Estimating Impacts 



One approach to estimating how much of an impact oil and gas activities 

 could have is to make a simple calculation (following an earlier suggestion to 

 consider worst-case scenarios). To do so, we assume a reasonable discharge of 

 contaminants that are transported into a small area at the canyon head or 

 axis. The accumulation is then assumed to be evenly distributed into a thin 

 layer of surface sediment and the resulting concentration is compared with 

 background concentrations. 



We assumed the contaminants would not settle disproportionately near the 

 drill site, but would be spread evenly throughout the 1 km^. We also assumed 

 that the materials would be mixed into a 1-cm thick surface area. (Biological 

 mixing of materials within sediments is often deeper than this, and in regions 

 of high current velocities, physical processes can mix sediments deeper than 1 

 cm. 



All three assumptions (area covered, depth of mixing, and amount 

 transported) yield a worse-case result--because the area covered and depth of 

 mixing is less then actually expected, and the amount transported is greater 

 than actually expected. 



Based on these worst-case assumptions, however, we can consider the 

 potential elevation of the metal chromium present in drilling mud at a 

 concentration of 200 mg/kg. If we assume a total discharge of 1 million kg of 

 drilling mud/well, evenly distributed over 1 square km of canyon floor, and 

 mixed to a depth of 1 cm, that would add 20 ppm to the chromium content of the 

 top centimeter of sediment. Background values average 45 ppm (ranging from 35 

 to 80 ppm) in Lydonia Canyon, so the increase is on the order of 45 percent. 

 However, this increase assumes that all of the drilling mud reaches the 

 canyon, and all of it is deposited in 1 square kilometer. For perspective, 

 consider that the world average chromium concentration in crustal rocks is 100 

 ppm. 



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