Welcome Slough is located on Puget Island in Wahkiakum County in the lower Columbia River estuary in Washington. Welcome Slough is a narrow, canal-like waterbody, closed at one end and opening to the Columbia River at the other. It is tidally influenced. In recent years, an infestation of the state-listed noxious freshwater weed Eurasian watermilfoil (Myriophyllum spicatum) and to a lesser extent native Canadian waterweed (Elodea Canadensis) has been impacting navigation, recreation, and aesthetics in the slough. At least 50 recreational boats and 13 commercial fishing vessels are moored in Welcome Slough and their owners reported significant problems in navigating out of the Slough. Because of these problems, Wahkiakum County sponsored the development of an integrated aquatic vegetation management plan for this area. At the time of this herbicide application, this plan had not yet been finalized, but unhappy residents pushed to have something done about the abundance of plants in 2003. To provide some relief for these residents, Wahkiakum County agreed to pay for an herbicide treatment (diquat) in 2003.
Because slough areas in the Columbia are known to be rearing areas for juvenile salmon, the Washington Department of Fish and Wildlife requested that the local citizens monitor water conditions before and after the herbicide treatment. The selected herbicide – diquat - is a contact herbicide that can rapidly burn back vegetation and the decomposing vegetation can sometimes lead to low oxygen concentrations with resultant fish kills. The purpose of monitoring was to document the effect of the diquat application on the dissolved oxygen concentrations in Welcome Slough.
Fish and Wildlife also requested that the applicator start his application at the end of the slough farthest from the river working towards the mouth of the slough. Treating in this pattern provided fish the opportunity to avoid the herbicide by moving out towards the Columbia River.
In August 22, 2003, 22.4 acres of the littoral zone of Welcome Slough were treated with the aquatic herbicide diquat (Reward®). Diquat was applied by a state-licensed applicator using sub-surface injection. A total of 44 gallons was applied. The application was timed to coincide with neap tides to increase the contact time between the herbicide and plants. No tidal exchange data have been collected from Welcome Slough, although the depth data from each station over the sampling time clearly shows the tidal influence. A group of citizen volunteers who live on Welcome Slough and were proponents for the herbicide treatment monitored Welcome Slough for dissolved oxygen (DO), water temperature, and presence of distressed or dead fish for two weeks after the herbicide application. The aquatic plant management company who applied the herbicide developed the monitoring protocol, trained the volunteers in instrument calibration and monitoring techniques, and loaned them the monitoring equipment.
Water: Four sampling stations were established at equal distances along Welcome Slough from the end of the slough to the mouth. Station 1 was located at the end of the slough furthest from the Columbia River and Station 4 was located at the mouth of the slough closest to the Columbia River. Station 2 and 3 were approximately equidistance between Stations 1 and 4 (see map).
Dissolved oxygen and temperature were measured at each station at approximately the same time each day immediately before treatment and daily for 13 days following herbicide application. At each station, a water column profile was taken from the surface and at one foot intervals to the bottom. These measurements were taken with a YSI® digital DO/Temperature meter. The data were recorded on field sampling sheets provided by the applicator. The volunteers were instructed to notify the applicator and County staff if there was a significant sag in DO levels from those observed prior to treatment. The depth at each station varied due to tidal fluctuations.
Fish: In addition to taking water measurements, the volunteers patrolled the waters of Welcome Slough by boat looking for dead or distressed fish. They were instructed to record the location of any dead or distressed fish, the number of fish observed, and the species (if known). In addition, they were asked to determine if the mortality was recent, to collect a sample, and to notify the applicator and the County.
The DO and temperature data from each Station is summarized in the tables below. Changes in maximum depth over time illustrate the large tidal influence on Welcome Slough. There was a 4-6 foot variation in maximum depth over the course of the monitoring. Because of this change in maximum depth, only the surface values at one foot were graphed for DO and temperature. The diquat application caused a measurable decrease in DO at stations 1-3 in the days following treatment. As the sampling sites moved away from the Columbia River, the DO decreased, with station 1 having lower DO than station 2; station 2 having lower DO than station 3; and station 3 having a lower DO than station 4. The DO concentrations at Station 4, closest to the Columbia, appear to have been very slightly depressed in the week after treatment. The low value at that station on day 8 may have been due to increasing water temperature at that station. Temperature appeared to be unaffected by the diquat treatment.
Washington has set a Water Quality Standard for a minimum DO concentration for surface water of 9.5 mg/L for salmon core areas. Dissolved oxygen in August was below this standard in Welcome Slough even before the diquat treatment. However, salmon can survive in 5 mg/L DO. Temperatures in Welcome Slough varied little between most sampling areas, but always exceeded the Water Quality Standard for salmon core rearing areas (16 degrees C).
Fish Observations: There were no distressed or dead fish observed during this two week period in Welcome Slough. It is possible that no salmonids were in the area or that they moved out of the slough into the Columbia River. The University of Washington will be conducting research to determine whether salmonids can detect and avoid herbicides such as diquat in 2004. The temperature and DO in Welcome Slough were not ideal for salmon prior to treatment so they may not utilize this area during the summer.
Efficacy: By the third day after treatment, volunteer monitor Jack McBride observed that the tips of the milfoil plants were starting to break off and the plant looked stressed. By the fifth day after treatment, Jack reported that the milfoil was starting to thin out and that the water was looking more clear. By the 2nd week after treatment, Jack reported that a high percentage of the Eurasian watermilfoil was effectively treated and that the slough was now clean and clear.
Station 1 – Dissolved oxygen - ppm
Station 2 – Dissolved oxygen - ppm
Station 3 - Dissolved Oxygen – ppm
Station 4 – Dissolved Oxygen - ppm
Station 1 – Temperature – °C
Station 2 – Temperature - °C
Station 3 – Temperature - °C
Station 4 – Temperature - °C
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