Dealing with subsurface contamination and risks: technical solutions and practical applications, from shallow to deep geological environments


Dealing with subsurface contamination and risks: technical solutions and practical applications, from shallow to deep geological environments
Convener: Felipe de Barros | Co-Conveners: Wouter Buytaert, Christine Stumpp, Elena Volpi
| Thu, 02 Jun, 08:30–10:00|Room Rondelet 2
| Attendance Thu, 02 Jun, 15:00–16:30|Poster area

Orals: Thu, 2 Jun | Room Rondelet 2

Chairperson: Felipe de Barros
Kouassi Aristide Aoulou, Severin Pistre, Yéï Marie Solange Oga, Benoît Dewandel, and Patrick Lachassagne

The Montagnes District is a metamorphic rocks area from Western Côte d’Ivoire covering an area of about 31,000 km2. Its capital is the city of Man. It is home to the largest industrial gold mine in Côte d'Ivoire, at Ity, with an annual production of 6 to 7 tons. Artisanal mining activities are also carried out in several places in the region. These activities can threaten groundwater, both in terms of quantity and quality. Therefore, a good understanding of the structure and functioning of aquifers in these mining areas is needed to prevent these impacts and to develop mining activities that are as environmentally friendly as possible.

This problem raises more generally the question of the validity of a hydrodynamic model explaining the origin of the permeability of hard rock aquifers in order to optimize groundwater resource management methods and better understand their vulnerability. To address this question, this work was able to rely on a statistical analysis of a database of 1654 boreholes.

Results shows that the structure of the aquifer is similar to that observed in several other hard rock areas in the world: it developed due to weathering processes, comprises the capacitive saprolite, 10–20 m thick on average, and an underlying transmissive fractured layer, overlying the unweathered impermeable hard rock. The fractured layer is about 80 m thick, the first 40 to 45 metres being its most productive zone, with a 11 m3/h median productivity.

The characterization of groundwater flow has led to the proposal of a new approach in the context of scarce piezometric data.

Finally, this research shows that the impacts of mining activities are local, limited mainly to the downstream part of the topographic watersheds where the mines are sited. There, groundwater quality and quantity may be affected, as well as the streams that drain these aquifers. In fact, surface water drains groundwater and therefore collects possible contaminants, downstream of the mines. A new approach to assessing the vulnerability of hard rock aquifers to contamination from mining has therefore been proposed.

How to cite: Aoulou, K. A., Pistre, S., Oga, Y. M. S., Dewandel, B., and Lachassagne, P.: Application of a new hydrogeological conceptual model of basement aquifers: Structuring and distribution of hydrodynamic properties. Critique and revision of piezometric mapping methods in a sub-basin of the Cavally River in western Côte d'Ivoire, IAHS-AISH Scientific Assembly 2022, Montpellier, France, 29 May–3 Jun 2022, IAHS2022-46, https://doi.org/10.5194/iahs2022-46, 2022.

Dimitri Rambourg and Philippe Ackerer

A reliable description of aquifer heterogeneities is crucial for solute transport modelling. Inverse methods, a field of research that has been developing greatly in recent decades, can provide the horizontal structure of heterogeneities, with the collection of piezometric data as a cornerstone. But the latter are very little sensitive to the vertical structure of the aquifer, leaving its estimation dependent on complex and expensive field methods (electrical resistivity and radar tomography, self-potential methods, cross-hole testing, hydraulic tomography) or lab analysis (grain-size analysis from core sample, secondary permeability tests).

In order to take advantage of the possibilities offered by the inversion techniques, while sidestepping the inconvenience of geophysical and field approach, a method is proposed using 2D inversion of flow (solely reliant on piezometric series) as parameterization constraints for a 3D hydrogeological model (see Figure 1).

The methodology is tested via a synthetic example, ensuring full knowledge and control of the aquifer's structure. It is composed of 5 lithofacies, distributed according to a sedimentary pattern, the level of heterogeneity for hydraulic conductivity spans 3 orders of magnitude and groundwater is unconfined. This synthetic example provides both the piezometric chronicles used to inverse 2D flow parameter fields and the lithological logs used to interpolate the 3D lithological model. Finally, the parameters of each facies are obtained through an optimization loop, minimizing the difference between the 2D and the 3D transmissivity (and specific yield).

The method results in the estimation of parameters very close to the known parameters, even with a scarce piezometric and lithological data sampling. The maximal discrepancy is 61% of the initial value for the permeability and 16% for the specific yield (mean error being respectively 18 and 4%). Although the methodology does not prevent interpolation error, it succeeds in reconstructing flow and transport dynamics very close to the synthetic control data. Due to the inherent limitations of the 2D inversion approach, the method only applies to the saturated zone at this point.

How to cite: Rambourg, D. and Ackerer, P.: 3D hydrogeological parametrization using scarce piezometric data, IAHS-AISH Scientific Assembly 2022, Montpellier, France, 29 May–3 Jun 2022, IAHS2022-112, https://doi.org/10.5194/iahs2022-112, 2022.

Jean-Pierre Faillat

The hydrodynamics and hydrochemistry of magmatic and metamorphic rocks, fissured, altered and with low relief, such as the Armorican massif (France), and this only for such conditions, leads to a redox organization of groundwater, which results in beyond 30-50 m deep, depending on the organization of the hydrogeosystems, there is a quasi-generalized reducing level of nitrates. This is due to the organization of underground hydrodynamic flows, the transport conditions and the redox stability range of nitrates.

Also, the modification of the distribution of nitrogen inputs in the areas to be protected as a priority (perimeters of protection of catchments along rivers, coastal watersheds, etc.) or more generally, provides a solution to the nitrate pollution of water. This consists in the interdiction or the strict control of nitrogen inputs in the parts of the catchment basins close to the rivers, where the deep denitrification of the nitrated groundwater cannot be done, because the hydraulic current lines reaching to rivers remain in the superficial oxidizing zone. The manuring of inputs would then only be admissible far enough from rivers so that the passage of streamlines through the deep denitrifying zone is inevitable (Fig. 1).

An experimentation or the practice should make it possible to best approach the delimitation of these preferential manuring zones. Numerical simulations show that significant improvements are possible in less than 5 years, the polluting nitrate stocks being mainly located in the oxidizing zone close to rivers, the best renewed part of the water tables (Fig. 2).

Finally, the constraints generated by this approach and the management measures intended to facilitate the deep infiltration of water and to prevent surface runoff as much as possible, such as plowing perpendicular to the slopes of the hillsides, the removal of drains in sewage fields, etc., would be minimal and have no significant impact on agricultural production. This should facilitate their application, especially since they would be accompanied by other types of measures relating to the modification and the amelioration of agricultural practices.