Water Flowpath and Bedrock Geology Control Pyrite Weathering Across Spatiotemporal Scales

Abstract:

Pyrite, a ubiquitous iron sulfide mineral, oxidizes to produce sulfuric acid (H₂SO₄). Stream chemistry is often used to investigate pyrite oxidation because rivers integrate the byproducts of weathering. When studying larger river systems, distinguishing pyrite-derived sulfate is difficult because there are many sources of sulfate in streams (e.g., atmosphere, evaporites, agriculture). Here, we utilize stream chemistry and machine learning to distinguish pyrite-derived sulfate in Shavers Creek, the main stream in the Susquehanna Shale Hills Critical Zone Observatory (SSHCZO) in Central Pennsylvania. We also utilize public datasets from the Water Quality Portal (WQP) to upscale our study to the Susquehanna River Basin in order to evaluate pyrite oxidation on a broader spatial scale. Our machine learning model deciphered that in Shavers Creek and the Susquehanna River there are three main sources of riverine sulfate, which we attribute to acid rain, pyrite oxidation, and fertilizers. Downstream water chemistry from both basins show increasing sulfate concentrations with increased distance downstream and then a constant sulfate concentration after a certain distance. Additionally, we found that the spatial pattern in source contribution is similar between Shavers Creek and the Susquehanna River. In both systems, sulfate in the headwaters are predominantly acid rain-derived (i.e., >50%), which decreases to <25% at the river outlet. Moreover, we observe that across all sites, concentrations of pyrite-derived sulfate are highest in the summer and fall months and lowest in the winter and spring months. We interpret the spatial and temporal trends in pyrite weathering to be a product of lithology and flowpath. Moving downstream in Shavers Creek, the watershed transitions from a low-sulfide shale to a high-sulfide carbonate, which could explain the increase in pyrite-derived sulfate downstream. Temporally, we observe the highest concentrations of pyrite-derived sulfate during low flow, when the streamflow is sourced from deeper groundwater. Overall, the patterns in sulfate chemistry observed in Shavers Creek are also observed in the greater Susquehanna River, which might indicate that Shavers Creek in the SSHCZO is a good model for weathering processes taking place on a larger spatial scale.