U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2007–5042

Prepared in cooperation with the National Park Service

Reconnaissance Study of the Hydrology of 

American Memorial Park, Island of Saipan, 

Commonwealth of the Northern Mariana Islands

Reconnaissance Study of the Hydrology of 

American Memorial Park, Island of Saipan, 

Commonwealth of the Northern Mariana 

Islands

By Jeff A. Perreault

Prepared in cooperation with the National Park Service

Scientific Investigations Report 2007–5042

U.S. Department of the Interior

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

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2007

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Suggested citation:

Perreault, J.A., 2007, Reconnaissance study of the hydrology of American Memorial Park, Island of Saipan, Commonwealth of the Northern Mariana Islands: U.S. Geological Survey Scientific Investigations Report 2007–5042, 31 p.
iii

Contents

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

Introduction.....................................................................................................................................................3

Purpose and Scope ..............................................................................................................................4

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

Climate...........................................................................................................................................5

Geology..........................................................................................................................................7

Hydrogeology................................................................................................................................9

History .........................................................................................................................................10

Land Use......................................................................................................................................10

Previous Investigations......................................................................................................................13

Acknowledgments..............................................................................................................................13

Reconnaissance...........................................................................................................................................13

Network Components and Installation............................................................................................13

Collection Methods.............................................................................................................................13

Surface- and Ground-Water Salinity......................................................................................13

Ground-Water Elevations .........................................................................................................15

Results ..................................................................................................................................................16

Surface-Water Salinity .............................................................................................................17

Ground-Water Salinity ..............................................................................................................19

Ground-Water Elevations .........................................................................................................20

Hydrologic Model of American Memorial Park......................................................................................24

Surface Water.....................................................................................................................................24

Ground Water......................................................................................................................................26

Additional Data and Analysis Needs........................................................................................................27

Summary and Conclusions.........................................................................................................................28

References....................................................................................................................................................39

Figures

1. Location map of American Memorial Park on the Island of Saipan, 

Commonwealth of the Northern Mariana Islands...................................................................2

2. Location map of features and facilities within American Memorial Park on 

the Island of Saipan, CNMI.........................................................................................................4

3. Distribution of mean annual rainfall on the Island of Saipan, CNMI (modified from 

Lander, 2004)..................................................................................................................................6

4. Mean monthly rainfall at Saipan International Airport during period 1954–99 ..................7

5. Generalized geologic map of the Island of Saipan showing (A) surficial geology, and 

(B) a geologic cross section (modified from Cloud and others, 1956; Carruth, 2003)........8

6. Geologic cross section A–A’ (see fig. 2 for location) near southeast perimeter of

natural wetland in American Memorial Park on the Island of Saipan 

(modified from Ogden, 1998) .....................................................................................................10
iv

7. Hydrogeologic cross section D–D’ through the Island of Saipan showing details of the 

ground-water-flow system (modified from Carruth, 2003)...................................................11

8. Aerial photographs of the Island of Saipan taken before (ca. 1936), during (ca. 1945), 

and after (ca. 1970) World War II provide long-term evidence of a persistent natural

wetland within the current boundaries of American Memorial Park 

(provided by D. Minton, 2006) ...................................................................................................12

9. Location map of municipal well fields on the Island of Saipan 

(modified from Carruth, 2003)....................................................................................................14

10. The American Memorial Park ground-water-monitoring network was installed to 

enable the collection of ground-water data (water-level and salinity).............................15

11. A surface-water-stilling well was paired with a ground-water-monitoring well to 

determine whether surface water and ground water were 

connected within the park’s natural wetland ........................................................................17

12. Location map showing the four discrete zones of surface water observed during the 

reconnaissance of American Memorial Park........................................................................18

13. A ground-water-monitoring well was installed within surface-water zone 

4 of the natural-wetland area of American Memorial Park.................................................19 

14. Ground-water-monitoring well AMP03 was installed near the constructed wetland 

(shown in the near background); in the far background is one of the hotels utilizing 

reverse-osmosis to obtain potable water, and releasing highly-saline wastewater 

into the storm-drainage system as a bi-product of the process ........................................19

15. Conductivities of ground-water samples withdrawn from wells in the installed 

ground-water-monitoring network indicated that ground-water salinity 

was geographically related ......................................................................................................20

16. Surface-water data, collected within the natural wetland, reflect a pattern 

consistent with a diurnal tidal signal.......................................................................................21

17. Synoptic water-level survey data reflect the areal distribution of ground-water 

levels within American Memorial Park...................................................................................22

18. Low-tide ground-water levels as indicated by the synoptic water-level survey data....23

19. High-tide ground-water levels as indicated by the synoptic water-level survey data...25

Tables

1. Ground-water monitoring network configuration.................................................................16

2. Surface-water specific electrical conductance (μS/cm @ 25°C) data collected in

the natural wetland zones on August 26, 2005.......................................................................17

3. Well purging and ground-water quality data collected on September 13 - 18,

2005 ...............................................................................................................................................21

4. Tidal efficiencies and lag times computed from data collected on September 16,

2005 ...............................................................................................................................................24
Conversion Factors

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square mile (mi2) 2.590 square kilometer (km2) 

Volume

ounce, fluid (fl. oz) 0.02957 liter (L) 

gallon (gal) 3.785 liter (L) 

million gallons (Mgal) 3,785 cubic meter (m3)

Flow rate

foot per day (ft/d) 0.3048 meter per day (m/d)

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

°F=(1.8×°C)+32

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

°C=(°F-32)/1.8

Vertical coordinate information is referenced to mean sea level.

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

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

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

25°C).

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

or micrograms per liter (μg/L).

Abstract

American Memorial Park, a unit of the National Park 

Service on the Island of Saipan, includes among its features a 

27-acre estuarine system that has become a rarity within the 

Commonwealth of the Northern Mariana Islands. The estuarine system’s mosaic of marshy areas interspersed with emergent wetlands and mixed wet forests provides critical habitat 

for various migratory and resident waterfowl, including two 

Federally listed endangered species: the Marianas gallinule 

(Gallinula chloropus guami) and the nightingale reed warbler 

(Acrocephalus luscinia). With sensitivity to the park’s ecologic 

assets and the uncertainty associated with locally rapid urbanization, a need to better understand the hydrology of American 

Memorial Park was recognized. To address that need, a reconnaissance study of the park was undertaken during August and 

September 2005. The goals of the study were (1) to describe 

the occurrence and salinity of surface and ground water within 

the park; (2) to develop a hydrologic model of the park area of 

the island, with emphasis on the 27-acre estuarine system; and 

(3) to identify additional data needed to further develop this 

model.

With regard to surface water, three freshwater inputs to 

the park’s natural wetland are possible: direct rainfall, seaward-flowing ground water, and overland flow. Direct rainfall, 

which is an important source of freshwater to the wetland, 

commonly exceeds evapotranspiration both seasonally and 

per storm. The seaward flow of ground water is likely to be 

a source of freshwater to the wetland because ground water 

generally has an upward vertical component in the nearshore 

environment. Overland flow upgradient of the park could 

potentially contribute a significant input of freshwater during periods of intense rainfall, but roads that flank the park’s 

perimeter act as a barrier to surficial inflows.

During the reconnaissance, four discrete bodies, or zones, 

of surface water were observed within the park’s natural 

wetland. Conductivity within these surface-water zones typically ranged from 1,540 to 4,370 microsiemens per centimeter 

(μS/cm) at 25°C, although values as low as 829 and as high as 

8,750 μS/cm were measured. As a result of these observations, 

the American Memorial Park wetland area meets the definition 

criteria of an estuarine system that is dominantly oligohaline. 

Conductivity was also measured in a constructed wetland that 

was built within the park to augment the storm-drainage infrastructure of the village of Garapan. Reverse-osmosis facilities, 

in operation at hotels adjacent to the park, have historically 

discharged highly saline wastewater into the storm-drainage 

system. This collective storm and wastewater flow is routed 

into the constructed wetland and from there into the ocean. 

The conductivity of water in the constructed wetland ranged 

from 45,000 to 62,500 μS/cm, exceeding nominal seawater 

values by as much as 25 percent, with the highest conductivities recorded near discharging storm drains.

With regard to ground water, the reconnaissance included 

installation of a ground-water-monitoring network. Data collected from this network identified the presence of freshwater 

underlying the park and indicated that surface water is directly 

connected to ground water in the natural wetland because the 

water levels of both surface water and ground water directly 

varied with the tide. Conductivities of ground-water samples 

from wells in the monitoring network indicated that groundwater salinity was geographically related: conductivities were 

lower (801–2,490 μS/cm) in surficially dry areas, intermediate (6,090–9,180 μS/cm) in natural-wetland areas, and higher 

(18,250–27,700 μS/cm) in areas adjacent to the constructed 

wetland and its associated ocean-discharge channel.

Synoptic water-level surveys were made to enhance 

understanding of the spatial expression of the water table; they 

were scheduled to overlap with peak and trough tidal signals 

to enable limited characterization of local hydrologic properties. The water-level surveys indicated that ground-water 

levels were directly influenced by ocean tides because tidal 

fluctuations were reflected in ground-water levels. The surveys 

also indicated that at any given time ground-water levels were 

higher than ocean-water levels and that this water-level offset 

was greater at low than at high tide and greater in the western 

Reconnaissance Study of the Hydrology of 

American Memorial Park, Island of Saipan, 

Commonwealth of the Northern Mariana Islands

By Jeff A. Perreault 
LOCATION MAP 

20 ̊S

0 ̊

20 ̊N40 ̊N

120 ̊E 140 ̊E 160 ̊E 180 ̊ 160 ̊W

Philippines

Australia

Japan

SAIPAN

Hawaii

Wake

Guam

PACIFIC OCEAN 

0 1,000 MILES 0 1,000 KILOMETERS 

INDONESIA

AMERICAN

SAMOA

FEDERATED STATES OF  MICRONESIA 

PAPUA NEW GUINEA 

FIJI

Tutuila

REPUBLICOF PALAU 

COMMONWEALTH

OF THE NORTHERN 

MARIANA ISLANDS 

REPUBLIC OF THE 

MARSHALL ISLANDS 

MañagahaIsland

Western Coastal 

Central

Southern

Plateau

Uplands

Plain

145 ̊4

145 ̊44'

15 ̊16'

15 ̊12'

15 ̊08'

Mt. Tagpochau 

Fina Sisu Hills 

Kagman

Peninsula

San Antonio 

Chalan Kanoa 

Susupe

San Jose 

Garapan

PuertoRico

Tanapag

San Roque 

Saipan

International

Airport

Agingan Pt 

I Naftan Pt 

LK

SaipanSea Port 

Muchot Pt 

Tanapag

Harbor

Laolao

Bay

Lake

Susupe

PACIF

Rapugau Str

South Fk Talufofo Str

PHILIPPINE

SEA

00 1 

Base modified from U.S. Geological Survey topographic map, Island of Saipan, scale1:25,000, 1983. Shaded relief from U.S. GeologicalSurvey 10-meter digital elevation model.

American Memorial Park boundary 

Figure 1. Location map of American Memorial Park on the Island of Saipan, Commonwealth of the Northern Mariana Islands.
than in the eastern part of the park. Tidal lags were calculated 

for the entire park, and tidal efficiencies were calculated for 

the natural-wetland area. These data suggest that hydraulic 

conductivity in subsurface strata may be higher in the eastern 

than in the western part of the park; mean lag time in the eastern part of the park was 21 minutes, in contrast with 115 minutes in the western part. Tidal efficiencies in the eastern part 

of the park ranged from 0.38 to 0.85, with a mean of 0.64.

With regard to continuing information gaps, additional 

monitoring wells would need to be installed to constrain 

ground-water flow through the park, including the vertical 

flow component, and to improve understanding of the effects 

of nearby faults and injection wells on local hydrology. 

Understanding of the effects of the perimeter-flanking roads 

on freshwater flow, both surface and subsurface, could be 

enhanced by expanding the ground-water-monitoring network 

and by monitoring surface flows that are being artificially 

diverted around the park to the ocean. A long-term plan that 

includes monitoring of surface and subsurface water levels 

and salinities could be useful to identify trends that could 

affect the park’s critical habitat, including its endemic and 

endangered species. Development of a numerical model, 

addressing coupled surface-water/ground-water flow and solute transport, could assist in assessing hydrologic changes and 

serve as a communication tool to inform interested parties of 

such changes to the park’s hydrologic characteristics, including those changes that may affect critical habitats.

Introduction

American Memorial Park is centrally located along the 

western coastal plain of the Island of Saipan in the Commonwealth of the Northern Mariana Islands (CNMI) (fig. 1). Land 

on which the park now resides was set aside as part of a 1977 

Covenant between the United States and the fledgling CNMI 

Government that established the commonwealth as an independent nation. The 133-acre park contains a combination of 

constructed and natural features (fig. 2). In addition to such 

recreational facilities as athletic fields, jogging paths, tennis 

courts, a public marina, and a 1,200-seat amphitheater, the 

park features numerous World War II structures and memorials and contains the landing site of the first major Carolinian 

immigration to Saipan.

The western part of the park, in which many of the aforementioned manmade features are located, is approximately 

bisected by a constructed wetland that was built to augment 

the storm-drainage infrastructure of the village of Garapan. 

Reverse-osmosis facilities, in operation at hotels adjacent to 

the park, have historically discharged highly saline wastewater into the storm-drainage system. This collective storm 

and wastewater flow is routed into this recently (ca. 1990s) 

constructed wetland within the park and from there into the 

ocean. A phaseout of surface disposal of reverse-osmosis 

wastewater in favor of subsurface disposal by way of injection 

wells has been planned. Data collected during the reconnaissance indicated that such a phaseout had not yet been 

completed, but were inconclusive with regard to evidence of 

subsurface injection.

Whereas much of the western part of the park is dominated by manmade features, the eastern part is predominantly 

within a Protected Natural Area Zone that contains a 28-acre 

expanse of mangrove shoreline and a 27-acre estuarine system 

(Wagner, 1990). By definition, an estuarine system has an 

ocean-derived salinity greater than 5 parts per thousand (ppt) 

and is at least intermittently tidally flooded (Cowardin and 

others, 1979). The park’s natural wetland meets this definition 

because it is dominantly oligohaline (conductivity, 800–8,000 

μS/cm), its salinity is ocean derived, and it is intermittently 

tidally flooded; however, this flooding is extremely infrequent 

and may occur only during large tropical cyclones and associated storm surges, or by a tsunami.

Viable estuarine systems have become rare within the 

CNMI, and as a result, the park’s natural wetland has been 

designated a Mitigation Policy Resource Category 2 site by 

the U.S. Fish and Wildlife Service (Wagner, 1990). The natural wetland and its mosaic of marshy areas are interspersed 

with emergent wetlands and mixed wet forests of mangrove, 

casuarina, and pandanus that provide critical habitat for 

various migratory and resident waterfowl, including two 

Federally listed endangered species: the Marianas gallinule 

(Gallinula chloropus guami) and the nightingale reed warbler 

(Acrocephalus luscinia). Human activity and development 

are reducing or eliminating similar wetlands throughout the 

CNMI, leaving American Memorial Park as the only known 

Federally protected area possessing viable populations of 

these species (U.S. Fish and Wildlife Service, 1991, 1998).

The park wetland is being threatened by Saipan’s rapid 

population expansion and associated urban development 

(Wagner, 1990). Official population figures indicate a growth 

of 329 percent between the years 1980 and 2000 (from 14,549 

to 62,392; U.S. Census Bureau, 2003); other sources cite 

continuing growth, placing the 2006 population at more than 

82,000 (U.S. Central Intelligence Agency World Factbook, 

URL https://www.cia.gov/cia/publications/factbook/geos/

cq.html, accessed Sept. 15, 2006). The park is undergoing a 

commensurate increase in visitation as a direct result of this 

population expansion (Charles Sayon, National Park Service, 

oral commun., 2005).

The motivation for this study came from recognition of 

the ecologic value of the American Memorial Park wetland, 

the loss of similar wetlands within the CNMI, and the increasing threat posed by rapid development of areas adjacent to 

the park, specifically urbanization of the village of Garapan 

and commercialization of the Puerto Rico harbor facilities. 

To address this concern and to identify the need for additional 

data, the U.S. Geological Survey (USGS), in cooperation 

with the National Park Service (NPS), conducted a reconnaissance study of the hydrology of American Memorial Park, the 

results of which are described in this report.

Introduction 
145°43' 145°43'20" 145°43'40" 145°44'

15°12'40"

15°13'

15°13'20"

15°13'40"

0 0.1 0.2 0.3 0.4 0.5 MILES

Base modified from U.S. Geological Survey digital 

raster graphics, scale 1:25,000, 1999. Universal 

Transverse Mercator projection, zone 55, NAD83 datum.

Constructed

wetland

Discharge

channel

WWII

motor pool

Carillon

Belltower

Flag Circle Pre-WWII road

EXPLANATION

WETLAND AREA (modified from

 Cloud, 1956)

LINE OF SECTION A-A’ IN FIGURE 6

TRACE OF MATANSA FAULT 

 (modified from Cloud, 1956).

Fault

A

A‘

TRACE OF MATANSA 

FAULT (modified from 

Cloud, 1956).”

Figure 2. American Memorial Park on the Island of Saipan, Commonwealth of the Northern Mariana Islands, showing locations of 

selected features and facilities. The eastern part of the park is in a dominantly natural condition, and includes a 27-acre estuarine 

wetland, the last known example of this ecosystem in the Northern Mariana Islands. The western part of the park is largely developed, 

and includes the park’s administrative infrastructure, numerous athletic fields and recreational beach facilities, and a small-boat 

harbor. A constructed wetland was built within the park in the 1990s to moderate the effects of storm runoff from the adjacent village of 

Garapan. Additional features shown include the trace of the Matansa Fault flanking the park’s southeast perimeter, and the location of a 

geologic transect developed in 1998 (see fig. 6).

Purpose and Scope

The purpose of this report is to (1) describe the occurrence of ground water and the salinity of ground and surface 

water within American Memorial Park; (2) present a hydrologic model of the park area of Saipan, with an emphasis 

on the 27-acre estuarine system; and (3) identify additional 

data needed to further develop this model. To determine the 

occurrence of ground water, a 13-well ground-water-monitoring network was configured, incorporating two preexisting monitoring wells coupled with 11 new piezometers. The 

piezometers were installed during the reconnaissance and were 

used not only for monitoring water levels but also for collecting water samples to measure ground-water salinity. Manual 

synoptic water-level surveys were performed, and continuousrecord pressure transducers (hereafter referred to simply as 

transducers) were installed in selected wells, and at the nearby 

harbor for use as a tide gage. The salinity of both ground water 

and surface water, as indicated by conductivity measurements 

made at most of the ground-water sites and selected surfacewater sites, was used to assess the mixing of fresh ground 

water with ocean-derived saltwater. The hydrologic model 

presented in this report, developed through compilation and 

analysis of previous investigations and publications, in combination with measurements and observations made during 

the 2005 reconnaissance, is intended to be a foundation upon 

 Reconnaissance Study of the Hydrology of American Memorial Park, Island of Saipan, CNMI
which future efforts will be built. This report concludes with 

a list of additional data and analyses that would be needed to 

augment the model’s development. This report in not intended 

to suggest and (or) prioritize specific future actions but rather 

to provide a list of topics and efforts that would, if undertaken, 

enhance understanding of the hydrology of American Memorial Park.

Description of Study Area

The Island of Saipan, with an area of 48 mi2, is the largest 

of the 14 islands composing the CNMI. Situated in the western 

Pacific Ocean between longitude 145°40′–145°50′ E. and latitude 15°05′–15°18′ N., Saipan is about 3,700 mi west-southwest of Honolulu, Hawaii, and about 120 mi north-northeast 

of Guam (Carruth, 2003). American Memorial Park lies on the 

western coastal plain and is centrally situated along the long, 

north-south axis of the island. The village of Garapan borders 

the park along its south boundary, and the major port facilities 

of Puerto Rico are less than 1 mi northeast of the park (fig. 1).

Climate

The climate of Saipan is typical for the western Pacific 

region, and this report will capitalize on this similarity to use 

climate data from nearby islands, most notably Guam, when 

local data are absent. Mean annual rainfall ranges from about 

75 in. on the southern plateau to more than 90 in. on the 

central interior highlands (fig. 3). Rainfall along the western 

coastal plain is toward the low end of this range, with rainfall 

in the area of the park averaging 78 in/yr. Rainfall varies seasonally, with the wet-season months of July through December 

receiving more than 72 percent of the mean annual total (fig. 

4; Lander, 2004).

Evapotranspiration is the volume of water directly evaporated from soil and water surfaces and transpired from vegetation through a plant’s stomata (Brutsaert, 1982). Potential 

evapotranspiration, the maximum rate possible given sufficient 

moisture in the soil and on the surface, can be assumed equal 

to evaporation on the basis of pan experiments on Hawaii 

(Ekern, 1966). Actual evapotranspiration is the rate at which 

true vaporization is occurring, given inplace available water 

in the soil (for evaporation as well as plant uptake) and on the 

surface. The distinction between potential and actual evapotranspiration is important, owing to the punctuated frequency 

(seasonality) and intensity (per storm) of rainfall on Saipan; 

actual evapotranspiration will, by definition, always be less 

than or equal to potential evapotranspiration.

Evapotranspiration rates have not been directly studied 

within the CNMI; however, previous research has indicated 

that pan-evaporation rates in Guam, about 120 mi southsouthwest of Saipan, are about 77 in/yr (Gingerich, 2002). 

Additional studies have suggested that evapotranspiration 

rates change minimally in this tropical region, by about 0.4 

in/yr per degree of latitude, decreasing with distance from the 

Equator (Nullet, 1987). Therefore, an evapotranspiration rate 

for Saipan of about 76 in/yr, or about 0.21 in/d, is a reasonable approximation. The daily rate is critical with regard to the 

park wetland because it represents an amount that commonly 

is exceeded by direct rainfall, both seasonally and per storm; 

rainfall in excess of the evapotranspiration rate represents a net 

freshwater input to the wetland. For example, in the 2 months 

during which the reconnaissance took place (Aug.–Sept. 2005) 

hourly rainfall, as measured at Saipan International Airport, 

equaled or exceeded 0.21 in/d on 18 of 61 days, on which days 

the average rainfall was 1.29 in/d. Though not intended to 

represent a rigorous analysis of long-term rainfall patterns on 

Saipan, these data do indicate that direct rainfall on the island 

commonly exceeds the approximate rate of 0.21 in/d; therefore, direct rainfall likely represents a frequent net freshwater 

input to the wetland.

The Northern Mariana Islands lie within the formative 

region for the northwestern Pacific tropical-cyclone basin and 

so may be threatened on a year-round basis by such storms. 

(The northwestern Pacific tropical-cyclone basin is unique 

among global formation basins in this regard.) This is significant in that the primary cause of above-average rainfall in 

the region is tropical cyclones. For example, when Tropical 

Cyclones Carmen, Winnie, and Tess passed over the island 

in August 1978, Saipan recorded its highest monthly rainfall total of 73.25 in., almost 95 percent of the mean annual 

rainfall for this area (Lander, 2004). Typhoon Carmen was 

also responsible for Saipan’s largest single-storm rainfall total 

when 44.5 in. fell during a 48-hour period on August 10–12, 

1978 (van der Brug, 1985). During the reconnaissance, Saipan 

was affected by Supertyphoon Nabi; though not on the order 

of the 1978 series of tropical cyclones, it is still illustrative that 

more than 7 in. of rainfall was recorded at Saipan International 

Airport in association with this storm, an amount that represents approximately 10 percent of mean annual rainfall for the 

island.

An additional climatically driven factor affects relative 

sea level in the western Pacific basin. El Niño/Southern Oscillation (ENSO) events have been recognized as causing interannual changes in Pacific Basin weather patterns (U.S. National 

Oceanic and Atmospheric Administration, URL http://www.

elnino.noaa.gov/, accessed Sept. 15, 2006), but they also 

cause local ocean-level variations that can equal or exceed 

that of astronomic influence. For example, on Saipan, diurnal 

astronomic tides commonly vary by about 2 ft. In comparison, 

the 1997 El Niño event resulted in a local relative ocean-level 

variation of about 1.5 ft (Lander, 2004). These variations are 

expressed as a drop in relative sea level in the western Pacific 

Basin that reaches its lowest levels in December of an El Niño 

year, recovering by the following spring. Conversely, a La 

Niña event describes a rise in relative sea level in the western Pacific Basin. A La Niña event that closely follows an El 

Niño event compounds the change in local relative sea level. 

Therefore, with regard to American Memorial Park, although 

the effects of an El Niño and (or) La Niña event on relative 

sea level can make it difficult to discern long-term trends, the 

Introduction 
80

80

80

80

90

90

76.23

77.34

73.48

78.18

84.38

77.91

84.71

95.52

I Naftan Pt 

Agingan Pt 

Muchot Pt 

Sabaneta Pt 

Laolao Kattan Pt 

Garapan

San Jose 

Susupe

Chalan Kanoa 

Dan Dan 

Kagman

San Antonio 

Koblerville 

Laolao

Bay

Fanonchuluyan 

Bay

PA C I F I C

O C E A N

P H I L I P P I N E

S E A

American Memorial Park boundary 

EXPLANATION 

80

77.91

LINE OF EQUAL MEAN ANNUAL 

 RAINFALL, IN INCHES 

RAIN-GAGING STATION AND 

 MEAN ANNUAL RAINFALL, 

 IN INCHES 

Base modified from U.S. Geological Survey 

digital line graphics, scale 1:25,000, 1999.

Universal Transverse Mercator projection, 

zone 55, NAD83 datum. 

0 0.5 1 2 MILES

145°48'

145°46'

145°44'

145°42'

15°16'

15°14'

15°12'

15°10'

15°08'

Figure 3. Distribution of mean annual rainfall on the Island of Saipan, CNMI (modified from Lander, 2004).

 Reconnaissance Study of the Hydrology of American Memorial Park, Island of Saipan, CNMI
direct connection between ground water and surface water in 

the wetland makes the identification of such trends critically 

important.

Geology

The Island of Saipan is a subareal expression of the 

1,565-mi-long Mariana volcanic island arc, itself the result of 

the Izu-Bonin-Mariana subduction zone wherein the Pacific 

Plate is being subducted beneath the Philippine Plate. The 

island’s ancient volcanic core is dominantly overlain by a 

series of terraced limestone formations (fig. 5). The axial 

uplands descend to the coast in a series of terraced benches, 

some of which are sheer on the seaward side. On the west 

side of the island the limestone formations give way to a 

coastal plain of calcium carbonate (lime) sand and volcanic 

outwash (clay). Furthermore, most of the punctuated topography described by these terraces is probably related to axially 

aligned faults, including the interface between the western 

coastal plain and the uplands to the east (Cloud and others, 

1956).

Saipan’s volcanic core is of Tertiary origin, as are 

many of the Mariana Islands (Cloud and others, 1956). The 

oldest exposed geologic unit, represented by the volcanic 

Sankakuyama Formation, is of mid-Eocene origin, radiometrically dated at ~41 m.y. Significant active volcanism appears 

to have ended during the Miocene, as evidenced by K/Ar ages 

of the youngest volcanic rocks, represented by the Fina-Sisu 

Formation, of 13 m.y. (Meijer and others, 1983).

Changes in relative sea level since the volcanic emplacements have resulted in the island’s volcanic core being 

overlain with numerous and distinct limestone formations, the 

age of the oldest of which, the Tagpochau Limestone, has been 

constrained through fossil analysis as Miocene, most likely 

early Miocene (~23 m.y.). The youngest rocks, the Tanapag 

Limestone, are Pleistocene but remain locally constructional 

(Cloud and others, 1956).

These changes in relative sea level have been episodic, 

and the island’s limestone formations have a typical geometry 

as a result; the formations are typically terraced, and are characterized by horizontal or slightly eastward sloping benches 

separated by seaward-facing scarps or steeply sloping surfaces 

(Cloud and others, 1956). Additionally, the abundance of 

rainfall has resulted in the formation of significant karst topography, evidence of which is plentiful throughout the island but 

most striking in the extensive subterranean caverns along the 

South Kalabera Cliffs, and in the shoreline sinkholes dotting 

Saipan’s northeast and east coast.

Seismic activity is common in the Mariana Trench 

region, and some of the episodic changes in relative sea 

level may be related to tectonic events, as well as being of 

glacioeustatic origin. As evidence of such events, numerous 

trench-parallel faults dissect the island, generally trending 

south-southwest and dipping west-northwest. Some exposed 

faults are expressed as weathered gaps in the limestone 

country rock, suggesting the presence of subsurface structures 

that may be acting as zones of locally higher permeability. 

Conversely, cuttings from boreholes drilled adjacent to some 

faults have shown compressional deformation of the country 

rock, suggesting areas of locally lower permeability. Cloud 

and others (1956) identified faults on the southern part of 

the island as purely dip-slip from examination of observable 

slickensides, suggesting that the fault-plane surfaces have been 

altered through compression to a more erosion resistant state, 

a condition that has been equated with locally lower permeability in other areas. Cloud and others observed evidence of a 

major nearshore fault complex, identified as the Matansa, and 

suggested that this fault complex extends to the present-day 

boundary of American Memorial Park (fig. 2); the fault complex approximately describes the interface between the western coastal plain and the topographic rise to east. The fault 

complex is generally colinear with Pale Arnold (also known 

locally as Middle Road), which itself describes the southeast 

boundary of the park. Because it is parallel to, proximal to, 

and upgradient of the southeast boundary, the fault complex 

may affect ground-water flow patterns with regard to the park 

as a whole and to the natural-wetland area in particular.

Cloud and others (1956) also suggested that the western 

coastal plain has been seismically downdropped below existing sea level and is therefore in a constructional phase, and 

that this plain is the most westward of the subaerial terraced 

limestone structures formed by trench-parallel faults. This 

suggestion is supported by the U.S. Department of Agriculture 

(1989), who identified the soil in the western part of the park, 

and in the eastern part seaward of its wetland, as Shioya loamy 

sand “formed in water-deposited coral sand” areas parallel 

to the shoreline. They further identified the wetland-area soil 

0

2

4

6

8

10

12

JAN JUNE JULY AUG SEPT OCT NOV DEC

MONTH

MEAN MONTHLY RAINFALL, IN INCHES 

FEB MAR APR MAY 

Figure 4. Mean monthly rainfall at Saipan International Airport 

(SIA) during period 1954–99 (data from Lander, 2004).

Introduction 
0

0

1

1

2

2 KILOMETERS

MILES

Sabaneta Pt 

Laolao Kattan Pt 

15 ̊06'

15 ̊08'

15 ̊10'

15 ̊12'

15 ̊14'

15 ̊16'

145 ̊44'

145 ̊48'

I Naftan Pt 

Agingan Pt 

Muchot Pt 

Mt. Achugao 

Pidos

Kalahe

Mt. Tagpochau 

Mt. Talufofo 

Mt. Laulau 

Mt. Achugao A

A'

Pidos

Kalahe

Mt. Tagpochau 

Mt. Talufofo 

Mt. Laolao

Qr

Qta

Qm

Tf

Ts

Td

Th

Qr

Qta

Qm

Tf

Tt

Ts

Td

Th

Mañagaha

Island

Tanapag I Spring 

(Bobo Agatan) 

Tanapag II Spring 

(Bobo Mames) 

Papago

Spring

A

GEOLOGIC UNIT 

LINE OF SECTION 

FAULT−High angle or vertical 

SELECTED SPRING NEAR SECTION LINE AND NAME 

MAP

limestones volcanic rocks 

A A'

Tt

Td

Th

Ts

Tf

Qr

Qta

Qm

Tt

Td

Th

Ts

Tf

Qr

Qta

Qm

Papago

Spring

EXPLANATION 

Quaternary Deposits−Emerged limesand, beach, wetland, 

artificial fill, and volcanic outwash

Tanapag Limestone−Raised fringing-reef limestone, generally

below elevations of 100 feet

Mariana Limestone−Clastic and reef limestone with argillaceous

rubbly facies

Tagpochau Limestone−Dominated by a massive compact,

inequigranular pinkish limestone, but also includes rubbly, marly,

tuffaceous and transitional carbonate facies

Fina-Sisu Formation−Interlayered andesitic marine tuffs and andesite

flow rocks

Densinyama Formation−Includes marine transitional rocks and

volcanically derived conglomerates, tuffaceous sandstone, and

andesitic breccia

Hagman Formation−Andesitic breccia, tuff, lava flows, conglomerate,

and tuffaceous limestone

Sankakuyama Formation−Dacitic tuff, breccia, and flow rocks

American Memorial 

Park boundary 

Base modified from U.S. Geological Survey

digital line graphics, scale 1:25,000, 1999. 

Universal Transverse Mercator projection, 

zone 55, NAD83 datum.

Figure 5. Generalized geologic map of the Island of Saipan, showing (A) surficial geology, and (B) a geologic cross 

section (modified from Cloud and others, 1956; Carruth, 2003).

 Reconnaissance Study of the Hydrology of American Memorial Park, Island of Saipan, CNMI
A A'

SEA LEVEL SEA LEVEL 

FEET

Qr

Qm Tt

Th

Th

Th Th Th

Th Th Th

Th

Td

Td

Td

Tt

Tt

Qm

Qm Tt

?

?

?

Th

Th

Th Th Th

Th Th Th

Th

?

Td

Td

Td

Tt

Mt. Talufofo 

? ?

?

?

Tt

Tt

?

?

Qr QtaQr

?

Qm

Qm

?

-800

-600

-400

-200

200

400

600

800

B

GEOLOGIC UNIT 

FAULT−High angle or vertical; arrows show direction 

of thrust 

SECTION

limestones volcanic rocks 

Tt

Td

Th

Tf

Qr

Qta

Qm

Tt

Td

Th

Tf

Qr

Qta

Qm

EXPLANATION 

Quaternary Deposits−Emerged limesand, beach, wetland, 

artificial fill, and volcanic outwash

Tanapag Limestone−Raised fringing-reef limestone, generally

below elevations of 100 feet

Mariana Limestone−Clastic and reef limestone with argillaceous

rubbly facies

Tagpochau Limestone−Dominated by a massive compact,

inequigranular pinkish limestone, but also includes rubbly, marly,

tuffaceous and transitional carbonate facies

Fina-Sisu Formation−Interlayered andesitic marine tuffs and andesite

flow rocks

Densinyama Formation−Includes marine transitional rocks and

volcanically derived conglomerates, tuffaceous sandstone, and

andesitic breccia

Hagman Formation−Andesitic breccia, tuff, lava flows, conglomerate,

and tuffaceous limestone

0

0

0.5

0.5 1 KILOMETER

1 MILE

Figure 5. Continued.

as Mesei Variant muck, with a surface typically below the 

high water-table elevation, and areas inland of the wetland as 

Chinen very gravelly loam, a well-drained soil developed over 

limestone plateau formations.

At the behest of the U.S. Army Corps of Engineers, 

several studies were undertaken in and around American 

Memorial Park during the period 1980–2004, the earlier of 

which focused on finding solutions to problems associated 

with frequent flooding of the village of Garapan. The later 

studies, beginning in 1992 and continuing through 2004, were 

intended to assess and document the composition and extent 

of hydrocarbon pollutants anthropogenically introduced into 

soils and ground water in and adjacent to the park, to develop 

remediation methods to address the pollution, to track the 

progress of remediation as it was implemented, and to plan for 

and document the removal of infrastructure associated with 

the remediation effort (Woodward-Clyde Consultants, 1992; 

Matrix Remedial Technologies, Inc., 1995; Ogden Environmental and Energy Services Co., Inc., 1997, 1998, 1999a, b, 

2002; Wil Chee-Planning, Inc., 2004a–c). Ogden Environmental and Energy Services Co., Inc.’s (1998) study included 

trenching in and inland of the park wetland for the purpose of 

identifying shallow lithologic units, the presence of cultural 

artifacts, and evidence of introduced hydrocarbons. Monitoring wells were then installed to assess the spatial extent of 

introduced hydrocarbons. The results of that study indicate 

that the soils beneath and adjacent to the wetland grade from 

silty clay (underlying the wetland), through silty coralline 

sandy fill, to poorly sorted, gravelly coralline sand (fig. 6). 

These results agree with previous studies by Cloud and others 

(1956) and the U.S. Department of Agriculture (1989).

Hydrogeology

The hydrogeology of Saipan, on an island scale, is well 

understood and documented on the basis of previous studies in the Mariana Islands and other places that are generally 

analogous to Saipan geologically (Gingerich, 2002; Carruth, 

2003). The basic conceptual model for an ocean-island aquifer 

describes a basal freshwater lens overlying ocean-derived saltwater, a configuration resulting from fluid-density differences; 

freshwater has a density of about 1 g/mL, whereas saltwater 

has a density of about 1.025 g/mL. This fluid-density difference, under hydrostatic conditions, would result in a sharp 

interface at a depth below sea level that is 40 times the height 

of the water table above sea level, commonly referred to as the 

Introduction 
Ghyben-Hertzberg relation (Todd, 1980). Under typical field 

conditions of mixing influences, a salinity gradient is present 

instead of the theoretical sharp interface.

On Saipan, this basic conceptual model is augmented 

by the presence of higher-level freshwater bodies, used for 

domestic production, on the east-central Kagman Peninsula, 

and in the volcanic basement rocks and overlying limestone 

in the central highlands (fig. 7; Carruth, 2003). In general, the 

limestone has a relatively high permeability due to its method 

of formation and (or) postdepositional dissolution, whereas the 

volcanic basement rocks have a relatively low permeability, 

a factor that has so far precluded their use as a developable 

resource. Additionally, previous studies have generalized 

that the basal freshwater lens becomes brackish inland of 

the shoreline, owing to the highly permeable limestone that 

encircles the island’s volcanic core.

History

Saipan was the site of intense conflict during World War 

II, when vast tracts of the island were so extensively bombed 

that surface soils were stripped away, leaving only exposed 

bedrock. The wetland area of the (as yet undesignated) park 

was said to have been spared this level of damage (Israel 

Sablan, National Park Service, oral commun., 2005), an assertion supported by aerial photographs taken before, during, and 

after the conflict that all show the wetland with approximately 

the same position and geometry as at present (fig. 8). Additionally, postconflict studies funded by the U.S. Department 

of Defense identify the wetland as Quaternary marshes, citing 

it as “once a brackish pond closed off by the [Point Muchot] 

spit” (Cloud and others, 1956; Davis, 1958).

American Memorial Park was created as part of a 1977 

Covenant with the United States to grant CNMI its independence. Article VIII, section 803(e) of Public Law 94–241 set 

aside 133 acres “to honor the American and Marianas people 

who died in the World War II Marianas Campaign” (CNMI 

Law Revision Commission, URL http://cnmilaw.org/covenant.

htm, accessed Sept. 15, 2006).

Land Use

Historically, the areas of highest population density on 

Saipan have been clustered along the southwestern coastal 

plain and the southern upland peninsula. Recent development 

has shifted toward the central part of the island: the Kagman 

?

?

?

?

?

?

?

EXPLANATION

WATER LEVEL DURING 

 DRILLING 

LITHOLOGIC CONTACT 

INFERRED LITHOLOGIC

 CONTACT 

INORGANIC CLAYS OF LOW 

 PLASTICITY, GRAVELLY CLAYS, 

 SANDY CLAYS 

SILTY GRAVELS, GRAVEL- 

 SAND MIXTURES 

WELL GRADED GRAVELS, 

 SAND MIXTURES, LITTLE TO 

 NO FINES 

INORGANIC SILTS AND VERY 

 FINE SANDS, CLAYEY SILTS 

 WITH SLIGHT PLASTICITY 

SILTY SANDS, SILT MIXTURES 

WELL-GRADED SANDS, 

 GRAVELLY SANDS, LITTLE 

 TO NO FINES 

UNDETERMINED LITHOLOGY 

CL

GM

GW

ML

SM

SW

UL

-4

0

-2

2

4

6

8

10

0 100 200 300 400 500 600 700 800 900

DISTANCE, IN FEET 

ELEVATION, IN FEET RELATIVE TO MEAN SEA LEVEL 

ML

ML/SM ML/SM

ML/SM

GW

SW

UL

SW/GW

SW/GW

SW

SW

SW/GW

SW/GW

SM/CL

SW

UL

ML

SW/GM

SW

SW/GW

UL

SM/GM

SW

UL

GM

SW/GM

UL

WETLAND

ROAD

A A‘

Figure 6. A geologic cross section A–A’ (see fig. 2 for location) was developed near the southeast perimeter of the American 

Memorial Park’s natural wetland in conjunction with a series of environmental impact and remediation studies undertaken in the 1990s 

(modified from Ogden, 1998).

10 Reconnaissance Study of the Hydrology of American Memorial Park, Island of Saipan, CNMI
peninsula on the east and the Garapan area on the west. The 

village of Garapan has become a center of commerce, with 

most tourism-focused development on its north end (directly 

adjacent to the southwest boundary of the park), the island’s 

primary hospital and medical facilities (directly adjacent to 

the south-central boundary of the park), a small-boat marina 

(within the park but administered by the CNMI Department of 

Fish and Wildlife), and a large commercial port facility. Separating the park from this port facility is an abandoned solidwaste dump along the northeast boundary of the park.

Production wells fields, providing water for domestic and 

nondomestic supply, are located upgradient of the park (fig. 

9). Numerous private wells, including production (withdrawal) 

and waste-disposal (injection) wells that have been drilled 

on hotel properties adjacent to the southwest boundary of the 

park, are largely unmonitored, with regard to their condition 

and rates of withdrawal or injection, by government agencies.

The easternmost area of the park was used as a motor 

pool by the U.S. military during, and for a brief period of time 

after, World War II (figs. 2, 8B). Clustered across what is now 

the southeast boundary of the park were fuel-service facilities 

used for both military and civilian purposes (Ogden Environmental and Energy Services Co., Inc., 1998). Additionally, 

numerous fuel pipelines ran the length of the park’s southeast 

perimeter alongside what is now Pale Arnold. All of these 

features were known to have been the source of chemical spills 

(Ogden Environmental and Energy Services Co., Inc., 1998). 

In the 1990s, several studies were undertaken at the behest of 

the U.S. Army Corps of Engineers to assess the contamination, including the installation of monitoring and remediation 

(injection) wells (Woodward-Clyde Consultants, 1992; Matrix 

Remedial Technologies, Inc., 1995; Ogden Environmental and 

Energy Services Co., Inc., 1997, 1998). Remediation efforts 

were completed in 2002 (Ogden Environmental and Energy 

Services Co., Inc., 2002). The monitoring wells were subsequently sealed by the U.S. Army Corps of Engineers (Wil 

Chee-Planning, Inc., 2004c).

D

D'

WEST EAST

D D'

? ?

?

?

?

?

250

? ?

?

BEND IN SECTION 

10,000

250

10,000

?

?

?

? ?

?

? ?

?

0 5,000 FEET

VOLCANIC ROCK 

(low permeability) 

LIMESTONE

(high permeability) 

LIMESTONE

(high permeability) (

Vertical scale greatly exaggerated 

-300

-200

-100

100

200

300

400

500

600

700

800

900

1,000

1,100

1,200

FEET

SEA LEVEL

Spring

discharge

or gaining 

stream

Recharge

Runoff

Evaporation

Precipitation

Ground-water

flow

Water table 

Water table 

Basal freshwater lens Basal freshwater lens 

High-level freshwater aquifer 

FRESHWATER

TRANSITION ZONE AND SALTWATER 

LINE OF EQUAL CHLORIDE 

 CONCENTRATION, IN MILLIGRAMS 

 PER LITER--Dashed where approximate or 

 inferred, interval is variable. 

EXPLANATION

250

Figure 7. Hydrogeologic cross section D–D’ through the Island of Saipan, showing details of the ground-water-flow system (modified 

from Carruth, 2003).

Introduction 11
Figure 8. Aerial photographs of the Island of Saipan taken before (ca. 1936), during (ca. 1945), and after (ca. 1970) World War II provide lo

evidence of a persistent natural wetland within the current boundaries of American Memorial Park (provided by D. Minton, 2006).

145°43' 145°43'20" 145°43'40" 145°44'

15°12'40"

15°13'

15°13'20"15°13'40"

0 0.1 0.2 0.3 0.4 0.5 Miles

American Memorial Park boundary

145°43' 145°43'20" 145°43'40" 

15°12'40"

15°13'

15°13'20"15°13'40"

0 0.1 0.2 

(C) 1970

American Memorial Park boundary

145°43' 145°43'20" 145°43'40" 145°44'

15°12'40"

15°13'

15°13'20"15°13'40"

0 0.1 0.2 0.3 0.4 0.5 Miles

(B) 1945

145°43' 145°43'20" 145°43'40" 145°44'

15°12'40"

15°13'

15°13'20"15°13'40"

0 0.1 0.2 0.3 0.4 0.5 Miles

American Memorial Park boundaryAmerican Memorial Park boundary

(A) 1936
Previous Investigations

The principal documents describing Saipan’s geology and 

hydrology are USGS publications (Cloud and others, 1956; 

Davis, 1958; van der Brug, 1985; Carruth, 2003). Cloud and 

others described the island’s geology but also touched upon 

other topics, including Saipan’s potable-water resources. Davis 

addressed Saipan’s hydrology and provided the first complete 

list of surface- and ground-water resources on the island. Van 

der Brug built on these early efforts by compiling a comprehensive assemblage of Saipan’s hydrologic resources and data. 

Most recently, Carruth brought together the numerous factors 

influencing ground-water availability, including land use, geology, geography, climate, aquifer configuration, and current 

production facilities.

Acknowledgments

This research was supported by the NPS, with funds 

administered by the USGS Pacific Islands Water Science Center. I thank Dwayne Minton (NPS) for his advice and expertise 

in site knowledge and access facilitation, Israel “Sid” Sablan 

(NPS) for his dedication and hard work with instrument installation and data collection, and Dale Nishimoto (USGS) for his 

field expertise and assistance. I also appreciate the hard work 

of Rob Carruth (USGS) for his Saipan facilitation efforts, 

ground-water-network installation, and subsequent data collection, and of Delwyn Oki (USGS) for providing project 

guidance and advice.

Reconnaissance

The hydrology of Saipan is well documented on an island 

scale; however, local areas, such as American Memorial Park, 

generally are poorly understood. To improve understanding 

of the hydrology of the park and to enable identification of 

additional information needs, a reconnaissance of the park was 

conducted from August 24 through September 19, 2005. The 

primary goal of the reconnaissance was to determine whether 

surface- and ground-water bodies were connected within the 

park wetland. A secondary goal was to determine the spatial 

distribution of salinity with regard to ground water and, to a 

lesser extent, surface water. Conductivity was measured to 

enable this determination; the results of such measurements 

are commonly used as a proxy for salinity, as in this report.

Network Components and Installation

To achieve the goals of the reconnaissance, a 13-well 

ground-water-monitoring network was configured, incorporating two preexisting monitoring wells coupled with 11 new 

piezometers (fig. 10). The piezometers, which were installed 

during the reconnaissance, were used not only for monitoring 

water level but also for collecting water samples for measuring 

ground-water salinity. Four of these wells (AMP01, AMP02, 

AMP05, AMP13) were located in surficially dry areas of the 

park; two wells (AMP03, AMP04) were located adjacent 

to, and strongly influenced by, the constructed wetland and 

its associated discharge channel; and the other seven wells 

(AMP06 through AMP12) were located in or near the natural 

wetland. The 11 newly installed piezometers were manually 

driven to depths ranging from 6 to more than 19 ft (table 1). 

Locations where significant driving resistance was encountered at depths less than 6 ft below grade, or which did not 

achieve a minimum depth of 3 ft below observed water level, 

were rejected to accommodate expected tidal variation. A 

measuring point referenced to mean sea level was established 

on each wellhead, and all the measuring points were surveyed 

to a vertical precision of 0.01 ft, using U.S. National Geodetic 

Survey (NGS) stations (resurveyed in 2003) as reference 

points.

Although the method of piezometer installation (impact 

slidehammer) did not produce cuttings that could be examined, 

site preparation included excavation of material to a depth of 

12 to 18 in. Materials observed during the excavation ranged 

from coarse calcareous sand to coralline cobbles, as much as 6 

in. in diameter. At places along the southeast boundary of the 

park, coherent layers of Tanapag Limestone were encountered, 

which precluded the installation of piezometers.

At a site within the natural wetland a surface-water-stilling well (AMP09S) was paired with a ground-water-monitoring well (piezometer AMP09G) to determine whether surface 

water and ground water were connected. The stilling well, 

which was directly attached to the piezometer riser pipe, was 

constructed of polyvinyl chloride (PVC), with holes drilled at 

regular intervals to allow for the free flow of water (fig. 11).

A tide gage, installed in the immediate vicinity of the 

park, was used to constrain the effects of ocean tides on the 

local ground-water system. The gage, which was included in 

the monitoring-network leveling effort, was similarly surveyed 

to a precision of 0.01 ft, using NGS stations as reference 

points. The tidal data used throughout this report were collected at this site.

Collection Methods

Surface- and Ground-Water Salinity

Conductivity, commonly used as a proxy for salinity, was 

measured, and data were collected, in both surface water and 

ground water, using a calibrated handheld conductivity meter. 

Surface-water measurements were made as the wetland and 

surrounding park areas were initially reconnoitered. Groundwater measurements were made after the ground-water-monitoring-network wells were installed and purged.

Wells were purged in accordance with USGS protocols, as described by Wilde and Radtke (2005). The primary 

Reconnaissance 13
Laolao

Bay

Tanapag

Harbor

Fanonchuluyan

Bay

Lake

Susupe

145o48'

15o16'

15o12'

15o08'

145o44'

Sabaneta Pt 

Muchot Pt 

San

Antonio

Chalan

Kanoa

Base modified from U.S. Geological Survey

topographic map, Island of Saipan, scale 

1:25,000, 1983. Shaded relief from U.S. Geological 

Survey 10-meter digital elevation model. 

Puerto

Rico

Calhoun

Sablan

Quarry

and

Maui IV 

Marpi

Quarry

Gualo

Rai

Chalan

Kiya

San Vicente Kagman

Peninsula

I Naftan Pt 

Agingan Pt 

San Roque 

0

0

1

1

2

2 KILOMETERS

MILES

Mt. Tagpochau 

Saipan

International

Airport

Akgak

Capital

Hill

Dan Dan 

Kagman

Koblerville Isley Field 

and Obyan 

PACIFIC

OCEAN

PHILIPPINE

SEA

Puerto

Rico MUNICIPAL WELL FIELD AND NAME 

American Memorial 

Park boundary 

Figure 9. Numerous municipal well fields have been developed on the Island of Saipan, including the Calhoun, Sablan 

Quarry, and Maui IV fields that are hydrologically upgradient from American Memorial Park (modified from Carruth, 2003).

14  Reconnaissance Study of the Hydrology of American Memorial Park, Island of Saipan, CNMI
145°43' 145°43'20" 145°43'40" 145°44'

15°12'40"

15°13'

15°13'20"

15°13'40"

0 0.1 0.2 0.3 0.4 0.5 MILES

Base modified from U.S. Geological Survey digital 

raster graphics, scale 1:25,000, 1999. Universal 

Transverse Mercator projection, zone 55, NAD83 datum.

Constructed

wetland

Discharge

channel

AMP02

AMP04

AMP05

AMP06

AMP08 AMP09 AMP12

AMP13

AMP10

AMP07 AMP11 AMP03

AMP01

EXPLANATION

AMP01

GROUND-WATER-MONITORING WELL

 AND NUMBER

TIDE GAGE

Figure 10. The American Memorial Park ground-water-monitoring network was installed to enable the collection of ground-water 

data (water-level and salinity). A tide gage was installed at Tanapag Harbor to enable tidal lags to be calculated from water-level data 

collected during the synoptic surveys.

protocol requires purging a minimum volume of water equal 

to 3 times the casing volume before withdrawing a sample for 

measurement. Secondary protocols allow for exceptions to this 

method for wells with sluggish recovery rates. When the rate 

of purging is high, the permeability of the aquifer is low, or the 

connection between the monitoring well and aquifer is poor, 

water can be completely evacuated from the well. In such situations, water quality can be measured after the water level has 

recovered to at least 90 percent of the level measured before 

evacuation, provided that full recovery occurs within 24 hours 

of evacuation. Of the 13 wells employed for the reconnaissance, 4 qualified for this exception.

Ground-Water Elevations

To develop a spatial representation of ground-water elevations within American Memorial Park and, most importantly, 

within the natural wetland, water-level data needed to be 

collected from spatially distributed sites over a broad range of 

temporal conditions. Two methods were used in the collection 

of these data: installation of transducers, and manual synoptic 

water-level surveys.

Transducers were installed in the surface-water-stilling 

well (AMP09S), in its colocated ground-water-monitoring well 

(piezometer AMP09G), and in the tide gage. Initially installed 

Reconnaissance 15
on September 7, 2005, the transducers at the colocated site 

were intended to determine whether ground water and surface 

water were connected. However, ground-water data from this 

site were unavailable because of equipment failure; nonetheless, the surface-water data do provide evidence indicative of, 

and consistent with, connection to ground water. The transducer used as a tide gage, installed at a former U.S. National 

Oceanic and Atmospheric Administration (NOAA) site from 

15:15 Chamorro standard time (Ch.s.t.) September 14 through 

17:30 Ch.s.t. September 18, 2005, continuously collected data 

at 5-minute intervals.

The other ground-water-monitoring wells (AMP06, 

AMP10) that were installed with transducers were selected 

to expedite synoptic water-level surveys of the wetland area. 

Later analysis indicated that these wells may have been completed in areas of very low permeability, on the basis of the 

attenuated amplitude of the tidal signal and the long recovery 

times observed during well purging. Water-level data from 

these wells have not been incorporated into this report because 

of these anomalies; however, these sites merit further study 

because they may represent distinct hydrogeologic regimes 

within the park.

Water-level data were collected by way of synoptic waterlevel surveys at the 10 ground-water-monitoring wells not 

installed with transducers. These 10 wells were subdivided 

into two clusters: a western group (wells AMP01 through 

AMP05) and an eastern group (wells AMP07, AMP08, 

AMP11, AMP12, AMP13). Characterization of the natural 

wetland was a priority of the reconnaissance; accordingly, 

both high- and low-tide data were collected from the eastern 

(natural-wetland) group, whereas only high-tide data were collected from the western group.

On the final day of the reconnaissance, an attempt was 

made to constrain head differences with depth by temporarily 

installing an additional piezometer at each of the 13 network sites until ground water was encountered. Comparative 

water-level measurements were then made in both the network 

well and the additional piezometer, after which the additional 

piezometer was removed for reuse.

Table 1. Ground-water monitoring network configuration. Depth to water measurements were collected on September 15, 2005.

[ft, feet; AMSL, above mean sea level; dd/mm/ss.s, degrees/minutes/seconds; μS/cm @ 25°C, micro Siemens per centimeter at 25°C; °C, degrees Celsius;

– –, not available; N/A, not applicable]

Well ID

Well 

depth 

(ft)

Screened-interval 

elevation1

(midpoint)

(ft AMSL)

Measuring-point

elevation

(ft AMSL)

Depth to water

Latitude 

(NAD83)

Longitude 

(NAD83)

Water 

depth 

(ft)

Water 

level 

(ft AMSL)

Time of

measurement2

AMP01 8.63 −2.56 5.57 4.88 0.69 12:00 p.m. N. 15°13 02.9" E. 145°43'01.8"

AMP02 7.66 −1.83 5.33 4.61 0.72 12:04 p.m. N. 15°13 07.7" E. 145°43'04.8"

AMP03 11.81 −4.77 6.54 6.25 0.29 12:10 p.m. N. 15°13'00.3" E. 145°43'07.3"

AMP04 8.80 −3.33 4.97 4.45 0.52 11:49 a.m. N. 15°13'03.2" E. 145°43'12.0"

AMP05 6.21 – – 3.65 1.85 1.80 11:53 a.m. N. 15°12'55.4" E. 145°43'15.8"

AMP06 19.53 −15.56 3.47 3.39 0.08 10:51 a.m. N. 15°12'55.3" E. 145°43'19.9"

AMP07 9.62 −4.16 4.96 4.51 0.45 11:47 a.m. N. 15°12'58.7" E. 145°43'21.7"

AMP08 11.81 −5.48 5.83 5.67 0.16 11:53 a.m. N. 15°12'56.2" E. 145°43'25.2"

AMP09G 12.45 −7.42 4.53 4.11 0.42 10:34 a.m. N. 15°12'55.4" E. 145°43'31.3"

AMP09S N/A N/A 1.52 N/A N/A N/A N. 15°12'55.4" E. 145°43'31.3"

AMP10 14.87 −4.13 10.24 10.03 0.21 11:21 a.m. N. 15°12'51.1" E. 145°43'32.8"

AMP11 12.63 −7.76 4.37 4.43 -0.06 12:01 p.m. N. 15°13'01.2" E. 145°43'33.3"

AMP12 6.24 −2.72 3.02 2.66 0.36 12:09 p.m. N. 15°12'55.0" E. 145°43'36.0"

AMP13 6.59 – – 3.45 3.12 0.33 12:04 p.m. N. 15°13'02.9" E. 145°43'36.5"

1

 Midpoint of the screened interval is 0.5 ft less than well depth. Configurations of pre-existing monitor wells AMP05 and AMP13 are unavailable, so 

midpoints are unknown.

2

 All times are Chamorro Standard Time (ChST). For context, a reading of −1.31 ft AMSL was recorded at the tide gage at 12:00 p.m. ChST, September 15, 

2005.

16  Reconnaissance Study of the Hydrology of American Memorial Park, Island of Saipan, CNMI
Results

During the reconnaissance, four discrete bodies, or zones, 

of surface water were observed within the natural wetland 

(fig. 12). Surface-water salinity was comprehensively measured in all four surface-water zones on August 26, 2005. Further surface-water salinity measurements were made as needed 

throughout the park during the reconnaissance, including in 

the constructed wetland and its associated discharge channel. 

Ground-water water level and salinity were measured throughout the reconnaissance.

Surface-Water Salinity

Surface-water salinity was observed to vary spatially and 

temporally. The standard conceptual hydrologic model for 

an island aquifer system suggests that surface-water salinity should increase with proximity to the ocean. Contrary to 

this expectation, field measurements in the wetland surfacewater zones displayed a general pattern of decreasing salinity 

with proximity to the ocean except in surface-water zone 4, 

as discussed below (fig. 12, table 2). Conductivities within 

surface-water zone 2, the largest and most central zone, ranged 

from 1,540 to 4,370 μS/cm. The lowest salinities were measured in surface-water zone 1, the most seaward zone, where 

conductivities were less than 1,000 μS/cm, with a minimum 

of 829 μS/cm; the highest salinities were measured in surfacewater zone 3, the most inland zone, where conductivities were 

greater than 5,000 μS/cm, with a maximum of 8,750 μS/cm, 

the single highest conductivity measured within the wetland 

during the reconnaissance. Note that these measurements 

were made after an 8-day period during which less than 0.5 

in. of rainfall was recorded at the Saipan International Airport 

climate station. (The long-term average for an 8-day period 

in August on Saipan is slightly more than 3.0 in.) Given the 

Figure 11. A surface-water-stilling well (AMP09S) was paired 

with a ground-water-monitoring well (piezometer AMP09G) 

to determine whether surface water and ground water 

were connected within the park’s natural wetland. Weatherresistant boxes contained vented transducer cable “tails” that 

accommodated changes in barometric pressure.

Table 2. Surface-water specific electrical conductance (μS/cm @ 25°C) data collected in the natural wetland zones on August 26, 

2005.

[μS/cm @ 25°C, micro Siemens per centimeter at 25°C; °C, degrees Celsius]

Surface-water

zone

Number of

measurements Median Range

Low High

1 1 3 873 829 973

2 2 9 3,220 1,540 4,370

3 3 3 5,760 5,510 8,750

4 4 13 6,910 4,430 8,290

1

 Discrete measurements in zone 1: {829; 873; 973}.

2

 Discrete measurements in zone 2: {1,540; 2,200; 2,540; 3,160; 3,220; 3,310; 3,490; 3,780; 4,370}.

3

 Discrete measurements in zone 3: {5,510; 5,760; 8,750}.

4

 Discrete measurements in zone 4: {4,430; 4,540; 4,600; 6,620; 6,620; 6,820; 6,910; 7,640; 7,760; 7,980; 8,010; 8,060; 8,290}.

Reconnaissance 17
145°43' 145°43'20" 145°43'40" 145°44'

15°12'40"

15°13'

15°13'20"

15°13'40"

0 0.1 0.2 0.3 0.4 0.5 MILES

Base modified from U.S. Geological Survey digital 

raster graphics, scale 1:25,000, 1999. Universal 

Transverse Mercator projection, zone 55, NAD83 datum.

Constructed

wetland

Discharge

channel

Pre-WWII road

EXPLANATION

SURFACE-WATER ZONE AND NUMBER

 (VALUES IN PARENTHESES REPRESENT RANGE

 OF FLUID SPECIFIC-ELECTRICAL CONDUCTANCE 

 MEASURED AT SELECTED SITES WITHIN THE 

 ZONE, IN MICROSIEMENS PER CENTIMETER)

(829−973)

(5,510−8,750)

(4,430−8,290)

(45,000−62,500

(1,540−4,370)

2

4

1

(829−973) 1

3

Figure 12. Four discrete zones of surface-water were observed during the reconnaissance of American Memorial Park. Conductivities 

measurements of surface-water within these zones indicated a general, and counterintuitive, pattern of decreasing salinity with 

proximity to the ocean.

approximate evapotranspiration rate of 0.21 in/d, or about 

1.68 in. over an 8-day period, evapotranspiration should have 

elevated the salinity of all wetland surface water absent some 

additional input of freshwater.

Surface-water zone 4, in which ground-water-monitoring 

well AMP06 was centrally located, had been recognized and 

documented before the reconnaissance for its anomalous vegetation; a previous survey identified the vegetation within this 

part of the wetland as Paspalum distichum, a type of saltgrass, 

bordered by Scirpus littoralis, a species known as a preferred 

habitat by the Federally listed endangered species Marianas 

gallinule (Gallinula chloropus guami), or common moorhen 

(fig. 13; Tenorio, 1979; Raulerson and Rinehart, 1989). Conductivities in surface-water zone 4 ranged from 4,430 to 8,290 

μS/cm; the higher value, 8,290 μS/cm, was the second-highest 

surface-water conductivity value measured within the natural 

wetland. Furthermore, unique to this surface-water zone, these 

measurements indicated that the salinity decreased radially 

outward from the approximate center of surface-water zone 4.

The conductivity of surface water in the constructed 

wetland ranged from 45,000 to 62,500 μS/cm, with the highest 

values proximal to the storm-drain inflow (into which reverseosmosis wastewater was being discharged) and decreasing 

with distance from this point of inflow (fig. 14). These values 

exceeded nominal seawater conductivity by as much as 25 

percent providing strong evidence that surface disposal of 

reverse-osmosis wastewater was occurring during the reconnaissance.

18  Reconnaissance Study of the Hydrology of American Memorial Park, Island of Saipan, CNMI
Figure 13. A ground-water-monitoring well (AMP06) was 

installed within surface-water zone 4 of the natural-wetland 

area of American Memorial Park. Aspects of zone 4 which 

were anomalous include the extensive presence of saltgrass, 

the dominant vegetation shown immediately surrounding the 

ground-water-monitoring well, and a radial pattern of outwardly 

decreasing surface-water salinities from the approximate center 

of the zone.

Figure 14. Ground-water-monitoring well AMP03 was installed 

near the constructed wetland (shown in the near background); in 

the far background is one of the hotels utilizing reverse-osmosis 

to obtain potable water, and releasing highly-saline wastewater 

into the storm-drainage system as a bi-product of the process.

Significant temporal changes in surface-water salinity were recorded in association with Supertyphoon Nabi; 

salinities were locally lower after the storm passed over the 

island on August 31, 2005. Surface-water measurements in the 

approximate center of surface-water zone 4 indicated that the 

prestorm conductivity of 8,290 μS/cm had dropped more than 

57 percent to 3,530 μS/cm.

Ground-Water Salinity

Conductivities of ground-water samples from wells in 

the monitoring network indicated that ground-water salinity 

was, at the time of the reconnaissance, geographically related: 

conductivities were lower (801–2,490 μS/cm) in surficially dry 

areas, intermediate (6,090–9,180 μS/cm) in natural-wetland 

areas, and higher (18,250–27,700 μS/cm) in areas adjacent to 

the constructed wetland and its associated ocean-discharge 

channel (fig. 15; table 3).

The anomalously high conductivity (14,800 μS/cm) of 

ground water sampled from monitoring-well AMP06 may 

have been caused by the depth of the screened interval (midpoint) of the well relative to the rest of the ground-water-monitoring network; at 15.56 ft below sea level it was significantly 

deeper than the mean screened-interval depth (4.07 ft below 

sea level, calculated exclusive of AMP06) for the network as 

a whole. Alternatively, monitoring well AMP06 was unique in 

that little resistance to driving was encountered during piezometer installation, resulting in the greater piezometer depth at 

this site. This difference may represent a lithologically based 

(possibly karstic) preferred pathway for seawater movement 

into the wetland, and could also explain the higher surfaceReconnaissance 19
water salinities, the radially outward pattern of diminishing 

surface-water salinity, and the unique vegetative profile in the 

immediate area of ground-water-monitoring well AMP06.

Ground-Water Elevations

As previously discussed, two methods for determining 

ground-water elevations were used: selected installations of 

transducers, and synoptic water-level surveys. Data collected 

from the transducer installed in surface-water stilling well 

AMP09S reflect a varyingly declining surface-water level. 

(The surface-water level had previously been drastically raised 

by direct rainfall associated with Supertyphoon Nabi.) Once 

detrended, these data reflected a pattern consistent with a 

diurnal tidal signal. Concurrent ground-water-level data from 

this site are unavailable because of a failure of the transducer 

installed in the monitoring well (piezometer AMP09G); nonetheless, data from surface-water-stilling well AMP09S indicate 

that surface water may be directly connected to ground water 

within the natural wetland because the water level at this site 

varied in accordance with a tidal signal (fig. 16).

Synoptic water-level surveys were performed on September 15–17, 2005; timespans were selected to monitor the 

effects on ground-water levels in the natural wetland of tidal 

peaks and troughs (fig. 17). Data collected from the surveys 

were used to calculate tidal efficiencies and lag times, parameters that are commonly used to gain insight into the hydrologic properties of local subsurface strata. (Tidal efficiency 

is a normalized measure, commonly expressed as a fraction, 

relating the amplitude of ground-water-level fluctuations in 

145°43' 145°43'20" 145°43'40" 145°44'

15°12'40"

15°13'

15°13'20"

15°13'40"

0 0.1 0.2 0.3 0.4 0.5 MILES

Base modified from U.S. Geological Survey digital 

raster graphics, scale 1:25,000, 1999. Universal 

Transverse Mercator projection, zone 55, NAD83 datum.

Constructed

wetland

Discharge

channel

Pre-WWII road

1

2 3

4

EXPLANATION

801

SURFACE-WATER ZONE AND NUMBER

GROUND-WATER-MONITORING WELL

 AND FLUID SPECIFIC-CONDUCTANCE

 VALUE, IN MICROSIEMENS PER CENTIMETER

TIDE GAGE

27,700

1,685

18,250

950

14,800

9,180 no sample

8,130

2,490

6,090

7,020 8,640

801

1

Figure 15. Conductivities of ground-water samples withdrawn from wells in the installed ground-water-monitoring network indicated 

that ground-water salinity was, at the time of the reconnaissance, geographically related; conductivities were lower (801–2,490 μS/cm) 

in surficially dry areas, intermediate (6,090–9,180 μS/cm) in natural-wetland areas, and higher (18,250–27,700 μS/cm) in areas adjacent to 

the constructed wetland and its associated ocean-discharge channel.

20 Reconnaissance Study of the Hydrology of American Memorial Park, Island of Saipan, CNMI
Table 3. Well purging and ground-water quality data collected on September 13 - 18, 2005.

[mL, milliliter; μS/cm @ 25°C, microsiemens per centimeter at 25°C; °C, degrees Celsius; – –, not available]

Well Geographic location

Screened-interval 

elevation (midpoint) 

(ft)

Amount purged (mL)

Specific electrical 

conductance

(μS/cm @ 25°C)

Temperature (°C)

Surficially dry areas

AMP01 Western −2.56 1,470 801 29.7

AMP02 Western −1.83 1,355 1,685 29.1

AMP051 Western – – 2,540 950 30.9

AMP131 Eastern – – 2,170 2,490 29.0

Constructed wetland

AMP03 Western −4.77 2,390 27,700 29.8

AMP04 Western −3.33 1,360 18,250 29.3

Natural wetland

AMP062 Eastern −15.56 2,410 14,800 29.0

AMP07 Eastern −4.16 1,515 8,640 28.7

AMP08 Eastern −5.48 2,505 9,180 28.5

AMP09 Eastern – – – – – – – –

AMP10 Eastern −4.13 1,580 6,090 29.8

AMP11 Eastern −7.76 1,540 7,020 29.2

AMP12 Eastern −2.72 1,420 8,130 30.5

1

 Configurations of preexisting monitor wells AMP05 and AMP13 are unavailable, and so midpoints are unknown.

2

 Specific electrical conductance is likely anomalously high in response to the depth of the screened interval.

-0.30

09/08/2005 09/09/2005 09/10/2005

-0.25

-0.20

-0.15

-0.10

-0.05

0.00

0.05

MEASURED WATER LEVEL 

DETRENDED WATER LEVEL 

OCEAN LEVEL 

(SCALED AND LAGGED) 

LINEAR TREND 

WATER LEVEL, IN FEET RELATIVE TO MEAN SEA LEVEL 

00:00 06:00 12:00 18:00 00:00 06:00 12:00 18:00 00:00 06:00 12:00

Figure 16. Surface-water elevations were recorded by a 

transducer-equipped surface-water-stilling well (AMP09S) 

installed in surface-water zone 2 of the park’s natural wetland. 

Once detrended, these data reflect a pattern consistent with a 

diurnal tidal signal.

response to tidal fluctuations at the coastal boundary; the tidal 

efficiency at the coast is commonly set to 1.00, representing a 

maximum for the study area. Tidal lag is an inverse measure of 

the propagation of a tidal signal as it moves inland through an 

aquifer; the tidal lag at the coast is commonly set to 0, representing a minimum for the study area, because this is the point 

of origin for the signal. Tidal lag is expressed in this report as 

the time difference between a low or high ocean tide and the 

corresponding low- or high-water level measured at groundwater-monitoring sites.)

Water-level data were collected from the entire groundwater-monitoring network over a timespan that bracketed an 

oceanic tidal trough occurring at 12:25 Ch.s.t. September 16, 

2005, thus enabling a spatial representation of ground-water 

level concurrent with the low tide (fig. 18). Calculated tidal 

lag times suggest that areal rates of hydraulic conductivity in 

subsurface strata may be higher in the eastern part of the park 

than in the western part; mean lag time in the eastern part of 

the park was 21 minutes versus 115 minutes in the western 

part (table 4).

An additional interesting pattern appears if the western 

part of the park is further subdivided into surficially dry areas 

and areas adjacent to the constructed wetland and its associReconnaissance  21
AMP12

2.5

2.0

1.5

1.0

0.5

0.0

-0.5

-1.0

-1.5

START OF DAY IN SEPTEMBER 2005 START OF DAY IN SEPTEMBER 2005

AMP11

AMP08

2.5

2.0

1.5

1.0

0.5

0.0

-0.5

-1.0

-1.5

AMP07

AMP04

2.5

2.0

1.5

1.0

0.5

0.0

-0.5

-1.0

-1.5

AMP03

AMP02

2.5

2.0

1.5

1.0

0.5

0.0

-0.5

-1.0

-1.5

AMP01

WATER LEVEL, IN FEET RELATIVE TO MEAN SEA LEVEL

SURFICIALLY DRY AREAS CONSTRUCTED-WETLAND AREA NATURAL-WETLAND AREA

15 16 17 18 19 15 16 17 18 19

MEASURED WATER LEVEL, IN FEET

 RELATIVE TO MEAN SEA LEVEL

OCEAN LEVEL AT TIDE GAGE, IN FEET 

 RELATIVE TO MEAN SEA LEVEL

Figure 17. Synoptic water-level survey data, collected September 15–18, 2005, reflect an areal distribution of groundwater levels; the ocean’s tidal signal, as it propagated through ground water, was attenuated in surficially dry areas 

relative to the park’s natural wetland. Water levels in the western, surficially dry areas of the park were generally higher 

than in the eastern, natural-wetland area. Intermediate water levels and tidal-signal attenuation were observed at sites 

adjacent to the constructed wetland and its associated ocean-discharge channel.

22  Reconnaissance Study of the Hydrology of American Memorial Park, Island of Saipan, CNMI
145°43' 145°43'20" 145°43'40" 145°44'

15°12'40"

15°13'

15°13'20"

15°13'40"

0 0.1 0.2 0.3 0.4 0.5 MILES

Base modified from U.S. Geological Survey digital 

raster graphics, scale 1:25,000, 1999. Universal 

Transverse Mercator projection, zone 55, NAD83 datum.

Constructed

wetland

Discharge

channel

Pre-WWII road

1

2 3

4

0.77

0.73

0.54

1.75 0.15

no measurement

0.38

0.38

0.35

-1.35

-0.08 0.49 0.36

EXPLANATION

0.77

SURFACE-WATER ZONE AND NUMBER

GROUND-WATER-MONITORING WELL

 AND WATER LEVEL, IN FEET ABOVE

 MEAN SEA LEVEL

GROUND-WATER-MONITORING WELL

 NOT MEASURED DURING SYNOTPIC SURVEY

TIDE GAGE AND OCEAN LEVEL, IN FEET

 RELATIVE TO MEAN SEA LEVEL

-1.35

1

Figure 18. Low-tide ground-water levels were extracted from synoptic water-level survey data collected on the afternoon of 

September 16, 2005.

ated discharge channel. In this more granular breakdown, 

the mean lag time for ground-water-monitoring wells associated with the constructed wetland and its discharge channel 

(wells AMP03, AMP04) was 66 minutes versus 148 minutes 

for those in the surficially dry area (wells AMP01, AMP02, 

AMP05). This additional breakdown does not materially contribute to the more significant divergence between lag times in 

the eastern and western parts of the park, or to the likely areal 

difference in subsurface strata responsible for that divergence; 

however, it does indicate that within the western part of the 

park, the constructed-wetland discharge channel may be acting as a locally preferred pathway for ground-water flow in 

response to variation in ocean tides.

Water-level data were collected from wells located in 

the natural wetland over a timespan that bracketed an oceanic 

tidal peak occurring at 19:35 Ch.s.t. September 16, 2005, thus 

enabling a spatial representation of ground-water concurrent 

with the high tide (fig. 19). Similarly calculated tidal lags were 

consistent with those calculated from low-tide data. Calculated 

tidal efficiencies ranged from 0.38 to 0.85, with a mean of 

0.64 (table 4).

An attempt was made to discern differences in head with 

depth at the ground-water-monitoring-network sites. Data 

do not indicate differences in head with depth at any of these 

sites. Possible reasons why such differences were not evident 

include that (1) the vertical separation between the coupled 

well and additional piezometer was insufficient, (2) locally 

high rates of hydraulic conductivity (vertical and horizontal) made head differences with depth difficult to detect, (3) 

an annulus immediately surrounding the well and (or) the 

additional piezometer was created as an artifact of the installation process, (4) the piezometer riser joints were sufficiently 

Reconnaissance  23
Table 4. Tidal efficiencies and lag times computed from data collected on September 16, 2005.

[AMSL, above mean sea level; ft, feet; hh:mm, hour:minute; – –, not available]

Well

Low water values High water values

Tidal

efficiency Time

(hh:mm)

Height 

(ft AMSL)

Offset 

from tide 

(ft)

Lag time

(hh:mm)

Time

(hh:mm)

Height 

(ft AMSL)

Offset from

tide (ft)

Lag time

(hh:mm)

Tide Gage1 0.52 -1.36 0.00 0.00 0.82 1.25 0.00 0.00 1.00

Western wells

AMP012 0.60 0.68 2.04 0.08 – – – – – – – – – –

AMP022 0.63 0.64 2.00 0.11 – – – – – – – – – –

AMP032 0.55 0.27 1.63 0.03 – – – – – – – – – –

AMP042 0.58 0.49 1.85 0.06 – – – – – – – – – –

AMP052 0.63 1.71 3.07 0.12 – – – – – – – – – –

Mean Values 0.60 0.76 2.12 0.08 – – – – – – – – – –

Eastern wells

AMP06 – – – – – – – – – – – – – – – – – –

AMP072 0.57 0.41 1.77 0.05 0.88 1.39 0.14 0.06 0.38

AMP082 0.52 0.15 1.51 0.00 0.82 2.07 0.82 0.00 0.74

AMP091 0.52 0.38 1.74 0.00 0.82 2.05 0.80 0.00 0.64

AMP10 – – – – – – – – – – – – – – – – – –

AMP112 0.52 -0.08 1.28 0.01 0.82 2.14 0.89 0.00 0.85

AMP122 0.53 0.38 1.74 0.00 0.82 2.08 0.83 0.01 0.65

AMP132 0.54 0.33 1.69 0.02 0.83 1.81 0.56 0.02 0.57

Mean Values 0.53 0.26 1.62 0.02 0.83 1.92 0.67 0.01 0.64

1

 Data are from transducers.

2

 Data are from synoptic surveys.

incoherent as to allow equilibration between surface-water and 

ground-water levels through leakage, or (5) some combination of these factors. Reasons 3 and 4 have been included for 

reasons of rigor; however, the inability to discern any differences in head with depth is most likely due to reasons 1 and 

2 because the vertical and horizontal permeability rates of 

inplace strata probably range from hundreds to thousands of 

feet per day. Future efforts to constrain this critical variable 

will likely require deeper piezometer placement.

Hydrologic Model of American 

Memorial Park

A hydrologic model of American Memorial Park has 

been developed as a result of the efforts associated with the 

reconnaissance. Though determined to be connected, and 

therefore components of a holistic system, surface water and 

ground water are discussed separately in this report. Surfacewater topics will include freshwater-input sources and salinity 

distribution. The discussion of ground water similarly includes 

these topics but also addresses flow mechanisms and controls 

and potential stressors to the park’s current hydrologic regime.

Surface Water

Three major potential contributors of fresh surface water 

to the park, and especially to the natural-wetland area, have 

been identified: direct rainfall, ground-water discharge, and 

overland flow resulting from storms.

Direct rainfall contributes freshwater to the park; the 

reconnaissance was interrupted by Supertyphoon Nabi, and 

this input was observed and monitored. Access paths to 

ground-water-monitoring network sites in the wetland were 

flooded with standing water, in some places to depths of more 

than 1 ft, and large areas of the western part of the park were 

ponded for several days after the storm had passed.

Attempts to constrain a vertical component of the generally seaward flowing ground water were inconclusive. How-

24  Reconnaissance Study of the Hydrology of American Memorial Park, Island of Saipan, CNMI
145°43' 145°43'20" 145°43'40" 145°44'

15°12'40"

15°13'

15°13'20"

15°13'40"

0 0.1 0.2 0.3 0.4 0.5 MILES

Base modified from U.S. Geological Survey digital 

raster graphics, scale 1:25,000, 1999. Universal 

Transverse Mercator projection, zone 55, NAD83 datum.

Constructed

wetland

Discharge

channel

Pre-WWII road

1

2 3

4

2.07 2.05

2.08

1.78

1.24

2.14 1.35

EXPLANATION

1.35

SURFACE-WATER ZONE AND NUMBER

GROUND-WATER-MONITORING WELL

 AND WATER LEVEL, IN FEET ABOVE

 MEAN SEA LEVEL

GROUND-WATER-MONITORING WELL

 NOT MEASURED DURING SYNOTPIC SURVEY

TIDE GAGE AND OCEAN LEVEL, IN FEET

 RELATIVE TO MEAN SEA LEVEL

1.24

1

Figure 19. High-tide ground-water levels were extracted from synoptic water-level survey data collected on the evening of September 

16, 2005.

Hydrologic Model of American Memorial Park  25

ever, some sites within the natural wetland are below sea level 

(for example, the saltgrass area surrounding ground-watermonitoring well AMP06, fig. 13), and ground water can be 

expected to provide surface-water input at these sites, given 

that surface water and ground water are connected and that 

short-term inputs from direct rainfall, and associated overland 

flow, are insufficient to reverse the flow gradient. Pressure 

changes recorded by the transducer installed in surface-waterstilling well AMP09S were consistent with a pattern indicative 

of tidal signal. These measurements indicate that at least in 

some sites within the wetland, surface water and ground water 

may be directly connected and so, in the absence of other 

inputs, ground water may contribute to surface water.

The coincident occurrence of Supertyphoon Nabi during 

the reconnaissance enabled a denial of freshwater input from 

overland flow to the natural-wetland area of the park by the 

current configuration of roads, and associated diversion channels, to be observed and documented. Although events at the 

peak of the storm were not directly observed, a visit to the 

park within hours of the passing of the eye showed that the 

diversion infrastructure was successful in channeling all overland flow around the park’s natural-wetland area and directly 

into the ocean. (Poststorm inspection of the vegetation flanking the park provided no evidence that the diversion infrastructure had failed to accommodate storm-related overland 

flows). This observation does not suggest, however, that there 

are not rainfall rates, and resulting overland-flow conditions, 

that could not overwhelm the capacity of this infrastructure but 

only that such rainfall rates and conditions were not observed 

during the reconnaissance in association with Supertyphoon 

Nabi.

Hydrologic Model of American Memorial Park  25
Although the distribution of surface-water salinity varies 

spatially within the wetland, all surface-water conductivities measured during the reconnaissance fall approximately 

within the oligohaline range (800–8,000 μS/cm). Generally, 

salinity appears to decrease with proximity to the shoreline; 

this counterintuitive observation may result from denial of 

freshwater overland flow into the wetland (as described above) 

or indicate spatially varying discharge of fresh ground water. 

More data are needed to better constrain this spatially varying 

surface-water-salinity regime. The radially outward pattern 

of diminishing salinity in surface-water zone 4 (near groundwater-monitoring well AMP06) could indicate a lithologically based (possibly karstic) preferred pathway between this 

surface-water zone and the ocean or, more simply, between 

the surface water in this zone and more saline ground water at 

depth. The relative ease of piezometer installation here, and 

the subsequently measured ground-water conductivity—far 

greater than at any other site—would be consistent with some 

form of karstic pathway beneath surface-water zone 4.

Temporally, surface-water salinity varied within the wetland during the period of reconnaissance; the greatest variation 

coincided with the freshwater input (direct rainfall) associated 

with Supertyphoon Nabi. For example, the salinity in surfacewater zone 4 decreased by about 57 percent (from a prestorm 

conductivity of 8,290 μS/cm to a poststorm conductivity of 

3,530 μS/cm). Similarly wide variations over short timeframes 

are probably related to direct rainfall events, such as Supertyphoon Nabi; however, within the wetland, some temporal 

variation in surface-water salinity is probable at all times, 

resulting from such factors as ground-water discharge and 

evapotranspiration effects. Further study is needed to constrain 

such temporal variations, as well as to establish seasonally 

influenced baseline values or, more likely, ranges of values.

Ground Water

Data collected through direct measurements of groundwater levels indicate that Saipan’s islandwide freshwater 

lens, during the period of reconnaissance, extended beneath 

American Memorial Park. Inputs to this lens come from three 

primary sources: (1) semi-immediate infiltration from direct 

rainfall; (2) delayed infiltration from ephemeral surface-water 

bodies that form as soil-infiltration rates are exceeded, owing 

to periods of intense rainfall and localized overland flow (as 

evidenced by the transducer data from surface-water-stilling 

well AMP09S); and (3) seaward-flowing ground water.

Semi-immediate infiltration from direct rainfall occurs in 

all environments at rates initially constrained by surficial perviousness and subsequently limited by vertical-hydraulic-conductivity rates. Soils in the park wetland have high infiltration 

and rapid percolation (permeability) rates; the U.S. Department of Agriculture (1989) characterized the dominant soil 

type in the wetland (Mesei variant muck) as moderately rapid 

with regard to permeability, with rates of about 0.6 to 2.0 in/hr. 

Given that rainfall occurs frequently along the western coastal 

plain of Saipan (Lander, 2004), direct rainfall can be expected 

to significantly contribute to ground water within the park.

Surface-water ponding will occur when local soil-infiltration rates are exceeded by direct rainfall, or by overland 

flow introduced from nearby areas where such rates have 

been exceeded. Although soil-infiltration rates in the wetland 

are moderately rapid, storm-related rainfall in the Tropics 

commonly exceeds these rates, resulting in localized surfacewater ponding. Subsequently, this ponded water will infiltrate 

through the soil surface, then percolate through subsurface 

soils until it reaches ground water (absent being captured or 

diverted by other processes, such as root uptake or soil moistening). Data from the transducer installed in surface-waterstilling well AMP09S were consistent with this mechanistic 

description; they indicated that surface water, ponded from 

direct rainfall associated with Supertyphoon Nabi, slowly 

infiltrated through the soils underlying the wetland. That these 

data were consistent with a tidal-response pattern may also 

be evidence of direct connection between surface water and 

ground water, one of the primary goals of the reconnaissance 

(fig. 16).

Extension of the islandwide freshwater lens beneath 

American Memorial Park suggests a generally seaward flow 

of fresh ground water upgradient of the park and, therefore, a 

freshwater input to ground water underlying the park; however, data collected during the reconnaissance and by previous 

investigators are insufficient to characterize this flow’s magnitude or local flowpath(s) and the effects of seasonality on these 

characteristics. Given that this input is likely to be the single 

largest contributor to fresh ground water beneath the park, 

especially underlying the natural wetland, additional efforts 

to collect and analyze ground-water data would be beneficial 

to understanding the hydrology of the park. A ground-water 

model could be constructed to facilitate this understanding, for 

use as a communication tool to disseminate this understanding, and as a long-term method of tracking changes to the 

park’s hydrology.

The synoptic water-level surveys conducted during the 

reconnaissance indicated that the local water table varied 

directly with the ocean tide, generally ranging from about 0.5 

to about 2 ft higher than the temporally corresponding data 

collected from the tide gage (table 4). Differences in elevation 

(offsets in water level) between the water table and the ocean 

were generally greater during periods of low tide and smaller 

during periods of high tide, and were generally greater in the 

western than in the eastern part of the park (figs. 18, 19).

Data collected from the synoptic water-level surveys were 

also used to determine tidal lags throughout the park, and tidal 

efficiencies in the natural-wetland area. These data indicated 

that hydraulic conductivities in the immediate area of the wetland were extremely high; however, lag times increased with 

distance from the wetland, including in wells that are seaward 

of the wetland. Lag times were greater still in the western part 

of the park. The granularity of the data, as well as the short 

timeframe sampled, preclude discrete spatial assessment; 

additional data would facilitate such assessment and be critical 

to construction of a ground-water model.

26  Reconnaissance Study of the Hydrology of American Memorial Park, Island of Saipan, CNMI
Further influences to ground-water flow in the park could 

include local faults and the reverse-osmosis injection wells. 

Faults on Saipan and nearby Tinian have been observed to 

act as areas of both higher and lower relative permeability 

(Carruth, 2003). Data collected in the 1980s in the Puerto Rico 

area (northeast of the park; see fig. 1) suggest that a major 

nearshore fault complex, identified by Cloud and others (1956) 

as the Matansa, may be acting as a zone of lower relative 

permeability because ground-water levels were significantly 

higher upgradient of the complex. Local topography suggests 

that this fault complex extends southwestward, immediately 

landward of the park’s boundary-flanking road, Pale Arnold. 

Further study is needed to definitively determine whether this 

fault complex does extend into areas proximal to the park and, 

if so, how the fault complex may be influencing the hydrology 

of the park.

Subsurface disposal of reverse-osmosis wastewater by 

way of the injection wells could be causing a local mounding 

of ground water in the western part of the park. Data collected 

from the synoptic water-level surveys indicated that groundwater levels are highest in this area (fig. 18), and there is no 

evidence of any type of impounding formation (or caprock, as 

it is commonly referred to in Hawaiian geology) seaward of 

the western part of the park that could account for these raised 

ground-water levels. Such ground-water mounding, if it is 

occurring as a result of reverse-osmosis wastewater injection, 

could perturb ground-water flow throughout the park, including in the Federally listed natural-wetland area.

In addition to identifying the freshwater lens underlying the park, the spatial distribution of ground-water salinity suggests an enhanced ground-water connection with the 

nearby ocean in the immediate area of the natural wetland 

and, possibly, in a strip underlying and flanking the constructed wetland’s discharge channel to the ocean. Habitat 

may be strongly influenced, even controlled, by ground-water 

salinity, especially in areas with a strong ground-water/surface-water connection, such as the natural wetland appears 

to represent. The reconnaissance was not intended to address 

biotic regimes, only the hydrology of the park. However, given 

the influence of hydrology on habitat, a cooperative effort to 

further constrain the quantity and quality of surface water and 

ground water, and the potentially adverse effects of environmental changes on the park’s biota, might be prudent.

Additional Data and Analysis Needs

The habitat associated with the 27-acre estuarine system 

is primarily a reflection of areal surface-water distribution 

and salinity. The areal distribution of surface water varies on 

an hourly to daily basis with regard to storms, on a seasonal 

basis as a reflection of rainfall and the upward discharge of 

ground water, and on a multiyear to multidecadal basis owing 

to long-term cyclical patterns (for example, El Niño) or global 

changes in climate and sea level. The reconnaissance study 

addressed the hourly to daily temporal variation, but characterization of longer-term variation requires additional data and 

analysis. The reconnaissance study was not intended to comprehensively map the areal distribution of surface water within 

the wetland or to characterize how that distribution could vary 

with changing conditions.

Equally important as areal distribution is the salinity 

of the surface water within the estuarine system. Surfacewater conductivities measured during the reconnaissance fell 

predominantly into the oligohaline category of salinity. Given 

that some types of vegetation within the wetland are critically 

sensitive to specific ranges of salinity (for example, mangroves 

can tolerate higher salinities than can casuarinas), a spatial 

baseline needs to be established, along with an enhanced 

understanding of what factors are contributing to the range of 

salinities measured throughout the wetland. Measuring localized differences in head with depth could provide insight into 

the vertical flow of ground water and, by extension, the spatial 

pattern of salinity within the wetland and adjacent areas.

Similarly, an enhanced spatial and temporal dataset of 

ground-water quality with regard to salinity, including profiles 

of the brackish-water transition zone (zone of freshwater/saltwater mixing), would augment understanding of the hydrology 

of American Memorial Park. Given the increasing urbanization near the park and probable commensurate increases in 

freshwater withdrawal and wastewater injection, establishment 

of baseline conditions would enable short-term changes and 

long-term trends to be identified that could affect the park’s 

critical habitats. With regard to the western part of the park, 

it may already be too late to establish a historical baseline; 

however, recent correspondence with the CNMI Department 

of Environmental Quality has indicated that local hotels have 

yet to fully transition from surface disposal of reverse-osmosis 

wastewater to subsurface injection (Brian Bearden, CNMI 

Department of Environmental Quality, written commun., 

2006). To better constrain ground-water salinity, clustered 

piezometers of differing depths could be installed at selected 

sites within the park.

The effects of subsurface disposal of reverse-osmosis 

wastewater on the hydrology of the park could be significant. 

If ground-water mounding occurs as a result of subsurface 

injection then ground-water flow within the park will likely 

be changed. The reconnaissance study was not intended to 

characterize ground-water flow in the western part of the park; 

however, additional efforts should address the potential effects 

such an injection mechanism could introduce, recognizing that 

their influence may be parkwide. This reconnaissance study 

was not intended to characterize ground-water flow in the 

western part of the park; however, additional efforts should 

address the potential effects of such injection techniques, recognizing that the effects may be parkwide.

The road flanking the southeast boundary of the park, 

Pale Arnold, is currently preventing overland flow from entering the wetland. At present, water is being diverted around the 

park in a series of surface drains and conduits. The effect of 

this diversion on the wetland could be studied to determine its 

Additional Data and Analysis Needs  27
significance to the hydrology and, by extension, the critical 

habitat of the park.

Numerous fault complexes bisect Saipan parallel to the 

island’s long axis. Cloud and others (1956) suggested that one 

of these fault complexes, the Matansa, is responsible for the 

significant change in relief proximal to, and in alignment with, 

the southeast boundary of the park (fig. 2). Data collected in 

the Puerto Rico area, about 1 mi from the park, suggest that 

this fault complex may be acting as a locally low permeability 

barrier to the generally seaward flow of fresh ground water, 

resulting in higher ground-water levels upgradient of the fault 

complex. Given that the local discharge of seaward-flowing 

ground water may be significant to maintaining the wetland’s 

brackish salinity, and that the fault complex may be acting as 

a ground-water-flow control, an improved understanding of 

the effects of this fault complex on the hydrology of the park 

could be informative.

Summary and Conclusions

American Memorial Park on the Island of Saipan, CNMI, 

represents a significant local asset with its varied mix of 

historical, recreational, and natural-resource features, which 

include a 28-acre Protected Natural Area Zone and a 27-acre 

estuarine system. These natural-resource features provide 

critical habitat for various migratory and resident waterfowl, 

including two Federally listed endangered species: the Marianas gallinule (Gallinula chloropus guami) and the nightingale 

reed warbler (Acrocephalus luscinia). Estuarine systems have 

become rare within the CNMI, and so the park’s estuarine 

system has been identified as a Mitigation Policy Resource 

Category 2 site by the U.S. Fish and Wildlife Service, the only 

such Federally protected area within the CNMI possessing 

viable populations of these endangered species.

In recognition of the park’s critical habitat, and the 

integral relation between this habitat and the local hydrologic 

regime, a need to enhance the understanding of the hydrology of American Memorial Park was identified. To address 

this need, a reconnaissance study of the park was undertaken 

during August and September 2005. The goals of the study 

were (1) to describe the occurrence and salinity of surface and 

ground water within American Memorial Park; (2) to present 

a conceptual model of the hydrology of the park area, with 

emphasis on the 27-acre estuarine system; and (3) to identify additional data needed to further develop this model and 

address continuing information gaps.

With regard to surface water, three possible freshwater 

inputs to the park’s natural wetland were identified: direct 

rainfall, seaward-flowing ground water, and overland flow. 

Direct rainfall is likely an important source of freshwater to 

the wetland. Evapotranspiration is probably occurring at a rate 

of about 0.21 in/d, a rate that will frequently be exceeded by 

direct rainfall, both seasonally and per storm; infiltration of 

direct rainfall in excess of this evapotranspiration rate would 

represent a freshwater input to the wetland. The seaward flow 

of ground water is likely to be a source of freshwater to the 

wetland because ground water commonly expresses an upward 

vertical component in the nearshore environment; however, 

attempts to constrain this vertical flow component were unsuccessful, likely owing to the relatively high rates of hydraulic 

conductivity. Overland flow upgradient from the park could 

contribute a significant input of water during periods of 

intense rainfall, but perimeter-flanking roads and infrastructure 

currently act as a barrier to such surface inflows.

Specific electrical conductance, or conductivity, was measured within the four discrete bodies, or zones, of observed 

surface water in the park’s natural wetland. These surfacewater measurements yielded conductivities that typically 

ranged from 1,540 to 4,370 μS/cm (surface-water zone 2), 

although conductivities as low as 829 μS/cm (surface-water 

zone 1) and as high as 8,750 μS/cm (surface-water zone 3) 

were recorded. As a result of these measurements, the natural-wetland area meets the definition criteria of an estuarine 

system that is dominantly oligohaline: (1) its salinity is ocean 

derived, (2) its salinity is primarily within the conductivity 

range 800–8,000 μS/cm, and (3) it is tidally flooded at times, 

although this flooding may be extremely infrequent, occurring 

only during large tropical cyclones and their associated storm 

surges, or by a tsunami. Counterintuitively, these natural-wetland surface-water conductivities indicated a general trend 

of increasing salinity with distance from the ocean, possibly 

resulting from the denial of overland freshwater flows, owing 

to the previously discussed road and related infrastructure 

barrier. Further study is needed to understand this apparent 

anomaly.

Surface-water zone 4, containing ground-water-monitoring-well AMP06, was unique in that salinity decreased radially outward from a central, near-sea-level area vegetated by 

saltgrass. Conductivities measured later in the reconnaissance 

within this surface-water zone, after the passing of Supertyphoon Nabi and the associated addition of significant freshwater input to the wetland in the form of direct rainfall, indicated 

that surface-water salinity had decreased by about 57 percent, 

a significant temporal variation in surface-water salinity. These 

changes may represent the norm rather than the exception, 

given Saipan’s position within the formative region for the 

northwestern Pacific tropical-cyclone basin.

Surface-water conductivity was also measured in the 

constructed wetland. A phaseout of surface disposal of 

reverse-osmosis wastewater had been planned in favor of 

subsurface disposal by way of injection wells; however, the 

reconnaissance coincided with this planned transition, and 

so both surface (by way of storm drains) and subsurface (by 

way of injection wells) disposal of reverse-osmosis wastewater may have been occurring concurrently. Conductivities of 

surface water in the constructed wetland exceeded nominal 

seawater levels by as much as 25 percent, ranging from 45,000 

to 62,500 μS/cm, with the highest values recorded near the 

storm drains discharging directly into the constructed wetland. 

These data indicate that surface disposal of reverse-osmosis 

28  Reconnaissance Study of the Hydrology of American Memorial Park, Island of Saipan, CNMI
wastewater was occurring during the reconnaissance. Analysis 

of water-level data collected from wells located downgradient from the injection wells was inconclusive in identifying 

any ground-water mounding as a result of subsurface injection; however, if such mounding is occurring, or does occur in 

the future, it will modify ground-water flow within the park. 

Further study is needed to constrain the affects of subsurface 

wastewater disposal.

Previous investigations of Saipan’s ground-water 

resources have determined that a freshwater lens underlies 

most of the island, including the western coastal plain on 

which American Memorial Park lies. Data collected by way of 

the ground-water-monitoring network installed as part of the 

reconnaissance identified the presence of fresh ground water 

underlying the park and indicated that surface water is directly 

connected to ground water in the natural wetland because the 

water levels of both varied directly with the tide. This fresh 

ground water is likely continuous with the islandwide freshwater lens identified by previous investigations; however, 

this reconnaissance study was not intended to determine such 

continuity.

Conductivities of ground-water samples collected from 

wells in the monitoring network indicated that ground-water 

salinity was, at the time of the reconnaissance, geographically related: conductivities were lower (801–2,490 μS/cm) 

in surficially dry areas, intermediate (6,090–9,180 μS/cm) in 

natural-wetland areas, and higher (18,250–27,700 μS/cm) in 

areas adjacent to the constructed wetland and its associated 

ocean-discharge channel.

Synoptic water-level surveys, conducted to enhance 

understanding of the spatial expression of the water table, 

were scheduled to overlap with peak and trough tidal signals 

to enable limited characterization of local hydrologic properties. The water-level surveys indicated that ground-water 

levels were directly influenced by ocean tides because tidal 

fluctuations were reflected in ground-water levels. The surveys 

also indicated that, at any given time, ground-water levels 

were higher than ocean-water levels; this offset in water level 

was greater at low than at high tide, and greater in the western 

than in the eastern part of the park. Tidal lags calculated for 

the entire park, and tidal efficiencies calculated for the naturalwetland area, suggest that areal rates of hydraulic conductivity 

in subsurface strata may be higher in the eastern than in the 

western part of the park: mean lag time in the eastern part of 

the park was 21 minutes versus 115 minutes in the western 

part. Tidal efficiencies in the eastern part of the park ranged 

from 0.38 to 0.85, with a mean of 0.64.

Additional data are needed to develop a better understanding of the hydrology of American Memorial Park. 

Installation of additional monitoring wells would be needed 

to constrain ground-water flow through the park, including 

the vertical component of flow, which would be evidenced 

by differences in head with depth. Additional ground-water 

measurements (water level and salinity) in the western part of 

the park, in coordination with varying amounts of subsurface 

reverse-osmosis wastewater disposal from nearby facilities, 

could lend insight into the effects of such disposal practices 

on ground-water flow within the park. Ground-water measurements (water level and salinity) bracketing faults that are 

proximal to, parallel to, and upgradient from the southeast 

boundary of the park can lead to improved understanding of 

the potential effects of these faults on the local hydrology. 

Understanding of the potential negative effects of perimeterflanking roads and associated infrastructure on freshwater 

flow, both surface and subsurface, could be improved by 

installing an enhanced monitoring-well network, and by monitoring surface flows that are currently and artificially being 

diverted around the park to the ocean.

Modeling local and islandwide hydrologic characteristics 

could assist in identifying short- and long-term hydrologic 

trends that could adversely affect the park and its critical 

habitats. Development of a water-budget model on a larger, 

possibly island, scale could enhance understanding of upgradient influences on the local hydrologic regime in the park. 

Development of a numerical model, on a park scale, to address 

coupled surface-water/ground-water flow and solute transport could enhance understanding of local processes, enable 

implementation of response-specific (adaptive) management 

practices, and serve as a visual-communication tool to inform 

interested parties of changes to the park’s hydrologic characteristics, including critical habitats, resulting from both local 

(for example, proposed urban development projects, such as 

the subsurface disposal of reverse-osmosis wastewater by way 

of injection wells) and farfield (for example, anthropogenic 

climate and (or) sea-level changes) influences.

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