Stable isotope patterns and dynamics in a sandy urban aquifer — The Brunswick Aquifer in Northern Germany

Sustainable drinking water supply from groundwater reservoirs in urban areas is becoming increasingly important against the backdrop of global change. Knowledge of groundwater recharge processes is an important prerequisite for future-oriented resource management. The sandy aquifer beneath the northern German city of Braunschweig (Brunswick), with a population of 250,000, is used for the production of 500,000 m³ of drinking water per year for municipal water supply. The aquifer is an unusual  example of a drinking water resource, as it is characterized by a high proportion of sealed surfaces within the densely built-up urban area. By analyzing 114 groundwater samples (from 92 monitoring wells) and 47 river water samples for stable water isotopes (δ¹⁸O, δ²H), we aimed to identify patterns in isotopic compositions to gain insights into groundwater recharge processes.

Despite the small study area (40 km²), the groundwater from the Brunswick aquifer exhibited a fairly high heterogeneity in its isotopic composition. δ²H values for groundwater samples ranged from –60.4 ‰ to –41.3 ‰ (median –57.0 ‰), and δ¹⁸O values from –8.7 ‰ to –4.7 ‰ (median –8.1 ‰). Groundwater and surface water were isotopically similar, precluding the quantification of recharge from river infiltration. Compared to the Local Meteoric Water Line (LMWL) for Hannover, many of the well waters exhibit lower deuterium excess, indicating an evaporation influence before or during recharge. The shallow groundwater (< 15 m below ground level) at seven wells showed little seasonal variation in isotope composition, especially for δ²H in some wells. Deeper groundwater presented more negative δ-values, suggesting older groundwater and cooler recharge temperatures. A comparison with amount-weighted annual mean δ‐values of precipitation in Hannover (50 km distance) indicates that mainly winter precipitation (November to April) dominates recharge. The low isotope variability of shallow groundwater confirms that infiltrating precipitation contributes to groundwater recharge in heavily sealed urban areas. Further investigations need to quantify these findings and to validate groundwater recharge calculations derived by water balance modeling.