Characterizing a Shallow Groundwater System beneath Irrigated Sugarcane with Electrical Resistivity and Radon (222Rn) Puunene, Hawaii

In this study, we use a combination of electrical resistivity profiling and radon (Rn-222) measurements to characterize a shallow groundwater system beneath the last remaining, large-scale sugarcane plantation on Maui, Hawaii. Hawaiian Commercial & Sugar Company has continuously operated a sugarcane plantation on the western flank of Haleakala Volcano since 1878. The sugarcane is irrigated with a combination of surface water brought through tunnels from the wetter, eastern flank of Haleakala Volcano and groundwater from wells within the plantation. To assess the flow of irrigation water through the shallow subsurface, we collected a representative topo-sequence of four 2-D resistivity profiles that sample different topographic and hydrologic settings within the plantation. The profiles show a down-slope-thickening (0 to 20 m), surficial low-resistivity (10-200 Ohm-m) layer extending from the upslope limit of irrigated sugarcane to the lowest elevations of the plantation. At a canal crossing, the low resistivity layer thickens and is less resistive upslope of the canal. Beneath a reservoir at mid-elevation, the layer thickens to 20 m and curves down slope beneath the reservoir and up to the base of the field beyond. At the base of the slope, the low resistivity layer is 20-m thick below both fields and a second reservoir. An increase in radon concentration in the down-flow direction within the canal system at one location suggests groundwater infiltration into the canal. We attribute the low-resistivity layer to irrigation water that has infiltrated below the root zone and leaked from canals and reservoirs within the plantation. The water flows down slope to the base of the slope and there flows vertically, recharging the basal aquifer. We suggest that seepage from the canals and reservoirs is in part controlled by the local pressure head within the shallow flow system.