Tidal influences on unconfined water levels in wells.
Tidal effects have the potential to measurably affect water level measurements and the estimation of groundwater flow direction and gradient. This has significant implications for the prediction and monitoring of contaminant plumes in coastal areas. Studies were conducted at two near-coastal sites in San Diego County, California, to evaluate the impact of tidal variability on the interpretation of water level data. For the two sites studied, Santa Clara Point and Spanish Landing Flats, variations in measured water levels ranged up to 2.2 feet in response to a 6-foot tidal variation in Mission Bay and up to 0.15 feet in response to a 5- to 8-foot tidal variation in San Diego Bay. Gradient direction estimated from time-specific groundwater contour maps varied up to 25 degrees over several tidal cycles. These studies and those conducted by previous workers emphasize the importance of understanding the type and magnitude of tidal response within a monitoring network when evaluating contaminant plumes in coastal areas.