Radium radioactivity in soil profiles following long term irrigation with high radioactivity fossil groundwater

Abstract

High radioactivity in a relatively saline confined aquifer water in different locations of the Disi area (South East of Jordan) was reported by some authors who recommended further investigation on the impact of that water on irrigated soils. Five well water samples (jointly used for irrigation and drinking) and 28 surface and subsurface soil samples from five profiles were collected from that area for the purpose of this study. Selected mineralogical, chemical, and physical properties of the soil samples were determined. Mineralogical compositions of the 28 soil samples were identified using x-ray diffraction, x-ray fluorescence, and ICP-MS techniques. Determination of activity concentrations of 238U, 226Ra and 228Ra in the five well waters and 22 soil samples from 4 sites (including a native soil) were determined using γ-ray spectroscopy. The results showed low salinity levels of both soil and water samples and low clay and organic matter contents in all soil samples. Kaolinite and mica were the dominant clay minerals with Fe substituting Al in the octahedral layer of these minerals. The average activity concentration of 226Ra and 228Ra in the well-water samples were 0.31 ± 0.09 and 1.74 ± 0.12 BqL−1, respectively. Such a high specific activity could be ascribed to the water enrichment with 228Ra diffusing from 232Th-rich sandstone geologic strata. Average concentrations of 238U and 232Th in the soil samples (0–120 cm depth) were 1.48 ± 0.38 mg kg−1 and 4.78 ± 1.55 mg kg−1, respectively. High correlation between these two radionuclides (R2 = 0.90) indicated no specific enhancement of these two metals from external sources, especially through chemical precipitation from irrigation water. Average activity concentration of 238U, 226Ra, and 228Ra in the soil samples were substantially low (20.8 ± 5.6, 16.94 ± 4.48, and 20.7 ± 6.2 Bq kg−1, respectively). No particular changes were observed when comparing concentration or radioactivity of these radionuclides with depth of a given soil or between irrigated and native soil samples at comparable depths. This could lead to the conclusion that there was no appreciable precipitation or adsorption of these radioactive metals from the percolating irrigation water onto the sandy soil complex.