Sr-87/Sr-86 ratios as a tracer for geochemical processes in an unconsolidated sediment aquifer (Liebenau, NW-Germany)

Groundwater of the Quaternary unconsolidated sediments in the catchment are a of the waterworks of Liebenau II (NW-Germany) is characterized by heterog eneous distributions of dissolved constituents, In order to determine sourc es of solutes and to estimate flowpaths, Sr isotope analyses were carried o ut on groundwater, precipitation, and soil samples. Sr-87/Sr-86 ratios of t he groundwater samples vary between 0.708 and 0.719 and allow to discrimina te four groundwater levels. The near-surface groundwater level is character ized by low Sr-87/Sr-86 ratios in the range of 0.7099 and 0.7130. At greate r depths Sr-87/Sr-86 ratios increase up to a value of 0.7187 in the interme diate groundwater level. Deep groundwaters which can be divided into two le vels exhibit a remarkable decrease of Sr-87/Sr-86 ratios. Groundwater of de ep level I shows Sr isotope ratios between 0.7118 and 0.7128, whereas deep level II shows Sr-87/Sr-86 ratios of 0.7082 to 0.7087. Differences in the Sr-87/Sr-86 ratios are the result of varying sources of dissolved strontium in the groundwater. These encompass mineral weathering, precipitation, and anthropogenic inputs. Low Sr-87/Sr-86 ratios of 0.710 t o 0.713 indicate the influence of precipitation and probably fertilizers on the near-surface groundwaters. At greater depths, groundwater chemistry is dominated by weathering processes. Due to significant differences in the S r-87/Sr-86 ratios the dissolution of silicate minerals can be distinguished from the dissolution of carbonate minerals. For the intermediate groundwat er level a dominant influence of silicate weathering is confirmed by radiog enic Sr-87/Sr-86 ratios between 0.713 and 0.719 and saturation indices < -3 .5 for calcite. In contrast, low Sr-87/Sr-86 ratios of 0.708 to 0.709 indic ate dissolution of carbonate minerals in the deep groundwater, where satura tion for calcite is observed. A schematic diagram of groundwater flowpaths basing on the analyzed Sr-87/S r-86 ratios, and geochemical parameter is displayed. The main aquifer is re presented by gravel-bearing sands which form the base of the Quaternary dep osits. A good agreement in the chemical and isotopic composition of groundw ater from wells EB9, EB7 and groundwater of the deep level I indicates the dominance of groundwater pumping from depths of 20 to 35 meters. Deep groun dwater levels are partially separated from the near-surface groundwater by intercalations of silt and clay lenses in the intermediate aquifer. These i ntercalations are characterized by a reduced permeability and longer reside nce time of groundwater. Vertical transport of solutes is partially reduced . This is confirmed by the larger variation of Sr-87/Sr-86 ratios in vertic al direction compared to the rather constant isotope ratios in horizontal f low direction.