Derelict Metal Mining as a Cause of Environmental Impact in the Aquatic Ecosystems (NW Spain)

The Caudal River is one of the main rivers in the Asturias region (NW Spain). Its average flow exceeds 20 m3 s−1 and its basin (654 km2) is highly anthropized, hosting numerous closed coal and metal (copper, mercury) mines. The latter is of particular interest since arsenic (As) is found in the mineral paragenesis of mercury (Hg) deposits. The geochemical environmental mobility of this metalloid is higher than that of Hg and therefore poses a greater environmental risk. In the mining sites, closed in the last century, when the environmental regulations were not as restrictive as in the present, old facilities and tailings continue to release these elements into the environment. The watercourses (Caudal River tributaries) in the lower part of the basin do not reach a good ecological state, according to the Hydrological Planning Office, and in some cases, their status is deficient. In this work, 17 samples of fluvial sediments and 9 of water have been taken, both in areas affected by anthropic activity and in pristine areas at the head of the basin, in order to establish the background levels for the critical elements. The sediment particle size fraction of less than 125 microns was analyzed. The As content in all sampled sediments is above the Threshold Effect Levels (TEL). The possible effect range within which adverse effects occasionally occur, according to the Canadian Sediment Quality Guidelines for the Protection of Aquatic Life. This fact is related to a naturally high geochemical background in the basin, due to the existing mineral deposits. On the other hand, the PEL threshold (the probable effect range within which adverse effects frequently occur) is exceeded by more than an order of magnitude in the sediments downstream of the Hg mines. In these points, the As content in the water, exceeding 500 μg L1 As, is also above the quality standard established in Spanish legislation. Although the concentrations decrease with distance from the source, these findings justify the low ecological quality of the affected watercourses.