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.
Cost-efficient targeted nitrogen (N) regulation of agriculture with low impact on the environment is the new N regulation paradigm. It requires detailed knowledge on the geological and geochemical conditions of the subsurface that is crucial for assessing the nitrate flowpaths and reduction processes. An integral part of this is analysis of the subsurface redox structures to determine the locations of nitrate reduction. This knowledge has so far not been easy to access because of lacking technology. Here we present a new concept consisting of integration and interpretation of data from the geophysical towed transient electromagnetic method, borehole information on lithology, sediment colour descriptions, geochemistry and groundwater chemistry. The concept is demonstrated in three small first-order hydrological catchments. National GIS screening analyses show that the new concept is highly needed in large parts of Denmark where the redox structures are complicated e.g. in marine landscape types and in glacial moraine landscapes but less needed in areas dominated by homogeneous meltwater plains. Providing subsurface knowledge for locally targeted N regulation of agriculture is paramount in many developed countries with intensive agriculture to lower the environmental impact, and it could also be critical in developing countries to support sustainable economic and environmental development.
Complex geology in glacial terrain makes groundwater exploration challenging for water supply companies. Abrupt lateral changes of geological conditions, for example, raise the risk of unsuccessful exploration drilling. At some locations, local water supply companies experience a large number of dry wells, raising the need for better exploration approaches. The use of geophysical techniques enables the detection and characterization of subsurface structures in terms of aquifers and aquitards.We present two examples in which seismic and resistivity methods are combined for subsurface characterization. A thrust structure is detected in the Varde study area in Denmark and a channel structure is mapped in the Hamburg-Sülldorf study area in Germany. The different imaging characteristics of seismic P‑ and S‑waves are demonstrated. The combination of seismic and resistivity methods enables delineating groundwater-bearing and groundwater-barrier layers down to a depth of 150 m.
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