Universität Tübingen
Observation-Based Conceptual Site Modeling Framework Combining Surface Geophysical, Direct Push-Based, Hydrogeochemical and Stable Isotope Methods
Abstract
Initial environmental and analytical data quality plays an important role in the cost-effective and successful cleanup of impacted soil and groundwater systems. In the absence of complete subsurface data, mathematical models are commonly developed to simplify complex dynamic interactions of natural processes. However, ecosystem functions are beset with high uncertainties that are too poorly understood to permit good modeling with a generalized numerical code. Due to sparse subsurface information and lack of flexibility, traditional methods of observation also lack sufficient uncertainty management characteristics. To capture significant contributors to data uncertainty and minimize faulty conclusions, an adaptive conceptual site model (CSM) approach of site investigation is therefore required to test hypotheses. The approach differs from the traditional methods in terms of the strategic work plan, targeted sampling, and the ability to minimize irrelevant field mobilizations. Under the framework of the CSM approach, carefully selected multimethods tend to iteratively leverage each other’s strengths. This dissertation examines the combined use of surface geophysical, direct push, and laboratory analytical tools in the evaluation of a subsurface CSM at the Wurmlingen study site (between Rottenburg am Neckar and Tübingen), southwest Germany. The general objective of this study was the assessment of the subsurface conditions in an attempt to quantify solute sources, fluxes, fate and transport at the nitrate plume scale. Geoelectrical resistivity measurements identified the presumable distribution of the subsurface structures and revealed a linearized anomalous low-resistivity feature aligned with the northwest – southeast trend of the investigated nitrate plume. Seismic refraction in conjunction with the direct push-based reverse vertical seismic profiling evaluated the subsurface structural integrity in relation to the bedrock structure. A channelized low P-wave velocity zone was detected and associated with the identified low-resistivity feature. Direct push-based soil electrical conductivity signatures and soil lithostratigraphy corroborated the channelized structure as an alluvium-bedrock interface capable of promoting groundwater exchange between the channelized deeper aquifer compartment and the surrounding shallower aquifer compartment. On the floodplain part of the study site, solute concentrations of groundwater samples collected using a direct push-based multilevel sampling device revealed steep geochemical gradients indicating higher fluxes of solutes associated with the channelized structure, which appears to represent a preferential flow path and a potential chemical hotspot within the nitrate plume. Stables isotope ratios also indicated that the source of the nitrate transported from the land surface through recharging water into the aquifer has been biogeochemically transformed and distinctly partitioned into soil organic nitrogen in the shallower compartment and animal (manure)/septic waste in the deeper flow compartment. It is observed from the combined evaluation of the groundwater chemical and dual stable isotope data that a diffusion-limited hydrologic transport mechanism created by the channelized aquifer system coupled oxic nitrate removal process in the shallower compartment depleted in the dual isotopes of nitrate to nitrate source admixture and remineralization processes in the deeper channelized compartment enriched in the dual isotopes of nitrate. Nevertheless, I hypothesize that explicit consideration of the role played by microbes in cycling the nitrogen nutrients transferred to them will lead to a clearer understanding of these alternative nitrogen-cycling pathways. A key finding emerging from this study is that there is a major shift in the nitrogen-cycling routes commonly associated with the canonical theories of oxic nitrification and anoxic denitrification. Another major finding is that the geohydrology of the area exerted dominant control on the complex nitrogen biogeochemical transformation pathways. In particular, the observation of anoxic nitrification in the deeper aquifer compartment appealed to field evidence for the resolution of the occurrence of what Granger and Wankel (2016) (see Supplementary Information - S1) reported as the “freshwater conundrum.” The complexities in the identified hydrobiogeochemical pathways suggest that the understanding of nitrogen-cycling processes in aquifer systems is far from complete and requires further research. This study highlights the necessity of realistically acknowledging the high degree of aquifer physical and biogeochemical heterogeneity, as well as varying solute source origins in the CSM evaluation procedure. Logical incorporation of nitrogen-processing information into the resolution of aquifer structural conditions not only allows for a better definition of mobility, persistence and transformation pathways of nitrate and other dissolved constituents but also alleviates concerns regarding the transparency of decision-making.
Author and committee
dc:creator, dc:contributor.*- Author
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- Utom, Ahamefula Udume
Identifiers
dc:identifier.*- Identifier
- hdl:10900/89073