Natural radionuclides in major aquifer systems of the Paraná sedimentary basin, Brazil

Abstract

This paper describes the natural radioactivity of groundwater occurring in sedimentary (Bauru and Guarani) and fractured rock (Serra Geral) aquifer systems in the Paraná sedimentary basin, South America that is extensively used for drinking purposes, among others. The measurements of gross alpha and gross beta radioactivity as well the activity concentration of the natural dissolved radionuclides ⁴⁰K, ²³⁸U, ²³⁴U, ²²⁶Ra, ²²²Rn, ²¹⁰Po and ²¹⁰Pb were held in 80 tubular wells drilled in 21 municipalities located at São Paulo State and its border with Mato Grosso do Sul State in Brazil. Most of the gross alpha radioactivity data were below 1 mBq/L, whereas values exceeding the gross beta radioactivity detection limit of 30 mBq/L were found. The radioelement solubility in the studied systems varied according to the sequence radon>radium>other radionuclides and the higher porosity of sandstones relatively to basalts and diabases could justify the enhanced presence of dissolved radon in the porous aquifer. The implications of the data obtained in terms of standards established for defining the drinking water quality have also been discussed. The population-weighted average activity concentration for these radionuclides was compared to the guideline value of 0.1 mSv/yr for the total effective dose and discussed in terms of the choice of the dose conversion factors.

Introduction

Estimates of the volumes of various kinds of water on, above, and under the surface of the earth reveal that fresh water makes up only 2.5% of all the water on earth, with groundwater comprising 99% of the earth's available fresh water (Delleur, 1999). Groundwater can be found in all three classes of rocks, i.e. igneous, sedimentary and metamorphic. In fractured rock aquifers, groundwater is stored in the fractures, joints, bedding planes and cavities of the rock mass; water availability is largely dependent on the nature of the fractures and their interconnection. However, sedimentary aquifers, also called unconsolidated aquifers, generally contain the greatest amounts of water due to their greater porosity. For instance, sandstones typically exhibit porosity between 5% and 30%, in contrast to the values of 0–10% often reported for the fractured crystalline rocks (Delleur, 1999). Therefore, sedimentary basins are an important source of groundwater worldwide, where many of them contain extensive multi-layered aquifer systems consisting of permeable sediments laid down at various times in the past.

In recent years, there has been growing public concern about the problems of groundwater quantity and quality as many aquifers are affected by adverse water quality and issues of the hydrological resources are faced with these challenges as a result of the actions or inactions of the responsible entities. Even in the well-known aquifers there are increasing concerns about pollution from diffuse source inputs and an ever-increasing need to balance resources spent on the groundwater management as compared with other priority regions. In managing it, increasing use is being made of information generated from hydrochemical studies held for understanding the release of constituents to the liquid phase due to water–rock interactions.

Potential health hazards from natural radionuclides in consuming water have been considered worldwide, with many countries adopting the guideline activity concentration for drinking water quality recommended by WHO (2008). In general, the recommendations apply to routine operational conditions of water supply systems, however, special attention must be given when groundwaters are utilized for public water supplies.

⁴⁰K is the only radioactive isotope of potassium and is released to water bodies as a consequence of water/soil–rock interactions (Davis, 1963). Thorium has been considered a highly insoluble element in water due to its presence in minerals of difficult dissolution (Langmuir and Herman, 1980). Uranium tends to be mobile under conditions present at the earth's surface; its concentration generally ranges from 0.1 to 10 μg L⁻¹ in rivers, lakes and groundwaters (Fritz and Fontes, 1980, Ivanovich and Harmon, 1992). WHO (2008) defined guidance levels in drinking water for ²³⁸U and daughters like ²³⁴U, ²³⁰Th, ²²⁶Ra, ²²²Rn, ²¹⁰Pb, and ²¹⁰Po, whose activity concentration evaluation involves sophisticated and time-consuming procedures and are generally performed if the gross alpha and beta measurements have exceeded 0.5 and 1 Bq/L, respectively.

Sandstone aquifers have been recognized of importance on the generation of enhanced radioactivity levels worldwide mainly due to ²²²Rn and radium isotopes ²²⁶Ra and ²²⁸Ra (Vengosh et al., 2009). Such rock types are widely spread in Paraná sedimentary basin, South America; however, igneous basic intruse rocks also outcrop in large surface area. This paper performs a comparative study of the radiation dose provided by sedimentary/fractured rock aquifers in Paraná basin, taking into account gross alpha/beta measurements and activity concentration data of radionuclides belonging to the ²³⁸U decay series.