A contribution to occurrence and origin of uranium in German mineral and tap waters

Abstract

Uranium (U) is a natural, but chemo- and radiotoxic heavy metal. The exposure of humans to U is mainly determined through uptake by drinking waters. The objective of the research work presented here was to contribute to the knowledge of U in German mineral¹ and tap waters. U and the content of 65 other chemical elements, radioactivity in 307 mineral water brands and 476 tap water samples of German origin were evaluated at the former Institute of Plant Nutrition and Soil Science (FAL-PB), recently the Institute for Crop and Soil Science of the Julius Kühn Institute, Federal Research Institute for Cultivated Plants (JKI-PB) in Braunschweig. The mineral water brands were collected between 2000 and 2007, the tap waters were sampled in 2006. The research work yielded the following main results. The mean U content in mineral waters was 3.08 μg/L U, in tap waters 0.43 μg/L U. The median was 0.13 μg/L U in both groups. 82 % of the samples had U concentrations < 2 μg/L U in both groups. The lower U concentrations (below the lower limit of detection of ICPQMS: 0.015 μg/L U) were found in wells of quarternary pore groundwaters. The highest U concentrations showed up in the spa water of the Nürtinger Heinrichsquelle in Nürtingen, Baden-Württemberg (474 μg/L U), the mineral water Purio Aqua Römer Mainhard of the Römerquelle in Mainhardt-Baad, Baden-Württemberg (27.4 μg/L U) and the mineral water of Alwa Bonalwa-Quelle in Bad Peterstal-Griesbach, Baden-Württemberg (24.5 μg/L U). The tap waters with the highest U concentrations derived from Gunzenhausen and Kulmbach with 8.5 μg/L U, followed by Darmstadt and Jena with 5.8 μg/L U. Comparing the analytical values of 66 elements as well as HCO3 and dose rate of the mineral, spa and tap waters analysed, the by far highest variation coefficient value of all parameters tested in mineral and spa waters U had with 820 %. In the tap waters this value was 208 % for U. In the mineral waters tested, the following 30 out of a total of 65 chemical elements correlated weakly but significant (< 0.05) with U: As, B, Be, Ca, Ce, Cl, Cu, Dy, Er, F, Fe, Ge, Gd, Ho, K, Li, Mg, Mn, Na, Ni, Mo, Nb, S, Se, Ta, Ti, V, W, Y and Zn as well as dose rate and total dissolved matter. In addition, the concentrations of 226Ra and Th and the parameters total dissolved matter and dose rate showed only weak correlations with U. In the tap waters the elements B, Ca, Ce, Co, Cu, K, Mg, Mn, Mo, S and Sr correlated weakly but significant (p < 0,05) with U. Principal component analysis and multiple regression analysis confirmed that the U content in mineral and tap waters was only conditionally related to the concentrations of other chemical elements. The U concentrations of the German mineral waters tested with > 2 μg/L U can be classified in the following regional water groupings with higher U contents (from north to south): Fulda - Oberweser, Southwestern Harz foreland, Northeastern Harz foreland (eastern Subhercyian Basin), Saale-Unstrut, Thüringer Wald and Forelands, Rhön, Bad Kissingen, Southern

German Keuper area and Northern Black Forest. Within the framework of a country-wide overall regional geological survey, all mineral and tap waters with a content of > 2 μg/L U could be explained genetically with high probability by geogenetic factors. The German mineral waters containing U in concentrations > 2 µg/L U are nearly completely related to the permotriassic sediment complexes of Hessonia, Lower Saxony, Saxony-Anhalt and Southwestern Germany. Most important U source rocks are Buntsandstein and Keuper strata, only secondarily crystalline and Permian aquifers: - Keuper: Northern Germany: Steinmergel or Gypsum Keuper (Extal); Southern Germany: Burgsandstein or Stubensandstein (Bad Überkingen, Mainhardt-Baad and Nürtingen) - Buntsandstein, primarily Middle Buntsandstein, partially Upper Buntsandstein (Bad Driburg, Bad Pyrmont, Fuldatal, Hecklingen-Gaensefurth, Katlenburg, Hessberg, Hessische Rhön, Saale-Unstrut/Leisslingen, Steinheim-Vinsebeck, Volkmarsen and Warburg) - Perm (Bad Kissingen, Friedrichroda, Hecklingen-Gaensefurth, Saale-Unstrut/Leisslingen and Schmalkalden) - Paragneisses of the Rench area, Black Forest (Bad Peterstal-Griesbach). Until now anthropogenic loads are not significant for the U concentrations in mineral waters. The results of this research work indicate however, that especially U from mineral phosphorous fertilisers may be already a threat for the quality of tap waters. This hypothesis is corroborated by the high weight (19 %) of boron (B) in a multiple regression of concentrations of 15 chemical elements in tap waters on the U concentration. Like U, B is applied in agriculture in significant amounts with mineral phosphorus fertilisers and like U, B has a high mobility in the soil matrix. The elements K and Mg show a similar behaviour in the multiple regression. Because mineral waters are not of juvenile but largely of meteoric origins, from a hydrogeologic view, it will be only a question of time until the surface/groundwater circle is closed for these anthropogenic pollutions. This question will demand increased attention and research in the future.

¹In the context of this research work the term „mineral water“ comprises natural spa, mineral and table waters.