Suspended Sediments, PVC Use, and Water Quality Effects Related to Geoduck Aquaculture

The aquaculture of geoduck clams in the intertidal zone has been pioneered as a commercial enterprise in the Puget Sound over the past 15 years by several growers. Environmental interactions from the culture of the clams and gear used in culture are under investigation.  As part of on-going development of best management practices for the industry, we have conducted a variety of impact studies related to the culture and harvest of geoduck clams.  Among the information collected, we are presenting results from studies designed to address two principal issues related to the culture and harvest practices: (1) the potential for the polyvinyl chloride (PVC) pipe used during the grow-out phase to leach plastics, microplastic particles, or metals used in PVC manufacture into the surrounding sediments and water, and (2) the transport and potential impacts of sediments suspended during a harvest cycle on water quality, and the potential risks of these sediments on fisheries resources.  One foot PVC drain pipe pieces are used as a predator protection method for culturing juvenile clams, and a ‘tube field’ of  up to 35,000 tubes/acre may remain in the sediment for two years during the initial grow-out phase of culture.  Sediment sampling for metals within a tube field and reference area indicated: (1) no statistically significant differences among sample areas, with the exception of beryllium and manganese, (2) beryllium was nominally, though statistically, significantly higher in the tube field, (3) manganese was significantly higher in the references area, (4) tidal height (beach elevation) was a significant source of variation for sediment metal concentrations of zinc, iron, nickel, and beryllium, and  (5) the mean concentrations of metals in sediments from both the reference and tube field were consistently below those reported across a range of sediment monitoring stations in Puget Sound, with the exception of nickel, for which the mean concentration was essentially equivalent with the mean of the tube field samples.  During an intertidal geoduck harvest, which typically occurs at low tide, sediment is loosened with hydraulic hoses to retrieve the buried clams.  Field monitoring data of past harvests indicate that water quality compliance for short-term increases is achievable within a 150 ft radius from the source generating the turbidity.  Incorporating suspended sediment data into a risk assessment model indicates the concentrations of suspended sediments in the plume along shore might lead to avoidance behavior in salmonids, but would not lead to significant physiological harm.