3 Candidata: Relatore:
Simona De Filippo Prof.ssa Patrizia Macera Correlatore:
Prof. Luigi Marini Correlatore:
Dott.ssa Matia Menichini TITLE: Hydrochemistry and isotope ratios of H, O and C for some springs of the Apuan Alps:
a contribution for the definition of groundwater pathways.
This thesis is aimed at studying the groundwater resources of the mountain basins situated on the coastal side of the Apuan Alps, in the Montignoso, Seravezza, Stazzema, and Pietrasanta municipalities, through the use of geochemical and isotopic techniques. The specific objectives of this research include the qualitative assessment of the resource, the definition of groundwater pathways, the identification of recharge areas, and the investigation of water-rock interaction phenomena.
To these purposes, two sampling campaigns were carried out at the end and beginning of
rainy periods, that isin May and September 2009, respectively. A total of 43 samples were collected
from springs situated in the study area. In the field, different sample aliquots were collected (raw, filtered, filtered-acidified), intrinsically unstable physical-chemical parameters (temperature, pH and Eh) were measured, electrical conductivity, total alkalinity, and ammonia were determined, and the flow rate was estimated when possible. Some samples of precipitations were also taken at different altitude. Finally, rock representative of the lithologies constituting the major aquifers of the Apuan Alps and active stream sediments were also sampled.
Water samples were analyzed for major (Ca, Mg, Na, K, Cl, HCO3, SO4), minor (NO3, F,
SiO2, B), and trace constituents (Cr, Mn, Fe, Ni, Cu, Zn, As, Pb), the 2H/1H and 18O/16O isotope
ratios of water, and the 13C/12C isotope ratio of total dissolved inorganic carbon, TDIC. The rock
and the stream sediments samples were described macroscopically and microscopically and
analyzed for major oxides (SiO2, TiO2, Al2O3, Fe2O3, MgO, CaO, MnO, Na2O, K2O, P2O5, LOI).
Sampled springs waters were attributed to different hydrochemical facies through the classifications methods proposed by Langelier - Ludwig and Piper. Based on the adopted chemical classification most waters belong to the calcium-bicarbonate facies, while sodium-chloride and calcium-sulfate families are less frequent. Calcium-bicarbonate waters come from aquifers hosted in carbonate lithologies, sodium-chloride springs discharge immature waters related to relatively short circuits in low-permeability rocks of the basement and in debris deposits; finally, calcium-sulfate waters proceed from aquifers hosted in the Calcare Cavernoso formation.
On the basis of the 2H/1H and 18O/16O isotopic ratios of water it was established that spring
waters have meteoric origin and that these isotopic ratios depend only on recharge elevation. Springs of low flow rate (< 0.5 l/s) and with an easily identifiable recharge area, not too different in altitude from the discharge point, were selected to identify the vertical isotopic gradient of the study area. This gradient was then used to evaluate the mean recharge elevation of the springs draining major aquifer systems.
4
On basis of the 13C/12C isotopic ratio the degree of water-rock interaction and the origin of
TDIC were assessed. In particular, sodium-chloride waters, representative of rainwater with low
residence time in shallow circuits, derive their TDIC mainly through addition of biogenic CO2 from
soils; the TDIC of the waters belonging to the other hydrochemical facies comes from both addition
of biogenic CO2 from soils and the dissolution of carbonate rocks, in agreement with the
geological-hydrogeological context and the higher residence times in the aquifer.
Subsequently, water-rock interaction phenomena were modeled using the EQ3/6 software package. Initially, the saturation state with respect to calcite was calculated for the springs. Then, the reaction path of the dissolution of local carbonate rocks was modeled. Most springs are positioned along the theoretical path for the dissolution of carbonate rocks, except calcium-sulfate waters, resulting mainly from dissolution of sulfate minerals. Reaction path modeling also suggests that the concentrations of As, Cu, and Zn do not derive from dissolution of carbonate rocks but are initially present in rainwaters, whereas the concentrations of Fe and Mn are controlled not only by dissolution of carbonate rocks but also by precipitation of Mn-bearing iron(III) oxy-hydroxides.
Finally, we have considered the special case of Porta’s springs, comprising three emergencies located at the base of a limestone outcrop consisting of the Calcare Cavernoso and Calcare a
Rhaetavicula Contorta formations. Hydrodynamic, isotopic, and geochemical data indicate that
their recharge area cannot be constituted only by the carbonate rocks cropping out immediately upstream of the springs, as claimed in some works. Based on available isotopic and geochemical data as well as on geological evaluations, a much larger recharge area, whose characteristics are consistent with the examined springs, was identified.
