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A pesticide is a substance, or mix of substances, used to kill or control insects, weeds, plant diseases and other pests. Commercial applicators, farmers and homeowners in the United States apply about 1.1 billion pounds a year to farmland, other land and urban areas. Pesticides are meant to help, but they can also contaminate groundwater and surface water, harming aquatic life and water supplies.
How pesticides reach water
- Point sources — spills, disposal sites, or drift while spraying — contaminate water directly.
- Non-point sources are the main route: farm and urban runoff, erosion, leaching from where pesticides were applied, and rain or snow contaminated by spraying upwind.
Figure 1. Routes pesticides can take from the land surface to surface water and groundwater. Chemigation means applying pesticides through irrigation systems.
Nevada's programme
Under the Federal Insecticide, Fungicide, and Rodenticide Act, the U.S. Environmental Protection Agency (EPA) requires every state to run a pesticide management programme. The Nevada Department of Agriculture (NDA), with EPA help, runs one of education, regulation and monitoring.
- 1997: the NDA and the U.S. Geological Survey began drilling boreholes and sampling a network of shallow wells for pesticides in groundwater.
- 2007: they added a surface-water network — the subject of this fact sheet.
Pesticides of concern. Some pesticides worry the NDA because they are widely used and they, or their breakdown products, leach easily into water, where they could approach or exceed human-health or environmental limits. A pesticide joins the NDA's list if it is detected often, detected more and more often, detected widely (in several counties), or found at more than 10 percent of its health and environmental threshold. The list is reviewed every 5 years, and a pesticide can come off it as detections fall.
An early warning. The NDA combines the low-level results with information on crops and land use into an early warning system and a best-management-practices tool, and uses them to teach applicators, farmers and the public how to cut pesticide loads early, while concentrations are still low.
How the sampling works
Passive samplers
Polar organic chemical integrative samplers (POCIS) trap polar organic compounds — including some pesticides and their breakdown products — as water flows through them. Compounds build up over time until there is enough to detect in the lab, so a POCIS shows whether a pesticide is absent or present nearby, and roughly how much.
Figure 2. A deployment canister holding three samplers on a central bracket.
From 2012 to 2019 the NDA and USGS deployed POCIS at 26 sites in 5 hydrographic basins, usually up to six sites in one watershed at a time. Sites were placed downstream of farms and towns and beside USGS stream gauges, to track streamflow while the samplers were out — usually about 30 days. Detections are assumed to come from sources upstream, and relative concentrations document which pesticides are present. Results were compared with drinking-water criteria and health advisories.
Figure 3. Pesticide monitoring areas in Nevada.
| Where | Basin | Year | Sites |
|---|---|---|---|
| Walker River (agriculture, range) | Walker | 2012, 2019 | 4, then 6 (two resampled) |
| Truckee River (urban) | Truckee | 2012 | 2 |
| Stillwater agricultural ditches and drains | Carson | 2014 | 6 |
| Humboldt River | Humboldt | 2015 | 6 |
| Virgin River* | Lower Colorado–Lake Mead | 2017 | 2 |
| Muddy River* | Lower Colorado–Lake Mead | 2017 | 1 |
| Las Vegas Wash* | Lower Colorado–Lake Mead | 2017 | 1 |
| Carson River | Carson | 2018 | 4 |
* Single "grab" samples rather than passive samplers.
Grab samples
A discrete sample, taken at one moment, is a snapshot; because pesticide use, runoff and flow vary so much, pesticides are usually below reporting levels in such samples, which is why passive samplers are preferred. But in 2017 the NDA lab's equipment broke down, so grab samples from the Virgin and Muddy rivers and the Las Vegas Wash went to the USGS National Water Quality Laboratory in Lakewood, Colorado, for 225 pesticide compounds.
Glyphosate, a widely used weedkiller in homes and on cropland, was added in 2018 to learn how homeowners and farmers use it. POCIS cannot measure it, so grab samples from the Carson River (2018) and Walker River (2019) went to the USGS pesticide laboratory in Kansas. The EPA's drinking-water limit (maximum contaminant level, MCL) for glyphosate is 700 micrograms per litre (µg/L); long-term exposure above it may damage the kidneys and reproduction.
What was found
From the passive samplers
Each POCIS was tested for nearly 80 pesticides by the NDA lab in Sparks, Nevada; results in nanograms per sampler were converted to concentrations for comparison with EPA limits and USGS health-based screening levels.
Herbicides were the most common. The three found most often are general-use weedkillers — sold without a licence — used by farmers, pest-control firms and homeowners:
| Herbicide | Share of sites | Findings |
|---|---|---|
| 2,4-D | 88% | kills broadleaf weeds but spares most grasses and crops; in every watershed except the Truckee, at least three orders of magnitude below its MCL (70 µg/L) |
| Diuron | 69% | every watershed except the Truckee, mostly in main river channels below farmland (9 of 12 river sites, leaving out the six Stillwater ditches and drains); all four Walker River sites in 2012 (0.0017–0.0122 µg/L), none there in 2019; at least four orders of magnitude below its health advisory (100 µg/L) |
| Hexazinone | 46% | every river except the Truckee and Humboldt, far below its health advisory of 2,000 µg/L |
- Atrazine, a restricted-use herbicide (it needs an applicator's licence), turned up at most Walker River sites, at least two orders of magnitude below its 3 µg/L MCL.
- Bromacil, a general-use herbicide, was found in four of the six farm canals and drainage ditches near Fallon, including the highest concentration any sampler recorded: 0.0627 µg/L.
- In the Truckee River only atrazine and bromacil appeared, at least four orders of magnitude below EPA health advisories.
Insecticides on the NDA list were found less often:
- Imidacloprid (general use) in the main Walker and Carson channels below towns, at 0.0001–0.0106 µg/L.
- Oxamyl (restricted use) only in 2019, in the four main-stem Walker River samples below Mason, the highest four orders of magnitude below its health advisory of 35 µg/L.
The most detections in one sampler came from Eagle Valley Creek, a Carson River tributary carrying urban runoff from Carson City, sampled in 2018: six of the seven pesticides of concern — five herbicides and one insecticide — all well below EPA limits.
From the grab samples
- 2017: nothing above reporting levels in the Virgin or Muddy rivers. The Las Vegas Wash — a large urban return-flow channel into Lake Mead — held three insecticides and a herbicide: acephate (0.0834 µg/L), fipronil (0.0045), imidacloprid (0.0331) and terbuthylazine (0.0118). None has an EPA MCL or health advisory.
- Glyphosate was found at three sites each on the Carson and Walker rivers, from 0.02 to 2.9 µg/L — the highest in Eagle Valley Creek — far below the 700 µg/L MCL.
Sources
- Huntington, J.M. (U.S. Geological Survey), Entz, D.C. (Nevada Department of Agriculture), and Thodal, C.E. (retired, U.S. Geological Survey), 2020, Early warning pesticide monitoring in Nevada's surface waters: U.S. Geological Survey Fact Sheet 2020–3070. https://pubs.usgs.gov/publication/fs20203070
- The figures, the site table and the reference levels come from the fact sheet's PDF. The site table's "principal land use" column holds numbers for six rows; those entries are left out.
- Words on this page from people and organisations outside the federal government are paraphrased; rewritten in hubnx's own words.
Licença: CC0 1.0 (domínio público) · Adaptado de pubs.usgs.gov
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