M. PRICE writes: Younger & Elliot (1995) used a radon diffusion model to derive apertures and field measurements to determine spacings for fissures (discontinuities through which flow takes place) in the Chalk. Their paper confirmed much existing knowledge, but when they attempted to use their data to calculate hydraulic conductivity values for chalk, they made a fundamental error that invalidated the main conclusions.

Although there are various assumptions, the average estimates of 0.45mm for the apertures and 9.4/m for the frequencies of fissures are in line with expectations. Applying these results to linear laminar flow in three orthogonal sets of smooth, plane, parallel-plate fissures gives a theoretical fissure porosity of 1.35 per cent and, for pure water at around 10°C, a hydraulic conductivity of 98 m/day. The porosity value is similar to the specific yield of much of the Chalk, but this hydraulic conductivity is much higher than is seen for the bulk of the Chalk in the absence of secondary (solutionenlarged) fissures (Price 1987).

However, in the paper, Younger & Elliot derived a value of hydraulic conductivity of 0.001 m/day. They therefore concluded that these fissure apertures and spacings could not explain the hydraulic conductivity of the Chalk even in interfluve areas (p. $46), and that much larger flow conduits must be present to explain the observed hydraulic conductivity values for Chalk beneath valleys. This difficulty is attributable to a simple error; their equation (11) yields results in m/see, not m/day as stated, so the calculated hydraulic conductivity values.

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