Total Alkalinity, Electrical Conductivity and pH as Potential MRV Proxies for Enhanced Weathering A Guided Tour Through Our Data From A Two-Year Greenhouse Experiment

[Geoengineering News]

https://www.researchgate.net/publication/400049972_Total_Alkalinity_Electrical_Conductivity_and_pH_as_Potential_MRV_Proxies_for_Enhanced_Weathering_A_Guided_Tour_Through_Our_Data_From_A_Two-Year_Greenhouse_Experiment

Authors: Dirk Paessler, Jens Steffen Hammes, Ingrid Smet, Anna Anke Stöckel, Melissa J Murphy, Ralf Steffens, Jens Hartmann

January 2026

Abstract

In this document we take you on a guided tour through the extensive dataset that we have gathered from our greenhouse experiments over the time of 3 years in 2023/2024/2025. These experiments entail more than 400 lysimeters with combinations of various soils and feedstocks that we maintained and monitored in our greenhouse. Roughly once a month we emptied the lysimeters’ tanks and measured various parameters of the leachate. We found several interesting aspects and correlations in these data, so let’s dive in! Enhanced weathering (EW) is a promising carbon dioxide removal (CDR) strategy, but its monitoring, reporting, and verification (MRV) remains challenging due to subtle signals and high measurement costs. We are conducting a multi-year greenhouse lysimeter experiment (18 soils × 18 rock/feedstock dust treatments) to investigate carbon removal based on total alkalinity (TA) in leachate, the primary indicator of CO₂ sequestration, and assess potential low-cost proxy measurements. Leachate samples from hundreds of soil columns were analyzed monthly for TA, pH, and electrical conductivity (EC), while major cations/anions were measured quarterly. Here we investigate whether the relationships between leachate EC, TA, pH and our other data points can serve as potential proxies for mineral weathering and alkalinity generation. At the aggregate dataset level, we found that leachate EC correlates strongly with TA (Pearson r ≈ 0.95), almost as tightly as leachate Ca²⁺ and Mg²⁺ concentrations (r ≈ 0.98). We found that easy-to-assess EC data can be used to reduce the number of necessary, more laborious TA measurements without sacrificing data quality, and EC/TA ratio spikes can indicate initial flushing and fertilizing events which might affect MRV results. Leachate pH is not a good quantitative indicator of CDR. In more detail, however, our dataset shows that our observations are dependent on the individual soil/feedstock combination. EC/TA correlations, along with EW performance, depend strongly on the specific soil and feedstock pairing, and proxy relationships that are valid “in general” can fail in particular soil/feedstock combinations. When working with EC as a proxy for CDR, frequent re-calibration is necessary, continued cross-checks against TA (and possibly major cations, e.g. Ca²⁺ and Mg²⁺) measurements are required, and on some soils these relationships seem to break down completely. These aspects call for further investigations, including additional soil analyses (e.g. soil extractions, cation retention, secondary carbonates, biomass) and a more diverse range of soil-feedstock-crop-climate-land use combinations. Limitations: We explicitly acknowledge that our dataset, while extensive, is limited to our specific greenhouse conditions (e.g. warm, no dry/wet cycles), a finite set of soils from Germany and our selection of feedstock types. Nonetheless, these findings demonstrate that continuous EC monitoring, when combined with periodic TA measurements, can enable cost-effective, high-frequency tracking of EW-induced carbon removal in experimental and field settings, while field deployments should validate proxy performance under their specific conditions.

Source: ResearchGate