<strong>The</strong> <strong>Geomorphology</strong> <strong>and</strong> <strong>Sediments</strong> <strong>of</strong> <strong>Cockburn</strong> <strong>Sound</strong> Figure 2: A Digital Elevation Model <strong>of</strong> the Rottnest Shelf <strong>and</strong> Perth Coastal Plain showing the major geomorphic features <strong>of</strong> <strong>Cockburn</strong> <strong>Sound</strong> <strong>and</strong> the adjacent coast. 5
<strong>The</strong> <strong>Geomorphology</strong> <strong>and</strong> <strong>Sediments</strong> <strong>of</strong> <strong>Cockburn</strong> <strong>Sound</strong> ANTHROPOGENIC CHANGES Considerable human alteration <strong>of</strong> <strong>Cockburn</strong> <strong>Sound</strong> has occurred since European settlement <strong>of</strong> the area during the 1800s. This includes industrial development <strong>and</strong> building <strong>of</strong> ship loading facilities on the mainl<strong>and</strong> coast, dredging for shipping channels within Parmelia Bank <strong>and</strong> the Eastern Shoal, <strong>and</strong> construction <strong>of</strong> a causeway across the southern opening between the mainl<strong>and</strong> <strong>and</strong> Garden Isl<strong>and</strong> (Fig. 1). Extensive industrial development in <strong>Cockburn</strong> <strong>Sound</strong> began in 1954 with the construction <strong>of</strong> an oil refinery on its eastern shore. Subsequent development has been rapid <strong>and</strong> the area now includes iron, steel, alumina <strong>and</strong> nickel refineries <strong>and</strong> processing plants, chemical <strong>and</strong> fertilizer plants, a wastewater treatment plant, electricity station <strong>and</strong> a bulk grain terminal (Department <strong>of</strong> Environment, 2005). Industrial discharge <strong>of</strong> pollutants along the east coast has included the metals Cd, Cu, Fe, Pb <strong>and</strong> Zn (Murphy, 1979; Rosman et al., 1980), organic compounds (pesticides <strong>and</strong> petroleum products) <strong>and</strong> nutrients, most notably nitrogen. Tributyltin, from anti-fouling paint, is also an issue in the southern section <strong>of</strong> the bay (DAL, 2001). <strong>The</strong>se industrial waste discharges into <strong>Cockburn</strong> <strong>Sound</strong> were made on the incorrect assumption that the waters <strong>of</strong> the sound are regularly flushed. Efforts were made to reduce pollution in industrial discharges that were identified in studies conducted in the 1970’s (Department <strong>of</strong> Conservation <strong>and</strong> Environment, 1979) <strong>and</strong> by the early 1980’s water quality had significantly improved (Department <strong>of</strong> Environment, 2005). However, water quality again declined in the late 1980’s <strong>and</strong> it was subsequently found that up to 70% <strong>of</strong> the total nitrogen load to the sound was entering the bay through the groundwater system (Department <strong>of</strong> Environmental Protection, 1996). Contamination <strong>of</strong> the groundwater by nitrogen is a significant environmental issue, with N concentrations up to 220 mg L -1 in shoreline sediment pore water <strong>and</strong> up to 130 mg L -1 in <strong>of</strong>fshore bottom sediment pore water (Smith et al., 2003). Groundwater N loads to the sound have recently been estimated as 234 ± 88 t yr -1 (Smith et al., 2003). Groundwater flows into <strong>Cockburn</strong> <strong>Sound</strong> from the east through Quaternary <strong>and</strong> recent sediments that form the regional unconfined aquifer. Discharge occurs both along the shoreline <strong>and</strong> at several <strong>of</strong>fshore sites. Significant discharge has been measured in a narrow zone along the eastern shore, while submarine discharge points have been inferred from echosounder pr<strong>of</strong>iles <strong>of</strong> the water column <strong>and</strong> bottom sediment pore water chemistry (Smith et al., 2003). However, the <strong>of</strong>fshore sites <strong>of</strong> discharge have not been directly identified on the seabed. Possibly, they are related to solutional features in the Tamala Limestone, the porous limestone bedrock (Smith et al., 2003). Large quantities <strong>of</strong> metal contaminants have been discharged into <strong>Cockburn</strong> <strong>Sound</strong> from heavy industry on the eastern shoreline. <strong>The</strong>se discharges impact the water quality <strong>and</strong> can settle out <strong>of</strong> the water column or bind with the fine fraction <strong>of</strong> surficial sediments. Analysis <strong>of</strong> sediments, compared to water monitoring, provides a longer-term assessment <strong>of</strong> the state <strong>of</strong> the marine environment. When compared with sediment quality guidelines that have been produced by the Australian <strong>and</strong> New Zeal<strong>and</strong> Environment <strong>and</strong> Conservation Council (ANZECC, 2000), this also provides a tool for the assessment <strong>of</strong> environmental health. <strong>The</strong> 1976-1979 <strong>Cockburn</strong> <strong>Sound</strong> Environmental Study found widespread metal contamination <strong>of</strong> sediments in the sound (Department <strong>of</strong> Conservation <strong>and</strong> Environment, 1979). Talbot <strong>and</strong> Chegwidden (1983) also examined the build up <strong>of</strong> several heavy metals (Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb <strong>and</strong> Zn) in sediments within Owen Anchorage, <strong>Cockburn</strong> <strong>Sound</strong> <strong>and</strong> Warnbro <strong>Sound</strong>. Anomalous <strong>and</strong> background values were estimated for each metal measured within <strong>Cockburn</strong> <strong>Sound</strong> <strong>and</strong> Owen Anchorage, <strong>and</strong> more than half <strong>of</strong> the metals in sediments from <strong>Cockburn</strong> <strong>Sound</strong> were above background levels. <strong>The</strong> contamination was found to be related to industrial discharge from Woodman Point (metropolitan sewage discharge) <strong>and</strong> James Point (steel mill, oil refinery <strong>and</strong> 6