Groundwater Research Articles

Diverse Responses of Upper Ocean Temperatures to Chlorophyll‐Induced Solar Absorption Across Different Coastal Upwelling Regions

AbstractChlorophyll in phytoplankton absorbs solar radiation (SR) and affects the thermal structure and dynamics within upwelling regions. However, research on this process across global‐scale coastal upwelling systems is still lacking. Here, we use a coupled ocean‐biogeochemical model to investigate differing responses to chlorophyll‐induced solar absorption between Pacific and Atlantic coastal upwelling regions. Chlorophyll‐induced solar absorption leads to colder Pacific coastal upwelling but warmer Atlantic coastal upwelling. In the Pacific, the shading effect of the surface chlorophyll maximum leads to colder subsurface water, which is then upwelled, contributing to cooling. The more stratified upper ocean leads to shallower mixed layer depth, intensifying offshore transport and upwelling. In the Atlantic, the absorption of SR by the subsurface chlorophyll maximum causes warmer and weaker upwelling. The processes described, in turn, trigger positive feedback to ocean biogeochemistry and potentially interact with climate dynamics, underscoring the necessity to incorporate them into Earth system models.

Tritium content in water bodies in regions of peaceful nuclear explosions

Sites of peaceful nuclear explosions pose a potential radiation hazard to the territories of the Russian Federation, primarily due to the possible release of radioactivity from the explosion cavity into aquifers and onto the earth's surface. Therefore, it is essential to conduct regular monitoring of anthropogenic radionuclides in drinking water sources in settlements located near the sites of peaceful nuclear explosions. Tritium serves as an indicator of the potential release of other anthropogenic radionuclides. Monitoring its levels in water bodies in regions where peaceful nuclear explosions were perfomed, and comparing this data with that from Roshydromet across the Russian Federation, allows for an assessment of the reliability of the engineering barriers between the central explosion zone and the environment with respect to preventing radionuclide migration into aquifers. One method for evaluating the reliability of these barriers is the assessment of tritium specific activity in drinking water sources. This article presents results of the study involving 220 water samples collected from drinking water sources (wells, boreholes, springs, central water supply systems) and surface waters within 167 settlements across 17 subjects of the Russian Federation, where 50 peaceful nuclear explosions were conducted between 1965 and 1988. The samples were collected between May and September 2024 in the settlements within a 30 km radius of a peaceful nuclear explosion site. Measurements of tritium specific activity were performed using the Quantulus 1220-003 alpha-beta spectrometric radiometer. The research revealed that the specific activity of tritium in underground water sources is significantly lower (Student's test p<0,05) than in surface waters. The average specific activity levels of tritium in boreholes, rivers, and lakes were 3.0, 3.45, and 4.31 Bq/kg, respectively. The specific activity of tritium in drinking water sources within the regions of peaceful nuclear explosions was found to be at the background levels recorded by Roshydromet, ranging from 1.1 to 5 Bq/kg.

Groundwater quality assessment for drinking and irrigation purposes in the Ayad river basin, Udaipur (India)

Globally, about 5.25 billion people depend on groundwater for their water needs. However, groundwater quality significantly impacts human health and agriculture, influenced by factors such as land use, waste seepage, soil properties, and geological settings. In Rajasthan, the primary groundwater quality issues involve fluoride, nitrate, chloride, and calcium. This study addresses the gap in the understanding of the spatial and temporal variations of these contaminants and how the variations are linked to geology and land use. The basis for the analysis is data spanning 2000 to 2021from the Ground Water Department (GWD), the Central Ground Water Board (CGWB), and citizen science data from 2022 to 2023, focusing on the Ayad River Basin. The research aims to evaluate groundwater quality for drinking and irrigation by assessing physico-chemical parameters and using the Weighted Arithmetic Water Quality Index (WAWQI) method to calculate the Groundwater Quality Index (GWQI) from 2000 to 2023. The findings suggest a decreasing GWQI trend from west to east in the basin, with good groundwater quality (GWQI below 50) in the southern regions near the cities Umarda, Ramgiri, Undri, and Hariyab. The highest index values were near Bhoyana, Khemli, and Sisarma. The results of the salinity hazard test showed that salinity is a major issue in the eastern part of the basin. Though the groundwater is notably hard, a comprehensive analysis of various parameters nevertheless suggested its suitability for irrigation purposes. These results provide new insights in the quality of the groundwater resources in the Ayad River basin and valuable insights for policymakers and for decision-makers to develop strategies to preserve the groundwater quality.

The role of farm subsidies in changing India’s water footprint

Dwindling groundwater supplies threaten food security and livelihoods. Output subsidies for farmers are a ubiquitous agricultural policy tool, yet their contribution to growing groundwater stress remains poorly quantified. We show how output subsidies guaranteeing the purchase of crops at higher than market prices may have contributed substantially to declining water tables in India. Our analysis suggests that these policies may have led to a 30% over-production of water intensive crops. In the northwestern state of Punjab, rice procurement can potentially account for at least 50% of the groundwater table decline over 34 years. In the central state of Madhya Pradesh, wheat procurement adopted in the late 2000s appears to have driven a 5.3 percentage point increase in dry wells and a consequent 3.4 percentage point increase in deep tubewells. These results suggest that well-intentioned but poorly designed subsidies can impose harmful externalities on the environment and undermine long-term sustainable development.

Characterization of hydrogeochemical elements in determining the ground water quality for irrigation potential and its correlation with climatological parameters of chennai basin aquifer system, southern india.

An attempt has been made to comprehend the ground water quality and climate impacts of the Chennai River basin, which is aimed at its main socio-economic growth of the state of Tamil Nadu. The ground water samples collected from the study area were analyzed for its hydrogeochemical elements. The ground water quality and irrigation suitability were determined using several water quality assessment metrics. Ground water is extensively utilized for irrigation in the entire basin area for the past two decades, especially in the 38 over-exploited Firkas out of the 109 Firkas of the basin. It is inferred that the phreatic aquifer ground water quality is fresh in about 20%, as indicated by the EC value (< 750µs/cm) at 25°C. In about 63% of the ground water indicating the moderately fresh showing the EC varies between 751 and 2250µs/cm at 25°C, 11% of ground water exerted an EC ranging between 2251 and 3000µs/cm at 25°C indicating that the ground water is slightly mineralized, and in about 6% of groundwater, the EC is > 3000µs/cm at 25°C indicating that the ground water is highly mineralized. There were no water samples that exceeded the permissible limit of chloride either in phreatic aquifer or in fracture aquifer. The changes in rainfall frequency and atmospheric temperature affect the ground water movement and storage directly and indirectly. Similarly, the temperature data shows a positive relationship with the concentration of fluoride and nitrate ions in the water.

Detachment and transport of composition B detonation particles in rills

The partial detonation of munitions used in military exercises leaves behind energetic particles on the surface of soil. Energetic particles deposited by incomplete detonations can then dissolve and be transported by overland flow and potentially contaminate ground and surface waters. The objective of this study was to evaluate the mechanisms of transport of Composition B, a formulation that includes TNT (2,4,6-trinitrotoluene) and RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine) during overland flow. The transport of Composition B was examined using a rill flume with three flow rates (165-, 265-, and 300-mL min−1) and four energetic particle sizes (4.75–9.51 mm, 2.83–4.75 mm, 2–2.83 mm, and &amp;lt;2 mm). After each erosion simulation, energetic particles remaining on the soil surface were measured along with energetics dissolved in runoff, in suspended sediment, and in infiltration. Smaller particle sizes led to greater transport in both solution and sediment. The properties of the energetic compounds also influenced transport. More TNT was transported in runoff than RDX, likely due to TNT’s higher solubility and dissolution rates, however, overall, dissolved energetics in runoff and infiltration accounted for very little of the total transport. Most transport of Composition B was the result of the physical movement of energetic particles and flakes by erosion forces. This study’s results allow for improved prediction of Composition B transport during overland flow.

A Novel Investigation on Microstructural Analysis of a Rupture GRE Composite Pipe for Underground Fire Water System

This study investigates how underground fire water systems using Glass Reinforced Epoxy (GRE) pipes are affected by aging. Over time, GRE pipes can degrade, potentially reducing their expected lifespan. The study compared damaged and undamaged areas of pipes to assess how damage impacts their structural integrity. Microscopic and chemical analyses (SEM and EDX) were performed on pipe samples from different locations of the pipe. Mechanical testing (following ASTM standards) was conducted beforehand. Analyses revealed that as damage increased (closer to damaged area), the levels of carbon (C) and oxygen (O) changed significantly with the weight percentage exposed 45.62% difference and 11.62% difference, respectively, as compared with the undamaged area. This suggests that the epoxy resin degrades with age, releasing these elements. Additionally, the presence of foreign objects around the pipes can affect how they bear loads. These findings highlight the importance of understanding how aging and surrounding conditions can influence the performance of GRE pipes in real-world fire water systems.

Assimilation of Sentinel‐Based Leaf Area Index for Modeling Surface‐Ground Water Interactions in Irrigation Districts

AbstractVegetation‐related processes, such as evapotranspiration (ET), irrigation water withdrawal, and groundwater recharge, are influencing surface water (SW)—groundwater (GW) interaction in irrigation districts. Meanwhile, conventional numerical models of SW‐GW interaction are not developed based on satellite‐based observations of vegetation indices. In this paper, we propose a novel methodology for multivariate assimilation of Sentinel‐based leaf area index (LAI) as well as in‐situ records of streamflow. Moreover, the GW model is initially calibrated based on water table observations. These observations are assimilated into the SWAT‐MODFLOW model to accurately analyze the advantage of considering high‐resolution LAI data for SW‐GW modeling. We develop a data assimilation (DA) framework for SWAT‐MODFLOW model using the particle filter based on the sampling importance resampling (PF‐SIR). Parameters of MODFLOW are calibrated using the parameter estimation (PEST) algorithm and based on in‐situ observation of the GW table. The methodology is implemented over the Mahabad Irrigation Plain, located in the Urmia Lake Basin in Iran. Some DA scenarios are closely examined, including univariate LAI assimilation (L‐DA), univariate streamflow assimilation (S‐DA), and multivariate streamflow‐LAI assimilation (SL‐DA). Results show that the SL‐DA scenario results in the best estimations of streamflow, LAI, and GW level, compared to other DA scenarios. The streamflow DA does not improve the accuracy of LAI estimation, while the LAI assimilation scenario results in significant improvements in streamflow simulation, where, compared to the open loop run, the (absolute) bias decreases from 75% to 6%. Moreover, S‐DA, compared to L‐DA, underestimates irrigation water use and demand as well as potential and actual crop yield.

Drought vulnerability assessment and its impact on crop production and livelihood of people: An empirical analysis of Barind Tract

North-Western section of Bangladesh is experiencing a protracted decrease in precipitation, irregular rainfall, and the depletion of ground water, which results in water scarcity and extreme dry weather that impedes the production of agricultural commodities and threatens the people's way of life. Analyzing the precipitation deficit and ground water deficit, along with vegetation cover, temperature condition, and the condition of the vegetation is a crucial component of drought vulnerability assessment. Rajshahi Zilla, a region of Bangladesh located in the middle of the Barind tract, is experiencing a severe water shortage. The irregular rainfall, decrease in rainfall, prolonged absence of rainfall and ground water depletion results in drought. This study aims to access the vulnerability of drought by analyzing precipitation rates, ground water depletion levels, temperature condition, vegetation condition and the vegetative droughts to find out the severe condition of droughts and the severe effects of this in the livelihoods of the farmers and their crop production practices. In this case the study focuses on determining NDVI, NDWI, NDMI, VCI, TCI, and VHI. The VHI results show a significant increase in extreme drought conditions from 2013 (4 %) to 2021 (7 %). By conducting a few questionnaire surveys and Focus Group Discussion the present situation of crop production and the livelihoods of people has been analyzed. Almost 18.3 % of farmers have made a permanent move away from agriculture, and 80 % of permanently relocated farmers report an improvement in their quality of life. Nearly 60 % of farmers believe that the construction of deep ditches has enhanced their crop yield. Once again, more than 20 % are in the same predicament as previously, with over 19 % reporting that deep irrigation has lowered agricultural yield. Comprehending the potential consequences of drought will enable planners and decision-makers to implement mitigation measures aimed at improving the communities' ability to manage drought risk. After analyzing data, it has been found that Rajshahi is facing a critical drought problem, which has led to water scarcity, severely affecting agricultural production and livelihoods.

Pervasiveness and classification of microplastics in Landfill Leachate: Impacts, Risks, and Treatment Efficiency

Microplastics (MPs) in surface and groundwater in Bangladesh are a significant issue. The purpose of this research was to assess the possibility of landfill leachate acting as a potential origin of MPs and to determine if the surrounding surface water (SW) and groundwater (GW) act as recipients. Furthermore, this research assessed the leachate treatment plant MP removal efficacy and MP risk assessment. The findings show that discharge leachate from the Matuail landfill contributes 3.5 × 108 particles per hour to the surrounding aquatic environment, with an average of 350 ±10 MPs/L. MPs was found highly in SW and then in GW with an average of 1683 ± 70 and 614 ± 40 MPs/L, respectively, with 48.9% of MPs ranged from 0.1mm to 0.5mm. The dominant shapes were fibers and fragments. The Fourier transform infrared spectroscopy (FTIR) analysis revealed low density polyethylene (LDPE), high density polyethylene (HDPE), and Polypropylene (PP) was the most common, and Polyurethane (PUR), Polymethyl methacrylate (PMMA), and Polyacrylonitrile (PAN) posed the greatest threat to the environment. The inefficient treatment method resulted in the release of 83.33% of MPs, indicating a low removal efficiency in the leachate. The inefficient removal rate leads to the highest pollutant load index for surface water (2.18). Ultimately, the analysis of the nemerow pollution index (NPI), contamination factor (CF), pollution load index (PLI), polymer hazard index (PHI), and potential ecological risk (Ei), revealed a minimal to extremely high range of contaminations. A clear link was obsevered between the particles shape and size throught the principal component analysis (PCA). Moreover, it highlights the need for ongoing national surveillance of MPs considering the gravity of this contamination and indicates the importance of proactive management of landfill sites.

IDENTIFICATION OF AQUIFERS BASED ON THE VERTICAL ELECTRICAL SOUNDING (VES) METHOD SCHLUMBERGER CONFIGURATION CASE STUDY: PULAU BAAI KAMPUNG MELAYU SUB-DISTRICT, BENGKULU CITY, INDONESIA

Investigation of the groundwater potential in the Pulau Baai area, Kampung Melayu Sub-district, Bengkulu City, must be carried out in such a way that the activities and needs of the people in the area can be fulfilled and the needs of the population in the area can be met. This study aims to determine the status of groundwater using the Schlumberger configuration geoelectric method. Measurements were made using a resistivity meter, and the results for each configuration depended on changes in resistivity. Measurements for each configuration depend on changes in resistivity at depth, the vertical direction (sounding), and the lateral direction (mapping), so hydrogeological analysis in this activity aims to get the maximum use of groundwater / underground water in aquifers for raw water needs. The dominant rock structures in the study area are clay, alluvium, siltstone, and sandstone, as well as some rocks with suitable porosity and permeability as water carriers, such as sand and gravel. However, what appears to have considerable potential is that groundwater is found at depths of 4-53 meters in VES 1, VES 2, VES 3, VES 5, and VES 10. The results of the analysis show that the location of the Pulau Baai, Kampung Melayu Sub-district, Bengkulu City Priority Utilization Area is within the groundwater storage area, so it can be used to meet the raw water needs of the study area.

Evaluation of groundwater in the city of Sousse in eastern Libya for some heavy metals and nitrate ions

Evaluation of groundwater potential and its quality assessment for irrigation has recently become a major concern, especially in developing countries due to various constraints. The aim of this study is to evaluate the quality of groundwater and establish whether they are safe for irrigation usage in Sousa city, Libya. The aim of the study to evaluating of groundwater polluted with Lead, Cadmium and Chromium metals and nitrates ions. Five water samples were collected from different wells were and analyzed. For understanding the hydro chemical and polluted characteristic with some heavy metal as (Pb, Cd and Cr ) respectively, and the suitability for irrigation purposes. Nine parameters were considered for calculating the irrigation water quality indices which are pH , electrical conductivity (EC), ions ( cations and anions) such as Calcium, magnesium, potassium sodium, chloride ,bicarbonate, sulphate,, in addition to some calculated parameters for as SAR, Adj. SAR, RSC, TDS and pHc, furthermore some heavy metals and nitrates ions were investigated. The results of polluted with heavy metals and nitrate ion, showed that the most water samples did not exceed the maximum limit according to FAO and Libyan local standard, except W05 exceeding the permissible limit for lead and nitrate ion, and W02 for pb. The results of this study emphasis that routine monitoring of groundwater play a vital role in maintaining its quality and serve as guideline for sustainable management of water quality in region study. Keywords: irrigation water, heavy metals, nitrate, ground water.

Helium enrichment theory and exploration ideas for helium-rich gas reservoirs

Using gas and rock samples from major petroliferous basins in the world, the helium content, composition, isotopic compositions and the U and Th contents in rocks are analyzed to clarify the helium enrichment mechanism and distribution pattern and the exploration ideas for helium-rich gas reservoirs. It is believed that the formation of helium-rich gas reservoirs depends on the amount of helium supplied to the reservoir and the degree of helium dilution by natural gas, and that the reservoir-forming process can be summarized as “multi-source helium supply, main-source helium enrichment, helium-nitrogen coupling, and homogeneous symbiosis”. Helium mainly comes from the radioactive decay of U and Th in rocks. All rocks contain trace amounts of U and Th, so they are effective helium sources. Especially, large-scale ancient basement dominated by granite or metamorphic rocks is the main helium source. The helium generated by the decay of U and Th in the ancient basement in a long geologic history, together with the nitrogen generated by the cracking of the inorganic nitrogenous compounds in the basement rocks, is dissolved in the water and preserved. With the tectonic uplift, the ground water is transported upward along the fracture to the gas reservoirs, with helium and nitrogen released. Thus, the reservoirs are enriched with both helium and nitrogen, which present a clear concomitant and coupling relationship. In tensional basins in eastern China, where tectonic activities are strong, a certain proportion of mantle-derived helium is mixed in the natural gas. The helium-rich gas reservoirs are mostly located in normal or low-pressure zones above ancient basement with fracture communication, which later experience substantial tectonic uplift and present relatively weak seal, low intensity of natural gas charging, and active groundwater. Helium exploration should focus on gas reservoirs with fractures connecting ancient basement, large tectonic uplift, relatively weak sealing capacity, insufficient natural gas charging intensity, and rich ancient formation water, depending on the characteristics of helium enrichment, beyond the traditional idea of searching for natural gas sweetspots and high-yield giant gas fields simultaneously.

Foam separation of radio-molybdenum oxyanions (99MoO42−) incorporated into Co(II)/Al(III) layered double hydroxide using anionic surfactant

Foam separation of radio-molybdenum oxyanions (99MoO42−) incorporated into in-situ formed Co(II)/Al(III) layered double hydroxide (LDH) particles was applied for recovery of these anionic species from aqueous solutions. The results showed that Co(II)/Al(III) molar ratios of 2 and 2.5 resulted into not only high recovery values for 99MoO42− (R > 0.97), but also high decontamination factor (DF = 33 and 36, respectively). Almost complete recovery was achieved for 99MoO42− coprecipitated with Co(II)/Al(III) LDH in the pH range 9.9–10.5. Ageing time of 5 min was sufficient to completely coprecipitate the concerned anionic species. Sodium dodecyl sulfate (SDS) concentrations in the range 1.5 × 10−3 - 3 × 10−3 mol/L had the ability to efficiently form hydrophobic 99MoO42--Co(II)/Al(III)-LDH particles, where recovery values of about 0.96 were achieved with high DF values. The influence of the coexistence of different foreign anions (Cl−, NO3−, HCO3−, CO32− and SO42−) during coprecipitation process of 99MoO42− and foam separation of the resultant particles was investigated. The suggested strategy in the present study was effectively applied for recovery of 99MoO42− anions from ground water (R ≈ 0.98 and enrichment ratio (ER ≈ 7172) and radioactive process wastewater (R ≈ 0.96 and ER = 3887). Based on characterization of Co(II)/Al(III) LDH using Fourier transform infrared (FTIR) and X-ray diffraction (XRD) before and after foam separation process using SDS and the obtained data, the recovery mechanism of 99MoO42− was proposed.

Using hydrological modeling and satellite observations to elucidate subsurface and surface hydrological responses to the extreme drought

Climate change and anthropogenic activities have intensified extreme weather events globally. In the summer of 2022, the Yangtze River Basin (YRB) in China experienced an extreme drought, significantly impacting the ecosystems and society. However, the specific effects of this extreme drought on surface and subsurface hydrological dynamics remain unclear. Here we employed satellite-observed terrestrial water storage anomaly (TWSA) and a modified hydrological model with consideration of reservoir operation, human water consumption, and water diversion engineering to quantify how subsurface and surface water in YRB responded to such an extreme drought in 2022. Validation against a series of observations shows that the modified model has good performance in reproducing daily streamflow, reservoir water storage, lake water storage, and snow water equivalent. It achieved more precise GRACE TWSA estimates in the YRB with significant human intervention, and therefore it can fairly accurately quantify both surface and subsurface hydrological responses to the 2022 extreme drought. Compared to the same months (July-December) in 2015–2021, the drought in 2022 resulted in a decrease in precipitation and discharge of 373 km3 (36 %) and 324 km3 (50 %), respectively, while an increase in evapotranspiration of 156 km3 (29 %) in the YRB. In general, the surface water storage (SWS) is relatively low from July 2022, followed by subsurface water storage (SSWS) from August 2022, indicating an approximately one-month lag from the former to the latter. During the latter half year of 2022, the SWS and SSWS reduced by 48 km3 and 83 km3, respectively, suggesting the changes in the latter dominated the TWS variations. This study sheds light on the responses of surface and subsurface hydrology to extreme droughts, and the hydrological modeling framework with consideration of human activities proposed here holds applicability beyond the YRB.

Contrasting nitrogen dynamics across the Mid‐Atlantic Bight shelfbreak front: Insights from nitrate dual isotopes and nitrifier gene abundance

AbstractObservations and model studies suggest that front dynamics can enhance phytoplankton productivity. This study tested whether frontal systems also increase the abundance of nitrifying microbes and nitrogen recycling during repeat sampling transects across the Mid‐Atlantic Bight shelfbreak in July 2019. We measured ammonium concentrations, nitrate dual isotopes (δ15N, δ18O), and ammonia monooxygenase subunit A (amoA) genes of ammonia‐oxidizing archaea (AOA) and bacteria (AOB). In subsurface shelf waters, ammonium concentrations exceeded 2 μmol L−1, due to a temporary imbalance in regeneration from sinking particles and subsequent nitrification. The inverse correlation between nitrate δ15N values and ammonium concentrations confirmed nitrate was partially or entirely from local nitrification on the shelf. In contrast, the shelfbreak frontal zone and slope sea subsurface waters had much lower ammonium concentrations (0.1–0.2 μmol L−1) due to tight coupling between ammonium regeneration and nitrification. The deviation of nitrate δ15N and δ18O from algal uptake‐driven 1 : 1 ratio suggests concurrent nitrification in the euphotic zone. The shelfbreak front acted as an ecological boundary where AOA and AOB amoA gene numbers were partitioned, with AOAs abounding in slope waters and AOBs in shelf waters, likely due to ammonium availability. At certain slope stations, deep‐water nutrient inputs via isopycnal lifting induced by Gulf Stream intrusions caused unexpectedly high phytoplankton biomass, which doubled nitrifier abundance and potentially stimulated both ammonium regeneration and nitrification. These findings demonstrate distinct distributions of nitrifying microbes along the salinity gradient from shelf to slope and highlight the significant influence of coastal ocean‐western boundary current interactions on nitrogen biogeochemistry.

Characterisation of dissolved organic matter in two contrasting arsenic-prone sites in Kandal Province, Cambodia

Aquifers throughout Asia are impacted by the release of geogenic arsenic (As) into groundwater by microbial reduction of As-bearing Fe(III) (oxy)hydroxide minerals, severely impacting water quality. Groundwater dissolved organic matter (DOM) is likely key to As release, mainly as electron donor or electron shuttles. This study used optical analyses and ultra-high resolution mass spectrometry to examine the sources and composition of groundwater DOM in the As-prone aquifers of Kandal Province, Cambodia, at boreholes with differing host lithology (clay- and sand-dominated). Groundwater and surface water DOM composition were related to As concentrations, to infer the potential role of DOM in promoting As release. Optical and molecular-level analyses indicated an overall dominance of terrestrial-derived DOM in the groundwater samples, with higher freshness index and relative abundance (RA) of aliphatic compounds in clay compared to sand-dominated lithology. Compared to surface water, groundwater DOM had relatively lower O/C ratios and nominal oxidation state of carbon (−0.19 to −0.13 compared to 0.04 for ground and surface water, respectively), with a lower %RA of aliphatic compounds and higher %RA of carboxyl-rich alicyclic molecules, suggesting microbial processing of DOM since percolation into the aquifer. Concentrations of As across both sites were negatively correlated with DOM tryptophan:fulvic-like fluorescence and the %RA of aliphatics, potentially indicating microbial degradation of biolabile DOM in connection with As release, which is consistent with its role as an electron donor source. Together these data support DOM composition as an important control on microbial mediated As release.

Evidence of Subsurface Control on the Coevolution of Hillslope Morphology and Runoff Generation

AbstractTopography is a key control on runoff generation, as topographic slope affects hydraulic gradients and curvature affects water flow paths. Simultaneously, runoff generation shapes topography through erosion, affecting landscape morphology over long timescales. Previous modeling efforts suggest that subsurface hydrological properties, relative to climate, are key mediators of this relationship. Specifically, when subsurface transmissivity and water storage capacity are low, (a) saturated areas and storm runoff should be larger and more variable, and (b) hillslopes shorter and lower relief, assuming other geomorphic factors are held constant. However, it remains uncertain whether subsurface properties can exert such strong controls on emergent properties in real landscapes. We compared emergent hydrological function and topography in two watersheds with very similar climatic and tectonic history, but very different subsurface properties due to contrasting bedrock lithology. We found that hillslopes were systematically shorter and saturated areas more dynamic at the lower transmissivity site. To test whether these features could be the result of coevolution between topography, hydrological function, and subsurface properties, we estimated all parameters of a coupled groundwater‐landscape evolution model for each site. Limitations were revealed in the model's ability to reproduce aspects of morphology and hydrologic behavior, however, model results suggested differences in hillslope length and variably saturated area between the sites could be explained by differences in subsurface properties, and not by differences in geomorphic process rates alone. This work demonstrates one way subsurface hydrology can profoundly affect landscape evolution.

Constraining Ocean and Ice Shell Thickness on Miranda from Surface Geological Structures and Stress Modeling

Images from the Voyager 2 mission revealed the small Uranian satellite Miranda to be a complex, dynamic world. This is exemplified by signs of recent geological activity, including an extensive fault system and the mysterious coronae. This has led to speculation that Miranda may have been tectonically active within the geologically recent past and could have hosted a subsurface liquid water ocean at the time. In this work, we aim to constrain the thickness ranges for the ice shell and potential subsurface ocean on Miranda. Here, we present the results for our geological mapping of craters, ridges, and furrows on the surface. We also present the results for our comparison of the geographic distribution of these features to the predicted geographic distribution of maximum tidal stress based on stress models. We model eccentricity tidal stress, ice shell thickening stress, true polar wander stress, and obliquity tidal stress and compare the predicted surface stress pattern for each to what pattern can be inferred from the surface geology. Our results show that a thin crust (≤30 km) is most likely to result in sufficient stress magnitude to cause brittle failure of ice on Miranda’s surface. Our results also suggest the plausible existence of a ≥100 km thick ocean on Miranda within the last 100–500 million yr. This has implications for the dynamical history of Miranda and its status as a potential ocean world.

Late-stage evolution of hypogene caves at Tyuya-Muyun (Kyrgyzstan): Quantitative insights from mineral deposits

The Tyuya-Muyun massif in SW Kyrgyzstan hosts a number of caves some of which contain Ra- and U-bearing minerals that were extensively mined in the early 20th century. Previous studies have suggested that the caves of the Tyuya-Muyun have experienced a complex speleogenetic history, including epigene and hypogene processes. Here, we reconstruct the late stages of hypogene processes by studying subaqueous (calcite and barite spars, cave clouds), near-water table (rafts, folia) and vadose (flowstone) mineral deposits in Great Barite and Surprise caves. We determined the chronology (230Th dating), stable isotope composition (δ18O, δ13C and Δ47), and formation temperature (fluid inclusion microthermometry and Δ47) of these minerals and reconstructed the oxygen isotopic composition of the paleo-water (δ18Ow). In contrast to previous hypotheses about cave evolution, we found no evidence of an initial epigenic karst phase. The earliest mineral deposits suggest already hydrothermal conditions, with calcite and barite forming at ~40–50 °C prior to 600 ka. Cave clouds in Great Barite Cave mark the beginning of the lowering of the groundwater table and also record a decrease in water temperature from 29.7 ± 8.3 °C to 12.7 ± 5.6 °C, as shown by Δ47 thermometry. The latter temperature remained stable, within analytical uncertainties, during the past 600 ka, as indicated by Δ47 results of near-water table and vadose speleothems in Surprise Cave. At the same time, the decrease in δ18Ow suggests a reduced contribution of thermal water and an increased input of colder meteoric water. By around 540–450 ka Surprise Cave emerged from the phreatic zone, as indicated by 230Th ages of the highest folia and flowstone. This lowering of the water table continued until 84 ka and may have been related to the final uplift phase of the Tyuya-Muyun massif and the concomitant incision of the Aravan River forming the Dangi Gorge.