Nevertheless, Al and Ni concentrations cannot be linked logically to the spatial relationships identified in the sediment or to impacts arising from the LACM spill. Although floodplains are known accumulation zones for sediment, this study found that surface floodplain samples exhibited lower total Cu concentrations (Max = 180 mg/kg, GM = 50 mg/kg – Table 2) compared to channel surface samples CDK activation (Max = 540 mg/kg, GM = 63 mg/kg – Table 1). This pattern of higher metal values in channel sediment than floodplain materials is the reverse of what Taylor and Hudson-Edwards (2008) reported from the much bigger ephemeral Leichhardt River, at Mount

Isa, some 140 km to the south-east. Given that the Saga and Inca creeks rise in a semi-arid environment, this system is also characterised by short periods of flooding and longer periods of limited or no water flow. According to

Ladd et al. (1998), under such conditions slack water drapes of fine-grained material can form, covering coarser deposits in channel beds, which may act as storage zones for heavy metals and result in localised zones of contamination (Hudson-Edwards et al., 2005). Thus, targeting the sampling from these sediment accumulation zones may have contributed to the finding HSP inhibitor that channel sediment-metal concentrations were higher compared to floodplain deposits. Measurement of the lateral (up to a maximum of 200 m wide) and vertical (0–2 cm) footprint of floodplain Cu deposition from the LACM spill within the Saga and Inca systems allows the total volume of contaminated floodplain sediment to be estimated at 41,300 m3 (∼16.5 Olympic swimming pools); benchmarked relative to the ISQG – low guideline values (ANZECC and ARMCANZ, 2000).

Floodplain surface sediments (0–2 cm) are significant because they are the most accessible component to cattle and native animals. Stone and Droppo (1996) assessed the distribution of Cu, Pb and Zn in agricultural catchments of southern Ontario, Canada, 3-mercaptopyruvate sulfurtransferase and showed that the potential sediment-metal bioavailability increased with decreasing grain size. Although grain size studies were not undertaken specifically in the Saga and Inca creek catchment, it is well documented that fine-grained sediment is the dominant particle size fraction of floodplain alluvium (Brewer and Taylor, 1997, Graf et al., 1991, Marron, 1989, Moore et al., 1989, Reneau et al., 2004, Taylor, 1996 and Walling and Owens, 2003). In contrast, coarse fractions generally relate to bed load sediment deposited in the channel (Malmon et al., 2002). Therefore, despite the lower floodplain sediment Cu values, it is reasonable to hypothesise that the accidental ingestion of fine-grained floodplain surface sediment (0–2 cm) during grazing poses the greatest potential risk to cattle compared to channel sediment-metals, in part because of the longer time spent grazing than drinking water.