Modeling of Regional Groundwater Flow in the Northeast Quadrant of Mauna Loa Volcano: Influence of Sea Level, Orographic Rainfall and Volcano Geometry

Abstract:

Exceptionally permeable lava flows in the northeast quadrant of Mauna Loa Volcano (Hawaiʻi) form an unconfined aquifer that is bounded laterally by a low-permeability rift zone in the south, the Mauna Kea Volcano in the north, and the ocean to the east and northeast. Estimated mean annual recharge is 2.7 km³, which discharges to 24 km of shoreline. Because groundwater data are limited, flow patterns were modeled using a variable-density groundwater model (SEAWAT version of MODFLOW) with a 1 km grid spacing. Mauna Loa lavas overlie older Mauna Kea lavas and drilling data suggests that volcanic ash and soil on the surface of Mauna Kea can form a low permeability layer between the Mauna Loa and Mauna Kea lavas. The three-dimensional geometry of the Mauna Loa aquifer is strongly controlled by variations in the elevation of the Mauna Kea surface, which within the aquifer ranges from 1,800 m above sea level to >3,000 m below sea level. Orographic forcing of precipitation contributes to a zone of intense recharge (5,000 mm yr^-1) at mid-elevations while recharge is moderate near the coastline and low near the summit (due to an atmospheric inversion zone). Complex flow patterns are influenced by spatial variations in recharge amount, strong variations in aquifer thickness, and the geometry of the coastline. According to the model, groundwater flow is downslope along the Mauna Kea surface in much of the upland area; some of this water pools against the low permeability rift zone. At lower elevations, sea level is main influence on groundwater levels. Along one 5 km stretch of coastline, large amounts of groundwater are vigorously flowing seaward through a thin (~150-300 m) aquifer, preventing full development of a basal lens. Here, salinity near the base of the aquifer (and half a km inland) is about 22% that of seawater. Along a different stretch of coastline, the aquifer saturated thickness exceeds 1,000 m. Here, 5 km inland the midpoint of the transition zone is 400-600 m below sea level. While basaltic sequences are known to be strongly anisotropic, sensitivity analysis suggests that the flow field and water table elevation are relatively insensitive to anisotropy of hydraulic conductivity. The extent of low-permeability zones and layers is a source of uncertainty for the aquifer conceptual model.