Locally Sponsored Research

Guam Hydrologic Survey

Data Availability Reports

Chapter 4: Tide and Coastal Zone Data
by: David Moran & John Jocson

This chapter discusses tidal data collection activities, and provides points of contact of current work related to the impact of fresh water discharge on coastal water quality.

Tidal Data

Data Collection Activities & Points of Contact

US Geological Survey

Under the Comprehensive Monitoring Program, the United States Geological Survey (USGS) operates a tidal gage at the top of the ramp to the commercial boats in the Agana Boat Basin (Fig. 4.1). Water level is recorded every 15 minutes by a digital recorder. USGS personnel from the Honolulu office visit Guam quarterly to download the data. With the re-opening of the USGS Guam Field Office during FY99, the gage will be serviced at least monthly.


USGS Data Archive

USGS has published daily mean tidal levels plus statistical summary information for the Agana tide gage since May 1983. USGS annually published daily sea level information from 1986 through water year 1989 in Volume 2 of Water Resources Data--Hawaii and Other Pacific Areas. Copies of the reports are available at GEPA (1982-1984 and 1986-1989 reports), WERI (1979 and 1982-1989 reports), and at the UOG RFK Memorial Library (1989 report). The reports can be purchased from the US Department of Commerce at NTIS, 5285 Port Royal Road, Springfield, VA 22161.

USGS also documented tidal data in printed reports entitled Guam Water Data Management System Annual Reports (from 1980 to FY 1996, FY 1997 is in preparation). Hard copies of these reports are available at GEPA and WERI. Published data are also available on the USGS web site at http://waterdata.usgs.gov/nwis-w/HI/. Historical data are also available on a commercial CD-ROM database, Hydrodata, prepared by Hydrophere®. WERI maintains a subscription to Hydrodata. There is typically about a one-year delay between data collection and publication in the CD-ROM database.


National Oceanographic Atmospheric Administration (NOAA)

The NOAA station is located at the western entrance to Sumay Cove Marina in outer Apra Harbor (Fig. 4.1). It normally contains an acoustic gage, a back-up analog bubbler gage with a chart recorder, and a tsunami device. Up until the passage of Typhoon Paka on December 16-17, 1997, tide levels were measured at six-minute intervals, with the data sent via satellite telemetry to NOAA Data Collection Center in Seattle. Typhoon Paka put the gage station out of service, however. Plans are underway to rebuild the station but actual construction will be dependent on the availability of funds, which are not expected to be available until the next fiscal year. Point of contact for the station is:

Mr. Mickey Moss
NOAA Ocean Systems Pac. Reg. Office
7600 Sand Way, NE Seattle, WA 98115-0700
Phone: (206) 526-6360



NOAA data can be obtained through the National Ocean Service website
http://www.opsd.nos.noaa.gov or through the University of Hawaii website at

Evaluation and recommendations

The USGS tide gage station is a reliable site for good sea level measurements, but it is currently the island’s only operating tidal data collection site. Until the NOAA station is restored there will be no backup for failure of the Agana gage. Installation of a second station by USGS would provide backup to the existing gage. It should also be noted that the island has no gage on the Pacific Ocean side of the island; there is therefor no means of evaluating tidal differences between the two sides of the island.

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Data Collection Activity & Points of Contact

Guam Environmental Protection Agency (GEPA)

GEPA currently collects coastal water quality data along selected beaches and river mouths. Beach areas are monitored weekly. Water samples are tested in the GEPA laboratory. Point of contact is:

Mr. Vance Eflin, Chief of Surveillance
P.O. Box 22439, GMF
Barrigada, GU 96921
Phone: (671) 475-1665
Fax: (671) 477-9402


Until 1990, GEPA sent their data to the USEPA STORET archival system. Since 1990, however, data have not been systematically sent to the STORET database. Data from 1990 to present are kept in hard copy at GEPA and are currently being transferred into Excel spreadsheet files.

Evaluation and recommendations

Although coastal waters are sampled regularly, groundwater discharging at the coast is not being systematically sampled. Groundwater discharging from the lens can entrain contaminants, including sewage from leaking lines or septic tanks, agricultural chemicals and fertilizers. Testing groundwater discharging in coastal spring for plant nutrients, particularly nitrates and phosphates, as well as for selected pesticides and other potential contaminants, is an important means of determining whether contaminants are being released in the groundwater sub-basin. It is particularly important to obtain baseline data against which to evaluate the affects of development and the incidence of pollution from human activities. Moreover, since contaminants from fresh water discharging at the coast can affect the quality of near-shore waters which support recreation and food collection, the quality of the fresh water entering the coastal zone is an essential piece of information for evaluating the impact of development and human activities on the coastal zone and for determining what sort of remedial steps might be needed.

From 1986 until 1996 as part of a series of studies on nutrient flux into the bay, Dr. Ernest Matson, University of Guam, collected bi-weekly groundwater samples in Tumon Bay. Currently, there is no ongoing sampling of coastal spring discharge into Tumon Bay or anywhere else along the coast. A program of regular sampling of groundwater coastal discharge should be instituted immediately. This program would provide baseline data from which to determine trends in selected nutrient (e.g., nitrate and phosphate) species that could affect coastal water quality, as well as occurrence of selected chemical or biological indicators storm water and sewage contamination.

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