The Input of Phosphate & Vanadium into the Lake Laacher See by Dissolution of Volcanic Rocks (East Eifel, Germany)

<p>The lake Laacher See (East Eifel, Germany) is affected by eutrophication due to high phosphorous concentration for stagnant waters averaging 34 &#181;g/l (range of 25&#8211;40 &#181;g/l P since the year 2000). These elevated concentrations have been monitored since the 1950s and an oligotrophic status could not be achieved despite various measures [2]. A linear correlation was determined between dissolved phosphate (211 &#8211; 643 &#956;g/l) and vanadium (6.7 &#8211; 28.4 &#956;g/l) in groundwaters of the Ringseitert volcanic complex (West Eifel) with implications for drinking water use [4]. These issues are the motivation to investigate the geogenic input of phosphorous into Laacher See and of vanadium into groundwaters by fluid-rock-interaction.</p><p>Laacher See is the water-filled crater area of the Plinian-erupted Laacher-See-volcano, which belongs to the East Eifel Volcanic Field with an active volcanism for 0.46 Ma [6]. The catchment of this lake includes rocks of siliciclastic Lower Devonian basement, scoria cones and lava flows of basanitic and tephritic compositions, and phonolithic to foiditic tephra from the Rieden volcanic complex. The youngest volcanic unit is the phonolithic, strong geochemically zoned tephra from the Laacher-See-volcano, which ejected 6 km&#179; magma in form of pumice and ash 12.9 ka ago [3,6].</p><p>The dissolution of apatite (primary) and vivianite (secondary phosphate phase) is assumed to be the reason of geogenic input of phosphorous and vanadium by groundwater flow. A particularity of the Laacher See area is the occurrence of mofettes that lead to elevated concentrations of CO₂, decreased pH and enhanced apatite dissolution [5]. The hydrochemical modelling program PHREEQC is used to investigate the equilibrium state of phosphate and vanadium in groundwaters at specific Eh/pH-conditions, p(CO₂)-values and compositions. In addition, residence times calculated by hydraulic conductivities and dissolution rates from batch experiments are used to distinguish between rate-limited or equilibrium process in phosphate dissolution. Data on phosphorous concentrations and pH of soils in the vicinity of the Laacher See and their equilibrium solutions are evaluated to their geological, geochemical and anthropogenic background and provide clues to phosphorous sources [1].</p><p>In future studies, bulk rock concentrations will be measured using XRF and total digestion, and detailed dissolution rates will be measured using extended batch experiments to combine these findings into a conceptual hydrogeological model of geogenic phosphate and vanadium input to lake Laacher See.</p><p><strong>References:</strong></p><p>[1] Armbruster, M. & Wiesler, F. 2012. Ermittlung der P-Gehalte entlang von 10 Transekten am Laacher See. LUFA Speyer, Speyer. P.24 (unpublished)</p><p>[2] Block, U.<em> et al.</em> 2015. &#220;bersicht &#252;ber die Phosphatthematik am Laacher See. Fachhochschule Bingen. P.41</p><p>[3] Bogaard, P.v.d. & Schmincke, H.U. 1984. The eruptive center of the late quaternary Laacher see tephra. <em>Geologische Rundschau</em>, 73, 933-980, http://doi.org/10.1007/BF01820883.</p><p>[4] H&#228;rter, L.M.<em> et al.</em> 2020. Vorkommen von Vanadium im Grundwasser der Vulkaneifel. <em>Grundwasser</em>, 25, 127-136, http://doi.org/10.1007/s00767-020-00447-x.</p><p>[5] Pan, H.B. & Darvell, B.W. 2009. Calcium Phosphate Solubility: The Need for Re-Evaluation. <em>Crystal Growth & Design</em>, 9, 639-645, http://doi.org/10.1021/cg801118v.</p><p>[6] Schmincke, H.-U. 2007. The Quaternary Volcanic Fields of the East and West Eifel (Germany). <em>In</em>: Ritter, J.R.R. & Christensen, U.R. (eds) <em>Mantle Plumes: A Multidisciplinary Approach</em>. Springer Berlin Heidelberg, Berlin, Heidelberg, 241-322, http://doi.org/10.1007/978-3-540-68046-8_8</p>