Chapter 12 Association of dissolved organic carbon with stream discharge and dissolved metals concentrations in black shale-draining streams

Abstract

Abstract Watershed characteristics influence the concentrations of dissolved organic carbon (DOC) that affects the levels, fate, partitioning, bioavailability, and toxicity of dissolved metals in aquatic ecosystems. Despite increasing interest in black shales research, there are limited data relating influence of bedrock type upon DOC and dissolved metals in streams whose watersheds are dominated by black shales. Four streams in North-Central Arkansas, USA, were selected for study, three of which (Trace Creek, Begley Creek, and Cove Creek) drain black shales-dominated watersheds. A fourth nearby stream (Mill Creek) that flows over a geologically distinct limestone bedrock, was selected as a reference site to allow comparisons between these watersheds and the assessment of the role of bedrock lithology on DOC and metal concentrations. Spatial and temporal variations in DOC and dissolved metals among stream sites were determined. Streams draining black shales had significantly higher conductivity, and lower pH than the stream draining limestone. Dissolved metal concentrations at all sites were highest in June 2003 and lowest in September 2003, which coincided with a stormflow and a baseflow, respectively. The concentrations of DOC and metals (e.g. Cd, Cu, Hg, Ni, Pb, and Zn) in water collected from Trace, Begley, and Cove creeks were significantly higher than those of Mill Creek. Metals in streams draining black shales whose concentrations approached or exceeded US EPA's surface water criteria included Cr, Cu, Hg, Pb, and Se. A linear regression analysis of DOC with discharge as the predictor variable showed that variations in DOC concentrations were strongly explained by stream discharge with r 2 -values ranged from 0.85 to 0.95, whereas DOC was inversely related to conductivity ( r 2 =0.47–0.89). The study also revealed that bedrock type was a significant predictor of the observed spatial variations in DOC. Streams draining black shale watersheds transported significantly higher ( p