Imaging spectroscopy to evaluate the contamination from sulphide mine waste in the iberian pyrite belt using hyperspectral sensors (Huelva, Spain)

Laburpena

Monitoring of mine waste on sulphide deposits through hyperspectral remote sensing data contributes to predicting surface water quality quantitatively estimating acid drainage and metal contamination on a yearly basis. The mineralogy of surface crusts loaded with highly soluble salts is a record of available humidity and temperature along the year. A temporal monitoring of salt efflorescence on mine wastes at a mine site in the Iberian Pyrite Belt (Spain) has been mapped in this work using hyperspectral airborne Hymap data. Climate change estimations are made based on oxidation stages derived from well-known sequences of minerals tracing sulphides oxidation intensity, using archive spectral libraries. Therefore, mine waste weathering products of sulphide mapped from airborne hyperspectral remote sensing data can be used as a short-term record of climate change, providing a useful tool for assessing environmental geoindicators in semi-arid areas. Hyperspectral remote sensing deals with airborne and spaceborne imaging spectrometers which have a similar spectral resolution to the field and laboratory instruments. A large number of channels in two dimensions require image processing procedures able to manage high dimensionality on data. The main technical features of the Hymap airborne and Hyperion spaceborne hyperspectral sensors used in this study are described. The pre-processing chain to convert raw data to reflectance on both sensors is drafted. The interpretation of hyperspectral remote sensing requires specific algorithms able to manage high dimensional data. The image processing methods to extract information are described, and the sequence of algorithms used to produce maps, both on mine sites and river paths. The mineralogical identification capability of algorithms to produce maps based on archive spectral libraries is discussed. Trends of mineral growth differ spectrally over time according to the geological setting. Subtle mineralogical changes are described using the spectral response and their meaning as indicators of pyrite oxidation intensity on mine waste piles, pyrite mud tailings and river sediments. A new exploration method, the Automated Modified Gaussian Model, intends to estimate further spectral variability explaining the geological meaning of the Hymap spectra associated to the units of river sediments mapped. Hyperspectral low spatial resolution Hyperion data are used to map mine waste from massive sulphide ore deposits, mostly abandoned, on the Iberian Pyrite Belt. Hymap high spatial resolution data are used for crossed interpretation. Mine dams, mill tailings and mine dumps in a variable state of pyrite oxidation are recognizable. Ponds of acid water in the mine sites are spectrally outstanding and, with simple image processing procedures, mappable. In addition, acid water with different chemical composition is mappable with hyperspectral data, whether of high or low spatial resolution. A sequence of hyperspectral image processing algorithms used to produce the maps is suggested. The Odiel River carries acidic water originating from mine waste contamination, including massive sulfide ore deposits. Acid Mine Drainage processes taking place in numerous mine sites drain acid and contaminated water to the river, causing acid pH, and increasing the transport of heavy metals and other contaminating substances. Changing climatic conditions control both the geochemical evolution of secondary minerals that precipitate over the river sediments and the hydrodynamic regime of the river, as much as flow rate. Mapping contamination products from sulphide mine waste on a river flow path with hyperspectral data involves a careful mapping of the river sediments and minor topographical features changing the water dynamics. The minor topography provides the favourable locations to monitor contaminant mineral development. The use of a reference mineralogical spectral library, developed in the laboratory, requires a careful geological context evaluation to provide efficient environmental information on contamination parameters. Based on the hyperspectral analyses of critical spectral features, river locations that may be crucial for tracing significant, future contaminant fluctuations were identified. The observations of the proposing group point to spectral and geomorphological indicators which can be monitored through image processing supported by field and laboratory spectral data, on a repeatable basis. Hyperspectral image analysis is included on the methods selected by the Joint Research Centre of the European Community (Yspra, Italy) to study abandoned mine sites to assess the enforcement of the European Mine Waste Directive, published in 2006. The pyrite belt in Andalucía has been selected as one of the core mission test sites for the PECOMINES II program (Cracow, November 2005) using imaging spectroscopy. The use of this technique as a monitoring tool aims to be implemented by the Environmental Net of Andalucía (REDIAM, Spain).