Comment on “Seasonal fluctuations in the chemistry of limestone springs” by Evan T. Shuster and William B. White

Abstract

Shuster and White have made a conceptual division of carbonate aquifers into 'diffuse' and 'conduit ' flow systems. Their field work has shown the relationship between flow systems and water hardness. Flow measurements and underground water tracing were not an integral part of the work by Shuster and White but the use of quantitative hydrological methods in the British Isles corroborates their findings from the United States. On the Mendip Hills of Somerset, U.K., forty years of discharge records are available for two springs draining the Carboniferous Limestone at Rickford and Langford. The five small streams which flow off the near by impermeable sandstone and sink on reaching the limestone have been traced to both Rickford and Langford using the spores of Lycopodium clavatum. Their contribution to spring flow was measured using small weirs1); it was found to vary between nil and 15 per cent. This value was found to bear a close relationship to the total hardness of the two springs (Fig. l.). The steeper decline of hardness at Langford with increasing proportions of sinking ('swallet') water shows that it is closer to the conduit end-member of Shuster and White's division than is Rickford. Analysis of flow duration curves confirms the greater variability of flow at Langford, typical of a conduit system. Having used the relationship between flow components as a dynamic variable in predicting water hardness at two adjacent springs in Somerset, the author attempted to derive regional values in other parts of the British Isles. The average ratio of sinking (swallet) water to percolation water was shown to be related to hardness variability, except in those aquifers in which percolating water enters widened fissures close to the surface and flows rapidly through the springs. This shows the difficulty of using field definitions of the two components of flow. In the exceptional cases flows which are diffuse for