Temperature-driven groundwater convection in cold climates

Maria Engström,Bo Nordell

doi:10.1007/s10040-016-1420-0

Abstract

The aim was to study density-driven groundwater flow and analyse groundwater mixing because of seasonal changes in groundwater temperature. Here, density-driven convection in groundwater was studied by numerical simulations in a subarctic climate, i.e. where the water temperature was <4 °C. The effects of soil permeability and groundwater temperature (i.e. viscosity and density) were determined. The influence of impermeable obstacles in otherwise homogeneous ground was also studied. An initial disturbance in the form of a horizontal groundwater flow was necessary to start the convection. Transient solutions describe the development of convective cells in the groundwater and it took 22 days before fully developed convection patterns were formed. The thermal convection reached a maximum depth of 1.0 m in soil of low permeability (2.71 · 10−9 m2). At groundwater temperature close to its density maximum (4 °C), the physical size (in m) of the convection cells was reduced. Small stones or frost lenses in the ground slightly affect the convective flow, while larger obstacles change the size and shape of the convection cells. Performed simulations show that “seasonal groundwater turnover” occurs. This knowledge may be useful in the prevention of nutrient leakage to underlying groundwater from soils, especially in agricultural areas where no natural vertical groundwater flow is evident. An application in northern Sweden is discussed.

Highlights

Fertilizer is applied to fields at different time of the year in the north and in the south of Sweden
Stable thermal convection occurs when Nu >1 and Rayleigh Number (Ra) > Rac. This means that thermal convection is influenced by soil permeability, horizontal groundwater flow, thermal properties of the soil, temperature difference, and the distance between the uppermost and undisturbed groundwater
Stable solutions were not found though Nu > 1, which means that part of the heat transport must be a result of convective heat transfer

Summary

Introduction

Fertilizer is applied to fields at different time of the year in the north and in the south of Sweden. It is considered more efficient to apply fertilizer in the autumn in the north, while it is done in the spring in the south. Nutrient loss to groundwater leads to overfeeding of lakes and watercourses, negatively impacting flora and fauna. By understanding the mechanisms behind the leakage, fertilizing could be done more effectively. Such knowledge would be helpful in preventing and counteracting other types of contamination by leakage from the ground surface to groundwater (Kyllmar 2004). Ground heat exchangers, used for extraction of thermal energy for space heating/cooling, are affected by such groundwater convection (Hellström et al 1988)

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