A new method is presented to estimate average diffuse aquifer recharge of water table aquifers in temperate climates using time series analysis of water table level fluctuations. An accurate estimate of the recharge caused by rainfall requires an accurate estimate of the influence of evaporation. In temperate climates, evaporation imprints a seasonal component in the water table fluctuations. As such, recharge is estimated from time series models fitted to observed heads under the additional constraint that the seasonal harmonic of the observed head is reproduced as the sum of the transformed seasonal harmonics present in precipitation, evaporation, and pumping. An explicit equation is presented, in terms of the model parameters, for the damping and phase shift of the response to the seasonal harmonic of the stresses. Taking into account the seasonal harmonic of the observed heads results in more reliable recharge estimates compared to standard time series analysis. The method is limited to systems that are sufficiently linear and that remain unaltered over the analysis period. Head fluctuations and stresses should contain a seasonal harmonic that can be estimated with accurately. Runoff must be negligible or quantifiable. The method is applied to measured heads obtained from piezometers situated on and around the ice‐pushed sand ridge of Salland in the Netherlands and compares well with recharge estimates based on the saturated zone chloride mass balance.
Time series analysis is applied to identify and analyze a transition in the groundwater regime in the aquifer below the sand ridge of Salland in the Netherlands, where groundwater regime refers to the range of head variations throughout the seasons. Standard time series analysis revealed a discrepancy between modeled and observed heads in several piezometers indicating a possible change in the groundwater regime. A new time series modeling approach is developed to simulate the transition from the initial regime to the altered regime. The transition is modeled as a weighted sum of two responses, one representing the initial state of the system, the other representing the altered state. The inferred timing and magnitude of the change provided strong evidence that the transition was the result of significant dredging works that increased the river bed conductance of the main river draining the aquifer. The plausibility of this explanation is corroborated by an analytical model. This case study and the developed approach to identify a change in the groundwater regime are meant to stimulate a more systematic application of time series analysis to detect and understand changes in groundwater systems which may easily go unnoticed in groundwater flow modeling.
We examined the spatiotemporal changes of microbial communities in relation to hydrochemistry variation over time and space in an aquifer polluted by landfill leachate (Banisveld, The Netherlands). Sampling in 1998, 1999, and 2004 at the same time of the year revealed that the center of the pollution plume was hydrochemically rather stable, but its upper fringe moved to the surface over time, especially at distances greater than 40 m away from the landfill. Complex and spatiotemporal heterogeneous bacterial and eukaryotic communities were resolved using denaturing gradient gel electrophoresis (DGGE) of 16S and 18S rRNA gene fragments. Large fluctuations were noted in the eukaryotic communities associated with strongly polluted and cleaner groundwater. The bacterial communities in strongly polluted samples were different from those in cleaner groundwater in 1998 and 1999, but no longer in 2004. The temporal variation in microbial communities was greater than the spatial variation: the 1998 bacteria communities in strongly polluted groundwater were more related to each other than to those recovered in 1999 and 2004. During the three sampling periods, the bacterial communities were more stable close to the landfill than at larger distances from the landfill. Overall, pollution appears to have only a minor influence on microbial communities. The considerable spatiotemporal variation in microbial community composition may contribute to better biodegradation of pollutants. Designing management strategies for natural attenuation of aquifer pollution will benefit from further long‐term, high‐density monitoring of changes in microbial communities, their diversity and physiological properties, in relation to changes in hydrochemistry.
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