Abstract

Abstract. The groundwater flow models currently used in the western part of The Netherlands and in other similar peaty areas are thought to be a too simplified representation of the hydrological reality. One of the reasons is that, due to the schematization of the subsoil, its heterogeneity cannot be represented adequately. Moreover, the applicability of Darcy's law in these types of soils has been questioned, but this law forms the basis of most groundwater flow models. With the purpose of assessing the typical heterogeneity of the subsoil and to verify the applicability of Darcy's law, geo-hydrological fieldwork was completed at an experimental field within a research area in the western part of The Netherlands. The assessments were carried out for the so-called Complex Confining Layer (CCL), which is the Holocene peaty to clayey layer overlying Pleistocene sandy deposits. Borehole drilling through the CCL with a hand auger was completed and revealed the typical heterogeneous character of this layer, showing a dominance of muddy, humified peat which is alternated with fresher peat and clay. Slug tests were carried out to study the applicability of Darcy's law, given that previous studies suggested its non-validity for humified peat soils due to a variable horizontal hydraulic conductivity Kh with head differences. For higher humification degrees, the experiments indeed suggested a variable Kh, but this appeared to be the result of the inappropriate use of steady-state formulae for transient experiments in peaty environments. The muddy peat sampled has a rather plastic nature, and the high compressibility of this material leads to transient behavior. However, using transient formulae, the slug tests conducted for different initial groundwater heads showed that there was hardly any evidence of a variation of the hydraulic conductivity with the applied head differences. Therefore, Darcy's law can be used for typical peat soils present in The Netherlands. The heterogeneity of the subsoil and the apparent applicability of Darcy's law were taken into account for the detailed heterogeneous model that was prepared for the research area. A MODFLOW model consisting of 13 layers in which 4 layers represent the heterogeneous CCL was set up for an average year, assuming steady-state conditions; and for the winter of 2009 to 2010, adopting transient conditions. The transient model was extended to simulate for longer periods with the objective of visualizing the flow paths through the CCL. The results from these models were compared with a 10 layer model, whereby the CCL is represented by a single layer assuming homogeneity. From the comparison of the two model types, the conclusion could be drawn that a single layer schematization of the CCL produces flowpath patterns which are not the same but still quite similar to a 4 layer representation of the CCL. However, the single layer schematization results in a considerable underestimation of the flow velocity, and subsequently a longer travel time, through the CCL. Therefore, a single layer model of the CCL seems quite appropriate to represent the general flow behavior of the shallow groundwater system, but would be inappropriate for transport modeling through the CCL.

Highlights

  • In the western part of The Netherlands there is a great need to understand the flow of groundwater in lowland peat areas and its interactions with surface water

  • The groundwater flow models currently used in peaty lowlands are likely a too simplified representation of the hydrological reality because of the following two reasons: 1. The Complex Confining Layer (CCL) is often represented as a one model layer whereby different parameter values in this layer are lumped together

  • The CCL may be considered as a single peaty model layer representing a phreatic aquifer resting on top of a Pleistocene sandy aquifer with horizontal flow

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Summary

Introduction

In the western part of The Netherlands there is a great need to understand the flow of groundwater in lowland peat areas and its interactions with surface water In these areas an aquitard consisting of peat and clay overlays a sandy Pleistocene aquifer. The aquitard or semi-permeable layer is often referred to as a Complex Confining Layer (CCL) due to its lithological and hydraulic heterogeneity (Dufour, 2000; Bierkens, 1994; Weerts, 1996) This layer, which covers a large area of The Netherlands, plays an important role in the interactions of surface water and groundwater systems in the low lying parts of the country, such as protecting groundwater from pollution. The models were compared with each other to assess the effects of simplifying the CCL and to test the applicability of Darcy’s law in peat

Research area
Field and laboratory work
Results of the field investigations
Validity of Darcy’s law
H5 H4 H4 H6 H4 H6 H7
Modeling results
Layer schematization for conceptual model
Boundary conditions for conceptual model
Model code and data input
Hydro-geological parameters
Time and model boundary heads
Groundwater recharge and the surface water system
Sensitivity analysis and model calibration
Comparison of detailed and simplified models
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