Up and Down--How Can We Assess Hydraulic Properties from Tidal Fluctuations in Coastal Aquifers?

Abstract:

Detailed estimates of hydraulic properties are important for models that are used to assess groundwater availability and quality. The attenuation of tidal variations in groundwater levels with distance from the coast reflects an aquifer’s hydraulic properties. We present a regional compilation of tidal-attenuation information for Hawaii, and estimate hydraulic properties using groundwater-level records from more than 250 wells in volcanic rock and more than 200 wells in coastal sediments. Results are combined in a regional context with respect to the hydrogeologic framework of each island in Hawaii. The tidal response in groundwater levels can help to distinguish and identify hydrogeologic units. For example, no tidal signal was observed in dike-intruded volcanic rock, and the lower-permeability coastal sediments caused a damping effect of the tidal signal. Aquifer-diffusivity and hydraulic-conductivity estimates in dike-free volcanic rock decrease with island age, consistent with a reduction in permeability through weathering. The results will be useful for developing island-wide numerical groundwater models of Hawaii.