End-member Mixing Analysis Research Articles

Phosphorus Fluxes in a Temperate Forested Watershed: Canopy Leaching, Runoff Sources, and In-Stream Transformation

Declining foliar phosphorus (P) levels call increasing attention to the cycling of this element in temperate forests. We explored the fluxes of P in a temperate mixed deciduous forest ecosystem in six distinct hydrological compartments: Bulk precipitation and throughfall, soil water draining laterally from three different soil depths (0-15, 15-150, 150-320 cm below soil surface), groundwater, creek and spring discharge, which were sampled at daily to bi-weekly resolution from March 2015 to February 2016. Atmospheric P fluxes into the ecosystem were equally partitioned between wet and dry deposition. Approximately 10% of the foliar P stock was lost annually by foliar leaching during late summer. The concentrations of dissolved P in soil water from the forest floor and upper mineral topsoil followed a pronounced seasonal cycle with higher concentrations during the vegetation period. The concentrations of P dissolved in soil water decreased with increasing soil depth. Using an end member mixing analysis (EMMA) we found that P sources feeding the spring water were both soil water from greater depths or groundwater with season specific contributions. Atmospheric P fluxes into the ecosystem determined in this study and P-release from weathering reported for the research site were large enough to compensate P losses with runoff. This suggests that declining foliar P levels of forests are unlikely the result of a dwindling total P supply, but rather caused by tree nutrition imbalances or alternative stressors.

Partitioning groundwater recharge sources in multiple aquifers system within a desert oasis environment: Implications for water resources management in endorheic basins

The global expansion of irrigation-based agricultural practices is causing an overexploitation of groundwater resources. A thorough understanding of groundwater recharge processes is necessary for restoring groundwater and improving water resources management, especially in desert oasis aquifers in endorheic basins. In this paper, the Ejina Oasis in the downstream of Heihe River Basin was selected as the representative case for such an endeavor. Based on stable isotopes and end member mixing analysis, we found that the shallow groundwater was modern water, with 13% originating from local precipitation and 87% from river water infiltration. The semi-confined groundwater is a mixture of shallow groundwater (30%), lateral groundwater flowing from the middle Heihe River Basin (MHR; 52%), and confined groundwater (18%). Confined groundwater was old water formed in paleo-climatic humid and cold conditions. Shallow groundwater levels were restored substantially after implementation of the Ecological Water Diversion Project (EWDP) began in 2000 in the Heihe River Basin, but the semi-confined groundwater levels still declined slowly. These different responses to EWDP are attributed to the differences in primary recharge sources of the two aquifers. Because river water infiltration was the main sources of shallow groundwater, increases in river discharge into the Ejina Oasis resulted in increased shallow groundwater levels over a short time scale. Continued over-pumping of groundwater for irrigation in the MHR reduced lateral groundwater flow into the lower Heihe River Basin, which was the primary source of semi-confined groundwater. For successful groundwater restoration and maintenance of groundwater for the future, EWDP implementation must integrate water management of river water and groundwater diversion across the whole basin. Our results can provide useful information on policy decision makers for sustainable use of water resources in the oases of other similar watersheds around the world.

Characterization of Mechanisms and Processes Controlling Groundwater Recharge and its Quality in Drought-Prone Region of Central India (Buldhana, Maharashtra) Using Isotope Hydrochemical and End-Member Mixing Modeling

A thorough study on understanding of groundwater recharge sources and mechanisms was attempted by integrating the hydrogeological, geochemical and isotopic information along with groundwater dating and end-member mixing analysis (EMMA). This study was necessitated due to prolonged dryness and unavailability of freshwater in semi arid Deccan trap regions of Central India. In addition, groundwater resources are not characterized well in terms of their geochemical nature and recharge sources. The hydrogeochemical inferences suggest that aquifer I consists of recently recharged water dominated by Ca–Mg–HCO3 facies, while groundwater in aquifer II shows water–rock interaction and ion exchange processes. Presence of agricultural contaminant, nitrate, in both aquifers infers limited hydraulic interconnection, which is supported by unconfined to semi-confined nature of aquifers. Groundwater in both aquifers is unsaturated with respect to carbonate and sulfate minerals indicating lesser water–rock interaction and shorter residence time. This inference is corroborated by tritium age of groundwater (aquifer I: 0.7–2 years old and aquifer II: 2–4.2 years old). Stable water isotopes (δ2H, δ18O) suggest that groundwater is a mixture of rainwater and evaporated water (surface water and irrigation return flow). EMMA analysis indicates three groundwater recharge sources with irrigation return flow being the dominant source compared to others (rainwater and surface waters). A conceptual model depicting groundwater chemistry, recharge and dynamics is prepared based on the inferences.

Coupled benthic–hyporheic responses of macroinvertebrates to surface water pollution in a gravel-bed river

Interactions between the hyporheic and benthic zones play a vital role in maintaining the integrity of river ecosystems, but simultaneous assessments of hyporheic and benthic macroinvertebrate responses to surface water pollution are limited. We examined how hyporheic (50 cm below surface) and benthic (surface) invertebrates responded to water pollution that originated from a point source (wastewater treatment plant) in a gravel-bed river in Hokkaido, Japan. Dissolved nutrient levels and algal biomass were significantly higher at sample sites down- than upstream of the point source. Furthermore, nitrate concentrations increased gradually from up- to downstream sites, which suggest the presence of diffuse pollution sources in the surrounding areas. Closely synchronized fluctuations of water temperature and similar water quality in the surface and hyporheic zones indicated that the hyporheic zone was highly hydrologically interactive with the surface water. Chloroperlidae (mostly Alloperla ishikariana) had a disproportionately high abundance in the hyporheic zone relative to the benthic zone, regardless of the surface water quality. Numerically dominant invertebrate taxa increased in abundance below the point source of pollution in both the benthic and hyporheic habitats. High N stable isotope ratios (SIR) of the invertebrates in both habitat types indicated that invertebrates assimilated anthropogenic N via the ingestion of pollution-contaminated food resources. End-member mixing analyses based on C SIR demonstrated that assimilation of anthropogenic N occurred because of the increase in available food resources originating from the biofilm. The trophic structure of invertebrates changed less markedly in the hyporheic zone than in the benthic zone, suggesting that the hyporheic assemblage was less sensitive than the benthic assemblage to the changes in surface water quality. Overall, our findings showed that the numeric responses of the invertebrate assemblage to anthropogenic changes in surface water quality were similar and coupled between the benthic and hyporheic zones.

Evaluating the contributions of different organic matter sources to urban river water during a storm event via optical indices and molecular composition

Dissolved organic matter (DOM) in river water dynamically changes with respect to its major sources during heavy rain events. However, there has been no established tool to estimate the relative contributions of different organic sources to river water DOM. In this study, the evolution in the contributions of ten different organic matter (OM) sources to storm water DOM was explored with a selected urban river, the Geumho River in South Korea, during storm events via an end-member mixing analysis (EMMA) based on fluorescence indices and Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). The OM source materials included treated effluent, road runoff, groundwater, topsoil, deep soil, leaves, reeds, riparian plants, attached algae, and suspended algae. The EMMA results provided quantitative estimates of the variations in the dominant OM sources with the progress of storms. Treated effluent was the prevailing source at the beginning period of the storm, while topsoil, leaves, riparian plants, and groundwater predominated during and after the peak period. The fluorescence indices-based evaluation was consistent with the statistical comparison of the molecular formulas derived from FT-ICR-MS conducted on the ten potential OM sources and the storm samples. The observed variations in the OM sources agreed with the typical characteristics of urban rivers in connection with anthropogenic inputs and the impact of surrounding impervious surfaces. This study demonstrates the application of intuitive and facile tools in estimating the relative impacts of OM sources in urban watersheds.

Integrated approach to study river fluxes, water and sediment sources apportionment in sparsely monitored catchment

Abstract. The paper presents an integrated approach for the apportionment of water and sediment sources and pathways in ungauged territories. The approach comprises various elements such as a large-scale semi-distributed hydrological and erosion model (using the SWAT tool), a statistical assessment of regular discharge data, the discrimination of runoff sources using end-member mixing analysis (EMMA), the apportionment of sediment sources using fingerprinting technique and sediment concentration and water discharge hysteresis rating curves analyses. The approach has been applied to Western Dvina River Basin (WDRB). The results discussed in the paper indicate main features of the water and sediment regime. The components of the presented framework provide a guide for detailed water and sediment studies making it an inexpensive way to develop a roadmap to understand important hydrological processes at a watershed scale.

Geochemical and sedimentological characteristics of surface sediments from Ashtamudi Estuary, Southern India: implications for provenance and modern sedimentary dynamics

Geochemical and end-member mixing analyses (EMMA) of the grain-size distribution were conducted on recent surface sediments from Ashtamudi Estuary, Southern India to investigate the hydrodynamic factors that influence the depositional processes in the region. The complex interplay between the natural (fluvial and marine) and anthropogenic influences on the Ashtamudi Estuary have been delineated based on the inter-relationships between geochemical elements and the end members (EM) derived from the grain-size parameters. Si, Al and Fe are the major elements in the sediment of Ashtamudi Estuary constituting 26–47%, 9–21%, and 8–14%, respectively. Four grain-size end members were identified with dominant modes of 7ϕ (EM1), 6ϕ (EM2), 4.5ϕ (EM3), and 3.3ϕ (EM4) corresponding to fine silt, medium silt, coarse silt, and fine sand, respectively. The high contributions of Al, Fe, Cr and Ni combined with the EM1 and EM2 in the upstream of the estuary indicated that the fine-grained sediments were derived from the fluvial inputs into the basin. The marine/tidal influenced lower estuary is characterised by high concentrations of Si, Ti, Ca, Sr and coarse fraction end member (EM3). The elemental concentration of Cu, Zn and Co along with EM4 shows higher values along the shore region suggesting human interventions in the basin. This integrated geochemical analysis and EMMA provide advancement in the knowledge of the transportation mechanisms and regional sediment dynamics from the estuarine system in Southern India.

Inverse relationship between south-west and north-east monsoon during the late Holocene: Geochemical and sedimentological record from Ennamangalam Lake, southern India

The southern peninsula of the Indian sub-continent is characterized by moisture source from both the south-west (SW) and north-east (NE) monsoon. However, the long-term climate variability associated with these two moisture sources and their relative contribution in the region is less known. In this study, we have used a multiproxy approach (geochemistry, clay mineralogy and end member mixing analyses of the grain size parameters) on the radiocarbon dated sediment profile from Ennamangalam Lake, southern India to reconstruct the past moisture sources in the region. Based on our systematic investigation, we have identified three hydrological stages in the region: stage 1 (ca. 4800 to 3150 cal BP)-relative drier condition, marked by low detrital content, and higher contribution of relatively fine-grained end member (EM) 2; stage 2 (ca. 3150 to 1640 cal BP)-transition phase, high sedimentation rate as compared to the preceding stage; and stage 3 (1640 cal BP to present)-represented by enhanced detrital content, intense chemical weathering (shown by increasing CIA and continuous declining value of Mg/Al) and dominance of EM 1 characterized by coarse grain sediments indicating high energy condition due to the intense precipitation. The regional comparison of paleoclimate records demonstrate that the increase in precipitation observed in Ennamangalam region during the late Holocene is in contrast to the records from the core monsoon zone. The overview of regional records indicate an inverse relationship between the south-west and the north-east monsoon strength during the late Holocene affected by the increasing ENSO variability.

Bayesian hydrograph separation in a minimally gauged alpine volcanic watershed in central Chile

This study examines the question of how much information one can extract from a tracer-based hydrograph separation in a remote and minimally gaged alpine catchment in Chile. We combine PCA-based endmember mixing analysis to identify the sources of flow contribution to the Diguillín River with a hierarchical Bayesian mixing model to integrate spatial and temporal variability in endmember concentration and quantify the source contributions to streamflow over time. The PCA-analysis shows that precipitation isotopes do not vary by elevation (e.g. snow and rainfall had identical signatures) but vary significantly by season, and that a third endmember is necessary to bound streamflow variability at the basin outlet, which was not captured by our field sampling. One of the main advantages of Bayesian methods is the quasi-machine learning capabilities, where we treated the third endmember as a parameter from which the mixing model could both estimate proportional contributions as well as posterior estimates for the tracer concentrations.The two tracer, three endmember hydrograph separation revealed groundwater to be the largest and precipitation (rain and snow) to be the smallest contributor, on average, to streamflow with the third unknown endmember contributing around 40% of streamflow during the Winter wet season. We hypothesize that interflow is occurring as the third endmember in the Alto Diguillín subwatershed, based on inferred tracer values and the presence of alluvium atop impermeable bedrock along certain reaches. More work is necessary to observe and sample these flowpaths, which was not possible during this study. The results of this work have implications for water resource management, since groundwater sustains the majority of streamflow in the Diguillín, and climate change will impact the timing and quantity of baseflow and interflow. Overall, we demonstrate the utility of combining PCA with Bayesian statistical modeling and inference to extract maximum information from a limited field dataset in a remote alpine catchment. The findings of this work can guide future water management in the Diguillín, but also provide clear questions for future research.

Hydrologic separation and their contributions to N loss in an agricultural catchment in hilly red soil region

The impacts of hydrological processes on N loss is of great value to understand the N transport at catchment scale, which is far from clear. Rainfall, soil water, groundwater and stream water and their N concentrations were monitored from March 2017 to February 2018 in Sunjia agricultural catchment of the red soil critical zone. Objectives of this study were: (1) to determine the dynamics of N concentration of different waters and their N loads; (2) to assess their contributions to N load of streamflow in the paddy and upland mixed agricultural catchment. Our results showed that the N concentrations of soil water (4.8 mg L−1) and groundwater (6.0 mg L−1) were the highest, approximately 2 to 5 times higher than those of stream water (2.7 mg L−1), rain water (1.7 mg L−1) and irrigation water (1.2 mg L−1). The N net loss of the catchment (38.2 kg ha−1 yr−1) accounted for 15% of the total fertilizer N input. Rainy season (April–June) was a high-risk period of N loss, contributing to more than one third of the total annual loss amount. Using end-member mixing analysis model (EMMA), we found groundwater (whose discharge accounted for 25% of the catchment streamflow) was an important source for the N loss in the agricultural catchment. Even in this catchment with coexisting upland and paddy field ecosystems, identified end-members could be used to predict the N load well (R2>0.87, p<0.001). These results can deepen our understanding of the relationship between hydrological process and N transport in the red soil critical zone and are also helpful to improve the water and fertilizer management in subtropical agricultural catchment.

Interaction of Surface Water and Groundwater Influenced by Groundwater Over-Extraction, Waste Water Discharge and Water Transfer in Xiong’an New Area, China

Understanding the interaction of surface water and groundwater affected by anthropogenic activities is of great importance for water resource and water quality management. The Xiong’an New Area, located in the North China Plain, has been designated a new building area by China’s government. Groundwater has been over pumped and artificial water was transferred to meet the water supply in this region. Therefore, the natural interaction of surface water and groundwater has been greatly changed and there has been a complex impact of the groundwater from anthropogenic activities. In this study, we used water chemical ions and stable isotopes of δ2H and δ18O to assess the interaction of surface water and groundwater in the Xiong’an New Area. We carried out field surveys and water sampling of the Fu River (domestic waste water discharge), Lake Baiyangdian (artificial water transfer), and the underlying groundwater along the water bodies. Results show that the artificial surface water (discharged and transferred) became the major recharge source for the local groundwater due to the decline of groundwater table. We used groundwater table observations, end-member mixing analysis of the stable isotopic composition and chloride tracers to estimate the contributions of different recharge sources to the local groundwater. Due to the over pumping of groundwater, the lateral groundwater recharge was dominant with a contribution ratio ranging from 12% to 78% in the upper reach of the river (Sections 1–3). However, the contribution of lateral groundwater recharge was estimated to be negligible with respect to the artificial water recharge from Lake Baiyangdian. Seepage from the Fu River contributed a significant amount of water to the connecting aquifer, with a contribution ranging from 14% to 75% along the river. The extent of the river influence into the aquifer ranges as far as 1400 m to the south and 400 m to the north of the Fu River. Estimations based on isotopic fractionation shows that about 22.3% of Lake Baiyangdian water was lost by evaporation. By using the stable isotopes of oxygen and hydrogen in the lake water, an influencing range of 16 km west of the lake was determined. The interaction of the surface water and groundwater is completely changed by anthropogenic activities, such as groundwater over pumping, waste water discharge and water transfer. The switched interaction of surface water and groundwater has a significant implication on water resources management.

Spatiotemporal Heterogeneity of Water Flowpaths Controls Dissolved Organic Carbon Sourcing in a Snow-Dominated, Headwater Catchment

The non-uniform distribution of water in snowdrift-driven systems can lead to spatial heterogeneity in vegetative communities and soil development, as snowdrifts may locally increase weathering. The focus of this study is to understand the coupled hydrological and biogeochemical dynamics in a heterogeneous, snowdrift-dominated headwater catchment (Reynolds Mountain East, Reynolds Creek Critical Zone Observatory, Idaho, USA). We determine the sources and fluxes of stream water and dissolved organic carbon (DOC) at this site, deducing likely flowpaths from hydrometric and hydrochemical signals of soil water, saprolite water, and groundwater measured through the snowmelt period and summer recession. We then interpret flowpaths using end-member mixing analysis in light of inferred subsurface structure derived from electrical resistivity and seismic velocity transects. Streamwater is sourced primarily from groundwater (averaging 25% of annual streamflow), snowmelt (50%), and water travelling along the saprolite/bedrock boundary (25%). The latter is comprised of the prior year’s soil water, which accumulates DOC in the soil matrix through the summer before flushing to the saprolite during snowmelt. DOC indices suggest that it is sourced from terrestrial carbon, and derives originally from soil organic carbon (SOC) before flushing to the saprolite/bedrock boundary. Multiple subsurface regions in the catchment appear to contribute differentially to streamflow as the season progresses; sources shift from the saprolite/bedrock interface to deeper bedrock aquifers from the snowmelt period into summer. Unlike most studied catchments, lateral flow of soil water during the study year is not a primary source of streamflow. Instead, saprolite and groundwater act as integrators of soil water that flows vertically in this system. Our results do not support the flushing hypothesis as observed in similar systems and instead indicate that temporal variation in connectivity may cause the unexpected dilution behavior displayed by DOC in this catchment.

Assessment on applicability of common source tracking tools for particulate organic matter in controlled end member mixing experiments

In this study, ideal mixing behaviors of the three commonly-used source tracking tools, which include fluorescence spectroscopy, stable isotope ratios, and lipid biomarkers, were evaluated in controlled particular organic matter (POM) mixtures of two contrasting end members e.g., soil and algae. In parallel, three different extraction methods based on water (WEOM) and two alkaline solutions were compared to identify the preferential pre-treatment option that leads to a good performance of the spectroscopic source tracking indices on the dissolved organic matter (DOM) extracted from the POM. Based on three evaluation criteria concerning the linearity of the relationships, the discrimination sensitivity, and the conservative mixing behavior of fourteen tested indices, the fluorescence proxies such as the relative distribution of the fluorescence components and the modified fluorescence index YFI of the WEOM were found to be reliable and robust indices for POM source tracking. The carbon stable isotope ratios of the POM samples exhibited an ideal mixing behavior even after the DOM extraction, while the biomarkers of sterols/stanols did not present a good linear and conservative behavior at similar conditions. In addition, the WEOM was identified as the preferred extraction method for the application of spectroscopic indices for POM source discrimination. This study provides a guideline to select the optimum indices for the POM source discrimination via the end member mixing analysis.

Constraining hydrological model parameters using water isotopic compositions in a glacierized basin, Central Asia

Water stable isotope signatures can provide valuable insights into the catchment internal runoff processes. However, the ability of the water isotope data to constrain the internal apportionments of runoff components in hydrological models for glacierized basins is not well understood. This study developed an approach to simultaneously model the water stable isotopic compositions and runoff processes in a glacierized basin in Central Asia. The fractionation and mixing processes of water stable isotopes in and from the various water sources were integrated into a glacio-hydrological model. The model parameters were calibrated on discharge, snow cover and glacier mass balance data, and additionally isotopic composition of streamflow. We investigated the value of water isotopic compositions for the calibration of model parameters, in comparison to calibration methods without using such measurements. Results indicate that: (1) The proposed isotope-hydrological integrated modeling approach was able to reproduce the isotopic composition of streamflow, and improved the model performance in the evaluation period; (2) Involving water isotopic composition for model calibration reduced the model parameter uncertainty, and helped to reduce the uncertainty in the quantification of runoff components; (3) The isotope-hydrological integrated modeling approach quantified the contributions of runoff components comparably to a three-component tracer-based end-member mixing analysis method for summer peak flows, and required less water tracer data. Our findings demonstrate the value of water isotopic compositions to improve the quantification of runoff components using hydrological models in glacierized basins.

Hydrologic functioning of the deep critical zone and contributions to streamflow in a high‐elevation catchment: Testing of multiple conceptual models

AbstractHigh‐elevation mountain catchments are often subject to large climatic and topographic gradients. Therefore, high‐density hydrogeochemical observations are needed to understand water sources to streamflow and the temporal and spatial behaviour of flow paths. These sources and flow paths vary seasonally, which dictates short‐term storage and the flux of water in the critical zone (CZ) and affect long‐term CZ evolution. This study utilizes multiyear observations of chemical compositions and water residence times from the Santa Catalina Mountains Critical Zone Observatory, Tucson, Arizona to develop and evaluate competing conceptual models of seasonal streamflow generation. These models were tested using endmember mixing analysis, baseflow recession analysis, and tritium model “ages” of various catchment water sources. A conceptual model involving four endmembers (precipitation, soil water, shallow, and deep groundwater) provided the best match to observations. On average, precipitation contributes 39–69% (55 ± 16%), soil water contributes 25–56% (41 ± 16%), shallow groundwater contributes 1–5% (3 ± 2%), and deep groundwater contributes ~0–3% (1 ± 1%) towards annual streamflow. The mixing space comprised two principal planes formed by (a) precipitation‐soil water‐deep groundwater (dry and summer monsoon season samples) and (b) precipitation‐soil water‐shallow groundwater (winter season samples). Groundwater contribution was most important during the wet winter season. During periods of high dynamic groundwater storage and increased hydrologic connectivity (i.e., spring snowmelt), stream water was more geochemically heterogeneous, that is, geochemical heterogeneity of stream water is storage‐dependent. Endmember mixing analysis and 3H model age results indicate that only 1.4 ± 0.3% of the long‐term annual precipitation becomes deep CZ groundwater flux that influences long‐term deep CZ development through both intercatchment and intracatchment deep groundwater flows.

Quantifying the contributions of snow/glacier meltwater to river runoff in the Tianshan Mountains, Central Asia

Abstract This study investigated the contributions of snow/glacier meltwater to river runoff in the northern and central Tianshan Mountains (Central Asia). Based on end- member mixing analysis (EMMA), the hydrograph separation was carried out in six benchmark catchments with different glacier area ratios (GARs) and snow area ratios (SARs) during a typical snow melt period (TSMP) and a typical glacier melt period (TGMP). The results indicate that the contribution of snow/glacier meltwater to river runoff is positively correlated with GARs or maximum SARs. The contribution ratios of snowmelt water vary from 22% to 49% in TSMP. The contribution ratios of glacier meltwater vary from 12% to 59% in TGMP. The contribution ratios of snow meltwater in the northern Tianshan Mountains (36%) are higher than central Tianshan Mountains (31%), while the contribution ratios of glacier meltwater in the northern Tianshan Mountains (36%) are lower than central Tianshan Mountains (42%). The contribution of rainfall to river runoff is higher in TGMP (9%–23%) than in TSMP (5%–9%). Baseflow is the most important recharge source to river runoff, with the contribution ratios vary from 44% to 73% in TSMP, and vary from 30% to 75% in TGMP. The spatial and temporal variations in tracer concentrations in streamflow components were found to be responsible to the uncertainties in hydrograph separation.

How landscape heterogeneity governs stream water concentration-discharge behavior in carbonate terrains (Konza Prairie, USA)

Mounting evidence suggests ecosystem changes that alter subsurface water fluxes and carbon dioxide concentrations in carbonate terrains may drive measurable changes in chemical weathering rates, stream water chemistry, and flow path evolution on human timescales. We test this idea by exploring if the encroachment of woody vegetation into grasslands in a carbonate terrain landscape at the Konza Prairie (KS, USA) has resulted in differences in landscape-stream connectivity and, thus, the behavior of stream water solutes. Woody encroachment (up to 60% cover) at Konza has been observed on watersheds, particularly those that experience a fire return interval of four years or greater. We focus on three headwater catchments (two grassland and one woody-encroached) and a downstream confluence, and analyze stream water discharge and chemistry (major anions, cations, and dissolved nutrients) measured from 2015 to 2016.We observe that the woody-encroached watershed exhibits a greater area-normalized solute flux and greater degree of chemodynamic behavior for most geogenic species compared to the less encroached grassland watersheds. The downstream confluence exhibits the most chemostatic behavior for these same solutes compared to the low order watersheds. We interpret the chemodynamic behavior of the woody-encroached watersheds to arise from a greater diversity of flow paths and solute sources that contribute to this stream. End member mixing analysis (EMMA) supports this hypothesis but also indicates a possible “missing” end member which we interpret to be solutes likely derived from clay weathering along limestone-mudstone boundaries. We invoke differences in rooting systems between grass and woody species to explain the differences in flow paths and solute generation between these headwater sites given that they sit adjacent to each other, dissect the same nearly horizontal (dip 0.1–0.21°NW) lithologic units, and experience the same climate. If these processes hold true at other sites, then the globally observed encroachment of woody vegetation into grasslands may deepen flow paths and enhance chemical weathering fluxes from ecosystems, and over long-time periods alter the trajectory of soil development and landscape evolution.

Multi-tracer assessment of seasonal water source changes in coastal water systems along the southeastern coast of Ivory Coast (West Africa)

ABSTRACTProper management of coastal freshwater resources depends on an understanding of processes controlling their chemistry and seasonal flowpaths. A quantitative approach involving the coupling of major solutes and isotopes (δ18O, δ2H) of 180 samples in end-member mixing analysis (EMMA) was adopted to elucidate seasonal patterns of hydraulic exchanges amongst coastal waters along the Ebrié Lagoon catchment, Ivory Coast. The results show that the Ebrié Lagoon is a hydrologically dynamic system. In the dry season, evaporation and seawater inflow are the dominating processes, while in the wet season, river discharge is the main water source in the lagoon. Regional geology plays a significant role in aquifer recharge patterns. The Quaternary aquifer responds faster to precipitation, while the Mio-Pliocene aquifer is recharged indirectly via floodplain seepages. Salinization of over 90% of wells arises from hydrological exchanges with the Ebrié Lagoon. A diluted seawater effect was recorded in wells during the wet season owing to the relative increase in freshwater inflow.

Quantifying the contribution of tile drainage to basin-scale water yield using analytical and numerical models

The Des Moines Lobe (DML) of north-central Iowa has been artificially drained by subsurface drains and surface ditches to provide some of the most productive agricultural land in the world. Herein we report on the use of end-member mixing analysis (EMMA) models and the numerical model Soil and Water Assessment Tool (SWAT) to quantify the contribution of tile drainage to basin-scale water yields at various scales within the 2370 km2 Boone River watershed (BRW), a subbasin within the Des Moines River watershed. EMMA and SWAT methods suggested that tile drainage provided approximately 46 to 54% of annual discharge in the Boone River and during the March to June period, accounted for a majority of flow in the river. In the BRW subbasin of Lyons Creek, approximately 66% of the annual flow was sourced from tile drainage. Within the DML region, tile drainage contributes to basin-scale water yields at scales ranging from 40 to 16,000 km2, with downstream effects diminishing with increasing watershed size. Developing a better understanding of water sources contributing to river discharge is needed if mitigation and control strategies are going to be successfully targeted to reduce downstream nutrient export.

Hydrochemical evolution of groundwater in a riparian zone affected by acid mine drainage (AMD), South China: the role of river–groundwater interactions and groundwater residence time

Investigations were undertaken in the riparian zone near Shangba village, an AMD area, in southern China to determine the effects that river–groundwater interactions and groundwater residence time have had on environmental quality and geochemical evolution of groundwater. Based on the Darcy’s law and ionic mass balances as well as the method of isotopic tracer, the results showed that there were active interactions between AMD-contaminated river water and groundwater in the riparian zone in the study area. River water was found to be the main source of groundwater recharge in the northwestern part of the study area, whereas groundwater was found to be discharging into the river in the southeastern part of the study area throughout the year. End-member mixing analysis quantified that the contributions of river water to groundwater decreased gradually from 35.9% to negligible levels along the flow path. The calculated mixing concentrations of major ions indicated that water–rock reactions were the most important influence on groundwater quality. The wide range of Ca2+ + Mg2+ and HCO3− ratios and the change of groundwater type from Ca2+–SO42− type to a chemical composition dominated by Ca2+–HCO3− type indicated a change of the major water–rock reaction process from the influence of H2SO4 (AMD) to that of CO2 (soil respiration) along a groundwater flow path. Furthermore, the kinetics interpretation of SO42− and HCO3− concentrations suggested that the overlapping time of their kinetics triggered the hydrochemical evolution and the change of major weathering agent. This process might take approximately 8 years and this kinetic time will be continued when a steady source of contamination enter the aquifer.