Borehole Diameter Controls Thermal‐Induced Convection and Evaporation From a Shallow Water Table

Abstract

Understanding air exchange via boreholes, shafts, and wells is important for Earth and environmental sciences because of their potential role for increased evaporation and greenhouse gas emissions. Here, we investigated the effect of atmospheric temperature variability on air transport via a borehole as a function of its diameter. Field experiment included five boreholes with diameters ranging from 0.10 to 0.38 m and a depth of 3 m with an artificial water table at −2.7 m. Temperature was recorded in each borehole at 1-min intervals for 1 year. Diurnal and seasonal air temperature changes triggered thermal instability, mainly during winter nights, which initiated thermal-induced convection within the boreholes. The thermal-induced convection penetrated deeper into the borehole as the diameter increased, doubling the water table evaporation rate for the largest borehole relative to the smallest. Borehole diameter plays an important role in controlling air exchange rates between the subsurface and the atmosphere. Plain Language Summary The Earth's surface is dotted with different types of boreholes and shafts with varying diameters and depths, which act like "straws" penetrating the Earth's subsurface whereby different types of gases can be emitted into the atmosphere from subsurface layers that were previously unexposed. To be able to calculate the quantities and rates of gas exchange, it is therefore important to understand the gas transport mechanisms within these "straws." In this study, we examined water vapor loss from a shallow water table using field measurements from five different human-made boreholes (3 m in depth). High-resolution temperature profiles and evaporation rates from the water table located 30 cm above the bottom of each borehole were used to explore gas transport. We show the importance of thermal-induced convection as a major air transport mechanism and its dependence on both borehole diameter and atmospheric conditions. The mechanism of thermally induced convection is similar to what controls cloud formation, in which warm (light) air moves upward to replace the cold (heavy) air above it. We found that the air transfer rate increases as borehole diameter increases and that boreholes emit more gas into the atmosphere, and at a higher rate, during winter than during summer.

Passive Breathing of Earth‐Air: “Fingerprint” Evidence From Moisture Records

“…As the soil has high specific heat and a good ability to absorb moisture, we do not usually see direct changes in temperature and RH due to the presence of earth‐air. However, earth‐air and atmospheric exchange in fact have very important influence on shallow soil heat flux (Levintal, Dragila, Lensky, et al., 2020; Levintal, Dragila, Zafrir, et al., 2020; Perrier & Moueel, 2016).…”

“…It is clear that AP-induced earth-air movement is the major mechanism responsible for surface-atmosphere air exchange, when compared with air molecular diffusion (Levintal, Dragila, Lensky, et al, 2020;Levintal et al, 2019;Levintal, Dragila, Zafrir, et al, 2020;Weisbrod, 2014). Under the induction of AP, the atmosphere enters the soil, but the RH variation in the pore-space could hardly be monitored, which indicates that the water supply from the surface of soil particles is very timely, and diffusion plays an important role in the transport and exchange of mass and energy in the front end of earth-air migration.…”

Квазистационарный Эффект Свободной Тепловой Конвекции В Водонаполненных Буровых Скважинах

“…Влияние диаметра скважины на интенсивности конвекции ранее отмечалось и в неглубоких воздухонаполненных скважинах [33].…”