The Science

The hyporheic zone (HZ) is a subsurface environment where groundwater and river water mix, causing various physical, chemical, and microbial processes to occur simultaneously. Dynamic river flow conditions can lead to fluctuations in the delivery of oxygen and organic carbon to the HZ, thereby impacting biogeochemical processes and contaminant transport. Despite their importance, these impacts are currently poorly understood. Laboratory experiments were designed to evaluate the effects of variable oxygen availability, mimicking the effects of dynamic river and groundwater mixing processes. The results were used to develop a numerical model that was used to evaluate the relative importance of hydrologic (water flow) and biogeochemical (material transformation) processes on contaminant (chromium, Cr) migration and to predict the effectiveness of the HZ as a barrier to Cr transport into the river environment.

The Impact

Dynamic hydro-biogeochemical conditions are especially prevalent in highly managed waterways such as rivers controlled by hydroelectric dams. This study provides important insights into the role of the HZ as a region of transition between biological communities for processing contaminants, as well as a natural redox barrier for immobilizing (sequestering) chromium under dynamic hydrological conditions. These results can support a holistic approach to river and groundwater management.

Summary

Hydrological, geochemical, and biogeochemical processes affect the supply, delivery, mixing, and residence times of microbes, chemicals, and other organisms that meet within the HZ. Until now, the impacts of hydrodynamic processes in rapidly changing river flow conditions have been poorly understood.

In this study, researchers from the Pacific Northwest National Laboratory, the China University of Geosciences, and the Southern University of Science and Technology in China used sediment samples from the Columbia River HZ in the U.S. Department of Energy’s Hanford 300 Area, which is located downstream of several chromium contaminant plumes. Chromium is a common contaminant in soils, sediments, surface water, and groundwater. In low concentrations, it is a human nutrient, but at higher concentrations it can be toxic, depending in part on its chemical form.

Researchers performed laboratory experiments to derive biogeochemical kinetic models, which were then incorporated into a reactive transport model to simulate chromium, iron (Fe), oxygen, and organic carbon interactions under field hydrological conditions—that is, those conditions in the Columbia River’s HZ zone. The modeling results were used to assess chromium reductive immobilization in the HZ and to estimate the rate of high-concentration chromium discharge to the Columbia River. The combined experimental and modeling results highlight the importance of Fe(II) regeneration during anoxic periods to chromium immobilization. This in turn is highly dependent on the availability of organic carbon, which can be reintroduced into the HZ by intrusion of river water containing particulate and/or dissolved organic matter. Without this dynamic process, organic matter can be fully consumed leading to loss of chromium reductive capacity, but incorporating this process into the model predicts sustainable reduction of chromium in the HZ, limiting chromium movement into the river environment.

Looking to the future, long-term monitoring systems are needed to evaluate the applicability of the new reactive transport model. The model, initial, and boundary conditions developed in this study are based on the Hanford HZ, but they can be readily adapted for other sites.

Principal Investigator

Tim Scheibe
Pacific Northwest National Laboratory
Tim.Scheibe@pnnl.gov

Program Manager

Paul Bayer
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
paul.bayer@science.doe.gov

Funding

This research is supported as part of the Subsurface Biogeochemical Research (SBR) program of the Office of Biological and Environmental Research (BER), within the U.S. Department of Energy Office of Science, through the Pacific Northwest National Laboratory SBR Science Focus Area (SFA) project. F. Xu and C. Liu also acknowledge support from Research Funds from the National Natural Science Foundation of China and China Postdoctoral Science Foundation.

References