Summary and Conclusions

Since 1999, the Hood River Pesticide Stewardship Partnership (HRPSP) has encouraged voluntary adoption of best management practices to reduce water-quality impacts associated with pesticide use in the Hood River basin. The Oregon Department of Environmental Quality collected and analyzed pesticide samples every year since the inception of the HRPSP to monitor the distribution and concentrations of pesticides in the basin’s salmonid-bearing streams. Water sampling coincided with peak pesticide application in the basin; 90 percent of samples were collected in March through June and in September.

Seven of 10 pesticides analyzed from 1999 through 2009 were detected at least once: two triazine herbicides and five organophosphate insecticides. Most pesticide detections were at Lenz Creek at mouth and Neal Creek at mouth. Simazine was the herbicide detected most frequently and at the highest concentration. It was detected at approximately the same frequency since 2007 as prior to 2007. Simazine was present at concentrations within an order of magnitude (approximately a factor of 10) of those known to cause sublethal effects on fish and aquatic invertebrates. Azinphos-methyl was the most frequently detected insecticide and typically was measured at the highest concentrations, although the maximum detected insecticide concentration was for chlorpyrifos. Azinphos-methyl was detected at concentrations greater than the Oregon water-quality criterion for chronic exposures every year except 2008. The presence of azinphos-methyl in Lenz Creek in September 2009 indicates that effluent water from fruit-packing facilities remains a potential source of organophosphate insecticide contamination to streams.

The frequency of detection of pesticides monitored since 1999 has declined; however, the analysis of trends in detections was confounded by two main factors: (1) monitoring was not consistent across sites, years, or months, and (2) reporting limits changed within and across years. For example, reduced detection frequency could be caused by (a) true reductions of instream concentrations, (b) a decreased number of samples collected since 2007 during months when, in preceding years, pesticides were detected most frequently or were detected at the highest concentrations, or (c) increased reporting limits in later years.

Fourteen of 100 pesticides analyzed in 2009 were detected; 12 of those were analyzed for the first time in 2009. Eight of the detected pesticides were measured at low concentrations relative to those known to cause toxicity or sublethal effects to aquatic organisms or were detected too infrequently to warrant concern. Endrin was the only pesticide detected for the first time in 2009 at a concentration exceeding any national or State water-quality criterion.

Instream mixtures of pesticides can cause less-than-additive, additive, or synergistic (greater-than-additive) toxicity to aquatic organisms. Triazine herbicides and carbamate insecticides are classes known to potentiate organophosphate toxicity to aquatic invertebrates and salmonids, respectively. Thus, even at low concentrations, the presence of some detected pesticides is of concern because of their synergism with organophosphate pesticides. Simazine (triazine) and carbaryl (carbamate) were among the most common pesticides detected in mixture samples in this dataset. In 1999–2009, 12 percent of samples had two or more pesticides detected in the same sample. In 2009, 31 percent of samples had mixtures of pesticides detected. The increase in 2009 likely was due to the expanded list of pesticides analyzed that year. Both values likely under-represent the presence of pesticide mixtures in streams since many pesticides known to be used in the basin were not analyzed for this project. Effects to salmonids of the observed pesticide mixtures at concentrations detected in the basin are unknown.

Trace elements can also cause deleterious effects to salmonids, including olfactory stimulation, avoidance, and toxicity. Limited data (mostly from 1999–2002) indicate that most analyzed trace elements likely are not of concern; however, eight exceeded or were within an order of magnitude of water-quality criteria set to protect aquatic life: aluminum, cadmium, copper, iron, nickel, selenium, silver, and zinc. Factors confounding the trace element analysis include (1) data are not current, (2) sample counts were low (n = 1–2) at most sites, (3) samples were not collected throughout the year or at the same time of year, (4) water-quality criteria depend on water hardness, for which data are not available for many trace element samples, and (5) most trace element data represent the total recoverable rather than the dissolved fraction.

The lack of measurements of pesticide and trace element concentrations throughout the year with regular periodicity makes it difficult to assess potential impacts to salmonids, which are present year-round in the basin. Monitoring for particle-bound pesticides would also provide useful information. Analysis of pesticides that are commonly used in the basin but have not been analyzed for this project could be used to determine environmentally relevant mixtures of pesticides for future pesticide exposure studies on salmonids.

First posted June 17, 2011