Long-Term Radiological Dose and Cancer Risk Assessment at Hongyum/Pongyum Quarry site, Mpape Using RESRAD-Onsite 7.2

Abstract

The RESRAD-Onsite 7.2 code was applied to assess the long-term total radiological dose and excess cancer risk at the Hongyum/Pongyum Quarry site, Mpape, Federal Capital Territory (FCT), Nigeria, over an area of 183,800 m² and a zone thickness of 2.00 m, for a simulation period of 1.0 × 10³ years. Mean activity concentrations of 930.57 ± 61.89 Bq kg⁻¹, 25.34 ± 4.32 Bq kg⁻¹, and 76.28 ± 4.81 Bq kg⁻¹ were determined for ⁴⁰K, ²²⁶Ra, and ²³²Th, respectively, from six quarry sampling points and one control site using a NaI(Tl) gamma spectrometry system. Concentrations of ⁴⁰K and ²³²Th exceeded the UNSCEAR (2000) world average values at all quarry sampling points, while ²²⁶Ra remained below the world average at five of the six sampling points. The total dose at t = 0 years for all pathways was 7.878E-01 mSv yr⁻¹. The maximum total dose of 1.156E+00 mSv yr⁻¹ was obtained at t = 37.65 ± 0.08 years, exceeding the Basic Radiation Dose Limit (BRDL) of 1.000E+00 mSv yr⁻¹. The total excess cancer risk at t = 0 years was 3.306E-03, which is 11.02 times higher than the WHO recommended limit of 3.00 × 10⁻⁴. A remediation scenario using a 1.25 m clean soil cover reduced the total dose by 82.6% and the total excess cancer risk by 85.5%; however, cancer risk continued to exceed the WHO recommended limit at all time points, due to residual ²²⁶Ra-driven radon breakthrough. The Hongyum/Pongyum Quarry site presents an unacceptable radiological cancer burden under current unmitigated conditions. While the 1.25 m clean soil cover is an effective first remediation step, supplementary radon attenuation measures are required alongside the soil cover to bring the site within the WHO-acceptable cancer risk threshold.