MODELING ANNUAL VARIATIONS OF SPRING AND GROUNDWATER TEMPERATURES ASSOCIATED WITH SHALLOW AQUIFER SYSTEMS

Annual periodic temperature variations in spring water and groundwater can provide information on the boundaries and properties of shallow, thin aquifer systems. Illustrative numerical simulations were carried out for four different aquifer systems and then were compared with temperature measurements from natural systems. Annual temperature fluctuations in the subsurface are characterized by the annual amplitude AMP and phase difference DELTAphi with respect to the Earth's surface temperature (approximated by the air temperature). When periodic surface temperature variations with the amplitude A0 are transported by vertical conductive heat flux, then a direct interdependence obtains between AMP and DELTAphi independently of material properties or other parameters AMP = A0exp(- DELTAphi). Given uniform surficial recharge and no other sources of fluid, aquifers of constant depth with groundwater flow parallel to the Earth's surface may exhibit the same interdependence between amplitude and phase difference. A constant-temperature lateral groundwater influx influences annual variations in spring water and groundwater temperatures by lowering the amplitude (here the phase difference remains constant). Mixing of waters with different values of AMP and DELTAphi can be identified and, given supplemental information, the mixture ratio can be determined. Lateral influx of groundwater from a source with independent annual periodic temperature changes occurs in nature in, for example alluvial aquifers characterized by bank filtration. The convective heat transport from the river into the aquifer causes a phase difference DELTAphi between river water and groundwater temperature. There is no direct relationship between AMP and DELTAphi. The groundwater temperatures are characterized by higher AMP values than those predicted by the conductive model. The phase difference of the groundwater may approximate to that of the bank filtration component, and bank filtrate component and its net residence time in the aquifer can be determined if the heat capacity of the aquifer is known.