Shale Hills, GRAD STUDENT
Scientists across disciplines have made concerted research efforts to predict the dominant geochemical controls on solute-sediment interactions. The physical and chemical properties of sediments are a relevant control on stream chemistry, particularly via the storage of organics, solid-aqueous phase interactions, and the creation of redox gradients that support diverse microbial metabolisms. However, the predictability of water-sediment dynamics is often complicated by human activities. For example, mining activities can enhance natural weathering rates by upwards of three orders of magnitude, and agricultural fertilizer can influence soil nutrient budgets by increasing nutrient concentration and mobility.
This dissertation evaluates the impacts of natural and anthropogenic processes on surface water-sediment interactions. In Chapter 2, I discuss how sediments in the hyporheic zone (e.g. the subsurface around a stream channel where shallow groundwater and surface water mix) facilitate the storage and exchange of essential nutrients in organo-mineral aggregates of a relatively pristine headwater stream. In Chapter 3, I discuss the impact of levee removal and floodplain-river reconnection on nitrogen cycling in a formerly agricultural watershed along the Cosumnes River, California. Microbial processes and nitrogen concentrations will change during flood events due to the rewetting of previously unsaturated soils. I found that flood type impacts the redistribution of carbon and fine-grained sediments across the floodplain, and thus creates the spatial framework that enables reduced and oxidized forms of N to coexist in small micro-environments within soils. Therefore, the amount of annual nitrogen loss or retention in the Cosumnes River floodplains depends on grain size distribution and nutrient availability in floodplain sediments. Finally, I use batch experiments to simulate how rain events and changes in mining practices could impact the pH and redox chemistry in channels downstream of a gold mine, with a focus on the interactions between arsenic and streambed sediments (Chapter 4). Results from these simulations indicate that arsenic is currently retained by sediments in neutral to high pH waste streams treated with lime. However, if the mine ceases to add lime and acid drainage conditions develop, then arsenic could be released from waste sediments via desorption and dissolution of hydrous ferric oxides.
The findings presented in this dissertation highlight the importance of streambed sediments as a regulator of stream chemistry in both pristine and human-impacted catchments. In each chapter, sediments are found to influence the concentration, form, and timing of solutes that are transported to downstream aquatic ecosystems and water resources.
Hoagland, Nell Elizabeth (2018): SURFACE WATER-SEDIMENT INTERACTIONS: EXAMPLES FROM PRISTINE AND CONTAMINATED CATCHMENTS. Doctor of Philosophy, Geosciences and Biogeochemistry, The Pennsylvania State University, p. 270.
This Paper/Book acknowledges NSF CZO grant support.