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

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

High resolution multi-facies realizations of sedimentary reservoir and aquifer analogs

Geological structures are by nature inaccessible to direct observation. This can cause difficulties in applications where a spatially explicit representation of such structures is required, in particular when modelling fluid migration in geological formations. An increasing trend in recent years has been to use analogs to palliate this lack of knowledge, i.e., exploiting the spatial information from sites where the geology is accessible (outcrops, quarry sites) and transferring the observed properties to a study site deemed geologically similar. While this approach is appealing, it is difficult to put in place because of the lack of access to well-documented analog data. In this paper we present comprehensive analog data sets which characterize sedimentary structures from important groundwater hosting formations in Germany and Brazil. Multiple 2-D outcrop faces are described in terms of hydraulic, thermal and chemical properties and interpolated in 3-D using stochastic techniques. These unique data sets can be used by the wider community to implement analog approaches for characterizing reservoir and aquifer formations

Physicochemical properties and 3D geostatistical simulations of the Herten and the Descalvado aquifer analogs

This data set presents a comprehensive characterisation of the sedimentary structures from important groundwater hosting formations in Germany (Herten aquifer analog) and Brazil (Descalvado aquifer analog). Multiple 2-D outcrop faces are described in terms of hydraulic, thermal and chemical properties and interpolated in 3D using stochastic techniques. For each aquifer analog, multiple 3D realisations of the facies heterogeneity are provided using different stochastic simulations settings. These are unique analogue data sets that can be used by the wider community to implement approaches for characterising aquifer formations

High-resolution aquifer analog of fluvial-aeolian sediments of the Guarani aquifer system

The Guarani aquifer system (GAS) represents one of the biggest aquifers in the world and is the most relevant groundwater resource in South America. For the first time, by combining field and laboratory measurements, a high-resolution aquifer analog model of fluvial-aeolian sediments of the GAS in São Paulo State (Brazil) is constructed. Three parallel sections of frontal outcrops, 28 m × 5.8 m, and two parallel sections of lateral outcrops, 7 m × 5.8 m, are recorded during open-pit mining of sandy sediments and describe in detail the three-dimensional distribution of the local lithofacies and hydrofacies. Variations of hydraulic conductivity, K, and porosity, n, are resolved on the centimeter scale, and the most permeable units of the fluvial-aeolian facies association are identified. The constructed aquifer analog model shows moderate hydraulic heterogeneity and a mean K value of 1.36 × 10-4 m/s, which is greater than the reported range of K values for the entire GAS in São Paulo State. The results suggest that the examined sedimentary unit constitutes a relevant portion of the GAS in São Paulo State in the context of groundwater extraction and pollution. Moreover, the constructed aquifer analog is considered an ideal basis for future numerical model experiments, aiming at in-depth understanding of the groundwater flow and contaminant transport patterns at this GAS portion or at comparable fluvial-aeolian facies associations. © 2013 Springer-Verlag Berlin Heidelberg