Report Number: 13
Year: 1980

Well Water Quality on Moen Island, Truk

No abstract was published. A summary of the Introduction and Results follows.

A study of the quality of well water was conducted on Moen Island, located in the eastern part of Truk Lagoon in Truk Atoll in the eastern Caroline Islands. Moen's groundwater resources are found in deeply weathered zones in the volcanic rock, primarily in a saddle between Tanaachau and Mount Teroken. Fourteen production wells are currently in service, pumping from 40 to 314 L/min. Chloride concentrations in new wells were initially low (14 to 120 mg/L) and did not appear to be affected by subsequent pumping.

Six production wells (Wells # 1, 7, 9, 10, 13, and 15) were monitored on a monthly basis for one year (June 1979 through May 1980) for basic physical, chemical and bacteriological water quality parameters in accordance with Standard Methods for the Examination of Water and Wastewater (1975). These parameters included nitrate-nitrogen, nitrite-nitrogen, reactive phosphorous, chloride, alkalinity, pH, turbidity, conductance, temperature, total residue, total non-filterable residue, total soluble residue, and carbon dioxide. Additional parameters which were measured on a less frequent basis included total Kjeldahl nitrogen, total phosphorous, sulfate, and total and fecal coliform bacteria. Ammonia-nitrogen and reactive silicate were measured once.

Mean turbidity levels of all six wells exceeded the territorial water quality standards at least once during the sampling period. Values ranged from 0.52 to 31 NTU, with a maximum of 160 NTU. Mean total residue values ranged from 145 to 695 mg/L, with four wells (1, 10, 13 and 15) exhibiting similar measurements. Total residue values, as well as total filterable residue (TFR), were generally higher around July and November 1979 and January 1980, although Well # 9 peaked again in April 1980. Total non-filterable residue (TNFR) values were similar (mean values ranged from 4.7 to 29.0 mg/L) for all wells except Well # 9 (mean = 70.2 mg/L).

Specific conductance readings were taken at 1 to 5-minute intervals. Readings were steady, ranging from 195 to 1400 μmhos/cm, at all wells except for Well # 9, whose values continually fluctuated. Mean pH ranged from 6.21 to 6.54, with the lowest readings occurring in October 1979 and February and March 1980 and the highest readings occurring from July to September 1979 and November 1979 to January 1980. Again, Well # 9 deviated from the norm with the widest range of pH values (5.8 to 7.3).

Mean temperatures ranged from 27.5 to 28.0 ºC, and mean chloride levels ranged from 12.4 to 260 mg/L, with Wells # 1 and 9 being the lowest and highest, respectively, of both parameters. Chloride concentrations in Well # 9 increased dramatically starting in January 1980, exceeding the territorial water quality standard (400 mg/L) with a measurement of 604 mg/L in May 1980. Wells # 1 and 10 also increased in chloride concentration between February and May 1980.

Alkalinity occurred in the form of bicarbonate, HCO3-, with mean values ranging from 116 to 230 mg/L. A maximum value of 407 mg/L occurred in Well # 9 in December 1979, while the other five wells peaked in October 1979. Alkalinities of all wells decreased after January 1980. Free carbon dioxide was generally high, with mean values between 70 and 145 mg/L. Levels in all wells peaked in October 1979 (between 160 and 550 mg/L) and March 1980 (from 120 to 240 mg/L).

Nitrogen was primarily in the form of nitrate-nitrogen. Levels were generally below detectable limits. Mean nitrite-nitrogen levels ranged from 0.35 to 2.51. Ammonia-nitrogen levels, sampled in May 1980, were negligible. Mean total Kjeldahl nitrogen (TKN = organic nitrogen + ammonia nitrogen) ranged from 0.16 to 0.38 mg/L. Total nitrogen (TKN + nitrite/nitrate-nitrogen) concentrations averaged 1 mg/L. Mean total phosphorous levels ranged from 0.07 to 0.13 mg/L. Orthophosphate levels ranged from 0.05 to 0.12 mg/L. Organic phosphate (total minus orthophosphate) levels were negligible.

Sulfate and reactive silicate concentrations were low in all six wells, ranging from <1.0 (Well # 1) to 6.7 mg/L (Well # 15) and 8.6 (Well # 15) to 11.8 mg/L (Well # 10), respectively. Total (TCB) and fecal coliform bacteria (FCB) contamination was present in all six wells, violating territorial safe drinking water standards. Geometric means ranged from 3 (Well # 10) to 11 colonies/100-mL (Well # 9) for TCB and from 1 (Wells # 10 and 13) to 8 colonies/100-mL (Well # 15) for FCB.

Well # 1 typically had the lowest sulfate levels and the highest nitrite-nitrogen, total Kjeldahl nitrogen, total phosphorous, orthophosphate levels. Well # 9 was consistently anomalous compared to the other five wells in terms of TFR, TNFR, conductance, alkalinity, temperature and pH. Well # 10 had the highest reactive silicate levels and the lowest mean coliform bacteria, nitrite-nitrogen, total phosphorous and orthophosphate levels.

The frequent presence of total coliform bacteria and turbidities exceeding 1 NTU in Wells # 7, 9, 13 and 15 constitutes localized groundwater pollution. The occurrence of fecal coliform bacteria implies groundwater contamination from sewage. Not only do these contaminants pose a human health risk, but they can cause extensive corrosion and incrustation of well structures. Disinfection of well water using chlorine is recommended to eliminate bacteriological contaminants. Frequent occurrences of low pH values, elevated temperature readings, and high free carbon dioxide, chloride, total filterable residue levels can all be considered indicators of corrosive groundwater as well.

Additional recommendations include: continuing required monthly monitoring plus full-spectrum (physical, chemical and bacteriological) analyses on an annual basis; examining particulates microscopically to determine the presence of iron- and sulfate-reducing bacteria; analyzing background heavy metal (arsenic, barium, chromium, lead, mercury, silver and iron) concentrations.

Russell N. Clayshulte
William J. Zolan