# Residual Level of Chlorpyrifos in Lettuces Grown on Chlorpyrifos-Treated Soils
## 1 Introduction  
Pesticides are generally utilized to manage agricultural pests, such as fungi, insects, and weeds, in order to increase crop productivity. Chlorpyrifos is a broad-spectrum chlorine-containing organophosphorus pesticide that is used for controlling agricultural and household insect pests [1, 2]. This compound possesses strong inhibition on acetylcholinesterase adhesive-active activity and is also potentially toxic to non-target animals or humans [3]. With the results of some studies appraising the safety of chlorpyrifos for mammals, it was found that chlorpyrifos might be related to various health disorders [4, 5]. Chlorpyrifos is registered in Korea for formulating a single or mixed product with some pyrethroids to control mites, aphids, and lepidopteran insects on apples, grapes, pears, peaches, cucumbers, cabbage, and mandarins [6]. After pesticides are applied, they can retain in the crop; thus, residuals have caused anxiety amongst consumers. In particular, foliar-sprayed pesticides can remain on the leaves of the target plant or drop onto the nearby soil, while the crop is growing [7]. The levels of pesticides in cultivation soils are periodically monitored by the government agency. Three pesticides such as endosulfan, procymidone, and chlorpyrifos, were frequently detected in Korean upland soil. In addition to thisresult, chlorpyrifos has also been shown to be one of the most detected pesticides in Korean orchard soil and in China [8; 9]. Results from a four-year analysis of pesticide residues in Good Agricultural Practices (GAP) certified agricultural products reported that chlorpyrifos was one of the pesticides that showed over a 5% detection rate from samples analyzed. Besides, chlorpyrifos is one of the violating compounds that were not registered on the detected crops [10]. These results could have been due to the use of an unregistered pesticide on the crops or translocation from soil contaminated with the pesticide because some of the detected pesticides were systemic chemicals [11]. In addition to the concern of residual unregistered pesticides in crops, the Positive List System (PLS) has been introduced. In order to ensure the safety of the consumption of foodstuffs presented in pesticide residues, the maximum residue limits (MRL), which is a non-risk level, has been established for each pesticide and crop by scientific evaluation using toxicity data and food intake. However, since MRL values for the registered pesticides are varied for each crop, it is hard to secure safety when the pesticide, that is not registered for use, is sprayed or left in a crop. Therefore, the PLS has been used on unregistered pesticides for crops on a non-detected basis and the standard of non-detection in PLS is established as 0.01 ppm (mg/kg) or less. In this regard, various reports have well-documented the levels of retained pesticides in crops/plants translocated from contaminated soils [12; 13; 14; 15; 16; 17; 18], especially for persistent organochlorine insecticides. The long-term persistence of chlorpyrifos results from its high stability in acidic and neutral pH conditions, while hydrolytic degradation is more rapid at alkaline pH conditions [19]. It is persistent in conditions with organic matter, leaf characteristics, decreased temperature, and ultraviolet light [20; 21]. In addition to this persistence of chlorpyrifos, other parameters can affect its uptake, such as soil type. Despite the observed effect of soil type on crop uptake, there have been no apparent means of estimating the absorbed level of contaminants in various plants in different types of soils [16; 17; 22; 23]. The uptake of \(O\)-methylcarbamoyloloximes and substituted ureas with numerous lipophilic from water solutions was studied by using barley roots [24]. They concluded that root factors (RCFs, C\({}_{\text{root}}\)/C\({}_{\text{water}}\)) and translocation were not proportional to the octanol-water partition coefficient (\(K_{\text{ow}}\)) values of those compounds.