Radon migration, diffusion coefficients and relationships with petrophysical properties of subsoil in a wetland may differ from those in similar subsoil in non-wetland areas due to water constituents of the subsoil material of the wetland as well as irregular radon interactions with air and water. The level of radon in subsoil and influence of the subsoil properties within the vadose zone of a wetland intended for use for intensive cultivation for increased food security by small holder Farmers was investigated in the current study. For this purpose, concentration levels of Rn-222 were measured in-situ using active measurement technique RAD7 at 0.2 m depth interval to total depth of 1.0 m, and 20 m sampling interval corresponding to points of increased and decreased resistivity with depth inferred from the electrical resistivity tomogram (ERT) of the wetland along 8 traverses. Diffusion coefficients were estimated empirically, while the subsoil properties, namely, Atterberg limits, porosity, permeability, and hydraulic conductivity, were determined employing appropriates American Standard of Testing of Materials (ASTM) procedures using disturbed soils samples recovered from 1.0 m deep pits dug at each sampling point. The subsoil which made up the vadose zone comprises silty sands of high plasticity (SMHP) as well as clayey sand (SC) of low plasticity (SCLP), medium plasticity (SCMP), and high plasticity (SCHP). The increasing order of abundance of median Rn-222 concentrations in each subsoil: SMHP (544.5 Bqm− 3) < SC (775 Bqm− 3), and SCLP (715.25 Bqm− 3) < SCMP (792.0 Bqm− 3) < SCHP (797.0 Bqm− 3) suggests that Rn-222 concentrations in soils are dependent on the plasticity of the soil and/or the amount of clay present. The increasing order of mean moisture contents and porosity with 222Rn concentrations: SCLP (14.50%, 715.25 Bqm− 3) < SCMP (19.92%, 792.0 Bqm− 3) < SCHP (22.75%, 797.0 Bqm− 3) and SCLP (44.60%, 715.25 Bqm− 3) < SCMP (44.61%, 792.0 Bqm− 3) < SCHP (45.13%, 797.0 Bqm− 3), respectively, suggests direct interrelation between 222Rn concentration and each of the moisture contents and porosity for the sub-clayey sands. These are in agreements with relationships obtained for soils other than wetland and reported in several literatures. The inverse correlation between 222Rn concentration and each of the moisture contents and percentage of gravel in clayey sands of high plasticity, however, was inconsistent with that found for soils other than wetland. Similarly, the inverse relationship between radon diffusion coefficient and porosity as exemplified by increased order of mean and median radon diffusion coefficients of the clayey sands subsoil: SCHP (1.16 10− 6 m2 s− 1, 1.11 × 10− 6 m2 s− 1) < SCMP (1.32 10− 6 m2 s− 1, 1.34 × 10− 6 m2 s− 1) < SCLP (1.91 × 10− 6 m2 s− 1, 1.88 × 10− 6 m2 s− 1), and decreased order of the mean porosity: SCHP (45.13%) > SCMP (44.61%) > SCLP (44.60%), was inconsistent with that found for soils other than wetland. However, the inverse relationship between increasing order of radon diffusion coefficient: SCHP (1.16 10− 6 m2 s− 1, 1.11 × 10− 6 m2 s− 1) < SCMP (1.32 10− 6 m2 s− 1, 1.34 × 10− 6 m2 s− 1) < SCLP (1.91 × 10− 6 m2 s− 1, 1.88 × 10− 6 m2 s− 1) and decreased order of the moisture contents: SCHP (22.75%) > SCMP (19.92%) > SCLP (14.50%) holds even for soils other than wetland. The median 222Rn concentrations of 1150 Bqm− 3 in the subsoil within the vadose zone of the wetland could be considered of relatively low risk having been less than 10,000 Bqm− 3 Swedish Criteria. This indicates low 222Rn radon enrichments in soils suggesting no or low risk to damage of the microbial community.

Soil gas radon (Rn 222) in vadose zone of A cultivated wetland in Southwest Nigeria

期刊论文
发表日期：

2025年11月26日

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Mustapha Kola Lawal,
Michael Olatunde Oni,
Moruffdeen Adedapo Adabanija,
Emmanuel Abiodun Oni,
Riliwan Damilola Abioye,
Abraham Adewale Aremu

摘要

Radon migration, diffusion coefficients and relationships with petrophysical properties of subsoil in a wetland may differ from those in similar subsoil in non-wetland areas due to water constituents of the subsoil material of the wetland as well as irregular radon interactions with air and water. The level of radon in subsoil and influence of the subsoil properties within the vadose zone of a wetland intended for use for intensive cultivation for increased food security by small holder Farmers was investigated in the current study. For this purpose, concentration levels of Rn-222 were measured in-situ using active measurement technique RAD7 at 0.2 m depth interval to total depth of 1.0 m, and 20 m sampling interval corresponding to points of increased and decreased resistivity with depth inferred from the electrical resistivity tomogram (ERT) of the wetland along 8 traverses. Diffusion coefficients were estimated empirically, while the subsoil properties, namely, Atterberg limits, porosity, permeability, and hydraulic conductivity, were determined employing appropriates American Standard of Testing of Materials (ASTM) procedures using disturbed soils samples recovered from 1.0 m deep pits dug at each sampling point. The subsoil which made up the vadose zone comprises silty sands of high plasticity (SM_HP) as well as clayey sand (SC) of low plasticity (SC_LP), medium plasticity (SC_MP), and high plasticity (SC_HP). The increasing order of abundance of median Rn-222 concentrations in each subsoil: SM_HP (544.5 Bqm^− 3) < SC (775 Bqm^− 3), and SC_LP (715.25 Bqm^− 3) < SC_MP (792.0 Bqm^− 3) < SC_HP (797.0 Bqm^− 3) suggests that Rn-222 concentrations in soils are dependent on the plasticity of the soil and/or the amount of clay present. The increasing order of mean moisture contents and porosity with ²²²Rn concentrations: SC_LP (14.50%, 715.25 Bqm^− 3) < SC_MP (19.92%, 792.0 Bqm^− 3) < SC_HP (22.75%, 797.0 Bqm^− 3) and SC_LP (44.60%, 715.25 Bqm^− 3) < SC_MP (44.61%, 792.0 Bqm^− 3) < SC_HP (45.13%, 797.0 Bqm^− 3), respectively, suggests direct interrelation between ²²²Rn concentration and each of the moisture contents and porosity for the sub-clayey sands. These are in agreements with relationships obtained for soils other than wetland and reported in several literatures. The inverse correlation between ²²²Rn concentration and each of the moisture contents and percentage of gravel in clayey sands of high plasticity, however, was inconsistent with that found for soils other than wetland. Similarly, the inverse relationship between radon diffusion coefficient and porosity as exemplified by increased order of mean and median radon diffusion coefficients of the clayey sands subsoil: SC_HP (1.16 10^− 6 m² s^− 1, 1.11 × 10^− 6 m² s^− 1) < SC_MP (1.32 10^− 6 m² s^− 1, 1.34 × 10^− 6 m² s^− 1) < SC_LP (1.91 × 10^− 6 m² s^− 1, 1.88 × 10^− 6 m² s^− 1), and decreased order of the mean porosity: SC_HP (45.13%) > SC_MP (44.61%) > SC_LP (44.60%), was inconsistent with that found for soils other than wetland. However, the inverse relationship between increasing order of radon diffusion coefficient: SC_HP (1.16 10^− 6 m² s^− 1, 1.11 × 10^− 6 m² s^− 1) < SC_MP (1.32 10^− 6 m² s^− 1, 1.34 × 10^− 6 m² s^− 1) < SC_LP (1.91 × 10^− 6 m² s^− 1, 1.88 × 10^− 6 m² s^− 1) and decreased order of the moisture contents: SC_HP (22.75%) > SC_MP (19.92%) > SC_LP (14.50%) holds even for soils other than wetland. The median ²²²Rn concentrations of 1150 Bqm^− 3 in the subsoil within the vadose zone of the wetland could be considered of relatively low risk having been less than 10,000 Bqm^− 3 Swedish Criteria. This indicates low ²²²Rn radon enrichments in soils suggesting no or low risk to damage of the microbial community.