Weathering, biomass production and groundwater chemistry in an area of dominant anthropogenic influence, the Chaves-Vila Pouca de Aguiar region, North of Portugal

Using a combination of a grouping algorithm and a geochemical mole balance algorithm, we could identify and quantify the contributions made by water-r ock and water-biomass interactions to the composition of shallow groundwate rs in northern Portugal, in an area with a sizable anthropogenic input (the Chaves-Vila Pouca de Aguiar region). The first algorithm, based on the mat hematical concept of equivalent relations, allows for the grouping of water samples with similar chemistries. The second algorithm, which uses the sto ichiometries and mole/charge balances in weathering reactions and biologica l processes, provides the possibility to relate the average chemistry of ea ch identified group to water-mineral and water-biomass interactions. This a lgorithm hinges on ratios of dissolved silica and bicarbonate, constituents considered to be derived only from chemical weathering and biomass product ion. Background information on the petrology, mineralogy and chemistry of b edrocks and soils, on climatic differences, on the history of deforestation and reforestation of the study area, and on the application and compositio n of fertilizers and manure, was used to identify the most realistic weathe ring reactions and biological processes which contribute to the water chemi stry. The concentrations of chloride, sulphate and nitrate were considered to represent sources other than chemical weathering or biomass changes, lik e cyclic salts, fertilizers and manure. Despite the high background concent rations derived from such sources, we were able to quantify the contributio ns of the identified weathering reactions and botanical uptake to the groun dwater composition. The spatial distributions of the various groundwater gr oups shows a good correlation with the geology, land use and climatic diffe rences in the area.