Hydrogeochemistry of two contrasting atoll island aquifers, Roi-Namur, Republic of the Marshall Islands

Abstract

Groundwater resources on low-lying atoll islands are extremely vulnerable to climate change and sea-level rise because they are typically 3 m above mean sea level and are composed of relatively permeable rock that drains vital groundwater resources. Because thick, topical vegetation covers large amounts of the landscape, evapotranspiration is high and recharge to aquifers is limited. Since atoll islands are isolated from any mainland, rainwater catchment is their primary source of water supply, but they also rely heavily on groundwater resources, especially during times of drought. Land-use/landcover (LULC) change and managed aquifer recharge (MAR) have shown promise in increasing groundwater supply on Roi-Namur Island, Republic of the Marshall Islands. Roi-Namur has two lobes, each with contrasting island settings and aquifers. Roi is estimated to have 8.6 x 105 m3 of potable groundwater in the freshwater lens (FWL), compared to only 1.6 x 104 m3 on Namur. This is mostly attributed to the removal of vegetation and MAR on Roi, which was implemented as result of a U.S. military installation on the island. A suite of monitoring wells were sampled on Roi and Namur and a Wilcoxon-rank sum test indicates statistically significant water quality differences between the contrasting aquifers. Results indicate dissolution of carbonate rock in the freshwater lens and overlying vadose zone of Roi has been reduced seven-fold as a result of LULC and MAR. Geochemical modeling with PHREEQC is used to investigate differences in geochemical processes. The more dominant geochemical process on Roi is mixing with seawater due to flushing of the aquifer. In contrast, equilibrium processes and dissolution-precipitation non-equilibrium reactions are more dominant on Namur, at least in part due to longer residence times relative to the rate of geochemical reactions. Results suggest LULC and MAR has successfully increased groundwater supply on Roi- Namur; these methods should be considered as a viable adaptation measure to climate change stressors on inhabited atoll islands.