Water Resources Research Act Program

Details for Project ID 2014GU274B

Measurement of Groundwater Salinity for Selected Production Wells, Island of Saipan, CNMI

Institute: Guam
Year Established: 2014 Start Date: 2014-03-01 End Date: 2015-02-28
Total Federal Funds: $5,826 Total Non-Federal Funds: Not available

Principal Investigators: Elena Alexandrova, Heidi Yelin

Abstract: Groundwater resources of Saipan, Northern Marianas Islands, provide a majority of the public drinking supply. Fresh water pumped from the basal lens is replenished continuously by rainfall. Precipitation percolates through the unsaturated limestone host unit to reach the upper levels of the fresh water aquifer. The aquifer supplies potable water at several well fields. Some of these fields have historically shown elevated salinity (measured as chloride ion) as a result of high pumping rates that exceed the recharge rates of the reservoir, very deep pump placement, and proximity to the transition zone between the fresh and saltwater layers. This water quality problem has been addressed by past researchers (Van der Brug, 1985, Carruth, 2003); however, current information has not been compiled in recent years. While the USEPA recommended concentration for chlorides is 250 mg/L (EPA, 1986) measurements from some wells have exceeded 2,000 mg/L. Monitoring for chlorides is required and performed by CUC on a quarterly basis, for all wells. Recent changes in island population (a significant decrease) and an increase in utilities costs have also resulted in lower water consumption over the last five years. The project will be conducted in two parts. The first task is compilation of historical salinity data from selected wells. The data source is the CUC Water Quality Laboratory records. A period of 2009 through 2013 is considered for this data compilation activity. The second task consists of frequent, scheduled collection of groundwater data (salinity measurements by conductivity and chloride argentometric quantification) of selected wells throughout 2014. This work will produce a current record of salinity variation in correlation with pumping rates and precipitation. Monthly pumping rate records for the selected wells will also be collected from CUC Water operations. This study has not considered the GWUDI study as it has different objectives. GWUDI targets 10 wells in an attempt to confirm or deny their relationship to surface water recharge. GWUDI measures parameters of turbidity, and microbial presence as a result of precipitation. Our investigation is focused on variation of salinity levels in groundwater, this does not coincide with the GWUDI study goals. However, aspects of both studies can complement and revalidate each other. The combination of data collected and logged by the CUC Water Quality Laboratory together with new field measurements will add to the current understanding of the island's groundwater quality. This project addresses needs listed in the WERI/CNMI Advisory Council document reviewed at the Saipan meeting of October 17, 2013.