New observations by visualizing age stratification and internal dynamics of freshwater lenses in heterogeneous media - laboratory experiments and numerical simulations
Abstract
We performed a series of multi-tracer laboratory scale experiments in a transparent sand-box model to visualize (a) processes during the genesis of freshwater lenses and (b) their internal dynamics. For physical modeling an acrylic glass box was used to simulate a cross section of an island, similar to Stoeckl & Houben (2012). Degassed salt water with a density of 1023 kg/m3 was injected from the bottom, saturating the sand inside the model. Fluorescent tracer dyes uranine, eosine and indigotine were used to mark the infiltrating fresh water from the top. All experiments were filmed and analyzed using fast motion mode. We performed two different types of experimental set-up according to Vacher (1988): (1) Layers of different hydraulic conductivity: By filling the sand-box model with sand of different grain sizes, layers of different hydraulic conductivity could be simulated. (2) Recharge distribution: By recharging the island heterogeneously we could observe shifts in the geometry of the freshwater lens. A novel approach of using different tracer colors and varying them spatially and over time within the recharge waters allowed us to visualize and measure internal flow processes. Age stratification and flow paths could therefore be investigated. Moreover, a combination of temporal and spatial tracer color variation in one single experiment enabled us to measure flow velocities of freshwater movement. Additionally, by injecting small amounts of tracer in the salt water environment, movements near the interface between fresh- and saltwater could be observed. Using the finite element model FEFLOW we could model the density driven dynamics of our small scale freshwater lens, including its formation and the degradation after turning off the recharge water. This is important to fill the gap between our physical sand-box model and ongoing field investigations. The main focus of this work is the effects of climate change as well as geological and morphological heterogeneities on the dynamics of freshwater lenses in the North Sea. Stoeckl, L., Houben, G. (2012): Flow dynamics and age stratification of freshwater lenses: experiments and modeling. J. Hydrol., 21 (458-459), pp. 9-15. Vacher, H.L. (1988): Dupuit-Ghyben-Herzberg analysis of strip-island lenses. Geolog. Soc. Am. Bul., 100, pp. 580-591.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.H52B..02S
- Keywords:
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- 1829 HYDROLOGY / Groundwater hydrology;
- 1831 HYDROLOGY / Groundwater quality;
- 1847 HYDROLOGY / Modeling