Marc Alperin’s work is focused on physical and biogeochemical processes in riverine, estuarine, and coastal marine sediments. The overall strategy is to combine field measurements, laboratory experiments, and numerical models to constrain carbon, nitrogen, sulfur, and metal cycling in a variety of biogeochemically active sedimentary environments. Field approaches used include concentration profiling, rate measurements using isotopic tracers, and natural stable- and radio-isotope distributions. Examples of specific projects are described briefly below:

• Impact of Sediment Processes on Estuarine Water Quality

Sediment-water interactions play an important role in regulating water quality in many estuaries. In these systems, the ratio of bed area to channel volume is high, the freshwater flushing time and sediment contact time are long, and a large portion of the organic matter produced in the water column settles to the sediment surface. Sediment processes influence the water column in three ways: (1) by serving as a long-term repository of oxygen demand, (2) by recycling fixed nitrogen (primarily ammonium) to the water column, and (3) by serving as a sink for nitrate via denitrification. We are conducting field measurements and long-term laboratory experiments to understand the controls on benthic oxygen and nutrient fluxes, sediment denitrification rates, and the response time of the sediment system to long term changes in nutrient loading.

• Benthic-boundary layer dynamics

We are developing a sediment process model that simulates interactions between the water column and sediment. The model is designed as a module for coupling with a pelagic water quality model, but can also function in the stand-alone mode. Emphasis has been placed on capturing the details of the sediment-water interface—in particular, the diffusive boundary layer—in order to accurately simulate the impact of bottom water currents and concentrations on sediment processes and benthic fluxes. The sediment process model provides a tool for making quantitative predictions as to how sediment oxygen demand, benthic ammonium flux, and denitrification rates will respond to a legislated reduction in nitrogen loading. The modeling project involves collaborations with members of the Computer Sciences Department at UNC-Chapel Hill as well as the Engineering Department at UNC-Charlotte.

• Geomicrobiological Models

Micro-scale reaction-transport models (also known as “geomicrobiological models”) are ideal for understanding the “thermodynamic ecology” of microbial symbionts. These models are useful for testing specific hypotheses, checking for internal consistency in field data, and can guide the optimization of experimental protocols. We have been working on a micro-scale model of anaerobic methane oxidation. Novel aspects of our high-resolution (~0.1 nanometer) model include: implicit coupling between reaction-transport physics and thermodynamic energy yield; realistic geometry of microbial aggregates; and differential transport kinetics and metabolic functions for microbial cell components such as lipid membrane, cell wall, and cytosol. See Figure 3.9.


• Water quality patterns in urban streams

We are collaborating with faculty from the Geography Department to investigate water quality patterns in the major stream network draining the UNC campus. This area is heavily


developed with a dense academic core, major athletic facilities and a large biomedical and hospital complex. We are particularly interested in establishing nutrient source areas as they vary through time and space with wetness conditions and groundwater levels in areas of different ground cover, land management and development activities, and retention processes resulting from sediment/water interactions. Information generated through the project is being made publicly available through web sites and other outlets, and the project will effectively help to establish the campus as an outdoor laboratory.



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