Free phase radon in the Earth's subsurface: Occurrence, sources, transport pathways, and insights

氡是一种放射性元素，以溶解和自由相气体形式存在。它是癌症的第六大原因（以及吸烟后肺癌的第二大原因），但人类感官无法察觉。氡在室内空气、浅层土壤和地下水中普遍存在且变化很大，但其来源尚不清楚。虽然浅层土壤和房屋中的氡通常归因于浅层（不到100米左右的深度）富含铀的土壤和岩石，但这一点尚未在研究中得到证实。了解主要的地下来源和运输途径，是负责氡的无处不在，但空间异质性的发生将有助于减轻其重大的公共健康影响。虽然我们对地下氡的主要来源和传输途径知之甚少，但我们有一些重要的线索。氡通常以相对高的浓度（沿着二氧化碳）共存于火山、洞穴、地下矿井和一些地质断层的游离相气体排放中。其在大陆上空的浓度高于海洋，与大陆来源一致。 这项研究将评估Rn在重要深度广泛产生的可能性（即，> 1 km），并在浮力作用下作为自由相气体通过广泛的通道网络输送到地下。这个概念在概念上是具有挑战性的，因为放射性衰变导致Rn浓度每3.82天减少一半，因此从深度的传输必须迅速。 虽然很少进行现场研究，但通过裂缝和孔隙的自由相气体运输早已报道，速率高达每天数十公里，然而，这是可能的。 我将与10名学员（5名理学士、3名硕士、1名博士生和1名博士后）一起对深层气体和相关流体（包括含气泉和池塘、威尔斯井、间歇泉和地质断层）进行采样，气泡从地下发出。氡将与一套其他参数一起进行分析，以深入了解氡和其他流体的来源和深度，以及相对的地下停留时间。这项工作将深入了解氡的来源和途径，以支持政府和工业界减轻氡的癌症风险，特别是在家庭中。 这项研究还将深入了解地球向大气中的碳排放（除了火山排放外，全球范围内的碳排放限制很差）。 第三，这项工作将深入了解地球地下的自由相气体传输路径和速度，我们对此知之甚少，但需要对设施周围进行适当的监测和测量（例如泄漏的石油威尔斯，气体储存地点，碳捕获和储存地点），以便加拿大人能够有效地减少温室气体排放。

Radon is a radioactive element that occurs in dissolved and free phase gas forms. It is the 6th leading cause of cancer (and 2nd leading cause of lung cancer after smoking), but imperceptible to the human senses. Radon is ubiquitous and highly variable in indoor air, shallow soil, and groundwater, but its source(s) are not clear. Although radon in shallow soil and homes is most often attributed to shallow (less than 100 metres or so depth) uranium-rich soil and rocks, this has not been borne out in research studies. An understanding the major subsurface source(s) and transport pathways that are are responsible for radon's ubiquitous but spatially heterogeneous occurrence would help mitigate its significant public health impacts. Although we know very little about the main source(s) and transport pathways of subsurface radon, we have some important clues. Radon often co-occurs at relatively high concentrations (along with carbon dioxide) in free phase gas emissions from volcanoes, caves, underground mines, and some geologic faults. Its concentrations are higher in air over the continents than the oceans, consistent with a continental source.  This research will evaluate the possibility that Rn is widely produced at significant depths (i.e., > 1 km) in the earth's continental crust or mantle, and transported to the subsurface under buoyancy as free phase gas through a widespread pathway network. This notion is conceptually challenging because radioactive decay causes Rn concentrations to decrease by one-half every 3.82 days, so the transport from depth has to be rapid.  Although little field research has been conducted, free phase gas transport through fractures and pores has long been reported, with rates up to tens of kilometers per day, however, so this is possible.    I will work with ten trainees (five BSc, three MSc, and one PhD student and one PostDoc) to sample deep sources of gases and associated fluids (including gassy springs and ponds, wells, geysers, and geologic faults) where gas bubbles issue from the subsurface. Radon will be analysed alongside a suite of other parameters to provide insight into the source(s) and depth(s) from which radon and other fluids originated, and also relative subsurface residence times. This work will providing insight into radon sources and pathways to support governments and industry to mitigate  cancer risk from radon, especially in homes.  This research will also provide insight into geogenic carbon emissions to the atmosphere (which are poorly constrained globally, except for volcanic emissions).  Thirdly, this work will  provide insight into free phase gas transport pathways and velocities in the earth's subsurface about which we know relatively little, but is needed for appropriate monitoring and measurement around facilities (e.g. leaking petroleum wells, gas storage sites, and carbon capture and storage sites) so Canadians can greenhouse gas emissions effectively.