Locally Sponsored Research

Guam Hydrologic Survey

Data Availability Report

Chapter 3: Groundwater Data
by: Mauryn E. Quenga & John W. Jenson

This chapter discusses groundwater data collection activities for water table elevation, groundwater production, chloride, and other water quality parameters. Current points of contact for the agencies and activities that collect or archive groundwater data are summarized in Table 3.1.

Table 3.1. Summary of contacts, addresses, and numbers (in alphabetical order by agency/activity).

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WATER TABLE ELEVATION DATA

Data Collection Activities and Points of Contact

GEPA???s Water Resources Management Program (WRMP) began collecting monthly single-measurement water-level measurements at the Agana Springs station (in the Agana Sub-basin) in January 1976. Since then, the WRMP added seven observation wells (Figure 3.1) to their water-level measurement program. Of the seven observation wells, two are in the Yigo-Tumon Sub-basin: EX-6 (along Macheche Road in Mogfog, Dededo) and GHURA-Dededo (at the Guam International Country Club). Five are in the Agana Sub-basin: Agana 147 (near the Agana McDonald???s), EX-1 (Chochogo, near San Miguel Elementary School), EX-4 (in Mangilao, near Father Duenas Memorial School), EX-9 (in Barrigada, near P.C. Lujan Elementary School), and the FD well (behind Father Duenas High School). Two additional observation wells, at which monthly single-measurement water levels were previously taken by GEPA???H-107 (along the road to Two Lover???s Point in the Harmon Annex) in the Yigo-Tumon Sub-basin, and EX-8 (in Northwest Field on Andersen AFB) in the Agafa-Gumas Sub-basin???are completely out of service due to blocking or cave-ins.

Data Archive

USGS published annual reports of daily mean water levels plus statistical summary information from 1972 to 1976 in Water Resources Data ??? Hawaii and Other Pacific Areas. Data from water years 1977 through 1989 are in Volume 2 of Water Resources Data ??? Hawaii and Other Pacific Areas. Copies of the reports are held at GEPA (1982-1984 and 1986-1989 reports), WERI (1979 and 1982-1989 reports), and at the UOG RFK Memorial Library (1989 report). Reports can be purchased from the US Department of Commerce at: NTIS, 5285 Port Royal Road, Springfield, VA 22161.

The USGS also documented both GEPA and USGS groundwater data in annual reports titled Guam Water Data Management System Annual Reports, from 1980 to FY 1996. The FY 1997 report is in preparation. Guam groundwater data are not yet available on the USGS website, but GEPA and WERI have hard copies of the reports. Historical data (from the 1970s through 1997) are available on Hydrodata, prepared by Hydrosphere??. WERI maintains a subscription to Hydrosphere??, but there is about a one-year delay between data collection and publication in the Hydrodata database.

Recommendations

Lack of coverage of the Finegayan Sub-basin???which contains no observation wells at internal locations???is the most serious current shortfall in water level data collection capability, especially since the Finegayan Sub-basin is already well developed, is undergoing ongoing development, and may yet possess significant reserve. Priority should be given to installation of at least an additional well or two in the central and northern portions of the sub-basin, where production wells are concentrated. Ideally, at least one should be placed in a para-basal location (where the base of the fresh water lens is in contact with the volcanic basement rock), and one in a basal location (where the fresh water lens is underlain salt water permeating the limestone).

In the Yigo-Tumon Sub-basin, the largest producer of the six sub-basins, the USGS currently collects continuous water-level data at four observation wells (EX-7, EX-10, M-10A, and M-11) and GEPA collects monthly single-measurement water levels at two observation wells (EX-6 and GHURA-Dededo). Spatial distribution of these data collection sites is probably inadequate for the long-term management of this important sub-basin, however. The current observation wells are concentrated near the southern end of Dededo. There are no observation wells at the head of the Yigo Trough, nor along its axis between Dededo and Yigo. Since these are areas known to possess substantial concentrations of groundwater, and probably a significant portion of the remaining reserve, plans should be made to install at lest one observation well in each of in these areas and to cover both para-basal and basal zones.

After the Yigo-Tumon Sub-basin, the Agana Sub-basin contains the second largest number of production wells. Spatial coverage in the Agana Sub-basin???in which USGS collects continuous water-level data at four observation wells (A-16, A-20, ACEORP Tunnel, and BPM-1) and GEPA collects monthly single-measurement water levels at six sites (Agana Spring, Agana 147, EX-1, EX-4, EX-9, and FD)--is the best of the six sub-basins, although there is no continuous observation in the west-central portion of the sub-basin. In this area, data collection could probably best be improved by installing continuous data recording equipment at Agana-147 or EX-1. It is important to maintain existing coverage in this sub-basin since it is heavily developed and some production wells appear to have begun to exhibit higher and increasing chloride concentrations. Additional observation wells might eventually be installed as areas of special concern are identified. Because the aquifer in the Agana Sub-basin is dominated by the less permeable Argillaceous Member of the Mariana Limestone, it exhibits classic karst features (such as blind valleys and disappearing streams) that make generalizations regarding the hydrology of the aquifer extremely uncertain. Locations for additional observation wells in this sub-basin should be based on needs to characterize specific anomalies or important local variations in aquifer behavior, as they are identified.

Development in the Mangilao Sub-basin is generally a higher risk than in the others because of its limited reserve and the difficulty of locating productive well sites. Presently, there are only seven potable water production wells in this elongated sub-basin, all located near its southern end (Figure 3.2A). If these wells are to be kept in production, a co-located observation well would be useful for managing these wells and assessing the response of the local groundwater body to seasonal changes and storm water infiltration.

The Agafa Gumas and Andersen Sub-basins are also without active observation wells, but both are virtually undeveloped since they are overlain mostly by federal property. Basic hydrologic data are important for evaluating development potential and establishing baseline characteristics to support management when they are eventually developed. Rehabilitation of EX-8 in the Northwest Field area of Andersen Air Force Base would provide a good starting point for gathering baseline data in the Agafa Gumas Sub-basin. On Andersen Air Force Base there are numerous observation wells installed to monitor for chemical contaminants in support of the US Department of Defense Installation Restoration Program. One or more of these might eventually be equipped to collect coninuous water level data if plans are made to install water production wells in the interior of the sub-basin.

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GROUNDWATER PRODUCTION DATA

Data Collection Activities and Points of Contact

Under current environmental regulations, the groundwater production of Guam???s 172 permitted production wells (Table 3.2) is monitored by well owners and reported to GEPA. Guam Waterworks Authority, with 109 wells, administers the majority of the island???s production wells. GWA wells are listed by sub-basin in Table 3.2A. Contact information for GWA is provided below:

Table 3.2A. GWA wells by sub-basin.

* permit for M-20A is pending

For the privately-owned wells on Guam, points of contact, numbers of wells, well IDs, and types of water use are shown in Tables 3.1B (northern Guam) and 3.1C (southern Guam). Locations for all production wells are shown in Figure 3.2A (northern Guam) and 3.2B (southern Guam).

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Table 3.1B. Northern Guam well owners, points of contact, well IDs, and water use (alphabetical order by owner).

Table 3.1C. Southern Guam well owners, points of contact, well IDs, and water use (alphabetical order by well owner).

Data Collection

Well owners must obtain a well operating permit from GEPA???s Water Resources Management Program before a well is allowed to begin production. The operating permit requires well owners to report monthly production by the 10th of each month to the WRMP. An annual report for each well itemizing monthly production, alternations, repairs, and rehabilitation is due to WRMP by January 15th each year. Specific information can be obtained from the WRMP Director (Table 3.1).

Data Archives

GEPA???s WRMP maintains island-wide well production records. Data prior to 1980 are primarily limited to GWA, Navy, and Air Force public water supply wells and are stored on hard copy in WRMP files. Data from 1980 through FY 1996 are compiled in Guam Water Data Management System Annual Reports by the USGS. The FY 1997 report is currently in preparation. These data are not currently available on the USGS web site, but copies of the Annual Reports are available at GEPA and at WERI. Production data from 1993 through the present are maintained on Excel spreadsheets and can be obtained from GEPA or WERI. Beginning in FY1999, the Guam Hydrologic Survey Program at WERI will maintain, in the Guam Water Resources Data Library, annual summary production statistics compiled from the GEPA consolidated data base. Data in the Guam Water Resources Data Library can be accessed via the WERI/GHS web site (http://uog2.uog.edu/weri/ghs/).

Data Evaluation

Data collection by private well owners generally appears to be accurate, consistent, and timely. The most recent GWA production report submitted to GEPA was for September 1997.

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Groundwater Cholera Data

Data Collection Activities & Points of Contact

Chloride Profiles

From 1985 through FY 1996, USGS constructed quarterly chloride-depth profiles of up to seven deep observation wells (EX-1, EX-4, EX-6, EX-7, EX-9, EX-10, and GHURA-Dededo) by measuring the chloride concentrations at selected elevations in the groundwater column. (Deep wells are defined in this document as wells of sufficient depth to penetrate the entire thickness of the fresh water lens down to 100% seawater.) The last of the chloride profile data-collection ceased at the end of FY 1996, when sampling equipment could no longer penetrate EX-7 and EX-9. Implementation of the WERI/USGS data-collection program in FY 1999 will re-establish chloride profiling activities at each of these seven wells. (Note: All wells except for EX-4 are uncased. Most of them were taken out of service as they became blocked from spalling-off of the rock in the bore hole.)

Wellhead Sampling

The Guam Waterworks Authority (GWA) laboratory collects and analyzes source water at each of its wells once per quarter. Contact information is provided below:

Environmental regulations require owners of potable water wells to monitor chloride concentrations. Monitoring frequencies (Table 3.2) vary depending on water use (see Tables 3.1B and C). Non-potable water wells may also be tested if background chloride levels are high. Results are submitted to GEPA???s Water Resources Management Program. GEPA confirms well owners??? results by conducting annual wellhead chloride sampling of all production wells, regardless of water use and background chloride levels.

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Table 3.2. Wellhead chloride sampling for production wells (alphabetically by owner).

Data Archive

USGS data are not currently available on their web site, but USGS chloride profiles were annually published in Water Resources Data???Hawaii and Other Pacific Areas from 1972 through 1976 and in Volume 2 from 1977 through water year 1989. Copies of the reports are available at GEPA (1982-1984 and 1986-1989 reports), WERI (1979 and 1982-1989 reports), and 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.

GEPA and USGS data from 1980 through FY 1996 have been compiled in Guam Water Data Management System Annual Reports by the USGS. The FY 1997 report is currently in preparation. The GWA Laboratory maintains quarterly chloride data on hardcopy from 1973 to the present and on Excel spreadsheets for FY 1997. Data from 1993 through FY 1997 are available on Excel spreadsheets. The WRMP and WERI maintain copies of the Annual Reports and Excel spreadsheets. Limited chloride and other water quality data (from 1976 to 1996) are available on Hydrosphere???s?? EPA Storet CD-ROM data base. WERI maintains a subscription to Hydrosphere??, but there is about a one-year delay between data collection and publication in the EPA Storet database.

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Data Evaluation and Recommendations

The fundamental limit to production from island and coastal aquifers is the amount of fresh water that can be withdrawn without contaminating the production zone of the fresh water lens with salt water. Monitoring chloride concentration in the groundwater is the single most important means for assessing whether current management practices and estimated sustainable yields are appropriate. Rehabilitation of the USGS chloride profiling program (at EX-1, EX-4, EX-6, EX-7, EX-9, EX-10, and GHURA-Dededo) (Figure 3.1 and Appendix I) is an important move towards adequate data collection.

It is important to note, however, that even after all seven of the wells are brought back on line, chloride profiles will still be obtained only in the Agana and Yigo-Tumon Sub-basins (and, at EX-10, along the boundary between the Yigo-Tumon and Finegayan Sub-Basins). The Finegayan Sub-Basin should be given high priority for the installation of additional observation wells for chloride profiles. The Mangilao well cluster should also be given high priority if the production wells in that sub-basin are to be kept in production. Before development proceeds in the Agafa Gumas and Andersen Sub-basins, chloride profiling should be initiated in at least one well located in the approximate center of the area from which production is anticipated. Rehabilitation of EX-8 (currently inactive) for chloride profiling and water level measurement would an important step for characterizing the freshwater lens in the Agafa Gumas sub-basin. This should be given priority following rehabilitation of the seven wells currently planned for the FY 1999 under the WERI/USGS data collection program.

It should be noted that even with the implementation of the WERI/USGS data collection program, current plans provide for only quarterly or monthly collection of chloride profile data. It would be useful to eventually equip some wells with electronic instruments capable of collecting continuous chloride profile data. GEPA is currently pursuing plans to obtain such equipment. Installation of continuous recorders at selected observation wells will enable hydrologists to observe responses to storms, droughts, and nearby human activities (such as storm water diversion). Periodic profiling of the wells under the WERI/USGS data collection program will provide data for keeping electronic instruments calibrated. These two programs, once implemented, could be operated concurrently and would therefore provide complementary data.

Wellhead chloride sampling of production wells is useful and should be continued. It is important to note however, that wellhead sampling gives no information on the thickness of the lens, and the chloride concentration at the well head is a function of the relative location of the well screens within the lens, as well as the lens thickness. Comparisons of differences in chloride data from well to well must therefore take into account differences in well screen depths and local lens geometry. Temporal trends at a given well, however, can provide useful information for identifying changes in fresh water lens conditions. Chloride data would be most accessible and useable for systematic study if it were entered into the STORET database, along with other water quality data.

In addition to the ongoing collection of chloride data, a useful and inexpensive addition to the data collection program would be to evaluate the pre-development chloride profile to the depth of the well in each new production well. Although in most cases this would not penetrate the entire thickness of the lens, information of the vertical gradient of the chloride concentration might be useful in characterizing the lens and diagnosing any future problems.

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Other Water Quality Parameters

Observation Wells

In addition to chloride, the USGS also constructed profiles based on specific conductance from 1980 to FY 1996. These data can be found in the reports described in the ???Data Archive??? section of this chapter.

Production Wells

GEPA requires sampling for certain water quality parameters before a well is issued an operating permit. These tests must be repeated every five years, when the well operating permit is renewed. Testing parameters depend on the intended use of the well. Wells used as potable water sources must be tested for all Primary Safe Drinking Water parameters while wells intended for non-potable use have less extensive sampling requirements. In addition, certain well owners are required to periodically test for pesticides and fertilizers (in both ground and surface water). GEPA???s Water Resources Management Program can be contacted for more detailed information (see Table 3.3).

Underground Injection Control (UIC) Wells

GEPA requires UIC well owners to periodically sample injection wells, in which storm water runoff is allowed to drain through a borehole. The WRMP Director can be contacted for more information (see Table 3.1).

Military Groundwater Monitoring

Since 1983 the Andersen Air Force Base Installation Restoration Program (IRP) has investigated possible groundwater contamination as a result of Air Force activities. IRP documents are available at the Nieves M. Flores Memorial Library in Agana and UOG???s RFK Memorial Library. For more information the Remedial Project Manager can be contacted at the addresses and numbers listed in Table 3.1.

The Navy also conducts groundwater contamination investigations as part of the Base Realignment and Closure (BRAC) program and other projects. The Navy Public Works Center (PWC) Environmental Services Supervisor can be contacted for more information at the addresses and numbers listed in Table 3.1.

References

Barrett, Harris & Associates, Inc., 1982, Northern Guam Lens Study.

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