Rn in Natural Waters

Burnett N. Dimova H. Dulaiova D. Lane-Smith, B. Parsa and Z. Szabo  

There are three naturally-occurring isotopes of radon Rn ( radon  

Of all the possible parameters which may effect the equilibration time, the only ones which could easily be controlled to some extent are the flow rates of the re-circulated air and water. In our previous experimental work with Rn, we had relied on the internal air pump of the RAD-7 which is fixed at a flow rate of about 1 L/min. Since the very short half-life of thoron dictates that we perform the analysis as quickly as possible, we have now installed an external air pump so we can vary the flow rate of the air as well as the water (Fig. 3). We report here our observations on the effect of different water and air flow rates on the response to a constant source of thoron. 

The thoron source is made from acrylic fiber impregnated with Mn02 which is used as a thorium/radium adsorber. There are two closed air loops (1) circulating through the exchanger driven by an external air pump and (2) a separate loop between the RAD-7 and the primary air path driven by the internal pump of the RAD- 7. 

3 THORON AS A PROSPECTING TOOL 3.1 Background and General Strategy 

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Ra. Exceptions to this are environments where Rn is lost from the system by degassing (e.g., see Condomines et al. 2003), or aqueous systems where the insoluble nature of °Pb leads to its preferential removal. The speciation of Pb in natural waters is rather complex and heavily depends on the availability of organic complexing agents for which Pb has the highest affinity. In the oceans, Pb has a very short residence (30-150 yrs) and is rapidly scavenged by particles. 

Rn-220 is another isotope of radon and belongs to the thorium decay series. Due to its short half life of 55.6 s, reports on its concentrations in those gases and in natural water are still scant. They are also important for a better estimate of our exposure to natural radioactivity and also for the geochemical study of the forma tion of those radon isotopes and their underground movement. 

Some of the earliest work on Rn in estuaries indicated that an enrichment in Rn in pore waters occurred from Rn recoil from solids and its overall inert natural character (Hammond et al., 1977). This work in the Hudson River estuary (USA) also concluded... 

With very few exceptions, surface and near surface waters contain an excess of Rn (Table I l-III) compared to Ra (Table 11-lV) and U (Table 11-VI). This Rn must come from Ra in solids such as rocks, soils and sediments. The solubility product of Ra salts is seldom reached in natural waters, because invariably it is adsorbed onto sulphates and carbonates at the surfaces of rocks and minerals. In the zone of oxidation it is also coprecipitated by hydrous oxides of Fe and Mn. Only in the vicinity of strong sources of very saline waters do Ra concentrations rise to 10 or even 10 g/L. 

The highly fractionated nature of the and Th series nuclides is illustrated by the measured activities in some representative waters in Figure 1. The highest activities are typically observed for Rn, reflecting the lack of reactivity of this noble gas. Groundwater Rn activities are controlled only by rapid in situ decay (Table 1) and supply from host rocks, without the complications of removal by adsorption or precipitation. The actinide U, which is soluble in oxidizing waters, is present in intermediate activities that are moderated by removal onto aquifer rocks. The long-lived... 

Out of these radical species, H and ej are reducing in nature, while OH radicals are oxidizing in nature. Employing suitable additives, such as inorganic salts and dissolved gases, it is possible to convert these primary radicals of water radiolysis to new radicals. The radiation chemical reactions leading to the conversion of these primary species into different ROS and RNS are given below. 

Within ocean sediments, the decay of uranium and thorium isotopes leads to the creation of Rn, which is released to sedimentary pore waters and subsequently diffuses into the over-lying seawater. Near the seafloor, excess Rn can be seen against the background of a natural standing stock of this isotope in the water column, which is produced by in situ decay of Ra, a long-lived and relatively uniformly distributed isotope. Because of its short half-life, the existence of this excess isotope some several hundred meters above the seafloor implies a significant flux into the bottom waters, and the shape of the profiles has been modeled as a vertical diffusive balance with radioactive decay of radon and in situ... 

Tanner, A. B. 1964. Physical and chemical controls on distribution of Ra-226 and Rn-222 in ground water near Great Salt Lake, Utah. In The natural radiation environment, ed J. A. Adams and W. M. Lowder, pp. 253-76. Chicago Univ. of Chicago Press. 

Diffusion out of sediments forms a significant input for Ra isotopes, Ac and Rn into overlying water. All these nuclides are produced in sediments through a-decay (Figure 1). The recoil associated with their production enhances their mobility from sediments to pore waters from where they diffuse to overlying sea water. Their diffusive fluxes depend on the nature of sediments, their accumulation rates, and the parent concentrations in them. is another isotope for which supply through diffusion from sediments may be important for its oceanic budget. 

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