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

Abstract

Soil water budgets in floodplains are distinct from uplands because there are more potential sources of water yet remain poorly understood and poorly represented in the empirical literature. Stable isotopes of hydrogen (²H) and oxygen (¹⁸O) in water are useful as tracers of water movement and have improved conceptual understanding of soil hydrological processes. We sampled two adjacent microsites at a ridge-swale sequence in a forested floodplain Louisiana, USA to determine temporal and spatial soil water isotopic variability in soils of similar climate and ecosystem but contrasting textures and hydrological processes. Repeated soil borings indicated that soil water isotopic variability was greater than any source water sampled (range of δ¹⁸O (‰): −11.9 to −0.9, range of δ²H (‰): −44 to 0; n = 783) including throughfall (range of δ¹⁸O (‰): −7.6 to +4.7, range of δ²H (‰): −40 to +11; n = 162), groundwater (range of δ¹⁸O (‰): −6.2 to 0, range of δ²H (‰): −31 to −7; n = 39), ponded water within the swale (range of δ¹⁸O (‰): −6.3 to +0.2, range of δ²H (‰): −33 to −4; n = 16), and free water from boreholes (range of δ¹⁸O (‰): −21 to +5, range of δ²H (‰): −4.5 to +5.8; n = 64). Soil water isotopic variability among samples of the same depth and date was not related to soil texture, organic content, or water content, but was instead likely related to seasonal wet up. Soil water isotopic mixing models indicated soil water recharge regimes varied by microsite: Swale soils were dominated by replacement of event water and ridge soils were dominated by conservative mixing at the profile scale. The isotopic composition of bound water was less variable spatially within structured clay soil than it was in coarser textured soil (average difference in s² 0.241 for δ¹⁸O and 2.6 for δ²H) apparently because the microtopography of the site allowed for greater inundation and more persistent source waters among swale soils.