U.S. Department of the Interior

Hydrologic Resources of Guam

By Stephen B. Gingerich

U.S. Geological Survey Water-Resources Investigations Report 03-4126

144o50' G i n g e r c h – H y d o l R s u f a m W I 0 3 - 4 1 2 6

EA S

E IN P IP Tumon Bay IL H o P 13 30'

Hagatna Bay

144o40' N A E C O

Apra Harbor IC IF C A P Pago Bay

Agat Bay

Ylig Bay

13o20' Talofofo Bay

2003

Prepared in cooperation with the WATER AND ENVIRONMENTAL RESEARCH INSTITUTE (WERI) UNIVERSITY OF GUAM U.S. DEPARTMENT OF THE INTERIOR PREPARED IN COOPERATION WITH THE WATER-RESOURCES INVESTIGATIONS REPORT 03-4126 U.S. GEOLOGICAL SURVEY WATER AND ENVIRONMENTAL RESEARCH INSTITUTE (WERI), Climate, Geology, Hydrology–SHEET 1 OF 2 UNIVERSITY OF GUAM Gingerich, S.B., 2003, Hydrologic Resources of Guam

INTRODUCTION HYDROLOGY The U.S. Territory of Guam, which lies in the western Pacific Ocean near latitude 13°28’N and longitude Rainfall can infiltrate the soil, return to the atmosphere by evapotranspiration, or runoff to streams. The amount Freshwater-lens systems are recharged by direct infiltration of rainfall, and by inflow from perched ground-water The degree of saltwater intrusion depends on several factors, which include the hydraulic properties of the rocks, 144°45’E (fig. 1), is the largest (211 mi2) and southernmost of the islands in the Mariana chain. Ground water of runoff is determined by rainfall, topography, soil type, and land use. Runoff is typically high where rainfall is systems. Discharge from the freshwater-lens system in northern Guam is by diffuse seepage near the coast and to supplies about 80 percent of the drinking water for the island’s 150,000 residents and nearly one million visitors per recharge rate, pumping rate, and well location and depth. Even if wells are designed and placed to minimize local high and slopes are steep and where rain falls on less permeable land surfaces. On Guam, streams are present only in subaerial and submarine coastal springs. In southern Guam, much of the fresh ground water discharges directly to year. In northern Guam, water is obtained from wells that tap the upper part of a fresh ground-water lens in an upconing, the lens will gradually shrink to a size that is in balance with the withdrawal. This regional shrinkage the south where low-permeability volcanic rocks slow the infiltration of rainwater and allow ground-water discharge stream valleys above sea level where the ground surface intersects the water table. In highly permeable rocks, aquifer composed mainly of limestone. About 180 wells, nearly all in the north, withdraw about 35 Mgal/d of water raises the transition zone closer to the wells, potentially can change the size and distribution of the “para-basal” to streams. In northern Guam, the exposed limestone at the surface allows virtually all rainfall to infiltrate. freshwater flow is mainly horizontal and toward the coast, but in low-permeability rocks, vertical flow can be with chloride concentrations ranging from 6 to 585 mg/L. In southern Guam, the main source of freshwater is from zone. Shrinkage of the freshwater lens due to dry weather can also contribute to high salinity in wells. Evapotranspiration is the loss of water to the atmosphere by the combination of transpiration by plants and direct significant where there are substantial vertical head gradients. surface water that runs off the weathered volcanic rocks that are exposed over much of the area. About 9.9 Mgal/d of evaporation from land and water surfaces. Water stored in the soil is available to plants or can eventually flow Ground-water withdrawal from wells, saltwater upconing, and regional lens depletion.--Some fraction of Perched Ground-Water Systems freshwater is obtained using surface reservoirs. The island’s freshwater resources are adequate to meet current downward to recharge the ground-water system. recharge can be withdrawn continuously by wells for drinking water, in effect capturing a fraction of the natural (2003) needs, but future demands will eventually be higher. To better understand the hydrology of the island, the Perched water is found in areas where low-permeability rocks impede the downward movement of ground water discharge. The most efficient means of pumping water from a thin lens is to locate widely spaced shallow wells U.S. Geological Survey (USGS) entered into a cooperative study with the Water and Environmental Research sufficiently to allow a perched water body to develop within otherwise unsaturated rocks. Minor perched systems are where the lens is thickest and to maintain low, uniform pumping rates at each well (fig. 5a). This configuration of Institute of the Western Pacific (WERI) at the University of Guam. The objective of the study was to provide a better Ground Water found in some of the higher-altitude limestone overlying the volcanic rocks of southern Guam. The height of the wells spreads withdrawal over a wide area and skims freshwater from the lens. Uniform pumping rates will cause understanding of the water resources of the island through analysis of data collected by the USGS on Guam. In Guam, the major fresh ground-water systems are below the lowest water table and are freshwater-lens systems. water table above sea level and the extent of the perched system are respectively dependent on the altitude and areal less mixing in the transition zone when compared to variable pumping rates. Saltwater intrusion can contaminate This report provides a description of the general hydrologic principles of the island’s ground-water systems, as Minor perched systems can exist above the lowest water table. Ground water flows from areas of high water level to extent of the low-permeability rocks and the rate and duration of recharge. Discharge from a perched system can be wells if the lens is too thin, if wells are too deep, or if too much water is withdrawn from a given area (fig. 5b). well as of the rainfall and geology of Guam. Hydrologic data described in the report include water levels, chloride areas of low water level, which in general, is from the interior of the island to the coast. to springs, streams, downward to a lower water table, or to the atmosphere by evaporation. concentrations, and pumpage from ground-water wells and streamflow data from southern Guam. Freshwater-Lens System On oceanic islands such as Guam, freshwater commonly forms a body of water called a freshwater lens that floats on saltwater and is separated from the saltwater by a transition zone of brackish water (fig. 3). The freshwater, brackish water, and saltwater are all part of the freshwater-lens system. Salinity in a freshwater-lens system is gradational, from an upper freshwater core through the underlying transition zone to saltwater. A chloride 120° 140° 160°E 180° 160°W 40° concentration of 250 mg/L is the maximum contaminant level (MCL) for drinking water recommended as a 0 2 JMaIpLaESn secondary standard by the USEPA (U.S. Environmental Protection Agency, 1991). Secondary standards are not mandatory requirements, but instead establish limits for constituents that may affect the aesthetic qualities of 0 2 KILOMETERPS ACIFIC OCEAN

o drinking water (taste and odor, for example). In this report, freshwater is defined as water having a chloride COMMONWEALTH 144 50' OF THE NORTHERN Hawaii MARIANA ISLANDS concentration less than or equal to 250 mg/L. Seawater has a chloride concentration around 19,200 mg/L (Thurman, A B Wake Pumped well 20°N Philippines 1990). Pumped wells Pumped well REPUBLIC OF THE The Ghyben-Herzberg principle is commonly used to estimate the depth where brackish water in the transition Guam MARSHALL ISLANDS 85 95 zone has a salinity about 50 percent that of seawater. The Ghyben-Herzberg principle describes a freshwater-

FEDERATED saltwater relation for conditions in which the two fluids do not mix (no transition zone) and the freshwater flow is STATES OF REPUBLIC MICRONESIA OF PALAU horizontal. For these conditions, the freshwater-lens thickness below sea level is directly proportional to the height 0° Philippine Water table Philippine Water table 90 of the top of freshwater above sea level. In principle, at a place where the water table stands 1 foot above sea level, Sea Water table Sea AMERICAN 100 SAMOA for example, 40 ft of freshwater will be below sea level, and the freshwater lens will thus be 41 ft thick. This relation INDONESIA PAPUA NEW GUINEA Tutuila exists because seawater is about one-fortieth denser than freshwater. In a real freshwater-lens system, mixing creates FIJI Australia a transition zone and reduces the amount of freshwater above the transition zone. In some places, especially near the Freshwater 20 S B Freshwater Br Freshwater ° coast, where freshwater is extensively mixed with saltwater, brackish water may exist immediately below the water ra ac ck w ki EA table. wa is at sh S te h er Transition-zone thickness depends on the extent of mixing between freshwater and saltwater and is generally r E IN 105 several tens of feet thick in northern Guam. Mixing in the transition zone results from tidal and pumping fluctuations PP ? 110 LI Tumon Bay ? 105 HI superimposed on the gravity-driven flow of freshwater toward the shore. Under conditions of steady recharge and no Saltwater Saltwater P o 13 30' pumping, the lens would ultimately have a fixed size. In reality, rainfall is episodic and seasonal, and lens volume 100 fluctuates naturally with time. Ground water discharges continuously throughout the year, but the lens shrinks during Hagåtña Bay ? ? 95 90 dry periods when recharge diminishes or ceases, and expands when recharge increases. 144o40'90 95 105 NOT TO SCALE NOT TO SCALE Apra Harbor 100 Water table West East Figure 5. Diagram showing the effects of pumping on the freshwater lens, northern Guam, (A) widely spaced wells pumping at uniform rate causing slight rise in transition zone, (B) closely spaced wells causing 85 110 brackish water from the transition zone to rise into the wells and reducing the area with para-basal water. Pago Bay Spring N Agat Bay A E 105 C 95 Ylig Bay O Philippine h Sea Pacific Water table Ocean IC 115 Sea 5 F 4 I level 0 C A About Freshwater P 40h Qa Freshwater

540 ? ? 540 115 Talofofo Bay Saltwater ? o 5 13 20' ? 4 Brackish Saltwater 0 water 144˚50' A' ? QTm

0 18 110 NOT TO SCALE

180 105 EXPLANATION 540 LIMESTONE 90 VOLCANIC ROCKS Cocos B Island 100 95 GENERAL DIRECTION OF FRESH GROUND-WATER FLOW Tbl ZONE WHERE FRESHWATER IN LIMESTONE IS IN DIRECT CONTACT 540 540 Base from U.S. Geological Survey 0 2 Miles WITH FRESHWATER IN UNDERLYING VOLCANICS (PARA-BASAL) Island of Guam, 1:62,500, 1953. Elevation from Digital Elevation Model, 1:24,000 0 2 Kilometers

Figure 3. Diagram showing generalized ground-water occurrence, northern Guam. EXPLANATION 540 100 LINE OF EQUAL MEAN ANNUAL RAINFALL, IN INCHES PER YEAR 540 On Guam, freshwater-lens systems are found in limestone and volcanic rocks but the most important sources of Ta ground water are from the freshwater parts of these systems in the high-permeability limestone rocks of northern Guam (fig. 4). In the most permeable limestone, the water table is no more than a few feet above sea level, and the 180 40 slope of the water table is nearly flat. The freshwater lens in these rocks is relatively thin and is commonly referred 5 Figure 1. Rainfall distribution (1950–- 99), Guam (modified from Charles Guard, Water and Environmental Research Institute, University to as “basal” water after similar occurrences in Hawaii (Meinzer, 1930). Wells and springs in this part of the system B' of Guam, written commun., 2000). generally have water-level altitudes less than 8 ft. In some areas, the freshwater lens is thick enough to extend into Tbl the low-permeability volcanic rocks underlying the limestone. The freshwater in the limestone is commonly referred to as “para-basal” water where it is in contact with the underlying poorly permeable volcanic basement (Mink, 1974). Ward and others (1965) recognized that the low-permeability volcanic rocks must be saturated below the RAINFALL water table. Therefore, ground water in the limestone and the underlying volcanic rocks is all part of a freshwater- 13˚30' lens system but the depth of freshwater in the volcanics is unknown. Figure 4 shows wells in which the theoretical The source of all freshwater on Guam is rainfall, which averages about 85 to 115 in/yr (Charles Guard, thickness of the freshwater lens (40 times the water level) extends into the volcanic rocks, the extent of which was University of Guam, written commun., 2000). Although the island is relatively small and the mountains are low 180 delineated by Vann for a Master’s thesis project (David Vann, University of Guam, written commun., 2000). Water (1,300 ft or less), a significant orographic effect causes rainfall totals from recording stations to differ by as much as levels and springs as much as several hundred feet in altitude are where the freshwater-lens system is vertically 540 30 in/yr (fig. 1). Guam has distinct wet and dry seasons. About 70 percent of the total annual rainfall is recorded extensive in low-permeability volcanic rocks (fig. 3). Ground water in these systems can also be considered basal during the wet season (July through December) with the rest falling during the dry season (January through June). ground water using Meinzer’s (1930) original definition of basal ground water. In such systems, where low- The heaviest rainfall is usually associated with the passage of typhoons during the wet season but only about 12 144˚40' permeability rocks cause significant vertical hydraulic-head gradients, the Ghyben-Herzberg principle does not percent of average annual rainfall can be attributed to typhoons passing within 180 nautical mi of Guam (Lander, accurately predict the depth where the brackish water in the transition zone has a salinity about 50 percent of 1994). Exceptionally dry years recur about once every 4 years in correlation with episodes of El Niño/Southern 540 seawater. Although the depth to saltwater has not been measured in wells in these systems, where flow is downward, Tol 180 Oscillation (ENSO) in the Pacific (Lander, 1994). QTm the freshwater lens is likely thinner than predicted by the Ghyben-Herzberg principle. GEOLOGY 540 Volcanic rock forms the foundation of the island and is exposed over about 35 percent of the island’s surface, predominantly in southern Guam (fig. 2). Neogene limestone overlies the volcanic rock and is exposed over about 60 Qa 0 1 2 MILES 180 percent of the island, mainly in northern Guam. The high porosity of the limestone gives it high permeability, 180 900 0 1 2 KILOMETERS QTm 0 whereas the texture and poor sorting of the volcanic basement material usually gives it much lower permeability. All 54 major limestone units have undergone pervasive meteoric diagenesis, thus most porosity and permeability is secondary. Faults transect the island throughout, complicating the structure and permeability of the rock units. Age EXPLANATION 540 determinations of Guam geologic units are based mainly on fossil remains and correlations with other islands in the 900 A 1 Pacific. 80 SURFACE EXPOSURE OF VOLCANIC ROCKS (MODIFIED FROM 5 Seven major geologic units are shown in figure 2. The Facpi and Alutom Formations contain the oldest exposed TRACEY AND OTHERS, 1964) 40 rocks, are of Eocene and Oligocene age, and underlie all other exposed rock units (Tracey and others, 1964; Reagan Ta SUBSURFACE VOLCANIC ROCKS ABOVE SEA LEVEL IN NORTHERN GUAM and Meijer, 1984). These units crop out in the highlands from central to southern Guam and in the highest elevations (modified from David Vann, 2000, University of Guam, written commun.) 0 of northern Guam. These exposures cover about 20 percent of the surface of the island. The maximum thickness of 54 each unit is at least 1,000 ft (Reagan and Meijer, 1984). The formations contain a series of pillow basalts and water- WATER LEVEL IN WELL INDICATING FRESHWATER LENS IN: 18 EXPLANATION laid pyroclastic rocks of volcanic origin ranging from tuffaceous shale to coarse boulder conglomerate and breccia. Limestone (basal) 0

The permeability of the formations is considered low (Ward and others, 1965). Limestone overlying volcanic rocks (para-basal) 540 The Umatac Formation of Oligocene – late Miocene age (Tracey and others, 1964) is separated from the Tol Qa BEACH DEPOSITS, REEF DEPOSITS (Merizo Limestone), Volcanic rocks ALLUVIUM, AND ARTIFICIAL FILL (Holocene) underlying Alutom Formation by a structural unconformity. The Umatac Formation crops out over about 15 percent 180 of the land surface, principally in the south-central highlands and plateaus. The unit thickens to the south away from GENERALIZED DIRECTION OF GROUND-WATER FLOW

0 QTm MARIANA LIMESTONE (Pliocene to Pleistocene)

0 the Alutom Formation and is at least 1,050 ft thick along the southwest side of the island (Tracey and others, 1964). 9 Water level information from Ward and others (1965); 18 18 0 Tbl BARRIGADA LIMESTONE (Miocene to Pliocene) Here, the Umatac Formation is composed (from oldest to youngest) of about 250 ft of reef and forereef limestone Mink (1974); Camp Dresser & McKee Inc. (1982); 0 Guam Environmental Protection Agency (1986); (Maemong Limestone Member), about 750 ft of tuff breccia and volcanic conglomerate (Bolanos Pyroclastic 0 4 Tol OLDER LIMESTONE (Miocene to Pliocene)-- Torikai (1997); Guam Hydrologic Survey (unpublished 5 Member, Schroeder Flow Member), and about 50 ft of basalt flows (Dandan Flow Member) (Meijer and others, includes Bonya Limestone, Alifan Limestone, and data); U.S. Geological Survey, Hawaii District 9 18 00 0 C' 1983; Reagan and Meijer, 1984). The permeability of the formation is considered low (Ward and others, 1965). (unpublished data). Janum Formation

Three limestone units, which locally overlie the Alutom and Umatac Formations, are grouped together as Older Tu UMATAC FORMATION (Oligocene to Miocene) Limestone for this report. These three units, from Miocene to Pliocene age, are the Bonya and Alifan Limestones, C 1 260 Ta ALUTOM FORMATION (Eocene to Oligocene) and the Janum Formation (fig. 2). These three units are facies distinctions of limestones of the same age. Together 13˚20'

9 these units cover only about 5 percent of the island’s surface and individually the units are each only about 70 to 200 0 Tf FACPI FORMATION (Eocene) 0 ft thick. The Bonya Limestone (periplatform and embayment facies of mostly jointed and fractured detrital limestone) and the Janum Formation (deep forereef facies of mostly tuffaceous limestone) are considered permeable 180 CONTACT

but contain little water because of the limited extent of the deposits (Ward and others, 1965). The Alifan Limestone 180 FAULT-dashed where approximately located, dotted where covered 9 18

(shallow reef margin facies of poorly to well-consolidated detrital limestone and conglomeritic clay marl [Talisay 0

Tf 0 0 Member]) is moderately to highly permeable and feeds some perennial contact springs in southern Guam. QTm A A' LINE OF SECTION 0 The Barrigada Limestone, of late Miocene to Pliocene age, covers about 9 percent of the island’s surface but 4 Tu 5 0 forms the bulk of the aquifer underlying northern Guam. This formation consists of fine-grained, pure foraminiferal- 54 detrital limestone with a thickness of greater than 540 ft (Tracey and others, 1964). The formation supplies most of Southern boundary of RELATIVE DIRECTION OF MOVEMENT ALONG FAULT the wells in northern Guam and the permeability is high (Ward and others, 1965; Contractor and Srivastava, 1990; Vann study 540 900 Jocson and others, 2002). In places, the Barrigada Limestone grades upward and outward into the overlying Mariana 180 Limestone. 180 180 TOPOGRAPHIC CONTOUR--In feet. Interval is 180. Datum is The Mariana Limestone is of Pliocene to Pleistocene age and is the most extensive unit at the surface. The Guam mean sea level. 90 Mariana Limestone comprises about 45 percent of the surface area, covering most of the northern half and parts of 0 ROAD southeast Guam. The maximum thickness of the formation is at least 500 ft and may thin to zero inland. The 180 180 Mariana Limestone is composed of reef and lagoonal limestone containing a wide range of lithologies (Tracey and 540 others, 1964). The limestone is clay-rich near the older volcanic uplands. The permeability of the deposits without clay is considered very high due to the presence of many solution fissures and channels (Ward and others, 1965; Contractor and Srivastava, 1990; Jocson and others, 2002). The permeability of the clay-rich deposits is considered moderate to high. Minor reef beach deposits, reef limestone, and alluvium are of Holocene age. These deposits cover about 7 percent of the surface of Guam, and may be as much as 200 ft thick at the mouths of some rivers. The beach deposits are composed of poorly consolidated sediments, mostly calcareous sand and gravel thrown onto beaches by waves Qa but some deposits of volcanic sand can be found where streams drain volcanic uplands (Tracey and others, 1964). The Merizo Limestone is a reef limestone up to 12 ft thick. Deposits of alluvial clay fill stream valleys and cover the inner parts of coastal lowlands. The overall permeability of alluvial deposits may be moderate but hydrologic information is limited (see Ayers and Clayshulte, 1983). Base from U.S. Geological Survey Base from U.S. Geological Survey Digital Line Graph, Island of Guam, 2000 Island of Guam, 1:62,500, 1953

Figure 4. Water levels in selected wells, Guam

A A' FEET 1400 1000 QTm QTm 600 QTm QTm Qa QTm 200 Qa SEA LEVEL Tbl Tbl 200 QTm Tbl Tbl 600 Ta Ta Ta Ta Ta 1000 VERTICAL EXAGGERATION 1.25x

B FEET B' 1400 1000 Tol QTm Qa 600 QTm 200 QTm Tbl SEA LEVEL 200 Ta 600 Ta 1000 VERTICAL EXAGGERATION 1.25x

C FEET C' 1400 1000 Tu Tol Tol Fena Valley Reservoir 600 QTm QTm QTm Tol Qa Tol Qa 200 Qa Qa QTm Qa SEA LEVEL 200 Tu Tf 600 Ta Ta Tol Tu 1000 VERTICAL EXAGGERATION 1.25x 0 1 MILE

0 1 KILOMETER

Figure 2. Geologic map and sections, Guam (modified from Tracey and others, 1964 with additional modifications from Galt Siegrist, University of Guam, 2001, written commun.).

HYDROLOGIC RESOURCES OF GUAM by Stephen B. Gingerich U.S. DEPARTMENT OF THE INTERIOR PREPARED IN COOPERATION WITH THE WATER-RESOURCES INVESTIGATIONS REPORT 03-4126 U.S. GEOLOGICAL SURVEY WATER AND ENVIRONMENTAL RESEARCH INSTITUTE (WERI), Water Levels, Salinity, Ground-Water Withdrawals –SHEET 2 OF 2 UNIVERSITY OF GUAM Gingerich, S.B., 2003, Hydrologic Resources of Guam

HYDROLOGIC DATA Salinity Measurements in Deep Monitor Wells Surface Water The collection and analysis of basic hydrologic data is critical in helping to understand the hydrologic resources Periodic measurements of specific conductance and chloride concentration, indicators of salinity, in deep wells The USGS has at various times operated 22 continuously recording gages in southern Guam to record streamflow The Fena Valley Reservoir, constructed in 1951, captures surface water from three main streams with a combined of an area. Hydrologic data-collection activities of the USGS in northern Guam include recording rainfall at selected that penetrate into or through the freshwater/saltwater transition zone provide information about the thickness and (fig. 12 and table 1). Hydrographs of two rivers, the Ylig and Maulap Rivers, show total flow and base flow drainage area covering 5.88 mi2 and is the major source of domestic water in southern Guam (Nakama, 1992). sites, ground-water levels in wells and the ocean level in Hagåtña Bay, and measuring the salinity of ground water in seasonal variation of the freshwater lens. Samples of water from various depths in the deep wells were collected estimated from a computerized hydrograph-separation technique (Wahl and Wahl, 1995) for the period January Combined flow into the reservoir averaged about 21.3 ft3/s since 1972, about 15.3 ft3/s (9.9 Mgal/d) of which was deep monitor wells that penetrate through the transition zone (fig. 6). In southern Guam, the USGS operates gaging starting in the early 1980’s and were analyzed for specific conductance and chloride concentration. Beginning in 1980–September 2000 (fig. 12). The gaged streams of southern Guam have a ratio of base flow to total flow ranging diverted from the reservoir and a nearby spring for domestic and military use during 1999-2000 (Naval Facilities stations that measure streamflow, reservoir level, and rainfall. 2000, profiles of salinity have been obtained using a conductivity/temperature/pressure sensor lowered into the deep from 0.24 to 0.57 indicating that the low-permeability aquifer causes a significant part of streamflow to be from Fena Water Treatment Plant, written commun., 2000). wells. Plots (fig. 9) depict the depths at which the chloride concentration or salinity of water in the well was equal to ground-water discharge (table 1). In other words, ground-water discharge constitutes 23 to 57 percent of total 10, 50, and 90 percent of the chloride concentration or salinity of seawater. Each sample date allows the approximate streamflow in the gaged basin. Similar ratios (0.38 to 0.56) are reported from eastern Kauai, Hawaii, where the low- depth of freshwater (less than 10 percent seawater for these plots) and the approximate thickness of the transition permeability lava flows that form the aquifer have allowed water levels to build up hundreds of feet above sea level zone (between 10 and 90 percent seawater) to be estimated and the time series of samples shows the seasonal (Izuka and Gingerich, 1998). variations in the depth and thickness of the transition zone. The freshwater thickness is largest at well EX-1, ranging o from about 150 to 200 ft thick and the transition-zone thickness also is the largest, ranging from about 100 to 250 ft 144 50' thick. The reasons for such a thick transition zone are not understood but may be related to the relatively lower permeability of the sandy limestone in this area. The freshwater lens farther to the north is significantly thinner, EX-8 about 100 ft thick, at wells EX-7 and EX-9 and the seasonal variability is less pronounced. The transition-zone thickness at well EX-7 ranges from 15 to 50 ft thick. The data from all three wells show how the freshwater lens gets thinner around the end of the dry season when recharge is reduced and thicker after increased recharge during the Hagåtña Bay Andersen AFB wet season. Hagåtña o H ANDERSEN AFB RAIN GAGE (National Weather Service) a A 35 144 40' g E EX-10 å S Yigo t ñ

a 30

E EX-7 Dededo GHURA-Dededo R IN 25 P M-11 Dededo IP Tumon Bay L EX-6 I M-10A 20 H o P 13 30' Apra Harbor

15 Hagåtña Bay Hagåtña Tide Gage ACEORP

H tunnel 10 o a g A-16 144 40' å t N ñ Hagåtña a A M O N T H L Y R A I F , C E S R EX-1 EX-9 E 86200 C 5 O Mangilao Apra Harbor BPM 1 0 P A-20 IC 86500 ag IF o EX-4 C WELL EX-1 Pa A R go 0 P i R v iv e er Pago Bay r Pago Bay 100

Y lig EXPLANATION Agat Bay N Riv a er m Ylig Bay o er M-11 200 10 percent Riv WATER-LEVEL MONITORING WELL Agat AND WELL NAME A 80765 Y 50 percent Agat Bay lig EX-1 300 E TRANSITION-ZONE PROFILE WELL N Ri Talofofo AND NAME S a ver M aulap m N R Fena Valley Dededo Reservoir RAIN GAGE AND NAME 400 o Ylig Bay er Alma alofofo Riv A gosa R T River 90 percent 85800 Talofofo Bay 13o20' E R Agat 80830 g 500 n o I m C

WELL EX-7 0 O

E Fena Valley 84500 Umatac

D E P T H B L O W M A N S V , I F 100 10 percent Reservoir 50 percent N

90 percent I 200 C Inarajan P M 85000 Merizo au84850 I 84800 lap 84900 84600 WELL EX-9 P R Talofofo F Cocos 0 I Island I L A 84810 fo I lm alo fo C 100 10 percent agosa R T River 50 percent Base from U.S. Geological Survey 0 2 MILES A H Talofofo Bay Island of Guam, 1:62,500, 1953 90 percent o 84700 200 0 2 KILOMETERS 13 20' P P R 85500

Figure 6. Location of selected wells, rain gages, and tide gage, Guam g

n 300 o 1980 1990 2000 I m 85450 HAGÅTÑA TIDE GAGE DATE 80950 2 80940 1 Figure 9. Rainfall at Andersen AFB rain gage and chloride concentration relative to seawater at various 80960 depths and times in selected monitoring wells, Guam. (Andersen AFB data from Western Regional -0 Climate Center, 2002) 0 2 MILES -1 Umatac 0 2 KILOMETERS -2 ACEORP TUNNEL 84000 4 Ground-Water Withdrawal and Chloride-Concentration Data 81600

3 About 180 wells are used on Guam to withdraw ground water from the freshwater lens (fig. 10). The distribution and magnitude of pumping is based on available data provided by Guam Waterworks Authority (GWA), Guam 83500 2 Environmental Protection Agency (GEPA), the U.S. Navy (USN), and the U.S. Air Force (USAF). The pumping WELL M-11 rates shown are the average pumping rate of each well between January 1996 and December 1998, the last period 7 EXPLANATION when data were readily available. The total average daily withdrawal during that period was about 35 Mgal/d with 6 82100 Inarajan most of the wells individually withdrawing less than 0.4 Mgal/d. The two wells that withdrew the most water are Merizo 5 well A-30 (1.1 Mgal/d) and well Y-15 (0.8 Mgal/d). On an island, the chloride concentration of water is an indicator of the amount of seawater that has mixed with ACTIVE STREAM-GAGING STATION (2003) 4 the fresh rainwater infiltrated to the freshwater-lens system. For the 180 wells with recent data, chloride concentrations ranged from 6 to 585 mg/L. Of the 234 wells and springs for which historical chloride-concentration 3 DISCONTINUED STREAM-GAGING STATION data are available (pre-1962 to 1997), 32 percent have exceeded 250 mg/L in at least one sample (fig. 11). Another 5 WELL A-20 45 percent have exceeded 200 mg/L and may be approaching the 250-mg/L guideline. The samples with the highest 83500 ABBREVIATED STATION NUMBER--Complete Base from U.S. Geological Survey number is 16835000 concentrations are generally clustered near the center of the island. Factors that may contribute to the high Island of Guam, 1:62,500, 1953 44 concentrations include: (1) excessive rates of ground-water withdrawal from a single well, (2) a cluster of wells that DRAINAGE BASIN 43 are too closely spaced, (3) wells that are drilled too deeply into the freshwater lens, or (4) higher permeability geologic formations that allow increased mixing of freshwater with the underlying saltwater (McDonald, 2001). 42

41

W A T E R L V I O M N S , F 40

39 DEDEDO RAIN GAGE (National Weather Service) 2.5

2.0

1.5

1.0

0.5 STREAM-GAGING STATION 85800 Ylig River ne D A I L Y R N F , C H E S ar Yona 0.0 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 1 1,000 SEPTEMBER-OCTOBER 1995 TOTAL FLOW BASE FLOW Figure 7. Tides at Hagåtña tide gage, water-level response in selected wells, and rainfall at Dededo rain gage (September 8 to October 1, 1995) Guam. (Dededo data from National Climatic Data Center, 100 2000) Table 1. Data for selected gaged stream basins, Guam [ft3/s, cubic feet per second, drainage basins shown in figure 12; complete gaging-station number is preceded by 16 and ends in 0; nd, not determined; na, not applicable; values of n and f in the base-flow index program (Wahl and Wahl, 1995) for all streams were 4 and 0.9, respectively; rates of flow expressed in ft3/s can be converted to Mgal/d by multiplying the rate by 0.64632; currently (2003) active 10 Water-Level Records continuous-record stations shown in bold.] USGS Ratio of base OND

EC

Water levels in wells may rise and fall in response to short- and long-term changes in ocean level and to changes gaging- Average Average flow to total S 1 R in recharge and pumping. In wells closest to the coast in the high-permeability limestone, water levels fluctuate daily station Gaging-station Period of record available total flow base flow flow 1 3 3

number location for base-flow analysis (ft /s) (ft /s) (in percent) ET PE as much as 0.5 ft in response to ocean tides. Wells in the high-permeability limestone interior of the island typically E Aplacho River at F show much smaller daily fluctuations. Although data are sparse, wells in the low-permeability volcanic rocks 80765 10/1999 to 09/2001 nd nd nd Apra Heights probably have minor or no daily water-level fluctuations from ocean tides but show more response to local recharge. UBIC 0.1

80830 Finile Creek at Agat 04/1960 to 12/1982 1.41 0.80 57 IN C

The record of water level during September 8 to October 1, 1995, in two wells penetrating limestone (ACEORP , tunnel and well M-11) shows how the water table fluctuates in response to the ocean tide and to recharge (fig. 7). Cetti River near 80940 03/1960 to 09/1967 4.44 1.21 27 STREAM-GAGING STATION 84850 The daily tidal signal from the ocean is attenuated as it travels through the aquifer and the daily fluctuation of the Umatac HARGE La Sa Fua River near Maulap River near Agat water level decreases with increasing distance from the ocean. The daily water-level changes in the ACEORP tunnel, 80950 na nd nd nd 100 Agat

2,780 ft from the coast, average about 0.5 ft. Well M-11, 12,200 ft from the coast, has daily water-level changes that AN DISC TOTAL FLOW

05/1953 to 06/1960, E La Sa Fua River near average about 0.05 ft. At well A-20, no ocean tidal fluctuations are measurable, mainly because the well penetrates 80960 10/1976 to 04/1984, 4.56 1.31 31 M BASE FLOW Umatac Y 06/2000 to 09/2001 an area of low-permeability volcanic rocks at sea level (fig. 7). EXPLANATION HL Umatac River at 10 Water-level fluctuations caused by changes in recharge depend on the amount and location of rainfall and the 81600 10/1952 to 12/1976 8.66 3.07 35 WELL AND AVERAGE WITHDRAWAL RATE Umatac

MONT hydraulic properties of the aquifer in the area near the well. During September 12–16, 1995, about 8 inches of Geus River near 82100 05/1953 to 09/1975 3.01 0.72 24 rainfall were recorded at the NWS Dededo rain gage. The water level in well M-11 increased nearly 3 ft after 3 days less than 0.2 Mgal/d Merizo of rain but the water level in ACEORP tunnel increased only about 0.4 ft. Water levels in both wells declined to pre- Inarajan River near 1 83500 10/1952 to 01/1983 17.44 6.61 38 between 0.2 and 0.4 Mgal/d Inarajan rainfall conditions in about 20 days. The water level in well A-20 increased by about 4 ft after 8 to 10 days and only Tinaga River near declined slightly after about 20 days. This response indicates that recharge near well A-20 takes longer to reach the between 0.4 and 0.6 Mgal/d 84000 11/1952 to 02/1986 5.62 2.03 36 Inarajan water table and longer for the water to flow away in the aquifer. Tolaeyuus River near 84500 10/1951 to 06/1960 21.06 7.21 34 0.1 In addition to the daily fluctuation in water level caused by the daily ocean tides, water levels in high- equal to 0.8 Mgal/d Agat 1980 1990 2000 06/1994 to 05/1995, permeability rocks also vary over longer times in response to non-periodic changes in ocean level (fig. 8). A plot of Tolaeyuus River at DATE 84600 10/1996 to 03/1997, nd nd nd the daily average water level removes the daily tidal fluctuations from the water-level record. The daily average mouth near Agat equal to 1.1 Mgal/d 07/1997 to 07/2002 ocean level at the Hagåtña tide gage averaged 0.3 ft and ranged from –1.0 ft to 1.5 ft altitude during October 1983 to 04/1960 to 03/1971, April 2000. Water levels in well M-11 averaged 3.4 ft and ranged from 2.5 ft to 8.0 ft altitude during October 1982 Imong River near 84700 10/1971 to 03/1994, 9.52 4.19 47 Agat to April 2000 and water levels in ACEORP tunnel averaged 2.5 ft and ranged from 1.7 ft to 4.3 ft altitude during the 07/1997 to 09/2001 Figure 12. Stream-gaging stations and selected hydrographs, Guam same period. The water-level records with the tidal fluctuations removed show variations over several-month time Almagosa Springs 10/1951 to 12/1967, 84800 3.63 0.83 26 scales that correspond closely to the ocean-level record. near Agat 12/1971 to 09/1975 Water levels in well A-20 show a seasonal pattern of variations that is significantly larger than that in the wells 04/1972 to 03/1992, Almagosa River near penetrating limestone at sea level. Water levels averaged 41.2 ft and ranged from 32.6 ft to 56.2 ft altitude during the 84810 01/1993 to 04/1994, 5.98 2.05 36 Agat period October 1982 to April 2000. Generally, the lowest water levels were recorded during July and August before 03/1997 to 09/2001 the onset of the annual typhoon season. Maulap River near 01/1972 to 03/1994, Figure 10. Distribution of ground-water withdrawal, 1996-98, Guam. (From data provided by Guam Waterworks Authority, Guam Environmental 84850 5.14 2.18 42 Protection Agency, U.S. Navy, U.S. Air Force) Agat 07/1997 to 09/2000 84900 Fena Dam Spillway 10/1951 to 09/2001 na na na Talofofo River near 85000 12/1951 to 06/1962 50.95 15.38 30 Talofofo Ugum River above 06/1977 to 06/1995, CONVERSION FACTORS, DATUMS, AND ABBREVIATED WATER-QUALITY UNITS 85450 24.29 13.33 55 Talofofo Falls 03/1997 to 09/2000 Multiply By To obtain DEDEDO RAIN GAGE (National Weather Service) Ugum River near inch (in) 2.54 centimeter 12 85500 06/1952 to 09/1970 29.44 15.48 53 Talofofo foot (ft) 0.3048 meter 07/1952 to 03/1986, mile (mi) 1.609 kilometer 85800 Ylig River near Yona 04/1987 to 05/1995, 27.13 9.23 34 square mile (mi2) 2.590 square kilometer 8 07/1997 to 09/2000 cubic foot per second (ft3/s) 0.02832 cubic meter per second Lonfit River near 86200 10/1951 to 03/1960 10.42 2.80 27 million gallons per day (Mgal/d) 0.04381 cubic meter per second Ordot 10/1951 to 09/1983, 4 Pago River near 11/1980 to 12/1980, 86500 26.05 7.10 27 Datums Ordot 03/1981 to 07/1981, Vertical coordinate information is referenced relative to Guam mean sea level.

D A I L Y R N F , C H E S 09/1981 to 01/1983 1 0 Base-flow index from Wahl and Wahl (1995) Horizontal coordinate information is referenced to the Guam Datum of 1963.

HAGÅTÑA TIDE GAGE Abbreviated water-quality units used in report 2 mg/L, milligrams per liter

1

0

-1 SUMMARY AND CONCLUSIONS WELL M-11 9 Ground water supplies about 80 percent of the drinking water for the island’s 150,000 residents and nearly one Izuka, S.K., and Gingerich, S.B., 1998, Ground water in the southern Lihue basin, Kauai, Hawaii: U.S. Geological Survey Water-Resources Investigations Report 98-4031, 71 p. 8 million visitors per year. In northern Guam, water is obtained from wells tapping a fresh ground-water lens in a Jocson, J.M.U., Jenson, J.W., and Contractor, D.N., 2002, Recharge and aquifer response: Northern Guam Lens Aquifer, Guam, Mariana 7 highly permeable limestone aquifer. Nearly 180 wells withdraw about 35 Mgal/d of water with chloride Islands: Journal of Hydrology, v. 260, p. 231-254. 6 concentrations ranging from 6 to 585 mg/L. In southern Guam, where low-permeability volcanic rocks are exposed Lander, M.A., 1994, Meteorological factors associated with drought on Guam: Technical Report No. 75, Water and Energy Research Institute 5 over most of the area, about 9.9 Mgal/d of freshwater is obtained from surface reservoirs. of the Western Pacific, University of Guam, 39 p. 4 The high permeability of the limestone in northern Guam allows rapid infiltration of rainfall so surface runoff McDonald, M.Q., 2001, Analysis of saltwater intrusion in the Northern Guam Lens Aquifer based on chloride ion concentrations at production and observation wells: M.S. Thesis, University of Guam, variously paged. 3 EXPLANATION occurs locally only after intense rain. The limestone also offers little resistance to ground-water flow so only a thin Meijer, Arend, Reagan, Mark, Ellis, Howard, Shafiqullah, Muhammad, Sutter, John, Damon, Paul, and Kling, Stanley, 1983, Chronology of 2 freshwater lens has developed. Water levels in the freshwater lens vary several feet daily and seasonally in response WELL OR SPRING AND MAXIMUM volcanic events in the eastern Philippine Sea in Hayes, D.E., ed., The tectonic and geologic evolution of southeast Asian seas and to ocean tides, recharge, and ground-water withdrawal. The thickness of the freshwater lens varies seasonally, islands: part 2: American Geophysical Union, Geophysical Monograph 27, Washington, D.C., p. 349-359. ACEORP TUNNEL CHLORIDE CONCENTRATION 5 mainly in response to seasonal variations in recharge. Ground-water withdrawal has led to localized areas of Meinzer, O.E., 1930, Ground water in the Hawaiian Islands in Stearns, H.T., and Clark, W.O., Geology and water resources of the Kau less than 200 milligrams per liter increased chloride concentration, an indicator of saltwater intrusion or upconing. Factors that may contribute to the District, Hawaii: U.S. Geological Survey Water-Supply Paper 616, p. 1–28. 4 high concentrations include excessive rates of ground-water withdrawal from a single well, a cluster of wells too Mink, J.F., 1974, Groundwater resources of Guam: Occurrence and development, report to the Public Utility Agency, Guam, variously paged. between 200 and 250 milligrams per liter Nakama, L.Y., 1992, Storage capacity of Fena Valley Reservoir, Guam, Mariana Islands, 1990: U.S. Geological Survey Water-Resources 3 closely spaced, wells that are drilled too deeply into the freshwater lens, or higher permeability geologic units that Investigations Report 92-4114, 17 p. between 250 and 500 milligrams per liter allow increased mixing of freshwater with the underlying saltwater. 2 National Climatic Data Center, 2000, Daily rainfall data, Dededo Rain gage, Guam: http://www4.ncdc.noaa.gov/cgi- greater than 500 milligrams per liter The low-permeability volcanic rocks of southern Guam allow less rainfall infiltration and slower flow of ground win/wwcgi.dll?wwDI~StnSrch~StnID~20030347 accessed September 4, 2000. 1 water. Heavy rainfall and high runoff cause intense flooding in the stream drainage basins. Because the volcanic Reagan, M.K., and Meijer, Arend, 1984, Geology and geochemistry of early arc-volcanic rocks from Guam: Geological Society of America rocks have low permeability, ground water saturates the aquifer to tens to hundreds of feet above sea level and Bulletin, v. 95, p. 701-713. WELL A-20 Thurman, H.V., 1990, Essentials of oceanography (3d ed.): Columbus, Ohio, Merrill Publishing Company, 398 p. 60 discharges at springs and into stream valleys. Ground-water discharge constitutes 24 to 57 percent of total 55 Torikai, J.D., 1997, Rainfall, ground-water, and ocean-tide data, Guam, 1996: U.S. Geological Survey Open-File Report 97-239, 67 p., 1 pl. in streamflow in the gaged stream basins. pocket. 50 Tracey, J.I., Jr., Schlanger, S.O., Stark, J.T., Doan, D.B., and May, H.G., 1964, General geology of Guam: U.S. Geological Survey 45 REFERENCES CITED Professional Paper 403-A, 104 p., 3 pl. A V E R G D I L Y W T O M N S , F 40 U.S. Environmental Protection Agency, 1991, Secondary maximum contaminant levels (section 143.3 of part 143, National secondary Ayers, J.F., and Clayshulte, R.N., 1983, Feasibility study of developing valley-fill aquifers for village water supplies in southern Guam: 35 drinking water regulations): U.S. Code of Federal Regulations, Title 40, Parts 100 to 149, revised through July 1, 1991. Technical Report No. 41, Water and Energy Research Institute of the Western Pacific, University of Guam, 91 p. 30 Wahl, K.L., and Wahl, T.L., 1995, Determining the flow of Comal Springs at New Braunfels, Texas: Proceedings of Texas Water ’95, A Camp Dresser & McKee Inc., 1982, Northern Guam lens study, Groundwater management program, Aquifer yield report prepared for the Component Conference of the First International Conference on Water Resources Engineering, American Society of Civil Engineers, 1980 2000 1990 Government of Guam, Guam Environmental Protection Agency, variously paged. August 16–17, 1995, San Antonio, Tex., p. 77–86. YEARS Contractor, D.N., and Srivastava, R., 1990, Simulation of saltwater intrusion in the Northern Guam Lens using a microcomputer, Journal of Ward, P.E., Hoffard, S.H., and Davis, D.A., 1965, Hydrology of Guam: U.S. Geological Survey Professional Paper 403-H, 28 p., 1 pl. Hydrology, v. 118, p. 87-106. Western Regional Climate Center, 2002, Anderson AFB Guam, Pacific Ocean monthly total precipitation data: http://www.wrcc.dri.edu/cgi- Guam Environmental Protection Agency, 1986, Groundwater management program annual report, Agana, Guam, 140 p. bin/cliMAIN.pl?piande accessed December 5, 2002. Figure 8. Rainfall at Dededo rain gage, tide at Hagåtña tide gage, and water-level response in selected Figure 11. Distribution of maximum chloride-concentration values (pre-1962 to 1997), Guam. (Data from Ward and others, 1965; Torikai, 1997; wells, Guam. (Dededo data from National Climatic Data Center, 2000) Mink, 1974; Guam Environmental Protection Agency, 1986; Guam Water and Environmental Research Institute, written commun., 2000)

HYDROLOGIC RESOURCES OF GUAM by Stephen B. Gingerich