SEASONAL HYDROGEOCHEMICAL LINKS AMONG BASEFLOW, SAPROLITE-HOSTED GROUNDWATER, AND WEATHERING DEPTH IN SMALL WATERSHEDS AT A CRITICAL ZONE RESEARCH SITE IN THE NORTH CAROLINA PIEDMONT

Mountainous regions with thin soils and steep slopes have received significant attention in watershed-scale critical zone studies, yet less is understood about watersheds formed on crystalline bedrock weathering profiles in the southern Piedmont terrane, with their thick soils and relatively gentle slopes. The objective of this study is to examine watershed-scale critical zone (CZ) function, its relationship to stream and groundwater chemistry, and to assess landscape curvature (i.e., hilltop/convex vs riparian/concave) as a potential CZ factor. These concepts are examined at the new (2019-) Redlair Observatory (RO), located in Gaston County, North Carolina. RO’s geology consists of felsic to mafic metavolcanic rocks of the Charlotte Terrane. These rocks have weathered to form a thick saprolite (>21 m b.l.s). We instrumented three 1st-order watersheds in agricultural to forested land use with 17 saprolite wells from 4-25 m depth.

In 2019-2020, groundwater depths (GWD) were highly variable seasonally (ranges of 1.6-3.1 m for the shallowest well depth completion and 10.7-12.5 m for the deepest). GWDs seem to have risen slightly earlier in the winter season in the shallow wells and fallen earlier in the summer season near hilltop areas. Depth to water was generally greater in the deeper wells, especially those in the more forested mid-slope/riparian regions. Preliminary analysis of groundwater chemistry shows a lower Ca/Na ratio in the shallow wells than the deep wells (overall range 0.12-1.64). This relationship implies shallow wells (lower Ca/Na) are dominated by Na-plagioclase weathering in the highly weathered yet shallow saprolite, while deeper wells (higher Ca/Na) are influenced by Ca-plagioclase weathering in the less weathered, deeper saprolite. In general, available stream baseflow Ca/Na ratios are intermediate between shallow and deep saprolite, suggesting mixing in terms of weathering depths that contribute to streams. Finally, preliminary water isotope (δD, δ¹⁸O) analysis shows no pattern of seasonality within the three watersheds. This suggests that subsurface waters are well mixed on a seasonal time scale in these small forested watersheds and/or that precipitation isotopic composition is not variable enough to record seasonality. Work in progress includes a 4-season synthesis of water chemistry across the concave and convex landscapes at RO.