Most of the public water supply in the Lahaina District, west Maui, Hawai‘i, is pumped from a freshwater lens in volcanic rocks. Because of population growth, groundwater withdrawals from wells in this area are expected to increase from about 5.8 million gallons per day in 2007 to more than 11 million gallons per day by 2030. Currently (2011), the salinity of water pumped from some of the wells in the area exceeds acceptable limits for drinking water. The expected increasing demand for water in an area in which the salinity of water is already unacceptable has led to concern over the long-term sustainability of withdrawals from existing and proposed wells.
A three-dimensional numerical groundwater flow and transport model was developed to simulate the effects of hypothetical withdrawal and recharge scenarios on water levels and on the transition zone between freshwater and saltwater. The model was constructed using time-varying recharge, withdrawals, and ocean levels. Hydraulic characteristics used to construct the model were initially based on published estimates but ultimately were varied to obtain better agreement between simulated and measured water levels and salinity profiles in the modeled area during 1926–2008. Scenarios included groundwater withdrawal at 2008–09 rates and locations with projected recharge (based on 2000–04 land use, no agricultural irrigation, and the rainfall record for the period 1926–2004) and withdrawal at redistributed rates and locations with several different recharge scenarios.
Simulation results indicate that continuing the 2008–09 withdrawal rates and distribution (6.3 million gallons per day from 21 wells) into the future would result in decreased water levels, a thinner freshwater lens, and increased salinity of water pumped from wells. Groundwater demand projections and proposed new well sites were used to produce a projected withdrawal rate and distribution during 2010–39. Simulation results from this projected withdrawal scenario (11.2 million gallons per day from 28 wells, including 10 proposed wells) also indicate decreased water levels, a thinner freshwater lens, increased water salinity, and unacceptable salinity at several current withdrawal sites, mainly in the Honokōwai Aquifer System; however, more groundwater is available than in the previous scenario. A simulation in which injection of treated wastewater is stopped indicates that several wells will have increased salinities compared to the scenario in which injection continues.
A scenario in which increased groundwater withdrawal was redistributed in an attempt to maximize withdrawal while maintaining acceptable salinities in the withdrawn water was simulated. The redistributed withdrawal simulates 20.7 million gallons per day of withdrawal from 26 wells or well fields in the Lahaina District. Simulation results indicate the following: (1) average water levels decrease by about 0.5–1 feet and the transition zone rises 20–50 feet in some areas after 30 years, mainly in the Launiupoko Aquifer System near the proposed wells, and (2), all wells produce water with salinities in the acceptable class (less than one-percent seawater salinity) after 30 years.