Multiphase transport of tritium in unsaturated porous media-bare and vegetated soils

Tritium is a short-lived radioactive isotope (T 1/2=12.33 yr) produced naturally in the atmosphere by cosmic radiation but also released into the atmosphere and hydrosphere by nuclear activities (nuclear power stations, radioactive waste disposal). Tritium of natural or anthropogenic origin may end up in soils through tritiated rain, and may eventually appear in groundwater. Tritium in groundwater can be re-emitted to the atmosphere through the vadose zone. The tritium concentration in soil varies sharply close to the ground surface and is very sensitive to many interrelated factors like rainfall amount, evapotranspiration rate, rooting depth and water table position, rendering the modeling a rather complex task. Among many existing codes, SOLVEG is a one-dimensional numerical model to simulate multiphase transport through the unsaturated zone. Processes include tritium diffusion in both, gas and liquid phase, advection and dispersion for tritium in liquid phase, radioactive decay and equilibrium partitioning between liquid and gas phase. For its application with bare or vegetated (perennial vegetation or crops) soil surfaces and shallow or deep groundwater levels (contaminated or non-contaminated aquifer) the model has been adapted in order to include ground cover, root growth and root water uptake. The current work describes the approach and results of the modeling of a tracer test with tritiated water (7.3×108 Bq m−3) in a cultivated soil with an underlying 14 m deep unsaturated zone (non-contaminated). According to the simulation results, the soil’s natural attenuation process is governed by evapotranspiration and tritium re-emission. The latter process is due to a tritium concentration gradient between soil air and an atmospheric boundary layer at the soil surface. Re-emission generally occurs during night time, since at day time it is coupled with the evaporation process. Evapotranspiration and re-emission removed considerable quantities of tritium and limited penetration of surface-applied tritiated water in the vadose zone to no more than ∼1–2 m. After a period of 15 months tritium background concentration in soil was attained.Peer ReviewedPostprint (published version

Aquifer recharge from intensively irrigated farmland: several approaches

En las últimas décadas la literatura se ha centrado en la estimación de la recarga natural y los parámetros que la controlan, incluyendo clima, vegetación, suelo, y topografía. Por el contrario, pocos son los trabajos centrados en la recarga de acuíferos a partir de zonas cultivadas intensamente regadas. Aunque estos han mejorado la compresión sobre el proceso de recarga, todavía resultan incompletos a la hora de estimarla a partir del mencionado uso de suelo. En este contexto, los objetivos de esta tesis son: (i) mejorar la compresión de la recarga a partir de zonas intensamente regadas, y (ii) proporcionar nuevas herramientas para su caracterización. Algunas de las metodologías proporcionadas pueden ser fácilmente reproducidas por profesionales para inferir información cuantitativa. Ensayos en campo, incluyendo diferentes tipos de cultivo y prácticas agrícolas, son llevados a cabo en al área del Campo de Cartagena, sureste de España, una región semi-árida donde la agricultura intensiva es el principal uso de suelo. El desarrollo de metodologías con observaciones en el medio saturado y no saturado, junto con la modelación numérica, fueron usados para mejorar la compresión de los procesos que controlan la recarga a partir de zonas intensamente regadas. Las aproximaciones desarrolladas pueden ser resumidas como sigue: Ensayo de campo a lo largo de seiscientos días bajo cultivos hortícolas anuales y riego por goteo. La distribución del contenido de agua en la zona de raíces y bajo esta fue simulada considerando un modelo de flujo no saturado. El contenido de agua y la succión fueron medidos a diferentes profundidades y empleados para la calibración y validación del modelo. Ensayo de trazador (tritio) en campo en una parcela experimental con riego por goteo y cultivos hortícolas anuales durante cuatrocientos treinta días. El movimiento de trazador a lo largo del perfil de suelo fue simulado considerando transporte multifásico. Los perfiles de concentración de trazador, a partir de un espaciado y limitado número de muestras destructivas, fueron usados para calibrar y validar el modelo. Experimento de larga duración (nueve años hidrológicos) para diferentes tipos de cultivo: cultivos hortícolas anuales, cultivos hortícolas perennes y árboles frutales. La recarga producida por cada tipo de cultivo fue estimada a partir del balance de agua en suelo, zona no saturada y acuífero. Las fluctuaciones del nivel freático registradas a lo largo del mencionado periodo fueron usadas para calibrar y validar el modelo. Esta experiencia permitió evaluar la fiabilidad de las estimaciones de recarga a partir de las otras dos metodologías previas (a corto plazo) para el tipo de cultivo coincidente (cultivos hortícolas anuales). Para las tres aproximaciones, el cubrimiento de suelo por las plantas y el crecimiento de raíces han sido incluidos en la condición de contorno superior. La evapotranspiración se ha dividido en evaporación y transpiración como una función del índice de área de hoja, y es limitada por el contenido de agua en el suelo. La transpiración a su vez ha sido distribuida a través del perfil de suelo como una función del contenido de agua y profundidad de raíces. Valores similares de recarga han sido obtenidos a partir de las tres técnicas, aunque el modelo de flujo no saturado la sobreestima ligeramente. La evapotranspiración real fue siempre más baja que la potencial, ya que el contenido de agua en suelo fue insuficiente para mantener la extracción de agua por parte de las raíces, a pesar de la alta frecuencia de riego. Aunque las prácticas agrícolas por parte de los agricultores son las correctas, con una alta eficiencia de riego, se obtuvieron altos valores de recarga. La lluvia es distribuida de manera irregular en unos pocos eventos intensivos, algo por otro lado muy común en regiones semi-áridas, lo que contribuye de manera significativa a la percolación profunda, debido al constante alto contenido de humedad en el suelo.In the past decades a large body of literature has focused on the assessment of the natural recharge and parameters of control (including climate, vegetation, soils, and topography). On the contrary, only few papers focused on aquifer recharge from intensively irrigated farmland. Although findings have improved the understanding of recharge phenomena, they still fail to characterize many features of aquifer recharge from the mentioned land use. In this context, the aims of this thesis are: (i) to improve the understanding of aquifer recharge from intensively irrigated farmland, and (ii) to provide new tools for its characterization. Also, this thesis provides a framework that can be easily used by practitioners to infer quantitative information. Field tests, including different crop types and agricultural management, were carried out in the Campo de Cartagena area of southeast Spain, a semi-arid region where intensive irrigated agriculture is prevalent. The development of methodologies, with field observations in both saturated and unsaturated media, along with the application of numerical modelling were used to understand the processes governing the recharge from irrigated farmland. The developed approaches can be summarized as follows: A field experiment with annual row crops and drip irrigation. Soil moisture dynamics through the root zone and below were simulated from unsaturated flow approach. Soil moisture and pressure head data at different depths were recorded along six hundred days for model calibration and prediction. A tracer test (tritium) in field along four hundred thirty days. The test was carried out in an experimental plot with drip irrigation and annual row crops. The tracer transport in soil was simulated considering a multiphase approach. Tracer concentration profiles, from a limited and sparse number of destructive samples, were used to calibrate and validate the modelling approach. A long-term field experiment (based on nine hydrologic years) for different crop types, annual row crops, perennial vegetables and fruit trees. The recharge produced from each crop type was estimated from a water balance approach, including soil, vadose zone and aquifer. Water table fluctuations, recorded along the mentioned period, were used for model calibration and predictions. This long-term approach permits to evaluate recharge estimates reliability of the two previous methodologies (short-term) for the type of crop overlapped (annual row crops). For the three approaches, ground cover and root depth are assumed as upper boundary conditions. Evapotranspiration is allocated to evaporation and transpiration as a function of leaf-area-index and is limited by soil moisture content; transpiration is distributed through the soil profile as a function of soil moisture and root depth. Similar recharge values have been obtained from the three techniques, although the unsaturated flow approach slightly overestimates values. Actual evapotranspiration was always lower than potential evapotranspiration, because soil moisture was insufficient to sustain the potential uptake, despite high irrigation frequency. Although the agricultural practices from farmers are sound, high irrigation efficiency, high recharge values are achieved. Rainfall is unevenly distributed into a few intensive events, likewise very common in semi-arid regions, and it meaningfully contributes to deep percolation, due to consistently high soil water content.Postprint (published version

Conference on Planetary Volatiles

Initial and present volatile inventories and distributions in the Earth, other planets, meteorites, and comets; observational evidence on the time history of volatile transfer among reservoirs; and volatiles in planetary bodies, their mechanisms of transport, and their relation to thermal, chemical, geological and biological evolution were addressed

Conference on Planetary Volatiles

Initial and present volatile inventories and distributions in the Earth, other planets, meteorites, and comets; observational evidence on the time history of volatile transfer among reservoirs; and volatiles in planetary bodies, their mechanisms of transport, and their relation to thermal, chemical, geological and biological evolution are addressed

SURFO Technical Report No. 2006-01

The 2005 technical reports written by undergraduate students participating in the SURFO (Summer Undergraduate Research Fellowships in Oceanography) Program while at the University of Rhode Island