Identification and Explanation of a Change in the Groundwater Regime using Time Series Analysis.

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.

A Simple Analytical Formula for the Leakage Flux Through a Perforated Aquitard.

In this methods note, we present a simple analytical formula to quantify the steady-state leakage flux (Q) over a perforated aquitard. The flux depends on the aquitard thickness (D), the radius of the perforation (R), the hydraulic conductivity of the material inside the perforation (kfill ), the conductivities of the overlying and underlying aquifers (k1 and k2 , respectively), and the head difference between the two aquifers (ΔH): [Formula: see text]. This equation assumes an aquitard separating two homogeneous and infinite aquifers (R ≪ aquifer thickness) in which radial flow to and from the perforation occurs, with no other recharge or discharge boundaries near the perforation. The flux through a perforation in a hypothetical case study with D = 10 m, k1 = 10 m/d, k2 = 20 m/d, R = 0.072 m, and ΔH = 1 m ranges between less than 1 mL/d if the hole is backfilled with bentonite (k(fill ) = 10(-4) m/d), to several liters per day if the perforation is backfilled with sand from the overlying aquifer (k(fill) = 10 m/d), to several m(3) /d if the perforation forms an open conduit (k(fill) = 10(5) m/d). The leakage fluxes calculated with this model agree well with those calculated using a numerical model (MODFLOW).

Modeling time series of ground water head fluctuations subjected to multiple stresses.

The methods behind the predefined impulse response function in continuous time (PIRFICT) time series model are extended to cover more complex situations where multiple stresses influence ground water head fluctuations simultaneously. In comparison to autoregressive moving average (ARMA) time series models, the PIRFICT model is optimized for use on hydrologic problems. The objective of the paper is twofold. First, an approach is presented for handling multiple stresses in the model. Each stress has a specific parametric impulse response function. Appropriate impulse response functions for other stresses than precipitation are derived from analytical solutions of elementary hydrogeological problems. Furthermore, different stresses do not need to be connected in parallel in the model, as is the standard procedure in ARMA models. Second, general procedures are presented for modeling and interpretation of the results. The multiple-input PIRFICT model is applied to two real cases. In the first one, it is shown that this model can effectively decompose series of ground water head fluctuations into partial series, each representing the influence of an individual stress. The second application handles multiple observation wells. It is shown that elementary physical knowledge and the spatial coherence in the results of multiple wells in an area may be used to interpret and check the plausibility of the results. The methods presented can be used regardless of the hydrogeological setting. They are implemented in a computer package named Menyanthes (www.menyanthes.nl).