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.

Evidence for high-elevation salar recharge and interbasin groundwater flow in the Western Cordillera of the Peruvian Andes

Improving our understanding of hydrogeological processes on the western flank of the central Andes is critical to communities living in this arid region. Groundwater emerging as springs at low elevations provides water for drinking, agriculture, and baseflow. Some springs also have recreation or religious significance. However, the high elevation sources of recharge and specific groundwater flowpaths that support these springs and convey groundwater to lower elevations in southern Peru remain poorly quantified in this geologically complex environment. The objectives of this study were to identify recharge zones and groundwater flowpaths supporting natural springs east of the city of Arequipa in the volcanic mountain terrain, particularly, the potential for recharge within the high-elevation closed-basin Lagunas Salinas salar. We used geochemical and isotopic tracers in springs, surface waters (rivers and lakes), and precipitation (rain and snow) sampled from March 2019 through February 2020. We obtained monthly samples from six springs, bimonthly samples from four rivers, and various samples from high-elevation springs during the dry season. We analyzed stable water isotopes (δ18O and δ2H) and general chemistry of springs, rivers, local rainfall, and snow from Pichu Pichu volcano. The monthly isotopic composition of spring water was invariable over time, suggesting that the springs receive a stable source of groundwater recharge and are not supported by relatively short groundwater flowpaths. The chemistry of springs in the low- and mid-elevations (2500 to 2900 masl) point towards a mix of recharge from the salar (4300 masl) and mountain-block recharge (MBR) in or above a queñuales forest ecosystem at ~4000 masl on the adjacent Pichu Pichu volcano. Springs at higher elevation closer to the salar and in a region with a high degree of faulting had higher chloride concentrations indicating higher proportions of interbasin groundwater flow from the salar. We conclude that while the salar is a closed basin, surface water from the salar recharges through the lacustrine sediments, mixes with mountain-block groundwater, and is incorporated into the regional groundwater flow system. Groundwater flow in the mountain block and the subsequent interbasin groundwater flow is accommodated through extensive faulting and fracturing. Our findings provide valuable information on the flowpaths and zones of recharge that support low-elevation springs in this arid region. In this study, high-elevation forests and a closed-basin salar are important sources of recharge. These features should be carefully managed to prevent impacts to the down-valley springs and streams.

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.

Stable Isotopes of Water and Nitrate for the Identification of Groundwater Flowpaths: A Review

Nitrate contamination in stream water and groundwater is a serious environmental problem that arises in areas of high agricultural activities or high population density. It is therefore important to identify the source and flowpath of nitrate in water bodies. In recent decades, the dual isotope analysis (δ15N and δ18O) of nitrate has been widely applied to track contamination sources by taking advantage of the difference in nitrogen and oxygen isotope ratios for different sources. However, transformation processes of nitrogen compounds can change the isotopic composition of nitrate due to the various redox processes in the environment, which often makes it difficult to identify contaminant sources. To compensate for this, the stable water isotope of the H2O itself can be used to interpret the complex hydrological and hydrochemical processes for the movement of nitrate contaminants. Therefore, the present study aims at understanding the fundamental background of stable water and nitrate isotope analysis, including isotope fractionation, analytical methods such as nitrate concentration from samples, instrumentation, and the typical ranges of δ15N and δ18O from various nitrate sources. In addition, we discuss hydrograph separation using the oxygen and hydrogen isotopes of water in combination with the nitrogen and oxygen isotopes of nitrate to understand the relative contributions of precipitation and groundwater to stream water. This study will assist in understanding the groundwater flowpaths as well as tracking the sources of nitrate contamination using the stable isotope analysis in combination with nitrate and water.

Identifying Karst Aquifer Recharge Areas using Environmental Isotopes: A Case Study in Central Italy

Water resources management is one of the most important challenges worldwide because water represents a vital resource for sustaining life and the environment. With the aim of sustainable groundwater management, the identification of aquifer recharge areas is a useful tool for water resources protection. In a well-developed karst aquifer, environmental isotopes provide support for identifying aquifer recharge areas, residence time and interconnections between aquifer systems. This study deals with the use of environmental isotopes to identify the main recharge area of a karst aquifer in the Upper Valley of Aniene River (Central Italy). The analysis of 18O/16O and 2H/H values and their spatial distribution make it possible to trace back groundwater recharge areas based on average isotope elevations. The Inverse Hydrogeological Balance Method was used to validate spring recharge elevations obtained by the use of stable isotopes. Areas impacted by direct and rapid rainfall recharge into the study area were delineated, showing groundwater flowpaths from the boundaries to the core of the aquifer. The results of this study demonstrate the contribution that spatial and temporal isotope changes can provide to the identification of groundwater flowpaths in a karst basin, taking into account the hydrogeological setting.

Identifying Karst Aquifers Recharge Area Using Environmental Isotopes: Case Study in Central Italy

Water resources management is one of the most important challenges worldwide because water represents a vital resource for sustaining life and environment. In the aim of sustainable groundwater management, the identification of aquifer recharge areas is a useful tool for water resources protection. In a well-developed karst aquifer, environmental isotopes provide support for identifying aquifer recharge areas, residence time and interconnections between aquifer systems. This study deals the use of environmental isotopes to identify the main recharge area of a karst aquifer in the Upper Valley of Aniene River (Central Italy). The analysis of 18O/16O and 2H/H values and their spatial distribution in the aquifer, make it possible to trace back groundwater recharge areas based on average isotope elevations. The Inverse Hydrogeological Balance Method was used to validate spring recharge elevations obtained by the use of stable isotopes. Areas impacted by direct and rapid rainfall recharge into the study area were delineated, showing groundwater flowpaths from the boundaries to the core of the aquifer. The results of this study demonstrate the contribution that spatial and temporal isotope changes can provide to the identification of groundwater flowpaths in a karst basin, taking into account the hydrogeological setting