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Dacthal and Dacthal Degradates

Treatment Processes
Fate and Transport
Chemical Bonds
Chemical Bonds1476
CAS Number: 1861-32-1
Synonyms: DCPA, Dimethyl tetrachloroterephthalate, MTP, TPA, monomethyl tetrachlorophthalate, tetrachlororterephthalic acid
Contaminant Type: Chemical

Dacthal also known as DCPA, is a pre-emergence herbicide used to prevent the sprouting of seeds of annual grasses in turf and lawns, as well as in some vegetable crops including strawberries, onions and beans. [1473, 1474, 1475] Dacthal is a man-made organic chemical and does not occur naturally. It has been used since the 1960's. As of 1990, more than 80 percent of its use was for turf, including golf courses and home lawns. [7473] Available data indicate that Dacthal use declined significantly over the course of the 1990s. On July 27, 2005, in response to concerns about groundwater contamination (especially for one of the Dacthal degradates), the registrant voluntarily terminated most uses for products containing Dacthal. Dacthal is currently registered only for use on sweet potatoes, eggplant, kale, and turnips. [1475]


Dacthal is chemically known as dimethyl tetrachloroterephthalate (DCPA), a synthetic organic compound (SOC) marketed under the trade name "Dacthal". [1475]


Dacthal has been found at generally low levels in both surface and groundwater in various parts of the country. [1473] Dacthal is not particularly mobile or persistent in the environment. Biodegradation and volatilization are the primary dissipation routes. [1475]


Degradation of Dacthal forms two breakdown products, the mono-acid degradate (monomethyl tetrachloroterephthalate or MTP) and the di-acid degradate (tetrachloroterephthalic acid or TPA). The di-acid, which is the major degradate, is mobile and persistent in the field, with a potential to leach into water. [1475] Dacthal is not very mobile in soil, however, its degradates are suspected to move rapidly through the soil to the groundwater. [1474]


MTP and TPA are metabolites of Dacthal in humans and animals. Dacthal appears to be poorly adsorbed from the gastrointestinal tract in humans and animals. Low solubility in aqueous media would contribute to its poor intestinal uptake. Metabolically, Dacthal is converted first to MTP and them to TPA most likely by nonspecific esterases. The solubility of the degradates is likely to be greater than that for Dacthal since acids tend to be more soluble than esters. [1476]


Little is known about the distribution of Dacthal or its metabolites. The target organs for Dacthal suggest distributions to the liver, kidney, thyroid and lungs. Limited data from animal studies indicate that there is distribution for the metabolites to the liver and kidney; parent compound has been detected in adipose tissue. Most orally ingested Dacthal is excreted in the feces, unabsorbed; TPA and MTP have been identified in the urine of humans and animals treated with Dacthal; the levels of MTP far exceed this of TPA. There have been no toxicokinetic studies of MTP or TPA. TPA does not degrade in living systems based on projections of the META metabolism and biodegradation modeling program. [1476]


The limited studies referenced in [1476] indicate no observable effects in humans due to Dacthal.


Date of Literature Search: June 2009

1473 Department of Human Services Environmental Toxicology Section ; 1989; Health Effects Information - Dacthal; Oregon Department of Human Services; Input May 2009
1474 Health and Ecological Criteria Division (DECD); 2008; Summary from Health Advisory for Dacthal and Dacthal Degradates - Document Number 822-S-08-002; OW, U.S. EPA; Input May 2009
1475 US EPA; 2008; Regulatory Determinations Support Document for Selected Contaminants from the Secondary Drinking Water Contaminant Candidate List (CCL2); EPA Report 815-R-08-012; Input May 2009
1476 Health and Ecological Criteria Division; 2008; Drinking Water Health Advisory for Dacthal and Dacthal Degradates; U.S. EPA; Input May 2009