• Abstract and Figures
• Public Full-text

Carbonates Evaporites (2011) 26:235–254 DOI 10.1007/s13146-011-0066-1 ORIGINAL ARTICLE The alluvial aquifer of the Lower Magra Basin (La Spezia, Italy): conceptual hydrogeochemical–hydrogeological model, behavior of solutes, and groundwater dynamics Gianpiero Brozzo • Marina Accornero • Luigi Marini Accepted: 18 August 2011 / Published online: 6 September 2011 Ó Springer-Verlag 2011 Abstract The alluvial aquifer of the Lower Magra Basin largely contributed by water–rock interaction. Unexpect- (La Spezia Province, Italy) is mainly recharged by two edly, Li has non-mobile behavior owing to either incor- main rivers of the region, Magra and Vara, with a subor- poration into secondary minerals or sorption onto solid dinate contribution from rain waters infiltrating in the phases or both. The discovery of two natural sources, one eastern horst adjacent to the graben of the Lower Magra of Cl and the other of SO4, in the basin of the Aullella River. This conceptual model is consistent with the con- Creek, a tributary of the Upper Magra River, made possible straints provided by both piezometric maps and chemical a key understanding of groundwater dynamics. Every year, and isotopic data of local groundwaters and fluvial waters. during the summer, a mass of Cl and SO4-rich waters Chemical data also suggest that, in the considered system, originating from the Aullella Creek enters the alluvial Cl and SO4 have mobile (conservative) behaviour, and Na, aquifer of the Lower Magra Basin. Average groundwater K, and Sr have close-to-mobile behaviour; in other words, velocity and average permeability, representative of the the contents of these chemical components are essentially entire alluvial aquifer of interest, were derived based on the fixed by mixing relationships between Magra and Vara movement of this high-salinity water mass. river waters. In contrast, Mg, Ca, Ba, HCO3, and SiO2 are Keywords Groundwater Á Thermo-mineral water Á Alluvial aquifer Á Magra River Á Vara River Electronic supplementary material The online version of this article (doi:10.1007/s13146-011-0066-1) contains supplementary material, which is available to authorized users. Introduction G. Brozzo Laboratory of Environmental Control, ACAM Acque S.p.A., Chemical constituents dissolved in natural waters may be Via Francesco Crispi 132, 19124 La Spezia, Italy e-mail: [email protected] classified as conservative (or mobile or inert) and com- patible based on their behavior (e.g., Arno´rsson et al. 1983; M. Accornero Chiodini et al. 1991; Giggenbach 1991; Appelo and Postma D’Appolonia S.p.A., Via San Nazaro, 19, 1996; Berner and Berner 1996). The concentration of 16145 Genoa, Italy e-mail: [email protected] conservative components is essentially regulated by evap- oration and mixing, without any further gain or loss. For L. Marini (&) instance, Cl, Br, B and, to some extent, Li, Rb and Cs Laboratory of Geochemistry, Dip.Te.Ris., behave as conservative or comparatively conservative University of Genova, Corso Europa 26, 16132 Genoa, Italy constituents in most natural waters. In contrast, the con- e-mail: [email protected] centration of compatible components is controlled by sinks/sources, such as water–rock interaction and biologi- L. Marini cal processes. Aluminum, Si, Ca, Mg, Sr, and Ba usually Institute of Geosciences and Georesources, C.N.R., Area della Ricerca, Via G. Moruzzi 1, have compatible behavior in most natural environments 56124 Pisa, Italy due to attainment of saturation with low-solubility mineral 123 236 Carbonates Evaporites (2011) 26:235–254 phases (including oxides and oxy-hydroxides, silicates, mineral dissolution (feldspars, micas, chlorites, amphi- Al-silicates, carbonates, sulfates, etc.), whose precipitation boles, olivines, and pyroxenes) and clay mineral formation causes a loss of these elements from the aqueous solution. (kaolinitization, laterization, and illitization), dolomite Bicarbonate concentration may be controlled by CO2 par- dissolution and calcite precipitation (dedolomitization), tial pressure, where a large gaseous CO2 reservoir is con- dolomite formation (dolomitization), sulfate reduction and nected to the aqueous solution (e.g., Langmuir 1997). The pyrite formation, silica precipitation, evaporation, and contents of several cations may be governed by ion- cation exchange (Nordstrom 2003). These processes can be exchange provided that a generic cation exchanger (e.g., modeled by means of several software packages, whose clay minerals, zeolites, organic substances, Mn(IV) and results largely depend on completeness and quality of the Fe(III) oxyhydroxides, etc.) is active in the system of thermodynamic database (e.g., Oelkers et al. 2009), kinetic interest. The limit between conservative and compatible database (e.g., Schott et al. 2009), and adopted approach, constituents is not absolute. Some components (e.g., Na, K, especially for what concerns sorption–desorption on solids SO4) are conservative in some systems and compatible in (e.g., Kulik 2009). Sources of thermodynamic data are others. reported by several authors (e.g., Nordstrom and Munoz Focusing on the distribution of elements in river waters, 1994 and Grenthe and Puigdomenech 1997), whereas Gaillardet et al. (2003) defined the mobility index as the kinetic data on dissolution/precipitation reactions were ratio between average dissolved concentration and average compiled by Palandri and Kharaka (2004), Marini (2007), concentration in the upper continental crust (taken from Li and Bandstra et al. (2008). 2000). This generalization is certainly important as a global Keeping in mind this general picture, this work is reference framework. Nevertheless, the behavior of focused on the alluvial aquifer of the Lower Magra Basin chemical constituents must be carefully investigated in (or Magra alluvial plain; Fig. 1) and is aimed at: (1) each natural system of interest. reconstructing its conceptual hydrogeochemical–hydro- For instance, the conservative versus non-conservative geological model, (2) evaluating the behaviour of selected behavior in estuarine waters, that is during mixing between dissolved constituents, and (3) investigating groundwater seawater and river water was studied by several authors dynamics. This third task is of utmost interest. As detailed (e.g., Boyle et al. 1974; Liss 1976; Loder and Reichard below, it was made possible by the discovery of two natural 1981; Officer and Lynch 1981; Kaul and Froelich 1984). In sources, one of Cl and the other of SO4, both situated in the particular, Liss (1976) showed that the concentration of a basin of the Aullella Creek (Fig. 1), a tributary of the conservative constituent plotted against Cl concentration Upper Magra River (Brozzo and Marini 2005, 2009). lies on a straight line connecting the river water point and Intrusion of underlying or adjacent saline groundwater the seawater point, that is on the theoretical mixing line for or saline-water flow from adjacent or underlying aquifers the considered conservative constituent. If a dissolved were recognized as causes for the salinization of many component is added during mixing, a concave-down curve aquifers (Vengosh 2003 and references therein). These above the dilution line will occur. In contrast, if the con- phenomena are often triggered by exploitation or overex- stituent is removed with increasing Cl, a concave-up curve ploitation perturbing the natural hydrological balance below the dilution line will result. Although the picture between different aquifers (Vengosh 2003 and references may be further complicated by other factors, such as therein). The intrusion of high-salinity waters in shallow presence of a third end-member (Boyle et al. 1974), sea- aquifers hosting low-salinity waters is rather frequent, in sonal changes (Kaul and Froelich 1984), and slow flow of geothermal areas (e.g., Arno´rsson et al. 2007). waters (Officer and Lynch 1981; Loder and Reichard Several previous studies (e.g., Vuataz et al. 1984; Flores- 1981), chloride plots are simple and powerful tools to Ma´rquez et al. 2006; Aksoy et al. 2009) have shown that, in investigate the conservative versus non-conservative general, the intrusion of salty thermal waters in shallow behavior of chemical constituents in the system of interest. aquifer systems is close-to-steady, owing to the nearly When dealing with aquifers, after this initial screening, constant flowrate of uprising thermal waters, at least where conservative constituents can be used to study water the process is governed by natural causes only, in the dynamics, as they are not retarded with respect to water, absence of anthropogenic perturbations. This means that being affected by hydrodynamic dispersion only (e.g., the plume of salty thermal water is neither expanding nor Freeze and Cherry 1979). In contrast, compatible compo- contracting with time. nents may provide insights into active sinks/sources and, in In contrast to this typical scenario, the recharge process particular, on water–rock interaction processes. The most occurring in the alluvial aquifer of the Lower Magra Basin important among these are calcite dissolution and precip- is rather unusual, as clarified below, in that it is charac- itation, gypsum dissolution and precipitation, pyrite oxi- terized by remarkable seasonal variations, with periodical dation and formation of hydrous ferric oxide, silicate inputs of Cl and SO4-rich water during each summer. Inter 123 Carbonates Evaporites (2011) 26:235–254 237 Fig. 1 Schematic geo-lithological

Load more

  20

Copy Link