Influence of rainfall on physicochemical characteristics of runoff water and sediments in riverbeds affected by mining and agricultural activities.

Cartagena-La Union mining district (SE Spain) has unconnected dry riverbeds, which transport mining waste downstream under torrential rain events, polluting a coastal lagoon (Mar Menor). Additionally, in the cropland Campo de Cartagena, agricultural activities promote nitrate and phosphate accumulation in that lagoon. Therefore, this study aimed to evaluate the spatial-temporal trends of sediments and runoff water properties in El Albujón y La Carrasquilla riverbeds. The sampling campaign was carried out before (BR) and after (AR) rainy period. Georeferenced points were established from the headwaters to the mouth of two riverbeds, 12 sediment samples were collected in each riverbed. Meanwhile, runoff water in El Albujón consisted of 9 points covering the zone that continuously drained into Mar Menor lagoon. Also, water samples were collected in La Carrasquilla during the rainy period, from 18th to 28th of March, the samples were located in the headwater and the mouth of the dry riverbed. Sediments samples were analyzed for pH, electrical conductivity, organic/inorganic carbon, nutrients and heavy metals. To determine the pollution level by metal(loid)s in sediment and water, the reference background values established by World Health Organization (WHO) for water, and pollution indexes (contamination factor, pollution load index, potential ecological risk index) for sediments were used. The results of the runoff water of El Albujón exhibited no temporal variation, with a high concentration of NO ₃ ^- along the discharge zone, which exceeds the standards established by WHO (102 mg L ^-1 BR and 101 mg L ^-1 AR). In the sediments of El Albujón, the pollution indexes showed no contamination of metal(loid). Oppositely, La Carrasquilla dry riverbed presented metal(loid)s contamination in the sediment in all the sampling points for both sampling periods (BR and AR). In the case of runoff water, in La Carrasquilla, during the rain event, the results exhibited concentrations over the limits for Cd, Mn, and Zn in the headwater. The temporal variability between a dry and rainy period represented a significant variance, especially for the metal(loid)s concentration on both sites. These results can support the design of mitigation plans for reducing environmental impacts associated with these riverbeds.
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