Abstract
Contaminants that are highly soluble in groundwater are rapidly transported via fractures in mechanically resistant sedimentary rock aquifers. Hence, a rigorous methodology is needed to estimate groundwater flow velocities in such fractured aquifers. Here, we propose an approach using borehole hydraulic testing to compute flow velocities in an un-faulted area of a fractured carbonate aquifer by applying the cubic law to a parallel plate model. The Cadeby Formation (Yorkshire, NE England) – a Permian dolostone aquifer present beneath the University of Leeds Farm - is the fractured aquifer selected for this hydraulic experiment.The bedding plane fractures of this dolostone aquifer, which are sub-horizontal, sub-parallel and laterally persistent, largely dominate the flow at shallow (<~40 mBGL) depths. These flowing bedding plane discontinuities are separated by a rock matrix which is relatively impermeable (Kwell-test/Kcore-plug~104) as is common in fractured carbonate aquifers.In the workflow reported here, the number of flowing fractures - mainly bedding plane fractures - intersecting three open monitoring wells are found from temperature/fluid conductivity and acoustic/optical televiewer logging. Following well installation, average fracture hydraulic apertures for screened intervals are found from analysis of slug tests. For the case study aquifer, this workflow predicts hydraulic apertures ranging from 0.10 up to 0.54 mm. However, groundwater flow velocities range within two order of magnitude from 13 up to 242 m/day.Notably, fracture apertures and flow velocities rapidly reduce with increasing depth below the water table; the upper ~10 m shows relatively high values of hydraulic conductivity (0.30–2.85 m/day) and corresponding flow velocity (33–242 m/day). Permeability development around the water table in carbonate aquifer-types is common, and arises where high pCO2 recharge water from the soil zone causes calcite/dolomite dissolution. Hence, agricultural contaminants entering the aquifer with recharge water are laterally transported rapidly within this upper part.Computation of groundwater flow velocities allows determination of the Reynolds number. Values of up ~1, indicating the lower limit of the transition from laminar to turbulent flow, are found at the studied site, which is situated away from major fault traces. Hence, turbulent flow is likely to arise in proximity to tectonic structures, such as normal faults, which localize flow and enhance karstification. The occurrence of turbulent flow in correspondence of such tectonic structures should be represented in regional groundwater flow simulations.
Highlights
Application of the cubic law to a parallel plate model is considered reasonable in absence of karstic cavities for fractured aquifers with flow dominated by sub-parallel discontinuities which are separated by an impermeable matrix (Witherspoon et al, 1980; Berkowitz, 2002)
Contaminant transport occurs at relatively high velocity in the saturated parts of shallow fractured aquifers, and this may be very important in areas of arable and livestock farming activity
We report an investigation of the Critical Zone of a fractured dolostone aquifer (Permian dolostone of the Cadeby Formation) beneath the University of Leeds Farm, Yorkshire
Summary
A range of contaminants reach the saturated part of fractured aquifers in areas of the world which are devoted to agriculture and farming activity, including nitrate, sulphate, chloride and toxic organic compounds released by mineral fertilizers and pesticides, which are highly soluble in groundwater (Foster, 1976; Thorbjarnarson and Mackay, 1997; Danielopol et al, 2013; Meckenstock et al, 2004; Bloomfield et al, 2006; Rivett et al, 2007, 2008; Fouillac et al, 2007; Guy et al, 2009; Lerner and Harris 2009; Stein et al, 2010; Petitta et al, 2009; Stein et al, 2010; Zhang et al, 2013; Re et al, 2017). Application of the cubic law to a parallel plate model is considered reasonable in absence of karstic cavities for fractured aquifers with flow dominated by sub-parallel discontinuities which are separated by an impermeable matrix (Witherspoon et al, 1980; Berkowitz, 2002) This is the approach of our research for determination of groundwater flow velocities and flow regime (laminar or turbulent as indicated by the Reynolds number) within the fault blocks of a fractured aquifer. We selected a non-faulted block at specific field site, the University of Leeds Farm
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