description of Study Area 3
     



134°35'52" 134°36' 134°36'07"

7°30'45"

7°30'37"

0 200 400 FEET

0 40 80 METERS

Figure 2. Cross sections and water-quality sampling sites, Lake Ngardok, Palau, 1996.

CROSS SECTION AND NUMBER

WATER-QUALITY SAMPLING POINT

REED MAT

EXPLANATION

17

A

A'

B

C

D

E

E' T'

T F'

G'

H'

I'

F

G

C'

D'

B'

Inflow

Outflow

Lake-stage

gage

16891425

I

H

J

L

M

N

N N'

O

P

Q

L'

K'

M'

N'

O'

P'

Q'

R'

S'

K

J'

S

R

4 Storage Capacity and Water Quality of Lake Ngardok, Babeldaob Island, Republic of Palau, 1996–98
Data-Collection Methods 5

Eocene in age and consists of massive basaltic-andesitic

volcanic breccia with tuff breccia and a layer of interbedded tuff. This formation is found in the higher elevations of the drainage basin along the Rael Kedam

around Mount Ometochel and is not as deeply weathered as the rocks of Aimeliik Formation. Alluvium is

found in the floodplain of the Ngerdorch River. Lake

Ngardok was created by a natural dam formed of alluvial deposits from a small tributary (U.S. Army Chief of

Engineers, 1956, p. 192). The source of the deposits was

a large quantity of clay eroded during heavy rains (U.S.

Army Chief of Engineers, 1956,

p. 146).

Soils.--Soils in the study area are derived from the

weathering of the volcanic rocks. The resultant red soil

is highly erodible, acidic, high in iron and aluminum

oxides, and low in silica. On the basis of terminology

described by Smith (1983), the principal soil types in

the Lake Ngardok basin were determined to be the

Aimeliik-Palau complex (42 percent) and the Ngardok

silt loam (48 percent). Along the ridges of the Rael

Kedam, soils of the Babelthaup-Ngardmau complex (6

percent) exist. These three soils all have moderate permeability. The remaining soil (4 percent), found along

the stream valleys, is poorly drained muck classified as

Dechel-Mesei complex. This soil underlies Lake Ngardok and on the basis of field observations is composed

of very fine silt and clay. Percentages were determined

by geographic information systems (GIS) analysis of a

1:62,500-scale soil map (U.S. Army Chief of Engineers,

1956) which classified the soils on the basis of a 1938

system of soil classification.

Biota.--Vegetation surrounding the lake consists

of a mixture of tropical forest and open area with scrub

vegetation and ferns. The shoreline of Lake Ngardok is

completely surrounded by freshwater marshes. These

marshes have floating vegetated mats of Hanguana

malayana, a reed-like plant. During fieldwork, a large

section of mat was observed to have separated from one

shoreline edge and floated freely on the lake. The lake

is largely undisturbed and supports aquatic plants,

freshwater fishes, marine crocodiles, and several roosting and nesting waterbirds (AECOS, 1997). The marine

crocodile, Crocodylus porosus, is listed as an endangered species and the upper reach of the Ngerdorch

River around Lake Ngardok is one of the few remaining

breeding areas of the crocodile in Palau (AECOS Inc.,

1997).

DATA-COLLECTION METHODS

Fieldwork, which included the bathymetric survey

and water-quality sampling, was done in late March and

early April 1996 and in November 1997. The bathymetric survey was carried out by crossing the lake with a

total of 19 cross sections (fig. 2). The reference mark at

gage 16891425 was used as a reference datum for the

survey. Because the datum was estimated from a topographic map, the absolute accuracy is 20 ft. The first

surveyed cross section endpoint was referenced to the

reference mark and the location and elevation of each

subsequent cross section endpoint was referenced to the

preceding endpoint. A tagline was placed between endpoints for recording distance increments. Depth readings were recorded along each cross section at about a

10 ft interval from one shoreline to the other. Cross sections J–J′, K–K′, and L–L′ did not completely extend

across the lake because crocodiles were observed at

those locations.

Water depths were measured along the cross sections from a boat using a modified surveying rod with a

precision of 0.1 ft. The modification involved the addition of a perforated 8-inch diameter wood base attached

to the bottom of the rod to prevent the rod from sinking

into soft mud. The thickness of the base was taken into

account during the fabrication of the rod, so all readings

were direct. Depth was recorded from water surface to

the lake bottom. Elevation readings along the cross sections, above the water surface, were measured with

standard leveling equipment with a precision of 0.01 ft.

On the basis of the surveyed cross sections, storage

capacity and surface area of the lake were computed by

using a geographic information system (GIS) computer

program. First, the survey data points were converted to

a format of northings, eastings, and ground elevations

using the coordinates of the cross section endpoints, the

elevation of the water surface, measurements of distance between monuments, and measured depths. Then,

using GIS, cross sections were plotted as shown in figure 2 and manually interpolated contours were drawn to

create the bathymetric map of Lake Ngardok (fig. 3).

The contour interval was variable: a 2-foot interval was

used on the lake region of high relief and a smaller contour interval was used on regions of low relief. The

shoreline was entered into GIS from recent aerial photographs of the lake (Patrick Colin, Coral Reef Research

Foundation, written commun., 1997) to provide additional data in shaping the contour lines. Storage capac-
134°35'52" 134°36' 134°36'07"

7°30'45"

7°30'37"

0 200 400 FEET

0 40 80 METERS

Figure 3. Bathymetry of Lake Ngardok, Palau, 1996.

BATHYMETRIC CONTOUR--Interval, in feet, 

 is variable. Datum is mean sea level. Shoreline 

 is at 59.5 feet elevation

EXPLANATION

Outflow

Inflow

56 Lake-stage

gage

16891425

50

58

46

44

46

48

52

62

6467.5

59

58

56

52

62 64 67.5

59

58

56

48

49

52

52

59

62

64

67.5

56

58

59

62 6467.5

58 56

54

54

50

49

57

6 Storage Capacity and Water Quality of Lake Ngardok, Babeldaob Island, Republic of Palau, 1996–98
Storage Capacity 7

ity and surface area at 0.1 foot increments were

computed from the contour lines by a triangulated irregular network (TIN) numeric analysis in the GIS computer program.

Field measurements of temperature, specific conductance, and dissolved oxygen were collected at various depths at a location in the deep-water part of five

selected cross sections on April 5 and 7, 1996 (figs. 2

and 4 [fig. 4 at end of report]). For depths of less than

10 ft, measurements were made by submerging the

instrument probe. Field measurements for depths

greater than 10 ft were made in a bottle used to collect

the deep water samples. Field measurements and waterquality samples also were collected at the lake outlet on

April 3, 1996 and November 10, 1997 (fig. 2). Waterquality samples collected at the center of the outflow

were analyzed for concentrations of major ions, nutrients, trace metals, pesticides, herbicides, volatile

organic compounds, and radiation and radionuclides,

following U.S. Environmental Protection Agency

(USEPA) Safe Drinking Water Act methods (U.S.

Environmental Protection Agency, 1996).

STORAGE CAPACITY

Lake Ngardok is estimated to be about 2,400 ft in

length between the inflow and outflow points shown on

figure 3 and to have maximum width of about 700 ft.

The greatest water depth measured in the lake was 16.9

ft and the mean of all 751 depth measurements was 4.8

ft. The measured depth readings and above-water

ground elevations were originally referenced to an arbitrary datum. However, instead of relating the capacity

and surface area to an arbitrary datum, elevation readings were tied in to the datum associated with the USGS

lake-stage gage 16891425, Lake Ngardok near Melekeok. The gage datum is based on the elevation of the

reference marker at the gage that was estimated to be 70

ft above mean sea level from the 1:25,000-scale USGS

Ngermetengel topographic map (1983).

Using GIS, surface area and storage capacity were

calculated from results of the bathymetric survey (fig.

3). Lake-elevation data collected from April 3, 1996 to

March 3, 1998 are shown in table 1. Mean elevation for

the 22 complete months covered by this period was 59.5

ft. The storage capacity of the lake at this water-surface

elevation of 59.5 ft is 107 acre-ft with a corresponding

surface area of 24.1 acres. Lake elevation-surface area

and lake elevation-capacity curves are shown in figure

5 and presented numerically in table 2.

According to the lake-elevation data, daily mean

lake elevations fluctuated from 58.71 to 61.88 ft during

the study period. The fluctuations were generally in

response to rainfall as shown by the rainfall data from

rain gage 073037134361570 (Ngerdok rain gage near

Melekeok) (table 3). Graphical comparison of rainfall

and lake elevation (fig. 6) showed that high daily rainfall totals corresponded to days with high daily mean

lake elevation. Lake elevation returned to a mean level

within a few days after a heavy rain because of the natural outflow from the lake (tables 1 and 3).

Error analysis.--Three sources of error affected

the storage-capacity determination: survey error, digital

conversion error, and the floating reed mats. A discrepancy was found between the cross section lengths measured in the field using a tagline and the cross section

lengths computed as the distance between mapped locations of the land survey endpoints. Differences between

the two sets of distance readings ranged from 0 to 30

percent and averaged 13 percent. The error was corrected by arithmetically adjusting the computed cross

section lengths to conform with the measured tagline

lengths. This adjustment was justified by comparing

measured tagline lengths with the lake shoreline as

established from a recent aerial photo (Patrick Colin,

Coral Reef Research Foundation, written commun.,

1997). This comparison showed that the measured

tagline lengths of the cross sections corresponded well

with shore-to-shore distances measured on the aerial

photo. Using the aerial photo shoreline as the most

accurate source, cross sections that had tagline to landsurvey distances differences of greater than 5 percent

were shifted to match the shoreline indicated by the

aerial photographs. The irregular shape of the shoreline

aided in the identification of the approximate cross section endpoints. Assessment of the error associated with

the cross section adjustments was not possible.

Digitizing the shoreline from the aerial photographs also contributed error to the accuracy of the

computed lake area and volume. It was not possible to

rectify the aerial photoraphs for distortion before digitizing. On the basis of the digitizer error, the accuracy

of the computed surface area was within a reasonable

range of ±3 percent from measured data. The digitizer

error for the volume computation was ±5 percent.
8 Storage Capacity and Water Quality of Lake Ngardok, Babeldaob Island, Republic of Palau, 1996–98

Table 1. Daily mean lake elevation at Lake Ngardok near Melekeok, Palau

[Values are in feet above mean sea level; U.S. Geological Survey gage number 16891425; --, no data or not applicable]

1996 1997

DAY Apr May June July Aug Sep Oct Nov Dec Jan Feb Mar

1 -- 59.72 59.46 59.64 60.05 59.11 59.92 59.69 59.19 59.53 59.52 60.78

2 -- 59.66 59.75 59.59 60.05 59.10 59.79 59.94 59.18 59.45 59.46 60.44

3 61.60 59.54 59.57 59.50 60.00 59.09 59.64 59.79 59.16 59.38 59.33 60.25

4 60.71 59.45 59.42 59.92 59.92 59.10 59.78 60.60 59.15 59.39 59.24 60.21

5 60.19 59.39 59.37 59.80 59.88 59.35 59.64 60.42 59.13 59.35 59.41 60.79

6 59.96 59.50 59.48 59.62 59.72 59.62 59.50 60.53 59.11 59.28 59.35 60.82

7 59.83 59.93 59.61 59.70 59.64 60.24 59.41 60.33 59.10 59.24 59.40 60.39

8 59.72 59.82 60.02 59.65 59.66 60.48 59.34 60.04 59.09 59.22 59.31 60.15

9 59.71 59.73 59.74 59.54 59.72 60.23 59.32 59.94 59.08 59.22 59.24 60.07

10 59.69 60.23 60.00 59.44 59.76 59.94 59.33 59.82 59.07 59.22 59.31 59.99

11 59.59 60.18 59.93 60.29 59.58 59.68 59.37 59.67 59.59 59.22 59.60 59.82

12 59.76 60.02 59.90 60.25 59.47 59.49 59.37 59.61 59.66 59.22 60.48 59.71

13 60.00 59.85 59.82 59.91 59.39 59.36 59.38 59.64 59.35 59.21 61.16 59.65

14 59.75 59.73 59.66 59.72 59.35 59.28 59.30 59.77 59.20 59.21 61.78 59.60

15 59.62 59.64 59.55 59.79 59.43 59.21 59.45 59.61 59.15 59.21 61.17 59.52

16 59.51 59.58 59.54 59.76 59.33 59.18 59.51 59.49 59.14 59.20 61.31 59.44

17 59.43 59.55 59.59 59.73 59.25 59.18 59.45 59.42 59.14 59.19 61.30 59.37

18 59.37 59.53 59.49 59.77 59.21 59.18 59.45 59.35 59.13 59.17 61.40 59.31

19 59.54 59.97 59.83 59.94 59.18 59.18 59.48 59.29 59.12 59.16 61.85 59.27

20 59.51 60.09 59.74 60.05 59.19 59.18 59.39 59.25 59.22 59.15 61.14 59.26

21 59.54 59.80 59.57 60.54 59.22 59.20 59.29 59.26 60.29 59.14 60.69 59.26

22 59.51 59.65 59.72 60.60 59.18 59.90 59.25 59.24 60.04 59.14 60.36 59.26

23 59.54 59.57 59.97 60.23 59.18 60.44 59.30 59.23 60.00 59.13 60.12 59.26

24 59.55 59.54 59.82 60.18 59.18 60.20 59.48 59.23 59.89 59.12 59.97 59.25

25 60.07 59.51 59.62 60.24 59.19 60.15 59.30 59.23 61.20 59.52 61.36 59.25

26 60.10 59.45 59.77 60.02 59.18 60.33 59.26 59.22 61.44 60.04 61.69 59.23

27 59.81 59.38 59.95 59.99 59.16 60.87 59.31 59.22 60.61 59.81 61.82 59.22

28 59.65 59.33 59.75 60.94 59.15 60.68 59.30 59.22 60.19 59.59 61.26 59.21

29 59.63 59.43 59.88 61.04 59.14 60.20 59.23 59.20 59.94 59.44 -- 59.20

30 59.67 59.45 59.78 60.47 59.13 59.95 59.39 59.19 59.76 59.26 -- 59.19

31 -- 59.36 -- 60.20 59.11 -- 59.49 -- 59.63 59.38 -- 59.18

Mean -- 59.66 59.71 60.00 59.44 59.70 59.43 59.61 59.58 59.32 60.43 59.69

Max -- 60.23 60.02 61.04 60.05 60.87 59.92 60.60 61.44 60.04 61.85 60.82

Min -- 59.33 59.37 59.44 59.11 59.09 59.23 59.19 59.07 59.12 59.24 59.18

0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360

VOLUME, IN ACRE-FEET

40

72

42

44

46

48

50

52

54

56

58

60

62

64

66

68

70

LAKE ELEVATION, IN FEET ABOVE MEAN SEA LEVEL

38 36 34 32 30 28 26 24 22 20 18 16 14 12 10 8 6 4 2 0

SURFACE AREA, IN ACRES

Volume

Mean lake elevation (April 1996 to March 1998)

Figure 5. Elevation/surface-area and elevation/capacity curves for Lake Ngardok, 

1996, Palau.

Area
Storage Capacity 9

Water

elevation

(feet)

Storage capacity

(acre-feet)

Difference

in volume

per foot .0 .1 .2 .3 .4 .5 .6 .7 .8 .9

44.0 0.04 0.05 0.06 0.06 0.08 0.10

45.0 0.09 010 0.12 0.13 0.15 0.17* 0.18 0.20 0.22 0.24 0.17

46.0 0.26 0.30 0.35 0.41 0.47 0.55* 0.61 0.68 0.76 0.85 0.69

47.0 0.95 1.04 1.14 1.24 1.36 1.48* 1.60 1.72 1.86 2.00 1.21

48.0 2.16 2.33 2.51 2.70 2.92 3.14 3.33 3.54 3.75 3.98 2.06

49.0 4.22 4.48 4.74 5.03 5.33 5.65 5.96 6.28 6.62 6.98 3.14

50.0 7.36 7.76 8.17 8.61 9.07 9.55 10.0 10.5 11.0 11.5 4.64

51.0 12.0 12.5 13.0 13.6 14.1 14.7 15.3 15.9 16.5 17.1 5.73

52.0 17.7 18.4 19.1 19.8 20.5 21.2 21.9 22.7 23.4 24.2 7.22

53.0 25.0 25.7 26.4 27.2 28.0 28.8 29.6 30.4 31.2 32.0 7.91

54.0 32.9 33.7 34.6 35.5 36.4 37.4 38.3 39.2 40.2 41.2 9.29

55.0 42.2 43.1 44.1 45.1 46.1 47.1 48.1 49.2 50.2 51.3 10.2

56.0 52.4 53.5 54.6 55.8 57.0 58.2 59.4 60.6 61.9 63.1 12.1

57.0 64.4 65.7 67.1 68.4 69.8 71.2 72.6 74.0 75.4 76.9 14.0

58.0 78.4 80.0 81.5 83.2 84.8 86.5 88.3 90.1 91.9 93.8 17.3

59.0 95.7 97.9 100 102 105 107 109 112 114 117 23.4

60.0 119 122 124 126 129 132 134 137 139 142 25.4

61.0 144 147 150 152 155 158 160 163 166 168 26.7

62.0 171 174 177 179 182 185 188 191 193 196 28.0

63.0 199 202 205 208 211 214 216 219 222 225 29.0

64.0 228 231 234 237 240 243 246 249 252 255 30.4

65.0 259 262 265 268 271 274 277 281 284 287 31.7

66.0 290 294 297 300 303 307 310 313 316 320 32.9

67.0 323 327 330 333 337 340

Table 2. Elevation/surface-area and elevation/storage-capacity for Lake Ngardok near Melekeok, Palau, 1996

[Datum is mean sea level]

Water

elevation

(feet)

Surface area

(acres)

Difference

in area per

foot .0 .1 .2 .3 .4 .5 .6 .7 .8 .9

 44.0 0.09 0.10 0.10 0.11 0.12 0.08

45.0 0.13 0.14 0.14 0.15 0.16 0.17 0.21 0.26 0.32 0.39 0.35

46.0 0.48 0.52 0.55 0.59 0.63 0.68 0.72 0.77 0.82 0.87 0.44

47.0 0.92 0.97 1.02 1.08 1.14 1.20 1.31 1.44 1.57 1.72 0.96

48.0 1.88 1.91 1.95 1.98 2.02 2.05 2.16 2.27 2.39 2.51 0.76

49.0 2.64 2.73 2.82 2.92 3.02 3.12 3.30 3.50 3.70 3.91 1.50

50.0 4.14 4.23 4.33 4.43 4.53 4.63 4.73 4.84 4.94 5.05 1.02

51.0 5.16 5.27 5.38 5.49 5.60 5.72 5.93 6.16 6.39 6.62 1.71

52.0 6.87 6.94 7.01 7.08 7.15 7.22 7.29 7.36 7.43 7.50 0.70

53.0 7.57 7.64 7.70 7.77 7.84 7.91 8.09 8.28 8.47 8.66 1.29

54.0 8.86 8.94 9.03 9.12 9.20 9.29 9.38 9.47 9.56 9.65 0.88

55.0 9.74 9.83 9.92 10.0 10.1 10.2 10.4 10.6 10.9 11.1 1.57

56.0 11.3 11.5 11.6 11.8 11.9 12.1 12.2 12.4 12.6 12.8 1.72

57.0 13.0 13.2 13.4 13.6 13.8 14.0 14.2 14.4 14.7 14.9 2.09

58.0 15.1 15.5 16.0 16.4 16.8 17.3 17.8 18.4 18.9 19.5 4.91

59.0 20.0 20.8 21.6 22.4 23.2 24.1 24.2 24.4 24.5 24.6 4.72

60.0 24.8 24.9 25.0 25.1 25.3 25.4 25.5 25.7 25.8 25.9 1.31

61.0 26.1 26.2 26.3 26.4 26.6 26.7 26.8 27.0 27.1 27.2 1.34

62.0 27.4 27.5 27.6 27.7 27.9 28.0 28.0 28.2 28.3 28.4 1.10

63.0 28.5 28.6 28.7 28.9 29.0 29.1 29.2 29.4 29.5 29.6 1.25

64.0 29.8 29.9 30.0 30.1 30.3 30.4 30.5 30.6 30.8 30.9 1.27

65.0 31.0 31.1 31.3 31.4 31.5 31.7 31.8 31.9 32.0 32.1 1.25

66.0 32.3 32.4 32.5 32.6 32.8 32.9 33.0 33.1 33.2 33.4 1.22

67.0 33.5 33.6 33.7 33.8 34.0 34.1
10 Storage Capacity and Water Quality of Lake Ngardok, Babeldaob Island, Republic of Palau, 1996–98

Table 3. Daily total rainfall, in inches, from Ngardok rain gage near Melekeok, Palau

[U.S. Geological Survey gage number 073037134361570; --, no data or not applicable]

1996 1997

DAY July Aug Sept Oct Nov Dec Jan Feb Mar Apr May

1 -- 0.00 0.00 0.52 1.03 0.00 0.00 0.77 0.00 0.26 0.00

2 -- .00 .00 .00 .26 .00 .00 .51 .00 2.32 .00

3 -- .26 .77 .51 2.32 .26 .00 .00 .00 .00 .00

4 -- .52 .26 .26 .77 .00 .26 .26 .26 .00 .00

5 -- .00 1.29 .00 1.29 .00 .00 .51 2.83 .00 .00

6 -- .00 1.29 .00 1.03 .00 .00 .51 .26 .26 .00

7 -- .26 .52 .00 .00 .00 .00 .00 .00 .00 .00

8 -- .00 1.54 .00 .00 .00 .00 .00 .26 .00 .00

9 -- 1.29 .26 .00 .51 .00 .00 .00 1.03 .52 .00

10 -- .00 .26 .26 .00 .52 .00 .77 .00 .26 .00

11 -- .00 .00 .26 .00 1.54 .00 1.03 .00 1.29 .00

12 -- .00 .00 .26 .00 .00 .51 3.61 .00 .00 .51

13 -- .00 .00 .00 .51 .26 .00 1.29 .00 .00 1.29

14 -- .00 .00 .26 .26 .26 .51 3.35 .00 .00 .00

15 -- .00 .00 .77 .00 .00 .00 1.03 .00 .00 .00

16 -- .00 .00 .00 .26 .00 .00 1.29 .00 .00 .52

17 -- .00 .00 .26 .00 .00 .00 1.80 .00 .00 .00

18 -- .00 .00 .26 .00 .00 .00 2.57 .00 .00 .26

19 -- .00 .00 .26 .00 1.03 .26 .51 .00 .00 .00

20 -- .77 .00 .00 .00 2.57 .00 .00 .00 .00 .00

21 -- .00 1.55 .51 .00 .51 .00 .00 .00 .00 .00

22 -- .00 3.35 .00 .00 1.29 .00 .00 .00 .00 .00

23 0.00 .00 .26 1.03 .00 .51 .00 .00 .00 .00 .00

24 .51 .00 1.29 .00 .26 1.29 .00 .26 .00 .00 .00

25 .00 .00 2.06 .00 .00 4.12 1.80 4.12 .00 .00 .00

26 .00 .00 .00 .77 .00 .00 .26 2.06 .00 .00 .00

27 .51 .00 1.55 .51 .26 .00 .00 1.29 .26 .00 2.32

28 3.09 .00 .00 .26 .00 .00 1.03 .26 .00 .00 .00

29 .00 .26 .00 .26 .51 .00 .00 -- .00 .51 .00

30 .00 .00 .51 1.03 .00 .00 .00 -- .77 .26 .00

31 .00 .26 -- .00 1.29 -- .00 -- .00 -- --

Total -- 3.62 16.8 8.25 9.27 14.2 5.92 27.8 5.67 5.68 4.90
Storage Capacity 11

Table 3. Daily total rainfall, in inches, from Ngardok rain gage near Melekeok, Palau--Continued

[U.S. Geological Survey gage number 073037134361570; --, no data or not applicable]

1997 1998

DAY June July Aug Sept Oct Nov Dec Jan Feb Mar

1 0.00 0.00 0.00 0.00 0.00 0.00 3.35 0.52 0.00 0.00

2 .26 .00 .00 .77 .00 .00 .00 .00 .00 .00

3 .00 .00 .00 .77 1.03 .00 .00 .26 .00 .00

4 .26 .00 .00 3.09 .26 .00 .00 .26 .00 --

5 .00 .00 .00 .52 .00 .00 .26 .77 .00 --

6 .00 .26 .26 .26 .00 .00 .26 .00 .00 --

7 .00 .00 .26 .00 .00 1.29 1.29 .00 .00 --

8 .00 .77 2.58 .52 .26 .00 1.55 1.03 .00 --

9 .00 .26 .00 .00 .00 .00 .00 .00 .00 --

10 2.06 .26 .52 .00 .00 .00 .00 .00 .00 --

11 .00 2.32 .00 .00 .26 2.06 .00 .52 .00 --

12 1.03 .26 .00 1.03 .00 .00 .52 .26 .00 --

13 1.03 .00 .00 .52 .00 .00 .00 .00 .00 --

14 .00 .26 .00 .77 .52 .00 .52 .00 1.80 --

15 .52 .00 .26 .52 .77 .00 1.03 .00 .26 --

16 .00 .00 .00 .00 .52 .26 .00 .00 .00 --

17 .00 .00 .00 .00 .00 .00 .52 .00 .00 --

18 .00 .00 .00 .00 .26 .00 .00 .00 .00 --

19 .00 .52 .00 .52 1.29 .00 .00 .00 .00 --

20 1.03 .77 .00 .00 .00 .00 .00 .52 .00 --

21 5.92 .00 .00 .52 .00 .00 .00 .00 .00 --

22 .00 .00 .00 1.29 .00 .00 .00 .26 .52 --

23 .00 .00 .00 .26 .00 1.03 .26 .00 .00 --

24 .00 .00 .00 .00 .00 .00 .00 .00 .00 --

25 .00 .00 .00 2.58 .00 .52 .00 .26 .00 --

26 .00 .00 .00 1.29 .52 .26 .00 .52 .00 --

27 .00 .26 .00 .00 .00 .00 1.80 .00 .00 --

28 .52 .00 .00 .00 .00 .00 .77 .77 .00 --

29 .00 .00 .00 .00 .00 .00 .26 .00 -- --

30 1.80 .00 .00 .77 .26 .00 .00 .00 -- --

31 -- .00 .00 -- .00 -- .26 .00 -- --

Total 14.43 5.94 3.88 16.0 5.95 5.42 12.7 5.95 2.58 --
Figure 6. Rainfall and lake elevation at Lake Ngardok, Palau, from July 1996 to July 1997.

8

7

6

5

4

3

2

1

0

60.4

60.6

60.8

61.0

61.2

61.4

61.6

61.8

62.0

60.2

60.0

59.8

59.6

59.4

59.2

59.0

58.8

AUG OCT DEC JAN FEB MAR APR MAY JUNE JULY SEPT NOV

LAKE ELEVATION, IN FEET ABOVE MEAN SEA LEVEL RAINFALL, IN INCHES

1996 1997

12 Storage Capacity and Water Quality of Lake Ngardok, Babeldaob Island, Republic of Palau, 1996–98
Water Quality 13

The mat of reeds, which surrounds the lake shoreline, covers about 58 percent of the total lake surface

area. This mat of reeds affects the storage capacity analysis owing to the difficulty in determining which part of

the mat was floating and which part rested on the lake

bottom. The reason is that the root structure of the reed

mat may have hindered the penetration of the survey rod

and as a result the depth readings measured in those

locations may not reflect the true depth to the lake bottom. The cross sections in figure 4 show the highly

varying depths measured in the field. Therefore, in the

mat areas, depth readings and resulting storage capacity

estimates will contain some level of uncertainty. However, field observations indicated that errors in the measured depths are not large.

WATER QUALITY

Temperature, dissolved oxygen, and specific conductance were measured at various depths at a location

in the deep water part of five lake cross sections (figs. 2

and 4). Near-surface temperatures of water ranged from

28.0°C to 29.0°C while lake bottom water temperatures

ranged from 26.0°C to 27.8°C. Temperature also varied

with depth, showing a consistent 2°C drop from water

surface to the deepest point measured at all five cross

sections (table 4).

Dissolved oxygen is second only to temperature as

a critical factor for aquatic life in lakes (Goldman and

Horne, 1983). Dissolved oxygen measurements

decrease from upstream to downstream. Values at the 1

ft depth ranged from 6.0 mg/L at cross section C–C′ to

7.0 mg/L at cross section P–P′ (table 4, fig. 2). Dissolved oxygen at the sampling locations initially

increased slightly with depth before dropping sharply.

For example at cross section H–H′ (table 4) dissolved

oxygen at the water surface was 6.6 mg/L. The dissolved oxygen readings increased to 6.8 mg/L at a depth

of 2.0 ft before dropping sharply to 2.6 mg/L at a depth

of 12.0 ft. These decreases in dissolved oxygen values

with depth are similar to those observed in Fena Reservoir on Guam, which had values ranging from 10 mg/L

at the surface to 2 mg/L at the 15 ft depth (LaBaugh,

1985), approaching anaerobic conditions at lake bottom.

Lake Ngardok is stratified based on temperature

and dissolved oxygen differences. As noted earlier, air

temperatures on Palau have only minor seasonal variations. As a result, the stratification indicated by the measurements made for this study are likely indicative of a

pattern of persistent stratification in the lake. Hutchinson (1957, p. 634) made similar observations in tropical

lakes near the equator.

Specific conductance is a measure of ionic concentrations in water. Values measured in Lake Ngardok

showed no systematic variation across the lake or with

depth and ranged from 41 to 61 μS/cm (table 4). These

specific conductance values are quite low compared

with those in Fena Reservoir, which ranged from 250 to

290 μS/cm (LaBaugh, 1985). The low values in Lake

Ngardok probably indicate a negligible component of

ground-water seepage which typically has higher ionic

concentrations.

The color of the lake, observed during the survey

period, was brown to greenish-brown and light penetration diminished rapidly with depth. However, no Seechi

disk measurements were made to document these observations. The brown color was probably from organic

degradation of the surrounding reed mats. The green

color was probably from algae.

Results from the analyses of water quality at the

lake outflow are presented in tables 5 and 6. Results

show that no primary drinking-water standards were

exceeded (U.S. Environmental Protection Agency,

1996) for any of the constituents measured. The concentration of total iron did, however, violate the secondary

maximum contamination level of 300 μg/L (table 5),

U.S. Environmental Protection Agency, 1996). The

high iron concentration in the water may be a result of

high iron concentrations in the local soils (Smith, 1983).

The low dissolved oxygen (3.2 mg/L) measured at the

outflow probably indicated that the water flowing past

the outlet originated from the deeper, oxygen depleted

depths. Other physical properties (conductivity, turbidity, and temperature) and nutrient concentrations fell

within the range of averages observed in other streams

on Palau (van der Brug, 1984) and are similar to those

measured in Fena Reservoir (Labaugh, 1985). Recent

measurements by AECOS Inc. (1997) at a point 1.4 mi

downstream from the lake outlet (table 7) were similar

to values measured in this study.
14 Storage Capacity and Water Quality of Lake Ngardok, Babeldaob Island, Republic of Palau, 1996–98

Table 4. Temperature, dissolved oxygen, and specific conductance at selected cross sections, Lake Ngardok, Palau, 1996

[Cross sections shown in figure 2; °C, degrees Celsius; mg/L, milligrams per liter; μs/cm, microsiemens per centimenter; ft, feet]

Depth

Water

temperature

(°C)

Dissolved

oxygen

(mg/L)

Specific conductance

(μS/cm at 25°C)

Cross section C–C′; April 7, 1996; 1022 hours; total depth 11.7 ft

surface 29.0 6.2 41

1.0 29.0 6.0 50

2.0 28.5 5.4 48

3.0 28.5 5.2 51

4.0 28.0 4.8 52

5.0 27.5 4.8 54

6.0 27.5 4.5 54

7.0 27.0 4.6 50

8.0 27.0 4.7 51

9.0 26.5 3.2 44

10.5 27.0 2.8 45

Cross section F–F′; April 7, 1996; 1000 hours; total depth 16.0 ft

surface 29.0 5.9 48

1.0 29.0 6.5 48

2.0 29.0 6.9 48

3.0 28.5 4.6 51

4.0 28.0 5.4 52

5.0 27.5 5.4 52

6.0 27.5 5.8 56

7.0 27.5 5.3 52

8.0 27.0 5.6 50

9.0 26.5 4.1 51

11.0 27.0 3.5 46

13.0 27.0 3.6 46

15.0 26.5 3.3 44

Cross section H–H′; April 7, 1996; 0920 hours; total depth 12.8 ft

surface 28.5 6.6 50

1.0 28.5 6.8 50

2.0 28.5 6.8 51

3.0 28.0 6.5 55

4.0 28.0 5.4 54

5.0 27.5 5.5 55

6.0 27.5 5.5 57

7.0 27.0 6.2 55

8.0 27.0 5.5 55

9.0 26.5 3.6 54

10.0 27.0 2.9 49

12.0 26.5 2.6 46

Cross section N–N′; April 5, 1996; 1515 hours; total depth 10.7 ft

surface 28.0 6.7 54

1.0 28.0 6.6 56

2.0 27.5 7.5 57

3.0 27.0 5.5 55

4.0 26.5 5.7 54

5.0 26.5 4.8 47

6.0 26.0 3.9 45

7.0 26.0 3.4 44

8.0 26.0 3.2 42

9.0 26.0 3.9 42

Cross section P–P′; April 5, 1996; 1100 hours; total depth 7.4 ft

1.0 28.0 7.0 50

2.0 27.0 7.8 61

3.0 26.5 8.4 60

4.0 26.0 7.7 52

5.0 26.0 6.6 56

6.0 26.0 5.3 55

7.0 26.0 -- 57
Water Quality 15

Table 5. Water-quality data at outflow station on April 3, 1996 and November 10, 1997, Lake Ngardok, Palau

[<, actual value is less than value shown, value shown is detection limit; μs/cm, microsiemens per centimeter at 25°C; °C, degrees Celsius; mg/L,

milligrams per liter; μg/L, micrograms per liter,; --, not analyzed; pg/L, picograms per liter; pCi/L, picocurie per liter]

Property or constituent April 3, 1996 November 10, 1997

INORGANIC

Field measurements

Specific conductance (μs/cm at 25°C 54 --

pH (units) 6.6 --

Major ions, total(mg/L)

Calcium 2.9 3.4

Magnesium 2.7 --

Sodium <5.0 <5.0

Alkalinity, total (as CaCO3) 21.6 25.4

Alkalinity, bicarbonate (as CaCO3) 21.6 25.4

Alkalinity, carbonate (as CaCO3) <5.0 <5.0

Alkalinity, hydrox (as CaCO3) <5.0 <5.0

Sulfate 0.69 0.73

Chloride 3.4 3.3

Flouride <0.50 <0.50

Silica (as SiO2) 13.4 16.5

Nutrients, total (mg/L)

Nitrogen, nitrate (as N) <0.50 0.50

Nitrogen, nitrite (as N) <0.50 <0.50

Phosphorus, orthophosphate (as P) <0.50 <0.50

Metals, total recoverable (μg/L)

Aluminum 110 <100

Antimony 1.5 <1.0

Arsenic <5.0 <5.0

Barium <10 <10

Beryllium <2.0 <2.0

Cadmium <5.0 <5.0

Chromium <10 <10

Copper <20 <20

Iron 920 1000

Lead <1.0 <1.0

Manganese -- 46

Mercury <0.20 <0.20

Nickel <40 <40

Selenium <5.0 <5.0

Silver <10 <10

Thallium <0.10 <1.0

Zinc <20 <20

OTHER INORGANIC

Asbestos (structures/L) 0.23 × 106 <0.20 × 106

PHYSICAL PROPERTIES

Temperature, water (°C) 28.0 --

Turbidity (NTU) 3.5 2.9

Oxygen, dissolved (mg/L) 3.2 --

Solids, total dissolved (mg/L) 50 54

ORGANIC

Cyanide, total (mg/L) <0.010 <0.010

Pesticides and herbicides, total recoverable, (μg/L)

alpha-Chlordane <0.1 --

gamma-Chlordane <0.1 --

2,3,7,8-TCDD (Dioxins) <1.4 pg/L <1.3 pg/L

Hexachlorobenzene 0.1 <0.1
16 Storage Capacity and Water Quality of Lake Ngardok, Babeldaob Island, Republic of Palau, 1996–98

Table 5. Water-quality data at outflow station on April 3, 1996 and November 10, 1997, Lake Ngardok, Palau--Continued

Property or constituent April 3, 1996 Detection limit

RADIATION AND RADIONUCLIDES(pCi/L)

Gross alpha. . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.171 ± 0.278 0.392

Gross beta. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.226 ± 0.450 0.683

Radium 226 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.0657 ± 0.141 0.256

Radium 228 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.193 ± 0.479 0.823

Uranium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . <1.0 μg/L 1.0 μg/L

Property or constituent November 10, 1997 Detection limit

RADIATION AND RADIONUCLIDES(pCi/L)

Gross alpha. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.182 ± 0.29 0.597

Gross beta. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.99 ± 1.5 2.66

Radium 226 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.0298 ± 0.044 0.0727

Radium 228 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.0916 ± 0.21 0.479

Uranium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.015 ± 0.004 μg/L 0.101 μg/L
Water Quality 17

Table 6. Pesticides, herbicides and other volatile organic constituents found to be less than detection limits in water samples

from outflow station on April 3,1996 and November 10, 1997, Lake Ngardok, Palau

Organic

(total recoverable)

Detection

limit (mg/L)

Organic

(total recoverable)

Detection

limit (mg/L)

Alachlor (Lasso) 0.1 Di (2-ethylhexyl) phthalate 0.6

Aldicarb 0.5 Dinoseb 0.1

Aldicarb Sulfone 0.4 Diquat 0.4

Aldicarb Sulfoxide 0.5 Endothall 9.0

Aldrin 0.1 Endrin 0.01

Aroclor 1016 0.08 Ethylene dibromide (EDB) 0.01

Aroclor 1221 2.0 Glyphosate (Roundup) 6.0

Aroclor 1232 0.5 Heptachlor 0.04

Aroclor 1242 0.3 Heptachlor epoxide 0.02

Aroclor 1248 0.1 Hexachlorocyclopentadiene 0.1

Aroclor 1254 0.1 3-Hydroxycarbofuran 1.0

Aroclor 1260 0.2 Lindane (gamma-BHC) 0.02

Atrazine 0.1 Methoxychlor 0.1

Benzo(a)pyrene 0.02 Methomyl 0.5

Butachlor 0.1 Metolachlor (Dual) 0.1

Carbaryl 1.0 Metribuzin (Sencor) 0.1

Carbofuran 0.9 Oxamyl (Vydate) 1.0

Chlordane 0.2 Pentachlorophenol 0.04

2,4-D 0.1 Picloram (Tordon) 0.1

Dalapon 1.0 Propachlor 0.1

1,2-Dibromo-3-chloropropane 0.02 2,4,5-TP (Silvex) 0.1

Dicamba 0.1 Simazine 0.07

Dieldrin 0.1 Toxaphene 1.0

Di (2-ethylhexyl) adipate 0.6

Benzene 1.0 2,2-Dichloropropane 1.0

Bromobenzene 1.0 1,1-Dichloropropene 1.0

Bromochloromethane 1.0 cis-1,3-Dichloropropene 1.0

Bromodichloromethane 1.0 trans-1,3-Dichloropropene 1.0

Bromoform 1.0 Ethylbenzene 1.0

Bromomethane 1.0 Hexachlorobutadiene 1.0

n-Butylbenzene 1.0 Isopropylbenzene (Cumene) 1.0

sec-Butylbenzene 1.0 p-Isoropyltoluene (p-Cymene) 1.0

tert-Butylbenzene 1.0 Methylene chloride 1.0

Carbon tetrachloride 1.0 Naphthalene 1.0

Chlorobenzene 1.0 n-Propylbenzene 1.0

Chloroethane 1.0 Styrene 1.0

Chloroform 1.0 1,1,1,2-Tetrachloroethane 1.0

Chloromethane 1.0 1,1,2,2-Tetrachloroethane 1.0

2-Chlorotoluene 1.0 Tetrachloroethene 1.0

4-Chlorotoluene 1.0 Toluene 1.0

Dibromochloromethane 1.0 1,2,3-Trichlorobenzene 1.0

Dibromomethane 1.0 1,2,4-Trichlorobenzene 1.0

1,2-Dichlorobenzene 1.0 1,1,1-Trichloroethane 1.0

1,3-Dichlorobenzene 1.0 1,1,2-Trichloroethane 1.0

1,4-Dichlorobenzene 1.0 Trichloroethene 1.0

Dichlorodifluoromethane 1.0 Trichlorofluoromethane 1.0

1,1-Dichloroethane 1.0 1,2,3-Trichloropropane 1.0

1,2-Dichloroethane 1.0 1,2,4-Trimethylbenzene 1.0

1,1-Dichloroethene 1.0 1,3,5-Trimethylbenzene 1.0

cis-1,2-Dichloroethene 1.0 Vinyl chloride 1.0

trans-1,2-Dichloroethene 1.0 m- & p-Xylenes 1.0

1,2-Dichloropropane 1.0 o-Xylene 1.0

1,3-Dichloropropane 1.0
18 Storage Capacity and Water Quality of Lake Ngardok, Babeldaob Island, Republic of Palau, 1996–98 Table 7. Data from AECOS Inc. station 20, 1.4 miles downstream of Lake Ngardok outflow, Palau [Data from AECOS Inc., 1997; ft3/s, cubic feet per second;

μS/cm, microsiemens per centimeter at 25°C; °C, degrees Celsius; NTU, nephelometric tu

ligrams per liter;

μg/L, micrograms per liter; --, no data or not applicable]

Date Time

Instantaneous

discharge

(ft3/s)

Specific

conductance

(μS/cm)

pH

(standard

units)

Water

temperature

(°C)

Turbidity

(NTU)

Disolved

oxygen

(mg/L)

Total

nitrogen,

NO2+NO3

(μg/L as N)

Total

nitroge

(μg/L as 

04/03/96 1525 34.6 40 6.1 28.0 3.1 6.0 -- -- 04/06/96 1540 -- 42 6.7 29.0 5.0 6.5 <1 45 07/16/96 1535 5.65 41 7.1 27.0 1.1 6.9 <1 62 
Summary 19

SUMMARY

Because of the increasing population on Koror, the

Palau government plans to relocate its capital to Melekeok on Babeldaob Island. Lake Ngardok, near Melekeok, the largest freshwater lake in the Republic of

Palau, is being considered as a source of water for the

new capital. This report presents the methods and

results of measurements of the quantity and quality of

water in Lake Ngardok.

A bathymetric survey was done during March and

April 1996, to determine the storage capacity of the

lake. From analysis of bathymetric data, a capacity of

107 acre-feet was estimated with 24.1 acres of surface

area at an elevation of 59.5 feet (mean lake elevation for

the period April 1996 to March 1998). Elevation-surface area and elevation-capacity curves summarizing

the current relations between elevation, surface area,

and storage capacity along with lake-elevation data collected between April 1996 and March 1998 showed that

lake capacity fluctuated between 90.0 and 168 acre-feet

during this period. Rainfall data collected from July

1996 to March 1998 indicated that lake water levels

were correlated to rainfall with lake elevation rising in

response to heavy rainfall and then returning to normal

levels after a few days. A floating mat of Hanguana

malayana, a reed-like plant, covers about 58 percent of

the surface area of the lake. The vegetation was a source

of error reducing the accuracy of the storage capacity

computations.

Quality of water was sampled during April 1996

and November 1997, and results indicated that no U.S.

Environmental Protection Agency primary drinking

water standards were exceeded for the organic and inorganic compounds and radionuclides analyzed. With

suitable biological treatment, the lake water could be

used for drinking-water purposes. Temperature and dissolved-oxygen measurements at various depths showed

stratification of the lake with depth and near anaerobic

conditions at the lake bottom. Complete mixing of the

lake probably occurs rarely. The low dissolved oxygen

(3.2 mg/L) measured at the outflow indicated that the

water flowing past the outflow probably comes from the

deeper oxygen-depleted depths of the lake.

REFERENCES CITED

AECOS, Inc., 1997, Information report on aquatic environments for the Palau compact road EIS, Babeldaob Island,

Republic of Palau, 78 p. + appendices in U.S. Department of the Army, Draft of environmental impact statement for construction of the Palau Compact Road,

Babeldaob Island, Republic of Palau: U.S. Army Engineer District, Honolulu. v. 1 and 2, various pagination.

Goldman, C.R., and Horne, A.J., 1983, Limnology: New

York, McGraw-Hill, 464 p.

Hutchinson, G.E., 1957, A treatise on limnology, vol. 1,

Geography, Physics, and Chemistry: New York, John

Wiley, 1,015 p.

LaBaugh, J.W., 1985, Survey of Fena Reservoir, Island of

Guam--Limnological reconnaissance: Water-Resources

Investigations Report 83-4241, 17 p.

Nance, Tom, 1986, Evaluation of Proposed Sources of Supply for the Palau Rural Water Systems Project: Belt Collins & Associates, 29 p.

National Oceanic and Atmospheric Administration, 1995,

Local climatological data, annual summary with comparative data, Koror Island, Pacific: National Climatic

Data Center, 8 p.

Smith, C.W., 1983, Soil survey of Islands of Palau, Republic

of Palau: U.S. Department Agriculture, Soil Conservation Service, 120 p.

U.S. Army Chief of Engineers, 1956, Military geology of the

Palau Islands, Caroline Islands: Intelligence Division,

Office of the Engineer Headquarters, U.S. Army Forces

Far East, 285 p.

U.S. Environmental Protection Agency, 1996, Drinking

water regulations and health advisories: EPA 822-R-96-

001, 12 p.

van der Brug, Otto, 1984, Water resources of the Palau

Islands: U.S. Geological Survey Water-Resources Investigations Report 83-4140, 223 p.
40

80

45

50

55

60

65

70

75

FEET

C C'

0 100 200 FEET

VERTICAL SCALE GREATLY EXAGGERATED

0 20 40 METERS

REED MAT

WATER SURFACE

WATER SAMPLING

LOCATIONS

EXPLANATION

BEND IN

SECTION

BEND IN

SECTION

45

50

55

60

65

70

75

FEET

B B'

FEET

A A'

55

60

65

70

Figure 4. Bed elevation at cross sections, Lake Ngardok, Palau, 1996 (datum is mean sea level).

20 Storage Capacity and Water Quality of Lake Ngardok, Babeldaob Island, Republic of Palau, 1996–98
45

50

55

60

65

70

40

45

50

55

60

65

70

FEET

E E'

FEET

D D' BEND IN SECTION BEND IN SECTION BEND IN SECTION

Figure 4. Bed elevation at cross sections, Lake Ngerdok, Palau, 1996 (datum is mean sea level)--Continued.

Figure 4 21
45

50

55

60

65

70

75

45

50

55

60

65

70

75

FEET

I I'

FEET

H H'

45

40

50

55

60

65

70

45

50

55

60

65

70

75

FEET

G G'

FEET

F F'

BEND IN

SECTION

BEND IN

SECTION

BEND IN

SECTION

BEND IN

SECTION

BEND IN

SECTION

BEND IN SECTION

BEND IN SECTION

BEND IN SECTION

Figure 4. Bed elevation at cross sections, Lake Ngardok, Palau, 1996 (datum is mean sea level)--Continued.

22 Storage Capacity and Water Quality of Lake Ngardok, Babeldaob Island, Republic of Palau, 1996–98
55

60

65

70

75

50

55

60

65

70

FEET

J J'

FEET

K K'

55

60

65

70

FEET

L

FEET

M M

L'

45

50

55

60

65

70

Figure 4. Bed elevation at cross sections, Lake Ngardok, Palau, 1996 (datum is mean sea level)--Continued.

Figure 4 23
45

50

55

60

65

70

75

45

50

55

60

65

70

75

FEET

N N'

FEET

O O' BEND IN SECTION

Figure 4. Bed elevation at cross sections, Lake Ngardok, Palau, 1996 (datum is mean sea level)--Continued.

24 Storage Capacity and Water Quality of Lake Ngardok, Babeldaob Island, Republic of Palau, 1996–98
50

55

60

65

70

75

80

50

55

60

65

70

75

FEET

P P'

FEET

Q Q'

Figure 4. Bed elevation at cross sections, Lake Ngardok, Palau, 1996 (datum is mean sea level)--Continued. BEND IN SECTION BEND IN SECTION BEND IN SECTION BEND IN SECTION BEND IN SECTION

Figure 4 25
50

55

60

65

70

50

55

60

65

70

75

FEET

S S'

FEET

R R'

55

60

65

70

75

FEET

T T'

BEND IN

SECTION

BEND IN SECTION

BEND IN SECTION

BEND INSECTION

Figure 4. Bed elevation at cross sections, Lake Ngardok, Palau, 1996 (datum is mean sea level)--Continued.

26 Storage Capacity and Water Quality of Lake Ngardok, Babeldaob Island, Republic of Palau, 1996–98
Yeung and Wong–STORAGE CAPACITY AND WATER QUALITY OF LAKE NGARDOK, REPUBLIC OF PALAU, 1996-98–WRIR 99-4118