Differentiating Environmental Fluctuations from Pumping Signals in Tidally-Influenced Data

Periodic water-level and chloride measurements in numerous observation wells were analyzed to characterize the effects of large-scale groundwater development adjacent to an environmentally sensitive site. The observation wells are located on the leeward coast of the island of Hawaii where groundwater levels fluctuate over 2 feet in response to the ocean tide, potentially obscuring small pumping signals in the highly transmissive coastal aquifer. The injection of reverse osmosis concentrate within the wellfield further complicates the data analysis. Simple linear regression of the raw data indicates that the water level fluctuations are correlated to the ocean tide, while chloride concentrations are not correlated to either water levels or tidal forcing. Periodic sampling of a continuous synthetic water-level record created from a nearby tide gauge confirmed the importance of sampling frequency in correctly detecting trends in tidally-influenced data. Continuous water-level and conductivity records were available for background wells and an aquifer test and models developed from these continuous data were used to remove environmental fluctuations. Similar methods were tested to determine whether the periodic measurements could also be corrected for environmental fluctuations over the longer-term pumping period. The results were applied to determine an adequate sampling frequency for the tidally-influenced wells and high-transmissivity aquifer.