PART B
Identifying possible human health risks through contamination

Introduction

In Part A an assessment was made to determine whether spillages of pesticides caused contamination of soil or groundwater. In Part B it will be determined whether this contamination poses a potential risk to human health. If there are risks, measures must be taken to reduce them. These measures are described below in Part C. If no risks exist, the contamination can be ignored because it will have no effect on the health of humans. Either way, it is wise to take one or more samples to check whether the outcome predicted in this manual corresponds to the real concentrations in the soil and groundwater at the site. How to carry out such checks is described in Part C.

As in Part A, Part B includes several steps that must be taken in order to determine whether contamination poses risks to human health. Step 5 assesses whether there are points in the neighbourhood of the store where humans are exposed to the contamination. Such “exposure points” may be a house or a well, that is, places that are frequently visited or used by people. Step 6 determines the pesticide concentrations that can be expected at the exposure sites. Step 7 assesses how people are exposed. That is, it determines the exposure routes between contamination and humans. Finally, Step 8 determines whether the concentrations exceed the “permissible exposure levels” - the levels of contamination that are still considered safe for humans.

Step 5
Identifying exposure points

EXPOSURE POINTS IN THE AREA WHERE GROUNDWATER IS CONTAMINATED

Objects vulnerable to contamination by pesticides via groundwater are wells, springs, rivers, lakes, reservoirs and ponds. Draw up a list of all the vulnerable objects in the immediate surroundings of the store, within a radius of 300 metres.

TABLE 5.1

Not all possible exposure points, however, are at risk of becoming contaminated. Determine the risk by taking the following steps:

• Determine the principal direction of groundwater flow. In the absence of groundwater level measurements, use the direction of steepest descent of the regional topography.

• Determine the downstream quadrant by drawing two lines at an angle of 45° to the principal direction of groundwater flow, as shown in Figure 5.1.

• Check if any exposure points or vulnerable objects are located inside the downstream quadrant. Mark these objects as “at risk”.

Wells and springs are so-called point sinks. If a point sink is located outside the downstream quadrant, it may still be at risk of becoming contaminated.

FIGURE 5.1
Delineation of the downstream quadrant

Calculate the radius of influence (r) of all point sinks with the following formula:

For each point sink, check if its radius of influence overlaps with the downstream quadrant, as shown in Figure 5.2. If this is the case, then mark this point sink as “at risk”.

Repeat these steps for all vulnerable objects identified as “at risk” and list all relevant objects in Table 5.2.

TABLE 5.2

EXPOSURE POINTS IN THE AREA WHERE TOPSOIL IS CONTAMINATED BY WIND

Objects vulnerable to contamination via wind are objects where humans stay for prolonged periods of time, such as houses, schools, meeting places and work areas. Crops and livestock also need to be considered as vulnerable objects since human beings consume them. List all the vulnerable objects in the immediate surroundings of the store, within a radius of 300 metres.

TABLE 5.3

FIGURE 5.2
Delineation of the radius of influence of point sinks

Step 6
Predicting concentrations at the exposure points

POINTS EXPOSED TO GROUNDWATER CONTAMINATION

The calculation of the concentrations at the exposure points is described below. (See Appendix 7.)

Calculate the relative distance (d) between the storage site and each exposure point.

Note that once the dissolved pesticide has reached the groundwater it is assumed that it is transported horizontally in a straight line towards a vulnerable object at risk. The dissolved pesticide moves with the groundwater in a so-called dispersion front.

TABLE 6.1

Note: The calculation of the retardation factor (R) is based upon a formation with sufficient porosity (approximately 0.3) and extremely low organic matter content (≤ 0.1 percent). These are the types of formations most vulnerable to contamination by pesticides.

• Repeat the steps in Table 6.1 for all exposure points and list the results in Table 6.2, differentiating between wells, springs and rivers on the one hand and lakes and reservoirs on the other (within a 300-metre distance).

• Calculate concentrations in exposure points (e.g. wells, springs, rivers, ponds).

The contamination of the groundwater from pesticide leaching will gradually decrease as contaminated water mixes with clean water in the aquifer. First, calculate a mixing ratio. The concentration at the exposure point is calculated by multiplying the mixing factor with the original concentration below the storage site, corrected with a correction factor. See Table 6.3 and Figure 6.1.

TABLE 6.2

Calculate the mixing ratio (m_g) of a well, spring or river with discharge Q:

The larger the distance from the pollution source, the more the pesticide will be spread out through the aquifer, as a result of hydrodynamic dispersion. Ignore any dispersion perpendicular to the principal direction of flow of the groundwater, which in most cases will be small. Furthermore, it is assumed that the dispersion is equal to 10 percent of the distance (x) to the vulnerable object.

Use these assumptions to calculate a correction factor (f_g) that takes into account hydrodynamic dispersion. This correction factor has been plotted in Figure 6.1 as a function of the relative distance travelled by the centre of mass of the dispersion front. Use this figure to look up the value for f_g corresponding with the relative distance d.

Use the following formula to calculate the pesticide concentration (C_g) in a well, spring or river:

C_g = C₁ × f_g × m_g

FIGURE 6.1
The correction factor f_g as a function of the relative distance d

When a store is situated on the border of a lake or other large reservoir, calculate the concentration by calculating the mixing ratio (m_s) of a lake, reservoir or pond with water volume V:

Again, the larger the distance from the pollution source, the more the pesticide will be spread out through the aquifer, as a result of hydrodynamic dispersion. The assumption is that the pesticide will accumulate in the surface water body until the maximum concentration C₁ has been reached.

Use these assumptions to calculate a correction factor (f_s) for surface water that takes into account hydrodynamic dispersion. This correction factor has been plotted in Figure 6.2 as a function of the relative distance travelled by the centre of mass of the dispersion front. Use this figure to look up the value for f_s corresponding to the relative distance d.

Use the following formula to calculate the pesticide concentration (C_s) in the lake, reservoir or pond:

C_s = C₁ × f_s × m_s

After calculating the concentration at each of the identified exposure points, list the results in Table 6.3.

POINTS EXPOSED TO CONTAMINATION BY WIND

The distribution of pesticides by wind causes the deposition of pesticide dust on an area around the storage site. The fact that an area is contaminated by pesticide dust does not automatically mean that that area poses risks to human health.

FIGURE 6.2
The correction factor f_s as a function of the relative distance d

TABLE 6.3

TABLE 6.4

Concentrations may be too low to cause effects. Therefore, it is important to determine which concentrations are still permissible (i.e. which do not cause any health effects) and which are not. The permissible concentrations for direct contact and for the consumption of vegetables, livestock products or drinking-water may be found in Appendix 3 (these concentrations apply also to pesticides distributed by wind).

Note that whether or not a concentration is high enough to cause health risks is calculated by means of risk assessment computer models. The model used to create this manual (Csoil) is scientifically proven and internationally accepted. More information is given in Appendix 6.

Use Table 6.4 to fill in the values.

Figures 6.3 through 6.5 show the pesticide concentrations that can be found in the area around the store. This applies to all directions. Thus, a circle can be drawn around the store to indicate the area affected by wind. In fact, this is a worst-case scenario. By drawing a circle it is assumed that the prevailing wind blows in all directions. (A prevailing wind direction was used in the wind distribution model used to create Figures 6.3 through 6.5. It was assumed that 25 percent of the time the wind blew in the prevailing wind direction.)

FIGURE 6.3
Graph to determine spreading of contaminants by wind (high emissions)

FIGURE 6.4
Graph to determine spreading of contaminants by wind (intermediate emission)

FIGURE 6.5
Graph to determine spreading of contaminants by wind (low emissions)

Now that the permissible concentrations for the relevant pesticides are known, the area within which these concentrations are exceeded can be identified. A standard wind distribution model has been used to this end. The results are presented in Figures 6.3, 6.4 and 6.5 (one for each rate of emission, as determined in Step 4). More information about the model and the parameters used in the calculations is given in Appendix 6.

Now, with the help of Figure 4.1 (assessing the rate of emission), Table 6.4 (the permissible concentrations), and Figures 6.3 through 6.5 (relation concentration and distance), the deposition of pesticides at exposure points may be determined.

Step 7
Identifying exposure routes

To assess whether humans might be affected by contamination (a process called “risk assessment”) one must first identify the routes by which humans can be affected. Human exposure is divided into two pathways, direct and indirect exposure.

Direct exposure will occur from direct contact with soil. This is possible from:

• ingestion of soil;

• dermal contact with soil;

• inhalation of soil particles.

In these cases only the contamination of topsoil is relevant. Direct exposure to a contamination in deeper soil layers is only relevant should one start digging.

Indirect exposure is possible by various different pathways. These pathways are:

• consumption of crops, vegetables or fruits from a contaminated area;

• ingestion of animal products, such as meat, milk and fish;

• ingestion of contaminated drinking water.

Not all exposure routes will always be applicable. For instance, if no crops are grown in the contaminated area, the exposure route “ingestion of contaminated vegetables or fruits” is not applicable and therefore not relevant. Use Table 7.1 to list only the relevant exposure routes. In order to identify relevant exposure routes take into consideration the following:

Soil

• Are humans living within the contaminated area?

• Are schools or hospitals situated within the contaminated area?

• Are crops for human consumption being grown in the contaminated area?

• Do cattle graze within the contaminated area?

Water

• Is water being drunk?

• Is water being used for irrigation?

• Is water being used for fishing?

• Is water being used regularly for bathing, washing (clothes, food, dishes, etc.) or swimming?

Proceed with Step 8, but only for the relevant exposure routes.

TABLE 7.1

Step 8
Determining when permissible exposure levels have been exceeded

International bodies such as the World Health Organization (WHO) generally define the health risks to humans by chemicals as Acceptable Daily Intake (ADI). The ADI value indicates the maximum quantities of a given chemical that human beings can absorb in a day without showing any signs of illness.

PERMISSIBLE EXPOSURE LEVELS FOR GROUNDWATER

The consumption of contaminated groundwater may pose risks to human health. Consumption of groundwater with pesticide concentrations lower than the permissible concentrations, however, will not pose risks to human health. Of course, the permissible concentration is different for each pesticide and for each exposure route. Consult Appendix 3 for the permissible concentrations, also called exposure levels. Use Table 8.1 to compare these levels with the predicted concentrations (refer to Table 6.3) to determine whether or not permissible concentrations have been exceeded.

PERMISSIBLE EXPOSURE LEVELS FOR WIND

When pesticides are distributed by wind, they may contaminate the soil around the storage area. Direct contact with contaminated soil (e.g. soil ingestion, dermal contact) or indirect contact (consumption of fruits or vegetables cultivated on a contaminated site) may pose risks to human health, but only when permissible concentrations have been exceeded.

For contamination caused by wind, the permissible deposition is also important. Appendix 3 provides the permissible concentrations, depending on the relevant exposure route. To calculate the permissible deposition, calculate the number of hours during which total deposition occurred (N_d) as follows:

N_d = M / average emission level

where:

N_d - number of hours during which total deposition occurred [hours]

M - total amount spilled (see Table 2.3) [kg or litres]

average emission level - see Step 4 [kg/hour]

Calculate the permissible deposition as follows:

permissible deposition = (permissible concentration × 0.5 × 365 × 24) / emission hours [g/m2/year]

Once the permissible deposition is known, compare it with the predicted deposition (Step 6) to determine whether the predicted concentrations exceed the permissible exposure levels.

TABLE 8.1