Development and Modelling of a High-Resolution Aquifer Analog in the Guarani Aquifer (Brazil)

Abstract

A comprehensive and detailed knowledge about the spatial distribution of physical and chemical properties in heterogeneous porous aquifers plays a decisive role for a realistic representation of governing parameters in mathematical models. Models allow the simulation, prediction and reproduction of subsurface flow and transport characteristics. This work explains the identification, characterization and effects of small-scale aquifer heterogeneities in the Guarani Aquifer System (GAS) in São Paulo State (Brazil) and elaborates on the fate and transport of a hypothetical petroleum hydrocarbon contamination in the GAS. The GAS represents one of the biggest aquifers in the world and is the most relevant groundwater resource in South America. The leading questions behind this thesis are: (1) What is the hydraulic response characteristic for the sedimentary architecture of the Pirambóia Formation in the Guarani aquifer recharge area in São Paulo State (Brazil)? (2) How do the heterogeneously distributed Fe(III) oxides as solid state electron acceptors effect the natural attenuation potential in this hydrostratigraphic unit? (3) How well is Compound Specific Isotope Analysis (CSIA) suited for assessing the occurrence and extent of intrinsic biodegradation of contaminants in heterogeneous porous aquifers? In order to properly address these three lead questions, field and laboratory work as well as a series of reactive transport simulations have been performed. In an outcrop study the sedimentary heterogeneities in the upper part of the Pirambóia Formation have been investigated in regard to the distribution of lithofacies as well as determination of hydraulic properties and sedimentary Fe(III) content. The three-dimensional distribution of the local litho- and hydrofacies is described and presented in an aquifer analog model. The model depicts the variations of hydraulic conductivity (K), porosity (n), and sediment-bound ferric iron content resolved on the centimeter scale. The two-dimensional (2D) fields of hydraulic and chemical properties were transferred into a numerical model to study the effects of small-scale heterogeneities on the flow and reactive transport of an oxidizable organic contaminant. Additional analog models have been applied in a numerical experiment to study the evolution of carbon isotope signatures during reactive transport of hydrocarbons in heterogeneous porous aquifers. Hereby, the performance of CSIA to quantify intrinsic biodegradation has been assessed. The research project resulted in three main outcomes: (1) A quantitative description of the spatial distribution of hydraulic and chemical parameters in the Pirambóia Formation of the GAS and construction of an aquifer analog model; the identification of the most permeable units and determination of a higher mean K value than previously reported for this hydrostratigraphic unit. (2) The solid-phase ferric iron concentrations and hydraulic conductivity inversely correlate and consequently limit the natural attenuation potential at the investigated site. (3) The effects of small-scale physical aquifer heterogeneities strongly influence the degree of dilution/dispersion, which is highly site-specific and undergoes significant spatiotemporal variations. The findings indicate that if the degree of dilution/dispersion is not accurately accounted for - a difficult task at contaminated sites - correct quantification of CSIA based in situ biodegradation in heterogeneous porous aquifers remains hardly possible.