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

Authors: Blackstock, Joshua; Owens, Phillip; Moore, Philip; TORBENSON, MAX - Johannes Gutenberg University; Ashworth, Amanda; Anderson, Kelsey; BURGESS-CONFORTI, JASON - Bureau Of Land Management; Delhom, Christopher

Submitted to: Journal of Hydrology: Regional Studies
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/25/2024
Publication Date: N/A
Citation: N/A

Interpretive Summary: This study revealed numerous novel spatial and temporal patterns among tributary and river sites in the Upper Mulberry River watershed. Basin-wide changes in pH were discovered from changepoint analysis at multi-year timescales. This finding could suggest influences from climate variations that affect basin-wide watershed processes and stream chemistry. Ultimately, these findings provide framework for future monitoring. We note the correlations defined in this study have the applicability for estimating pH change in proximal watersheds within the region, as well. We also note the utility of the algorithm for changepoint analysis we used for unevenly sampled water quality data for other datasets. Meaning, the algorithm used presents a potentially useful tool in determining water quality trends from historical data in other regions often having unevenly sampled datasets.

Technical Abstract: The Mulberry River is a U.S. federally listed Wild and Scenic River. designated as impaired in the upper reaches under the U.S. Clean Water Act criterion stemming from low pH values. However, no analysis of spatial and temporal pH patterns or trends have been conducted, to-date. In this study, we analyzed pH measurements from both tributaries and river locations along the main stem of the upper reaches of the Mulberry River collected from March 2015 to June 2022. We assessed spatial and temporal pH variability by: 1, using pairwise sample comparisons of pH among tributaries and river sites; 2, quantifying longitudinal pH change, i.e. river pH slope, along river reach and potential long-term trends; 3, evaluating long-term pH trends at respective tributary sites; and 4, determining changepoints and assessing trends between changepoints for respective tributary pH time series. Results from our analyses revealed numerous novel spatial and temporal patterns among tributary and river sites in the Upper Mulberry River watershed. No significant long-term pH trends were present at tributary sites for the entire period of record, significant decreasing and increasing trends were present among selected changepoint segments. Near synchronous, basin-wide changes were discovered from the changepoint analysis at multi-year timescales and could suggest influences from hydroclimate variability that modulate basin-wide watershed processes affecting stream acid-base chemistry. Ultimately, these findings provide framework for future monitoring and potential applicability for estimating pH change in proximal watersheds within the region. We also note the utility of the CPOP algorithm for changepoint analysis of unevenly sampled water quality data. Given that water quality records in remote regions often constitute unevenly sampled datasets, CPOP presents a potentially useful tool in determining water quality trends from historical data up to through current monitoring programs.