Understanding co-occurrence and mobility of uranium and arsenic sources in groundwater flow systems in semi-arid zones in the Mexican Altiplano

Groundwater is the main source of water supply for the population in the semi-arid zones in Mexico. In this climatic condition, evapotranspiration originated from phreatophytes and evaporation is an important component of the water budget for shallow aquifers. Arsenic and uranium are trace elements generally found naturally in groundwater, as they can originate from the interaction of groundwater with igneous rocks and sediments. Despite their generally different redox properties and transport behavior in groundwater, they show a relationship (a potential common geogenic source).

This research addresses the effect of groundwater evapotranspiration on arsenic and uranium concentrations in different flow systems in Villa de Reyes and Cerritos basins in the Mexican Altiplano. The former is mainly characterized by volcanic rocks and the latter is located in an area dominated by limestone yet connected to felsic, partially mineralized rocks by a complex karst system. According to the Mexican legislation, the permissible limit for arsenic in water for human use and consumption is 25 µg/L; however, it does not consider a permissible limit for uranium. German legislation as well as the World Health Organization (WHO) established 10 µg/L.

The concentrations of arsenic and uranium found in the rocks in the two sites were in the range of 1.8-65 and 0.7-19 mg/kg respectively, in which it is observed that some sites exceed the local background values. The high arsenic and uranium concentrations have been found in felsic intrusive bodies and rhyolites in both sites. This confirmed an earlier study identifying rhyolitic lava flows and rhyolitic glass within the Villa de Reyes Basin as main arsenic and uranium geogenic sources.

The arsenic and uranium concentrations in groundwater at the Villa de Reyes basin showed that 100% of the groundwater samples were within the Mexican Legislation for arsenic but in the case of uranium, 6% of the samples were above the German and WHO regulations. The shallow granular aquifer in the Cerritos Basin, the groundwater values gave different results. Only 74% of the sites were within the Mexican regulation for arsenic and 36% above the critical value. For uranium, 90% of the sites were within the German and WHO regulations.

In addition, isotopic data (environmental stable isotopes from the water molecule ²H and ¹⁸O) showed that shallow groundwater in the Cerritos Basin was fractioned by evapotranspiration processes. These led to the observed high concentrations of especially arsenic as was also supported by geochemical modelling.

Especially as the climatic trend predicted higher temperatures for central to north Mexico, enrichment of arsenic and other potentially toxic elements by evapotranspiration will be encouraged. Increasing population requires a good quality water supply, understanding the behavior of arsenic and uranium in these areas may be useful for similar regions not only in Mexico but also in similar areas around the world.