Are “ramblas” the sewerage of metal pollution in mining areas with a semi-arid climate?

 “Rambla”are temporary watercourses, which are characteristic of mediterranean and semi-arid climates, with low rainfall and sporadic torrential rain. A few times a year, when the rainfall exceeds 80 mm in less than an hour, they are capable of evacuating high flows of water mixed with particulate material of different granulometries to the sea.

The Sierra Minera of Cartagena (Murcia, Spain) has two marine slopes, one of short distance and steep slope towards the Mediterranean Sea, and another of a greater extent that affects the Mar Menor, a specially protected saltwater lagoon.

The processes that can take place in the course of the ramblas that begin in the Sierra Minera, when water is present, are those of transport of particulate and soluble material, processes of dissolution, hydrolysis, redox, complexation, carbonation and precipitation among others. When the rainy season ends and the Surface water disappears, the sediments that have been left in the riverbed are subject to other secondary processes that lead to the formation of very particular mineralogical species, efflorescence of hydrated sulphates, carbonates and oxydroxides. This is due to the pore water which can remain for a long period of time in the sediments of the rambla bed. Depending on the degree of influence of the current mining sediments in the rambla, the water it transports can have an acid pH (3.5-5.5), being qualified as acid mine drainage.

The Rambla del Beal is one of the various watercourses that cross the Sierra and flow into the Mar Menor next to a wetland that is an Special Protection Area (SPA), which makes the study more interesting. Like other ramblas in the area, during different periods, it carried the waste from the mineral floatation plants to the sea, as if it were a natural pipe, so along its route there are abundant terraces formed by this waste. This is in addition to the materials dumped and the materials eroded from the ponds and dumps that are in its receiving basin.

A selective sampling has been carried out along the riverbed to its mouth, analysing the content of potential toxic elements (PTEs), granulometry, general characteristics and mineralogy.

The bioavailability and mobility of the different PTEs (As, Pb, Zn, Cd and Fe) and their relationship with the mineralogy have also been studied.

The results show that only in the central part of the rambla can sediments not affected by primary pollution be found (they are secondary pollution), and that the Rambla del Beal itself can be considered a focus or primary source of pollution by PTEs.