APPLICATION OF A DISCRETE-CONTINUUM NUMERICAL MODELING APPROACH FOR PROCESS AND SYSTEM UNDERSTANDING OF HAWAIIAN PSEUDO-KARST GROUNDWATER SYSTEMS

Heterogeneous Hawai`i flank lavas have rheogenic pseudokarst features where turbulent groundwater movement through pyroduct (lava tube) conduits can potentially govern preferential flows. Known Hawai`i pyroduct diameters range in size from 0.5m to 10m at their greatest extent and slopes follow the general angle of the ground surface (>2° to 7°). Furthermore, shallow pyroducts (e.g., Kaumana cave) exhibit flood flow behavior. Deeper conduits may often be fully saturated and transport water quickly through the lavas, depending on the conduit’s connectivity to the surrounding matrix, but the structures of deeper conduits are often not well known and are potentially influenced by lava fill-in, thermal downcutting, erosion, and compression or collapse at greater depths.

This study investigates a pseudokarstic system using the numerical code MODFLOW-CFPv2 to evaluate the significance of considering turbulent flow for Hawai`i groundwater systems. For this reason, Lyman and Ola`a flume springs, which seem to be hydraulically connected as an underflow-overflow system, were considered as the case study. The springs are located on the Northeast slopes of Mauna Loa at 603 m and 520 m, near the surface geological contact of Mauna Kea and Mauna Loa lava flows, likely discharging perched water gathered as recharge in the Mauna Loa lava flows. The springs (esp. Lyman), have peak-and-recession discharge behavior alike many karst spring systems. It is theorized in previous studies that there is a possible saturated conduit below portions of Kaumana cave, which exists on the same slope as Ola`a and Lyman springs.

This study comprises steady state and transient flow and solute transport computations covering time spans up to several months. Models are generated using synthetic and real recharge inputs and are performed to investigate alternative conceptual models of regional and local Hawai`i groundwater flow, adopting a multi-model approach. The study examines scales at which turbulent groundwater flow is significant and assesses the associated potential effects on groundwater contamination.