| Article ID | Journal | Published Year | Pages | File Type |
|---|---|---|---|---|
| 4699185 | Chemical Geology | 2012 | 14 Pages |
The construction of the new 56 km long Gotthard railway base tunnel in Switzerland opened a large number water conducting fractures and 122 water samples were collected along the Amsteg section of the tunnel and then chemically analyzed. The Amsteg section of the tunnel cuts across granites, gneisses and metarhyolites of the Alpine basement. The overburden of basement rocks above the tunnel axis is up to 2200 m thick with rock temperature reaching 45 °C. Sodium is the prime cation of most waters; calcium, potassium and magnesium are low to very low in concentration. The associated anions are carbonate/bicarbonate, sulfate and chloride in widely varying concentrations. High to ultra-high fluoride concentrations of up to 29 mg/L F and high pH values of up to 10.4 are significant properties of the waters. Six chemical types of water can be distinguished reflecting the different types of fractured rock present in the section. Most of the waters can be viewed as ternary mixtures of dissolved natrite (Na2CO3), thenardite (Na2SO4) and halite (NaCl).The geological and hydrogeological context suggests a meteoric origin of the waters. The unique water compositions and characteristics result from interaction with continental basement rocks along the flow path. The assemblage of major minerals of the different rocks is generally very similar, but the rocks differ with respect to minor minerals and accessories. Derived rock data suggest that dissolution and precipitation of calcite and fluorite, albite dissolution, leaching of salty inclusions, sulfide oxidation by CO2, alteration of biotite to chlorite, precipitation of stilbite and hematite are the important processes that create the observed water compositions. Although fracture water acquires its composition exclusively by chemical interaction with crystalline basement rocks with very similar mineral assemblages, the resulting waters are of great chemical variability.
► 122 water analyses from water conducting fractures in the world's longest tunnel ► Water composition is controlled by interaction with granite and gneiss. ► Water–rock reactions produce high pH up to 10.5; F and Si are very high. ► Most waters are ternary mixtures of Na2CO3–NaCl–Na2SO4. ► Sulfide to sulfate oxidation reduces C(4) to elementary carbon and methane.
