Groundwater flow velocities in karst aquifers; importance of spatial observation scale and hydraulic testing for contaminant transport prediction

Abstract

We review scale dependence of hydraulic conductivities and effective porosities for prediction of contaminant transport in four UK karst aquifers. Approaches for obtaining hydraulic parameters include core plug, slug, pumping and pulse tests, calibration of groundwater flow models, and spring recession curves. Core-plug and slug tests are unsuitable because they do not characterise a large enough volume to include a representative fracture network.Pumping test values match regional-scale hydraulic conductivities from flow modelling for the less intensively karstified aquifers: Magnesian Limestone, Jurassic Limestone and Cretaceous Chalks. Reliable bulk hydraulic conductivities were not available for the intensively-karstified Carboniferous Limestone due to dominance of flow through pipe-conduits in the Mendips. Here, the only hydraulic conductivity value found from springs recession is one order of magnitude higher than that indicated by pumping tests. For all four carbonate aquifers, effective porosities assumed for transport modelling are two orders of magnitude higher than those found from tracer and hydrogeophysical tests. A combination of low hydraulic conductivities and assumed flowing porosities, has resulted in underestimated flow velocities. The UK karst aquifers are characterized by a range of hydraulic behaviours that fit those of karst aquifers worldwide. Indeed, underestimation of flow velocity due to inappropriate parameter selection is common to intensively karstified aquifers of southern France, north-western Germany and Italy. Similar issues arise for the Canadian Silurian carbonates where high effective porosities (e.g., 5%) used in transport models leads to underestimation of groundwater velocities. We recommend values in the range of 1% -0.01% for such aquifers.

Review of groundwater flow and contaminant transport modelling approaches for the Sherwood Sandstone aquifer, UK; insights from analogous successions worldwide

“…Where fracture flow dominates, it may also be the case that REV can be defined at such a scale, taking into account that the spacing of bedding plane fractures and joints is smaller than 10m, provided that the effect of large faults is taken into account separately, where these occur on the 100m -1km scale (Medici et al 206;Bense et al 2013). However, where karst-like conduits are present, no REV can be defined (Schulze-Makuch et al 1999;Hartman et al 2014;Medici and West 2021). In such cases, the hydraulic conductivity will continue to increase with scale, up to the characteristic length of the connected conduits that can be detected by tracer testing.…”

“…In such circumstances, representation using the EPM approach is likely to represent the only practicable solution, whatever the modelling objective. This option requires to estimate the equivalent bulk properties of the modelled layers, for example the EPM hydraulic conductivity, and where transport is modelled, the effective porosity (Worthington et al 2012(Worthington et al , 2019Medici and West 2021;). The EPM methodology has commonly been applied to trace particles to abstraction wells for the purpose of wellhead protection due to uncertainties related to constraining the fracture network even in heavily fractured sandstones (e.g., Freeze and Cherry 1979).…”

“…New tracer tests in the highly transmissive valleys in west Cumbria might reveal higher flow velocities than 1.6x10 -3 m/s found in Liverpool by Barker et al (1998). More tracer testing would also determine the effective porosity which is inadequately characterised in the Sherwood Sandstone Aquifer and is necessary to run contaminant transport models (Allen et al 1997;Tellam and Barker 2006;Medici and West 2021). Note that, values of effective porosity used to run contaminant transport models in the range 0.1 -0.35 and are not experimentally supported (Bottrell et al 2006;Rivett et al 2012;Zhang et al 2010Zhang et al , 2011Zhang et al , 2016.…”

Identify Priority Control Pollutants and Areas of Groundwater in an Old Metropolitan Industrial Area—A Case Study of Putuo, Shanghai, China

“…The heavy metal pollution of groundwater was a prominent environmental problem due to the fast spread of contaminants in the subsurface caused by industrial activities and land use management [9][10][11]. Many studies related to industrial area pollution have been undertaken in China.…”

Numerical Study on the Effects of Regional Groundwater Flow on the Contaminant Migration through an Inactive Supply Well as a Preferential Flow Pathway

“…This theoretical research can also be applied in there-dimensional (3D) groundwater flow model. Meanwhile specific attention is given to the effects of the regional groundwater velocity on contaminant migration [27][28][29]. For example, the model is developed in a sandy aquifer with regional groundwater flow.…”