Metal mobility processes in an AMD-affected estuary: Huelva Estuary, (SW Spain)

LECOMTE K.L.; SARMIENTO AM; BORREGO J; NIETO JM

Congreso; V Regional Committee on Neogene Atlantic Stratigraphy. RCANS; 2013

Huelva Estuary is located in South West Spain; it is a partially enclosed body of water along the coast where acid rivers (Tinto and Odiel rivers) mix with salt waters from the ocean. This is a transitional region, where metal-rich acid waters interact with alkaline waters from the sea, giving rise to multiple processes that control dissolved and particulate geochemical and mineralogical characteristics. The aim of this work is to determine and quantify metal mobility (i.e. precipitation, sorption/desorption) during the pH gradient observed in estuarine mixing. Representative samples of both systems were collected in April 2013: Tinto River sample was collected at its source and sea water on the coast of Huelva (37° 7? 58?? N and 6° 51? 43?? W). A pH controlled test was carried out by mixing a known volume of river water with enough sea water to reach every selected pH. The sediment precipitated in each step was collected and prepared to make the sequential extraction (SE). In this work, preliminary results of the SE analysis are presented. Tinto River showed a pH of 1.66 and an electrical conductivity of 25.8 mS cm-1, whereas sea water had values of 7.87 and 55.2 mS cm-1, respectively. The sea water volumes required to achieve the different pH values were surprisingly enough that worth to be highlighted. The experience indicates that for each liter (L) of Tinto waters are needed 170 L of sea waters to reach a pH of 3; out of which about 30 L were consumed during buffering process exerted by ferric ions at a pH of 2.7. A similar buffer capacity was observed for aluminum at a pH of 4.5. Furthermore, sea water/Tinto water ratios of 280 and 320 were necessary to reach pH values of 3.5 and 4, respectively. Ratio near 500 and 850 are required to reach pH values of 5 and 6, respectively, and more than 1700 to get a neutral solution. In a hypothetical case that Tinto River reaches the estuary with a composition similar to that recorded in headwaters, it would be necessary more than 8500 m3 of sea water to neutralize a river discharge of 5 m3 s-1 of acid waters.