The overall objective of the proposed 1-year study is to provide information on how representative measured vertical salinity profiles from deep monitor wells are of conditions in the adjacent aquifer. A numerical modeling approach, incorporating hydraulic characteristics and recharge representative of the Pearl Harbor aquifer, will be used to evaluate the effects of borehole flow on measured salinity profiles from deep monitor wells. Borehole flow caused by vertical hydraulic gradients associated with the natural regional flow system and local ground-water withdrawals will be modeled. Model results will be used to estimate differences between vertical salinity profiles in open boreholes and the adjacent aquifer in areas of downward, horizontal, and upward vertical flow within the regional flow system, with and without nearby pumped wells. Results from this study will provide insight into the magnitude of the discrepancy between vertical salinity profiles from deep monitor wells and the adjacent aquifer, which is needed for management and predictive modeling purposes.
To meet the objective of this study, the USGS will develop a three-dimensional numerical model capable of simulating density-dependent ground-water flow and solute transport. For this study, previously published values for hydraulic characteristics and recharge representative of the Pearl Harbor aquifer will be tested in the model, although the model mainly will be conceptual in nature and incorporate a simplified geometry. A steady state condition that generally represents the distribution of measured water levels in the aquifer will be simulated and used as an initial condition for all other simulations.
Deep open boreholes will be introduced at selected sites within the natural regional flow system in areas of downward, horizontal, and upward flow. Flow within the borehole will be simulated with a suitable model for an open conduit. Simulated salinity profiles within the borehole will be compared to (1) the pre-existing distribution of salinity in the aquifer without the borehole, and (2) the distribution of salinity in the aquifer with the borehole present. The effects of the borehole on saltwater intrusion into the aquifer also will be quantified.
Pumped wells will be introduced at selected distances from the open boreholes to evaluate the immediate effects of ground-water withdrawals on salinity profiles and saltwater intrusion into the aquifer. The depths of simulated pumped wells will correspond to the depths of typical production wells in the Pearl Harbor aquifer. The effects of both vertical wells and horizontal shafts will be simulated. Pumped wells will be located about 100 and 5,000 ft from the open boreholes, and ground-water-withdrawal rates of about 1 and 10 million gallons per day will be simulated from each pumped well.
A sensitivity analysis, in which values of hydraulic characteristics are varied one at a time, will be conducted to evaluate how the magnitudes of hydraulic conductivity, dispersivity, and storage values may affect borehole flow. In addition, at least two simulations incorporating low- and high-permeability layers within the aquifer will be simulated.