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

Rasmus Bødker Madsen,Ingelise Møller,Troels Norvin Vilhelmsen,Hyojin Kim,Anders Juhl Kallesøe,Peter B E Sandersen,Birgitte Hansen,Anders Vest Christiansen,Thomas Mejer Hansen

doi:10.5194/hess-25-2759-2021

Rasmus Bødker Madsen, Ingelise Møller + Show 7 more

Open Access

https://doi.org/10.5194/hess-25-2759-2021

Copy DOI

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.

Highlights

The loss of reactive nitrogen (N) from agricultural soils results in adverse environmental and human health impacts (Schullehner et al, 2018; Temkin et al, 2019), including eutrophication of freshwater and marine ecosystems and nitrate contamination of groundwater and drinking water (Schullehner and Hansen, 2014)
We propose a novel way to combine the available information on hydrogeology and redox conditions by estimating a quantified uncertainty at unsampled locations in modeling using geostatistical simulation
multiple-point geostatistical (MPS) modeling with a bivariate training image (TI) of geology and redox presents some important features compared to previous mapping studies: (1) MPS effectively produces geology and redox following expectations and (2) TIs provide an intuitive and easy collaboration across different fields of expertise

Summary

Introduction

The loss of reactive nitrogen (N) from agricultural soils results in adverse environmental and human health impacts (Schullehner et al, 2018; Temkin et al, 2019), including eutrophication of freshwater and marine ecosystems and nitrate contamination of groundwater and drinking water (Schullehner and Hansen, 2014). Further actions are still required to improve the state of the aquatic ecosystems to meet the requirements of, e.g., the EU Water Framework Directive (European Commission, 2018; Hansen et al, 2019; Kallis and Butler, 2001). This must be achieved in a cost-effective manner for society and the agricultural industry. This creates a demand for new knowledge and new solutions for more efficient future N regulation of the agricultural sector both in Denmark and in other countries with intensive agriculture. The proposed direction is to introduce more targeted N regulation depending on the site-specific conditions at field level

Objectives

Methods

Findings

Discussion

Conclusion