MODELING AQUIFER RESPONSE TO WATER HARVESTING IN NUUANU AND KALIHI, OAHU, HAWAII

Groundwater is the primary source of fresh drinking water for residents of Hawaii. However, the island of Oahu may experience long-term recharge reduction due to the island’s growing population and the effects of potential climate change. A decrease in recharge can lead to a reduction in the volume of potable water that can be pumped from various wells throughout an aquifer. Aquifer Storage and Recovery is a method of surface water harvesting and conservation implemented to augment an aquifer’s groundwater supply. Injecting the surface water into an aquifer minimizes the loss of water through evaporation and the potential for contamination. Additionally, the approach is consistent with expected climate change scenarios by capturing from wet areas that are expected to become wetter and utilizing the captured water in dry areas that are expected to become dryer. In this study, surface water will be captured at the Nuuanu Reservoir 4 and injected into the unsaturated zone of the Kalihi aquifer downstream. It is crucial to simulate this process to ensure that the aquifers respond well to the subsurface injection. The two primary objectives of this research are (1) identifying the lithology of the Nuuanu and Kalihi aquifers and (2) modeling the aquifers to determine the appropriate locations and depth in which water should be injected. Numerical models require inputs, such as hydraulic conductivity, porosity, recharge, aquifer thickness, and pumping rates. The study employs borehole logs and electrical resistivity surveys calibrated against each other to deduce the general strata composition, which can identify the appropriate injection point and infer the hydraulic conductivities and porosities throughout the aquifers. Various parameters will be estimated based on observed water levels. Injection locations and depths can then be tested to establish the appropriate site to drill an injection well. The model is finally used to predict the aquifers’ response to potential decrease in recharge, future water injection, and change in water head levels. This will help with Oahu’s freshwater management strategies and potentially augment future drinking water capacities.