A numerical investigation to illustrate the consequences of hydraulic connections between granular and fractured-rock aquifers

Abstract

Natural or artificial hydraulic connections between a granular aquifer in contact with a fractured-rock aquifer can have significant physical and chemical impacts at both a local and a regional scale. In this study, numerical simulations are conducted in order to illustrate the hydrogeological consequences of such hydraulic relationships. The numerical investigation, based on a conceptual model, focuses on the effects of the hydraulic connections when conducting a pumping test in a well that is set into a granular confined aquifer overlying a fractured-rock aquifer which presents a few fractures directly connected to the granular aquifer. It is illustrated that when interpreting the pumping test with the conventional methods consisting of plotting the drawdown versus time, a bias is introduced on the estimation of the transmissivity of the granular aquifer due to groundwater flowing from the fractured-rock aquifer via connecting fractures. However, it is underlined that plotting drawdown log-derivative versus time helps to diagnose the existence of these hydraulic relationships and therefore avoids committing a bias on the transmissivity estimation of the granular aquifer. Numerical simulation results also illustrate that hydraulic connections between the two aquifers can have significant impacts on the hydrogeochemical signature of the granular aquifer under investigation.

Scenarios of groundwater chemical evolution in a region of the Canadian Shield based on multivariate statistical analysis

“…These samples could correspond to mixed or intermediate water types from granular and bedrock aquifers (Frape et al, 1984;Rajmohan and Elango, 2004;Subramani et al, 2010). This last observation highlights the possibility of hydraulic connections between the Canadian Shield bedrock and the granular deposits as recently demonstrated by Chesnaux et al (2012) and Richard et al (2014). The Cluster 4 samples would therefore correspond to more saline and probably more mature water from bedrock aquifers (Frape et al, 1984;Gascoyne and Kamineni, 1994).…”

“…The Cluster 2 and Cluster 3 intermediate or mixed groundwater samples are less strongly influenced by the two first factors than the other cluster samples. The Cluster 2 and Cluster 3 (Ca-Na)-HCO 3 groundwaters could correspond to mixed or intermediate groundwater between Cluster 1 and Cluster 4 from possibly interconnected granular and bedrock aquifers (Chesnaux et al, 2012;Richard et al, 2014). The Cluster 3 samples are more influenced by Factor 3 (Si 4+ ).…”

Consequences and mitigation of saltwater intrusion induced by short-circuiting during aquifer storage and recovery in a coastal subsurface

“…Santi et al (2006) evaluated tools to investigate cross-contamination of aquifers. Chesnaux et al (2012) used numerical simulations of theoretical cases to demonstrate the consequences of pumping tests and hydrochemistry of hydraulic connections between granular and fractured-rock aquifers, which clearly demonstrated the significant hydrochemical cross-contamination when short-circuiting aquifers have a distinct chemical composition. The impact of shortcircuiting on ASR has not been evaluated to date.…”

Identification and characterization of regional water flows contributing to the recharge of an unconfined aquifer

“…An analysis of the effect of regional flows in particular has hardly been studied or reported [3,4]. The predominance of one or another source of water in the system depends on the geological, geomorphological, edaphological and climatological properties of the system.…”

“…Within a hydrogeological system, the main source of direct recharge to a free aquifer is generally precipitation surplus [1,2]; however, in aquifers with significant thickness, lateral contributions from the host rock or from regional flows are able to add a significant amount to the volumes of water that are stored and circulate through porous media and fractured rocks.They can also influence the water's chemical properties.…”