National Institute for Water Resources, Water Resources Research Institute Program
8/01/2009 - 7/31/2010
A recent numerical-modeling study of coastal wells in Israel indicated an upward displacement of the borehole salinity in wells located in the coastal-discharge area of the aquifer while it is at a steady-state condition. Responding to the influence of ocean tides, the vertical flow in the borehole changes direction and the flow in the monitor well is three orders of magnitude larger than that in the aquifer. This indicated that the observed borehole salinity does not accurately represent the aquifer salinity (Shalev et al. 2009). Therefore, these monitor wells do not accurately monitor the actual freshwater-saltwater transition zone.
The overall objective of this study was 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 the hydraulic characteristics and recharge data representative of the Pearl Harbor aquifer, was 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 both the natural regional flow system and with local groundwater withdrawals were simulated. Model results were used to estimate differences between vertical salinity profiles in open boreholes and the adjacent aquifer in areas of downward, horizontal, and upward flows within the regional flow system—in areas both with and without nearby pumped wells. Results from this study provided insights into the magnitude of the discrepancy between current vertical salinity profiles from deep monitor wells and the actual salinities of adjacent aquifers. Such data is critically needed for management and predictive modeling purposes.