Characterizing the recharge regime of the strongly exploited aquifers of the North China Plain by environmental tracers

We employed environmental tracers (H-3-He-3, noble gases, stable isotopes O-18 and H-2) 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 H-3-He-3 ages of less than 40 years. H-3-He-3 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.