Regions of surface water and groundwater exchange are major sites for the transfer and transformation of solutes and nutrients between stream and subsurface environments. Conventional stream and groundwater exchange investigations are limited by methodologies that require intensive field investigations and/or the set-up of expensive infrastructure. These difficulties are exacerbated where hydraulic gradients are very low and stream discharge highly variable. This thesis uses a suite of environmental tracers (Cl-, Rn-222, H-2 & O-18, Sr-87/Sr-86) to characterise the extent of stream and groundwater exchange between a sand bed stream and adjacent alluvial aquifer in a subtropical catchment (the Wollombi Brook) of eastern Australia. The aims were to identify sources and relative contributions of different sources of groundwater to stream discharge and specifically to improve the methodology of using Rn-222 to obtain quantitative estimate of groundwater fluxes. The sensitivity of the Rn-222 technique for identifying groundwater discharge based on the Rn-222 concentration in stream water was improved via an iterative numerical approach to account for Rn-222 loss from stream water via turbulent gas exchange and radioactive decay. Optimal distances between stream sampling points for defining the magnitude of groundwater discharge to stream flow based on Rn-222 concentrations in stream water is a function of average stream velocity and water depth. The maximum allowable distance between sampling points for determining the magnitude of groundwater discharge to the Wollombi Brook was 2 km. This work showed that groundwater discharged to all reaches of the Wollombi Brook during baseflow and flood recession conditions. Alluvial groundwater contributed less than 30% of water to stream flow in the mid Wollombi Brook catchment. Dilution of steady-state Rn-222 concentrations measured in transects from the stream to the alluvial sediments showed that significant surface water and groundwater exchange occurs even when gradients between surface water and groundwater are low. Lateral stream water influx to the adjacent alluvial aquifer was more extensive in the lowland areas of the Wollombi Catchment during low flow than flood recession conditions. Extensive stream water influx to the adjacent alluvial aquifer occurs contrary to the net direction of surface water and groundwater flux (as indicated by hydraulic gradients toward the stream channel). The rate of stream and groundwater exchange within the adjacent alluvial aquifer appears to be greatest during baseflow conditions. Fresh alluvial groundwater appeared to provide a buffer against higher salinity regional groundwater discharge to the alluvial aquifer in some reaches of the Wollombi Brook catchment. Pumping of the alluvial aquifer and diversions of surface water may jeopardise the water quality and volume of the alluvial aquifer and induce water flow from the regional aquifer toward the stream, potentially salinising the fresh alluvial aquifer and subsequently the stream. The change in the Cl- concentration and the variation in slope of the deuterium � oxygen-18 line between consecutive stream sampling points could be used to differentiate between regional and alluvial groundwater discharge to stream flow. Incorporating this information with three-component end-member mixing using [Sr2+] and Sr-87/Sr-86 showed that stream and alluvial groundwater exchange within the stream channel was highest in the lowland floodplains during low flow conditions. The least stream and alluvial groundwater exchange occurred in the low streambed gradient mid reaches of the Wollombi Brook regardless of stream stage. The greatest difference in the degree of stream and alluvial groundwater exchange between high and low stream stages occurred in the lowland floodplains of the Wollombi Brook.

Surface mining of oil sands results in extensive land disturbance, earth movement and water usage. After mining, the disturbed landscapes must be reconstructed and reclaimed as natural landforms. There are numerous challenges associated with understanding the responses of these landforms over time, including a need to track and characterize water movement through closure landforms to understand the hydrological responses of these landforms over time. This study attempted to use natural stable isotopes of water (δD and δ18O) to identify and characterize source waters from various closure landforms at an oil sands mine site. The study area is Syncrude‟s Mildred Lake mine, an open pit oil sands mine located in northern Alberta. A variety of groundwater, surface water and soil samples from a variety of landforms (overburden dumps, composite and mature fine tailings areas, tailings sand structures and freshwater reservoirs) were collected in an attempt to fully represent the isotopic distribution of waters across the mine site. Laboratory analysis of δD and δ18O was done on all samples. The local meteoric water line first established by Hilderman (2011) was redeveloped with additional precipitation data and calculated to be δD=7.0(δ18O) -18.6‰. A natural evaporation line having a slope of 5.3 was calculated for the mine site with samples collected from three surface water ponds on the mine site. Five primary source waters were identified on the mine site: process affected water/tailings, rainfall, snow, interstitial shale water and Mildred Lake water. It was found that these sources of water generally have unique natural stable water isotope signatures. Process affected water at the site generally had an enriched signature compared to other mine waters. The enrichment was attributed to fractionation from the recycle water circuit and natural evaporation. The characterizations of these source waters were then used in several hydrogeological examples to demonstrate that natural stable water isotopes can be applied to water balance estimates and to identify water movement processes related to closure landforms.

The Newmarket Till is a regional aquitard in southern Ontario that overlies the Illinoian to Middle Wisconsinan Lower Sediments and is overlain by the Oak Ridges Moraine (ORM). Geological investigations have mapped the distribution of the till and it is understood that erosional channels, subsequently infilled with fluvial material, breach the till and may create enhanced hydraulic connection between overlying and underlying aquifers. However, little is known about the protective capability of the Newmarket Till where it is intact. This study used natural tracers to assess the extent of transport in the aquitard-aquifer system. Stable isotopes of water (δ18O and δ2H) showed a depletion trend versus depth. In the Newmarket Till most of the samples had isotope ratios similar to meteoric water data from the nearest location (Egbert, ON). The depleted values of δ18O in the Thorncliffe Formation suggest a remnant signature of early-Holocene precipitation (-16‰ at the depth of 60 m). Elevated levels of NO3- and Cl- were detected near the surface and because of the low permeability aquitard (Newmarket Till), they could not migrate to depth. Total extractable ammonium concentrations are ranging from 4.09 ppm from near the surface to 60 ppm in the lowest part of the Newmarket and then gradually increase to 514 ppm in the bottom of the Thorncliffe Formation. The combination of high NH4+ values and organic carbon content in the Thorncliffe Formation suggests a natural source from mineralization of organic N. The fractionation which happened between δ15Nsediment and δ15N-NH4 may have three explanations: (1) lighter isotopes diffuse faster than heavier ones, so the higher rate of diffusion can cause fractionation; (2) heavier isotopes partition to exchange sites, causing fractionation along the transport pathway; (3) dissociation of NH4+ to NH3 under anaerobic condition. Positive values for δ13C in groundwater in the Thorncliffe Formation are likely due to i) a contribution of carbonate mineral dissolution, and ii) methanogenesis - the Archea favor the lighter isotope of C (12C). Methanogenesis, therefore, enriches the δ13C-DIC was enriched; however, the δ13C in dissolved organic carbon (DOC) is depleted. These geochemical characteristics demonstrate a long residence time for the porewater in the system and indicate that the Newmarket till inhibits recharge of recent precipitation, thereby providing protection to the underlying aquifers from surface-derived contaminants.

La plaine de la Crau (Bouches-du-Rhône, France) renferme une nappe phréatique alluviale qualifiée de « ressource patrimoniale ». Débutée il y a près de 500 ans par la création d’un réseau de canaux, la mise en culture de prairies, est encore aujourd’hui, pratiquée selon une technique d’irrigation traditionnelle par submersion. Provenant d’un autre bassin versant, les eaux d’irrigation ont un effet majeur sur la recharge de la nappe. Néanmoins l’urbanisation progressive du territoire, l’augmentation des prélèvements, et le changement climatique, menacent l’équilibre actuel de la nappe de la Crau. La préservation de cette ressource nécessite donc la production d’un outil de gestion performant prenant en compte la globalité de cet hydro-système, ainsi que sa complexité géométrique et hydrologique. Cette thèse présente une approche originale de modélisation hydrogéologique en estimant à chaque étape les variables et paramètres nécessaires de manière indépendante, réduisant ainsi un problème récurrent d’équifinalité. Constituant donc un outil prospectif fiable, ce nouveau modèle est alimenté par des scénarios réalistes permettant d’observer les impacts du changement climatique, de l’évolution de l’occupation des sols planifiée et de réductions occasionnelles de la dotation en eaux d’irrigation à l’horizon 2030. Sous les effets combinés de ces réductions de la recharge (jusqu’à -19%), la nappe phréatique de la Crau pourrait être soumise à une diminution de sa surface piézométrique allant jusqu’à 2 m entrainant l’assèchement de zones humides rares et protégées. / The Crau plain (Southern France) contains an alluvial aquifer described as a regional "heritage resource". Started nearly 500 years ago by creating a network of canals, the cultivation of grasslands is even today performed using a traditional technique of irrigation by flooding. Derived from another watershed, irrigation water has a major impact on the recharge of the aquifer. Nevertheless, urbanization of the territory, increase of uptakes, and climate change threaten the existing balance of the Crau aquifer. The preservation of this resource therefore requires the production of an efficient management tool accounting for the whole hydro-system in its hydrological and geometric complexity. This thesis presents an original approach of hydrogeological modeling by independently estimating, for each step, the necessary variables and parameters, reducing a recurring problem of equifinality. Providing a reliable forecasting tool, this new model is implemented by realistic scenarios to observe the impacts of climate change, the evolution of the planned land-use, and occasional reductions of irrigation input in 2030. Under the combined effects of these recharge reductions (up to -19%) the water table could be subjected to local decreases up to 2 m, leading to the drying up of rare and protected wetlands.

Stable isotope ratios in rainwater and water vapor act as a tracers of the hydrological cycle. The data on stable isotopic composition in rainwater and water vapor are limited due to the lack of observational studies across the Indian subcontinent and over the ocean. These present day observations are important database to deduce the paleo-climatic condition from geological archives. In majority of the cases, isotopic records are translated into physical factors with the present background knowledge where relationships with physical variables are well established. In case of tropical region, the primary controlling factor identified behind the stable isotopic variations is the amount of rainfall. However recent observations contradict such relationships challenging the role of amount effect and identified source moisture effect on stable isotope ratios in rainwater at Indian stations. The thesis investigated the amount effect relationship of isotopes at seasonal time scales. The long period observation covering 4 years, where daily time interval rainwater samples collected were used for our study. We used two different station locations; Thiruvananthapuram and Bangalore, where monsoonal rainfall is received for Southwest and Northeast periods. The role of mesoscale and synoptic convection and rainout along the transport pathways were found responsible for the isotopic variance. The role of moisture source regions, rainout over the advection pathways, high rainfall producing systems such as depression and cyclones, and continental recycling of water on rainwater and water isotopes ratios were major findings of the thesis work which are detailed in the chapters. Upon knowing the atmospheric and oceanic condition at the moisture source region based on satellite data and reanalysis data set we modeled our observations using the governing equations of isotope fractionation and well accepted Rayliegh’s distillation model. The observations on rainwater and water vapor isotope ratios in oceanic region are extremely limited due to difficulties in conducting sampling. Water vapor and rainwater observations over the Southern Ocean during the expedition of 2013 and estimation of the moisture recycling across the latitudes over the oceanic regions is a novel work in this thesis.