Hydraulic Conductivity Estimation Using Low-Flow Purging Data Elaboration in Contaminated Sites

Abstract

Hydrogeological characterization is required when investigating contaminated sites, and hydraulic conductivity is an important parameter that needs to be estimated. Before groundwater sampling, well water level values are measured during low-flow purging to check the correct driving of the activity. However, these data are generally considered only as an indicator of an adequate well purging. In this paper, water levels and purging flow rates were considered to estimate hydraulic conductivity values in an alluvial aquifer, and the obtained results were compared with traditional hydraulic conductivity test results carried on in the same area. To test the applicability of this method, data coming from 59 wells located in the alluvial aquifer of Malagrotta waste disposal site, a large area of 160 ha near Rome, were analyzed and processed. Hydraulic conductivity values were estimated by applying the Dupuit’s hypothesis for steady-state radial flow in an unconfined aquifer, as these are the hydraulic conditions in pumping wells for remediation purposes. This study aims to show that low-flow purging procedures in monitoring wells—carried out before sampling for groundwater characterization—represent an easy and inexpensive method for soil hydraulic conductivity estimation with good feasibility, if correctly carried on.

Combined Well Multi-Parameter Logs and Low-Flow Purging Data for Soil Permeability Assessment and Related Effects on Groundwater Sampling

“…The depth measurement frequency was every one meter, waiting at each step for parameter stabilization (Figure 3). The stabilized water level measured in each well during the low-flow purging was used as an input data, as well as the well radius and depth, for assessing aquifer horizontal hydraulic conductivity K H [15].…”

“…The misleading common belief is that the low-flow purging is only related to the pumped value, whereas one has to consider the induced groundwater flow to the well, which depends on the aquifer properties too [12][13][14]. In this sense, the only measured parameter that can give information about the right conditions is the drawdown ∆H, or even better the ratio ∆H/H (where H is the thickness of the aquifer), which allows to scale the drawdown taking into account the aquifer geometry also [15,16].…”

“…Recent studies demonstrated that this technique might be useful for estimating the horizontal hydraulic conductivity (K H ) of the aquifer and for strengthening the preliminary site assessment, without further and expensive investigations [15,29,30].…”

Quali-quantitative considerations on low-flow well purging and sampling

“…The abundance of hydraulic data (drawdowns), which can be collected during the procedure, can be used to determine aquifer properties, such as horizontal hydraulic conductivity (K H ), usually assessed with slug tests (Binkhorst and Robbins 1994). K H can be easily assessed during low-flow conditions too, using water level data collected after the stabilization and following simple procedures of data processing, valid both for fully penetrating (De Filippi et al 2020;Robbins and Higgins 2018;Robbins et al 2009) and partial penetrating wells (Aragon-Jose and Robbins 2011). The evaluation of K H through these methods is not always easily possible, especially in low-permeable layers, where drawdowns may be too high to fully respect Dupuit's assumptions for radial flow and to neglect well losses and turbulence, even in low-flow conditions.…”

“…In Figure 1, three simplified sketches of real cases studied during low-flow purging are reported (De Filippi et al 2020). The screen length covers the entire saturated zone and the only simplification is layer homogeneity.…”