University of Hawaii team reports new geophysical and tracer results suggesting deeper freshwater flow paths near Red Hill

Don Thomas of the University of Hawai‘i School of Ocean and Earth Science and Technology (SOEST) briefed FTAC on Oct. 1 about multi‑method studies intended to improve understanding of groundwater flow around Red Hill and Pearl Harbor.

Thomas summarized geophysical work — gravity, resistivity and controlled‑source electromagnetic surveys — that the research team used to map subsurface structures and to distinguish salt‑saturated rock from zones of fresher groundwater. ‘‘In this case ... we have a significant freshwater saturated zone at a depth of, say, 75 meters down to about 225 meters below sea level,’’ Thomas said, adding that some survey lines indicate freshwater or “freshened” water extending below the cap rock and on to the submarine flank of Oahu.

Why it matters: Thomas said those deeper, freshened zones can provide additional pathways for groundwater flow and discharge and that the existence of deeper pathways changes conceptual models that previously assumed most recharge discharges only via shallow coastal springs.

Tracer tests and in‑well flow measurements: Thomas described a February dye injection and subsequent sampling program that detected tracer in several monitoring wells and in parts of the Red Hill shaft over months of monitoring. He also described in‑well colloidal borescope measurements that provide point measurements of lateral flow direction and velocity in specific well intervals; those measurements show variable flow directions in closely spaced wells in some locations and coherent flow in others. Thomas said the dye detections arrived slower than some modelers expected and that monitoring is continuing to better characterize travel times.

Next steps and modeling: Thomas said his group plans to continue integrating geophysics, geochemistry and tracer data into a two‑stage modeling approach: a regional model to set boundary conditions and a higher‑resolution local model for the Red Hill area. He said funding constraints limit the pace of modeling work and that some continued field work — notably ongoing dye‑tracer sampling — is expected to continue beyond the sponsored project period to inform the model.

Ending: Thomas said the team will continue to deliver data to inform Navy and regulator models and encouraged use of the publicly posted geophysical and tracer data as they are finalized.