## 1 Introduction  
The Guadalquivir River (SW, Spain) is the second longest river in Spain and the only one which allows navigation in the Estuary, being the main water source of a Spanish region with more than 7 million inhabitants. It reaches the Gulf of Cadiz (Atlantic Ocean), bordering Donana National Park Reserve which is the most important wintering site for many birds in Europe. However, this region has been exposed for centuries to mining activities which have greatly enriched the metal content of its water and soils [1].  
Integration of chemical concentration and toxicity responses is the best way to determine the sediment [2, 3]. It is known that sediments are able to transport contaminants much more easily than water [4], but also act as a sink and/or source of contaminants [5]. Therefore, biological effects based on laboratory tests can determine toxic responses. Sediment bioassays are simple tests that evaluate the responses of the tested organism to contaminated sediments under controlled conditions [6]. Despite of the fact that some reports have pointed out that biological tests or bioassays are decisive to understand the toxic effects of sediments [7, 8, 9], the final decision of the sediment status is based on a comparison between levels of contaminants measured in the sediments against different sediment quality guidelines (SQGs).  
A battery of tests was selected using different trophic levels to determine sediment toxicity: (1) a whole sediment survival bioassay using the amphiphod _Ampelisca brevicornis_,(2) the commercial Basic Solid Phase Test (BSPT) of Microtox(r) based on the bioluminescence of the bacteria _Vibrio fischeri_, (3) the _Paracentrotus lividus_ sea urchin fertilization success and (4) embryo-larval development bioassays, and (5) a whole sediment toxicity test with the oligochaete worm _Tubifex tubifex_.  
The acute bioassay with the _A. brevicornis_ amphipod has shown this species as a sensitive organism valid to assess the toxicity of contaminated sediments [10; 11; 12; 13; 14]. The commercial bioassay Microtox(r) has been used to detect the \"hot spots\" of field contamination in the screening procedure [15; 16], and also to assess oil contaminated sediments [11; 17; 18]. Sea urchin toxicity tests use fertilization and larval development as endpoints, which are considered to be useful tools for assessing toxicity in marine environments. These acute toxicity tests have been applied to determine the toxicity of sediments [19; 20; 21] under laboratory conditions, and have been included and standardized by several national environmental agencies [22; 23].  
The chronic toxicity test with the oligochaete _T. tubifex_ was widely used for sediment bioassessment in the Great Lakes of North America and included in environmental programs [24; 25]. In Europe, _T. tubifex_ was also used for sediment toxicity assessment [26; 27; 28], as well as in bioaccumulation studies [3; 29] and organic compounds [30; 31; 32]. The followed procedure was developed by [26], and recommended in the American Society for Testing Materials (ASTM) guide [33]. Therefore, they are considered ideal for determining the toxicity of contaminants [34; 35; 36] in freshwater sediments. This bioassay was selected to assess toxicity during low tide when salinity falls down, since the tidal influence comes up to the Alcala del Rio dam (maximum recorded high tide: 1.70 m).  
The main aim of this work is to characterize the sediment quality in different areas of the Guadalquivir River affected by different levels of contamination by linking both chemical concentrations and bioassay responses using a weight of evidence approach.