The Influence of Water Pollution on the Slowed Weathering Rates and Accelerated Biomass Productions Observed in Two Portuguese Granitoid Areas

Abstract

Quantitative geochemical mole-balance studies were first applied by Garrels and Mackenzie (1967) in the study of a pristine area (Sierra Nevada, USA); using the so-called 'balance-sheet' method, they could envisage which natural processes were responsible for the composition of springs and groundwater in their area. More recently, other mole-balance models were developed to address problems such as the influence on mineral weathering rates caused by the growth of biomass (Taylor and Velbel, 1991) or to analyse the chemical evolution of water along a flow path caused by the mixing of different water types _ plus the reaction with solid phases (Parkhurst, 1997). In the present paper, we present a summary of the most important conclusions of two studies, carried out in two Portuguese granitoid areas (Pacheco and Van der Weijden, 1996; Pacheco et al., submitted), which, among other things, analysed the influence of soil, saprolite and ground water pollution on the intensity of mineral weathering (plagioclase and biotite) and on the rates pine growth in those areas. In these two studies, a different mole-balance algorithm was applied, the so-coined SilicaBicarbonate (SiB) algorithm, which is able to separate the contributions to water chemistry associated with natural sources from those associated with anthropogenic and other pollution sources. The type and abundance of secondary minerals present in the soils derived from a certain rock type depend primarily on the environmental parameters climate and topography, as early recognized by Tardy et al. (1973), on the local drainage conditions, and on the mineralogy and chemistry of that rock type; in the case of a granite under temperate climatic conditions, the most important weathering reactants are plagioclase (major) and biotite (minor), and the most common associated secondary products are vermiculite (low rainfall/stagnant drainage conditions), halloysite (moderate to high rainfall/favourable drainage conditions) and gibbsite (very high rainfall/excellent drainage conditions). But the formation of a certain weathering product may also be influenced by factors such as pollution, as pointed out by Pacheco and Van der Weijden (1996) in a study of the Fund~o granitoid plutonite (central Portugal); they suggested that, under stagnant leaching conditions and raised levels of soil/ saprolite water contamination caused by the application of manure and fertilizers on farmland, vermiculite could result from the weathering of biotite, instead of halloysite the most stable product under the local rainfall conditions. The influence of soil and saprolite water pollution on the intensity of mineral weathering was later corroborated by Pacheco et al. (submitted) in a study of the Chaves-Vila Pouca area (northern Portugal), a granitoid and forested area located along a large-scale Hercynian fault; in this case, it was noticed that two water types linked to the weathering of the same plagioclase type (andesine-An40) could be associated to the formation of mixtures with 50% gibbsite plus 50% halloysite, when the water type was less polluted, and to mixtures with 30% less gibbsite when the water type was definitely contaminated. In pristine forested areas, the rates of biomass production and of elemental uptake depend on factors such as climate, morphology, soil fertility, tree species, and stand maturity; for instance, regarding tree species, the response of elemental uptake to the demand of biomass production is different when considering coniferous or deciduous species (coniferous species appear to meet all of their annual requirement of nutrients via the uptake process, while deciduous species meet only about 70% of their needs throughout that process, the rest being provided by translocation of nutrients from older tissue). Besides, most of the annual uptake of elements is provided by recycling from decay of previous year's litter and only a small part is removed from the mineral compartment through weathering.