WRRCTR No.172 Carbonate Geochemistry and Hydrogeology Relationships: Laura, Majuro Atoll, Marshall Islands

Abstract

In carbonate aquifers diagenetic alteration of the aquifer host rock and the chemical interactions between groundwater and the carbonate sediments and rocks are important for groundwater quantity and quality relations. These processes result not only in the alteration of groundwater chemistry, but also influence the development of porosity and permeability, and thus groundwater occurrence and movement in carbonate aquifers. Likewise, the rates, directions, and mechanisms of fluid transport through aquifer pore-networks are basic to an understanding of many diagenetic processes. In this study a hydrogeochemical approach is taken to decipher the occurrence and movement of groundwater in the Laura area of Majuro Atoll, Marshall Islands. The following questions are addressed: (1) what is the geochemical composition and depositional environment of the subsurface materials; (2) what are the diagenetic processes operating in the hydrologic regime; (3) what are the controls on these processes; and (4) what is the effect of depositional and diagenetic history on permeability and porosity characteristics of the aquifer? The stratigraphic section in Laura consists of two principal units: an upper unit of moderately permeable carbonate sediments, underlaid by an extremely permeable carbonate rock unit. The sediment and rock units contain a calcium carbonate freshwater-seawater system, in which the freshwater nucleus occurs on the lagoon side of the island. Its movement is governed by an an-axisymnetric distribution of lithofacies and an increase in permeability with depth. An effect of the an-axisymnetric distribution of fresh groundwater is a groundwater divide that is preferentially closer to the lagoon shoreline than the ocean shoreline; thus, more of the island's recharge is directed toward the ocean rather than to the lagoon. A longer flow path from the groundwater divide toward the ocean, coupled with a shallower high permeability carbonate rock unit, results in a thicker freshwater-seawater mixing zone in the sediments bordering the ocean than those bordering the lagoon. Thus, not only is the mixing zone thicker, but it also receives a greater amount of rainfall recharge. This thick, well-flushed mixing zone is the site of continuously occurring diagenetic reactions that are having a significant effect on the development of porosity and permeability. The diagenetic history of the lower carbonate rock unit and the upper carbonate sediment unit are the primary controlling factors in the occurrences and movement of groundwater in the Laura area of Majuro Atoll.