Ehsan Rasa

Mathematical model of Chlorella minutissima UTEX2341 growth and lipid production under photoheterotrophic fermentation conditions

To reduce the cost of algal biomass production, mathematical model was developed for the first time to describe microalgae growth, lipid production and glycerin consumption under photoheterotrophic conditions based on logistic, Luedeking-Piret and Luedeking-Piret-like equations. All experiments were conducted in a 2 L batch reactor without considering CO(2) effect on algaes growth and lipid production. Biomass and lipid production increased with glycerin as carbon source and were well described by the logistic and Luedeking-Piret equations respectively. Model predictions were in satisfactory agreement with measured data and the mode of lipid production was growth-associated. Sensitivity analysis was applied to examine the effects of certain important parameters on model performance. Results showed that S(0), the initial concentration of glycerin, was the most significant factor for algae growth and lipid production. This model is applicable for prediction of other single cell algal species but model testing is recommended before scaling up the fermentation of process.

Role of back diffusion and biodegradation reactions in sustaining an MTBE/TBA plume in alluvial media

A methyl tert-butyl ether (MTBE) / tert-butyl alcohol (TBA) plume originating from a gasoline spill in late 1994 at Vandenberg Air Force Base (VAFB) persisted for over 15 years within 200 feet of the original spill source. The plume persisted until 2010 despite excavation of the tanks and piping within months after the spill and excavations of additional contaminated sediments from the source area in 2007 and 2008. The probable history of MTBE concentrations along the plume centerline at its source was estimated using a wide variety of available information, including published details about the original spill, excavations and monitoring by VAFB consultants, and our own research data. Two-dimensional reactive transport simulations of MTBE along the plume centerline were conducted for a 20-year period following the spill. These analyses suggest that MTBE diffused from the thin anaerobic aquifer into the adjacent anaerobic silts and transformed to TBA in both aquifer and silt layers. The model reproduces the observation that after 2004 TBA was the dominant solute, diffusing back out of the silts into the aquifer and sustaining plume concentrations much longer than would have been the case in the absence of such diffusive exchange. Simulations also suggest that aerobic degradation of MTBE or TBA at the water table in the overlying silt layer significantly affected concentrations of MTBE and TBA by limiting the chemical mass available for back diffusion to the aquifer.

Effect of different transport observations on inverse modeling results: case study of a long-term groundwater tracer test monitored at high resolution

Conservative tracer experiments can provide information useful for characterizing various subsurface transport properties. This study examines the effectiveness of three different types of transport observations for sensitivity analysis and parameter estimation of a three-dimensional site-specific groundwater flow and transport model: conservative tracer breakthrough curves (BTCs), first temporal moments of BTCs (m1), and tracer cumulative mass discharge (Md) through control planes combined with hydraulic head observations (h). High-resolution data obtained from a 410-day controlled field experiment at Vandenberg Air Force Base, California (USA), have been used. In this experiment, bromide was injected to create two adjacent plumes monitored at six different transects (perpendicular to groundwater flow) with a total of 162 monitoring wells. A total of 133 different observations of transient hydraulic head, 1,158 of BTC concentration, 23 of first moment, and 36 of mass discharge were used for sensitivity analysis and parameter estimation of nine flow and transport parameters. The importance of each group of transport observations in estimating these parameters was evaluated using sensitivity analysis, and five out of nine parameters were calibrated against these data. Results showed the advantages of using temporal moment of conservative tracer BTCs and mass discharge as observations for inverse modeling.RésuméDes traçages conservatifs peuvent fournir des informations utiles pour caractériser diverses propriétés du déplacement en subsurface. Cette étude examine la portée de trois différents types d’observations sur le déplacement pour analyse de sensibilité et paramétrage d’un modèle tridimentionnel d’écoulement et de déplacement spécifiques sur un site donné : courbes d’apparition du traceur conservatif (BTCs), instant initial de BTCs (m1), et décharge cumulée du traceur (Md) par plans de contrôle combinés avec des observations piézométriques (h). Des données haute résolution obtenues lors d’une expérimentation contrôlée de 410 jours sur le terrain de la base US Airforce de Vandenberg, Californie, USA, ont été utilisées. Dans cette expérience, du brome a été injecté pour créer deux nuages adjacents contrôlés sur 6 sections transverses différentes (perpendiculaires à l’écoulement souterrain) avec un total de 162 forages de contrôle. Un total de 133 observations différentes de rabattement en régime transitoire, 1,158 de concentrations BTC, 23 de moment initial, et 36 décharge ont été utilisées pour analyse de sensibilité et estimation de neuf paramètres d’écoulement et de déplacement de l’eau souterraine. L’importance de chaque groupe d’observations de déplacement dans l’estimation de ces paramètres a été évaluée en utilisant une analyse de sensibilité, et cinq de ces neuf paramètres ont été calés sur ces données. Les résultats ont montré les avantages d’utiliser l’instant initial du traceur conservatif BTCs et le volume déchargé comme paramètres observés pour la modélisation inverse.ResumenLos experimentos de trazadores conservativos pueden proporcionar información útil para caracterizar varias propiedades de transporte subsuperficial. Este estudio examina la efectividad de tres diferentes tipos de observaciones de transporte para un análisis de sensibilidad y la estimación de los parámetros de un modelo de transporte y flujo de agua subterránea tridimensional en un sitio específico: curvas de ruptura del trazador conservativo (BTCs), primer momento temporal de BTCs (m1), y descarga de masa acumulativa del trazador (Md) a través de planes de control combinado con observaciones de carga hidráulica (h). Se han usado los datos de alta resolución obtenidos a partir de un experimento de campo controlado de 410 días en la Base de la Fuerza Aérea Vandenberg, California (EEUU). En este experimento, el bromuro fue inyectado para crear dos plumas adyacentes monitoreadas en seis transectas diferentes (perpendiculares al flujo de agua subterránea) con un total de 162 pozos de monitoreo. Se usaron un total de 133 diferentes observaciones de carga hidráulica transitoria, 1,158 de concentración de BTC, 23 de primer momento, y 36 de descarga de masa para el análisis de sensibilidad y estimación de los parámetros de nueve parámetros de flujo y transporte. Se evaluó la importancia de cada grupo de observaciones de transporte para estimar estos parámetros usando el análisis de sensibilidad, y cinco salidas de nueve parámetros fueron calibrados contra estos datos. Los resultados mostraron las ventajas de usar el momento temporal del trazador conservativo BTCs y la descarga de masa como observaciones para el modelado inverso.ResumoOs testes com traçadores conservativos podem fornecer informações úteis para a caraterização de várias propriedades de transporte subsuperficial. Este estudo analisa a eficácia de três tipos diferentes de observações de transporte para análise da sensibilidade e estimação de parâmetros de um modelo de escoamento subterrâneo e transporte tridimensional num local específico: curvas de ruptura (BTCs) de traçadores conservativos, primeiros momentos temporais das BTCs (m1) e a massa de descarga cumulativa do traçador (Md), através de planos de controlo combinados com observações do nível piezométrico (h). Foram usados dados de alta resolução obtidos num teste de campo controlado de 410 dias realizado na Base da Força Aérea de Vandenberg, na Califórnia (EUA). Nesta experiência, foi injetado brometo, de forma a criar duas plumas adjacentes monitorizadas em seis transetos diferentes (perpendiculares ao escoamento subterrâneo), num total de 162 furos de monitorização. Foram usadas um total de 133 observações diferentes de níveis piezométricos transitórios, 1,158 observações de concentração de BTC, 23 do primeiro momento e 36 de massa descarregada, para análise de sensibilidade e estimação de 9 parâmetros de escoamento e transporte. A importância de cada grupo de observações de transporte na estimação destes parâmetros foi avaliada através da análise de sensibilidade, e cinco dos nove parâmetros foram calibrados com estes dados. Os resultados mostraram as vantagens de se usar o momento temporal das BTCs dos traçadores conservativos e as massas de descarga como observações para a modelação inversa.