Damming Weakens the Groundwater and Surface Water Exchange and Its Nitrate Removal in Semi-arid Prairie Rivers

Surface water (SW)-Groundwater (GW) exchange plays a vital role in a prairie aquatic system and the biogeochemical cycling in such a system. Considering the inadequate understanding of damming on SW-GW exchange, a damming prairie river in Southeast Eurasian steppe was chosen to investigate variations of the SW-GW exchange and its influences on the fate of nitrate (NO3-). Both hydraulic and hydrochemical methods were applied to precisely depict the daily and seasonal exchange processes. The upstream and downstream reaches of the dam were observed to be upwelling and downwelling conditions respectively within a hydrologic year. Results obtained from multiple tracer methods and hydraulic method indicate that damming contributed to transfer the stream from the upwelling to the downwelling condition and weaken the SW-GW exchange in the downstream. The patterns of SW-GW exchange modulated the NO3- uptake or production between the SW and the GW. NO3- was mainly removed in the SW-GW exchange zone (SW-GW EZ) of the upwelling segment, while produced in the downwelling segment. Both the removal and production of NO3- were enhanced during snowmelt period, which might be an active period for the SW-GW exchange and NO3- fate. This study underscores the negative effect of damming on the SW-GW exchange and accompanied NO3- removal in prairie river systems.

Use of helium as an artificial tracer to study surface water/groundwater exchange

<p><span>Groundwater - surface water interactions (SGI) fundamentally control groundwater recharge. The according dynamics are, thus, key for sustainable (drinking) water management. SGI are particularly relevant in the context of climate change and re-naturalization of canalized rivers, which might affect the availability and quality of groundwater pumped near streams. SGI are often not directly observable due to their complex spatial and temporal patterns. To complement the few available tracer methods (dye, electric conductivity, heat, etc.) to analyze SGI, we developed a novel method to quantify riverine groundwater recharge by using helium (He) as an artificial tracer. </span></p><p><span>We injected gaseous He into a Swiss pre-alpine river (river Emme, canton of Berne) through perforated tubing which was placed on the riverbed. Dissolved He (as well as Ar, N</span><span>2 </span><span>and O</span><span>2</span><span>) concentrations were continuously monitored in the river (200 m downstream of the injection point) and in a piezometer (30 m away from the river) using a portable mass spectrometer allowing quantitative gas determination under field conditions (miniRUEDI, gas-equilibrium membrane-inlet mass spectrometer (GEMIMS), Gasometrix GmbH, Brennwald et al. (2016)). The He injection consisted of two pulses, each lasting around 8 hours, during which dissolved He became supersaturated by up to three orders of magnitude compared to the natural (atmospheric) He abundance in surface waters (concentration of air saturated water (ASW)). The two associated He pulses were clearly identifiable in the groundwater and appeared in the piezometer approximately one day after the injection phases. The measured He concentrations in the groundwater were four to six times higher than ASW.</span></p><p><span>In conclusion, our experimental setup allows the identification of the freshly infiltrated river water in an adjacent groundwater body in a concise, robust and straightforward manner. Our new method is also non-toxic and can thus often be implemented with minimal constraints. Such tracer methods provide useful observations to constrain physically based, surface water/groundwater models.</span></p>

Review: Advances in the methodology and application of tracing in karst aquifers

Tracer methods have been widely used in many fields of environmental and natural sciences, and also in human health sciences. In particular, tracers are used in the study of karst hydrogeology, typically focusing on phenomena such as sinkholes, sinking rivers and large karst springs. It is known that tracers have been used since antiquity. The aim of tracer tests has been to investigate underground flow paths, transport processes and water–rock interactions, and to get an insight into the functioning of a karst aquifer. In karst hydrogeology, tracer methods are the most important investigation tools beside conventional hydrological methods. In early times, tracer methods were applied only to investigate underground flow-paths. Later they were also used to elucidate transport processes associated with water flow, and today they are often the basis, together with detailed hydrological information, of groundwater protection investigations and aquifer modelling. Many substances (spores, microspheres, bacteriophages, salt tracers, fluorescent dyes, radioactive substances) have been investigated for their properties and potential usage in environmental investigations, in particular the often unknown and inaccessible underground systems of karst areas. A great number of analytical techniques is available. This includes instrumentation for laboratory applications and direct online, on-site or in-situ field measurements. Modern instruments have a high capability for data acquisition, storage and transmission in short intervals, as a basis for quantitative evaluation and modelling. This enables research on the hydrological and hydrochemical dynamics of aquifers and their response to different natural or anthropogenic impacts.

Groundwater Recharge in the Cerrado Biome, Brazil—A Multi-Method Study at Experimental Watershed Scale

Groundwater recharge is a key hydrological process for integrated water resource management, as it recharges aquifers and maintains the baseflow of perennial rivers. In Brazil, the Cerrado biome is an important continental recharge zone, but information on rates and spatial distribution is still lacking for this country. The objective of this work was to characterize the groundwater recharge process in phreatic aquifers of the Cerrado biome. For this, an experimental watershed representative of the referred biome was established and intensively monitored. The methodology consisted of an inverse numerical modeling approach of the saturated zone and three classic methods of recharge evaluation—hydrological modeling, baseflow separation, and water table elevation. The results indicated average potential recharge around 35% of the annual precipitation, average effective recharge around 21%, and higher rates occurring in flat areas of Ferralsols covered with natural vegetation of the Cerrado biome. As the level of uncertainty inferred from the methods was high, these results were considered a first attempt and will be better evaluated by comparison with other methods not applied in this work, such as the lysimeter and chemical tracer methods.

Regional Isotopic Signatures of Groundwater in Croatia

Tracer methods are useful for investigating groundwater travel times and recharge rates and analysing impacts on groundwater quality. The most frequently used tracers are stable isotopes and tritium. Stable isotopes of oxygen (δ18O) and hydrogen (δ2H) are mainly used as indicators of the recharge condition. Tritium (3H) is used to estimate an approximate mean groundwater age. This paper presents the results of an analysis of stable isotope data and tritium activity in Croatian groundwater samples that were collected between 1997 and 2014 at approximately 100 sites. The composition of the stable isotopes of groundwater in Croatia originates from recent precipitation and is described using two regional groundwater lines. One of them is applied to groundwater accumulated in the aquifers in the Pannonian part of Croatia and the other is for groundwater accumulated in the Dinaric karst of Croatia. The isotope content shows that the studied groundwater is mainly modern water. A mix of sub-modern and modern water is mostly accumulated in semi-confined porous aquifers in northern Croatia, deep carbonate aquifers, and (sub)thermal springs.

Options to enhance the veracity of Australian health service accreditation assessments

Background: Assessment processes applied within some health service accreditation programs have been criticised at times for being inaccurate, inconsistent or inefficient. Such criticism has inspired the development of innovative assessment methods. Objective: The Australian Commission on Safety and Quality in Health Care considered the use of three such methods: short-notice or unannounced methods; patient journey or tracer methods; and attestation by governing bodies. Method: A systematic search and synthesis of published peer-reviewed and grey literature associated with these methods. Results and Conclusion: The published literature demonstrates that the likely benefits of these three assessment methods warrant further evaluation, real-world trials and stakeholder consultation to determine the most appropriate models to introduce into national accreditation programs. Implications: The subsequent introduction of models of short-notice assessments and attestation by governing bodies into the Australian Health Service Safety and Quality Accreditation Scheme in January 2019 demonstrates how the findings presented in this article influenced the national change in assessment practice, providing an example of evidence-informed accreditation development.

Mobile or not mobile: exploring the linkage between deep mantle composition and early Earth surface mobility

<p>Numerical models of mantle convection help our understanding of the complex feedback between the plates and deep interior dynamics through space and time. Did the early Earth have plate tectonics, a stagnant lid, or something in between? The surface dynamics of the early Earth remain poorly understood. Current numerical models of mantle convection are constrained by present-day observations, but the behavior of the hotter, early Earth prior to the onset of plate tectonics is less certain. The early Earth may have possessed a large hot magma ocean trapped near the core-mantle boundary after formation during differentiation, and likely containing different elements from the surrounding mantle. We examine how composition-dependent properties in the deep mantle affect convection dynamics and surface mobility in high Rayleigh number models featuring plastic yielding. Our Newtonian models indicate that increased conductivity or decreased viscosity flattens basal topography while also increasing the potential for surface yielding. We vary the viscosity, thermal conductivity, and internal heating in a compositionally distinct basal magma ocean and explore the compositional topography, insulation effects and surface stresses for non-Newtonian rheology. Models are run using a variety of crustal compositions, such as the inclusion of primordial continental material before the onset of plate tectonics. We monitor the surface for plate-like behavior. Since convective vigour is very strong in the early Earth, specialized tracer methods are employed for increased accuracy. In our models, Stokes flow solutions are obtained using a multigrid method specifically designed to handle large viscosity contrasts and non-Newtonian rheology.</p>