Biogeochemical fluxes of iron from rainwater,rivers and sewage to a Galician Ria (NW Iberian Peninsula). Natural versus anthropogenic contributions

The total iron (TFe) concentrations in five rivers to a Galician Ria (averages 1.0–4.5 μM) was within the pristine range, but in rainwater it was higher (17 μM). TFe values of small sewage treatment plants (STP) ranged between 3 and 4 μM, whereas in the largest was 11 μM. Particulate iron in rivers was five times more abundant than dissolved iron, except in the Lagares where it was 20 times higher, but in the STDs the dissolved/particulate coefficients varied from 0.1 to 1.1 and in the rainwater it was lower than 0.4. Equations of water flow versus iron flux were obtained to quantify the iron contribution from the freshwater sources to the Vigo Ria. It receives annually 490 tons of iron (6% in dissolved form) and 90% of this comes from industries focused on metal processes. The contaminated Lagares River accounts for the main input of TFe (327 t a⁻¹), followed by rainwater (78 t a⁻¹), the Oitavén River (28 t a⁻¹) and Vigo STP (33 t a⁻¹). Anthropogenic activities have increased the amount of iron flowing into the Ria by roughly ten times and this could upset the biogeochemical cycle in similar coastal systems.