Tracing effects of decalcification on solute sources in a shallow groundwater aquifer, NW Germany

delta Sr-87 values and Ca/Sr ratios were employed to quantify solute inputs from atmospheric and lithogenic sources to a catchment in NW Germany. The aquifer consists primarily of unconsolidated Pleistocene eolian and fluviatile deposits predominated by >90% quartz sand. Accessory minerals include feldspar, glauconite, and mica, as well as disperse calcium carbonate in deeper levels. Decalcification of near-surface sediment induces groundwater pH values up to 4.4 that lead to enhanced silicate weathering. Consequently, low mineralized Ca-Na-Cl- and Ca-Cl-groundwater types are common in shallow depths, while in deeper located calcareous sediment Ca-HCO3-type groundwater prevails. delta Sr-87 values and Ca/Sr ratios of the dissolved pool range from 7.3 to -2.6 and 88 to 493, respectively. Positive delta Sr-87 Values and low Ca/Sr ratios indicate enhanced feldspar dissolution in shallow depths of less than 20 m below soil surface (BSS), while equilibrium with calcite governs negative delta Sr-87 values and elevated Ca/Sr ratios in deep groundwater (>30 m BSS). Both positive and negative delta Sr-87 values are evolved in intermediate depths (20-30 m BSS). For groundwater that is undersaturated with respect to calcite, atmospheric supplies range from 4% to 20%, while feldspar-weathering accounts for 8-26% and calcium carbonate for 62-90% of dissolved Sr2+. In contrast, more than 95% of Sr2+ is derived by calcium carbonate and less than 5% by feldspar dissolution in Ca-HCO3-type groundwater. The surprisingly high content of carbonate-derived Sr2+ in groundwater of the decalcified portion of the aquifer may account for considerable contributions from Ca-containing fertilizers. Complementary tritium analyses show that equilibrium with calcite is restricted to old groundwater sources. (C) 2009 Elsevier B.V. All rights reserved.