University of California, Riverside Cooperative Extension



A Quarterly Publication of the Department of Environmental Sciences

Winter 2002

Pesticide and Groundwater Quality


By Dr. Jay Gan

Cooperative Extension Water Quality Specialist



Nowadays it is an enormous challenge to use pesticides while maintaining water quality. On one hand, we all recognize the significant role of pesticides in ensuring a high quality of life, and acknowledge that the effective production of food and fiber relies on pesticides to control weeds, insects, and plant diseases. At the same time, we also need to be aware that pesticide application has affected and still can affect water quality. 

“Pesticides and Groundwater Quality" is a complex subject. I plan to address this topic in two separate articles: "Pesticides and Groundwater Quality" (this issue) and "How to Reduce Pesticide Leaching" (next issue). In this article, I will start with a little overview of the current state, and discuss briefly the processes that drive pesticide downward movement (or leaching) through soil.


National Picture


The detection of DBCP (1,2-dibromo-3-chloropropane) in thousands of wells in California over twenty years ago sparked an era of extensive research and monitoring for pesticides in the groundwater throughout the nation. Over the last two decades, through its National Water-Quality Assessment (NAWQA) program, the U.S. Geological Survey (USGS) has completed extensive monitoring projects.  Results show that pesticides are widespread in groundwater nationwide. The following conclusions can be made from these studies (Gilliom, 2001):


         In agricultural areas, at least one pesticide or metabolite was found in 61% of the monitored   wells.


         In urban regions, at least one pesticide or metabolite was found in 54% of the monitored wells.


         Pesticides were detected more frequently in shallow aquifers than major (deep) aquifers.


         About 25% of samples from wells in urban and agricultural areas contained two or more detectable pesticides.


         In most wells with detectable pesticides, concentrations were substantially below EPA's drinking-water standards, which were rarely exceeded.


California Picture


The state of California has arguably the most comprehensive pesticide regulations. This has occurred partly because of the widespread detection of DBCP and EBD throughout the state. Even twenty years after their cancellation, residues of DBCP and EDB are still being detected in aquifers in many regions. DBCP and EDB were once widely used soil fumigants. California Department of Pesticide Regulation (CDPR) and State Water Resources Control Board (SWRCB) have been monitoring for pesticides in groundwater throughout the state since 1984. The annual detection rates, as % of monitored wells, are graphed in Figure 1 (CDPR, 2002). These detection rates are apparently lower than the national average. This may be due the specific geophysical conditions, the mixture of pesticides used, and the regulatory practices in California.  In the state of California, the detection rate has been hovering around 10% over the last few years.


Although hundreds of pesticide active ingredients are used each year in California, only a limited few pesticides have ever appeared in the groundwater through monitoring. Table 1 summarizes the up to date detections of different pesticides in California wells since 1984.


Among the detected pesticides, DBCP, EDB, and 1,2-D were phased out by the early 1980s. The other pesticides, without any exception, are pre- or post-emergence herbicides.  To be an effective pre- or post-emergence herbicide, the product must be weakly adsorbing to soil and persistent so that when plants emerge and develop, pesticide residues are absorbed from the soil solution by plant roots (Trioano et al., 2001). This implies that it may be difficult to replace these herbicides with other herbicides, because the weak adsorption and long persistence are necessary for achieving weed control.


In California, these pesticides are placed by CDPR on the "Groundwater Protection List" list, or the "leachers" list.  One important approach for CDPR to protect groundwater from being contaminated by these products is the development of Pesticide Management Zones (PMZs). A PMZ is a land area where a pesticide has been detected in groundwater and where it has been determined that the contamination was due to legal agricultural use. Pesticide Management Zones have been established in various areas for atrazine, simazine, diuron, prometon, and simazine.  The total use of these potentially groundwater-contaminating pesticides in California since 1992 is shown in Figure 2 as total lbs of active ingredients used and total treatment areas in acres.  The overall use of these chemicals has been relatively steady, with over 2 million lbs being used each year for the last ten years or so.


Why Leachers?


Why is groundwater more vulnerable to these chemicals? Or why are these chemicals "leachers"?   Roughly speaking, these pesticides enter groundwater because of their specific properties, which give them a high leaching potential. Leaching potential is mostly a result of adsorption and persistence.  Adsorption indicates how strongly a chemical "sticks" to the soil while moving down with water.  Persistence is a measure of how long the chemical stays in its original form in soil.  Simply put, a pesticide that does not adsorb to soil readily but has a long persistence is a good candidate for leaching.


Adsorption:  What controls a pesticide's adsorption?  Pesticide adsorption is a property of the pesticide itself. Typically, as the water solubility increases, adsorption of a pesticide in soil decreases, simply because water-soluble pesticides tend to stay in the soil solution. Pesticide adsorption is also a function of soil properties, especially soil organic matter content. It is generally observed that pesticide adsorption decreases as soil organic matter content decreases. In sandy soils or subsurface soils, pesticides tend to move more freely due to weaker adsorption. When sandy soil texture is coupled with a shallow groundwater table, risks for pesticides to contaminate groundwater are higher. Soil textures and groundwater depths are two important factors that CDPR used for defining Pesticide Management Zones (PMZs) in California after the Pesticide Contamination Prevention Act (PCPA) was enacted in 1986 (Troiano et al., 2001).  Adsorption is generally expressed as Koc (mL/g or L/kg).  The greater the K, the stronger will be the adsorption ability of the pesticide.


Persistence: Like most other things that are organic, pesticides are not stable in soil and can undergo changes under soil microbial and chemical "attacks". This process is referred as "transformation" or "degradation". Degradation typically detoxifies a pesticide, and therefore is a desirable process from the perspective of environmental safety.  However, fast degradation may render a pesticide ineffective for pest control.  How to balance these two interests is a delicate question for those chemists who design pesticide molecules. Persistence is inversely correlated with pesticide degradation rate. Persistence depends closely on the properties of the pesticide itself.  Some pesticides are made more stable, and some are made more susceptible to degradation. The persistence of pesticides in soil is measured by dissipation half-life (DT1/2) - the time that it takes for a pesticide to decrease from its original concentration to a half of the original concentration. Apparently, the longer the persistence, the better will be the possibility for a pesticide to survive its journey through soil and reach the groundwater.


"GUS"   The importance of adsorption and persistence can be illustrated through the Groundwater Ubiquity Score (GUS) index (Gustafson, 1993). GUS is calculated using the following simple equation:


GUS = log(DT50) × (4 - log(Koc))


If GUS is > 2.8, the pesticide will likely be a "leacher".  If GUS is < 1.8, the pesticide will be a "non-leacher".  If 1.8 < GUS < 2.8, the pesticide is considered as a "marginal leacher". This model is an empirical regression. It incorporates only the properties of pesticides, and no information from the soil. Therefore, GUS indicates the intrinsic mobility of pesticides.


From above, pesticides that have been found in California groundwater and those that are included in the Groundwater Protection List are all relatively persistent but weakly adsorbed to soil (Table 1), with DBCP and EDB as the extreme examples.  The GUS indices for these pesticides are listed in Table 1 along with the Koc and DT50 values.  The leaching "herbicides" are also relatively persistent and weakly adsorbing, with GUS mostly exceeding 2.8.  It must be noted, however, the potential of a pesticide to contaminate groundwater is also influenced by many other factors, including soil conditions, application methods, and irrigation practices. These interactions will be discussed in the up-coming issue.


Table 1. Pesticides in wells sampled in CA as of December 1999 and resulting from nonpoint-source applications



Wells sampled

Wells with detections



















































































DCPR (California Department of Pesticide Regulation). Sampling for Pesticide Residues in California Well Water: 2000 Update of the Well Inventory Database.

Gilliom, R.J., Pesticides in the hydrologic system - What do we know and what's next? Hydrological Processes 15: 3197-3201 (2001).

Gustafson, D.I. Pesticides in Drinking Water. New York: van Nostrand Reinhold, 1993.

Troiano, J., D.Weaver, J. Marade, F. Spurlock, M. Pepple, C. Nordmark, and D. Bartkowiak. Summary of well water sampling in California to detect pesticide residues resulting from nonpoint-source applications. Journal of Environmental Quality 30:448-459, 2001.



An Interactive Website


Ever since I took on this position, I have always been thinking about developing a user-interactive Website with a focus on pesticides related water quality and environmental issues. Through six months of diligent effort by a professional programmer (Dr. Peter X. Pang), this Website is now in a good enough shape and can be accessed at:


The focus of this Website is the "Pesticide Wise" interactive tool that allows you to find and use critical information on almost any pesticide that is out there. You may type in a partial or full name (trade or chemical name) of a pesticide to start the search. For instance, you may type in "simazine".  After you make a selection, you will be presented with a list of critical pesticide property and toxicity parameters, along with the index for leaching or runoff potential.  When the leaching (or runoff) potential is "High", a "Risk" button will pop up.  Clicking the "Risk" button will take you to the interactive pages where you can determine the actual runoff or leaching risk for your specific site or application, and find out what mitigation options are available for reducing the risk.


This Website is unique in that it is interactive, and easy to use. In comparison, most other pesticide Websites are static, which forces the user to go through layers of static pages.  Being interactive also allows the user to input information specific to his/her situation, because where and how a pesticide is used makes huge differences! The purpose of this Website is for outreach and education. If you have time, please play with it. If you have ideas about how it can be improved, please email me at



Do You Know…


You can use hot water to kill weeds sprouting out between bricks or in gaps of concrete pavement around your house. Heard this one from the radio ... but it seems so obvious!