Northern Italy): Accounting for the Role of Deep Organic Carbon in Evaluating Palaeorecharge

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IAEA-CN-8/60 GROUND WATER DATING IN THE PO VALLEY AQUIFERS (NORTHERN ITALY): ACCOUNTING FOR THE ROLE OF DEEP ORGANIC CARBON IN EVALUATING PALAEORECHARGE

E. ALLAIS, A. CONTI XA0100905 Dipartimento di Scienze della Terra, LIGA, Universita di Torino, ITALY

G.M. ZUPPI Dipartimento. di Scienze Ambientali, Universita Ca' Foscari di Venezia, and Dipartimento di Scienze della Terra, LIGA, Universita di Torino, ITALY

E. COSTA, E. SACCHI Dipartimento di Scienze Mineralogiche e Petrologiche, Universita di Torino, ITALY

Since the 70s, water demand for various purposes in the most developed region of Italy is satisfied by the aquifers of the Po Valley [1]. Nevertheless, the recharge rate of those reservoirs, which are generally confined, is not very well known because of the large availability of surface waters and thus the scarcity of studies on groundwater recharge and groundwater turnover time. The Po Plain covers an area of approximately 46,000 km2. It is oriented E-W, and bound by the Alps and the by the Apennine chains respectively at the N- NW and at SW, with a natural flow to the Adriatic Sea in the Eastern part. The hydrodynamic system of the plain is constituted by marine sediments that filled a late-Tertiary palaeo-gulf. This process is ending at present with the deposition of alluvial sand and gravel deposits. Organic matter participated and still participates to the sedimentation processes giving origin to lignite, peat and organic clay deposits often forming aquicludes.

The organic matter [2,3] evolves during burial, producing reservoirs of isotopically depleted CH4, at different depths. Fresh groundwater, interacting with methane and organic matter, induce their partial oxidation, and at the same time increases its original content of dissolved carbon. Under particular conditions (fluvial erosion, tectonic subsidence, seismic activity) waters produce, at present, as well as in the past, a secondary generation of isotopically enriched authigenic carbonates, with a 813C sometimes as high as +4 %o [2].

In order to account for the different sources of C leading to the modification of the isotopic composition of DIC, a simple isotopic mixing model utilises 813C values of DIC from primary recharge and the admixed DIC from carbonate sediments. Such an approach is applicable only for a two-source system when the 513C signatures are known. Instead in the deep aquifers of the Po valley two sources of additional DIC must be taken in account: the mineralization and oxidation of sedimentary organic C, and the dissolution of secondary calcite. The combined presence of depleted and enriched carbon sources introduces a problem for the correction of 14C ages of groundwater. In fact, when correcting the age using the different classic models, groundwater appears to display either a modern age, when carbon-13 is controlled by the O.M., or an older age, when water dissolves deep CO2 or secondary carbonates.

Primary recharge DIC and the I4C-free DIC from the mineralization of SOC both originate from organic sources, and thus cannot be distinguished on the basis of their I3C content. Therefore, in aquifer systems where this process is of considerable magnitude, radiocarbon

55 IAEA-CN-8/60 dating using the 813C to quantify the primary recharge DIC provides false and sometimes unrealistic ages.

Reasonable estimates of groundwater ages have been obtained by correcting 14C data using several geochemical and isotopic mass balance calculations [2, 4].

Stable isotopes of the water molecule reflect, in the aquifers, the isotopic composition of the surface waters participating to the recharge. Stable isotopes in the portion of the aquifers influenced by the present rivers describe the evolution of the surface water network. 2H and I8O from deep groundwater indicate a change in the past climates on the Po Valley, and thus the change of the recharge conditions in agreement with water ages. For instance, the water of the Eridano basin (palaeo-name of Pleistocenic Po), is depleted in 18O of about 2%o, with respect to present day groundwater. This water can be found in the Pleistocene aquifers, as well as in the Pliocene, depending on the geological patterns. Older water, aged more than 40 Ka, displays an !8O content depleted of about 2.5 %o [2].

REFERENCES

[1] AGIP (1994) Acque dolci sotterranee. Inventario dei dati raccolti dall'Agip durante la ricerca di idrocarburi in Italia (dal 1971 al 1990). Agip, 515.

[2] CONTI A., SACCHI E., CHIARLE M., MARTINELLI G., ZUPPI G.M. (2000) Geochemistry of the formation waters in the Po plain (Northern Italy): an overview. Appl. Geoch., 15 (1), 51-65

[3] MATTAVELLI L., RICCHIUTO T., GRIGNAM D., SHOELL M. (1983) Geochemistry and habitat of natural gases in Po basin, Northern Italy. Am. Ass. Petrol. Geol. Bull., 67, 2239-2254.

[4] ARAVENA R., WASSENAAR L.I. (1993) Dissolved organic carbon and methane in a regional confined aquifer, southern Ontario, Canada: Carbon isotope evidence for associated subsurface sources. Applied Geoch., 8 (5), 483-493.