Modelling the influence of organic acids and rainfall inputs on the weathering of a volcanic ash deposit: a case study in the northern Ecuadorian Andes

Details

Supervisors

Delmelle, Pierre ; Calispa, Marlon

Faculty

Faculté des bioingénieurs

Degree label

Master [120] : bioingénieur en chimie et bioindustries, à finalité spécialisée

Abstract

The páramo in the tropical Andes is notable for its large water retention capacity, its high soil carbon content and rich biodiversity. The resulting ecosystem is the main source of water for many large Latin America cities. The current climate change affects these regions with predicted rainfall pattern alterations. In addition, human activities may lead to modifications in vegetation cover. However, very little is known about the impact of such changes on the weathering of the páramo soils. In this study, we innovatively used the reactive transport model approach to simulate the chemical weathering of a volcanic soil under different rainfall inputs and vegetation covers. We focus on the páramo of the Antisana region in northern Ecuador. Its soils developed on the ash deposit from the 800 years B.P. eruption of Quilotoa volcano. Successive model versions were iteratively elaborated to progressively correspond to actual measurements of soil chemistry. The computer simulations were run using the Crunchflow engine and were based on a 1 m dacitic porous ash deposit reacting with an aqueous solution of rainfall composition (infiltration rate: 0.4 m/yr). In order to take into account the soil respiration effect and to reproduce the pH values of the field soil solution, the CO2 had to be increased 100 times (30 000 ppmv) compared to atmospheric CO2. Due to the acidity consumption by the primary solid phases weathering, the pH increases with depth and decreases with time. Thus, the concentrations of SiO2(aq), Ca2+, Mg2+, Na+ and K+ increase with depth and decrease with time. Free Al3+ is the only cation which concentration decreases with depth and increases with time. Allophanes and gibbsite precipitation are visible. Then, to represent the two different vegetation types, i.e. tussock-like grasses (TU, low DOC) and cushion-forming plants (CU, high DOC), oxalic acid was incorporated in the weathering solution in two different DOC concentrations. The soil solution pH under CU is lower and the mobilisation of Al is stronger, compared to under TU. Allophanes are the only secondary solid phases that precipitate (∼12 %v at 5 kyr). Once the model was tuned, different water flows were tested to study the impact of changing precipitation patterns on the soil solution and mineralogy. A double rainwater flow decreases slightly the soil solution pH under TU and up to 1.5 units under CU. It also decreases the concentrations of SiO2(aq), Ca2+, Mg2+, Na+ and K+ in solution, with a soil solution almost free of Ca2+ and Na+ after 5 kyr of weathering, but with a free Al3+ concentration at the bottom of the profile is 100 times higher under CU than under TU. The depth at which the allophanes precipitate is also lowered by respectively, 5 and 25 cm under TU and CU. On the contrary, decreasing the flow by half has an opposite effect: the pH and the dissolved element concentrations increase and the allophanes precipitate at shallower depths. Further studies need to be carried out including for extrapolating consequences on the ecosystem and in particular on its water quality. However, it is also essential to significantly enrich the actual field data to further tune the model, specifically regarding pH, soil solution chemistry, as well as a robust determination of the water infiltration rate.