OUCC Proceedings 11 (1983)

Stephen Gale

Very little water tracing has so far been attempted in the western Picas de Europa, all known successful tests being summarised in Table 1. In an effort to increase our knowledge of groundwater linkages in the region, further tests were made during the course of the 1983 OUCC Expedition. However, only one of these yielded positive results. This was made to establish the route taken by water sinking in the northern end of the large closed depression of Covellona (43° 15` 20`` N, 1° 17` 00`` W with reference to the Madrid meridian), which forms the head of the valley of the Rio del Texu (Frontispiece).

A quantity of Rhodamine B dye was introduced into the sink at 12.13 on 18 July 1983. The dye began to reach visible concentrations at the resurgence of Fuente Escondida (43° 15` 30`` N, 1° l7` 10`` W with reference to the Madrid meridian) during the morning of 19 July 1983, and the peak of the dye pulse was clearly observed at 15.30 on the same day. At the time, the discharge of the resurgence was approximately 2.5 l/s

The straight-line distance between sink and resurgence is 375 m, giving a straight-line velocity for the time to dye-pulse peak of approximately 3.8x10^-3 m/s. This obviously provides only a minimum value for the true flow velocity. Nevertheless, this figure is significantly lower than the straight-line velocities measured by Atkinson and Smith (1973, 15) in the crystalline, massively-bedded White Limestones of Jamaica and the Carboniferous Limestones of Mendip. Two possible explanations may be given. First, the flow between Covellona and Fuente Escondida may be highly tortuous and flow velocities may be similar to those normally recorded for fissure flows in carbonate aquifers. Secondly, flow may have been diffuse rather than discrete, that is, taking place through a network of fissures rather than along a single conduit. Insufficient work has been done on diffuse flow to enable comparisons to be made, although velocities of diffuse groundwater flow under unit hydrological gradients in carbonate aquifers have been calculated as 1.03x10^-3 m/s for the Carboniferous Limestone of Mendip (Atkinson, 1977, 105) and as a maximum of 1.97x10-3 ms-1 in the most permeable parts of the Cretaceous Chalk of East Yorkshire (Foster and Milton, 1974, 497).

An attempt was also made during the 1983 Expedition to trace the outlet of the water flowing down Pozu Jorcada Blanca. Unfortunately, it was not possible to recover all the dye detectors from this test, and those detectors which were recovered (from Canal de la Raya, Fuente Puente Bolin, and from the streams above and to the west of Cain) all gave negative results. However, the test did establish that the water sinking at the bottom of The Font in Pozu Jorcada Blanca reappears at the Hot Tub before flowing down to the terminal sump. This suggests the existence of a hydrologically-important bypass which has captured and left inactive the northernmost section of the cave, that around One Step Beyond and The Sphinx.

References

• Atkinson, T.C., 1977. Diffuse and conduit flow in limestone terrain in the Mendip Hills, Somerset (Great Britain). J. Hydrol. 35, 93-110.
• Atkinson, T.C. and Smith, D.I., 1-3. Underground flow rates in cavernous limestones in Britain and Jamaica. Proc. Internat. Congr. Speleol. 6(4), 13-16.
• Austin, M. and Wilcock, J.D., 1965. Water analysis and hydrology. In Oxford University Expedition to Northern Spain 1961. Cave Res. Grp Ct Br. Pub. 14, 11-20.
• Collis, W.J.M.F., 1-5. Cueva el Gueya Reinazo. Proc. Oxford Univ. Cave Club 7, 36.
• Foster, S.S.F. and Milton, V.A., 1-4. The permeability and storage of an unconfined Chalk aquifer. Hydrol. Sci. Bull. 19, 485-500.
• Laverty, M., 1-9. Cueva del Frieru. Proc. Oxford Univ. Cave Club 9, 6.
• Willis, R.G., 1981. Pozu del Xitu - 1981 dye tests. Proc. Oxford Univ. Cave Club 10, 49-50.

   Table 1. Known successful water-traces in the western Picos de Europa, Asturias, Northern Spain