Uranium in the Surrounding of San Marcos-Sacramento River Environment (Chihuahua, Mexico)

1. Introduction

The State of Chihuahua is located at the north of Mexico. Its capital is Chihuahua City, which is located in a semiarid region that uses, above all else, San Marcos-Sacramento aquifer waters for human consumption.

In Chihuahua State, about 30 uranium anomalies have been found; most of them are located near Chihuahua City [1]. Although the uranium deposit of Peña Blanca has been studied in previous works by other authors, there are at least two other zones in the area that could contribute with radioactive minerals to the Chihuahua City basin: the Pastorias zone at SW and the San Marcos zone at NW from Chihuahua City. This later area was studied in this paper regarding the pollution caused by natural occurring uranium.

To assess the radiological contamination from either natural or anthropogenic sources, it is important to understand the behavior of radionuclides released to the environment [2]. The radioisotopes ²³⁸U, ²³⁵U, and ²³²Th are the first radionuclides of three natural decay chains, respectively.

Uranium is occurring naturally in the earth crust, and its average content will vary as a function of rock type. Generally uranium is concentrated in igneous, metamorphic, and sedimentary rocks: granite, lignite, and phosphate deposits [3]. The uranium geological cycles begin by weathering processes in the earth’s crust and continue with mobilization by surface and ground water. In rock systems, secular radioactive equilibrium is common, while surface and ground waters are characterized by significant disequilibria. Uranium exists dominantly in the +4 and +6 oxidation states in most geologic environments [4, 5]. The uranium transport generally occurs in oxic natural waters as uranyl species (U⁺⁶), mostly complexed with hydroxide, carbonate, fluoride, sulphate, or phosphate [6]. Natural uranium concentrations in ground water ranges from 0.1 to 10 mg L⁻¹ (or ppm) [7, 8], while dissolved uranium concentration in rivers ranges from 0.01 to 100 μg kg⁻¹ (or ppb) and have a global average of about 0.3 ppb [4].

Likewise, the uranium isotopes in soil and water may be transferred to the plants and animals. The main sources of radionuclides uptaken by plants are the geologic substratum, the atmospheric fallout, or direct discharge from industries into the surrounding environment. It is necessary to evaluate the soil to plant transfer...

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