Has the Discrepancy between Laboratory and Field Weathering Rates Been Resolved?

Geoscientists interested in the topic of chemical weathering, the process by which the surface of the Earth is removed chemically (i.e., dissolved), have been mystified for the better part of thirty years by the apparent discrepancy between laboratory- and field-determined weathering rates. Geoscientists have observed that rates in the field may be three to five orders of magnitude slower than they are in laboratory experiments. Since chemical weathering is important in the long-term regulation of the greenhouse gas CO₂ (weathering reactions use up CO₂), for predictive purposes, identifying the controls on chemical weathering rates and why they might be different between lab and field is essential.  Now, Dr. Kate Maher of Stanford University, and co-workers Carl Steefel of the Earth Sciences Division at Lawrence Berkeley National Laboratory and Art White and David Stonestrom of the U.S. Geological Survey, have proposed that data from weathering profiles located near the city of Santa Cruz, California, provide confirmation of one of the explanations for the discrepancy.

Figure 1. Satellite photograph of weathering zones developed on marine terraces of differing ages (youngest profile is at the bottom of the photo near the Pacific Ocean, oldest is highest on the slope) near the city of Santa Cruz, California. The study of Maher et al. (2009) focused on Terrace 5 (red), the oldest profile at 226,000 years.

Maher and her co-workers analyzed both vertical mineral profiles and pore-water chemistry at the oldest of the weathering sites (226,000 years), using the reactive transport software CrunchFlow developed by ESD scientist Carl Steefel over many years at Berkeley and Lawrence Livermore National Laboratories. The software takes a mechanistic approach to simulating chemical weathering, incorporating rate constants for mineral dissolution and precipitation that are then rigorously coupled to rainwater flow through the weathering profile. The software can integrate the chemical weathering process over very long periods of time, even out to the 226,000 age of the oldest weathering profile at Santa Cruz (Figure 1). This mechanistic, coupled treatment of the weathering processes in and of itself represents a significant improvement over previous, much simpler uncoupled simulation approaches, and is able to resolve at least part of the discrepancy between lab and field rates.

More importantly, however, a hypothesis developed earlier while Dr. Maher was a doctoral student at UC Berkeley was confirmed with the modeling:  the rates of clay precipitation (crystallization) driven by the buildup of chemical constituents in the pore water of the weathering profile can control the overall rate of chemical weathering, by regulating how close to chemical equilibrium the system is with respect to the primary weathering phases, especially feldspar (the most abundant mineral in the Earth’s crust).  Recent laboratory studies have shown that feldspar dissolution (weathering) rates slow down significantly as chemical equilibrium is approached. The chemical weathering process therefore turns out to be a complex one, in which the overall weathering rate is controlled by a series of coupled chemical reactions, all of which are linked to water flow rates through the profile. When the slow clay precipitation rates (confirmed by the pore water chemistry at the Santa Cruz site) and the decrease in feldspar dissolution as chemical equilibrium is approached are accounted for in the mechanistic reactive transport model, the “discrepancy” largely disappears. Laboratory mineral dissolution (weathering) rates can then be used directly—which makes it possible to incorporate chemical weathering into global carbon cycle models over geological periods of time.

Figure 2: Reactive transport simulations (solid lines) of mineral profiles after 226,000 years of chemical weathering at Terrace 5, Santa Cruz. The simulations are able to match the observed profiles even while using laboratory-determined chemical weathering rates.

Funding from the Office of Science at the U.S. Department of Energy through the Geoscience Program in the Office of Basic Energy Sciences (Contract No. DE-AC02-05CH11231) is gratefully acknowledged.

Maher, K., C.I. Steefel, A.F. White, and D.A. Stonestrom, 2009, The role of reaction affinity and secondary minerals in regulating chemical weathering rates at the Santa Cruz Soil Chronosequence, California, Geochim. Cosmochim. Acta, 73, 2804–2831.

New Below-Ground Monitoring Method for Microbial Activity Validated at Colorado Site

Geophysical monitoring approach provides “x-ray” images of subsurface biogeochemical processes

Geophysics at Rifle site. Vertical shot: Dr. Kenneth H. Williams (LBNL) and graduate student, Adrian Flores Orozco (University of Bonn), collecting surface spectral induced polarization data at the Rifle Integrated Field Research Challenge site to detect and delineate regions of naturally elevated subsurface microbial activity.

Results: Scientists as superheroes? Well, maybe, at least in their ability to “see” through subsurface soil and rock, by using a new technique for monitoring groundwater contamination that eliminates the need to drill wells. Scientists recently performed the first field demonstration of a minimally invasive monitoring approach for tracking subsurface biogeochemical changes accompanying the bioreduction of a uranium-contaminated aquifer. Their results showed that the approach, called surface spectral-induced polarization (SSIP), is both feasible and practical for remote monitoring of microbial activity stimulated during microbiological reduction.

The SSIP approach lets scientists track geochemical and mineralogical changes that occur when electron donors, such as acetate, are added to groundwater to stimulate subsurface microbial activity. SSIP was developed by scientists from Lawrence Berkeley National Laboratory, Pacific Northwest National Laboratory, the University of Bonn in Germany, and the University of California, Berkeley. The field demonstration took place at a former uranium mill tailings site near Rifle, Colorado.

Why it matters: Groundwater contamination by industry and nuclear weapons programs has spurred research into the use of microorganisms to facilitate remediation by isolating aqueous metals and radionuclides in forms in which they can’t move. Much of the research has focused on microorganisms capable of immobilizing contaminants, such as uranium, after introducing organic carbon compounds, such as acetate, lactate and ethanol.

But understanding just how microorganisms alter their physical and chemical environment during bioremediation is hindered by the inability to adequately assess subsurface microbial activity over dimensions relevant to a field site, which can encompass areas and depths of tens to hundreds of meters. The SSIP field monitoring approach makes it possible to monitor the subsurface with very high spatial resolution-areas as small as 0.3 m-and without the need for groundwater wells.

“Similar to how non-invasive medical imaging has reduced the need for invasive, exploratory surgeries, geophysical monitoring techniques, such as SSIP, allow us to monitor large volumes of aquifer sediments without having to drill groundwater wells, which saves money and disturbs less land,” said project lead Dr. Kenneth Williams, LBNL. “These methods are well suited for assessing the activity and end products of stimulated microbial activity and their relationship to contaminant remediation over long periods of time without relying on conventional and labor-intensive sampling approaches.”

Scanning electron microscope image of filter residue obtained from groundwater pumped from a Rifle well. (B) High-resolution TEM image of individual cell and surface-associated precipitates. (C) Energy dispersive X-ray spectrum of precipitates in (A).

Methods: The research team used SSIP to monitor stimulated microbial activity in the Rifle aquifer while acetate is added. They injected variable frequency currents into the ground and measured resulting voltage potentials using electrodes embedded in the ground surface. The electrical response was dependent upon the predominant metabolic processes active in the subsurface at a given point in time.

The accumulation of mineral precipitates, such as iron sulfide, and electroactive ions, such as ferrous and hydrogen sulfide) altered the ability of fluids in the subsurface pore spaces to conduct electrical charge.  This accounted for the anomalous electrical response and revealed the usefulness of such measurements for monitoring mineralogical and geochemical changes accompanying subsurface bioremediation.

What’s next: SSIP may also be used to extend geochemical data from a few boreholes that provide valuable information about remediation effectiveness over large areas. This will thus require less monitoring while providing a high level of assurance that the remedial process is working as intended.

Future work will focus on collecting SSIP over a much wider range of frequencies (e.g. 0.05-500 Hz), testing the method for detecting naturally occurring zones of bioreduction, and specifying system requirements for widespread application.

Acknowledgments: Kenneth Williams, LBNL, leads geophysical monitoring research for the Rifle Integrated Field Research Challenge, which is part of the Environmental Remediation Science Program, Office of Biological and Environmental Research (BER), U.S. Department of Energy.

The research team includes Susan S. Hubbard, LBNL; Jennifer Druhan, and Jillian F. Banfield, UC-Berkeley; Evan Arntzen, Phil Long, Michael J. Wilkins and Lucie N’Guessan, PNNL; and Andreas Kemna, University of Bonn. Electron microscopy was carried out at the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by DOE-BER and located at PNNL.

Reference: Williams KH, A Kemna, MJ Wilkins, J Druhan, E Arntzen, AL N’Guessan, PE Long, SS Hubbard and JF Banfield. 2009. “Geophysical monitoring of coupled microbial and geochemical processes during stimulated subsurface bioremediation.” Environmental Science & Technology 43(17):6717-6723 DOI: 10.1021/es900855j.

Field Trip! Australia– Completion of Well

By:  MaryAnn Villavert

ESD Staffers returned from their field trip (in early October 2007) to Otway, Australia and have these photos to share.

Here’s what Barry Freifeld (Hydrogeology) had to say about their experience. “We have successfully landed the assembly in the borehole. The most amazing thing was lifting the entire 120 ft-long bottomhole assembly and dropping it in using a 70-to crane and a 15-ton crane to perform a two-crane-lift over the borehole.

It was a Herculean effort to get everything down in three days but the workover crews were great.

Every few hours a new weather front would come through and dump buckets of water down on us.”

Find out more about the project in Otway by reviewing to the GEO-SEQ Project located in the Geologic Carbon Sequestration Program website.