Characterization of reciprocity gaps from interference tests in fractured media through a dual porosity model

Abstract

14We analyze drawdown reciprocity gaps emerging in interference tests performed 15 in a confined fissured karstic formation. Modeling the system as a dual porosity 16 continuum allows characterizing the dynamics of the relative contribution of the 17 connected fractures and the rock matrix to the total flow rate extracted at the pumping 18 wells. Observed lack of reciprocity of drawdowns can then be linked to the occurrence 19 of processes that are not accounted for in the classical flow models based on a single-20 continuum representation of the system through flow equations grounded on Darcy's 21 law only. We show that interpreting the system as a dual porosity continuum can cause 22 drawdown reciprocity gaps to emerge as a consequence of local effects associated with 23 an identifiable contribution of the matrix to the total fluid extracted at the well location 24 during pumping. These theoretical results are then employed to identify the contribution 25 to the flow being supplied to the pumping well by the low conductivity matrix 26 constituting the host rock formation, in contrast to that provided by the fractures. An 27 application to data from two interference tests performed at the Hydrogeological 28Experimental Site (HES) in Poitiers, France, illustrates the approach. We show that, 29 whenever the matrix is assumed to provide a contribution to the total flow rate 30 extracted, non-reciprocity is expected, the latter being linked to the occurrence of a 31 differential drawdown between fracture and matrix at the pumping well. This difference 32 decreases with time in the example presented, displaying a power-law late time 33 behavior, with non-reciprocity effects persisting up to remarkably long times.

Identification of groundwater flow parameters using reciprocal data from hydraulic interference tests

“…Even as our main results show that reciprocal data collected from hydraulic investigations performed between pairs of wells do not provide significant added value to inverse modeling compared with using additional data stemming from pumping from diverse locations, designing and performing hydraulic tomography campaigns has an intrinsically high value. Several aquifer characteristics and flow mechanisms may cause deviations of observed drawdowns from reciprocity (see, e.g., Delay et al, 2011Delay et al, , 2012Sanchez-Vila et al, 2016). As such, bi-directional tomographic experimental investigations are valuable in revealing emergences of reciprocity gaps in the monitored data.…”

“…In practice, redundancy of information (in the strict sense we define) only arises when considering model outputs, observed data being very often corrupted by measurement errors or displaying clearly identifiable reciprocity gaps. Analyses of observations displaying reciprocity gaps is performed by Delay et al (2012) with reference to vertically-averaged heads in unconfined aquifers and by Sanchez-Vila et al (2016) on the basis of a dual-continuum conceptualization of drawdowns observed in a confined fractured aquifer. Here, we are chiefly interested in the investigation of the relevance of introducing reciprocal information (eventually in the presence of noisy data, as corrupted by measurement errors) in an inverse modeling framework taking (1) as the process model and exploring the consequences of this choice on estimates of system parameters (i.e., S and T) and their uncertainty.…”

MFIT 1.0.0: Multi-Flow Inversion of Tracer breakthrough curves in fractured and karst aquifers

“…somewhere between the injection site and the monitoring point. Although injection artifacts may be involved in some cases (see Guvanasen and Guvanasen, 1987), tracer splitting most commonly originates from the spreading (transverse dispersion) of the solute into areas of contrasting flow velocities; see Moreno and Tsang (1991), Siirila-Woodburn et al (2015), and Boon et al (2017. More precisely, assuming a single-pulse tracer injection signal, multimodal BTCs reflect a three-step process: (1) tracer spreading into different flowing or nonflowing aquifer subdomains characterized by different transit or residence times, (2) tracer motion within each subdomain with little or no exchange between the different subdomains, and (3) convergence (mixing) of the subtracer fluxes somewhere upstream from, or at, the monitoring point.…”

“…Although spatially distributed numerical models (e.g., MOD-FLOW/MT3DMS or FEFLOW) can be used for this purpose, simpler (i.e., spatially lumped) models are generally used, at least in the early stages of tracer studies, either because of time constraints or because of a lack of model input data. A number of computer codes for BTC fitting have been developed in recent decades: CATTI (Sauty et al, 1992), TRACI (Käss, 1998(Käss, , 2004, OTIS (Runkel, 1998), STANMOD (van Genuchten et al, 2012), TRAC (Gutierrez et al, 2013), OM-MADE (Tinet et al, 2019), and OptSFDM (Gharasoo et al, 2019). Note that STANMOD integrates a number of former codes, including the widely used CXTFIT code developed by Parker and van Genuchten (1984) and Toride et al (1999).…”

MFIT 1.0.0: Multiflow inversion of tracer breakthrough curves in fractured and karst aquifers

“…CC BY 4.0 License (Delay et al, 2007;Riva et al, 2009;Delay et al, 2011;. Bodin et al, 2012;Sanchez-Vila et al, 2016;Le Coz et al, 2017), thedrawdown responses exhibit complex behaviors, which are likely due to the strong aquifer heterogeneity induced predominantly by the presence of karst features. In addition to the pumping test experiments, a number of cross-borehole tracer tests have been performed at the HES since 2011.…”