STREAM PH DRIVERS IN THE HUMID, SUBTROPICAL HEADWATERS OF THE MULBERRY RIVER, ARKANSAS, USA

The Mulberry River is a U.S. federally listed Wild and Scenic River located in the southwestern Ozarks region in the state of Arkansas (USA). However, the Mulberry River is also designated as an impaired waterbody in the headwater reaches due to periodically low stream pH (pH < 5.5). Low pH can negatively impact aquatic ecosystems within streams, but drivers of stream pH across space and time are often difficult to discern and vary through time. Here, we have analyzed measured stream pH from both tributaries and river locations of the headwater reaches of the Mulberry River watershed collected from March 2015 to June 2022 including estimation of total dissolved inorganic carbon (DIC) and related DIC species of H₂CO₃, HCO₃, and CO₃ from acid neutralizing capacity (ANC) titrations. Notably, we accounted for both low ionic strength and dissolved organic carbon (DOC) for individual pH measurements, which can affect DIC estimations from ANC titrations. After accounting for low ionic strength and alkalinity from DOC, H₂CO₃ concentrations were found to consistently explain over 60% of pH variation among river and tributary sample locations. Based on this relation, we inferred that stream pH and pH-H₂CO₃ relations are likely driven by the combination of: 1, H₂CO₃ production, specifically from soil respiration contributing H₂CO₃ to baseflow; 2, variable dilution of H₂CO₃ in baseflow from excessive infiltration or dilution of high H₂CO₃, low pH baseflow by runoff; and 3, variable contributions of ANC in tributaries. We hypothesize that similar pH and pH-H₂CO₃ patterns and controls are likely be present in other streams with siliciclastic dominated watershed lithology in the western Arkansas, across the Southeast U.S. and other humid, siliciclastic dominated catchments, globally, and warrant further investigation.