Pesticides can be very beneficial for growing crops, but they can also pose a serious risk for human health. Pesticides like DDT can cause hormonal problems or even cancer if someone is exposed to them at high enough concentrations. No one wants drinking water contaminated with pesticides, so it is important to understand how they can enter your drinking supply and how to get rid of them.
There are two ways that pesticides can enter the water supply: contaminating surface water through runoff, or contaminating ground water by leaching through the soil. Fortunately, when it comes to ground water contamination, it takes a very long time (often years) for pesticides to leach through the soil to the water below. Because of this slow process, pesticides that break down over time or do not pass well through soil are unlikely to contaminate groundwater. Contamination of surface water may be more likely, but Environmental Protection Agency (EPA) regulations help to minimize this risk. The EPA requires pesticide companies to vigorously test all pesticides and estimate the risks for both ground water and surface water contamination before any product is allowed to be brought to market. Even then, based on the risks of any particular pesticide, the EPA may put strict limits on its use.
In general, these regulations help to reduce the quantity of pesticides in public water sources, but do not eliminate them entirely. The United States Geological Survey (USGS) conducts a National Water-Quality Assessment (NAWQA) Program every ten years that helps to assess the degree of contamination in water sources including ground water, streams, and watersheds. In their 1992-2001 study, they found that ground water was the least likely to be contaminated, although they found pesticides in about half of the shallow wells tested. Pesticides were much more common in surface water; at least one pesticide was detected in every single stream tested. Although the presence of pesticide can vary at different times of year, pesticides were detected throughout most of the year in more than 94% of the streams tested. Fortunately, the amount of each of these pesticides is generally very low. Pesticide concentration was higher than the safe level for humans in less than 10% of the streams tested. Concentration in ground water was even lower, with only 1% of wells tested exceeding the safe level. They found that dieldrin, dinoseb, atrazine, lindane, cyanazine, and diazinon were the most common pesticides; however, both cyanazine and dieldrin had stopped being used by the end of the study. Additionally, any surface water that contributes to our drinking water system will go through treatment that may remove pesticides.
Most pesticides can be removed from water with relative ease. Activated carbon filters can be used to remove many pesticides; by passing contaminated water through the filter, pesticide molecules will be attracted to the filter and will be trapped as clean water passes through. Nanofiltration is another strategy. It involves the use a membrane, a thin barrier that allows some things through but traps others behind. The membrane in nanofiltration only allows very small things to pass through (like water molecules) but leaves larger things (like the pesticide molecules) behind. If you are worried that pesticides may still be present in your water after it leaves the water treatment facility, there are several at-home solutions that you can adopt. Activated carbon filters are a part of many pitcher filters, faucet filters, and even larger purification strategies like under the sink water purifiers, or whole house water purifiers. Nanofiltration may be a component of the larger purifying systems like the under the sink purifiers or whole house purifiers. For more information about these strategies, see our other articles.