Spraying Buffelgrass From the Air at Saguaro National Park: Where Does the Herbicide Go?

The Rincon Mountain District of Saguaro National Park, with buffelgrass mapped in 2012. USGS map.

A fire-prone invader

Buffelgrass (Cenchrus ciliaris) threatens desert ecosystems. The most common type in southern Arizona came from the Turkana region of northern Kenya and was planted for erosion control, near Saguaro National Park in the 1970s and 1980s.

• Spread: first recorded in the park in 1989 and seen as a threat by the early 1990s, it covered more than 2,000 acres by 2012, growing more than 25 percent a year.
• Why it's a problem: it grows fast on rocky hillsides and in washes, spreads by seed on wind, water and animals, and burns easily and hot, crowding out native plants and forming wildfire corridors. The Sonoran Desert historically burned rarely, in patchy, low-intensity fires. Big buffelgrass fires put saguaros and other long-lived species at risk, speed erosion, and can destroy scarce aquatic habitat that species such as the lowland leopard frog depend on.

Buffelgrass. Image from the USGS fact sheet.

The treatment, and the question

After a 2010 trial, the park began spraying glyphosate-based herbicides from the air; in 2014, more than 250 acres in the Rincon Mountain District, east of Tucson, were treated. It worked, and continues — but its effects on aquatic ecosystems had never been fully assessed.

Glyphosate itself hasn't proved highly toxic to tested animals, but commercial formulations include surfactants that can harm wildlife, especially amphibians. Surfactants are hard to measure, so the study used glyphosate as a stand-in for them.

The study

From 2015 to 2018, the USGS and the park sampled soil, surface water and fine sediment (silt and muck) in three treated watersheds — Box Canyon, Madrona Canyon and Loma Verde — to trace glyphosate and its main breakdown product, AMPA, from treated slopes to the bedrock pools (tinajas) and washes below.

Areas sprayed and detections of glyphosate-based herbicide in the Box Canyon and Madrona Canyon watersheds. USGS figure.

What they found

• Water: at normal low flow, detections were rare — about 19 percent of samples (9 of 48) had glyphosate, AMPA or both, six of them from one small, heavily treated Box Canyon tributary (Pool 00), usually within a few months of spraying. Three more came during runoff in Madrona Canyon (summer 2017) and Box Canyon (winter 2018).
• Sediment: roughly 25 percent (7 of 28) had detections, mostly in two Box Canyon pool areas below heavily treated slopes — and one in Lower Madrona Canyon pool 02, known lowland leopard frog habitat.
• Soil: Loma Verde, untreated for 592 days, still had very low but measurable levels. In twice-sampled Box Canyon areas, half the glyphosate disappeared in about 75 days, and half the AMPA in about 125 days. The more ground sprayed, the more likely detections downstream, especially after storms.
• Levels: the highest water concentration was 0.2 μg/L, and about 90 percent of samples were below the detection limit of 0.02 μg/L — far below the drinking water limit (700 μg/L) and chronic benchmarks for fish and invertebrates (25,700 and 49,900 μg/L).

Water stage in Madrona Canyon with the timing of detections and aerial spraying. USGS figure.

What it means

Very little glyphosate moved downstream relative to what was sprayed. It appears to bind to fine sediment that stays near the surface or settles out in pools, limiting transport far downstream or into groundwater. Compared with urban and agricultural areas, median and maximum levels here were much lower — the park's once-a-year spraying in steep, rocky, semi-arid terrain may prolong persistence but limits transport.

Still, little is known about the effects of low, intermittent exposure, from both water and sediment, in a desert aquatic ecosystem — and surfactants, more harmful than glyphosate, weren't measured. The concentrations found are likely of low toxicity to the lowland leopard frog, but laboratory studies at realistic levels could show how the timing of exposure affects its metamorphosis.

A lowland leopard frog. Image from the USGS fact sheet.

More: Paretti and others (2021), Occurrence, fate, and transport of aerially applied herbicides to control invasive buffelgrass within Saguaro National Park Rincon Mountain District, Arizona, 2015–18, USGS Scientific Investigations Report 2021–5039.

Sources

Based on Nicholas V. Paretti and Bruce Gungle, "Occurrence and Transport of Aerially Applied Herbicides to Control Invasive Buffelgrass in Rincon Mountain District, Saguaro National Park, Arizona," Fact Sheet 2022–3029, July 2022, U.S. Geological Survey; a work of the United States government in the public domain. The maps, graph and photographs are restored from the fact sheet's PDF; a chart using a stock photograph is not reproduced.