Isotopic Composition Of Stream Water Research Articles

Spatiotemporal dynamics of stable isotopes of different water sources in western Himalayas: Insights into regional hydrological processes

The spatiotemporal variations in stable isotope ratios are primarily related to atmospheric moisture transport. However, the isotopic composition of stream water is significantly influenced by the diverse landscape of glaciated and snow-dominated basins. The present study investigates the spatiotemporal variation of the isotopic composition of stream water and potential water sources such as rain, snowmelt, glacier melt, lakes, and groundwater in the Upper Jhelum River Basin (UJRB). Our findings reveal that the significant seasonal variability of precipitation is mainly associated with the transition between westerlies (WDs) and the Indian Southwest Monsoon (ISWM), each conveying distinct moisture to the UJRB. The isotopic signature in rain is actively controlled by the combined effect of temperature (T) and relative humidity (RH) during WDs. However, the impact of “amount effect” with lower condensation temperature is dominant during the latter half of ISWM. Seasonal variations in δ18O and d-excess in stream water are distinct, influenced by different contributing sources and topographic controls. A linear relationship is observed between δ18O vs. altitude and glacier area ratio (GAR). Lower δ18O in stream water and higher d-excess values are observed in the southwest at higher elevations in glacier-fed tributaries. On the northern side, higher δ18O in stream water indicates contributions from non-glacier fed tributaries and enhanced groundwater contributions rather than merely following the lower δ18O of precipitation along the strengthened WDs gradient. The δ18O isotopic lapse rate (ILR) of rain (−0.19‰/100 m) is higher compared to ILR for the river (−0.05‰/100 m), reflecting the mixing of water sources in river water. However, both are lower than the global average (−0.28‰/100 m). This study helps in understanding the influence of meteorological, topographic factors, and other water sources on stream water, a prerequisite for projecting a potential future change in the complex Himalayan region.

Catchment-scale impacts of shallow landslides on stream water chemistry

Catchment water quality plays an important role in ecosystem and water resource management in mountainous areas. Shallow landslides triggered by earthquakes or heavy rainfall can cause a sudden and long-term deterioration in stream water quality by releasing contaminants into streams. Although many studies have been undertaken on the relationship between a single landslide and the water chemistry of a nearby river, little is known about the impact of densely distributed shallow landslides on stream water chemistry at the catchment scale. To this end, this study determined the major ion concentrations and isotopic compositions of stream water along with the shallow landslide area/catchment area ratio (LCR) in 37 headwater subcatchments in the southern part of Hokkaido, Japan, where an earthquake caused more than 6000 shallow landslides on September 6, 2018. In subcatchments with a high LCR, stream water exhibited significantly higher Ca2+ and HCO3− concentrations, while there was no correlation between the LCR and concentrations of Na+ and Cl−. The δ18O and δD values of stream water plotted between the local meteoric water lines of summer and winter precipitation, indicating that they originated from meteoric water. Shallow landslides formed sliding surfaces, landslide deposits, and landslide-dammed lakes, which enhanced the interaction between the surface soil and stream water, leading to Ca–HCO3 type water. The results showed that shallow-landslide-driven changes in stream water quality could be linearly approximated by the fraction of the landslide area at the catchment scale, which is a more versatile approach than the local framework of a single landslide and a nearby stream. In future research, these findings could be combined with a slope stability model and the background climatic, geological, topographical, and water quality conditions of a watershed to evaluate water pollution triggered by shallow landslides at the catchment scale.

The value of water isotope data on improving process understanding in a glacierized catchment on the Tibetan Plateau

Abstract. This study integrated a water isotope module into the hydrological model THREW (Tsinghua Representative Elementary Watershed) which has been successfully used in high and cold regions. Signatures of oxygen stable isotope (18O) of different water inputs and stores were simulated coupling with the simulations of runoff generation. Isotope measurements of precipitation water samples and assumed constant isotope signature of ice meltwater were used to force the isotope module. Isotope signatures of water stores such as snowpack and subsurface water were updated by an assumed completely mixing procedure. Fractionation effects of snowmelt and evapotranspiration were modeled in a Rayleigh fractionation approach. The isotope-aided model was subsequently applied for the quantification of runoff components and estimations of mean water travel time (MTT) and mean residence time (MRT) in the glacierized watershed of Karuxung river on the Tibetan Plateau. Model parameters were calibrated by three variants with different combinations of streamflow, snow cover area and isotopic composition of stream water. Modeled MTT and MRT were validated by estimates of a tracer-based sine-wave method. Results indicate that (1) the proposed model performs well on simultaneously reproducing the observations of streamflow, snow cover area and isotopic composition of stream water, despite the fact that only precipitation water samples were available for tracer input; (2) isotope data facilitate more robust estimations on contributions of runoff components (CRCs) to streamflow in the melting season, as well as on MTT and MRT; (3) involving isotope data for the model calibration obviously reduces uncertainties in the quantification of CRCs and estimations of MTT and MRT, through better constraining the competitions among different runoff processes induced by meltwater and rainfall. Our results inform scientists on the high value of water isotope data for improving process understanding in a glacierized basin on the Tibetan Plateau.

Climate and landscape controls on spatio-temporal patterns of stream water stable isotopes in a large glacierized mountain basin on the Tibetan Plateau

The spatio-temporal variations of stream water stable isotopes are often assumed to follow atmospheric moisture transport over the Tibetan Plateau (TP). However, the isotopic composition of streamflow can be modified by the extensive variation in landscape properties in large glacierized mountain basins. In this study, the isotopic composition of stream water and its dominant controls in terms of spatial variation and potential water sources of rainfall, snow and glacier melt, and groundwater are analyzed based on synoptic water sampling from September 2018 to August 2019 over the Lhasa River basin (LRB) in the Southern TP. Results showed that: (1) δ18O variation in stream water is linearly proportional to longitude and latitude in the north. This spatial pattern is primarily controlled by cold mountainous environments, where stream water δ18O is more depleted and d-excess is higher towards the northwest and higher elevation in glacier-fed streams. Glacial melt could contribute considerably to streamflow generation, especially in the late monsoon season. (2) In the south, stream water δ18O does not simply follow depleted δ18O in precipitation along the strengthened Indian monsoon moisture gradient, but is enriched by strengthened local moisture recycling and increased groundwater contributions. The rainfall recharge is highly regulated and mixes with storage before it reaches the mainstem of the river. (3) The seasonal variations of stream water δ18O and d-excess are distinct, resulting from different contribution sources and catchment controls. In the pre-monsoon season, the strongest local moisture recycling obscures any simple stream water isotope lapse with elevation. These identified source areas and seasonal variations in the isotopic composition in stream water of LRB help us understand diverse water sources and flow paths to streams in this complex environment, which is a prerequisite for projecting potential future change.

Flux and influencing factors of CO2 outgassing in a karst spring-fed creek: Implications for carbonate weathering-related carbon sink assessment

Whether carbonate weathering on land represents an important terrestrial carbon sink has long been debated as the CO2 sequestered by the dissolution of carbonate may return to the atmosphere through CO2 outgassing from groundwater-feeding surface waters. In headwater area of karst terrain, the CO2 outgassing frequently happens in spring-fed streams, but its flux and influencing factors still remain under-researched. In this study, we conducted a monthly monitoring of hydrochemical and isotopic compositions of stream water along a karst spring-fed creek at Changliushui, SW China that has a steep gradient and in which tufa deposition occurs, in order to explore the process of CO2 outgassing and elucidate the outgassing flux across space and time and its influencing factors. The analyses of downstream geochemical evolution have revealed that two distinguishable types of CO2 outgassing exist along the flow path: (i) outgassing of physically dissolved soil CO2 during the process of carbonate dissolution, as the primary CO2 outgassing, and (ii) an additional outgassing of the CO2 arising from the precipitation of calcite. At a short distance from the spring, the aqueous CO2 escapes from water to a large degree through molecular diffusion driven by high gradient of partial pressure of CO2 (PCO2) between the water and the atmosphere. Such CO2 outgassing concomitantly causes the saturation index with respect to calcite (SIc) to rise, resulting in subsequent calcite precipitation and thus more CO2 outgassing. Spatiotemporal variations in the mass transfer of CO2 and CO2 outgassing flux show that the rates of both types of CO2 outgassing are significantly influenced by hydrodynamic conditions, such as flow discharge and streambed morphology. In particular, lower rates of CO2 outgassing are observed at higher flow discharge, and vice versa. In the studied reach of stream, the primary and additional CO2 outgassing have caused about 21%–35% and 4%–20% of the total soil CO2 sequestered in DIC of the feeding spring water to be released back to the atmosphere, respectively and the higher flow discharge, the less CO2 return. Based on these findings, we postulate that the CO2 outgassing, especially that induced by calcite precipitation should be relatively smaller in streams which have a lower gradient and elevated discharge rate and where the mixing or dilution of tributaries and increased in-stream metabolism occur. This provides more evidence for the previous conclusion that the H2O-carbonate-CO2-aquatic phototroph interaction could represent a net carbon sink at least over short timescales.

Effects of ecological construction on the transformation of different water types on Loess Plateau, China

Water is the most essential resource for the ecological and biological survival of organisms as well as being an important strategic socio-economic factor. Stable isotopes of δD and δ18O in water are important indicators of hydrological and ecological process. In this study, temporal and spatial variations in δD and δ18O and transformations between three water bodies (precipitation, stream water, and groundwater) under ecological construction were studied in two contrasting watersheds of the Wuding River, China. In order to understanding the spatial and temporal variation of stable isotopes and water transmission times (WTT) under ecological construction, a total of 1028 water samples were collected from the 30 sites, and 79 precipitation samples were collected at the weather station. The results show that variation range of three water bodies occurred in the order precipitation > stream water > groundwater, the local meteoric water line was above the level of the last two, and the isotopic composition of stream water and groundwater in controlling watershed is more enriched than that in natural watershed. WTT from precipitation to stream water in Jiuyuangou were 1.53 times those of Peijiamao. Similarly, WTT from precipitation to groundwater was about 7.6 times than that form precipitation to stream water. Supply ratios exhibited obvious seasonal variation. Precipitation and groundwater recharged stream water mostly in the dry season, while precipitation and stream water recharged groundwater during the wet season. Overall, this study shows that ecological construction measures extend WTTs and enhance water evaporation and fractionation. The results of this research are significant as they enhance our understanding of water transformation on the Loess Plateau under ecological construction.

Oxygen and hydrogen isotopic composition of waters in a past-mining area of southern Apuan Alps (Italy): Hydrogeological characterization and implications on the fate of potentially toxic elements

The oxygen and hydrogen isotopic composition of stream water, springs, groundwater tapped from irrigation wells and acid mine drainages were determined during two different surveys in 2015 in an area highly impacted by past-mining activity of Apuan Alps (Italy), as a guidance on good practices for water management. The isotopic local meteoric water line (LMWL) was built by monthly collecting rainwater between 2014 and 2018, given by δD = 7.02 ± 0.35 × δ18O + 8.54 ± 2.89. The obtained results indicate that acid mine drainages (AMD) are supplied by freshwater from karst systems which flow throughout the post-mining workings. Such waters contaminate by interactions with sulfides (pyrite) that remained unmined in the ore-bodies. During rainstorms, infiltration rainwater displaces water ponding within mine, sharply increasing the outflow rate of highly-contaminated AMD. Acidic drippings in tunnels show an isotopic shift in both δ18O and δD values attributable to pyrite oxidation and Fe hydrolysis. The data reveal that karst-springs represents the primary supply for the stream. The isotopic data also reveal that waters flowing in the bedrock carbonate aquifer represent the main feeding component for the overlying alluvial aquifer tapped by wells. The prevailing transfer of clean freshwaters from the carbonate aquifer towards the alluvial aquifer system mitigates the possible influence of contaminated water from stream seepage. However, these observations require a monitoring program on water quality to be established.

Multiyear Increase in the Stable Isotopic Composition of Stream Water From Groundwater Recharge Due to Extreme Precipitation

AbstractThe isotopic composition of surface and groundwater is impacted by many hydrologic processes. The long‐term stable water isotopic response of systems to hydrologic change is critical for interpreting isotopic information for streamflow generation, stream‐aquifer‐coupling, and recharge processes. To evaluate the response of stream‐aquifer systems to extreme precipitation events we use 743 surface and groundwater isotopes with drainage areas ranging from 0.1 to >800 km2. Results show multiannual trends from high to low isotopic compositions associated with increases in the composition of shallow groundwater. The year 2011 was one of the wettest years and the months of August and September were the wettest consecutive 2‐month period in the 123‐year record. This increase in the isotopic composition has long‐term impact on the isotopic composition of surface and groundwater highlighting the importance of groundwater sources of baseflow to streams and the transient storage and release mechanisms of groundwater at the catchment scale.

The fate of monsoonal atmospheric nitrate deposition in two forest catchments in Soyang lake watershed, South Korea: a mass balance and stable isotope approach

Over 60% of South Korea is covered by forests. Nitrate deposition plays an important role as nitrogen source in these forests. Nitrate input from deposition increases nitrogen availability leading initially to increases in productivity. However, later forests may become nitrogen saturated with nitrate leaching, denitrification and forest decline. Using a mass balance approach, we compared the atmospheric nitrate input and output by stream runoff in two forest sub-catchments, a deciduous and a mixed catchment, within the Soyang lake watershed, the main drinking water reservoir for the 20-million-person metropolis Seoul. A dual stable isotope approach was used to identify the origin of nitrate in stream discharge of the two sub-catchments. Extremely different monsoon seasons in 2013 and 2014 drove the nitrate discharge. Total nitrate-N export was closely related with rainfall events and intensities. Nitrate-N discharge at the deciduous forest was lower than nitrate-N deposition, thus sink conditions prevailed. Nitrate-N discharge at the mixed forest was consistently higher. During the heavy monsoon season of 2013 the mixed forest turned from a sink for atmospheric nitrate towards a source, i.e. nitrate amounts in the discharge were higher than atmospheric inputs. Nitrate isotopic compositions of stream water from both forests revealed soil nitrification as main nitrate source. However, under conditions of heavy monsoon rainfall direct runoff of atmospheric deposition nitrate was identified in the mixed forest. Lower nitrate assimilation capacities of conifers compared to broadleaf trees are probably the driver for a lower nitrate retention capacity making coniferous forests more prone to nitrate leaching.

Isotopic characteristics of precipitation, groundwater, and stream water in an alpine region in southwest China

The hydrological cycle has been enhanced under the warmer climatic conditions on the southeastern edge of the Tibetan Plateau, China. To investigate the complex hydrological system in the study area, δ18O and δ2H were measured on waters in a mountainous region—Huangbengliu watershed. The stream water tends to be isotopically more depleted in 18O and 2H than precipitation and groundwater, but more enriched than meltwater. A clear seasonality with higher isotopic values of stream water in summer and lower values in winter is evident, coinciding with the timing of the peak discharge. The correlation between δ18O and δ2H in stream water shows that the isotopic signature of precipitation is well preserved in stream flow, and the evaporation during the stream flow is only minor. The isotopic composition of stream water shows a decreasing trend from upstream to downstream, which results from tributary water inputs with heavy-isotope-depleted water. According to an isotopic mass-balance model, the fraction of meltwater inputs over the total stream flow ranged from 25.5 to 61.8 %. The study demonstrates that the ice–snow melting and tributary inputs are the prevailing mechanisms regulating stream isotope hydrology in alpine regions on the edge of the Tibetan Plateau.

Evaporative fractions and elevation effects on stable isotopes of high elevation lakes and streams in arid western Himalaya

Isotopes of oxygen and hydrogen in water from streams, snow, and lakes were used to model the ratio of evaporation to total inflow (E/I) of four high elevation lakes in closed basins in the Indian Himalaya. Air temperature and relative humidity (h) data from meteorological stations and global climate grids (GMAO-MERRA) were used as input to the model. A second model of the volume of inflow during snowmelt constrained the magnitude of seasonal variability in isotopic composition. Similar to other areas of the Himalaya, elevation was a strong determinant of isotopic composition of stream water, suggesting that heavier isotopes rain out at lower elevations. Deuterium excess (d) in stream water suggests that summer precipitation originating from the Bay of Bengal rather than winter precipitation from Central Asia is the dominant source of precipitation. For the largest and deepest lakes (>15 m), E/I was 77–87%, and 18–50% for two shallow lakes (<2 m). Modelled E/I was sensitive to h; ground-level measurements of h are needed to constrain E/I estimates in arid mountain regions, though h from GMAO also produced reasonable E/I values. The results suggest that some lakes in closed basins in the Himalaya lose a significant fraction of their inflow to groundwater, particularly shallow lakes with a low ratio of lake volume to watershed area. Isotopic values from the mainstem of the Indus River suggest that evaporatively enriched waters have limited impact on both wet and dry season discharge at the basin scale. Lakes with a high evaporative fraction may be uniquely sensitive to climate, and isotopic analysis can help identify lakes that may be vulnerable to climate fluctuations and change.

Storage dynamics in hydropedological units control hillslope connectivity, runoff generation, and the evolution of catchment transit time distributions.

We examined the storage dynamics and isotopic composition of soil water over 12 months in three hydropedological units in order to understand runoff generation in a montane catchment. The units form classic catena sequences from freely draining podzols on steep upper hillslopes through peaty gleys in shallower lower slopes to deeper peats in the riparian zone. The peaty gleys and peats remained saturated throughout the year, while the podzols showed distinct wetting and drying cycles. In this region, most precipitation events are <10 mm in magnitude, and storm runoff is mainly generated from the peats and peaty gleys, with runoff coefficients (RCs) typically <10%. In larger events the podzolic soils become strongly connected to the saturated areas, and RCs can exceed 40%. Isotopic variations in precipitation are significantly damped in the organic-rich soil surface horizons due to mixing with larger volumes of stored water. This damping is accentuated in the deeper soil profile and groundwater. Consequently, the isotopic composition of stream water is also damped, but the dynamics strongly reflect those of the near-surface waters in the riparian peats. “pre-event” water typically accounts for >80% of flow, even in large events, reflecting the displacement of water from the riparian soils that has been stored in the catchment for >2 years. These riparian areas are the key zone where different source waters mix. Our study is novel in showing that they act as “isostats,” not only regulating the isotopic composition of stream water, but also integrating the transit time distribution for the catchment.Key PointsHillslope connectivity is controlled by small storage changes in soil unitsDifferent catchment source waters mix in large riparian wetland storageIsotopes show riparian wetlands set the catchment transit time distribution

Study of stable isotopes for highly deformed aquifers in the Hsinchu-Miaoli area, Taiwan

This study was based on the analysis of isotopic compositions of hydrogen and oxygen in samples from precipitation, groundwater and stream water. In addition, parts of groundwater samples were dated by carbon-14 and tritium. These data are integrated to provide other views of the hydrologic cycle in the Hsinchu-Miaoli groundwater district. The groundwater district is principally composed of Pleistocene and Holocene aquifers. The Pleistocene aquifers are highly deformed by folding and faults into small sub-districts with areas of only tens of square kilometers. These aquifers are exclusively recharged by local precipitation. The Holocene aquifers cover narrow creek valleys, only tens of meters in thickness. The local meteoric water line (LMWL), constructed from rainfall samples in the Hsinchu Science Park, is described by the equation δD=8.02δ18O+10.16, which agrees with the global meteoric water line. In addition, the precipitation isotopic compositions can be categorized into two distinct end members: typhoon type and monsoon type. The groundwater isotopic compositions are perfectly located on an LMWL and can be considered a mixture of precipitations. Based on the mass balance of isotopic compositions of oxygen and hydrogen, infiltration is more active in the rainy season with depleted isotopic compositions. The amount of infiltration during May–September is roughly estimated to comprise at least 55% of the whole year’s recharge. The isotopic compositions of stream water are expressed by a regression equation: δD=7.61δ18O+9.62, which is similar to the LMWL. Although precipitation isotopic compositions are depleted during summer time, the isotopic compositions contrarily show an enriched trend in the upstream area. This is explained by the opposite altitude effect on isotopic compositions for typhoon-related precipitations.