Thermal profiling to quantify the spatial variability of ambient groundwater flow at a provisional groundwater abstraction site

Knowledge of the saturated hydraulic conductivities of aquitards in provisional groundwater abstraction sites is essential to assess the sustainability of future water production. The subsurface of the site in the southwest of The Netherlands has a simple layer cake stratigraphy. Temperature-depth profiles in 12 boreholes were measured and analysed to infer vertical fluxes across an aquitard at a depth (~100 m) below where the impact of recent surface warming could be detected. Hence, the analytical mathematical solution for coupled groundwater-heat flow described by Bredehoeft & Papadopulos in 1965 could be be employed for this purpose. The selection of the depth interval for the aquitard for which the solution is applied, is guided by scanning through the TDP to find depth-intervals for which both a low RMSE between observed temperature and solution is obtained as well as a high Peclet number indicative of significant vertical groundwater flow. Through comparison of the depth intervals with lithological data, temperature-depth profiling is shown to have the capacity to detect aquitards, provided that the approximate depth of the aquitard is known, as well as the flux direction and magnitude across the aquitard. In combination with observed hydraulic gradients, the spatial variability of hydraulic conductivity of the aquitard could be evaluated. These values range from 10 to 100 mm/d, where earlier estimated values using more traditional methods suggested a range of  5 to 10 mm/d.