Hydrogeochemical and hydrodynamic features affecting redox processes in groundwater

This research deals with some hydrogeochemical and hydrodynamic features that affect redox processes in groundwater, presenting three different case studies and methodological approaches. This information is vital when characterizing contaminated sites, planning monitoring, selecting remedial techniques, and assessing health risks. In the San Pedro Sula site case study (Honduras), new insights and an advanced method for the aquifer redox zonation were provided. The Multi-Collocated Factorial Kriging, a multivariate geostatistical technique borrowed from agriculture and soil science, identified simultaneous redox processes, acting at different scales and mainly due to eutrophicated surface-water/groundwater interaction: at short-range, Mn oxihydroxide reductive dissolution and Fe oxi-hydroxide colloidal phase precipitation; at long-range, Fe mobilization. The obtained results are supported by the Principal Component Analysis and hydrogeochemical numerical modeling. The interaction among different contamination events in the VR site allowed to get a deeper insight into the competition for organic substrate among different redox processes. To this purpose, a 1-D Advective-Reactive- Dispersive transport numerical model was implemented, and its results highlighted that the reductive dechlorination is inhibited by the inorganic Terminal Electron Accepting Processes, enriching groundwater in highly toxic by-products of the reductive dechlorination (i.e. Vinyl Chloride). As a matter of fact, the bacteria using inorganic Terminal Electron Acceptors to oxidize the organic substrate are more efficient than the halo-respiring bacteria, that favor reductive dechlorination. The contaminated site of Bussi Officine case study allowed to clarify the hydrodynamic effect on the redox processes and the anoxic condition distribution. The comparison between dissolved chlorinated solvents and the inorganic Terminal Electron Acceptors distribution in the aquifer pointed out that dispersion inhibits all the redox processes because it dilutes the organic substrate concentration in groundwater. On the other hand, the lacustrine deposits rich organic matter (i.e. peat) can be considered as “chemical reactors”, that releases by back-diffusion toxic by-product of the reductive dichlorination in groundwater.

Identification of shallow geohazard channels based on discontinuity seismic attributes in the South Caspian Sea

Identification of geomorphological features in seismic data is a key element of seismic interpretation. Channels in the shallow subsurface are potential geohazards. At deeper levels, they can be the actual targets for (horizontal) drilling. Either way, it is important to optimally delineate these features prior to well location positioning and drilling. We have studied a poststack 3D seismic data from the South Caspian Sea featuring shallow channels that are considered potential geohazards for drilling operations. In the first step, we attenuate the acquisition footprints along the inline direction using a geostatistics approach based on factorial kriging. To better visualize channels in the presence of stratigraphic dips, we create a dense set of horizons using an inversion-based flattening algorithm. In the next step, we compare various discontinuity attributes such as semblance, similarity, curvature, and the relatively new attribute based on the multiscale and multidirectional shearlet transformation to determine which one best images our features of interest. Curvature attributes clearly image channel levies (positive curvature) and channel centers (negative curvature). Lateral changes in the curvature magnitude infer sedimentation from the north. Similarity, semblance, and shearlet transform attributes also successfully delineate channel edges, but these attributes do not contain additional geologic information. In the final step, we qualitatively analyze channel thickness variations by the red-green-blue blending of three spectral components based on short window Fourier transforms.

Quantifying the scales of spatial variation in gravel beds using terrestrial and airborne laser scanning data

Previous studies measured gravel bed surfaces by terrestrial laser scanning (TLS) and close-range photogrammetry suggested the presence of at least two different scales of spatial variation in gravel bed surfaces. This study investigated the spatial variation of airborne laser scanning (ALS) point clouds acquired in gravel bed. Due to the large footprint of ALS systems, a smoother surface is expected, but there exists some uncertainty over the precise scale of ALS measurement (hereafter referred to as the spatial support). As a result, we applied the regularization method, which is a variogram upscaling approach, to investigate the true support of ALS data. The regularization results suggested that the gravel bed surface described by the ALS is much smoother than expected in terms of the ALS reported measurement scale. Moreover, we applied the factorial kriging (FK) method, which allows mapping of different scales of variation present in the data separately (different from ordinary kriging which produces a single map), to obtain the river bed topography at each scale of spatial variation. We found that the short-range and long-range FK maps of the TLS-derived DSMs were able to highlight the edges of gravels and clusters of gravels, respectively. The long-range FK maps of the ALS data shows a pattern of gravel-bed clusters and aggregations of gravels. However, the short-range FK maps of the ALS data produced noisy maps, due to the smoothing effect. This analysis, thus, shows clearly that ALS data may be insufficient for geomorphological and hydraulic engineering applications that require the resolution of individual gravels.