Spatiotemporal patterns of pH related to streamflow variability, drought conditions, and bedrock lithology in acid sensitive streams within a humid, subtropical catchment: Mulberry River, Arkansas, USA

Mulberry River, Arkansas, USA Low alkalinity streams within siliciclastic dominated catchments like the Mulberry River in the south-central United States experience episodic acidification. Since 2009, reaches of the Mulberry River have been classified as impaired due to low pH. We hypothesize that if pH time series patterns are similar among tributaries, external watershed factors like seasonal weather may be related to low stream pH. We investigated pH variability in the Mulberry River watershed including changepoint detection analysis. Patterns of pH and changepoints were compared with streamflow variability and drought conditions using Palmer Drought Severity Index (PDSI). Changes in pH were near synchronous and similar magnitude across the study area, despite observed increases in median tributary pH from east to west. From the comparison of pH time series and PDSI, low pH values tended to occur during more negative PDSI, i.e. drier periods. The specific mechanisms driving low pH variability remain elusive, but we infer low stream pH is influenced, in part, by seasonal weather variability leading to drier watershed conditions and lower pH. The spatial variability of median pH values from east to west, though, are more related to spatial variability in bedrock lithology. We hypothesize other acid-sensitive streams in the region likely share similar pH behavior and with low pH occurring during drier periods. • CPOP provided a changepoint analysis method for unevenly sampled pH time series. • Changepoint and spatiotemporal analysis revealed near synchronous pH changes. • General pH-PDSI relations reveal low pH related to drier watershed conditions. • East-west pH increases likely driven by watershed lithology spatial variability. • Other low TDS streams in Arkansas and Oklahoma may show general pH-PDSI relations.