Tidal effects on aquifer thermal regime: An analytical solution for coastal ecosystem management

An analytical model was developed to estimate the groundwater temperature change in the transition zone (intermediate zone between seawater and fresh groundwater) due to seawater and fresh groundwater temperature change in coastal aquifers. A set of type curves was developed in such a way that the curves account for the advection effect of groundwater flow and can be applied under different aquifer and tidal conditions to estimate the resulting temperature distribution. The proposed method will be important in evaluating the long-term effects of urbanization and climate change on coastal ecosystems where limited observation wells are available. The practical applicability of the composed methodology was tested in the Sendai plain. Continuous 1-h water level (from April, 2005 to July, 2007) and temperature (from May, 2007 to February, 2008) observations were made at three aquifer depths in each observation well at four locations to examine the temporal and spatial variations. Time series analysis was performed to find the correlations of the tidal and groundwater level fluctuations. Results of the preliminary analysis and the time series analysis indicated that the groundwater level within 20 m depth from the ground surface is more sensitive to the recharge from precipitation, while the depths below 20 m are greatly influenced by the tidal fluctuations. Reasonably high cross correlation (0.74) was found in tides with water level fluctuations, and it was also noted that the tidal effect on groundwater level fluctuation and temperature distribution significantly decays as the distance from the coast increases. The simulated temperature distribution from the proposed analytical solution shows good agreement with the observed temperature records. Among the hydrogeologic parameters, hydraulic conductivity has a robust influence in determining the pattern of temperature distribution within the sea water and fresh groundwater boundaries. Verified results in the Sendai plain indicated that the individual effect of seawater temperature change has a more profound effect on temperature change near to the coast than a fresh groundwater temperature change. Combined effects of temperature change at two boundaries within the range of ±1 °C will lead to a 0.4-1 °C temperature change at a distance 500 m away from the coast where in general, the coastal wetlands are located. These figures may be significant for maintaining or achieving the ecological balance of coastal ecosystems, and the findings of this research will assist planners and decision-makers in coastal environment management programs.