Senior Executive

FIELD TRIPof the Northern Guam

Lens Aquifer

2013

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Senior Executive

FIELD TRIPof the Northern Guam

Lens Aquifer

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Objective Preface

Examine, in a single, short trip, the four physical components of the Northern Guam Lens Aquifer that determine its production capacity and its vulnerability to contamination or misuse. Current conditions of the aquifer and steps that should be taken to support sustainable development and optimal management are also presented and discussed.

Hafa Adai!

On behalf of the faculty and staff of the University of Guam’s Water & Environmental Research Institute of the Western Pacific, I am honored to welcome our island’s senior leaders and policy-makers to this first offering of our senior executive short course on Guam’s primary water resource—our Northern Guam Lens Aquifer. As the island’s premier source of new scientific research, analysis, and information on Guam’s water resources, we are committed to supporting

informed policy choices and sustainable management of Guam’s water resources, especially given the anticipated expansion of our civilian economy and military activities over the next decade and beyond. For this field trip we have carefully selected four of the island’s most accessible and instructive sites for an informative “hands-on” explanation of the aquifer’s principal components, all of which can be visited well within the span of a single workday. We have also prepared this field trip guide to provide you with an uncomplicated introduction and concise ready reference on our island’s most important natural resource. We hope you will find this valuable and enjoyable, and that you will feel free to call on WERI at any time for whatever advice and assistance you might find useful in meeting present and future water resource needs for commercial and military activities on Guam, and for providing the highest quality of life for both its permanent residents and visitors.

Sincerely,

Shahram Khosrowpanah, Ph.D., P.E.Director, WERI

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Abstract Table of Contents

This field trip takes you to four sites, each of which is a premier example of the four basic components of the Northern Guam Lens Aquifer: the tight, nonproductive volcanic basement rock that constitutes the floor of the aquifer; the porous and soluble water-bearing limestone bedrock that is the source of our drinking water; the surface catchment that captures the recharging rainwaters; and the features of the surface that control and distribute the entry of water into the aquifer. At each site, we observe and discuss the geological and hydrological properties that control the capture, storage, and production of potable water, and the opportunities and challenges faced by geologists and engineers in exploring, developing, and managing groundwater production. While traveling between sites we observe features of regional importance to groundwater development and aquifer protection. We also discuss basic facts and statistics related to drinking water production, consumption, and conservation on Guam and nationwide.

The Northern Guam Lens Aquifer: Basic Facts 6The Northern Guam Lens Aquifer: Water Processes 7Surface of the Northern Guam Lens Aquifer 8Groundwater Basins of the Northern Guam Lens Aquifer 9Idealized profile of the Northern Guam Lens Aquifer 10Cross-section along the Yigo Trough 12

Field Trip Map and Schedule of Stops 14

Stop 1: The Floor of the Aquifer – the Volcanic Basement Rock 16

Stop 2: The Core of the Aquifer – the Limestone Bedrock 18

Stop 3: Plumbing of the Aquifer – Surface and Internal Drainage 20

Stop 4: The Roof of the Aquifer – the Water Catchment System 22

From Sustainable Yield to Sustainable Management 24Acknowledgments 26Further Reading 27Authors 28

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The Northern Guam Lens Aquifer: Basic Facts

The Northern Guam Lens Aquifer: Water Processes

➤ Geographic extent all of northern Guam➤ Total surface area 83 sq mi (214 sq km)➤ Average annual rainfall ~100 inches / year 467 million gallons per day➤ Estimated recharge 238 million gallons per day

➤ Current production1 44 million gallons per day (19% of estimated recharge)

➤ Number of production wells Active: 123 Other: 27

➤ Current consumption2

~30 million gallons per day*

N G L

A

GWA 39

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1Information on the number of wells and production is from Guam EPA, Guam Waterworks Authority Engineering Department, US Navy, and US Air Force.2The gap between current production and consumption is thought to be due to a combination of loss through leakage and undocumented consumption through unmetered usage.

RAINFALLEvaporation

Transpiration

Vadosepercolation(months to years) Vadosefast flow

(minutes to hours)

Overlandflow

RECHARGEPhreatic storage and flow

Overflow

Spring/Seep Discharge

SOIL (0-1 ft)EPIKARST

(10s of feet)

VADOSE ZONE

(100s of feet)

FreshwaterPHREATIC

ZONE(<~200 feet)

➤➤

➤

➤

DoD4

95

Private <1

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InfiltrationInternal runoff

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Surface of the Northern Guam Lens Aquifer

Groundwater Basins of the Northern Guam Lens Aquifer

GROUNDWATER SETTINGS: BASAL WATER PARABASAL WATER SUPRABASAL ZONE*

SIX BASINS Discharge boundaries color-coded by basin: l Hagåtña l Yigo-Tumon l Finegayan l Agafa Gumas l Andersen l Mangilao

Basement contours Sea level contour Basin boundary (fixed) Basin boundary (loose) Pago-Adelup fault

LIMESTONE TERRAIN VOLCANIC TERRAIN SINKHOLES and other closed contour depressions

Major geologic faults Pago-Adelup fault

* Supra-basal water is found in discontinuous patches and conduits within this zone.

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Idealized profile of the Northern Guam Lens Aquifer

SUPRA-BASAL WATER• underlain by basement rock• standing above sea level• invulnerable to sea water contamination• very high quality water — catchment “headwaters”• more responsive to wet-dry cycles than para-basal water• very hard to find (luck is important, even with map...)

BASAL WATER• underlain by seawater• vulnerable to sea water contamination• variable quality water• easy to find (underlies most of northern Guam)

PARA-BASAL WATER• underlain by basement rock• resistant to sea water contami-

nation • ”upstream” from surface threats• more affected by wet-dry cycles

than basal water• hard to find

VOLCANIC BASEMENT

LIMESTONE

SALTWATER

FRESHWATER LENS

Water table

Saltwater toe

Mixing zone

Mean sea levelReef

T H R E E T Y P E S O F G R O U N D W A T E R S E T T I N G S

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1:40

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Cross-section along the Yigo Trough

BASAL WATER

SALTWATER FRESHWATER LENS Saltwater toe NOTE: Vertical scale on this diagram is realistic. The following values are used: Horizontal and vertical scale: 1 inch = 3168 ft (1 km); Cross-section width 7.5 miles; Elevations: Mt. Santa Rosa (MSR) +830 ft, central plateau surface +400 ft, volcanic contact at the coast -800 m, volcanic contact offshore -1500 ft, water table +4 ft, 50% isochlor -200 ft

SU

PR

A-B

AS

AL

WAT

ER

PAR

A-B

AS

AL

WAT

ER

Water tableMea

n se

a le

vel

Water found only in isolated conduits and discontinuous patches

LIMESTONE

VOLCANICS

FRESH WATER

SALTWATER

Mt.

San

ta R

osa

~75% of northern Guam

Tum

on

Bay

<5% of northern Guam

~20% of northern Guam

This cross section depicts the actual proportions of the freshwater lens and geologic units along the axis of the basement valley that heads between Mount Santa Rosa and Mataguac Hill and extends from there beneath Dededo to Tumon. Called the “Yigo Trough” by local hydrologists, it collects and carries fresh water, much like a surface stream valley, beginning in the area around Yigo and discharges it into the ocean in Tumon Bay. The Yigo Trough is the aquifer’s most prolific producer of drinking water.

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Field trip map and schedule of stops

Starting point: Adelup Stop 1: Mt. Alutom Stop 2: DPW Quarry Stop 3: Mataguac Hill Stop 4: Mt. Santa Rosa

Itinerary

0800-0900 Van meets and pick up participants at Adelup

0930-1000 Stop : THE FLOOR OF THE AQUIFER (Alutom Formation, Mt. Alutom)

1045-1130 Stop : THE CORE OF THE AQUIFER (DPW Quarry, Dededo)

1145-1230 Stop : PLUMBING OF THE AQUIFER (Mataguac Hill Peace Memorial Park)

1245-1315 Stop : THE ROOF OF THE AQUIFER (Summit of Mt. Santa Rosa)

1345-1400 Van delivers participants to point of departure

Bottled water will be available. Boxed lunches will be provided on the way to Stop #3, Mataguac Memorial Peace Park, where we restrooms will also be available.

There will be no hiking, but participants are advised to wear appropriate casual field clothes, headgear, and footwear, and be prepared for intense sunlight. Sunglasses are advisable for the quarry visit. Safety gear will be provided for the quarry visit.

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1234

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Stop 1: The Floor of the Aquifer – The Volcanic Basement Rock

Hydrologic role of basement rock Named the Alutom Formation, this hydrologically “tight” volcanic rock forms subterranean hills and ridges beneath the overlying limestone bedrock. The basement topography is the single most important consideration for successful groundwater exploration on northern Guam. Beneath about one-fifth of the plateau surface the basement partitions the aquifer into six semi-contiguous groundwater basins. Descending groundwater concentrates along the axes of valleys above sea level and may even be impounded in some small subterranean reservoirs. Although very difficult to locate, such streams and patches of “supra-basal” water are the freshest water in the aquifer, and are invulnerable to seawater contamination. The water descending down the hills and valleys to sea level is concentrated into a “para-basal” rim of freshwater that is underlain by volcanic basement rock rather than seawater. Because it is thus very fresh and much less vulnerable to seawater contamination the surrounding “basal” water of the freshwater lens, the para-basal zone has long been the zone of choice for development and production of groundwater.

Alutom Formation, Mt. Alutom

Here, around the crest of Mount Alutom, are the island’s best outcrops of the practically impermeable volcanic basement rock that underlies the entire Northern Guam Lens Aquifer. The summit of Mount Alutom also provides an impressive view of the northern limestone plateau—the surface of the aquifer—which we will visit in the next three stops. To the far northeast can be seen Mount Santa Rosa, where the volcanic basement rock emerges above the surface of the northern limestone plateau. The summit of Mount Santa Rosa will be our fourth and final stop, where we will be able to see and discuss in more detail the surface features that control the amounts and rates of aquifer recharge.

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Stop 2: The Core of the Aquifer – the Limestone Bedrock

Barrigada Limestone, DPW Quarry, Dededo

Active quarrying of the limestone here in the “Dededo Coral Pit” provides some of the island’s best exposures of the rock that comprises the core of our aquifer. This limestone is actually not from coral, but is rather a granular, “detrital” limestone, formed in deeper waters mostly from accumulation of shells and fragments of shells left behind over millions of years by tiny organisms that colonize the bottoms of shallow banks and the water above. Fresh cuts in the quarry walls provide outstanding examples of the kinds of porosity that constitute the internal plumbing of the aquifer.

Hydrologic role of the Barrigada Limestone

The Barrigada Limestone is by far the most extensive and important of the three major limestone units of the aquifer. Even a casual inspection shows that this limestone is, overall, noticeably porous. A somewhat closer look, however, also reveals that the porosity of this rock can vary remarkably over the scale of just a few tens of feet. Thus, even though this rock is very porous at the regional scale, only about one in three or four exploratory wells usually proves suitable for production. Production from successful wells can be extremely high, however—500 to 750 gallons per minute. Inside the quarry, we will examine the qualities and features of the rock that determine the success or failure of new wells and the paths by which contaminants may enter and move through the aquifer.

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Hydrologic role of sinkholes, shafts, and caves

Recharging waters enter the aquifer and descend to the water table in two ways: 1) by slow (months to years) percolation of water from ordinary rainfall that infiltrates through the ground surface, and 2) by fast flow (minutes to days) of water that ponds in natural surface

Stop 3: Plumbing of the Aquifer – Surface and Internal Drainage

Surface water and sinkhole, Mataguac Hill Peace Memorial Park

In addition to its significance as a World War II historical site, the Peace Memorial Park provides an outstanding example of a natural sinkhole, with a visible active swallow hole at its bottom. The swallow hole has formed along the contact between the soluble limestone bedrock above and the insoluble volcanic basement below. Here, spring water that constantly flows from the nearby spring that forms in the volcanic rock of Mataguac Hill, and storm waters that occasionally run off the flank of Mataguac Hill, enter the aquifer and descend some 400 vertical feet to the water table.

depressions (“sinkholes”) during heavy storms. The fast flow process is permanently on display here in this sinkhole on the flank of Mataguac Hill, where the perennial Mataguac Spring provides a modest but steady flow of water to the nearby open cave (see photo on opposite page) and into the swallow hole located at the cave’s bottom (see photo below).

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Hydrologic role of the aquifer surface

All of the water that recharges the freshwater lens begins as rainwater that falls on the surface of the plateau. Some 70 percent of Guam’s rainfall arrives during its wet season, from July through December. Ongoing studies of cave dripwaters suggest that hardly any of the other 30 percent that falls during the dry season contributes to recharge of the lens. Consistent with these findings, a recent independent study of recharge suggests that about 50 percent of total annual rainfall evaporates or is taken up and transpired by vegetation growing on the plateau, while the other 50 percent descends through the limestone—either directly to the water table or onto the flanks of the basement hills and valleys that occupy the 20 percent of the aquifer within the supra-basal zone. Past studies suggest that over the long term up to about 30 percent of recharge may descend via fast flow routes, such as observed at the previous stop. Quantifying aquifer recharge is one of the outstanding challenges of hydrological research on Guam.

Stop 4: The Roof of the Aquifer – the Water Catchment System

Vista of entire aquifer, Summit of Mt. Santa Rosa

This final stop provides a spectacular view of the entire aquifer surface, including each of its six basins, from the Hagatña Basin at the extreme southwest; the Finegayan Basin on the far side of Mataguac Hill, to the west; the Agafa Gumas Basin to the northwest; the Andersen Basin just to the north; and the Mangilao Basin, which starts on the southern flank of Mount Santa Rosa and runs southwest along the Pacific coastline to the southeastern flank of Barrigada Hill. This view provides a particularly good perspective of the Yigo-Tumon Basin, which heads between Mount Santa Rosa and Mataguac Hill and runs southwest beneath the villages of Yigo, Dededo, Tamuning, and Tumon. The axis of the Yigo-Tumon Basin, termed the Yigo Trough by local hydrologists, is the single most prolific production zone of the aquifer.

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From Sustainable Yield to Sustainable Management

American Waterworks Association

U.S. daily indoor per capita water use: 69.3 gallonsU.S. daily household average: 350 gallons

And for Guam?

For 30 mgd consumptionand 200,000 people

daily per capita consumption = 150 gallons

➤ 1982 Northern Guam Lens Study: “The rate of production that can be sustained without unacceptably degrading water quality.” So… What is acceptable quality? Who decides? How?

➤ NGLS quality target: 150 mg/l chloride 1982 SY: 59 mgd 1992 SY: 80 mgd

➤ USEPA standard: 250 mg/l chloride Current production: ~45 mgd Current consumption: ~30 mgd

SHOWERS 11.6

17%

CLOTHES WASHERS 15.0

22%

DISHWASHERS 1.0

1% TOILETS 18.5

28%

BATHS 1.2

2%

LEAKS 9.5

14%

FAUCETS 10.9

16%

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Further Reading

Selected Current and Historical Technical Sources

AECOM Technical Services Inc., 2011, Guam Water Well Testing Study to Support US Marine Corps Relocation to Guam, Volume Contract Number N62742-06-D-1870, TO 036: Pearl Harbor, HI, Naval Facilities Engineering Command, Pacific.

CDM, 1982, Final Report, Northern Guam Lens Study, Groundwater Management Program, Aquifer Yield Report, Camp, Dresser and McKee, Inc. in assoc. with Barrett, Harris & Associates for Guam Environmental Protection Agency.

Contractor, D.N., and Jenson, J.W., 2000, Simulated Effect of Vadose Infiltration on Water Levels in the Northern Guam Lens Aquifer: Journal of Hydrology, v. 229, p. 232-254.

Drew, D. and Hötzl, H., 1999, Karst Hydrogeology and Human Activities. Impacts, Consequences and Implications: Balkema, Rotterdam, 322 pp.

Ford, D. and Williams, P., 2007, Karst Hydrogeology and Geomorphology: London, Wiley Chichester, 576 p.

Goldscheider, N. and Drew, D., Eds., 2007, Methods in Karst Hydrogeology: London, Taylor & Francis, 264 pp.

Habana, N.C., Heitz, L.F., Olsen, A.E., and Jenson, J.W., 2009, Vadose flow synthesis for the Northern Guam Lens Aquifer, WERI Technical Report No. 127: Mangilao, Water & Environmental Research Institute of the Western Pacific, University of Guam.

Jenson, J.W., and Jocson, J.M.U., 1998, Hydrologic data collection on Guam: FY 1998 Report, WERI Technical Report No. 83: Mangilao, Water & Environmental Research Institute of the Western Pacific, University of Guam.

Jenson, J.W., Keel, T.M., Mylroie, J.R., Mylroie, J.E., Stafford, K.W., Taboroši, D., and Wexel, C., 2006, Karst of the Mariana Islands: The interaction of tectonics, glacio-eustasy, fresh-water/salt-water mixing in island carbonates: Geological Society of America Special Paper, v. 404, p. 129-138.

Johnson, A.G., 2012, A water-budget model and estimates of groundwater recharge for Guam, Volume U.S. Geological Survey Scientific Investigations Report 2012–5028 , p. 53.

Krešić, N., 2007, Hydrogeology and Groundwater Modeling, Second Edition: Boca Raton, New York, London, CRC Press/Taylor & Francis, 807 p.

Lander, M.A., Jenson, J.W., and Beausoliel, C., 2001, Responses of well water levels on northern Guam to variations in rainfall and sea level: WERI Technical Report No. 94, p. 36.

Mylroie, J.E., and Carew, J.L., 1995, Karst development on carbonate islands, in Budd, D.A., Harris, P.M., and Saller, A., eds., Unconformities and Porosity in Carbonate Strata, Volume Memior 63, American Association of Petroleum Geologists, p. 55-76.

Mylroie, J.E., and Jenson, J.W., 2000, The Carbonate Island Karst Model applied to Guam: Theoretical and Applied Karstology, v. 13-14, p. 51-56.

Mylroie, J.E., Jenson, J.W., Taboroši, D., Jocson, J.M.U., Vann, D.T., and Wexel, C., 2001, Karst features on Guam in terms of a general model of carbonate island karst: Journal of Cave and Karst Studies, v. 63, p. 9-22.

Partin, J., Jenson, J., Banner, L., Quinn, T., Taylor, F., Sinclair, D., Hardt, B., Lander, M., Bell, T., Miklavič, B., Jocson, J. and D. Taboroši, 2012, Relationship between modern rainfall variability, cave dripwater and stalagmite geochemistry in Guam, USA: Geochemistry, Geophysics, Geosystems 13 (3).

Siegrist, H.G., and Randall, R.H., 1992a, Carbonate geology of Guam, in Richmond, R., ed., Proceedings of the Seventh International Coral Reef Symposium, Volume 2: Mangilao, Guam, University of Guam Marine Laboratory & Water & Energy Research Institute of the Western Pacific, p. 1195-1216.

—, 1992b, Carbonate Geology of Guam: Summary & Field Trip Guide, 7th International Coral Reef Symposium: Mangilao, Guam, Water & Energy Research Institute of the Western Pacific & Marine Laboratory, p. 39 p.

Siegrist, H.G., and Reagan, M.K., 2008, Geologic Map and Sections of Guam, Mariana Islands (1:50,000), Revision of original map from USGS Professional Report 403A, 1964, by Tracey, J.I., Jr., Schlanger, S.O., Stark, J.T., Doan, D.B. & May, H.G. Field interpretations for 2008 revision assisted by Randall, R.H. and Jenson, J.W., Water & Environmental Research Institute of the Western Pacific, University of Guam, Mangilao, Guam.

Tracey, J.I., Jr., Schlanger, S.O., Stark, J.T., Doan, D.B., and May, H.G., 1964, General Geology of Guam: US Government Printing Office, Washington, D.C., U.S. Geological Survey Professional Paper.

Simard, C.A., Jenson, J.W., and Lander, M.A., 2013, in review, Analysis of Salinity in the Northern Guam Lens Aquifer, in Savarese, M., and Glumac, B., eds., 16th Symposium on the Geology of the Bahamas and Similar Regions: Gerace Research Center, San Salvador Island, Bahamas.

Taboroši, D., 2004, Field Guide to the Caves and Karst of Guam: Honolulu, Bess Press, 105 pp.Taboroši, D., Jenson, J.W., and Mylroie, J.E., 2005, Karst features of Guam, Mariana Islands:

Micronesica, v. 38, p. 17-46.Ward, P.E., Hoffard, S.H., and Davis, D.A., 1965, Hydrology of Guam: US Government Printing

Office, Washington, D.C., U.S. Geological Survey Professional Paper 403-H.White, W. B., 1988, Geomorphology and Hydrology of Karst Terrains: New York, Oxford:

Oxford University Press.

Note: The on-line version of this field trip guidebook can be accessed on the WERI website by searching for ‘WERI Northern Guam Lens Aquifer Field Trip Guide’. It contains links to many of the documents listed above.

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Authors

John W. Jenson, PhDSenior Hydrogeologist & Professor of Environmental GeologyWater & Environmental Research Institute of the Western PacificUniversity of Guam, Mangilao, GU 96923jjenson@uguam.uog.edu

Ph.D. Geology, Oregon State UniversityM.A. Applied Economics, University of MichiganB.S. Economics, US Air Force Academy

Dr. Jenson has studied the Northern Guam Lens Aquifer for some two decades since joining the WERI faculty in 1993. His research encompasses both applied and theoretical groundwater hydrology and related environmental science. Recent work on Guam includes assessment of newly-drilled and rehabilitated wells, rehabilitation of long-out-of-service wells, and reconnaissance of promising sites for new production wells. Recent applied work also includes evaluation of the stormwater drainage potential of a large sinkhole on Andersen Air Force Base, in collaboration with Dr. Taboroši; and an updated study of spatial patterns and temporal trends in the salinity of water from Guam’s production wells. He is currently supervising the construction of a new aquifer database and an update of the map of the aquifer basement. Dr. Jenson’s basic scientific research activities include development of the Carbonate Island Karst Model and reconstruction of past wet-dry cycles and sea levels on Guam from field geology and geochemical clues in cave deposits on northern Guam. Dr. Jenson also served a 30-year career as a US Air Force officer, with tours in strategic missile operations and staff duty at HQ Strategic Air Command, HQ US Air Force, HQ US Forces-Japan, and HQ 13th Air Force.

Danko Taboroši, PhDDirectorIsland Research & Education Initiative, Palikir, Pohnpei, FM 96941taborosi@islandresearch.org

Ph.D. Earth Science, Hokkaido UniversityM.S. Environmental Science, University of GuamB.S. Marine Biology, College of CharlestonB.A. Geology, College of Charleston

Dr. Taboroši’s research focuses on carbonate geology, karst processes, and coastal and island geomorphology. His Masters research was a landmark inventory of the karst features of Guam, which included the first thorough maps of the island’s sinkholes and caves. He has remained active in Guam water resources research. His doctorate at Hokkaido University took him into detailed interdisciplinary studies of calcium carbonate precipitation, cave microclimates, limestone coastal processes. He has taught sedimentology, stratigraphy, and oceanography at the American University of Beirut in Lebanon, and a professional development course on the Northern Guam Lens Aquifer at the University of Guam in collaboration with Dr. Jenson. Dr. Taboroši is also the Pohnpei-based Island Research & Education Initiative for which his work includes mapping erosion, and studying water resources infrastructure and water use on Micronesian atolls. In addition to numerous professional papers, he has authored textbooks and children’s books for the Federated States of Micronesia as well as Guam, including Field Guide to Caves and Karst of Guam, Student Atlas of Guam, and Environments of Guam. Dr. Taboroši has traveled in over 100 countries and is fluent in seven languages.

Acknowledgments

Special thanks to Mr. Carlos Taitano and his colleagues at the University of Guam’s Professional and International Programs Office for logistic and administrative support in preparing the field trip and this guide. Thanks also to Mr. Nathan Habana at WERI for assistance with maps and photography, and to Mr. Alfred Santos at the Department of Public Works Quarry in Dededo for graciously assisting in preparation and hosting visits to the quarry.

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2013