Page 179 - Soil Degradation, Conservation and Remediation

168 6 Soil Pollution

• Hexachlorocyclohexane (lindane)

• DDT and related compounds, DDE and DDD

• Cyclodienes (aldrin, heptachlor, and others)
• Mirex and chlordecone
Eleven specific persistent pesticides within these groups were measured
during the Churchill County leukemia study (16 children diagnosed with acute
leukemia between 1997 and 2002 lived in Churchill County, Nevada; the probable
association of persistent pesticide residues could not, however, be proved; Rubin
et al. 2007 ): beta-hexachlorocyclohexane, gamma-hexachlorocyclohexane (lindane),
hexachlorobenzene, dieldrin, mirex, heptachlor epoxide, oxychlordane, trans -
nonachlor, o , p ,-DDT, p , p ,-DDT, and p , p ,-DDE.
Herbicides are applied to the soil in hopes of obtaining season-long weed control.
It is desirable for the chemicals to control weeds during the season of application,
but they should not remain long enough to affect subsequent crop growth. The length
of time that a herbicide remains active in the soil is called “soil persistence” or “soil
residual life.” Anything that affects the disappearance or breakdown of a herbicide
affects persistence. Many factors determine the length of time herbicides persist.
Most factors fall into three categories: soil factors, climatic conditions, and herbicidal
properties. These categories strongly interact with one another. Herbicides vary in
their potential to persist in the soil. Some herbicide families that have persistent
members include the triazines, uracils, phenylureas, sulfonylureas, dinitroanilines,
pigment inhibitors, imidazolinones, and certain plant-growth regulators. Several
common herbicides in these groups are shown below.
• S - triazines : atrazine (AAtrex, Atrazine), hexazinone (Velpar), prometon (Pramitol),
and simazine (Princep)

• Dinitroanilines : benefin (Balan), oryzalin (Surflan), pendimethalin (Pendimax,

Prowl), prodiamine (Barricade), and trifl uralin (Treflan, Tri-4, TRILIN)

• Others : bensulide (Betasan, Prefar), clomazone (Command), and tebuthiuron (Spike)
• Phenylureas : diuron (Karmex, Direx)
• Uracils : bromacil (Hyvar X) and terbacil (Sinbar)
• Imidazolinones : imazapyr (Arsenal), imazaquin (Scepter), and imazethapyr (Pursuit)
• Sulfonylureas : chlorimuron (Classic), chlorsulfuron (Telar), nicosulfuron
(Accent), primisulfuron (Beacon), prosulfuron (Peak), and sulfometuron (Oust)
• Plant-growth regulators : clopyralid (Stinger), picloram (Tordon), triclopyr (Garlon),
and chlorimuron
Pesticide persistence in soil is not the property of the pesticides alone; it also
depends on soils, climate, and management. At higher soil pH, persistence of some
pesticides decreases. At lower pH, the persistence of clomazone and the imidazolinones
(imazaquin and imazethapyr) decreases. Soil pH has little effect on the persistence
of some other herbicides. Various nutrients and cations in the soil affect both
herbicide activity and degradation. The CEC, principally a function of clay- type
and organic matter content, is directly involved in herbicide adsorption. Some
herbicides are more available in the presence of certain cations, whereas others may