Influencia de la escorrentía rural sobre el aporte de metales pesados y sedimentos al sistema fluvialanálisis a escala de cuenca

Resumo

Metals are a natural part of the environment and enter natural waters from a variety of sources, mostly by the weathering of bedrock and soils in the catchments. Besides the natural processes, anthropogenic factors also lead to an increase of metals in soils and waters. Elevated concentrations of metals pose potential hazards to human health because of their toxicity and persistence in the environment. The understanding of the processes controlling sediment and metal export is critical to assessing and anticipating impacts on the water courses. Catchment characteristics such as geology, slope, drainage, land use, climate conditions (magnitude, intensity, frequency and rainfall distribution, etc.), water flow paths (surface runoff, subsurface water and groundwater) and connectivity of source area with the river are important factors controlling the forms and quantities of sediments and metals that are transported to rivers. The transport mechanisms vary for each metal, depending on its abundance, solubility or distribution in particles, among other factors. In sum, the transport of metals and sediment from the terrestrial environment to rivers is strongly affected by the hydrological processes, which undergo spatial and temporal oscillations in response to changes in the characteristics of the drainage area. Besides hydrological processes, the transport of metals in fluvial systems is also controlled by a variety of geochemical processes, including amount and characteristics of both dissolved organic carbon and suspended particulate matter, redox conditions, and by precipitation/dissolution and adsorption/desorption reactions. The drainage basin is the fundamental unit to characterize hydrological processes, and analyze the dynamics of heavy metals and sediments as well as to diagnose water resources. The chemical composition of water only offers partial information of the aquatic environment. The variation along one or more hydrological years as well as during periods of intense or prolonged rainfalls provides much more information. Runoff events cause immediate transportation (ranging from a few hours to a few days) of large masses of solutes and sediments. Therefore, it becomes highly important to know the pattern of concentration-discharge relations and to quantify loads of metals and suspended solids in all hydrological conditions. The determinations over time of the physico-chemical composition of the water and the discharge, as well as simultaneous recording of meteorological elements, offer the opportunity to correlate he hydrological and physico-chemical responses in relation to rainfall, which allowing to know what hydrometeorological factors that are involved in the transport of such materials in rivers. Metals that enter aquatic environments become part of the water sediment system, and their distribution processes are controlled by a dynamic set of physicochemical interactions and equilibria. Knowledge of the total contents of metals in soils and sediments provides limited information regarding their potential behavior and bioavailability. Metals are associated with various soil and sediment components in different ways, and these associations determine their mobility and availability. Thus, metals can exist as water-soluble and exchangeable forms; precipitate in carbonates; occlude in Mn-, Fe- or Al-oxides; bound to organic matter; or appear in residual phases (incorporated into the crystalline lattices of clays). Comprehensive studies that examine the transfer of metals across soil-river bed sediment systems at catchment scale are scarce. An understanding of the distribution of metals among various fractions of soils and sediments within a catchment is fundamental to understand their environmental behavior and, thus, to define watershed management plans. At catchment scale studies on suspended solids (concentrations and loads) transported during events are also scarce, and those evaluating metal fluxes in rural catchments even much scarcer. Therefore, both the temporal variations of concentrations and loads of such materials between events and during events, and the reasons leading to concentrations and loads of metals increase with runoff during some episodes and not in others are still little studied...