Estimating the hydraulic properties of the fracture network in a sandstone aquifer

The potential hydraulic behaviour of the fracture network in a major Triassic sandstone aquifer in the UK has been evaluated. The properties of the fracture network were determined using results from detailed scan line surveys at 10 sites, television and geophysical borehole logging, and packer testing. Six sets of discontinuities common to all sites were identified and statistically characterised (dip, strike, orientation, density, size, and estimated transmissivity). A discrete fracture network model was then used stochastically to investigate the properties of the network. In general, the network is poorly connected: it is estimated that 9% of the discontinuities intersecting boreholes are transmissive. The hydraulic behaviour of the network is generally dominated by one sub-horizontal bedding plane fracture set, although when present, a relatively infrequent north-south striking, sub-vertical set modifies the bulk flow properties significantly. Ignoring this latter set, the network's minimum representative volume is about 35 x 35 x 35 m. The upscaled permeability is anisotropic, being typically 23 times greater in the horizontal than in the vertical. Tortuosity in the north-south direction is around 1.6.     

Preliminary indications from atomic force microscopy of the presence of rapidly-formed nanoscale films on aquifer material surfaces

The objective of this study was to determine if there is a nanoscale surface film on aquifer-like materials exposed to deep groundwaters, as has previously been found on surfaces exposed to surface and soil waters. Such surface films will modify surface properties that are so important in determining the mobility of many groundwater pollutants. Muscovite mica was used because a) it is a good analogue for the main sorbing phases of many clastic aquifers and b) its cleavage planes are atomically flat allowing high resolution imaging. Freshly-cleaved muscovite plates were exposed to groundwater from a sandstone aquifer for 30 min, and surface properties (morphology, coverage, roughness and tip–substrate force interactions) were measured using atomic force microscopy (AFM). A patchy surface film of several nanometres in depth, incorporating larger separate particles, was found on the mica surface. This film was associated with significantly increased roughness values and AFM probe–sample interaction forces compared with pure water and inorganic (synthetic groundwater) solution controls. Although the results reported are preliminary in nature, if confirmed, such films are likely to affect sorption reactions, surface-facilitated redox interactions, non-aqueous phase liquid wetting angles, and colloid–pathogen–rock attachment, and will thus be of importance in understanding natural attenuation and migration of dissolved, non-aqueous and particulate phases in groundwaters.     

Sandstones of unexpectedly high diffusibility

Measurements have been made of diffusion coefficients (D(i)=-mass flux/concentration gradient) using a double reservoir, steady-state method with two tracers, CaBr(2) and amino-G-acid, on intact samples of Triassic red-bed sandstone from northwest England. Diffusibility (D'=D(i)/diffusion coefficient in water) averages 0.124, ranging between 0.075 and 0.215 (porosity 0.1 to 0.24), very similar for the two tracers. Implied tortuosities (actual path length/straight line length) average 1.21 (range 1.06 to 1.47), with constrictivities close to 1. In comparison with limited red-bed sandstone data from elsewhere, these D' values are up to 4 times greater, and tortuosity correspondingly lower. Re-interpretation of formation factor data from previous studies on shallow sandstone samples also from northwest England confirms that diffusibility is significantly higher in these sandstones than others from similar palaeoenvironment/stratigraphic units. The lower tortuosities appear to result from the relatively high permeability, open fabric of the rock, properties likely to be present in shallow sandstone systems used for water supply. It is concluded that diffusion rates may, in some shallow freshwater-containing continental sandstone systems, be significantly greater than is implied by estimates of sandstone diffusibility current in the literature.     

Attenuation of landfill leachate by UK Triassic sandstone aquifer materials: 2. Sorption and degradation of organic pollutants in laboratory columns

The sorption and degradation of dissolved organic matter (DOM) and 13 organic micropollutants (BTEX, aromatic hydrocarbons, chloro-aromatic and -aliphatic compounds, and pesticides) in acetogenic and methanogenic landfill leachate was studied in laboratory columns containing Triassic sandstone aquifer materials from the English Midlands. Solute sorption and degradation relationships were evaluated using a simple transport model. Relative to predictions, micropollutant sorption was decreased up to eightfold in acetogenic leachate, but increased up to sixfold in methanogenic leachate. This behaviour reflects a combination of interactions between the micropollutants, leachate DOM and aquifer mineral fraction. Sorption of DOM was not significant. Degradation of organic fractions occurred under Mn-reducing and SO₄-reducing conditions. Degradation of some micropollutants occurred exclusively under Mn-reducing conditions. DOM and benzene were not significantly degraded under the conditions and time span (up to 280 days) of the experiments. Most micropollutants were degraded immediately or after a lag phase (32–115 days). Micropollutant degradation rates varied considerably (half-lives of 8 to >2000 days) for the same compounds (e.g., TeCE) in different experiments, and for compounds (e.g., naphthalene, DCB and TeCA) within the same experiment. Degradation of many micropollutants was both simultaneous and sequential, and inhibited by the utilisation of different substrates. This mechanism, in combination with lag phases, controls micropollutant degradation potential in these systems more than the degradation rate. These aquifer materials have a potentially large capacity for in situ bioremediation of organic pollutants in landfill leachate and significant degradation may occur in the Mn-reducing zones of leachate plumes. However, degradation of organic pollutants in acetogenic leachate may be limited in aquifers with low pH buffering capacity and reducible Mn oxides. Contaminants in this leachate present a greater risk to groundwater resources in these aquifers than methanogenic leachate.