Determination of major and minor constituents in anoxic thermal brines of deep sandstone aquifers in Northern Germany

Abstract

The common process of low energy geothermal exploitation is the doublet of production-and reinjection borehole. The quality of water reinjected into a clastic reservoir is essential for the reliability of an injection well. In order to estimate precipitation reactions it is necessary to obtain extensive reliable analysis data of the water for the use of thermodynamic modelling. For thermal anoxic brines, the analysis of major and especially minor ion content is difficult because of matrix effects and possible iron precipitation. A selection of analysing methods were applied to two anoxic thermal brines of deep sandstone aquifers of Northern Germany. Detection limits and measured data of the major constituents are presented of Na +, K +, NH + 4, Ca 2+, Mg 2+, Sr 2+, Fe total, Mn 2+, SiO 4− 4, B(OH) 3, Zn 2+, Pb 2+, Cd 2+, F −, Cl −, Br −, I −, SO 2− 4, SO 2− 3, S 2−, PO 3− 4, NO − 3, NO − 2 and DOC. The measurements were done with ICP-OES, ionselective electrodes, photometry, polarography, titration methods, ion chromatography and TOC-analyzer. Except for SO 2− 14 and Cl −, the anion analysis was done on-site, since the high iron content in the anoxic water requires acidification in order to prevent iron hydroxide precipitation. The minor constituents Zn 2+, Pb 2+, Cu 2+, Cd 2+, Cr 3+, Sc 3+, Co 2+, Y 3+, La 3+, Ce 3+, Al 3+, were enriched by trace matrix separation using the cation exchange resin Chelex ®100. The element concentrations in the acidic eluates of the Chelex ®100 columns were measured using ICP-MS. The pH dependency of the exchange equilibrium at pH values of 4, 5 and 6 (buffered and unbuffered) as well as the relation to the salt content between 35 and 250 gl −1 total dissolved solids of Na-K-Ca-Mg-Cl-SO 4 were evaluated by sensitivity analysis.