Machine learning deciphers CO₂ sequestration and subsurface flowpaths from stream chemistry

Endmember mixing analysis (EMMA) is often used by hydrogeochemists to interpret the sources of stream solutes, but variations in stream concentrations and discharges remain difficult to explain. We discovered that machine learning can be used to highlight patterns in stream chemistry that reveal information about sources of solutes and subsurface groundwater flowpaths. The investigation has implications, in turn, for the balance of CO2 in the atmosphere. For example, CO2-driven weathering of silicate minerals removes carbon from the atmosphere over ∼106-year timescales. Weathering of another common mineral, pyrite, releases sulfuric acid that in turn causes dissolution of carbonates. In that process, however, CO2 is released instead of sequestered from the atmosphere. Thus, understanding long-term global CO2 sequestration by weathering requires quantification of CO2- versus H2SO4-driven reactions. Most researchers estimate such weathering fluxes from stream chemistry, but interpreting the reactant minerals and acids dissolved in streams has been fraught with difficulty. We apply a machine-learning technique to EMMA in three watersheds to determine the extent of mineral dissolution by each acid, without pre-defining the endmembers. The results show that the watersheds continuously or intermittently sequester CO2, but the extent of CO2 drawdown is diminished in areas heavily affected by acid rain. Prior to applying the new algorithm, CO2 drawdown was overestimated. The new technique, which elucidates the importance of different subsurface flowpaths and long-timescale changes in the watersheds, should have utility as a new EMMA for investigating water resources worldwide.

Stable Carbon and Sulfur Isotope Characteristics of Stream Water in a Typical Karst Small Catchment, Southwest China

Surface water samples from the Maolan National Natural Reserved Park (MNNRP) were analyzed from Sept. 2013 to June 2014, for major ion concentrations (K+, Na+, Ca2+, Mg2+, Cl−, SO42−, HCO3−), δ13C-DIC and δ34S-SO42− to quantify the sources of solutes and chemical weathering. The results show that HCO3− and SO42− are the main anions in Banzhai watershed, which account for 86.2 and 10.4% of the total anion equivalent, respectively. While Ca2+ and Mg2+ account for 76.9 and 20.5%, respectively. Considerable Mg2+ in stream water indicates that it may be affected by dolomite weathering. stream water samples present the δ13C-DIC values in the range of −16.9‰~−10.8‰ (mean value was −13.9‰), which were lower than that of the groundwater. The δ34S-SO42− values ranged from −15.2‰ to 1.7‰ (mean value was −4.4‰). There was a negative correlation between HCO3− content and δ13C value, implying the result of the interaction of temperature and precipitation intensity in different seasons. The significant positive correlation between SO42− content and δ13C-DIC indicates that H2SO4 may be involved in the weathering process of carbonate rocks in small watershed scale. The content of SO42− in a school sample site was much higher than that of other sample sites for the interference from human sources. The δ34S values show that the average δ34S-SO42− in most sites is close to the δ34S isotopic values of Guizhou coal and rain, indicating that they may be affected by local coal.

Chemical and isotopic composition of nitrogen thermal waters of the Kuldur Deposit (JAR, Russia)

The article presents the results of the hydrogeochemical study of thermal groundwaters from the Kuldur spa (Jewish Autonomous Region, Russia). The main characteristics of these groundwaters are high temperature (73 ᵒС), low TDS (up to 0,38 g/l), and alkaline (pH 9). The dominated cation is sodium, and the anion is hydrocarbonate. Water is enriched with fluorine, silicon, aluminum, tungsten, molybdenum, and some other anionic elements. This study provides detailed data on the chemical and mineral composition of host rocks and sources of solutes. Isotopic data from bubbling gases show that the main component of the gas phase (N2) comes from an atmogenic source, while CH4 and CO2 are biogenic. Argon and oxygen are also derived from air, while helium is predominantly radiogenic. The ϬD and Ϭ18О in the aqueous phase indicate the undoubtedly meteoric genesis of groundwater with an extended circulation period. Our results suggest that the studied groundwaters are results of the penetration of meteoric waters to 4 km depth and heating up to 100 ᵒC. The solutes come to aquifer via the dissolution of rocks, but since groundwater circulates within the poorly soluble rock (granitoids), respectively, the water TDS remains low.

Machine Learning Deciphers CO₂ Sequestration and Subsurface Flowpaths from Stream Chemistry

Endmember mixing analysis (EMMA) is often used by hydrogeochemists to interpret the sources of stream solutes, but variations in stream concentrations and discharges remain difficult to explain. We discovered that machine learning can be used to reveal patterns in stream chemistry that pertain to information about weathering sources of solutes and also about subsurface groundwater flowpaths. The investigation has implications, in turn, for the balance of CO2 in the atmosphere. For example, CO2-driven weathering of silicate minerals removes carbon from the atmosphere over ~106-yr timescales. Weathering of another common mineral, pyrite, releases sulfuric acid that in turn causes dissolution of carbonates. In that process, however, CO2 is released instead of sequestered from the atmosphere. Thus, to understand long-term global CO2 sequestration by weathering requires quantification of CO2-versus H2SO4-driven reactions. Most researchers estimate such weathering fluxes from stream chemistry but interpreting the reactant minerals and acids dissolved in streams has been fraught with difficulty. We use a new machine learning technique in three watersheds to determine the minerals dissolved by each acid. The results show that the watersheds continuously or intermittently sequester CO2 but the extent of CO2 drawdown is diminished in areas heavily affected by acid rain. Sulfide oxidation contributes ~23 % to 62 % of sulfate fluxes. Without the new algorithm to deconvolve the mineral weathering, CO2 drawdown was always overestimated. The new technique, which also elucidated the importance of different subsurface flowpaths and long-timescale changes in the watersheds, should have great utility as a new EMMA for investigating water resources worldwide.

Cropland Soil Salinization and Associated Hydrology: Trends, Processes and Examples

While global food demand and world population are rapidly growing, land potential for cropping is steadily declining due to various soil degradation processes, a major one of them being soil salinization. Currently, approximately 20% of total cropland and 33% of irrigated agricultural land are salinized as a result of poor agricultural practices and it is expected that by 2050, half of the croplands worldwide will become salinized. Thus, there is a real need to better understand soil salinization processes and to develop agricultural practices that will enable production of the needed amount of food to feed humanity, while minimizing soil salinization and other degradation processes. The major sources of solutes in agricultural environments are: (i) the soil itself, and the parent geological material; (ii) shallow and salt rich groundwater; and (iii) salt rich irrigation water. The salinization of soil is a combination of transport of solutes towards the root zone to replenish evaporation and transpiration and limited washing of the soil by rain or irrigation. Therefore, most salinized soils are present in arid and semi-arid environments where precipitation is low and evaporation is high. In this manuscript, examples of soil salinization processes from croplands around the world will be presented and discussed to bring attention to this important topic, to present the latest scientific insights and to highlight the gaps that should be filled, from both scientific and practical perspectives.