Groundwater Resource Vulnerability to Climate Change: Outer Atolls of the Republic of the Marshall Islands

Based on modeling results of the freshwater availability, the outer islands of the Republic of the Marshall Islands (RMI) hold enough groundwater to satisfy the basic human needs of island residents; however, groundwater salinity often exceeds the drinking water standard. We used an adapted algebraic model of atoll freshwater lens volume to predict the size and shape of the groundwater lens on the populated islands of eight outer atolls. For each atoll island, we constructed 5-10 parallel transects, spanning from the atoll lagoon to ocean, to estimate groundwater area. Variables that impact the volume of the groundwater available include island geometry, mean annual precipitation, hydraulic conductivity, depth to the Thurber discontinuity, and the presence of a reef flat plate. Individual transect groundwater volumes were then complied to determine the magnitude of an island’s groundwater resource. On inhabited islands, available groundwater exceeds freshwater demand by more than an order of magnitude. We assumed a standard demand of 50 liters per person per day based on basic water requirements for human activities. In addition, the Environmental Protection Authority (EPA) of the RMI recommends drinking water contain less than 500 mg/L of total dissolved solids (TDS). While many atoll islands meet this salinity standard, several locations in the populated areas of atoll islands exceed 1500 mg/L TDS. Most residents of the outer atolls reside on the lagoon side of the atoll islands; however, the lagoon shore of the islands also coincides with where the freshwater lens is thinnest and more susceptible to salt water intrusion. The deepest part of the freshwater lens (with most abundant water) is near the midpoint between the lagoon and the ocean, thus creating a spatial divide between the location of residents and the availability of freshwater. As climate change continues to accelerate, the RMI will likely experience a change in precipitation, as well as a continued increase in sea level, thus intensifying the need for understanding the magnitude and distribution of freshwater resources in the RMI and other Small Island Developing States (SIDS)