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Name:
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Søren Erbs Poulsen
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Project title:
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Geology and groundwater flow on a barrier coast.
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Enrolled at:
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Aarhus Universitet
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Start and end date:
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01-09-2006 - 01-09-2009
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Address:
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Nattergalevej 6, 8362 Hørning
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Phone:
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+45 25 46 22 32
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Email:
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soeren.erbs@geo.au.dk
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Website:
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Advisor(s):
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Steen Christensen, Keld Rømer Rasmussen
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Source of funding:
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AUFF/AU, Cheminova, Ringkjøbing Amt.
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International collaboration:
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University of Adelaide
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Description of research:
Topic 1 abstract: A monitoring system including five groups of piezometers and five vertical multielectrode profiling probes (VMEP) have been installed in an aquifer beneath a coastal dune. In order to asses the salinity distribution within the aquifer, geoelectrical data were gathered in March, June and September 2008, by measuring a dipole-dipole and gradient array using multi electrode profiling. Interpretation of the processed resistivity data was performed by regularized inversion using a 1D, horizontally layered model of formation resistivity. The standard deviation on estimated layer log-resistivity was 0.01--0.03. By estimating two parameters of a power function, observed fluid conductivities derived from samples of porewater were related to corresponding estimated formation resistivities. The conductivity profiles correlate with a winter situation in March with high sea level, active recharge and significant wave activity, causing increased hydraulic heads, a thicker freshwater lens and saltwater overlying freshwater close to the sea. In June the thickness of the freshwater lens is reduced due to less recharge and prevailing offshore winds, imposing density-stable conditions and a sharper transition between fresh and brackish water. During the autumn aquifer recharge is enhanced and hydraulic heads increase, resulting in a thicker freshwater lens.
Topic 2 abstract: In this study, contaminant transport in a micro-tidal, shallow barrier aquifer under the influence of large-amplitude episodic storm-surge events is explored using numerical modelling. Emphasis is put on the near-shore response from overtopping salt-water during storm-surges and the landward movement of contaminants as a function of sealevel, recharge and the hydraulic head and salinity in the lagoon inland of the barrier. For a proper conceptualization, the FEFLOW model used for this study is a vertical 2D variably-saturated density-dependent finite-element model in which the type and spatial extent of the boundary conditions can be reformulated dynamically. Hydrodynamics within the beach and lagoon boundaries are re-produced using dynamic boundary conditions that represent flooding, seepage and recharge. The boundary condition along the topography of the beach and lagoon are dynamically reformulated from analytical functions describing the temporal variation in sealevel and the hydraulic head in the lagoon. The modelling results indicate that episodic storm-surge impacts serve to control the flow and salinity behaviour of the barrier aquifer and result in accelerated contaminant plume migration in the inland direction, inducing roughly a X\% increase in the discharge of contaminants to the lagoon compared to the situation of a constant sea-level. Furthermore, a significant proportion of the inland migration of contaminants is due to aquifer discharge into lagoon surface-water where surface water provides a pathway for rapid spreading of solutes. The rate of transport of contaminants into the lagoon is enhanced by approximately X\% during storm-surges due to the elevated hydraulic head at the beach. Rapid contaminant transport to the sea occurs following storm-surges due to extensive local flushing of contaminants from the flooded near-beach sediments. In general terms, the impact of storm-induced processes on the salinity distribution and transport of contaminants in a controlling factor in the overall flow and solute behaviour of the barrier aquifer.
Topic 3 abstract: Cores of sediment taken near a stream from an unconfined, sandy aquifer have been used in laboratory experiments to measure drainage responses to water-table drawdown. The responses can be sufficiently modeled by estimating the specific yield and five exponential time constants of a Moench/Boulton (Moench et al. 2001; Boulton 1955) type model of delayed drainage. The specific yield is thus estimated to 0.24. Two pumping tests were conducted near the stream. The drawdown observations were analyzed to estimate the hydrogeological parameter values and, in some cases, the specific yield and the time constants characterizing the release of water at the falling water table. The drawdown analyzes were made using a 3D numerical groundwater flow model facilitating use of the Moench/Boulton type boundary condition at the water table. The drawdown observations could be fitted nearly equally well whether the release of water at the water table was modeled as instantaneous or as delayed, and most estimated parameter values turned out realistic and nearly identical for the various calibration cases. However, estimation of the specific yield was found to be sensitive to whether drainage is modeled as instantaneous or as delayed. In the first case the estimated specific yield is about half of the estimate from the core experiments; in the second case the estimate is in better agreement with the drainage experiments. The analysis indicates that the existence of two drawdown-dependent sources of groundwater recharge, the storage and the stream, complicates pumping test design to obtain unique parameter estimation. Finally it is demonstrated that simulating the release of water from above the water table as delayed instead of instantaneous in itself caused just a slight difference in the simulation of early time stream flow depletion. For the studied field site it is therefore acceptable to make a prediction of stream flow depletion due to abstraction using a model that is based on the assumption of instantaneous drainage - if the prediction is based on realistic parameter values.
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Publications:
Poulsen, S.E., Christensen, S., Rasmussen, K.R., Mortensen, J.S., Viezzoli, A. 2008, "An Investigation of Groundwater Flow on a Coastal Barrier using Multi Electrode Profiling", in SWIM - 20th Salt Water Intrusion Meeting, Program and Proceedings, s. 209-212. Conference paper peer reviewed.
Poulsen, S.E., Christensen, S., Rasmussen, K.R. 2008, "An investigation of the interaction between the unsaturated zone, aquifer, and stream during a period of groundwater abstraction", presented at ModelCARE 2007 , Int. Conf. on Calibration and Reliability in Groundwater Modelling – Credibility in Modelling, 6 edt., Copenhagen, 9.9.2007 - 13.9.2007. Poster.
Numerical modeling of flow to a well in a water table aquifer considering delayed yield. Master Thesis. Geologisk Institut, Aarhus Universitet. 2006.
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