No abstract

[1] We employed environmental tracers ( 3 H-3 He, noble gases, stable isotopes 18 O and 2 H) to study groundwater recharge and residence times in the strongly exploited North China Plain aquifer system in the area of Shijiazhuang, the capital of the Hebei province. Groundwater in the unconfined parts of the piedmont plain contains tritium down to depths of about 100 m and exhibits 3 H-3 He ages of less than 40 years. 3 H-3 He ages correlate well with sampling depth but less so with distance along a transect from the mountains in the west through the recharge area in the piedmont plain, indicating a minor role of lateral inflow compared to spatially distributed recharge. The increase of groundwater age with increasing thickness of the saturated zone combined with the steady groundwater table descent over the last decades implies an effective recharge rate of around 0.3 m/yr, mostly due to infiltration of precipitation and irrigation water. Despite the recycling of irrigation water, a water balance deficit remains, causing depletion of the aquifers. Anthropogenic modifications of the natural recharge regime appear to be reflected by a trend of increasing stable isotope ratios in groundwater of the last decades.

Coupling between groundwater and surface water at Lake Willersinnweiher, a gravel pit lake in the Upper Rhine Graben without any surface in-or outfl ow, was investigated using both a groundwater model and the tracers 18 O and SF 6 . Based on groundwater modeling, recharge and discharge areas around the lake as well as the residence time of the lake water were determined in a regional context. The uncertainty of the simulated fl ow fi eld was assessed by sensitivity analysis. The tracers 18 O and SF 6 were measured in several observation wells and piezometers around the lake as well as in the lake's water column. They were used to verify groundwater fl ow directions found in the modeling. We found that the groundwater-lake interaction model had a large uncertainty even though relatively detailed information on input data was available. Independent information obtained from the environmental tracers allowed us to improve and verify the model.

We employed environmental tracers ((3)H-(3)He, SF(6)) in a study investigating the groundwater recharge in the North China Plain (NCP), a sedimentary aquifer system consisting of fluvial and alluvial river deposits near the city of Shijiazhuang. The (3)H-(3)He dating method revealed reasonable results for the young groundwater with ages covering the range of recent to ~40 a. SF(6) samples were taken in parallel for independent dating and to compare the applicability of both methods. However, the SF(6)-results are influenced and, in part, dominated by a systematic non-atmospheric component, revealing that the dating with SF(6) is unreliable in this region. A correlation of non-atmospheric SF(6) and (3)H-(3)He ages suggests a continuous accumulation of natural SF(6) in the groundwater of the NCP aquifers. Although terrigenic SF(6) has previously been associated with crystalline or igneous rocks, our results indicate that it can also be accumulated in sandy aquifers on the timescale relevant for SF(6) dating.