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Understanding groundwater–surface water (GW–SW) interactions is vital for water management in karstic catchments due to its impact on water quality. The objective of this study was to evaluate and compare the applicability of seven environmental tracers to quantify and localize groundwater exfiltration into a small, human‐impacted karstic river system. Tracers were selected based on their emission source to the surface water either as (a) dissolved, predominantly geogenic compounds (radon‐222, sulphate and electrical conductivity) or (b) anthropogenic compounds (predominantly) originating from wastewater treatment plant (WWTP) effluents (carbamazepine, tramadol, sodium, chloride). Two contrasting sampling approaches were compared (a) assuming steady‐state flow conditions and (b) considering the travel time of the water parcels (Lagrangian sampling) through the catchment to account for diurnal changes in inflow from the WWTP. Spatial variability of the concentrations of all tracers indicated sections of preferential groundwater inflow. Lagrangian sampling techniques seem highly relevant for capturing dynamic concentration patterns of WWTP‐derived compounds. Quantification of GW inflow with the finite element model FINIFLUX, based on observed in‐stream Rn activities led to plausible fluxes along the investigated river reaches (0.265 m3 s−1), while observations of other natural or anthropogenic environmental tracers produced less plausible water fluxes. Important point sources of groundwater exfiltration can be ascribed to locations where the river crosses geological fault lines. This indicates that commonly applied concepts describing groundwater–surface water interactions assuming diffuse flow in porous media are difficult to transfer to karstic river systems whereas concepts from fractured aquifers may be more applicable. In general, this study helps selecting the best suited hydrological tracer for GW exfiltration and leads to a better understanding of processes controlling groundwater inflow into karstic river systems., Karst aquifers represent an increased complexity when aiming to measure the interaction between groundwater and river water. Combining field‐based measurements on catchment scale and modelling, the applicability of ‘classical’ environmental groundwater tracers was compared to selected organic (micro)pollutants often considered as conservative and originally arising from a wastewater treatment plant. This study demonstrates that the choice of an appropriate tracer is crucial when either aiming to quantify groundwater exfiltration into karstic river systems, or indicating hydrological processes, applying (globally) omnipresent pollutants., German Research Foundation (DFG) http://dx.doi.org/10.13039/501100001659

Bank filtration is a cost-effective and sustainable method of improving surface water quality for drinking water production. During aquifer transit, natural biodegradation and physiochemical filtration improve the quality of the raw water by removing sediments, pollutants, and pathogens. Strict regulations prohibit the use of substances that can be used to estimate aquifer residence times to define water protection areas for bank filtration. In this study, we present a novel measurement and modeling framework for deriving mean aquifer residence times for bank filtrate using the natural tracer radon-222. The method is intended for application in the drinking water sector, where extraction wells are screened over the entire aquifer and pumps are operated at high production rates. Mean aquifer residence times are estimated using composite residence time distributions that account for flow path mixing and non-uniform residence times with multiple components including bank filtrate, shallow groundwater, and deep groundwater. The mathematical framework is demonstrated for a drinking water production facility. Radon activities for the six monitored extraction wells ranged between 4,400 and 8,400 Bq/m³. Estimated mean aquifer residence times for the wells range from5 days to 110 days and strongly depend on i) the type of residence time distribution model (exponential, gamma or piston flow), ii) the mixing ratio between bank filtrate and local groundwater, and iii) the heterogeneity in the groundwater endmember. By accounting for mixing processes, we can show that radon can be used beyond the "5-fold half-life" (~20 days) commonly described in the literature as the upper limit for age dating purposes for radon. This method provides a simple and cost-efficient way to quantify residence times of bank filtrate on a regular basis without any addition of external substances to the aquifer.

Dissolved organic carbon (DOC) constitutes the biggest portion of carbon that is exported from soils. During the last decades, widespread increases in DOC concentrations of surface waters have been observed, affecting ecosystem functioning and drinking water treatment. However, the hydrological controls on DOC mobilization are still not completely understood.We sampled two different topographical positions within a headwater catchment in the Bavarian Forest National Park: at a steep hillslope (880 m.a.s.l.) and in a flat and wide riparian zone (770 m.a.s.l.). By using piezometers, pore water samplers (peepers) and in-stream spectrometric devices we measured DOC concentrations as well as DOC absorbance (A254/A365 and SUVA254) and fluorescence characteristics (fluorescence and freshness indices) in soil water, shallow ground water and stream water in order to gain insights into the DOC source areas during base-flow and during precipitation events.High DOC concentrations (up to 80 mg L-1) were found in soil water from cascading sequences of small ponds in the flat downstream part of the catchment that fill up temporarily. The increase of in-stream DOC concentrations during events was accompanied by changing DOC characteristics at both locations, for example increasing freshness index values. As the freshness index values were approaching the values found in the DOC-rich ponds in the riparian zone, these ponds seem to be important DOC sources during events. Our preliminary results point to a change of flow pathways during events.

Investigation of river water/groundwater interaction aims generally at: (i) localizing water migration pathways, and (ii) quantifying water and associated matter exchange between the two natural water resources. Related numerical models generally rely on model-specific parameters that represent the physical conditions of the catchment and suitable aqueous tracer data. A generally applicable approach for this purpose is based on the finite element model FINIFLUX that is using the radioactive noble gas radon-222 as naturally occurring tracer. During the study discussed in this paper, radon and physical stream data were used with the aim to localize and quantify groundwater discharge into a well-defined section of a small headwater stream. Besides site-specific results of two sampling campaigns, the outcomes of the study reveal: (i) the general difficulties of conducting river water/groundwater interaction studies in small and heterogeneous headwater catchments, and (ii) the particular challenge of defining well constrained site- and campaign-specific values for both the groundwater radon endmember and the radon degassing coefficient. It was revealed that determination of both parameters should be based on as many data sources as possible and include a critical assessment of the reasonability of the gathered and used datasets. The results of our study exposed potential limitations of the approach if executed in small and turbulent headwater streams. Hence, we want to emphasize that the project was not only executed as a case study at a distinct site but rather aimed at evaluating the applicability of the chosen approach for conducting river water/groundwater interaction studies in heterogeneous headwater catchments.

The spatial distribution and temporal dynamics of groundwater inflow to rivers is often poorly defined but central to understanding water and matter fluxes. This is especially true for the Spree River which drains the Lusatia mining district, Brandenburg Germany. In the Spree catchment iron and sulphate fluxes to the river stem from the pyrite rich groundwater system, and the area’s history of open-pit lignite mining and re-flooding of many of these mines at the end of their lifetime. This iron flux threatens the river ecosystem, tourism in downstream communities (Spreewald) and the drinking water of Berlin. Iron is often observed as precipitates along the river bed, as well as colouring the river water yellow-brown, indicating the presence of iron (oxy)hydroxides such as ferrihydrite and goethite. In this work we have used radon as a natural groundwater tracer to delimited areas of active groundwater discharge to both the main Spree River and the Kleine Spree River to better understand the spatial destitution of groundwater input to the system. This was combined with mass-balance modelling to quantify the groundwater flux along the river using the FINIFLUX model. This was complemented by measurement of iron and sulphate concentrations in the steam and stream-near groundwater. During two measurement campaigns during 2018 the total groundwater inflow for a 20 km long reach of the Kleine Spree and a 34 km long reach of the Spree ranged between ~3,000 and ~7,000 m³ d-1 (Kleine Spree) and ~20,000 and ~38,000 m³ d-1 (Spree). Particularly high groundwater inflow was identified (up to 70% of total inflow) along the Spreewitzer Rinne, a local aquifer consisting of excavated mining materials. For the Kleine Spree the dominant groundwater and Fe flux occurred shortly before the confluence with the Spree. For these river reaches large amounts of dissolved iron and sulphate enters the rivers with inflowing groundwater as calculated from the radon data. Using the measured iron and sulphate loadings we calculated that up to 120 tons/day of iron (oxy)hydroxide was retained in the combined Spree and Klein Spree catchments, a large amount of which remains in the mining lakes.

Oxygen is one of the most crucial elements on earth and equally affects life and inorganic redox processes. After its transition to water with moderate solubility and slow diffusion rates, most aquatic organisms depend on permanent renewal of dissolved oxygen (DO). Recharge of this pivotal aqueous gas may become hampered by anthropogenic and climatic influences with so far unknown consequences for surface freshwater systems and entire ecosystems. Because rivers integrate biogeochemical information of catchments, their oxygen dynamics may also reflect ecosystem and landscape health. Here we summarize the most important sources and sinks of DO and its role in river systems. These considerations also extend to associated water compartments and fluxes including lakes, reservoirs, soils, groundwater and the hyporheic zone. In addition, for continental-scale considerations, we analysed the GLObal RIver CHemistry (GLORICH) database with 170,369 DO measurements. These analyses revealed that DO in rivers relates to water temperature, pH and nutrient availability. On larger scales, it is also influenced by catchment area, slope, ratios of forests to managed land and population density. Our review also highlights important links between physical and biological influences on DO transfer as well as its sources and sinks in streams and rivers. We conclude that DO monitoring should be combined with novel interdisciplinary tracing techniques such as stable isotope ratios, radon gas and biological analyses. Such combined analyses have the potential to improve our understanding of transfer and transformations of oxygen in rivers as essential integrators of landscapes.

Identifying groundwater discharge zones to lakes is important because the groundwater system can contribute a significant flux of water and solutes and potentially influence lake chemistry and ecology. We present a new instrument, Temperature Mapper (TM), to map groundwater discharge zones in lakes via temperature anomalies along the lake bottom, tested in Lake Constance at 2 former sand and gravel extraction sites. TM is essentially a 4 m wide sled with an array of 19 high-resolution sensors attached to an aluminum lattice towed along the lake bed behind a boat. It is able to map spatial temperature patterns representing groundwater discharge zones at a resolution of 20 cm laterally and 2.5 m in path direction. We conducted 2 field campaigns in February 2016 and February 2017, during which TM identified several temperature anomalies interpreted as groundwater discharging zones. Radon-222 was also analyzed at selected locations as an independent indicator of groundwater discharge. Overall, when comparing the different methods and different years, we observed similar spatial patterns of preferential groundwater discharge. TM offers a useful qualitative tool to identify and map potential groundwater discharge zones in lakes or coastal areas that can then be further studied by more quantitative, small-scale methods such as chemical tracers or seepage meters.

Coastal ecosystem health and sustainability is tightly coupled to submarine groundwater discharge (SGD) and associated nutrient, carbon and pollutant fluxes. However, there are few studies that systematically analyse the interaction between the terrestrial aquifer system, catchment morphology and coastal SGD. The objective of this study was to evaluate the role of catchment morphology and how this influences the spatial distribution, timing and volume of the SGD flux to a branched tidal creek system, on the barrier island Spiekeroog, Germany. The subsurface salinity was mapped using electrical resistivity tomography (ERT) and a hydrogeochemical survey of the shallow groundwater. Temporal and spatial analysis of geochemical tracers (radon-222, chloride) in the tidal creek water was integrated into a transient 222Rn mass balance model for quantification of SGD rates during two field campaigns that encompassed spring and neap tides. The ERT mapping indicated that fresh groundwater dominated under the dune ridges down to about 15 m depth, but became progressively more brackish seawards. This is likely due to frequent tidal and storm flooding of low-lying areas. The highest groundwater fluxes into the creek were indicated by high 222Rn activities (average 468 Bq m−3) towards the dune ridge. Chloride concentrations (up to 17.3 g L−1) increased seawards showing the progressive salinization of water in the creek. The freshwater component of SGD was highly variable in time but was highest at low tide, while the total SGD flux (saline + fresh) was highest when tides changed from inflow to outflow as the rapid pressure release on the local aquifer caused a large hydraulic gradient towards the creek. Comparing the freshwater component of mean daily SGD to the creek with estimated groundwater recharge rates in the catchment (665 m3 d−1) shows that the fresh groundwater discharge exceeds fresh recharge during spring tides (~120%) but is was lower than recharge during neap tides (~27%). In this study we show that tidal creeks and their relation to catchment morphology are relevant for understanding the spatial and temporal exchange of fresh and saline water between the catchment and coastal zone.

The flux of water, nutrients, carbon and salt through the subsurface at the land-sea interface is an important control on coastal nutrient processes, salinization of coastal aquifers and carbon balances of the coastal zone. However, these fluxes are often spatially and temporally complex and difficult to quantify, especially in high-energy mesotidal systems. Here we use vertical temperature profiles along a morphologically complex mesotidal high-energy beachface to map and quantify water infiltration and exfiltration on the island of Spiekeroog, Germany. Water fluxes were quantified using heat transport calculations from three solutions to the 1D heat transport equation, and include 1) a steady state analytical solution, 2) a non-steady state numerical model and 3) a non-steady state analytical solution. The temperature profiles could clearly map areas of upwelling warm (up to 10 °C) groundwater during the winters of 2018 and 2019. These upwelling zones were focused on an intertidal runnel system and at the low water line, consistent with the current understanding of the site based on visual observations and hydrogeological models. The steady state model provided good fits to the measured data in the winter when the seawater temperatures were not changing significantly, but was less able to reproduce the measured profiles in spring when seawater was warming. The steady state flux rates ranged from −110 to −43 mm d−1 in the runnel and low water line to +43 mm d−1 towards the high water line. The dynamic numerical model successfully captured the propagation of the seawater temperature signal into the subsurface and was able to reproduce the temperature profiles during both seasons. The flux estimates tended to be larger with the numerical model, with up to −150 mm d−1 in the runnel and +110 mm d−1 towards the high water line. The non-steady state analytical solution could only be applied to a limited time series due to the difficulty of logging temperatures in the subsurface at this highly dynamic site. Up to 1.5 days of data suggested fluxes that were considerably higher than the other two methods with best-estimates of −400 to −900 mm d−1. Thermal Peclet numbers ranged from 0.2 to 2 suggesting that both conduction and advection of heat is important. This study demonstrates that the morphology of the beach face is an important control on spatial distribution of down-welling and upwelling zones along the beach and that temperature measurement combined with heat modelling are potentially useful methods for understanding the interactions between groundwater and the sea.

Determining recharge rates is critical for understanding and managing groundwater systems. This study compares recharge rates estimated using different methods in a temperate catchment (the Ovens Valley, southeast Australia). A catchment water balance yields recharge rates of 110–410 mm yr−1, with the range reflecting uncertainties in evapotranspiration and the fraction of river water that represents surface runoff. The water table fluctuation method yielded recharge rates of 177–1296 mm yr−1 with a median estimate of 518 mm yr−1. Residual moisture in the unsaturated zone, which lowers the effective specific yield, probably results in this method overestimating recharge rates. Recharge rates estimated from chloride mass balance are also variable (5–712 mm yr−1) with a median value of 285 mm yr−1. Most uncertainties in those estimates arise from uncertainties in the mass of Cl exported directly by surface runoff. Recharge rates calculated from 3H activities using the well-mixed reservoir (renewal rate) model are 11 to 480 mm yr−1 with a median estimate of 159 mm yr−1. The lower recharge rates calculated from 3H may be due to the presence of older water in zones of restricted flow in the aquifers. Additional uncertainties in this technique arise from assumptions in the 3H input function and having to estimate the depth of the active recharge zone at the top of the aquifer. The wide range of recharge rate estimates from methods based on individual groundwater samples may also reflect spatially heterogeneous recharge, which creates difficulties for determining representative recharge rates. The Ovens catchment is typical of many globally in that recharge rates are estimated using infrastructure and data that were not specifically intended for that purpose. The catchment water balance and chloride mass balance are based on abundant commonly-available data and probably yield reasonable estimates of recharge in this and similar temperate catchments.

The potentially detrimental impact of groundwater discharge into rivers on the ecosystem services provided by the river makes the localization of groundwater discharge areas as well as the quantification of the associated mass fluxes an issue of major interest. However, localizing groundwater discharge zones and evaluating their impact are challenging tasks because of (i) the limited number of suitable tracers and (ii) the high spatio-temporal variability of groundwater/river water interaction in general. In this study, we applied the ubiquitous naturally occurring radioactive noble gas radon (&sup2, &sup2, Rn) as an aqueous tracer to localize and quantify groundwater discharge along a 60 km reach of the upper German part of the major river Elbe under drought conditions. All radon data processing was executed with the numerical implicit finite element model FINIFLUX, a radon mass balance-based approach, which has been developed specifically to quantify the groundwater flux into rivers. The model results were compared to the tritium (3H) distribution pattern in the studied river reach. The results of the study proved the applicability of both (i) the methodical approach (i.e., radon as tracer) and (ii) the application of FINIFLUX to drought conditions (with river discharge rates as low as 82 m3/s vs. a long time mean of 300 m3/s). Applying the model, the recorded dataset allowed differentiating between groundwater baseflow, on the one hand, and interflow and surface water runoff distributions to the river, on the other. Furthermore, the model results allowed assessing the location and the intensity of groundwater discharge into the river under low flow conditions. It was also shown that analysing discrete river water samples taken from distinct points in a major stream might lead to slightly incorrect results because of an incomplete mixing of river water and locally discharging groundwater. An integrating sampling approach (as applied for radon) is preferable here.

Understanding groundwater–surface water exchange in river banks is crucial for effective water management and a range of scientific disciplines. While there has been much research on bank storage, many studies assume idealized aquifer systems. This paper presents a field-based study of the Tambo Catchment (southeast Australia) where the Tambo River interacts with both an unconfined aquifer containing relatively young and fresh groundwater ( 2500 μS/cm and >10 000 years old). Continuous groundwater elevation and electrical conductivity monitoring within the different aquifers and the river suggest that the degree of mixing between the two aquifers and the river varies significantly in response to changing hydrological conditions. Numerical modelling using MODFLOW and the solute transport package MT3DMS indicates that saline water in the river bank moves away from the river during flooding as hydraulic gradients reverse. This water then returns during flood recession as baseflow hydraulic gradients are re-established. Modelling also indicates that the concentration of a simulated conservative groundwater solute can increase for up to ~34 days at distances of 20 and 40 m from the river in response to flood events approximately 10 m in height. For the same flood event, simulated solute concentrations within 10 m of the river increase for only ~15 days as the infiltrating low-salinity river water drives groundwater dilution. Average groundwater fluxes to the river stretch estimated using Darcy's law were 7 m3/m/day compared with 26 and 3 m3/m/day for the same periods via mass balance using Radon (222Rn) and chloride (Cl), respectively. The study shows that by coupling numerical modelling with continuous groundwater–surface water monitoring, the transient nature of bank storage can be evaluated, leading to a better understanding of the hydrological system and better interpretation of hydrochemical data. Copyright © 2015 John Wiley & Sons, Ltd.

Understanding groundwater inflows to rivers is important in managing connected groundwater and surface water systems and for protecting groundwater-dependant ecosystems. This study defines the distribution of gaining reaches and estimates groundwater inflows to a 62 km long section of Deep Creek (Maribyrnong catchment, Australia) using 222 Rn. During summer months, Deep Creek ceases to flow and comprises a chain of ponds that δ 18 O and δ 2 H values, major ion concentrations, and 222 Rn activities imply are groundwater fed. During the period where the river flows, the relative contribution of groundwater inflows to total river discharge ranges from ∼14% at high flow conditions to ∼100% at low flows. That the predicted groundwater inflows account for all of the increase in discharge at low flow conditions lends confidence to the mass balance calculations. Near-continuous 27 week 222 Rn monitoring at one location in the middle of the catchment confirms the inverse correlation between river discharge and relative groundwater inflows, and also implies that there are limited bank return flows. Variations in groundwater inflows are related to geology and topography. High groundwater inflows occur where the river is at the edge of its floodplain, adjacent to hills composed of basement rocks, or flowing through steep incised valleys. Understanding the distribution of groundwater inflows and quantifying the contribution of groundwater to Deep Creek is important for managing and protecting the surface water resources, which support the endangered Yarra pygmy perch.

Knowledge of groundwater residence times and recharge locations is vital to the sustainable management of groundwater resources. Here we investigate groundwater residence times and patterns of recharge in the Gellibrand Valley, southeast Australia, where outcropping aquifer sediments of the Eastern View Formation form an "aquifer window" that may receive diffuse recharge from rainfall and recharge from the Gellibrand River. To determine recharge patterns and groundwater flow paths, environmental isotopes (3H, 14C, δ13C, δ18O, δ2H) are used in conjunction with groundwater geochemistry and continuous monitoring of groundwater elevation and electrical conductivity. The water table fluctuates by 0.9 to 3.7 m annually, implying recharge rates of 90 and 372 mm yr−1. However, residence times of shallow (11 to 29 m) groundwater determined by 14C are between 100 and 10 000 years, 3H activities are negligible in most of the groundwater, and groundwater electrical conductivity remains constant over the period of study. Deeper groundwater with older 14C ages has lower δ18O values than younger, shallower groundwater, which is consistent with it being derived from greater altitudes. The combined geochemistry data indicate that local recharge from precipitation within the valley occurs through the aquifer window, however much of the groundwater in the Gellibrand Valley predominantly originates from the regional recharge zone, the Barongarook High. The Gellibrand Valley is a regional discharge zone with upward head gradients that limits local recharge to the upper 10 m of the aquifer. Additionally, the groundwater head gradients adjacent to the Gellibrand River are generally upwards, implying that it does not recharge the surrounding groundwater and has limited bank storage. 14C ages and Cl concentrations are well correlated and Cl concentrations may be used to provide a first-order estimate of groundwater residence times. Progressively lower chloride concentrations from 10 000 years BP to the present day are interpreted to indicate an increase in recharge rates on the Barongarook High.

222Rn is an important tracer for quantifying groundwater inflows to rivers, especially where groundwater and surface water have similar major ion and stable isotope geochemistry. Uncertainties in the 222Rn mass balance arise, however, from not accurately estimating the degree of degassing of 222Rn to the atmosphere and the extent to which interaction within the parafluvial zone provides an additional source of 222Rn. This study estimates both 222Rn production in the parafluvial zone and degassing along a 75km stretch of the King River, Australia, in order to more precisely quantify groundwater inflows. The contribution of 222Rn from the parafluvial zone (Fp) was estimated using 222Rn emanation rates from near-river sediments and assessment of the residence time of water in the parafluvial zone from 222Rn activities and Cl concentrations in water the alluvial sediments. Values of Fp range from 40,400Bq/m/day in the upper King to 8500Bq/m/day in the lower King, corresponding to differences in the mineralogy and the volume of the parafluvial zone. The gas transfer coefficient (k) was estimated by matching groundwater inflows to observed increases in river discharge during a period of low discharge; k decreases from 25day-1 in the upper King to 3day-1 in the lower King. These k values are higher than those from most empirical formulations, probably due to the extensive pool and riffle sections that promote degassing. The King River is gaining in its upper and middle sections but several of the lower reaches are losing. Groundwater inflows on a reach scale are as high as 10m3/m/day and cumulative inflows along the 75km stretch are up to 19,000m3/day. Groundwater inflows increase proportional to total flow reflecting the response of both groundwater and surface water systems to rainfall. Uncertainties in the calculated groundwater inflows are reduced by independently estimating k from the discharge data; however, calculated inflows are up to 40% higher if parafluvial flow were not taken into account. •Predicted Rn activities from emanation measurements agree with those measured in groundwater.•Contribution of Rn from parafluvial zone is assessed.•Rn degassing is independently assessed by comparison with flow gauge data.•Rn degassing is higher than predicted from empirical formulations due to pool and riffle sections.•King River transitions from gaining in upper sections to losing in lower sections.

Radon ( R 86 222 n ) as a hydrological tracer offers a method for studying short to medium term groundwater - surface water interactions. These high frequency processes play an important role in wetland hydrology and biogeochemistry and may influence their contribution to the global carbon cycle. Therefore, there is a definite need for robust methods to measure high resolution 222Rn time series in-situ. In this study we adapted and improved a membrane system to measure 222Rn continuously with a primary focus on a rapid response time and low power consumption. The membrane system was constructed using a hydrophobic capillary membrane and laboratory experiments were conducted to quantify the systems' response time to predefined 222Rn pulses. It was then deployed in a stream draining a riparian wetland. The new membrane system could reduce the response time by ≈ 60 % in comparison to the established silicone membrane. We could identify the behaviour of the system in response to dynamically changing 222Rn activities and suggest a new method using simple linear regression to quantify the systems’ response when the response time concept is inapplicable. Finally, we were able to measure high temporal resolution 222Rn activities reliably over an extended field deployment (68 d). We conclude that the improved system fills a gap ensuring high temporal resolution while maintaining extended maintenance intervals. This allows the user to study high frequency hydrological processes in remote areas. This new membrane system can be used to detect fast changes in 222Rn activities improving the comprehension of the underlying hydrological processes.

A robust correlation between electrical conductivity (EC) values and Cl concentrations in river water from southeast Australia allows detailed Cl fluxes to be calculated from continuous EC and river discharge records. Many Victorian rivers export significantly more Cl than is delivered to their catchments by rainfall. Cl* is defined as the mass of Cl exported in the rivers relative to that input by rainfall over a multi-year period (Cl*. =. 100% indicates that the river exports the same mass of Cl as is input by rainfall). There is a systematic relationship between catchment type and Cl*. Rivers draining cleared plains have Cl* values between 50 and 750%, rivers draining volcanic plains have Cl* values of 770-1600%, whereas rivers with large forested upland catchments have Cl* values of 50-110%. These values are minima as they do not account for Cl exported by groundwater from the catchments. The calculations are based on long-term (up to 22. year) records that span drought and high rainfall periods. The magnitude of Cl* is far higher than can be explained by errors in the calculations or variability in rainfall and runoff, and Cl/Br ratios preclude halite dissolution as a source of Cl. The excess Cl reflects hydrological changes in the catchments. Land clearing on the cleared plains has caused the rise of regional water tables which results in the export of Cl from saline groundwater via increased baseflow to the river systems. Drainage systems on the volcanic plains are re-establishing following impoundment by recent (

The interaction between groundwater and surface water along the Tambo and Nicholson rivers, southeast Australia, was investigated using 222Rn, Cl, differential flow gauging, head gradients, electrical conductivity (EC) and temperature profiles. Head gradients, temperature profiles, Cl concentrations and 222Rn activities all indicate higher groundwater fluxes to the Tambo River in areas of increased topographic variation where the potential to form large groundwater–surface water gradients is greater. Groundwater discharge to the Tambo River calculated by Cl mass balance was significantly lower (1.48 × 104 to 1.41 × 103 m3 day−1) than discharge estimated by 222Rn mass balance (5.35 × 105 to 9.56 × 103 m3 day−1) and differential flow gauging (5.41 × 105 to 6.30 × 103 m3 day−1) due to bank return waters. While groundwater sampling from the bank of the Tambo River was intended to account for changes in groundwater chemistry associated with bank infiltration, variations in bank infiltration between sample sites remain unaccounted for, limiting the use of Cl as an effective tracer. Groundwater discharge to both the Tambo and Nicholson rivers was the highest under high-flow conditions in the days to weeks following significant rainfall, indicating that the rivers are well connected to a groundwater system that is responsive to rainfall. Groundwater constituted the lowest proportion of river discharge during times of increased rainfall that followed dry periods, while groundwater constituted the highest proportion of river discharge under baseflow conditions (21.4% of the Tambo in April 2010 and 18.9% of the Nicholson in September 2010).

Documenting whether surface water catchments are in net chemical mass balance is important to understanding hydrological systems. Catchments that export significantly greater volumes of solutes than are delivered via rainfall are not in hydrologic equilibrium and indicate a changing hydrological system. Here an assessment is made of whether a saline catchment in southeast Australia is in chemical mass balance based on Cl. The upper reaches of the Barwon River, southeast Australia, has total dissolved solids, TDS, concentrations of up to 5860mg/L and Cl concentrations of up to 3370mg/L. The high river TDS concentrations are due to the influxes of groundwater with TDS concentrations of up to 68,000mg/L. Between 1989 and 2011, the median annual Cl flux from the upper Barwon catchment was 17.8×106kg (~140kg/a/ha). This represents 340-2230% of the annual Cl input by rainfall to the catchment. Major ion and stable isotope geochemistry indicate that the dominant source of solutes in the catchment is evapotranspiration of rainfall, precluding mineral dissolution as a source of excess Cl. The upper Barwon catchment is not in chemical mass balance and is a net exporter of solutes. The chemical imbalance may reflect the transition within the last 100ka from an endorheic lake system where solutes were recycled producing shallow groundwater with high TDS concentrations to a better drained catchment. Alternatively, a rise in the regional water table following land clearing may have increased the input of groundwater with high TDS concentrations to the river system.

There is little known about the short-term dynamics of groundwater-surface water exchange in losing rivers. This is partly due to the paucity of chemical techniques that can autonomously collect high-frequency data in groundwater bores. Here we present two new instruments for continuous in situ (222)Rn measurement in bores for quantifying the surface water infiltration rate into an underlying or adjacent aquifer. These instruments are based on (222)Rn diffusion through silicone tube membranes, either wrapped around a pole (MonoRad) or strung between two hollow end pieces (OctoRad). They are combined with novel, robust, low-cost Geiger counter (222)Rn detectors which are ideal for long-term autonomous measurement. The down-hole instruments have a quantitative response time of about a day during low flow, but this decreases to12 h during high-flow events. The setup was able to trace river water bank infiltration during moderate to high river flow during two field experiments. Mass-balance calculations using the (222)Rn data gave a maximum infiltration rate of 2 m d(-1). These instruments offer the first easily constructible system for continuous (222)Rn analysis in groundwater, and could be used to trace surface water infiltration in many environments including rivers, lakes, wetlands, and coastal settings.