To talk of many things;
Samples analyzed at the Food and Environmental Quality Laboratory may have their origins across the street or across the country. Field researchers collect these samples and either ship or bring them to us. When we do manage to get out of the laboratory and collect samples ourselves, it often feels like a class field trip. Such was the case last spring, when the analytical crew went to Ocean Beach, Wash. to collect sediment and water samples from Willapa Bay.
Growers treat oyster beds with carbaryl, a carbamate insecticide registered for control of burrowing shrimp. These shrimp adversely affect growth of commercially harvested oysters. In evaluation of carbaryl use rates and potential chemical alternatives for control, FEQL participated in a study sponsored by the Willapa-Grays Harbor Oyster Growers Association. Like Lewis Carroll's Walrus and Carpenter, the lab crew went down to the sea to visit the oysters.
The chemistry portion of the study was designed to examine pesticide concentration, distribution, and persistence in water and sediments. We used the insecticides carbaryl and imidacloprid, a potential alternative to carbaryl. Carbaryl, as Carbaryl 80W, was applied at 4.0 and 8.0 pounds active ingredient per acre (lb ai/A); imidacloprid, as AdmireÆ 2F, was applied at 0.25, 0.5 and 1.0 lb ai/A. Application took place at low tide on exposed sediment. Sediment was sampled immediately after application, the following day, and twice during the next month. Most water samples were collected during the first tide following application.
Of shoes‚and ships‚and sealing-wax‚
Designing a water sampling pattern for examining chemical distribution during tidal flow was an interesting problem. One possibility was to collect a set of samples, at different distances from the application site (10, 25, 50, 100 feet, etc.), at a specific time after the tide came in. This setup had two major problems: at shorter distances from the application site the pesticides would not be well mixed with the water. This added variability. The method would also require the largest number of sampling personnel, because one person would be needed at each sampling distance for each of the three treated plots.
The sampling plan used in the study put one person 500 feet from each treated site (on the shore side), collecting samples at 5 - 10 minute intervals as the tide came in. Instead of producing a 'snapshot' of pesticide distribution, this sampling plan would produce a temporal plot of pesticide movement. The distance was expected to be ample for mixing. Such a sampling scheme would give an idea of pesticide water concentrations as well as the speed at which the pesticides moved off site. We also collected water samples at 50 feet from the treated area on the day of application and directly above the plots the day after application.
Plans called for each person collecting samples to be stationed in a separate boat, with necessary labels, paperwork, ice chest, sample bottles, etc. When the day arrived for actual sampling, we took our places at the sampling stations. We stood our ground like real troopers, without chest waders and with water bottles stashed in all pockets, and looked longingly at the single boat aground on the far side of the plots. While empty water sampling bottles might work well as personal floatation devices, when full the word 'anchor' came more to mind. We managed to have enough pockets and stamina to last (despite the wind, rain and 65† air temperature ) until the water depth was nearly 3 feet.
Of cabbages‚and kings‚
We returned to the laboratory nursing colds, realized why we don't get out too often, and set to work analyzing samples. The sediment story is a short one ‚ the samples collected immediately following application showed that we applied what we had intended to apply, and at the correct concentration. Carbaryl concentrations averaged 2.8 ppm in sediment from the 8.0 lb ai/A plots, and imidacloprid concentrations averaged 0.42 ppm in sediment from the 1.0 lb ai/A plots. One day later, 97% of each pesticide was gone from the plots, and 99% was gone by the end of two weeks. Neither pesticide was found at significant concentrations the day after application at 100 to 800 foot distances from the treated plots.
And why the sea is boiling hot‚
The water samples proved to be fairly interesting. The highest pesticide concentrations were found at 50 feet from the plots but, as predicted, insufficient mixing added variability. Concentrations of carbaryl in the water varied among plots from 0.8 to 800 ppb. Imidacloprid concentrations varied from less than 0.5 to 20 ppb. Average concentrations were 280 ppb for carbaryl and 6.5 ppb for imidacloprid. Time-course sampling at the 500-foot distance proved less variable; much lower pesticide concentrations occurred here because of the large volume of water into which the pesticides were mixed. Carbaryl concentrations peaked 10 minutes after the tide water reached the sampling point, at a concentration of 13 ppb. Imidacloprid also peaked at 10 minutes; average concentration was 0.94 ppb (see figure).
Carbaryl was present in the water sample we collected prior to our pesticide applications, at a concentration of 0.7 ppb. This was most likely due to the background concentration of carbaryl in the bay. Similar concentrations were found throughout the study. Carbaryl was found in all but two of the 38 samples analyzed, including those collected one day and 28 days after applications. Imidacloprid was found in 10 of the samples, only among those collected during the first tide post-application.
And whether pigs have wings.'
Neither carbaryl nor imidacloprid were found at levels in the water that would lead to human health concerns. The drinking water standard for carbaryl is 574 ppb; imidacloprid does not have a standard, but it has similarly low mammalian toxicity. The carbaryl concentration estimated to be safe for marine organisms is 0.06 ppb (National Academies of Sciences and of Engineering, 1973). From aquatic toxicity data, we estimate the safe level for imidacloprid to be 0.4 ppb. For both insecticides, water quality criteria were slightly below the method detection limits. Thus, any detection represents a concentration above the criteria. Imidacloprid exceeded safe water concentrations in a brief period immediately following application; any impacts were expected to be confined to the application vicinity. Carbaryl concentrations were greater than protective levels in all but two samples, leading to concerns of adverse environmental impacts caused by its use.
These data suggest the need for more diversity in pest control strategies and more pesticide registrations, not fewer. Replacement of all carbaryl use in the bay by imidacloprid could easily result in regular detections of imidacloprid in the water. Without large-scale testing we cannot predict eventual background levels as compared to water quality criteria. Availability of both insecticides for control of burrowing shrimp should result in lower background levels of each. Use of a variety of products with differing modes of action would be most protective to the environment when pest control is necessary. The Walrus favored his oysters with bread and butter. Regardless of how you favor yours, there is no need for them to come at an environmental price.
Lewis Carroll's The Walrus and the Carpenter, from 'Through the Looking-Glass,'1923
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