Flowpath 2012. Percorsi di Idrogeologia 1 USE OF TRACER TESTS FOR THE ASSESSMENT OF THE RELATIONSHIP BETWEEN
SURFACE WATER AND A DRINKING-WATER SPRING (QUINCINETTO, TURIN)
Lasagna Manuela *,1 Clemente Paolo1, De Luca Domenico Antonio1, Dino Giovanna Antonella1, Forno Maria Gabriella1, Gattiglio Marco1
1. Dipartimento di Scienze della Terra, Università di Torino, Torino, manuela.lasagna@unito.it; domenico.deluca@unito.it; clemente.paolo@unito.it; giovanna.dino@unito.it; gabriella.forno@unito.it; marco.gattiglio@unito.it
* Presenting author
Key-words: drinking-water spring, surface water, tracer tests, fluorescein
1. Introduction
The Montellina Spring is the main source of drinking-water supply of the City of Quincinetto (Turin). It is characterized by a high discharge, varying on average from 50 to 150 l/s (Sea Consulting & GDP, 2010).
The Renanchio Stream is placed in the right watershed and likely feeding the Montellina Spring (Fig.1). The design of a derivation for an hydroelectric plant from the Renanchio Stream has required the execution of a hydrogeological study. This study allowed an analytical assessment of the hydrogeological relationship existing between the Renanchio Stream and the Montellina Spring.
2. Material and Methods
The Montellina spring is located in the right side of the Dora Baltea Valley. In this area the bedrock consists in the Eclogite Micaschist Complex of the Sesia Lanzo Zone (Compagnoni et alii, 1977). It is formed
by coarse polimetamorphic micaschist, with gneiss body and basic seams. Locally a Mesozoic cover also develops, represented by quartzite, marble and calcschist.
The bedrock shows a complex structural setting due to the main alpine deformation events. It is affected by striking fracture systems E-W, N-S, NW-SE and NE-SW favoring the landslides. The bedrock is diffusely covered by glacial deposits referred to both the Dora Baltea Valley and tributary Renanchio Valley glaciers.
The Montellina spring emerges in a landslide body of large elements, near the contact with the underlying glacial sediments. It is also located where the detachment niche crosses a striking NE-SW fracture.
The hydrogeological surveys consisted of: - measurements of the Renanchio Stream discharge using sodium-cloride (NaCl) tracer tests;
- measures of the overflow of the Montellina Spring employing sodium-cloride (NaCl) tracer tests;
- tracer tests of the Renanchio Stream with fluorescent tracer and its continuous monitoring at the Montellina Spring.
The tests were conducted during 3 measure campaigns between July and November 2011, to verify the results for different discharge of the Renanchio Stream.
In particular the discharge of the Renanchio Stream was evaluated by two tracer tests in two measuring stations (referred as upstream section and downstream section) for each campaign of detection; the tracer tests were carried out by slug injection of a known mass of NaCl in solution into stream.
The losses of the stream in the investigated sector were evaluated on the basis of the difference between the measure of discharge in the upstream section and in downstream section.
The measure of the overflow of the Montellina Spring, that is the surplus water flowing out to superficial network, was evaluated by 3 tracer tests using NaCl. The tracer tests were carried out, by slug injection into the stream created by water surplus of Montellina Spring of a known mass of NaCl.
The estimation of the rate of Renanchio water feeding the Montellina Spring was performed by 3 tracer tests of stream using a fluorescent tracer and its continuous monitoring at the spring. The used tracer is uranine, that showed no effect upon either the genotoxicity or the ecotoxicity tests (Behrens et al., 2001).
The tracer tests were carried out by slug injection of a known amount of fluorescein in solution into the stream in the upstream section. After the injection, the detection and record of fluorescein were started in the water of the Montellina Spring at regular time intervals. The used measuring equipment is a fluorometer, positioned in the spring water.
The rate of the Renanchio Stream feeding the Montellina Spring was calculated by assessing the mass of fluorescein which had reached the spring.
Then the following parameters were evaluated (De Luca et alii, 2012):
Flowpath 2012. Percorsi di Idrogeologia 2
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amount of water of the Renanchio Stream that feeds the Montellina Spring;-
percentage of water of the Renanchio Stream that arrives to the spring;-
percentage of the discharge of the spring which is derived from the Renanchio Stream discharge.3. Results
The discharge of the Renanchio Stream varied considerably between July and November 2011, ranging from 133 l/s and 1493 l/s in the upstream section and from 102 l/s and 1104 l/s in the downstream section; also the losses of the stream in the investigated section are different depending on the monitoring campaign, ranging from 31 l/s and 389 l/s.
On the contrary the discharge of the Montellina Spring remained fairly steady between 144 l/s and 181 l/s.
The rate of the Renanchio stream feeding the Montellina Spring, calculated by tracer tests with fluoresceine (Fig. 2), is lower than 19 l/s, both in lean period and in flood period. In details, the surface water moving to the spring ranges between the 1% and 14% of the Renanchio Stream discharge in the upstream section. At last the percentage of the Montellina spring discharge coming from the stream ranges between 8% and 13%.
4. Conclusions
Tracers tests using NaCl and fluorescein allowed to identify and quantify the relashionship between Renanchio Stream and Montellina Spring. Even if there is a large variation of stream discharge and stream losses during the study period, the discharge of spring remains fairly steady.
The quantization of the rate of the Renanchio Stream feeding the Montellina Spring proved that only a small part of stream losses moves to spring. As a consequence, the derivation of Renanchio stream water for hydroelectric use has only a moderate impact on Montellina Spring discharge.
References
Behrens H., Beims U., Dieter H., Dietze G., Eikmann T., Grummt T., Hanisch H., Henseling H., Kaess W., Kerndorff H., Libundgut C., Mueller Wegener U., Ronnefahrt I., Scharenberg B., Schleyer R., Scholz W. & Tilkes F. (2001). Toxicological and ecotoxicological assessment of water tracers; Hydrogeology Journal; 9 (3); 321-325.
Compagnoni R., Dal Piaz G.V., Hunziker J.C., Lombardo B. & Williams P.F. (1977). The Sesia-Lanzo Zone, a slice of continental crust with
alpine high pressure-low temperature assemblages in the Western italian Alps. Rend. Soc. It. Mineral. Petrog., 33, (1), 281-334.
Custodio E. & Llamas M.R. (2005). Idrologia sotterranea 1. Dario Flaccovio editore, pp. 1104. Custodio E. & Llamas M.R. (2007). Idrologia sotterranea 2. Dario Flaccovio editore, pp. 1100. De Luca D.A., Lasagna M., Clemente P., Forno M.G., Dino G. & Gattiglio M. (2012). Studio idrogeologico per la valutazione della quantità di acqua del Torrente Renanchio alimentante la sorgente Montellina (località Quincinetto, Torino) – relazione, pp. 43.
Sea Consulting & GDP (2010). Derivazione a scopi idroelettrici del Torrente Renanchio -Studio idrogeologico per la Sorgente Montellina. Rapporto finale, pp. 63.
Fig. 1 – Conceptual scheme of study area (Qt1 -discharge of the Renanchio Stream in the upstream section; Qt2 - discharge of the Renanchio Stream in the downstream section; P - losses of the Renanchio Stream; Qs - discharge of the Montellina Spring; Qt1s - rate of the Renanchio Stream feeding the Montellina Spring).
Fig. 2 – Fluorescein concentration in the Montellina Spring during the tracer test in the Renanchio Stream of September 2011.
