A new type curve method for estimating aquitard hydraulic parameters in a multi-layered aquifer system

- Aquifer drawdown history was approximated as piecewise linear segments.
- A new type curve method for estimating aquitard parameters was proposed.
- This method is suitable for long-term fluctuating aquifer drawdown condition.
- This method was tested and verified in a field multi-layered system case.

SummaryWe propose a new type curve method for estimating the hydraulic conductivity (K) and the specific storage (Ss) of an aquitard in a multi-layered aquifer system. Theoretical type curves of rate of aquitard compaction versus time are obtained through drawdown-time series in the aquitard, under a condition that the drawdown history of the underlying aquifer can be approximated as piecewise linear segments (PLS), in each of which the drawdown shows a linear increase trend with time. Comparing with previous analytical solutions, which are limited by the assumption of a constant head in the overlying unconfined aquifer, the superposition principle can be called when water levels fluctuate with time in both upper and lower aquifers adjacent to the aquitard. When applying the new PLS method to estimate K and Ss values of an aquitard, the starting point of drawdown history at adjacent aquifers should be determined at first. After that the drawdown history is approximated as PLS to obtain drawdown rate of change for each linear stage and turning points of drawdown history. The new PLS method is demonstrated at a field site in Changzhou City, Jiangsu Province of China. Estimated results show that K and Ss are, respectively, 9.80 × 10−10 m/s and 4.46 × 10−4 m−1 for a thick aquitard, and 2.34-3.50 × 10−10 m/s and 2.28 × 10−4 m−1 for a thin aquitard. Estimated field K values, as well as Ss values, through the new PLS method, are generally in accordance with the results obtained from the geologic method by Konikow and Neuzil (2007). They are also in agreements with the numerically calibrated results of Shi et al. (2008a) at the regional scale and the laboratory experimental values of in-situ soil samples collected by Geological Survey of Jiangsu Province of China.