Locally Sponsored Research

Guam Hydrologic Survey

Data Availability Report

Chapter 5: Geological Data
by: Danko S. Taboroši, John W. Jenson, John M.U. Jocson & H. Galt Siegrist

This chapter summarizes the historical regarding the basic geology and hydrogeology of Guam, geological research programs and data held or being compiled by WERI, and emerging programs to collect and archive new geological data on Guam.


Surface Geology

The first comprehensive geologic studies were done by United States Geologic Survey (USGS) during the two decades following WWII. A preliminary report by Cloud (1951) was followed by a series of USGS Professional Papers in the early 1960s, most notably Tracey, Schlanger, Stark, Doan, and May, (1964) report on the general geology of Guam, which included a 1:50,000-scale map of the surface geology of the island. (Other reports in the series were Emery, 1963; Stark and Tracey, 1963; Schlanger, 1964; Cole, 1963; Carroll and Hathaway 1963; Stensland, 1963; Johnson, 1964; Ward, Hofford and Davis, 1965; and Todd, 1966). Ward and Brookhart (1962) published a report on the Military Geology of Guam which included a water resources supplement.

The first comprehensive hydrologic study was published by Ward et al. in 1963. The next was in 1976 by Mink. In 1980, the Guam EPA administered the most exhaustive study of the aquifer to date, known locally as the Northern Guam Lens Study (NGLS). The chief contractor for the study was Barrett, Harris, and Assoc., who published the study report in 1982. Local contributions to the NGLS by WERI included a hydrogeologic analysis by Ayers (1981) and a preliminary study of aquifer discharge by Zolan (1982). In 1992, Barrett Consulting Group prepared a revision or the 1982 study for Guam EPA, but the report was not published or widely circulated. WERI has a copy in the Water Resources Data Library. A comprehensive bibliography of geologic and hydrogeologic studies of Guam has been compiled by Siegrist (1992). (The bibliography is available on the WERI-GHS web site, (http://uog2.uog.edu/weri/microbib/index.htm).

Since 1994 WERI has been compiling an unpublished map of sinkhole distribution on northern Guam. Recently, Lewis and Siegrist (1994-1995, unpublished data at WERI) mapped fracture orientations in southern Guam. WERI points of contact are summarized in Table 5.1.

Table 5.1. Geology points of contact: Guam Hydrologic Survey program, WERI.

Individual Phone Number Email Address
Dr. H. Galt Siegrist, Jr. (671) 735-2685 hsiegris(uog)uog.edu?subject=Contact - Dr. H. Galt Siegrist
Dr. John Jenson (671)735-2685 jjenson(uog)uog.edu?subject=Contact - Dr. John Jenson
Mr. John Jocson (671) 735-2693 jjocson(uog)uog.edu?subject=Contact - Mr. John Jocson
Mr. Danko Taboroši (671) 735-2146 dankot(uog)campus.univguam.edu.gu?subject=Contact - Mr. Danko Taboroši

Basement Geology

The first map of the basement topography prepared from comprehensive geophysical data was published as part of the 1982 NGLS. This map was slightly revised in 1992 by the Barrett Consulting Group, using the time domain electromagnetic technique (TDEM). Ongoing drilling by GWA and by consultants to the military installation restoration programs has demonstrated that these maps must be used with circumspection and should be systematically updated on the basis of the most complete and recent information, especially drilling log data.

Beginning in January 1998, under the GHS program, WERI began consolidating data on basement topography, including historical drilling logs obtained from Guam Waterworks Authority, Guam EPA, local engineering consulting firms, and local drilling companies. WERI now maintains copies of these data in the Guam Hydrologic Survey Data Library. Current points of contact for drilling data are summarized in Table 5.2. Work is underway at WERI to revise the basement map using this newly consolidated information.


Table 5.2 Drilling data points of contact: drilling companies and Guam EPA

Company/Agency Name Individual Responsible Phone Number
Earth Tech Mark Kramer (671) 647-6083
Pacific Drilling, Inc. Danny Mafnas (671) 646-6371
Marianas Drilling, Inc. Dan Ramos (671) 646-7710
JTTL Enterprises John Lund (671) 653-1897
Guam EPA Marilou Yamanaka (671) 475-1636

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New Data Collection Efforts

Surface Geology

In July 1998, WERI began a two-year field study to characterize the karst features of the Northern Guam Lens Aquifer. This includes a comprehensive inventory of sinkholes, coastal springs and seeps, cave systems, as well as the structural and stratigraphic features that influence or control the development of the karst.

Bedrock and basement geology

GEPA permitting regulations require drillers to maintain drilling logs and to collect rock samples during the drilling, according to their permit agreements. Under the GHS program, WERI is consolidating this information, to include archiving drilling cuttings. Bill 652 will require WERI to assist GEPA is setting data collection standards, and that permit applicants notify WERI prior to drilling so that a WERI geologist can supervise on-site data collection.


Data Archive

We are confident that most of the hydrologically significant reports and drilling logs that remain on the island have been located and archived in the GHS Data Library at WERI. Currently, the data have to be accessed physically at the locations where they are stored. WERI plans to make some of the data available on-line beginning August 1998. Access will be possible through our web page at uog2.uog.edu/weri/ghs.



Surface Geology

The 1:50,000 scale map of Guam by Tracey et al (1964) is still the only comprehensive geologic map of Guam. Although the map is of high quality for its scale, subsequent work (e.g., Randall and Siegrist 1996) has shown that some parts are incomplete and that revisions are needed. More importantly, the 1:50,000 scale of the map is inadequate to support the current and growing need for greater precision and finer resolution of the geologic features of Guam. The rapid economic growth of the island, with the accompanying needs for installation of more new infrastructure, site evaluations for development projects, and settlement of land use questions makes it imperative that selected portions of the island soon be remapped at 1:24,000 or larger scale. Currently, there are no plans for such mapping, however.

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Basement Geology

The kind, amount, and quality of hydrogeologic data collected varies widely because of the previous lack of a standard for on-site data collection. Drilling log data, however, are of central importance for accurately predicting the location of productive zones of groundwater and understanding the behavior of production wells. Implementation of the requirement under Public Law 24-247 to notify WERI prior to drilling and to have a WERI geologist supervise drilling data collection will ensure that scientific standards are met, data (including cutting samples) are properly archived, and that knew knowledge gained is exploited to update and revise existing maps and analyses.

In addition to collecting and keeping data from drilling associated with public and private water development projects, the island’s water resource managers would be well served by a comprehensive update of island bedrock (aquifer) and basement geology based on new seismic data and selected corings from key locations on the island. Improved accuracy and resolution of the basement map, for example, will allow us to more efficiently utilize the groundwater resource by being able to precisely locate areas of water accumulation and to make more accurate assessments of sustainable yield. There are currently no plans for systematic study of the bedrock or revision of the basement topography, however.



Ayers, J. F. (1981) Estimate of Recharge to the Freshwater Lens of Northern Guam. University of Guam, Water and Energy Research Institute of the Western Pacific. Tech. Report No. 21 March.

Barrett, Harris & Associates (1982) Northern Guam Lens Study, Summary Report, Guam Environmental Protection Agency, Harmon Guam.

Barrett Consulting Group (1992) Groundwater in Northern Guam, Sustainable Yield and Groundwater Development, prep. in assoc. with John F. Mink, for Public Utility Agency of Guam, Agana, Guam

Carroll D. and J. C. Hathaway (1963) Mineralogy of selected soils from Guam. U.S. Geol. Survey Prof. Paper 403-F: 42 p.

Cloud, P. E. Jr. (1951) Reconnaissance Geology of Guam and Problems of Water Supply and Fuel Storage: Military Geology Branch, U.S. Geol. Survey.

Cole W. S. (1963) Tertiary larger foraminifera from Guam. U.S. Geol. Survey Prof. Paper 403-E: 28 p. & 11 plts.

Emery, K.O. (1962) Marine Geology of Guam, U.S. Geol. Survey Prof. Paper 403-B

Johnson J. H. (1964) Fossil and recent calcareous algae from Guam. U.S. Geol. Survey Prof. Paper 403-G: 40 p. and 15 plts.

Mink, J. (1976) Groundwater Resources of Guam: Occurrence and Development. University of Guam, Water Energy Research Institute of the Western Pacific. Tech Report No. 1 Sept.

Randall, R. H. and H. G. Siegrist, Jr. (1996) The legacy of Terague Embayment and its inhabitants, Andersen AFB, Guam. Volume III: Geology, beaches and coral reefs.

Schlanger S.O. (1964) Petrology of the limestones of Guam with a section on petrography of the insoluble residues by J. C. Hathaway and D. Carroll. U.S. Geol. Survey Prof. Paper 403-D: 52 p. & 21 plts.

Siegrist (1992) Selected bibliography of 20th century geoscience and related scientific literature of Micronesia, part I: Mariana island arc. Micronesian Area Research Center, University of Guam.

Stark, J. T. and J. I. Tracey, Jr. (1963) Petrology of the volcanic rocks on Guam. U.S. Geol. Survey Prof. Paper 403-C: 32 p.

Stensland C. S. (1963) Description of soil profiles. In, D. Carroll and J. C. Hathaway, Mineralogy of selected soils from Guam. U.S. Geol. Survey Prof. Paper 403-F: 43-49

Todd R. (1966) Smaller foraminifera from Guam. U.S. Geol. Survey Prof. Paper 403-I: 41p.

Tracey, J. I., Jr., Schalanger, S. O., Stark, J. T., Doan, D. B., and May, H. G. (1964) General Geology of Guam: U.S. Geol. Survey Prof. Paper 403-A.

Ward, P. E., and Brookhart, J. W. (1962) Military geology of Guam. Water resources supplement. U.S. Army. Chief Eng., Intellig. Div., Headg. U.S. Army Pacific, Tokyo

Ward P. E., S. H. Hofford and D. A. Davis (1965) Hydrology of Guam, Mariana Islands. U.S. Geol. Survey Prof. Paper 403-H: 25 p. and map

Zolan, W. J. (1982) A Preliminary Study of Natural Aquifer Discharge on Guam. University of Guam, Water Energy Research Institute of the Western Pacific. Tech Report No. 34 Aug.

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