Hydrology of Guam GEOLOGICAL SURVEY PROFESSIONAL PAPER 403-H Prepared in cooperation with the Government of Guam Hydrology of Guam By PORTER E. WARD, STUART H. HOFFARD, and DAN A. DAVIS GEOLOGY AND HYDROLOGY OF GUAM, MARIANA ISLANDS GEOLOGICAL SURVEY PROFESSIONAL PAPER 403-H Prepared in cooperation with the Government of Guam UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1965 UNITED STATES DEPARTMENT OF THE INTERIOR STEWART L. UDALL, Secretary GEOLOGICAL SURVEY William T. Pecora, Director For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C., 20402 CONTENTS Page Hydrology Continued Abstract.._ ___________________________________ HI Ground water___----_---_--_--------------_---_ H12 Introduction. ____---_____--_---____-_________ 1 Northern Guam.____--_-__-___-______ 12 Purpose and scope.___-_-_______-_-_____-_. 1 Water in limestone______________ 12 Location and extent of the area of this report. 1 The fresh-water lens.-_-__---_ 12 Previous investigations and present work____ 2 Water perched on volcanic rock. 16 Acknowledgments __ ______________________ 3 Water in volcanic rock-___________ 18 Population and development._-____-_-_____ 3 Southern Guam._____________________ 18 Topography...__--___-_--_____-__________ 3 Water in volcanic rock.___________ 18 Climate.___--_-_____--_________--___-___ 4 Perched water in limestone ________ 18 Geology.___------------_----_---_-____-_ 5 Water in limestone at sea level. _ 19 Hydrology- ______----_.._-_----__-_-_-_---____. 6 Development of ground water._________ 19 Streamflow__ - __________________________ 6 Ground-water areas.__________________ 19 Umatac River__-__-___-_-_--_________ 6 Area l_____----_---_--___------- 23 Inarajan River. _____________________ 6 Area 2_---_--------_. 23 Ugum River_______----___-__________ 7 Area 3_____.-__----_. 23 Ylig River.__-____-_---___--__-____-_ 7 Area 4.________--__-,. 24 Pago River______..----_-__---______-- 7 Area 5_-------------- 24 Runoff characteristics__________________ 7 Area 6______------_-. 25 Rainfall-runoff relations____________ 7 Area 7__---_---- .. 25 Low flow.____--_-_____-_---_-___ 8 Chemical character of the water. 25 Flood flows. ___-__-_---_------_--. 8 References cited.___---_______----. 28 Development of surface water._________ 12 ILLUSTRATIONS PLATE 1. Water-resources map of Guam__________________.__________.________-_-________-----.-_.----------In pocket Page FIGURE 1. Map showing location of Guam____________________________________________________________________ H2 2. Map showing the distribution of rainfall______________________.________________________________________ 4 3. Graph of rainfall and runoff in the Pago River basin______________________-______-_-_-_____--_--____--__ 8 4. Duration curves of daily flow._______________________________________________________________________ 9 5. Low-flow frequency curves in the Ugum River. ________________________________________________________ 10 6. Low-flow frequency curves in the Pago River__________________________________________________________ 11 7. Composite frequency curve for floods_______________________________________________-_____-_____-__-___ 11 8. Curve showing relation of the mean annual flood with the ratio of drainage area to length of principal stream __ 12 9. Mass curve of outflow of Fena Valley Reservoir____________________________________________-___--__-___ 13 10. Approximate height of the water table in northern Guam._________________--___-_-_-_-_--__---------_--- 14 11. Diagram showing the occurrence of the fresh-water lens in northern Guam__________--_______---_--_-_-_- 15 12. Graph of the water level in well 82 showing the effect of tidal fluctuations_________________________________ 15 13. Graph of water levels in wells 79 and 82 showing the effect of storm waves.__________________________ 15 14. Graphs showing water levels in wells 79 and 82 compared to tide levels and rainfall.______________________ 16 15. Graphs showing rainfall and water levels in wells 147 and 148____________________________________________ 17 16. Maps of northern Guam showing effect of pumping on chloride content of water in limestone._______________ 17 17. Graphs showing analyses of water from various sources_---_-_____-________-_--____-_-_--___------_-_--_- 27 18. Analyses of water represented by polygons..___________________________________________________________ 28 in IV CONTENTS TABLES Page TABLE 1. Summary of rainfall at Agana Naval Air Station, Guam, 1952-62_________________________-___-_-_-_-___-_. _- H4 2. Rocksof Guam and their water-bearing properties-. __-----_------------_--------_-------------------- -- 5 3. Summary of discharge records of five representative streams on Guam---------------------------------------- 7 4. Records of wells and springs in Guam.___-______________________-_________---___---------_--------------- 20 5. Analyses of water from various sources in Guam,....._____.______________-_-_-______----__-_-------------- 26 GEOLOGY AND HYDROLOGY OF GUAM, MARIANA ISLANDS HYDROLOGY OF GUAM By PORTER E. WARD, STUART H. HOFFARD, and DAN A. DAVIS ABSTRACT scattered mostly along the streams. Limestone caps on some The average rainfall on Guam amounts to nearly a billion hills in southern Guam contain perched water, which dis­ gallons of water per day. The northern half of the island is charges at springs around the edge of the limestone. These largely a permeable limestone terrane which readily absorbs springs fluctuate widely in flow. Limestone extending below the rainfall and from which no streams flow. The southern sea level along parts of the coast of southern Guam contains half is mostly a complex of volcanic rocks that has overall low mostly brackish water. permeability and is deeply eroded by many streams. Because of low permeability of the volcanic rock, virtually The average runoff to the sea in streams is about 250 mgd no water has been obtained from wells drilled in southern Guam. (million gallons per day). A large part of the rainfall in rainy During World War II the daily pumpage from scores of wells seasons runs directly off, and at the end of the rains the stream- drilled in the limestone of northern Guam amounted to 6-8 flow quickly recedes to the low flow maintained by the slow million gallons. In later years the daily pumpage has been discharge of ground water from the volcanic rock. The runoff 2-4 million gallons, and diversions of probably 2 mgd has been during extended dry periods varies significantly from stream to made from various springs on the island stream. The lowest rate of recession among the major streams Water in volcanic rock and in. limestone not affected by sea- is in the Ugum River in which the lowest daily discharge during water intrusion has a dissolved-solids content of 200-300 parts the period of record was 3.4 cubic feet per second. per million. The dissolved constitutents are mostly calcium Heavy rainfall during typhoons commonly causes widespread and bicarbonate. Water in the volcanic rock is relatively high flooding, but the highest flood peaks on some streams have in silica. Mixing with sea water causes sharp rises in the con­ resulted from intense rainfall during local storms. The various tent of magnesium, sodium, sulfate, and chloride. basins are similar in their flood-producing characteristics, and flood-frequency curves for individual streams can be combined INTRODUCTION into a composite curve that is applicable to any site on most of PURPOSE AND SCOPE the streams. The largest development of streamflow on Guam is the Fena The purpose of this report is to describe the water on Valley Reservoir, which has an impounding capacity of 2,500 the surface and in the rocks of Guam. The report in­ million gallons. During the dry year of 1959 the reservoir could have maintained a constant draft of 12 mgd. cludes a summary of the climate and an outline of the Recharge to the limestone aquifer in northern Guam main­ geology of the island. It describes representative tains a fresh-water lens that floats in the slightly heavier sea streams and the character of the runoff, describes the water filling the rock below sea level. The height of the water occurrence of ground water in the various types of rock, table ranges from sea level at the shore to several feet above and gives information on the development of water and sea level in the interior of the island. The lens discharges at the shore, but, because of mixing of fresh water and sea water the chemical character of the water. in the permeable limestone, water flowing at the shore is mostly brackish or saline. Mixing is promoted by sea-level fluctua­ LOCATION* AND EXTENT OF THE AREA OF THIS tions caused by the tides and storm waves, by fluctuations in REPORT recharge which thicken and thin the lens, and by pumping Guam is a tropical island lying at lat 13°28' N., long from wells, which also changes the thickness of the lens. Heavy 144°45' E. It has an area of about 212 square miles pumping during World War II greatly reduced the lens and caused widespread intrusion of sea water into the aquifer. and is the largest and southernmost of the Mariana Is­ Locally, water is perched in the limestone on underlying volcanic lands (fig. 1). The island is 30 miles long, and from a rock, some of which discharges at a spring near sea level on width of 81/2 miles in the northern part it tapers to 4 the eastern shore of northern Guam. miles at the central waist, then widens again to ll1/^ The water table in the volcanic complex in the southern half miles in the southern part. Several islets lie along the of the island has high relief, rising from sea level at the shore to hundreds of feet above sea level under the uplands. Ground- fringing reef, the largest being Cocos Island off the water discharge is diffused at many seeps and small springs southern tip. Hi H2 GEOLOGY AND HYDROLOGY OF GUAM, MARIANA ISLANDS 100' I20 140° 160° 180° 160° 140° 120" 60° NORTH PACIFIC OCEAN HAWAIIAN ' ,« 4 ISLANDS ,co 145° 146° 4 SOUTHERN MARIANA ISLANDS SAIPANj T1NIAN M ICO f 10 A6UIJAN.