Impact of mining on geochemical signatures of riverine sediments in adjacent ecosystems

As a result of the open-cast lignite mining in Lusatia (Eastern Germany), large quantities of iron (Fe) and sulphate (SO₄^2-) are fed into small streams discharging into the Spree river system. The study examined whether the inputs of Fe and SO₄^2- lead to longitu­dinal and depth-dependent gradients in the riverine sediments downstream the mining region in terms of element composition and mineral formations.

We sampled the surface (upper 0-3, 3-6 cm) sediment using a gravity corer at 18 sites from the heavily mining impacted Spreewald region downstream 200 km to the Bänke at Lake Müggelsee. We also included sampling sites at a pit water purification system in Vetschau, one neutral mining lake and a reference site without mining impact. Sedi­ments were analysed for total C, N using an element analyser, for various elements (incl. Fe, S, Mn, Al, P, heavy metals) by ICP-OES after digestion with hot aqua regia. A sequential Fe-extraction from fresh sediments and XRD was performed to differenti­ate solid iron forms and other minerals, respectively.  Characteristic sediment signatures are investigated with the help of a Principal Component Analysis (18 sites, 19 parameters).

We discovered a decreasing sedimentary Fe-content in flow direction from 300 mg g-1 in Vetschau, 130 mg g-1 close to the mining region in Lübbenau down to 30 mg g-1 at Bänke near Lake Müggelsee. In contrast, the S-content increased with decreasing mining impact from 3 mg g-1 in Vetschau up to 35 mg g-1. Minimum Fe- and S-contents are similar to Bautzen reservoir as a non-mining impacted reference location with Fe 28 mg g-1 and S 4 mg g-1. The statistical analysis with the PCA revealed the longitudinal influence of mining products within Spree river. Two major groups emerge from the score plot. First, there are those samples, which are clearly influenced by mining activities. Second, there are samples, which include the reference point and samples more distant from mining, where we expect no or only minor mining impact. This separation becomes even more apparent after taking heavy metals into account. Furthermore, the Fe binding shifts from more easily reducible Fe/amorphous minerals to less easily reducible Fe/more crystalline minerals in flow direction, which probably has consequences for the microbial degradability of organic matter and the strength of the ability of Fe to bind phosphorus.  

We were able to prove that the sedimentary ele­ment composition and especially the Fe mineral characteristics are influenced by mining activities at least 100 km downstream the Spree river system, probably affecting the phosphorus availability and carbon turnover.