Hydrochemical Tracers Research Articles

Groundwater recharge through internally drained basins in a semiarid climate, Western Australia

Study regionThe study site, the Mount Bruce Flats, is located in the Pilbara, a semiarid region of northern Western Australia. Study focusSemiarid climates are characterised by an extreme water deficit, with evaporation exceeding precipitation several-fold. Groundwater recharge is episodical and occurs only after infrequent but very large volume precipitation, while all other precipitation events are quickly lost to evaporation. We investigated the contribution of an internally drained basin to localised groundwater recharge by combining the results of calculations from various tracer methods. We developed a new mobile-immobile recharge transport model coupled with a probability model based on Markov Chain Monte Carlo simulations that combines several age and hydrochemical tracers to estimate localised infiltration. New hydrological insights for the regionThe actual recharge varied greatly across the basin, being 4–5 times higher in the central part compared to the margins. The highest probability for recharge was calculated for 2.5 mm×y−1 and 12.6 mm×y−1, with a median value of 6.8 mm×y−1 while the median mobile effective porosity was low at ∼0.5 %. The flood plains overlying freshwater aquifers may contribute several times more to localised recharge than the upper parts of the catchments, which are characterised by fast runoff and a lower water retention time. Therefore, these endorheic basins could be a valuable source of water for groundwater recharge or constitute an additional hydrological challenge for mining.

Identifying river water sources using end‐member mixing analysis in a subtropical monsoon basin China

AbstractPressure on river water resources may increase continuously under global climate change and the increasing intensity of human activity, and the identification of river water sources is needed to maintain water resource sustainability. In this study, river water sources were studied using hydrochemical tracers (i.e. the major ions) and end‐member mixing analysis (EMMA) during two hydrological years, in the subtropical monsoon basin of the Xiangjiang River in the south‐central China. Various water types including precipitation, river water, and mountain creek water, were collected in the basin at a high frequency. The results of EMMA showed that the average contributions from the direct input of precipitation, near‐surface runoff (represented by mountain creek water), and groundwater (represented by river water sampled during the base flow period) were 28 ± 24%, 49 ± 24% and 23 ± 16%, respectively. The contributions from these water sources to the river water varied greatly within the year, which may indicate a shift in the major flow pathways from direct precipitation input and near‐surface flow pathways in the spring rainy months (e.g. from March to May), to a deeper flow pathway in the dry or rainless months (e.g. January, February, July and December). The shift of the major flow pathways may be due to the factors such as the seasonality of precipitation, evapotranspiration and soil wetness conditions. Groundwater and near‐subsurface runoff dominated the river water and the direct input of precipitation was negligible during the dry or rainless months, and consequently, the river water chemistry was less diluted and more influenced by water‐soil/rock interactions and anthropogenic pollution during these months. The creek water contribution to the river water was relatively high in summer, especially in June and July 2021, which may indicate the substantial inter‐seasonal carryover of precipitation that contributed to the river water. With the projected increased frequency of extreme weather events such as seasonal drought, streamflow in the subtropical monsoon regions may experience greater recharge by deeper flow pathways, with the enhanced role of base flow in regulating streamflow, and there may be an increased risk to the water resource security under the condition of the decreased river dilution capacity and increased anthropogenic pollution.

Variation in riverine dissolved organic matter (DOM) optical quality during snowmelt- and rainfall-driven events in a forested wetland watershed

The spring snowmelt period is an important time for organic matter mobilization and export in peatland and wetland dominated systems. The hydrologic flowpaths that transport melting snow and soil water to surface waters can vary over relatively short time periods (several days to weeks) depending on the timing and rate of melt and the occurrence of freeze–thaw cycles and rain-on-snow events. Dynamic flowpaths can alter dissolved organic carbon (DOC) concentrations and dissolved organic matter (DOM) optical properties in surface waters due to differential melt and mobilization of snow, ice, and soil waters. In 2018 and 2020, we measured the DOC concentration and DOM optical properties in a woody wetland-dominated watershed in northern Minnesota during spring snowmelt (2018 & 2020) and early growing season (2018 only) events. We examined patterns in riverine DOC, DOM, and hydrochemical tracers across event hydrographs to infer changes in hydrologic connectivity across this seasonal transition. Measurements of riverine DOM optical properties, calcium concentration, and water isotopic composition indicated that DOM shifted from a primarily groundwater source and more fresh character during the dormant season to a more shallow wetland soil source and recalcitrant character during the early growing season. Seasonal variations in DOM optical properties and highly complex hysteresis patterns of DOM indices (e.g., Fluorescence Index, Humification Index, β/α, E250:E365, and the fluorescence peak ratio T:C) during individual events may reflect spatial heterogeneity in soil thawing throughout the watershed and temporal variations in water table elevation, affecting flowpath depths and connectivity to DOM pools within the soil profile. This study provides an important seasonal and event-scale baseline against which to evaluate potential changes in riverine DOM quantity and quality associated with predicted changes in climate and snow cover in organic matter-rich ecosystems.

Quantifying streamflow sources to improve water allocation management in a catchment undergoing agricultural intensification

Changes in streamflow contributing sources in semi-arid catchments undergoing agricultural intensification are often poorly understood. As a result, pathways to increase resilience to environmental change are not well established. Mixing model analysis using stable water isotopes and hydro-chemical tracers was applied to evaluate streamflow sources in the Kaleya Catchment, Zambia. Results showed that strong agricultural intensification signal influenced streamflow sources in time and space. In the rainy season, streamflow mainly originated from stormwater runoff from non-irrigated areas (43 ± 13%), the perennial spring (39 ± 21%) and stormwater runoff from irrigated areas (18 ± 17%). But in the dry season, the spring (65 ± 15%) and irrigation return flows (35 ± 15%) became the important upstream sources, while downstream sources were irrigation return flows (73 ± 15%) and wastewater containing vinasse (27 ± 15%), both associated with water originally transferred from the adjacent Kafue River. Given the current importance of irrigation return flows to downstream users including ecosystems, social cooperate responsibility and/or water markets must help to improve irrigation efficiency while simultaneously ensuring downstream flows are maintained.

Assessing the role of groundwater recharge from tanks in crystalline bedrock aquifers in Karnataka, India, using hydrochemical tracers

The majority of India’s rural drinking water supply is sourced from groundwater, which also plays a critical role in irrigated agriculture, supporting the livelihoods of millions of users. However, recent high abstractions are threatening the sustainable use of groundwater, and action is needed to ensure continued supply. Increased managed aquifer recharge (MAR) using the > 200,000 existing tanks (artificially created surface water bodies) is one of the Indian government’s key initiatives to combat declining groundwater levels. However, few studies have directly examined the effectiveness of tank recharge, particularly in the complex fractured hydrogeology of Peninsular India. To address this gap, this study examined the impact of tanks in three crystalline bedrock catchments in Karnataka, southern India, by analysing the isotopic and hydrochemical composition of surface waters and groundwaters, combined with groundwater level observations. The results indicate that tanks have limited impact on regional groundwater recharge and quality in rural areas, where recharge from precipitation and groundwater recycling from irrigation dominate the recharge signal. In the urban setting (Bengaluru), impermeable surfaces increased the relative effect of recharge from point sources such as tanks and rivers, but where present, pipe leakage from public-water-supply accounted for the majority of recharge. Shallow groundwater levels in the inner parts of the city may lead to groundwater discharge to tanks, particularly in the dry season. We conclude that the importance of aquifer recharge from tanks is limited compared to other recharge sources and highly dependent on the specific setting. Additional studies to quantify tank recharge and revisions to the current guidelines for national groundwater recharge estimations, using a less generalised approach, are recommended to avoid over-estimating the role tanks play in groundwater recharge.

Analysis of Event-based Hydrological Processes at the Hydrohill Catchment Using Hydrochemical and Isotopic Methods

Abstract. Hydrochemical and isotopic techniques have been widely applied in hydrological sciences because isotopic tracers can identify water sources and hydrochemical tracers can discern runoff flow paths. To better understand the hydrological process, we combined hydrochemical and isotopic techniques under controlled experimental conditions to investigate hydrological process from rainfall to runoff in the Hydrohill experiment catchment, a typical artificial catchment in Chuzhou, China. Hydrochemical and isotopic data, i.e., pH, electric conductivity (EC), total dissolved solids (TDS), anions (Cl−, NO3-, SO42- and HCO3-), cations (K+, Na+, Ca2+ and Mg2+) and dissolved Si, 18O and D in water samples were collected during a rainfall event in 2016, and used to determine the hydrochemical and isotopic characteristics of rainfall and runoff components. We applied EC, TDS, SO42-, Ca2+, Mg2+, 18O and D as tracers to investigate rainfall-runoff processes in the experimental catchment. Runoff flow paths could be well identified by the relationship between 18O and EC, TDS, SO42-, Ca2+ and Mg2+. The quantity of flow flux and mass fluxes of main hydrochemical and isotopic tracers gauged at the catchment outlet shows applicable tracers (Ca2+, Mg2+, SO42-, and 18O) are mainly from deep groundwater runoff (from soil layer of 60–100 cm beneath ground surface). Contributions of the event water and pre-event water to the total runoff during the rainfall-runoff process are different. The quantitative results were very encouraging as a basis to develop hydrological models for further study.

Differential C-Q Analysis: A New Approach to Inferring Lateral Transport and Hydrologic Transients Within Multiple Reaches of a Mountainous Headwater Catchment

Concentration-discharge (C-Q) relationships have been widely used as “hydrochemical tracers” to determine the variability in riverine solute exports across event, seasonal, annual, and decadal time scales. However, these C-Q relationships are limited to investigating solute transport dynamics at individual sampling stations, such that they create an incomplete understanding of the solute behavior upstream or downstream of the sampling station. Therefore, the objective of this study is to develop, apply and assess a differential C-Q approach that can characterize spatial variability in solute behavior across stations, as well as investigate their controls, by following a different spatial scheme and organizing the river into multiple sections. The differential C-Q approach captures the difference in concentration in a river segment over the difference in discharge, thereby accounting for gains, losses or fractional solute turnover between sampling stations. Using water quality data collected over four water years (2014-18) in a mountainous headwater catchment of the East River, Colorado, this study compares traditional and differential C-Q relationships in predicting solute behavior between three sampling stations distributed throughout the river. Results from the differential C-Q analysis demonstrate significant differences in solute behavior within upstream versus downstream reaches of the East River watershed. In particular, the meandering downstream section is marked by significant gains in both groundwater and solute concentrations as opposed to the dilution and the declining trends observed in the high-relief, steep terrain upstream reach. Shale mineralogy was determined to have a major influence on in-stream concentrations pertaining to Ca, DIC, DOC, Mg, Mo, NO3 and SO4. The analyses further revealed that total P concentration in the downstream reach exceeded the U.S. Environmental Protection Agency’s desired goal for control of eutrophication (110 ppb). Overall, differential C-Q metrics yield a better understanding of the lateral storage and interactions within catchments than traditional analyses, and holds potential for aiding water quality managers in the identification of critical stream reaches that assimilate harmful chemicals.

Environmental isotopes and hydrochemical tracers applied to hydrogeological conceptual modeling of the southwest portion of the Amazon Aquifer System (Acre, Brazil)

Geochemical and environmental isotope techniques are successfully applied to understand the origin and evolution of groundwater in aquifers, with the aim of producing consistent conceptual models. In the present study, δ18O, δ2H, and δ13C stable isotope ratios and 14C and 3H levels were used for this purpose. One of the most important aquifer systems in South America, the Amazon Aquifer System (AAS), remains almost entirely unstudied in this aspect. The combination of hydrochemical and isotopic tracers was applied in a pioneer study to foster the understanding of the driving hydrogeological processes in the southwestern portion of the AAS, in the Acre Sedimentary Basin (ASB), northwestern Brazil. The study area comprises three aquifers: from top to bottom of the system, the Quaternary Aquifer, the Içá–Solimões Aquifer (Paleogene/Neogene), and the Tikuna Aquifer (Cretaceous). The aquifers have distinct hydrochemical characteristics, acidic pH and Na–HCO3 waters in the Quaternary Aquifer, pH ~8 and Ca–HCO3 and Na–HCO3 waters in the Içá–Solimões Aquifer, and neutral pH and Ca–HCO3 waters in the lower Tikuna Aquifer system. Ion exchange processes predominate in all three aquifers, but carbonate dissolution also occurs in some parts of the Içá–Solimões Aquifer. The Içá–Solimões Aquifer shows a geochemical evolution along the regional flow from southwest to northeast. δ18O and δ2H measurements of the groundwater align with the global meteoric water line, without a significant evaporative effect or dispersion. Groundwater from the Içá–Solimões and Tikuna Aquifers is more depleted than the weighted rainfall average (δ2H = −27.9‰ and δ18O = −5.35‰, respectively). The deuterium excess of the local rainfall is approximately +14‰. Residence times, estimated by 14C, range between 13 and 18 ka in the Içá–Solimões Aquifer and reach approximately 6 ka in the Tikuna Aquifer. In addition, tritium concentrations lower than 0.6 TU pointed to the absence of recent waters in the studied aquifers. Local structural and stratigraphic features in the Serra do Divisor area, where asymmetric folding and faulting occur, affect the functioning and features of these aquifer systems, originating from the erosive exposure of deeper and older layers to recent recharge. Thus, geologically older and subjacent aquifer units (Tikuna Aquifer) hold more recently circulating waters than the younger and shallower ones (Içá–Solimões Aquifer).

Recharge Estimation Using CMB and Environmental Isotopes in the Verlorenvlei Estuarine System, South Africa and Implications for Groundwater Sustainability in a Semi-Arid Agricultural Region

Groundwater recharge remains one of the most difficult hydrogeological variables to measure accurately, especially for semi-arid environments where the recharge flux is much smaller than in humid conditions. In this study, groundwater recharge was estimated using chloride mass balance (CMB) in the Verlorenvlei catchment, South Africa where the effects of recent severe drought conditions in an already semi-arid environment have impacted both agricultural activity as well as the RAMSAR-listed Verlorenvlei estuarine system. Chloride, 18O and 2H tracers were used to improve understanding of the groundwater flow patterns and allowed the fresh parts of the groundwater system, defined by Ca2+-HCO3− groundwater types, to be separated from those where additional salts were being introduced through groundwater mixing, and thus characterized as Na+-Cl− groundwater types. Recharge rates calculated from CMB in the fresh parts of the system were between 4.2–5.6% and 11.4–15.1% of mean annual precipitation for the headwater valley and mountains of the Krom Antonies and are largely consistent with previous studies. However, much lower recharge rates in the valleys where agriculture is dominant contrasts with previous results, which were higher, since groundwater-mixing zones were not recognised. Although the chloride concentration in precipitation is based on only one year of data between 2015 and 2016, where 2015 had on average 28% less precipitation than 2016, the results provide a snapshot of how the system will respond to increasing drought frequency in the future. The results suggest that low rates of groundwater recharge under dry spell conditions will impact on low flow generations which are required to sustain the Verlorenvlei estuarine lake system. Overall, the study highlights the importance of combining hydrochemical tracers such as bulk chloride and stable isotopes with numerical modelling in data-scarce catchments to fully understand the nature of hydrological resilience.

Groundwater connectivity of a sheared gneiss aquifer in the Cauvery River basin, India

Connectivity of groundwater flow within crystalline-rock aquifers controls the sustainability of abstraction and baseflow to rivers, yet is often poorly constrained at a catchment scale. Here groundwater connectivity in a sheared gneiss aquifer is investigated by studying the intensively abstracted Berambadi catchment (84 km2) in the Cauvery River Basin, southern India, with geological characterisation, aquifer properties testing, hydrograph analysis, hydrochemical tracers and a numerical groundwater flow model. The study indicates a well-connected system, both laterally and vertically, that has evolved with high abstraction from a laterally to a vertically dominated flow system. Likely as a result of shearing, a high degree of lateral connectivity remains at low groundwater levels. Because of their low storage and logarithmic reduction in hydraulic conductivity with depth, crystalline-rock aquifers in environments such as this, with high abstraction and variable seasonal recharge, constitute a highly variable water resource, meaning farmers must adapt to varying water availability. Importantly, this study indicates that abstraction is reducing baseflow to the river, which, if also occurring in other similar catchments, will have implications downstream in the Cauvery River Basin.

A Model of Mixing of Four River Runoff Recharge Sources Using Hydrochemical Tracers in the Problem of Hydrograph Separation

Methods for the analysis of the formation of river water chemistry based on mixing models are discussed. An analytical solution for a mixing model of four recharge sources is given along with the results of its adaptation based on the materials of hydrochemical and hydrological observations over 2013 and 2014 in drainage basins in Primorski Krai. The analysis and calculations revealed four components of river recharge in these years: rainwater, two types of subsoil water, and groundwater, the latter forming the base runoff. The estimates of the mixing model with four sources and its tests showed a high quality of modeling.

Carbonic springs as distal manifestations of geothermal systems, highlighting the importance of fault pathways and hydrochemical mixing: Example from the Jemez Mountains, New Mexico

We examine a series of carbonic warm and hot springs in northern New Mexico, USA: 1) Tierra Amarilla springs 2) Penasco Springs and 3) Soda Dam, Jemez and Indian hot springs. Spring geochemistry is compared to waters from the Valles Caldera geothermal system and to groundwater in nearby sedimentary aquifers. Multiple hydrochemical tracers are applied to quantitatively evaluate flow paths and mixing at varying distances from the caldera. We test three hypotheses for source and transport of waters to Tierra Amarilla and Penasco Springs: San Juan basin origin, meteoric flow from the Nacimiento Mountains, and/or influence from the Valles Caldera geothermal system. Geochemically, carbonic spring groups are distinct from meteoric and sedimentary aquifer waters. Based on isotopes of He and Sr, and concentrations of Cl, SO4, Li and B, we interpret these carbonic springs to be distal manifestations of fluid circulation along faults with a mixture of Valles Caldera geothermal waters, local meteoric and Paleozoic aquifer waters, with the potential for small contributions from the San Juan Basin aquifers. Semi-confined fault conduits, the Jemez fault and Nacimiento fault systems, provide connectivity and help explain geochemical similarities and mixing trends within carbonic spring systems, and between these systems and the distal Valles Caldera. In addition, Penasco Springs are interpreted to reflect a component of outflow from the geothermal system that crosses the Nacimiento Mountain basement block along NE-trending faults. Input of deeply sourced waters can degrade water quality by contributing significant salinity and trace metals to groundwater at distances of >50 km from geothermal systems, with faults acting as conduits for subsurface fluid flow.

Surface and subsurface water contributions to streamflow from a mesoscale watershed in complex mountain terrain

AbstractAn understanding of surface and subsurface water contributions to streamflow is essential for accurate predictions of water supply from mountain watersheds that often serve as water towers for downstream communities. As such, this study used the end‐member mixing analysis technique to investigate source water contributions and hydrologic flow paths of the 264 km2 Boulder Creek Watershed, which drains the Colorado Front Range, USA. Four conservative hydrochemical tracers were used to describe this watershed as a 3 end‐member system, and tracer concentration reconstruction suggested that the application of end‐member mixing analysis was robust. On average from 2009 to 2011, snowmelt and rainwater from the subalpine zone and groundwater sampled from the upper montane zone contributed 54%, 22%, and 24% of the annual streamflow, respectively. These values demonstrate increased rainwater and decreased snow water contributions to streamflow relative to area‐weighted mean values derived from previous work at the headwater scale. Young water (2.3 ± 0.8 months) fractions of streamflow decreased from 18–22% in the alpine catchment to 8–10% in the lower elevation catchments and the watershed outlet with implications for subsurface storage and hydrological connectivity. These results contribute to a process‐based understanding of the seasonal source water composition of a mesoscale watershed that can be used to extrapolate headwater streamflow generation predictions to larger spatial scales.

Evaluating Temporal and Spatial Variation in Nitrogen Sources along the Lower Reach of Fenhe River (Shanxi Province, China) Using Stable Isotope and Hydrochemical Tracers

Nitrate is one of the most common pollutants in river systems. This study takes the lower reach of Fenhe River as a case study, combined with a multi-isotope and hydrochemical as the tracers to identify nitrate sources in river system. The results show that all samples in the industrial region (IR) and urban region (UR) and 68.8% of the samples in the agriculture region (AR) suffer from nitrate pollution. NO3−–N is the main existing form of dissolved inorganic nitrogen (DIN), followed by NH4+–N, which account for 57.9% and 41.9% of the DIN, respectively. The temporal variation in nitrogenous species concentration is clear over the whole hydrological year. The spatial variation is smaller among different sampling sites in the same region but greater among different regions. The main source of nitrogenous species is from anthropogenic rather than natural effects. Multi-isotope analysis shows that denitrification is found in some water samples. Combined with the apportionment of nitrate sources by the IsoSource model and the analysis of the Cl− content, the main source of nitrate in the IR, UR and AR are industrial sewage and manure, domestic sewage and manure, and chemical fertilizers, respectively. Atmospheric nitrogen deposition is also a source of nitrate in the study area.

Application of environmental isotopes and hydrochemistry in the identification of source of seepage and likely connection with lake water in Lesser Himalaya, Uttarakhand, India

Oxygen (\({\updelta }^{18}\hbox {O}\)) and hydrogen (\({\updelta }^{2}\hbox {H}\) and \(^{3}\hbox {H}\)) isotopes of water, along with their hydrochemistry, were used to identify the source of a newly emerged seepage water in the downstream of Lake Nainital, located in the Lesser Himalayan region of Uttarakhand, India. A total of 57 samples of water from 19 different sites, in and around the seepage site, were collected. Samples were analysed for chemical tracers like \(\hbox {Ca}^{++}\), \(\hbox {Mg}^{++}\), \(\hbox {Na}^{+}\), \(\hbox {K}^{+}\), \({\hbox {SO}_{4}}^{--}\) and \(\hbox {Cl}^{-}\) using an Ion Chromatograph (Dionex IC-5000). A Dual Inlet Isotope Ratio Mass Spectrometer (DIIRMS) and an Ultra-Low Level Liquid Scintillation Counter (ULLSC), were used in measurements of stable isotopes (\({\updelta }^{2}\hbox {H}\) and \({\updelta }^{18}\hbox {O}\)) and a radioisotope (\(^{3}\hbox {H}\)), respectively. Results obtained in this study repudiate the possibility of any likely connection between seepage water and the lake water, and indicate that the source of seepage water is mainly due to locally recharged groundwater. The study suggests that environmental isotopes (\({\updelta }^{2}\hbox {H}\), \({\updelta }^{18}\hbox {O}\) and \(^{3}\hbox {H}\)) can effectively be used as ‘tracers’ in the detection of the source of seepage water in conjunction with other hydrochemical tracers, and can help in water resource management and planning.

Nitrate contamination in a shallow urban aquifer in East Ukraine: evidence from hydrochemical, stable isotopes of nitrate and land use analysis

A combined hydrochemical and stable isotope approach was used to investigate the origin of nitrate in the shallow unconfined groundwater of Kharkiv city, Eastern Ukraine. The contamination was investigated in the context of land use within the catchment area. The observed enrichment of sulfate, chloride and nitrate suggests significant groundwater contamination in the shallow urban aquifer, which is widely used as drinking water source for the urban population. Characteristic nitrate/chloride ratios as well as stable isotope ratios (N and O) of nitrate in the most contaminated springs confirmed that septic waste from leaky sewer systems was the main source of nitrate contamination in the groundwater. Nitrate contamination is linked to the type of land use and sewage treatment regime in the catchment area. It is also modulated by the regional hydrogeology, which determines the susceptibility of a given aquifer toward groundwater pollution. A more quantitative assessment of nitrate sources based on the nitrate isotope analysis alone is rather difficult. However, our study confirms that the combination of hydrochemical tracers, robust land-use analysis and nitrate stable isotope measurements represents a valuable approach to identify the origin of the nitrate contamination.

Groundwater flow and heterogeneous discharge into a seepage lake: Combined use of physical methods and hydrochemical tracers

AbstractGroundwater discharge into a seepage lake was investigated by combining flux measurements, hydrochemical tracers, geological information, and a telescopic modeling approach using first two‐dimensional (2‐D) regional then 2‐D local flow and flow path models. Discharge measurements and hydrochemical tracers supplement each other. Discharge measurements yield flux estimates but rarely provide information about the origin and flow path of the water. Hydrochemical tracers may reveal the origin and flow path of the water but rarely provide any information about the flux. While aquifer interacting with the lake remained under seemingly steady state conditions across seasons, a high spatial and temporal heterogeneity in the discharge to the lake was observed. The results showed that part of the groundwater flowing from the west passes beneath the lake and discharges at the eastern shore, where groundwater springs and high discharge zones (HDZs) are observed at the lake bottom and at seepage faces adjacent to the lake. In the 2‐D cross section, surface runoff from the seepage faces delivers 64% of the total groundwater inputs to the lake, and a 2 m wide offshore HDZ delivers 13%. Presence of HDZs may control nutrient fluxes to the lake.

Understanding runoff processes in a semi-arid environment through isotope and hydrochemical hydrograph separations

Abstract. The understanding of runoff generation mechanisms is crucial for the sustainable management of river basins such as the allocation of water resources or the prediction of floods and droughts. However, identifying the mechanisms of runoff generation has been a challenging task, even more so in arid and semi-arid areas where high rainfall and streamflow variability, high evaporation rates, and deep groundwater reservoirs may increase the complexity of hydrological process dynamics. Isotope and hydrochemical tracers have proven to be useful in identifying runoff components and their characteristics. Moreover, although widely used in humid temperate regions, isotope hydrograph separations have not been studied in detail in arid and semi-arid areas. Thus the purpose of this study is to determine whether isotope hydrograph separations are suitable for the quantification and characterization of runoff components in a semi-arid catchment considering the hydrological complexities of these regions. Through a hydrochemical characterization of the surface water and groundwater sources of the catchment and two- and three-component hydrograph separations, runoff components of the Kaap catchment in South Africa were quantified using both isotope and hydrochemical tracers. No major disadvantages while using isotope tracers over hydrochemical tracers were found. Hydrograph separation results showed that runoff in the Kaap catchment is mainly generated by groundwater sources. Two-component hydrograph separations revealed groundwater contributions of between 64 and 98 % of total runoff. By means of three-component hydrograph separations, runoff components were further separated into direct runoff, shallow and deep groundwater components. Direct runoff, defined as the direct precipitation on the stream channel and overland flow, contributed up to 41 % of total runoff during wet catchment conditions. Shallow groundwater defined as the soil water and near-surface water component (and potentially surface runoff) contributed up to 45 % of total runoff, and deep groundwater contributed up to 84 % of total runoff. A strong correlation for the four studied events was found between the antecedent precipitation conditions and direct runoff. These findings suggest that direct runoff is enhanced by wetter conditions in the catchment that trigger saturation excess overland flow as observed in the hydrograph separations.

Evaporation and concentration gradients created by episodic river recharge in a semi-arid zone aquifer: Insights from Cl−, δ18O, δ2H, and 3H

Summary This study has significantly advanced our understanding of the origin of groundwater recharge in a semi-arid zone region of the Darling River catchment, Australia. The generally accepted hypothesis in arid zone environments in Australia that river water forms the primary groundwater recharge source has proven difficult to monitor. This is due to the time lags between large floods, the remoteness and expense of studying these hydrologically complex systems in detail. In addition, the highly episodic nature of dryland rivers complicates the interpretation of the groundwater signal. A range of hydrochemical tracers (chloride, oxygen-18, deuterium and tritium) measured in rain, river water, soil water and groundwater were used in this multi-year study to trace the pathways of groundwater recharge under wet and dry climatic conditions. The evaporation and Cl concentrations observed in the unsaturated zone confirmed that small volumetric inputs from periodic rainfall were not the major recharge mechanism. Sampling which included an overbank flooding event in March 2012 provided firm evidence for groundwater originating from high flow episodic river recharge. The use of long-term environmental data to understand how economically important water resources respond to climate change with increasing temperatures is considered essential for future sustainability.

Delineating vulnerability of karst aquifers using hydrochemical tracers in Southwestern China

Karst groundwater is particularly vulnerable to contamination. Long-term hydrochemistry auto-monitoring combined with laboratory analyses has shown that the chemograph demonstrates changes in the system. In order to determine the primary groundwater vulnerability, a relationship between hydrochemical changes and the origin of vulnerability in six karst spring catchments in Southwestern China is discussed. The reason and physical meaning of the hydrochemical changes in the spring are explained based on tracer tests and physical–chemical process in the S31 spring. The results prove that the hydrochemical changes show the origin of vulnerability and also can be its expression. Generally, autogenic recharge in bare karst is concentrated in epikarst zone, forming bypass flow and shaft flow, which causes dilution of Ca2+. The chemograph reflects the percentage of concentrated and diffuse recharge. The higher the ratio of concentrated recharge, the higher the grade of groundwater vulnerability. Due to Ca2+, HCO3−, SEC, pH, and temperature in water can change for kinds of reasons during their transport, more karst systems with variable natural and human factors are discussed. Protective layer, allogenic water recharge, and transform of surface stream and subterranean river can change the origin shape of hydrochemical curve. Under this circumstance, more indicators need to be monitored, especially contaminant-related ions such as nitrate, in order to clearly understand the contribution of concentrated and diffuse recharge to water chemistry. After comparing the traditional groundwater vulnerability assessment and the methods based on hydrochemical change, rainfall, protective layer, epikarst, transient storage and karst network should be more concerned when the work is done. Auto-monitoring of karst is more and more popular in Southwestern China; therefore, it is suggested that the hydrochemical methods should be used as a primary expression of vulnerability, giving a direct reference for karst water management.