资源环境科技发展态势分析平台

Knowing the age distribution of water abstracted from public water supply wells helps to ensure customer trust in drinking water sources and underpin predictions of water quality evolution. We sampled the mixed water of 39 large public supply well fields for major ion chemistry, ³H, ³He, ¹⁸O, ²H, ¹⁴C_DIC, ¹³C_DIC and noble gases and determined the Noble Gas Temperature (NGT). We used a discrete travel time distribution model to quantify the age distributions using ³H, ⁴He, ¹⁴C apparent age and NGT as the 4 distinctive tracers. Helium-4 and NGT provided information on the older part of the age distributions and showed that the ¹⁴C apparent ages are often the result of mixing of waters ranging between 2,000 and 35,000 years old, instead of being discrete ages with a limited variance as previously assumed. The results reveal a large range of age distributions, comprising vulnerable well fields with >60% young water (<100 years) and well fields with >30% paleo groundwater (> 25,000 years) and all forms of intermediate distributions. The age distributions match the hydrogeological setting; well fields with age distributions skewed towards older ages appear in the Roer Valley Graben structure, where fluvial and marine aquitards and sealed faults provide protection from recent recharge. Waters from this graben exhibit paleoclimate signals, with a clear relation between NGT (range 3.2 to 9.3 °C), ⁴He (up to 3.3x10^-6 ccSTP/g) and δ¹⁸O (range -8.5 to -5.9 ‰_VSMOW). Moreover, ³He/⁴He ratios of these graben waters suggest a certain influx of He from mantle origin.

This article is protected by copyright. All rights reserved.