VADOSE ZONE ISOTOPIC MONITORING WITH A LARGE WEIGHING LYSIMETER. NORTH CHINA PLAIN

Interpretations of δ¹⁸O and δ ²H in groundwater and the unsaturated zone based on comparison with atmospheric input signatures and/or evaporative fractionation can provide useful insights into drainage and groundwater recharge processes. However, root zone hydrologic processes including preferential flow and root uptake can alter isotopic signatures relative to precipitation and complicate isotopic boundary conditions for groundwater. In this study, lysimeter leachates were collected daily for 23 months (2009-2011) in order to assess isotopic trends in sub-root zone drainage. The primary motivation was to better understand the patterns of isotopic variability of moisture entering the portion of the unsaturated zone beneath the root zone (e.g. “deep drainage”). Few isotopic studies using weighing lysimeter leachates have previously been reported. There is a tradeoff with core sampling approaches in that information on vertical variability is lacking but temporal resolution is high (under our experimental conditions, drainage is funneled into sampling receptacle when it reaches 2 m depth, approximately the base of the root zone). The study was undertaken at the Luancheng agricultural experimental station in the piedmont plain section of the North China Plain. There is high interest regarding agriculturally-affected water cycles in this area because of severe rates of groundwater depletion currently underway.

Isotopic results demonstrate that moisture draining beneath the root zone does not conform to either a simple bucket model or to piston flow. Although the total variance of isotopic values over the monitoring period can be attributed to meteoric controls, isotopic variability exceeding analytical error is found on weekly timescales, mostly likely resulting from preferential flow. Volume-weighted mean isotope values and D-excess values both deviate from precipitation, which should be taken into account in hydrologic and paleoenvironmental studies in the region. Consistent with earlier studies using cores, δ ²H /δ¹⁸O slopes are found to be significantly lower than precipitation (~ 2-3) because of fractionation during pore water evaporation. Seasonal variability of δ ²H /δ¹⁸O slopes is attributed to seasonal temperature and soil moisture conditions affecting kinetic fractionation factors.