“In 2007 alone, approximately 390 million kilograms of pesticides, including herbicides, insecticides and fungicides, were used in the United States…”
Pesticides play a large role in the modern day agricultural industry, and the need for larger yields, free of unwanted pest organisms, has led to large-scale applications of such products. What happens to those products when heavy rains saturate the soil and run-off into local waterways, or enter the groundwater supply by means of soil infiltration? Chemical contamination from things like pesticides can cause a number of threats to non-target systems. The use of contaminated water in cultivation can cause secondary contamination of crops and pass on to consumers.
The legalization of Cannabis in states like Oregon, Washington and Colorado have led to large scale grow operations, both indoor and outdoor. And just as with any other form of agriculture, the demand for higher yields is ever-growing. Inappropriate management and irrigation technology, in all forms of agriculture, can lead to chemical run-off from cultivation sites to nearby natural water sources, contaminating rivers, lakes and streams. Water with dissolved pesticides entering the soil can also leach contaminates deep into the groundwater supply through the vertical descending displacement of materials through the soil profile. These contaminated water sources can then cause secondary contamination of crops.
“The degree to which applied pesticides reach the groundwater is determined by factors like land-management practices, chemical and biological degradation rates and hydrogeological conditions like the thickness of the unsaturated zone of the soil profile, which can vary between different soil types…
There are several forms of water source contamination. This post will focus primarily on groundwater and surface run-off contamination. Within a typical soil profile, there are five layers called master horizons. Not all five horizons are present in every soil type, but every soil type displays at least one horizon. Below those five horizons lies bedrock, which is usually the parent material for the soil that lies above. Groundwater is the subsurface water that fills in the cracks of the bedrock and the spaces between individual soil particles throughout the soil horizons. These spaces are referred to as pore spaces.
Groundwater can move freely under the influence of gravity, and often will move horizontally towards stream channels. The degree to which applied pesticides reach the groundwater is determined by factors like land-management practices, chemical and biological degradation rates and hydrogeological conditions like the thickness of the unsaturated zone of the soil profile, which can vary between different soil types (Toccalino et al. 2014).
Variations in soil types can influences water contamination. Some soils may be more susceptible to the leaching of pesticides as water infiltrates the surface horizons. Infiltration is the process in which water falling onto the ground enters the soil through absorption. The rate of infiltration decreases as the soil becomes more saturated, and once the soil is fully saturated, infiltration can no longer proceed and the water is transported across the top of the soil as surface run-off.
A study by the U.S. Geological Survey’s (USGS) National Water-Quality Assessment (NAWQA) Program tracked changes in pesticide concentrations in U.S. groundwater over the course of nearly two decades, from 1993 through 2011. The assessment by Toccalino and her team revealed that pesticide concentrations infrequently surpassed human-health standards. However, there may still be a concern for secondary contamination of crops like Cannabis, which could potentially lead to detections of pesticides in flower samples submitted for regulatory analytical testing.
The measure of a pesticide’s capacity to dissolve in water is known as pesticide solubility, or pesticide mobility.As water moves through the soil, dissolved pesticides are drawn into the soil profile during infiltration. As of 2015, information on the plant uptake of soil-persistent pesticides was largely deficient, and the research that is currently available targets the detection of residues in edible plants like cucumbers, leafy greens and potatoes (Hwang et al. 2015). Recent studies have been initiated based on the alarms of reported pesticides in organic agricultural products, which seem to be an increasingly problematic issue for organic farms that do not directly apply pesticides to their crops.
As soil is eroded, soil particles also known as sediment are more easily transferred by water and wind. Chemicals attached to those eroded soil particles and dissolved in water are also transferred. Soil types that are sandier than clay, for example, have a greater rate of infiltration and water loss. Sand has a larger particle size than silt or clay, creating relatively bigger pore spaces between the particles of sand, leading to a greater rate of infiltration of water through the soil. As soils become more eroded, finer particles like silt and clay are moved away while larger particles like sand remain. Eroded soils are therefore more prone to infiltration and water loss and move water and dissolved chemicals in and out of the soil profile more rapidly (Brady and Weil, 2010).
Drawing water from wells for use in agriculture without proper filtration could be problematic. Wells draw water from deep below the soil surface where groundwater moves freely. Using contaminated groundwater to supply water through an incorrectly maintained delivery system could potentially cause secondary contamination. The cleanup of contaminated groundwater is nearly impossible and contamination can proceed for many years.