In this presentation, some of the possible explications for the laboratory-field rate discrepancy are briefly analyzed.  In years past, the general approach was to develop a lengthy “laundry list” of explanations for the discrepancy, and in the end simply state the rates were different, and perhaps irreconcilable.  But over the past 10 years, the range of possible explanations has narrowed considerably.  We argue here that this is due in part to the use of modern reactive transport software that allows one to consider field rates in a fully dynamic context, including explicitly the role of flow and/or molecular diffusion, the development of reaction fronts, and physical and chemical heterogeneity.  In some cases, the use of a reactive transport approach has reduced the so-called “discrepancy” by close to an order of magnitude, particularly where the geochemical system approaches equilibrium. <br/> <br/>At circumneutral pH where incongruent weathering dominates, Zhu et al (2004), Maher et al (2006), and Maher et al (2009) have demonstrated the importance of slow clay precipitation in regulating the approach to chemical equilibrium along a flow path.  When combined with nonlinear dissolution kinetics, as in Maher et al (2009), most of the lab-field discrepancy can be eliminated in relatively porous, uncemented reactive sands like those found in the Santa Cruz chronosequence.  In other words, the lab rates can be used directly.  At lower pH where incongruent reactions are less important, the lab-field discrepancy either disappears, or if present, can be traced to the role of physical and chemical heterogeneity.  In field analogue studies carried out in the laboratory (flow-through column experiments), there is no discrepancy at all where the porous medium is homogeneous (Noiriel et al, 2012; Molins et al, 2014).  In materials characterized by pore scale heterogeneity, reaction rates may be up to 10-20x slower in core flood experiments where the pore structure remains intact compared to those determined in well-mixed flow-through reactor experiments (Beckingham et al, 2016; 2017).  Most of this effect is due to the reduced accessibility of fluid to the reactive mineral surfaces at the pore to grain scale. One expects additional orders of magnitude reductions in the field rates when volume-averaged over larger regions, particularly where hierarchical physical heterogeneity (fast flow paths, as in fractures or high permeable material) are present.<br/>

Lab-Field rate discrepancy revisited

253rd National Meeting (2017)

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