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Tracer tests were utilised in an area of northwester Italy, in order to evaluate the hydrogeological relationship between the drinking water Montellina Spring and the Renanchio Stream, likely feeding the spring. The project of a derivation from the stream for an hydroelectric plant could affect the water regime of the spring. So an analytical assessment of the connection existing between the stream and the spring was required. Moreover, a geological survey of the spring area was carried out, with the aim to reconstruct the geological and hydrogeological context. The large amount of groundwater in the basin is linked to a high permeability of the shallow rock mass. The concentration of supplies to the spring is due to morphological reasons and the occurrence of major fractures that carry also a part of the losses of the stream. The purposes of the study are the evaluation of: the losses (l/s) of stream feeding the spring, the percentage of water of the stream that reaches the spring and the percentage of the spring discharge supplied by the stream. The hydrogeological surveys consisted of sodium-chloride tracer tests for the measurement of the stream discharge and of the overflow of the spring, and tracer tests of the stream with fluorescent tracer. The surveys were conducted in three measurement campaigns for different flows of the stream. Findings indicate that, despite the large variation of stream discharge and stream losses, the discharge of spring remains fairly steady and scarcely supplied by the stream. The case study, besides increasing the knowledge about this area, unusual for the hydrogeological features, provides a methodological approach to the evaluation of the relationship between rivers and groundwater.
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