THE POROUS FRACTURED CHALK OF THE NORTHERN NEGEV DESERT: LESSONS LEARNT FROM 10 YEARS OF STUDY

The northern Negev desert in Israel hosts the National Site for Hazardous Waste and has become a prime target for siting a variety of chemical industries. Despite the aridity of the area and the low permeability of the underlying Eocene chalk, groundwater contamination was found below the industrial sites and triggered decade-long investigations of the fractured chalk aquitard.

The chalk matrix contains biomicritic calcite, with up to 20% siliceous cements, zeolites and clays. Whereas the near-surface white chalk contains traces of organic carbon and has a very limited sorption capacity, the gray chalk dominates at depths > 20 m, contains ~1% of organic carbon (immature kerogen) and has a very high sorption capacity. The dominant pore-throat mean microns, accounting for the low matrix permeability (mean=0.2 mD) and implying that there is essentially no water flow or advective transport through the matrix. The chalk is highly porous (35%), resulting in extensive matrix diffusion (De=2 * 10-6 cm2/s), limiting potential remediation schemes.

The chalk is intensively fractured by both single-layered and large-extension, multilayer fractures, the dominant strike of which is NW-SE and NE-SW. The monitoring of the site was designed to capture the latter fracture systems by more than 90 boreholes. The distribution of the hydraulic conductivity values of the fractures is log normal with highest values limited to the upper 25 m and observed where two fracture systems intersected each other. Flow in single fractures is restricted to small segments along the fracture plane, generally associated with dissolution channels formed at fracture intersections. As little as 20% of the fracture void was found to account for > 80% of the fracture flow. Experiments of up to 5 days' duration did not result in steady-state flow, suggesting temporal variations in the effective fracture void. This instability is caused by particle shearing from the relatively soft fracture surfaces and the disintegration of fracture-filling materials. Under conditions of variable water content, the aperture, roughness and flow channels are transient properties. On-site experience suggests that cores, borehole tests and trenches provide by far more useful information for solute transport studies than fracture mapping in outcrops.