Chemicals Remaining from Sugar Cane Cultivation

• According to Crop Profile for Sugarcane in Hawaii, cane is drip-line fertigated with N, P, K, S and Ca.
• Prior to tillage [..] perennial weeds, such as guineagrass are removed chemically with glyphosate or hexazinone, or mechanically with a backhoe.  After the drip system is installed, a preemergence herbicide mix, consisting of one or more herbicides for broad-spectrum weed control (usually pendimethalin and atrazine), is broadcast by tractor.  [..] Field edges may receive additional applications (usually glyphosate and/or 2,4-D) [..] for easy maintenance and service.  Herbicides account for 97% of all pesticide use in Hawaii’s sugarcane. Five herbicides, atrazine, diuron, pendimethalin, glyphosate and ametryn, represent 80% of the total pesticide usage. Other herbicides used in significant amounts are trifluralin, 2,4-D and hexazinone.
• Historically, a few additional chemicals were used, although phased out by the 1990s: simazine (Princep), metribuzin (Lexone or Sencor) terbacil (Sinbar), asulam (Asulox), dicamba (Banvel) and metsulfuron (Ally).
• The final harvest was in 1993, so there is at least 16 years since last application of chemicals.
• A useful resource: EXTOXNET Extension Toxicology Network

2,4-D (extoxnet) (wikip)	Half-life is 7-10 days in soil, other studies give 1.5 to 16 days. Major component (about 50%) of Agent Orange. Only kills dicots, leaving behind monocots.
Ametryn (extoxnet)	Half-life is 70 to 250 days, depending on the soil type and weather conditions.  Ametryn is broken down into non-toxic substances by tolerant plants.
Atrazine (extoxnet) (wikip)	Half-life is 13 to 261 days, other source says "60 to 100+ days". Banned in the EU. Harmful to amphibians.
Diuron (extoxnet) (wikip)	Residues in soil are toxic to plants. Residue levels are lower in soils with low organic content. Residue half-lives are from one month to one year.
Glyphosate (extoxnet) (wikip)	Aka. Roundup. Tightly bound to the soil, little is transferred by rain. Beakdown primarily by microbes.  Half-life from "1 to 174 days", some sources say average of 2 months, others show at little as 3 days in warm climates.  Toxicity to animals is very low.
Hexazinone (extoxnet)	Half-life is 1-6 months depending on the climate and soil type.
Pendimethalin (extoxnet)	Half-life is 90 days in soil. Adsorbs strongly to soil organic matter and clay and does not leach through the soil.
Trifluralin (extoxnet)	The half-life 45 to 60 days (6) or about 6-8 months (2.5 kg/ha) (8). After six months to one year, 80- 90% of its activity will be gone (5).

• Given these half-lives, after 16 years in a warm climate with high rainfall, erosion, and strong solar radiation, there should not be detectable levels of any of the herbicides.  Also, most of them are not directly toxic to humans, so any tiny residue would be of concern only to plants.