Medical Hydrology and Balneology: Environmental Aspects

Application of geochemical modelling for
quantifying processes affecting high silicon
content in groundwaters of northern
Gran Canaria (Canary Islands, Spain)
Dobrzynski D (1) , Cruz T (2) , Kostka A (1) , Cabrera MC (2) , Hernández P (2)
(1) Department of Groundwater Geochemistry, Faculty of Geology,
University of Warsaw, Poland
(2) Department of Physics, University of Las Palmas de Gran Canaria, Spain
dobrzynski_dariusz@yahoo.co.uk
Introduction and Objectives
The chemistry of silicon-rich groundwater in the north part of Gran Canaria
(Canary Islands, Spain) has been investigated to elucidate processes responsible for
high Si concentrations. In the study area, basaltic and basanitic lavas and ignimbrites
of the Roque Nublo and Post-Roque Nublo groups and sediments of Las
Palmas detritic Formation outcrop. Groundwater comes mainly from Roque Nublo
group and underlying Miocene trachytic and phonolitic lavas. The island aquifer is
conceptualised as a stratified and heterogeneous water-body, with groundwater
flows from recharge area (at the summit) towards the coast. In the study area,
groundwater flows generally from the south to the coast, located at the north.
Materials and Methods
Waters in 19 large-diameter (2.5 and 3 m) wells were sampled. After complete
chemical analysis, geochemical (speciation and inverse mass balance) modelling
combined with data mining methods have been applied.
Results
Studied groundwater are fresh and mineral (SEC up to 2760 µS/cm), with temperature
ranges from 16.3°C to 25.5°C, and pH between 4.40 and 7.40. Hydrochemical
types of HCO3-(Cl)-Mg-Ca-Na prevail. In fresh groundwater of nearneutral
pH, silicon usually occurs at concentrations of 0.1-0.3 mM. In studied
groundwater Si concentrations are 0.42 to 1.82 mM (mean 1.28 mM), and show
positive correlation with ionic strength and temperature, and highest values at pH of
6.0-6.5.
In most groundwaters, silicon occurs as (ortho)silicic acid (H4SiO4), and is
mainly governed by temperature and pH. At chemical equilibrium in the groundwater-rock
system, Si concentration is controlled by solubility of minerals. In aquifers
of active zone, like in studied fissured volcanic aquifer, at non- or metastableequilibrium,
Si concentration meets quasi steady-state conditions, i.e. is a resultant
Balnea
2012, núm. 6, 451-452
451
ISBN: 978-84-669-1887-0
978-84-669-3482-4