Hydrogeologic characterization of coralline limestone aquifer at industrial facility in the Hawaiian Islands

Presented at Pacific Basin Conference on Hazardous Waste, Honolulu, Hawaii, November 8-12, 1993.
Joseph E. Odencrantz 

ABSTRACT: A hydrogeologic characterization was performed to determine horizontal and vertical flow directions, hydraulic conductivities, porosity and tidal influences. The monitoring well network is approximately two thousand feet inland from the shoreline in the uppermost unconfined aquifer. Four sets of ground-water well clusters consisting of three wells per cluster were installed as the main monitoring network. The well clusters were placed in a rectangular arrangement approximately four hundred feet apart. The well screens within each cluster were placed to divide the uppermost aquifer into three vertical intervals to evaluate vertical flow magnitude and direction. Hourly water level data was collected from the twelve wells for a two month period. In addition to the main monitoring network, three piezometers were installed near one of the well clusters to measure draw-down during three pumping tests. The pumping tests were performed in order to assess the vertical variability of hydraulic conductivity and storage coefficient within the aquifer. The interconnection of the different zones within the aquifer were apparent during the pumping tests. The aquifer is composed of varying degrees of fractured coral and limestone, and hydraulic conductivity increases with depth.

The hourly water level data was used to calculate vertical and horizontal gradients using average water levels over two different three-day averaging periods. Both the arithmetic averages and filtered, moving averages were calculated. Average horizontal gradients in the upper, middle and lower zones were 0.00018, 0.00015and 0.00087, respectively. The average vertical gradient between the upper-middle zone the middle-lower zone were 0.0042 and 0.0180,respectively. All average vertical gradients were in the downward direction. Time lag and attenuation between the monitoring well water levels and the hourly ocean level data was determined. Time lags and attenuation factors between the ocean stage and the aquifer were determined. The variability about the mean water levels in the monitoring wells and the ocean was assessed statistically over different time periods.