Dynamics of rainwater lenses on upward seeping saline groundwater

Abstract

Fresh water is generally a limited resource in coastal areas which are often densely populated. In low-lying areas, groundwater is mostly saline and both agriculture and freshwater nature depend on a thin lens of rainwater that is formed by precipitation surplus on top of saline, upward seeping groundwater. Understanding the dynamics of such lenses is vital for sustainable food production and development of natural vegetation and biodiversity under changing conditions like sea level rise and climate change. The thickness of the mixing zone between the fresh and saline water is substantial and characteristics of this mixing zone cannot be neglected. In this thesis we have studied the behaviour of these thin rainwater lenses and their mixing zones. To study the basic relations of such a system, we considered the development of a rainwater lens, starting from initially saline conditions using a numerical model. The ratio of seepage over precipitation, density difference and to a lesser extent the geometry of the flow domain, significantly influence the thickness of the lens and mixing zone. The thickness of the mixing zone is also significantly influenced by dispersion (as the lens grows), diffusion (at steady state) and distance from a drain (caused by convergence). Field observations show that geological layering influences these processes importantly and that head differences often overrule the effects of density difference. Groundwater salinity is furthermore influenced by re-mixing of soil- and rainwater, dual porosity and preferential flow. If there is significant seepage, the thickness of a lens can be estimated by an analytical solution. The effects of weather and climate variations were numerically studied using sinusoidal and actual net precipitation patterns. The average lens thickness is hardly influenced by weather fluctuations and we can relate minimum and maximum lens thickness to this average thickness. The thickness of the mixing zone can be derived from the “travelled distance” from its center: thickness increases with increasing vertical movement due to alternating precipitation and evaporation. Field observations confirm this, but show concentration of dynamics in the top of the lens. This is not contradictory since the mixing zone for the studied site starts very near the ground water table, so indeed the mixing zone thickness is influenced by precipitation events. Convolution theory can be used to determine the impulse-response function for a thin lens which enables derivation of the delay and amplitude of a lens reaction to changes in climate. The cation exchange process was investigated using a numerical model based on field data. On the short term, the process is characterized by the salt-shock caused by the large difference in concentration between rainwater and seepage water. The pore water quality changes quickly from saline to relatively fresh and from sodium and magnesium dominated to calcium dominated. On the long term, changes of the soil complex occur. This is a process of several centuries (even for the shallow systems studied), since the amount of cations in the fresh solution is very small compared to the amount of cations adsorbed to the soil. Initially, both calcium and magnesium in the sorption complex increase and later magnesium is outcompeted by calcium. The net flux downward has a much larger effect on the mixing process than short-term variations and the influence of tile drainage. Although the main flow component is horizontal, the water quality and soil complex are only influenced by the vertical flow that causes mixing of water with different composition. Comparison of the numerical model with field data from both study sites in Zeeland confirm our results and show that equilibrium has not yet been reached. Influence of saline groundwater on vegetation development has been assessed simulating combinations of different vegetation types, soil characteristics and groundwater levels and –salinities for two different climates. We assessed the relative importance of these parameters on the fresh water availability and stress experienced by vegetation and put this in perspective of stress due to drought and lack of oxygen. Soil type and climate are shown to be the most important parameters. Salinity stress is substantial, but small compared to stress caused by lack of oxygen and drought. For areas with groundwater with limited salinity, salt tolerance may be a parameter that can be used to improve sustainability of agriculture. Where groundwater is more saline, soil and ground water management are the most effective tools.