Diagnostic Plots Applied to Well-Tests in Karst Systems

Abstract

Pumping tests conducted on wells intersecting karst heterogeneities such as the conduit network are difficult to interpret. Nevertheless, this case can be solved by assimilating the horizontal karst conduit to a finite-conductivity vertical fracture. In this case, several flow patterns corresponding to the respective contributions of karst subsystems (fractured matrix, small conduits and main karst drainage network) can be identified on the diagnostic plot of drawdown derivative. This is illustrated on two examples from Mediterranean karst systems. A pumping test on a well crossing the main karst drainage network of the Cent-Fonts karst system shows (i) a preliminary contribution of the karst conduit storage capacity followed by (ii) linear flows into the fractured matrix. A pumping test on a well intersecting a small karst conduit of the Bas-Agly karst system shows the existence of (i) bi-linear flows within both the karst conduit an the fractured matrix at early times, followed by (ii) radial flows within the fractured matrix and (iii) finally the contribution of a major karst cavity.The use of diagnostic plots allows identifying the various flow regimes during pumping tests, corresponding to the response of the individual karst aquifer subsystems. This is helpful in order to understand the structure of the karst aquifer and flow exchanges between subsystems.

Improving hydrogeological understanding through well-test interpretation by diagnostic plot and modelling: a case study in an alluvial aquifer in France

“…It provides additional insight into the structure of the aquifer, which the basic Theis approach cannot achieve. Diagnostic plot analyses have been successfully applied in various heterogeneous aquifers such as karst (Maréchal et al 2008(Maréchal et al , 2014Jazayeri Noushabadi et al 2011;Spitzberg and Ufrecht 2013;Nassimi and Mohammadi 2017;Giese et al 2018), alluvial aquifers (Samani et al 2006;Mijinyawa and Gringarten 2008;Corbett et al 2012) or fractured aquifers (Illman and Neuman 2000;Beauheim et al 2004;Maréchal et al 2004;Hammond and Field 2014;Dewandel et al 2018). Usually presented in a log-log graph, the decomposition of the log-derivative signal into separate straight lines makes it possible to identify each hydraulic behaviour and eventually computes hydrodynamic properties (T, S) for each hydraulic behaviour.…”