Spatiotemporal Variability of Soil Water δ18O and δ2H Reveals Hydrological Processes in Two Floodplain Soils

The movement of water through soil is preferential and heterogeneous. Subsurface interactions between mobile flows and the soil matrix are not uniform and are therefore difficult to predict through time and space. The use of stable isotopes of hydrogen (2H) and oxygen (18O) as conservative tracers of water movement is improving understanding of soil hydrological processes, yet field-scale observations of isotopic variability remain scarce despite implications for identifying dominant hydrologic processes. We sampled two adjacent soils at a ridge-swale topography floodplain forest to determine soil water isotopic variability at a 20 cm depth resolution in soils of differing textures and structures. Soil water isotopic composition measured through direct vapor equilibration varied more in time and space than any mobile source water sampled including throughfall, groundwater, ponded water within the swale, and free water from boreholes. Repeated, replicate soil borings within a rolling 1 m2 area indicated that soil water isotopic variance by depth was greater in coarser textured ridge soil than in the heavily structured, cohesive clay soil in the swale. Soil water isotopic variance among samples of the same depth and date was not apparently related to soil texture, organic content, or water content. Instead, the timing of soil water isotopic variance seemed to be related to soil structural change as it relates to seasonal hydrologic processes and site topography. Three isotopic mixing models using isotopic composition of throughfall and pre-event soil water were tested to conceptualize the soil water recharge regime at each site. These models represented ideal scenarios of complete bypass flow in which there was no exchange between event water and the soil matrix (test of Two Water Worlds hypothesis), complete replacement by event water (test of complete translatory flow) and conservative mixing between antecedent soil water and event water. At the profile scale, swale soil water was best modeled as rapid replacement by event water and ridge soil water was best modeled as conservative mixing between event water and antecedent soil water. However, variance within the profile of the ridge site showed that conservative mixing was a poor physical model and isotopic variance of soil water was much higher than could be predicted using mean soil water and throughfall isotopic composition alone. These results illustrate that temporal frequency and spatial scale of sampling affect inferences as to what factors control isotopic variability within the critical zone