Seasonal turnover in groundwater

Abstract: This Licentiate Thesis presents a new approach of understanding leakage in agricultural land. Former studies concentrate on long term measurement of different pollutants in nearby watercourses and streams. The new approach is so far only numerically performed, but will soon be complemented by laboratory tests and field measurements. Our hypothesis is that nutrient leakage into groundwater is caused by thermally driven groundwater convection. The maximum density of water occurs at a temperature of near 4oC. Thus, a density increase of the groundwater occurs by heating from about 0oC in the north of Sweden (springtime) and by cooling from about 10oC in the south (autumn). The depth of the convection (leakage) depends on the size of the thermal gradient. This hypothesis consequently explains both why the nutrient leakage occurs during different seasons in the north and south of Sweden and also why the leakage reaches greater depths in the south. The numerical results show that convection is induced by a small horizontal groundwater flow. In the south of Sweden the lowest required permeability for convection to occur was K=6.7∙10-10m2. In this soil the convection cells reached to a maximum depth of 6 meters. The Rayleigh number (Ra) could be as low as 19 for convection to occur, the general critical Ra is 40 in porous media. In northern Sweden a permeability of K=6.1∙10-92 was required. In this soil and climate convection occurred to depths from 0.2 to 0.9 meters. Transient solutions showed that the required time for the convection pattern to fully develop was 22 days. The effect of frost lenses on the groundwater convection was also studied. Small lenses changed the convection rolls slightly, while large obstacles forced the convection rolls to change size and shape. The simulations showed that the required grain size for convection to occur was considerably greater than the grain size in typical agricultural soils. Still vertical groundwater movements exist. Other possible explanations to groundwater convection in agricultural soil in northern Sweden are to be investigated. Unstable groundwater convection or oscillating convection cells, infiltration of rain and melt water, pressure induced convection and the possibility that Coriolis force due to Earth´s rotation could cause secondary currents in groundwater flow.