RIVER CHEMISTRY OF THE SCIOTO, MIAMI, AND MUSKINGUM WATERSHEDS IN OHIO, USA

The Ohio River watershed is one of the six major basins of the Mississippi River, and over half of the state of Ohio drains into the Ohio River. To investigate the chemistry of Ohio’s rivers that flow into the Mississippi we sampled in three major watersheds that drain into the Ohio River and characterized them by lithology, land use, and urban influence. We identify major dissolved constituents in relation to watershed characteristics and describe concentration-discharge (c-Q) relationships of major ions and nutrients in the Scioto, Miami, and Muskingum watersheds. A total of 96 surface water samples were collected from 16 rivers in August 2020 (n=24), May to June 2021 (n=37) and September 2021 (n=35), and we compiled USGS Streamflow data from each sample day. We collected samples at higher flows (Scioto, Miami, Muskingum > Q50) in early summer and lower flows (<Q50) in late summer of each year. Rivers had high total dissolved solid (TDS) values, ranging between 218 and 754 mg l^-1. Differences in watershed geology were reflected in river chemistry. Sulfate (SO₄^2-) concentrations in the Scioto (169–1892 μM) and Muskingum (280–1468 μM) river systems both had a greater range than the Miami (86–564 μM) due to extensive shale bedrock within the sample drainage area. Miami watershed rivers had higher magnesium (Mg²⁺) concentrations (879–1699 μM) because dolostone bedrock is dominant in the upper watershed. The Scioto watershed, draining dolostone in its head waters, had a wide range of Mg²⁺ concentrations (324–2046 μM). Locations with high TDS values and elevated concentrations of Cl (>1000 μM), SO₄ (>1000 μM), NO₃+NO₂ (>300 μM), or PO₄ (>6 μM) were indicators of urban or agricultural input. We also observed variation in the c-Q relationship. Patterns in ion and nutrient enrichment and dilution were inconsistent among rivers, which we attributed to local natural (geological, land cover) and anthropogenic (agriculture, urbanization) factors. Nevertheless, we identified a common pattern of major ion dilution and nutrient enrichment in high flows. We present the relationships of river chemistry, geology, land use, and discharge of three major Ohio watersheds to identify the controlling environmental and anthropogenic variables of each watershed and discuss differences among these major tributaries of the Ohio River.