Distribution of injected wastewater in the saline-lava aquifer, Wailuku-Kahului wastewater treatment facility, Kahului, Maui, Hawaii

Abstract

Field studies and digital modeling of a lava rock aquifer system near Kahului, Maui, Hawaii, were employed to define and describe the distribution of planned injected wastewater from a secondary treatment facility. The lava rock aquifer contains water that is almost as saline as seawater. The saline water is below a seaward-discharging freshwater lens, and separated from it by a transition zone of varying salinity. Injection of wastewater at an average rate of 6.2 cubic feet per second is planned through wells open only to the aquifer deep within the saline water zone. The lava rock aquifer is overlain by a sequence of residual soil, clay, coral reef deposits, and marine sand that form a low-permeability caprock which semiconfines the lava rock aquifer. A two-dimensional finite-difference model was used to describe initial conditions within the freshwater part of the aquifer, and to indicate response of the freshwater part of the system to injection of wastewater. Steady-state distribution of the wastewater within the saline and the freshwater parts of the lava rock aquifer was simulated by use of a three-dimensional finite-difference model. This model, using vertical-to-horizontal ratios of hydraulic conductivity of 1:10 and 1:100, shows that wastewater with a lesser density than that of either the saline or freshwater aquifer, will form a roughly cylindrical column around the injection well base and rise through the transition zones into the freshwater lens flow system. The column diameter varies with the ratio, and the column, or plume, widens progressively through the transition zone, then maintains virtually constant size through the freshwater part of the lava rock aquifer. Seaward flow within the freshwater lens tends to distort the plume seaward. Under conditions measured and assumed for the study, it is evident that wastewater injected at the planned rate will move upward, then seaward without significant change. After reaching a new steady state, the wastewater will discharge into and through the caprock sequence within an area measuring approximately 1,000 feet inland, 1,000 feet laterally on either side of the injection site, and about 2,000 feet seaward. Little, if any, of the injected wastewater may be expected to reach the upper part of the caprock flow system landward of the treatment plant facility.