We work at the interface of energy, water, and environment. Our team focuses on a wide range of topics where hydrological and biogeochemical coupling between flow, transport, and reaction processes play a key role in determining surface and subsurface system dynamics. We work around a couple of major themes.

1. Developing predictive understanding of hydrobiogeochemical coupling at the watershed scale

Watersheds are the fundamental earth surface unit that supports river networks, the blood vessels of Earth’s skin. Watersheds partition water, mass, and energy into distinct compartments (atmosphere, trees, soil, groundwater aquifers) through a complex suite of hydrobiogeochemical processes that mobilize key elements (e.g., C, N, P, metals) into rivers, the fluxes of which govern water quality and support aquatic ecosystems downstream. These hydrobiogeochemical interactions are tightly and nonlinearly coupled through thermodynamic and kinetic laws of physics, chemistry, and biology. They are Earth’s hydrobiogeochemical complex systems that respond and adapt to changes in external forcings, maintaining a clement environment for life on Earth. These complex processes also govern contaminant spreading from non-point sources and regulate natural water quality and quantity. The Li group has recently developed modeling capabilities (RT-Flux-PIHM) that have enabled us to understand and predict such complex process coupling in pristine and human-impacted watersheds. Two projects are ongoing in this theme.

image Figure 1. A schematic representation of processes in different modules (different colors) of RT-Flux-PIHM. The model allows systematic understanding of coupled processes at the local grid and watershed scales.

1) Predictive Understanding of Metal Export from Coal Creek, Colorado

Coal Creek watershed (CO) is contaminated with metals from historical mining activities. Preliminary data at the site have indicated strong connections between metals such as Cd and Zn and dissolved organic carbon (DOC). In other words, in the high elevation Coal Creek watershed, most metal export occurs during the high flow - spring melt period. This project is set to develop predictive understanding of hydrology-driven soil carbon decomposition and metal export from Coal Creek, CO, which provide the drinking water resources for the town of Crested Butte, CO. Such understanding and predictive capabilities are important not only for Coal Creek, CO, but also for other human-impacted watersheds including those in Appalachian Basin that extends thousands of miles. Human activities including historical mining and current natural gas production have threatened water resources that are important for tens of million people. Predicting capabilities are also essential in understanding the responses of these contaminated watersheds to changing climate. We acknowledge the support by the Department of Energy Subsurface Biogeochemistry Research Program.

2) Using the Susquehanna - Shale Hills CZO to Project from the Geological Past to the Anthropocene Future

This project is part of the Critical Zone Observatory (CZO) program supported by NSF (http://criticalzone.org/national/). Our goal in this project is to understand watershed hydrogeochemical process coupling and how such coupling control the export of solute, including non-reactive tracer, geogenic species derived from chemical weathering, and biogeochemical relevant species such as organic C and nutrients (N, P) into aquatic systems. Predictive understanding gained here are important to project the response of watersheds to external perturbation, including contamination events, and external forcings, including the changing climate. Two manuscripts under this project are currently in review.

2. Understanding the role of spatial heterogeneities in determining reactive transport in the natural subsurface

Nature is ubiquitously heterogeneous. Preferential flow path, macro pores, and low-permeability clay-rich zones abound in nature. The spatial distribution patterns of these physical (water-conductive) and chemical (reactive) characteristics determine the interactions among water, rock, and biological components (e.g., bacteria, fungi, roots), as well as the natural attenuation and remediation of contaminants. Understanding the role of spatial heterogeneities in solute reactive transport in the subsurface remains a central theme for the Li reactive transport group. We have published work on the role of spatial heterogeneities in affecting:

1) Contaminant bioremediation

(see Li et al., 2009, 2010, 2011, and Bao et al., 2014 in publication page) image

Figure 2. Predicted spatial and temporal evolution of uranium precipitates during bioremediation at Rifle, Colorado.

2) Natural attenuation of Marcellus Shale waters

(see Cai and Li, 2017, and Cai and Li, in preparation) image

Figure 3. Set up of 2D flow cell for exploring natural attenuation of leaked MArcellus Shale water into natural aquifers

3) Water-rock interactions

(see Salehikhoo et al., 2013, 2014; Salehikhoo and Li, 2015 on publication page)
Figure 4. Introducing “reactive interface”. Here we compare profiles of effluent Mg(II) concentrations and local rate from a homogeneous column with evenly distributed magnesite and quartz to those in a One-zone column with a cylindrical magnesite zone. Data and RTM output shows that the One-zone column-scale rates are largely controlled by the “reactive interface” that sits at the immediate vicinity of magnesite-quartz boundary and dissolves at rates orders of magnitude higher than the rest of the domain (Li et al., 2014; Salehikhoo and Li, 2015). Understanding reactive interfaces is important for chemical weathering, ecosystem functioning, and biogeochemical elemental cycling. image

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