Spatio-temporal variations of δ2H and δ18O in precipitation and shallow groundwater in the Hilly Loess Region of the Loess Plateau, China

Groundwater is of utmost significance to socio-economic development and ecological recovery for the Loess Plateau. However, studies regarding the mechanism governing groundwater recharge over this area appear to be inadequate. This study is to examine the spatio-temporal variations of delta H-2 and delta O-18 in precipitation and shallow groundwater. On the basis of this, the mechanisms governing shallow groundwater recharge were explored. Precipitation and groundwater were sampled monthly from May to October during the period 2004-2006 at 13 sites in the Chabagou Catchment (187 km(2)). In the Caopingxigou Experimental Watershed (0.1 km(2)), meteorological variables were observed and rainfall larger than 5 mm was sampled immediately after each rain event. Across the area, 90% of the precipitation occurred from May to September primarily in the form of heavy rains or rainstorms with great spatial variability. There were about 30 localized rains in each year. It was indicated that there existed notable seasonality and pronounced spatial variability in precipitation isotopic compositions. Contributing factors and indications of isotopic compositions, as well as their climatic indications such as monsoon intensities and mixing processes of water vapor, were investigated. The delta H-2-delta O-18 relation of groundwater was found to be delta H-2 = 3.22 x delta O-18 - 38.1, deviating from the local meteoric water line delta H-2 = 7.57 x delta O-18 + 3.9. The range of delta values in groundwater is shrunken to be 15-21% of that in individual precipitations, and groundwater in the middle reaches shows a wider range of delta values. Isotopic results showed that groundwater originates from precipitation with hydrogen and oxygen isotopic compositions being -69 and -9.7aEuro degrees, respectively, and most groundwater experiences serious evaporation and adequate mixing with old water during infiltration or percolation in the aerated zone. It was also founded that obvious fluctuations of isotopic compositions in groundwater mainly appear in the middle reaches especially at sites that are close to valleys, suggesting varying sources of groundwater from precipitation, precipitation runoff, isotopically enriched surface water and/or lateral recharge of adjacent groundwater.