Hydrogeological applications of 222rn

Abstract

Groundwater resources management and protection require a quantitative understanding of various hydrogeological processes at different temporal and spatial scales. However, the identification and quantification of hydrogeological processes are difficult to assess using only conventional tools. Environmental tracers, such as isotopes, often provide a deeper insight into hydrogeological processes, especially when combined with other tracers and conventional tools. 222Rn (radon), a radioactive noble gas of lithological origin, has been revealed as a useful tracer in hydrology. Given its chemical characteristics that prevent precipitation, adsorption, or exchange, its potential as a lithological tracer in heterogeneous aquifer systems is considerable. For several decades, the use of radon for identifying groundwater discharge and flow-rate estimation was labour intensive, expensive, and limited. However, thanks to new analytical techniques, radon measurements are now easily performed using portable detectors. The general objective of the thesis is to evaluate the suitability of radon as the main tool to gain valuable scientific information on key hydrogeological processes occurring in large and complex hydrological systems, expanding its use as a singular qualitative and quantitative hydrogeological tool. Specifically, the aims of this thesis are to (1) evaluate the usefulness of radon to identify groundwater-surface water interactions between different hydrogeological settings and different surface water bodies, (2) identify groundwater sources and flow patterns in different complex multilayer aquifers, and (3) establish a sound methodology to quantify groundwater discharge to rivers using a radon mass balance model. To with comply these aims, this thesis features i) a state-of-the-art review of contemporary sampling and analytical techniques and the scientific progress on the use of radon in hydrogeological applications, which is complemented with a synthesis of the processes and factors controlling radon levels in groundwater, and ii) three hydrogeological applications of radon carefully designed to fulfil each of the thesis specific objectives, whose results originated the three papers published in first-order international journals included in this dissertation. This research originated in the Spanish-financed project REDESAC (CGL2009-12910-C03-03) and continued in the framework of several research projects funded by International Atomic Energy Agency in Argentina. The state-of-the-art review showed that recent improvements on in-situ radon measurements in water have increased this geochemical tracer's applications in hydrogeological studies worldwide in the last 15 years. Most of the published scientific literature using this tracer focused on identifying groundwater discharge areas; some on its quantification, requiring complementary variables often challenging to measure or estimate, and a few on the use of radon to trace the provenance of water sources. In addition, the comprehensive analysis of current radon sampling and measuring methods provided valuable information for professionals interested in using this tracer in the field, including different types of equipment available for measuring radon activities in water, as well as the factors affecting analytical accuracy. The three applications of radon designed to comply with the specific objectives and the main outcomes are as follows: The Mundo River (SE Spain) was selected to test the applicability of radon to identify and quantify groundwater discharge to the river, as these were poorly understood due to the complex geology of the area. The observed increases in radon activity in the river water pointed to significant diffuse groundwater discharge in four tracts of the studied river reach. The characterization of groundwater radon activity by measuring wells and springs allowed us to assess groundwater discharge to the river quantitatively. Radon was key to tracing groundwater discharge, especially in areas with minimal chemical and other isotopic contrast. A novel contribution was the estimation of an integrated uncertainty associated with the quantified groundwater discharge flow by radon mass balance, which increase the confidence of the results. The Loma de Úbeda aquifer system was carefully chosen to test the usefulness of radon as a tracer of groundwater provenance in thick multilayer aquifer systems with varied lithology, and as a potential tracer to complement hydrochemistry-derived conceptual models on endmember signatures. The groundwater flow network of this system is modified by intense exploitation and deep faults that favour the vertical mixing of waters from different layers and with distinct chemical compositions. This situation has induced water quality deterioration and fostered the risk of quantity restrictions. Radon provided insights to identify and quantify mixtures of multiple endmember waters via their different radon signatures in this large-scale system (102 km2 and >700 m deep wells), where the hydraulic gradients and flow velocities in the surroundings of the wells are increased due to intensive exploitation. Radon was paramount to discriminate the signatures of two chemically similar hydrogeological layers and to quantify their mixing proportions in samples from deep, very productive agricultural wells. The Esteros del Iberá Wetland Area was selected to evaluate the applicability of radon as a primary tracer to validate the hypothesis of regional discharge to large surface-water systems. These wetlands were suspected to be a regional discharge area of one of the largest transboundary aquifers in the world, the Guaraní Aquifer System (SAG, after the Spanish name). Radon was key to identifying deep groundwater discharge, both from the SAG and the Pre-SAG formations, into shallow groundwater layers, as well as the localised discharge of those deep groundwaters to one lagoon and adjacent stream, within an extensive wetland area (37,930 km2). Especially novel was radon's contribution to highlighting the role of geological structures, specifically regional steep faults, in controlling the groundwater mixtures and surface discharge by rapid upward flows. This thesis concludes that: (1) Radon has proven to be a key and distinctive tracer to i) identify groundwater discharge to different surface water bodies, especially in large wetland areas, where quite often most waters are very diluted by rainfall contribution and there are no noticeable chemical changes; ii) quantify discharge to streams; and iii) identify the hydrogeologic provenance and quantify the contribution of different groundwater sources in mixed groundwaters commonly found in large aquifers system. (2) Integrated in situ radon measurements provided precise and accurate results with very low uncertainty levels in surface water samples. The methodology developed and applied to different hydrological settings was robust and successful. In contrasts, the grab sampling technique was proven to be more adequate for measuring radon in groundwater samples, providing highly precise and accurate results with an analytical uncertainty of ± 2σ, a 95% confidence interval. (3) Using a multi-tracer approach is recommended to further constraint endmember identification when the spatial and temporal variability of radon in water causes considerable complexity (4) Water flow values estimated through radon mass balances will gain confidence and utility if they are joined by the estimation of the whole uncertainty involved in the measurement process. Among the most relevant research questions that should be addressed in the future, this thesis recommends further work in high-resolution and continuous long-term measurements of radon in water, as well as data analysis of radon in water time series and numerical modelling of radon generation and transport in groundwater systems.