|Main authors:||Meindert Commelin, Jantiene Baartman, Piet Groenendijk, Oene Oenema, Susanne Klages, Isobel Wright, Tommy Dalgaard, Morten Graversgaard, Jenny Rowbottom, Irina Calciu, Sonja Schimmelpfennig, Nicola Surdyk, Antonio Ferreira, Violette Geissen|
|FAIRWAYiS Editor:||Jane Brandt|
|Source document:||»Commelin, M. et al. 2018. Review of measures to decrease pesticide pollution of drinking water sources. FAIRWAY Project Deliverable 4.2, 79 pp|
Most of the drinking water used in the EU originates from groundwater (66%) followed by surface waters (30%) (Figure 8). The use of groundwater is dominant in Germany, France, Spain, Italy, Denmark, Belgium, and The Netherlands. The use of surface water is dominant in the United Kingdom, Portugal, Czech Republic, Finland, Estonia, and Ireland. The use of groundwater and surface waters greatly depends on the availability of fresh and clean groundwater and surface waters.
The pollution of groundwater and surface waters with pesticides from agriculture depends on the use of pesticides, the hydrological pathways and the pesticides removal/retention processes during transport. Here we discuss the hydrologic cycle, hydrological pathways and the factors that contribute to groundwater recharge and pesticides removal/retention processes during transport.
|1. The hydrological cycle|
|2. Pathways of pesticide movement through the environment|
|3. Factors controlling pesticide movement|
Solar radiation is the basic driver of the hydrological cycle. It ‘fuels’ evapotranspiration from plants, soil and water surfaces. The moist air moves up but once in cold air layers it condenses to form clouds, and thereafter returns to the surface as precipitation. Some of the rain evaporates back into the atmosphere, some enters surface waters through surface runoff, and some infiltrates the soil and percolates into groundwater and may ultimately seeps its way to rivers, lakes and oceans, and then is released back into the atmosphere through evaporation (Figure 9).
Groundwater is often divided in two subsystems
- the shallow groundwater with the (partly) unsaturated zone with rapid transport of solutes through shallow groundwater to local water courses (subsurface runoff) and
- the deep groundwater saturated zone with slow transport towards larger streams and rivers.
The infiltration capacity of the soil depends on its porosity, which depends on its texture and structure. When the rate of rainfall (intensity) exceeds the infiltration capacity of the soil, runoff will be generated and causes potential transport of applied pesticides. The vegetation exerts influence on the infiltration capacity of the soil; a dense vegetation cover often increases the infiltration capacity. Human activities that may affect runoff are the removal of vegetation and soil, grading the land surface, including terracing, and constructing drainage networks. These activities change runoff volumes and travel times to streams or other water bodies. Also, soil sealing in urban and infrastructural areas, and soil compaction by heavy machinery decreases the infiltration of water into the soil.
The residence time of water in groundwater systems is important for the prognosis of the long-term behaviour of groundwater systems in response to pesticide inputs. The longer the residence time, the older the water, the greater the chance that the groundwater has been influenced by anthropogenic influence, and the greater the chance that natural remediation can improve the quality of polluted groundwater.
Pesticide transport processes from sloping farmland to surface waters have been generally poorly documented (Tang et al., 2012). Depending on their chemical characteristics, pesticides can be either adsorbed to solid (soil) particles or dissolved in water. Thus, they can be transported in particulate (adsorbed) or dissolved form. The pathways of pesticide movement through the environment are mainly through the air (drift) and with water, i.e. following the pathways of water (Figure 10). Both include transport with sediment, i.e. in particulate form attached to soil particles, that can be moved by wind erosion and by water erosion over and through the soil.
Drift occurs during the application of the pesticides when they are sprayed on the field. Spray drift can pollute surface water and off-site locations. This depends mainly on application conditions as wind and humidity, and the used material (Reichenberger et al., 2007; Röpke, Bach, & Frede, 2004). Spray drift is an important route for pesticides into surface waters and should be taken seriously in view of the directness of the input and the high pesticide bioavailability (Tang et al., 2012). Its contribution to surface water pollution in European countries is however thought to be rather small (Neumann & Moritz, 2002; Tang et al., 2012)
Rittenberg et al., (2015) describe the movement of pollutants with the hydrological pathway. Depending on the so-called ‘hydrological land type’ (Figure 11) and the climate characteristics, pollutants either move over the soil surface (type A), partly infiltrate and move in a sub-surface layer (type B1), or vertically leach to the groundwater table (type B2).
- When the precipitation intensity exceeds the infiltration capacity of the soil (case A in Figure 11), infiltration excess overland flow, or Hortonian overland flow, will occur and pesticides may be moved either with the runoff water (in dissolved form) or with the soil particles as soil erosion (in particulate form) downslope. For pesticides showing a high sorption on organo-mineral soil particles, such as e.g. glyphosate, trifluralin, paraquat or organochlorine pesticides, transport by surface runoff is principally associated with the suspended soil particles generated by water erosion (Tang et al., 2012).
- If precipitation intensity is smaller than the infiltration capacity of the soil (case B1 in Figure 11), water and pollutants will infiltrate into the soil until a restrictive layer is encountered. Water and solutes will flow laterally over the restrictive layer. When the lateral transport capacity becomes less than the incoming lateral or vertical flux, layers become saturated. In these locations, exfiltration of water and solutes occurs as saturation excess overland flow.
- When the restrictive layer is deep or absent, water will predominantly flow vertically downwards (case B2 in Figure 11) via matrix and preferential flow until it reaches the water table. Preferential flow through macro pores is of particular interest in relation to the rapid transport of pesticides from farmland (Tang et al., 2012).
- A final pathway of pesticide pollution is direct point loss, which includes spray drift, but also spillage, the clean-up of pesticide application equipment and other operations. The importance of rapid direct point losses, like tank filling, spillages, faulty equipment, washing, waste disposal and overspray of surface waters has been confirmed by monitoring campaigns (Holvoet, Seuntjens, & Vanrolleghem, 2007).
Even though the general pathways of pesticide transport through the environment are known, as shown above, Borggaard and Gimsing (2008) noted that knowledge about subsurface leaching and surface runoff of glyphosate as well as the importance of this transport as related to ground and surface water quality is scarce, emphasising the very scarce direct knowledge of glyphosate transport by overland flow (Tang et al., 2012).
Pesticides are also degraded in the soil into other organic compounds. For glyphosate, this is mainly a microbial process, as practically no degradation has been observed in sterile soil (Borggaard & Gimsing, 2008). Pathways of microbial degradation of glyphosate are twofold, with one leading to the intermediate formation of sarcosine and glycine and the other leading to the formation of AMPA (Borggaard & Gimsing, 2008). Great variability is observed in the ability of soils to degrade glyphosate. It has been correlated with general microbial activity and thus with respiration rate (Borggaard & Gimsing, 2008), but also correlations with other factors have been found.
As can be deduced from the various pathways in which pesticides can reach ground- or surface waters, there are many factors that affect pesticide movement and these factors vary among locations and soil types. In this section, a brief overview is given of the most important factors controlling pesticide movement in and over the soil to ground- and surface waters.
Sorption to soil is one of the most important processes affecting the fate of pesticides in the environment. Strong sorption to soil solids results almost in immobilisation, while a weakly sorbed compound can be readily leached (Rittenburg et al., 2015). The tendency of pesticides towards sorption is expressed in terms of the sorption coefficient Kd defined as the ratio of the pesticide concentration in the sorbed phase to that in the aqueous solution phase (Tang et al., 2012). Sorption retards the transport of dissolved pesticides, but it can enhance the transport of particulate or colloid-associated forms if rainfall or irrigation triggers soil erosion (Tang et al., 2012). Pesticide physicochemical properties (e.g. solubility, polarity, polarizability, charge characteristics) in combination with soil chemical properties (clay content, pH, organic matter content) govern pesticide sorption in soils (Borggaard & Gimsing, 2008). For example, soil pH determines the electrical charge of glyphosate and therefore its adsorption on the mineral phase (Vereecken, 2005). Almost all pesticides are moderately to weakly sorbed in soils, mainly by soil organic matter (SOM), because most of the pesticide molecules are dominated by apolar groups (Borggaard & Gimsing, 2008). Glyphosate is an exception, as it is strongly sorbed by soil minerals due to its three polar functional groups (carboxyl, amino and phosphonate groups) that have a high affinity for aluminium and iron oxides (Borggaard & Gimsing, 2008). Therefore, the risk of ground and surface water pollution by glyphosate seems limited because of sorption onto variable-charge soil minerals, e.g. aluminium and iron-oxides, and because of microbial degradation (Borggaard & Gimsing, 2008). Glyphosate competes for sorption sites with phosphate, which may have a severe impact on glyphosate bonding, and hence leachability, especially on many agricultural soils in Europe, the USA and elsewhere that are saturated or nearly saturated with phosphate because of surplus fertilisation over many years (Borggaard & Gimsing, 2008).
As becomes clear from sorption, also physical and chemical soil characteristics play a role in determining the movement of pesticides through and over the soil. As Borggaard and Gimsing (2008) state: ‘Soil sorption and degradation of glyphosate exhibit great variation depending on soil composition and properties.’ As indicated above, soil pH and organic matter content affect sorption of pesticides to the soil solids. Soil structure and texture are important factors determining whether water with solutes move through the soil as matrix (or piston) flow or whether preferential flow occurs. In unstructured, uniform soils (e.g. sandy soils), mainly matrix flow occurs. As this is slower than preferential flow, pesticides have more change to sorb to the soil solids. In structured soils, preferential flow paths exist through which rapid flow to lower layers and the groundwater can occur. In clayey soils, preferential flow bypassing the soil matrix more or less, is common. Preferential flowpaths include macropores, including biopores and fissures / cracks between aggregates, but also bands of higher hydraulic conductivity such as sandy bands in between a clay matrix may occur (Borggaard & Gimsing, 2008). Figure 11 indicates that the presence in the soil of restrictive layers plays an important role, as this prevents vertically downwards flux and leads to more rapid subsurface flow. Similarly, the depth of the soil to either the bedrock or the groundwater affect pesticide transport.
Climate plays a role mainly in terms of rainfall occurrence and intensity. Several authors indicate the role of rainfall in high intensity storms that occur shortly after pesticide application. This increases the risk of both leaching of pesticides through subsurface flow and vertical flow to the groundwater, as well as the risk of loss of pesticides through overland flow (Borggaard & Gimsing, 2008; Tang et al., 2012; Vereecken, 2005). Figure 12 shows a map of Europe with the distribution of excess rainwater. For example, Tang et al. (2012) state that the first overland flow event usually causes the highest pesticide loss, especially after a long dry period during which numerous pesticide applications have been made. Another climatic factor is wind speed and direction, which plays a role in drift pollution.
For pesticide loss via overland flow (both dissolved as in particulate form), the factors that determine water erosion play a role. These include climatic factors (mainly rainfall intensity, but also amount and length of storms), soil properties, such as texture, structure, crust formation, soil moisture content and erodibility of the soil. These factors determine whether infiltration capacity of the soil is exceeded, leading to Hortonian overland flow, and how susceptible the soil is to erosion. For example, silt textured soils are more susceptible to erosion than sand or clay soils. The topography and geomorphology of the landscape plays a role, in determining the accumulation and redistribution of overland flow and thereby soil particles. Slope steepness is one of the most important factors determining the amount of soil erosion. Vegetation type and density are other major factors for soil erosion and deposition of (polluted) sediment, as well as for infiltration characteristics of the soil.
Topography and landscape position is a factor for subsurface transport of (dissolved) pesticides, because exfiltration may occur. Footslopes are more vulnerable to pesticide loss via overland flow than other parts of a catchment, as the soils in footslope positions receive subsurface flow from higher contributing areas.
Finally, there are technical and management factor that play a role in the risk of pesticide loss. These include the equipment design, pressure, droplet size and spray type (Gil, Sinfort, & Bonicelli, 2005; Tang et al., 2012). Clearly, the timing of application of pesticides relative to (expected) rainfall events is important, as well as the number of applications.
The very wide variation in pesticide movement through the environment is because very often, multiple factors play a role and these factors are different for different types of pesticides. For example, transport of glyphosate may be caused by an interaction of high rainfall events shortly after application on wet soils showing the presence of preferential flows (Vereecken, 2005).
Note: For full references to papers quoted in this article see