3D multiple-point geostatistical simulation of joint subsurface redox and geological architectures

Abstract

Abstract. Nitrate contamination of subsurface aquifers is an ongoing environmental challenge due to nitrogen (N) leaching from intensive N fertilization and management on agricultural fields. The distribution and fate of nitrate in aquifers are primarily governed by geological, hydrological and geochemical conditions of the subsurface. Therefore, we propose a novel approach to modeling both geology and redox architectures simultaneously in high-resolution 3D (25m×25m×2m) using multiple-point geostatistical (MPS) simulation. Data consist of (1) mainly resistivities of the subsurface mapped with towed transient electromagnetic measurements (tTEM), (2) lithologies from borehole observations, (3) redox conditions from colors reported in borehole observations, and (4) chemistry analyses from water samples. Based on the collected data and supplementary surface geology maps and digital elevation models, the simulation domain was subdivided into geological elements with similar geological traits and depositional histories. The conceptual understandings of the geological and redox architectures of the study system were introduced to the simulation as training images for each geological element. On the basis of these training images and conditioning data, independent realizations were jointly simulated of geology and redox inside each geological element and stitched together into a larger model. The joint simulation of geological and redox architectures, which is one of the strengths of MPS compared to other geostatistical methods, ensures that the two architectures in general show coherent patterns. Despite the inherent subjectivity of interpretations of the training images and geological element boundaries, they enable an easy and intuitive incorporation of qualitative knowledge of geology and geochemistry in quantitative simulations of the subsurface architectures. Altogether, we conclude that our approach effectively simulates the consistent geological and redox architectures of the subsurface that can be used for hydrological modeling with nitrogen (N) transport, which may lead to a better understanding of N fate in the subsurface and to future more targeted regulation of agriculture.

Upscaling of Denitrification Rates from Point to Catchment Scales for Modeling of Nitrate Transport and Retention

“…The geological structure of the study catchment is characterized by three geological elements defined as (1) quaternary sequence; (2) buried valley; and (3) pre-quaternary Paleogene clay. 22 A geological element is a lithological volume that displays similar geological and depositional characteristics. 22 The quaternary sequence covers the southern part of the catchment and consists of primarily glacial till deposits from the latest glaciation.…”

“…22 A geological element is a lithological volume that displays similar geological and depositional characteristics. 22 The quaternary sequence covers the southern part of the catchment and consists of primarily glacial till deposits from the latest glaciation. About a 1 km-wide buried valley was mapped in the lowland area in the northern part of the catchment near the catchment outlet 22,45 (Figure 1d).…”

“…Pre-quaternary Paleogene clay is impermeable thick clay, demarcating the no-flow boundary. 22 Therefore, this geological element was excluded in this study.…”

“…46 The sandy till acts as a "geological window" (Figure 1d), which allows infiltration of oxidants into the deeper subsurface below the overlying reduced clay till layer. 22 In the buried valley, no secondary oxic layer was observed, resulting in the planar type (i.e., a simple, 1-D transition from oxic to reduced conditions). 22 The water of the pre-quaternary Paleogene clay is always reduced because the clay is effectively impermeable.…”

“…This study was carried out in a catchment underlain by glacial deposits where the subsurface hydrogeological and redox structures are well characterized. 22 The main objectives of the study were (1) identification of the primary controls of the spatial distribution of denitrification zones by synthesizing geophysical, hydrological, and biogeochemical information; (2) parameterization of denitrification rates through field and laboratory measurements and literature reviews; and (3) development of a 3D TI to describe the structure of the denitrification zones with representative denitrification rates.…”