End-member Mixing Research Articles

Effects of bedrock groundwater discharge on spatial variability of dissolved carbon, nitrogen, and phosphorous concentrations in stream water within a forest headwater catchment

AbstractThe quantitative evaluation of the effects of bedrock groundwater discharge on spatial variability of stream dissolved organic carbon (DOC), dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorous (DIP) concentrations has still been insufficient. We examined the relationships between stream DOC, DIN and DIP concentrations and bedrock groundwater contribution to stream water in forest headwater catchments in warm‐humid climate zones. We sampled stream water and bedrock springs at multiple points in September and December 2013 in a 5 km2 forest headwater catchment in Japan and sampled groundwater in soil layer in small hillslopes. We assumed that stream water consisted of four end members, groundwater in soil layer and three types of bedrock groundwater, and calculated the contributions of each end member to stream water from mineral‐derived solute concentrations. DOC, DIN and DIP concentrations in stream water were compared with the calculated bedrock groundwater contribution. The bedrock groundwater contribution had significant negative linear correlation with stream DOC concentration, no significant correlation with stream DIN concentration, and significant positive linear correlation with stream DIP concentration. These results highlighted the importance of bedrock groundwater discharge in establishing stream DOC and DIP concentrations. In addition, stream DOC and DIP concentrations were higher and lower, respectively, than those expected from end member mixing of groundwater in soil layer and bedrock springs. Spatial heterogeneity of DOC and DIP concentrations in groundwater and/or in‐stream DOC production and DIP uptake were the probable reasons for these discrepancies. Our results indicate that the relationships between spatial variability of stream DOC, DIN and DIP concentrations and bedrock groundwater contribution are useful for comparing the processes that affect stream DOC, DIN and DIP concentrations among catchments beyond the spatial heterogeneity of hydrological and biogeochemical processes within a catchment.

Constraining the Water Cycle Model of an Important Karstic Catchment in Southeast Tibetan Plateau Using Isotopic Tracers (2H, 18O, 3H, 222Rn)

Understanding the connectivity between surface water and groundwater is key to sound geo-hazard prevention and mitigation in a waterscape such as the Jiuzhaigou Natural World Heritage Site in the southeast Tibetan Plateau, China. In this study, we used environmental isotope tracers (2H, 18O 3H, and 222Rn) to constrain a water cycle model including confirming hydrological pathways, connectivity, and water source identification in the Jiuzhaigou catchments. We established the local meteoric water line (LMWL) based on the weekly precipitation isotope sampling of a precipitation station. We systematically collected water samples from various water bodies in the study area to design the local water cycle model. The regional water level and discharge changes at one month after the earthquake indicated that there was a hydraulic connection underground across the local water divide between the Rize (RZ) river in the west and Zechawa (ZCW) lake in the east by the δ18O and δ2H measurements. We employed an end-member mixing model to identify and quantify Jiuzhaigou runoff-generating sources and their contributions, and we found that the average contributions of precipitation and groundwater to the surface runoff in the catchments are about 30% and 70%, respectively. The two branches of the Shuzheng (SZ) trunk were recharged by 62 ± 19% from the ZCW lake and 38 ± 19% from the RZ river, which was consistent with the fractions calculated by the actual discharge volume. 222Rn mass balance analyses were employed to estimate the water exchange between groundwater and river, which further confirmed this estimate. 222Rn concentrations and 3H contents showed that the groundwater had a short residence time and it was moderate precipitation, thought the contribution of groundwater to the river was 70%, according to the different tracers. A three-dimensional conceptual model of the water cycle that integrated the regional hydrological and geological conditions was established for the catchments.

Modeling organic matter sources of sediment fluxes in eroding landscapes: Review, key challenges, and new perspectives

The coupled relationship between soil erosion and soil organic matter (OM) dynamics has sparked considerable interest in scientific communities in recent decades. However, the question of whether soil erosion acts as a net source or sink for atmospheric carbon dioxide is still a pending issue, possibly due to the uncertainties regarding the fluxes and fates of laterally transported OM in eroding landscapes. Quantifying the OM sources of redistributed sediments on the earth’s surface provides new insight for understanding this issue. In this review, we offer a comprehensive cross-disciplinary review of three approaches to modeling (i.e., end-member mixing models, Bayesian isotope mixing models, and parallel factor analysis (PARAFAC) models) that are widely used to accurately quantify the sources of redistributed OM in its bulk and soluble phases. Furthermore, we present the current knowledge of the benefits and shortcomings of each model and identify key challenges for determining their feasibility. We find that Bayesian isotope mixing models, especially the stable isotope analysis in the R model, better incorporate the uncertainty and variation of tracers compared with end-member mixing models, demonstrating a greater capability for quantifying various bulk OM sources. Excitation-emission matrix spectroscopy (EEMs) - PARAFAC models can identify changes in the fluorescence of dissolved OM to enable tracing of individual fluorescent fractions, thereby usefully supplementing and enhancing bulk OM source apportionment. Finally, future perspectives are proposed to further our understanding of model performance for quantifying the fluxes and fates of redistributed OM across a range of conditions.

Deep Nonnegative Dictionary Factorization for Hyperspectral Unmixing

As a powerful blind source separation tool, Nonnegative Matrix Factorization (NMF) with effective regularizations has shown significant superiority in spectral unmixing of hyperspectral remote sensing images (HSIs) owing to its good physical interpretability and data adaptability. However, the majority of existing NMF-based spectral unmixing methods only adopt the single layer factorization, which is not favorable for exploiting the complex and structured representation relationship of endmembers implied in HSIs. In order to overcome such an issue, we propose a novel two-stage Deep Nonnegative Dictionary Factorization (DNDF) approach with a sparseness constraint and self-supervised regularization for HSI unmixing. Beyond simply extending one-layer factorization to multi-layer, DNDF conducts fuzzy clustering to tackle the mixed endmembers of HSIs. Moreover, self-supervised regularization is integrated into our DNDF model to impose an effective constraint on the endmember matrix. Experimental results on three real HSIs demonstrate the superiority of DNDF over several state-of-the-art methods.

Biogeochemical alteration and fluxes of dissolved organic matter and nutrients in coastal bays

We measured dissolved organic carbon (DOC), δ13C-DOC, fluorescent dissolved organic matter (FDOM), dissolved inorganic and organic nitrogen (DIN; DON), dissolved inorganic phosphate (DIP), and radium isotopes (223Ra, 224Ra, and 226Ra) in three different bays: Gwangyang Bay (GB), Suyoung Bay (SB), and Ulsan Bay (UB), Korea. The water residence times based on Ra isotopes were approximately 15 (±7) for GB and 1–3 days for SB and UB. UB and SB showed clear two (terrestrial and marine) endmember mixing trends for DOC, DIN, and DIP. In addition, significant “excess” DOC (10–50% of outer bay) was observed for SB under depleted nutrient conditions. GB, which had the longest water-residence time, showed depleted DIN and DIP, with large “excess” DOC (40–60% of the outer bay) and DON (10–40% of the outer bay) concentrations. The excess DOC observed in SB and GB was found to be marine in origin, although there was a slight influence of terrestrial DOC in the low-salinity waters of SB based on δ13C-DOC values. Terrestrial humic-like FDOM was conservatively mixed in the three bays. The net fluxes of these components, estimated using the water residence times, suggest that GB is a significant source of DOM (DOC, DON, and humic FDOM) but a sink of DIN and DIP. SB and UB are the sources of inorganic nutrients as well as DOM to the open ocean. Therefore, our study reveals that the residence time of coastal embayment play an important role in the biogeochemical production and alteration of nutrients and DOM.

Identification and quantification of contributions to karst groundwater using a triple stable isotope labeling and mass balance model

Although karst groundwater systems provide critical ecosystem services in many regions worldwide, anthropogenic contamination has seriously degraded groundwater quality. Properly elucidating geochemical processes, quantifying contributions of natural and anthropogenic end members, and then protecting karst aquifer systems remain challenging from scientific and engineering aspects. To identify the hydrochemical processes and quantifying contributions of end members (especially, contamination end members), 49 samples were collected from cave waters (CW), artesian springs (AS), and gravity springs (GS) in a karst watershed in Guiyang, China. With increased anthropogenic contamination, the CW, AS, and GS characterized by a Ca–Mg–SO42--HCO3- composition often had pH and SO42− concentrations exceeding USEPA secondary drinking water standards. That is attributed to the influence of water-rock interaction, rainfall, and anthropogenic sources (mainly, sewage and fertilizers), in agreement with the compositions of δ34SSO4, δ18OSO4, and 87Sr/86Sr as well as the results of principal component analysis and correlation coefficients. Based on an end-member mixing model, contributions of rainfall and anthropogenic sources were 47% and 33% of GS, 52% and 41% of CW, and 58% and 35% of AS, respectively. It suggests that the karst groundwater quality is predominantly controlled by rainfall and anthropogenic sources (especially, land use). Results may be applied to properly evaluate the impacts of natural and anthropogenic sources on karst aquifers, coupled with actions to efficiently control potential contamination end members.

Carbon dioxide sources and sinks in the delta of the Paraíba do Sul River (Southeastern Brazil) modulated by carbonate thermodynamics, gas exchange and ecosystem metabolism during estuarine mixing

Tropical estuarine deltas generally present poorly buffered waters in their freshwaters. Carbonate chemistry predicts that mixture of such warm freshwater with seawater will create rapid consumption of the freshwater carbon dioxide (CO2) by the carbonate buffering capacity of the seawater. In this study, we used the Paraiba do Sul River as a laboratory to investigate how thermodynamics compare with biological processes, gas exchange, and tidal advection from mangrove. We conducted three spatial surveys covering the salinity gradient of the main channel and surrounding mangrove waters and one 24-h mooring in a mangrove creek. In the main channel, dissolved inorganic carbon (DIC) and total alkalinity (TA) showed closely conservative distribution along the salinity gradient, increasing seaward. The partial pressure of CO2 (pCO2) followed a bell-shaped curve predicted by carbonate chemistry for conservative mixing of river and ocean endmembers. During high river flow, pCO2 sharply decreased between salinities 0 and 5 (1800 to 390 ppmv), a pCO2 drawdown attributed to riverine outgassing and thermodynamics. Indeed, the mixing of TA-poor freshwater (363 ± 16 μmol kg−1) with TA-rich seawater creates a deficit of dissolved CO2 not related to biotic processes. During low river flow, the entire mixing zone was undersaturated in pCO2 with an increasing trend seaward. However, observed pCO2 values were slightly above those predicted by conservativity. Approximately half of this deviation was attributed to biological activity (net heterotrophy), and remaining deviation was assigned to the effects of gas exchange (18%) and water heating (36%). The effect of gas exchange was higher in fresh and low salinity waters, reflecting the higher outgassing/ingassing of CO2, and lower buffering capacity. Water heating was more important in mid- to high-salinities as a result of diel patterns of solar irradiance. Heterotrophy was slight and not able to outcompete thermodynamics and generate outgassing during estuarine mixing. Consistently, stable isotopic signatures of DIC (δ13C-DIC) presented slight deviations below the conservative mixing, corroborating net heterotrophy in the main channel. Areas of CO2 uptake due to phytoplankton activity were identified but restricted to the freshwater endmember during low river flow, with lowest pCO2 (up to 41 ppmv) and the highest chlorophyll a (up to 21.3 μg L−1). The estuary was a CO2 sink during low river flow (−1.34 to −5.26 mmolC m−2 d−1) and a source during high river flow (5.71 to 19.37 mmolC m−2 d−1). In the mangrove creek, the pCO2, DIC, δ13C-DIC and TA presented deviations from the conservativity, with slopes between TA and DIC demonstrating organic carbon degradation mediated by aerobic respiration and sulphate reduction. Mangrove creek waters were a CO2 source (average of 134.81 mmolC m−2 d−1), exhibiting high values of pCO2 (up to 21,720 ppmv). The results reveal that the low buffering capacity in the main channel of tropical estuarine deltas can be the predominant driver of pCO2, generating CO2 undersaturation along the mixing zone, a process overlooked in estuarine systems. Moreover, air-water CO2 exchange, thermal variability, and biological activities contribute to deviation of the carbonate system from conservative mixing in specific estuarine areas, also modulating pCO2 variability.

Effects of human activities on soil organic carbon redistribution at an agricultural watershed scale on the Chinese Loess Plateau

Human activity is thought to perturb the dynamic change of organic carbon (OC), but its impact on the transportation and redistribution of erosion-induced OC is still poorly understood. Here, we use multi-source field sampling data and long-term hydrological observation data to clarify the transportation and redistribution of OC in an agricultural watershed on the Loess Plateau. The endmember mixing model based on the radiocarbon isotopes (14C) and a budget equation were used to assess the composition and budget of erosion-induced OC in a 187 km2 watershed. The results showed that compared with the period of agricultural activities (1960−1969), soil conservation activities in 1970−1999 and 2000−2019 reduced soil erosion by 31.5 % and 75.4 %, respectively. Additionally, land use changes significantly reduced the mobilized OC, from 14,370 ± 1966 (1960−1969) to 3311 ± 431 Mg C yr−1 (2000−2019). Check dam construction led to the mobilized OC was effectively buried at a rate of 2125 ± 478 and 1420 ± 282 Mg C yr-1 in periods of 1970−1999 and 2000−2019, respectively. Further radiocarbon isotopes analysis shows that the land use changes and check dam construction effectively reduced the output of ancient petrogenic organic carbon (OCpetro) and young biospheric organic carbon (OCbio) from the watershed, which has a positive impact on the regulation of atmospheric CO2 level. Our results emphasize the positive effects of soil conservation activities on terrestrial carbon sequestration, which are of great significance to clarify the effects of soil erosion on the carbon cycle.

A novel combined approach for compositional and Pb isotope data of (leaded) copper-based alloys: bronze coinage in Magna Graecia and Rome (5th to 2nd centuries BCE)

Lead is a ubiquitous and ambiguous component of ancient copper-based artefacts as it can be a residue of the ore or could have been deliberately added. In this study, we present compositional and Pb isotope data of a set of Roman, Syracusan and south Italian indigenous bronze coins from the 5th to 2nd centuries BCE. We describe an approach that considers the Pb content and its potential implications for provenance and technology as a variable alongside Pb isotope source characteristics. By combining elemental with isotopic data of lead and, in a subsequent step, with trace element signatures, this method allows to identify endmembers of natural and/or anthropogenic mixing and to outline their possible provenance. In a wider context, our results show how the existing bronze currencies transform parallel to the changing political situation and how Rome's bronze monetary instruments develop from a combination of local tradition and Greek practices into a coinage in its own right.

Helium and carbon isotopic signatures of thermal spring gases in southeast Yunnan, China

Late Palaeozoic and Cenozoic magmatism, including Pingbian Quaternary basaltic volcanoes, occurs in southeast Yunnan, China, where seismic tomography evidence indicates magma activity in the uppermost mantle. However, the characteristics of mantle fluid release remain poorly understood. Here, we report the chemical compositions of hot spring gases in SE Yunnan and measure the 3He/4He ratios, 4He/20Ne ratios, and δ13C-values (CO2, CH4) to determine their origins and the factors influencing their migration. Except for five samples contaminated by the atmosphere (4He/20Ne ratios, 0.39 ̶2.39), the Rc/Ra ratios (Rc is the air-corrected 3He/4He ratio, Ra is the air ratio with 1.4 × 10−6) range from 0.042 Ra to 0.470 Ra, indicating that the helium is mostly of crustal origin with minor addition of mantle-derived helium. The calculated maximum proportion of mantle helium was ~5% of total helium, indicating that mantle 3He leakage correlates with the ductile lower crust. The hot spring gases are fractionated by the partial dissolution of CO2 in groundwater, and the initial δ13CCO2 values are modelled by Rayleigh fractionation within a range of −26.5‰ to −6.14‰. The modelled CO2 in samples estimated by the HeC isotope coupling model originates from the mixing of limestone and organic sediment end-members. The isotopic signature of CH4 (−69.2‰ to 35.9‰) suggests a thermogenic source and microbial oxidation in some samples. Furthermore, the apparent isotopic temperatures are estimated using fractionation factors for CH4 and CO2, suggesting a lack of magmatism in the crust beneath the Red River fault zone (188–318 °C), but partial melting under the Bozhushan granitoid plutons (572–688 °C). The low emission of mantle-derived helium is attributed to the low fault permeability and minor active crustal deformation in present-day SE Yunnan, which restrain the upward migration of mantle fluids.

Variations in organic carbon sourcing along a trans-Himalayan river determined by a Bayesian mixing approach

Rivers transfer particulate organic carbon (POC) from eroding mountains into geological sinks. Organic carbon source composition and selective mobilization have been shown to affect the type and quantity of POC export, but their combined effects across complex mountain ranges remain underexplored. Here, we examine the variation in organic carbon sourcing and transport in the trans-Himalayan Kali Gandaki River catchment, along strong gradients in precipitation, rock type and vegetation. Combining bulk stable nitrogen, and stable and radioactive organic carbon isotopic composition of bedrock, litter, soil and river sediment samples with a Bayesian end-member mixing approach, we differentiate POC sources along the river and quantify their export. Our analysis shows that POC export from the Tibetan segment of the catchment, where carbon bearing shales are partially covered by aged and modern soils, is dominated by petrogenic POC. Based on our data we re-assess the presence of aged biospheric OC in this part of the catchment, and its contribution to the river load. In the High Himalayan segment, we observed low inputs of petrogenic and biospheric POC, likely due to very low organic carbon concentrations in the metamorphic bedrock, combined with erosion dominated by deep-seated landslides. Our findings show that along the Kali Gandaki River, the sourcing of sediment and organic carbon are decoupled, due to differences in rock organic carbon content, soil and above ground carbon stocks, and geomorphic process activity. While the fast eroding High Himalayas are the principal source of river sediment, the Tibetan headwaters, where erosion rates are lower, are the principal source of organic carbon. To robustly estimate organic carbon export from the Himalayas, the mountain range should be divided into tectono-physiographic zones with distinct organic carbon yields due to differences in substrate and erosion processes and rates.

Do stream water solute concentrations reflect when connectivity occurs in a small, pre-Alpine headwater catchment?

Abstract. Expansion of the hydrologically connected area during rainfall events causes previously disconnected areas to contribute to streamflow. If these newly contributing areas have a different hydrochemical composition compared to the previously connected contributing areas, this may cause a change in stream water chemistry that cannot be explained by simple mixing of rainfall and baseflow. Changes in stormflow composition are, therefore, sometimes used to identify when transiently connected areas (or water sources) contribute to stormflow. We identified the dominant sources of streamflow for a steep 20 ha pre-Alpine headwater catchment in Switzerland and investigated the temporal changes in connectivity for four rainfall events based on stream water concentrations and groundwater level data. First, we compared the isotopic and chemical composition of stormflow at the catchment outlet to the composition of rainfall, groundwater and soil water. Three-component end-member mixing analyses indicated that groundwater dominated stormflow during all events, and that soil water fractions were minimal for three of the four events. However, the large variability in soil and groundwater composition compared to the temporal changes in stormflow composition inhibited the determination of the contributions from the different groundwater sources. Second, we estimated the concentrations of different solutes in stormflow based on the mixing fractions derived from two-component hydrograph separation using a conservative tracer (δ2H) and the measured concentrations of the solutes in baseflow and rainfall. The estimated concentrations differed from the measured stormflow concentrations for many solutes and samples. The deviations increased gradually with increasing streamflow for some solutes (e.g. iron and copper), suggesting increased contributions from riparian and hillslope groundwater with higher concentrations of these solutes and thus increased hydrological connectivity. The findings of this study show that solute concentrations partly reflect the gradual changes in hydrologic connectivity, and that it is important to quantify the variability in the composition of different source areas.

Geologic Controls on Source Water Drive Baseflow Generation and Carbon Geochemistry: Evidence of Nonstationary Baseflow Sources Across Multiple Subwatersheds

AbstractThe contributions and composition of baseflow sources across an extended recession period were quantified for six subwatersheds of varying size in a structurally complex watershed in coastal California using endmember mixing analysis and related to catchment characteristics (e.g., topography, geology, land use, and soil characteristics). Both shallow subsurface and deep groundwater reservoirs were important contributors for streamflow during low flow periods, and the composition of baseflow sources across subwatersheds was directly related to geologic indices. A binary classification of underlying bedrock permeability (e.g., low vs. high) best explained the changes in shallow subsurface water and deeper groundwater inputs through the seasonal recession. Dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), and specific UV absorbance at 254 nm (SUVA254) were used to provide additional insight into endmember characteristics and their contributions to baseflow. Stream water DIC concentrations were broadly controlled by mixing of groundwater and shallow subsurface water endmembers with relatively constant DIC concentrations, while stream water DOC concentrations reflected both spatial and temporal changes in shallow subsurface water DOC. Results from this study show (1) the importance of considering baseflow as a dynamic mixture of water from multiple sources, (2) the effect of geology on source composition at the subwatershed scale during low flow conditions, and (3) the impact of shifting baseflow sources on stream water dissolved carbon concentrations and the utility of using dissolved carbon concentrations to obtain additional insight into temporal variability in baseflow sources.

Comparing Bayesian and traditional end-member mixing approaches for hydrograph separation in a glacierized basin

Abstract. Tracer data have been successfully used for hydrograph separation in glacierized basins. However, in these basins uncertainties of the hydrograph separation are large and are caused by the spatiotemporal variability in the tracer signatures of water sources, the uncertainty of water sampling, and the mixing model uncertainty. In this study, we used electrical conductivity (EC) measurements and two isotope signatures (δ18O and δ2H) to label the runoff components, including groundwater, snow and glacier meltwater, and rainfall, in a Central Asian glacierized basin. The contributions of runoff components (CRCs) to the total runoff and the corresponding uncertainty were quantified by two mixing approaches, namely a traditional end-member mixing approach (abbreviated as EMMA) and a Bayesian end-member mixing approach. The performance of the two mixing approaches was compared in three seasons that are distinguished as the cold season, snowmelt season, and glacier melt season. The results show the following points. (1) The Bayesian approach generally estimated smaller uncertainty ranges for the CRC when compared to the EMMA. (2) The Bayesian approach tended to be less sensitive to the sampling uncertainties of meltwater than the EMMA. (3) Ignoring the model uncertainty caused by the isotope fractionation likely led to an overestimated rainfall contribution and an underestimated meltwater share in the melt seasons. Our study provides the first comparison of the two end-member mixing approaches for hydrograph separation in glacierized basins and gives insight into the application of tracer-based mixing approaches in similar basins.

Gallium: A New Tracer of Pacific Water in the Arctic Ocean

AbstractDetermining the proportions of Atlantic and Pacific Ocean seawater entering the Arctic Ocean is important both for understanding the mass balance of this basin as well as its contribution to formation of North Atlantic deep water. To quantify the distribution and amount of Pacific and Atlantic origin seawater in the western Arctic Ocean, we used dissolved Ga in a four‐component linear endmember mixing model. Previously, nutrients, combined in their Redfield ratios, have been used to separate Pacific‐ and Atlantic‐derived waters. These nutrient tracers are not conservative in practice, and there is a need to find quantities that are conserved. Dissolved Ga concentrations show measurable contrast between Atlantic and Pacific source waters, shelf‐influenced waters show little impact of shelf processes on the dissolved Ga distribution, and dissolved Ga in the Arctic basins is conserved along isopycnal surfaces. Thus, we explored the potential of Ga as a new parameter in Arctic source water deconvolution. The Ga‐informed deconvolution was compared to that generated with the NO3:PO4 relationship. While distributions of the water masses were qualitatively similar, the Ga‐based deconvolution predicted higher amounts of Pacific water at depths between 150 and 300 m. The Ga‐based decomposition yields a smoother transition between the halocline and Atlantic layers, while nutrient‐based solutions have sharper transitions. A 1‐D advection‐diffusion model was used to constrain estimates of vertical diffusivity (Kz). The Ga‐based Kz estimates agreed better with those from salinity and temperature than the nutrient method. The Ga‐based approach implies greater vertical mixing between the Pacific and Atlantic waters.

An integrated approach to estimate the mixing ratios in a karst system under different hydrogeological conditions

Study regionThe karst system of the Syemareh Dam site with the reservoir capacity of 3.2 bm3, located in the Zagros fold belt, southwestern of Iran, was studied under two conditions: before and after the reservoir impoundment as pre-reservoir and post-reservoir conditions, respectively. Study focusDespite a mixing of several water resources including the dam reservoir contributed to the flow rate of the downstream springs, the number of end-members was still unknown and their mixing ratio was unquantified. In this study, firstly a statistical approach (e.g. loading plot and matrix plot) was utilized to identify possible chemical reactions. Then, End Member Mixing Analysis (EMMA) was used to determine the number of end members. Lastly, the MIX code was utilized to quantify the mixing ratios. Three scenarios (i.e. assuming three, four and five end members) were compared to find a reliable mixing model based on the best match between the observed and estimated values of species concentration. In order to improve the calibration of the reliable mixing model, the species concentrations were corrected by deduction of the effect of plausible chemical reactions. New hydrological insights for the regionThe five end-members mixing model consisted of three aquifers, deep groundwater, and the reservoir was suggested as the dominant mixing process. Leakage from the reservoir as an end member was quantified by calculating the mixing ratio of the reservoir through mixing modeling for the first time.

Tracing the Spatial Distribution of Whole-Lake Exchange of Groundwater and Lake Water in Low-Hydraulic Gradient Systems Using δ18O and Electrical Conductivity and Uncertain End-Member Mixing Analysis

δ18O and electrical conductivity (EC) were used successfully to trace the spatial distribution of whole-lake groundwater-lake exchange for a small (four ha) groundwater-fed lake situated in a low relief and low hydraulic gradient area. The method relies on quick sampling of shallow groundwater, direct analysis of EC in the field, and relatively in-expensive analysis of δ18O in the laboratory. Ternary uncertain end-member mixing analysis (precipitation, groundwater, and lake water) quantified the composition of water discharging to and recharging from the lake. The tracer distribution and mixing analysis were in agreement with the interpreted groundwater flow near the lake. The use of only one tracer (either δ18O or EC) gave the same results for the recharge segments, but the discharge segments changed the origin of the water from being groundwater to precipitation controlled. The two tracers complemented each other, especially with different signals in precipitation and groundwater. The uncertain end-members were assessed based on local (groundwater and lake water) and off-site (precipitation) data. The off-site data were found to be useful if it contained representative information on local-site seasonality (uncertainty, variance). Final end-member concentrations could explain the transience of the hydrology at the site (i.e., flooding of the area adjacent to the lake during periods with high precipitation, and variability of the δ18O signal in precipitation). This methodology potentially represents a new option to study groundwater-lake systems. The tracer information collected over only two days is useful by itself for developing the next steps like the quantification of fluxes based on other standard methods (Darcy approach, seepage meters, or temperature). The tracer information can provide quantitative estimation of inputs and outputs by using the mixing analysis.

Using Si, Al and Fe as Tracers for Source Apportionment of Air Pollutants in Lake Baikal Snowpack

The aim of this study was to select chemical species characterized by distinctly different proportions in natural and anthropogenic particulate matter that could be used as tracers for air pollutant sources. The end-member mixing approach, based on the observation that the chemical species in snow closely correlated with land use are those that exhibit differences in concentrations across the different types of anthropogenic wastes, was used for source apportionment. The concentrations of Si and Fe normalized to Al were used as tracers in the mixing equations. Mixing diagrams showed that the major pollution sources (in descending order) are oil, coal, and wood combustion. The traces of several minor sources, such as aluminum production plants, pulp and paper mills, steel rust, and natural aluminosilicates, were also detected. It was found that the fingerprint of diesel engines on snow is similar to that of oil combustion; thus, future research of the role of diesel engines in air pollution will be needed. The insufficient precision of source apportionment is probably due to different combinations of pollution sources in different areas. Thus, principles for the delineation of areas affected by different source combinations should be the subject of further studies.

A comparative study on hydrochemical evolution and quality of groundwater in coastal areas of Thailand and Bangladesh

In recent years, Thailand and Bangladesh have been drawing more attention due to groundwater pollution, but there is rarely a comparative study on hydrochemical evolution and quality assessment of groundwater in the areas In this paper, end-member mixing and Gibbs model are used to study the evolution of groundwater hydrochemistry based on the 74 water samples. Due to the seawater intrusion, the evolution path of groundwater in Bangkok Bay is Ca/Mg-HCO3 → Na (Ca/Mg)-HCO3 and Na (Ca/Mg)-Cl → Na-Cl, which led to increased Na+, K+, and HCO3– concentrations in groundwater and enrichment of fluorine in groundwater caused fluorine pollution. In contrast, although the evolution path of groundwater in southern Bangladesh shows a similar pattern as Ca/Mg-HCO3 → Na (Ca/Mg)-HCO3 and Ca/Mg-Cl → Na-Cl, the principal influence factors are residential, agricultural and industrial pollutants. The scope of seawater intrusion is only limited to estuary areas of the southern estuary, whereas most groundwater is received recharge of polluted surface water, which resulted in the enrichment of Fe and Mn. In addition, compared with shallow groundwater, deep groundwater is updated slowly and evolved to higher degree, and is in the Na (Ca/Mg)-HCO3 and Ca/Mg-Cl stage. For improving groundwater conditions in southern Bangladesh, enhancing the supply capacity of surface potable water and significantly reducing groundwater exploitation are priority. Strictly controlling the generation, storage and treatment of domestic waste, industrial and agricultural pollutants are also important. In Bangkok Bay, reducing the groundwater exploitation and dispersing the exploitation area is required.

A global review of sediment source fingerprinting research incorporating fallout radiocesium (137Cs)

Information on the main sources supplying deleterious sediment loads to river systems is needed to improve our understanding of soil erosion processes to better target erosion control measures. In particular, it is important to quantify the respective contributions of surface and subsurface sources to material degrading waterbodies. Radiocesium (137Cs), emitted during thermonuclear bomb testing (~1950–1980) and nuclear accidents provides significant discrimination between surface material exposed to atmospheric fallout and subsurface material sheltered from it. A systematic worldwide review of research articles (n = 123) that used 137Cs to trace sediment sources showed that the United Kingdom (n = 24), Australia (n = 23) and the United States (n = 20) had the highest number of publications utilizing 137Cs in a sediment tracing framework. In contrast, few studies were published for catchments in Africa (n = 9) or South America (n = 2). In the northern hemisphere, positive relationships were evident between 137Cs activities in surface and subsurface sources and the proportion of thermonuclear bomb fallout. However, given the low proportions of fallout received in regions between 0 and 20°N and 0–20°S, the potential application of 137Cs tracing techniques may be limited in this area of the world as well as in agricultural regions with severe soil erosion (i.e. Chinese Loess Plateau and South Africa). In total, 94% of the studies researching surface and subsurface sources that analysed 137Cs as a potential tracing property included this parameter in end-member mixing models. The main challenges for the future of this technique are related to the access to ultra-low background gamma spectrometry facilities and the development of surrogate tracers. Future research should ensure that basic catchment information and details on the sampling design are properly documented to ensure studies are reproducible. Moreover, researchers should provide raw measurement data to help improve our global understanding of 137Cs dynamics in soil erosion research.