**Should We Be Concerned with Long-Term Health Problems Associated with Pesticides in Namibian Groundwater?**

Benjamin Mapani1 and Rainer Ellmies2,3

*1University of Namibia, Geology Department, Windhoek, 2Federal Institute for Geosciences and Natural Resources (BGR), Hannover, 3BGR Project in the Geological Survey of Namibia, Windhoek, 1,3Namibia 2Germany* 

#### **1. Introduction**

422 Pesticides in the Modern World - Risks and Benefits

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In the modern world there are several sources of pesticides and other phenyl type organic compounds. Why do we use pesticides? Simply that the world's agricultural practices have commercialized to feed an overgrowing world population. Agriculture has also become a very lucrative business, where various players try to find niches to market their products. At the same time, societies have continued to become more aware about their natural environment and how to sustain it. As far back as 500 A.D., the city of Athens passed a law requiring all refuse to be disposed in a designated landfill outside the city walls (Zakrzewski, 1997); this was followed by many such laws in Europe. However direct linking of disease and health to environmental occupation was done by Dr. Percival Pott, in 1775 (quoted in Zakrzewski, 1997), who correctly linked the "chimney workers" with the unusual high rate of scrotal cancer, which he associated with exposure to soot in their work environment (Zakrzewski, 1997).

In a city like Windhoek, pesticides may arise from the control of grasses on road pavements (Mapani, 2005); or from small scale urban agricultural gardens behind homes. However the major source of pesticides is by far from the farming industry. It is estimated that in the United States alone, close to 98% of the sprayed insecticides and 95% of the herbicides finally reach an unintended sink (destination) (Miller, 2004). The effect of this is obvious when it comes to infiltration of water into the ground that has been used in the irrigation of crops. Pesticides are one type of compounds that undergo "biomagnification" in the food chain (US EPA, 2003). This aspect is especially dangerous as it ensures that the threat from the pesticide is not decreased with time, but rather increased. The United States Environmental Protection Agency (US EPA, 2003) has classified pesticides into four categories; namely; (i) organophospates; (ii) N-methyl carbomates; (iii) triazines and (iv) chloroacetanilides. Some specific pesticide compounds have common mechanisms of toxicity, and require cumulative risk assessment over a relative long period, in order to define the potential risks. However the danger to aquatic life as described below in section 2 is well known, but the danger to human health is ill defined in comparison; with only a few

Should We Be Concerned with Long-Term Health

situated some 200 km north of Keetmanshoop (Figure 1).

after a critical period of time.

is virtually non-existent.

**2.1 Effects of pesticides on groundwater** 

Problems Associated with Pesticides in Namibian Groundwater? 425

of point sources. When mixtures of point sources or multiple point sources occur in one particular area, then pesticides may present completely different mechanisms of toxicity. It has also now been known that pesticides can persist in rivers and in groundwater (Chilton, 2000) for long periods of time. This coupled with the fact that these xenobiotics are biocumulative in the ecosystems and in humans (USEPA, 2003), the concern is then raised as to how long their effects take or whether or not the effects will appear as diseases in humans

In this paper, the conditions are examined, that should raise concern on the possibility of severe effects appearing among the general population that uses groundwater obtained from areas that have been affected by pesticide exposure. These areas are mainly the Grootfontein-Tsumeb-Otavi (GTO) area in the northern part of the country, including the Tsumeb townlands; the Windhoek area and Okahandja towns, that are sustained from Kombat and Berg Aukas aquifers in the GTO area and the Stampriet area, in the south between Windhoek and Keetmanshoop where the Stampriet aquifer is both used as a water source for domestic use and for farming purposes (Figure 1); the Mariental area, which receives its water from the Hardap dam, whose catchment is a traditionally farming area,

**2. Effect of pesticides on groundwater, aquatic life, humans and other biota**  Pesticides degrade, and their products of degradation (degredates) can find their way into groundwater and surface water (Chilton et al., 2000; Foster et al., 1991). However as the some authors have pointed out, e.g., Barret (1996), Houtman et al (2004); the process of degradation takes very long that the effects of these xenobiotic compounds generally do affect aquatic life in surface waters. The danger to aquatic life is better defined compared to human health (USEPA, 2003). To human health pesticides have long term effects that need to be known and avoided where possible.As indicated above at least 98% of sprayed insecticides and 95% of herbicides reach an unintended destination, including surface and groundwater (Miller et al., 2004). This implies that pesticides are ending up in areas where they must not be present, and its here that the trouble begins. Once pesticides have entered the hydrological cycle, their elimination is difficult, and in most places impossible, especially so with groundwater. However when pesticides percolate together with surface water, towards the groundwater table, certain quantities may be trapped by clay minerals (Mapani & Schreiber, 2008). The effectiveness of this trapping process (attenuation) depends on the soil types developed in a particular area. In areas where montmorillonite clays are abundant, this process is much better, although it may not completely eliminate the danger. In arid areas such as Namibia, soils are not well developed and the trapping ability of soils

In the area around Windhoek, Mapani (2005) reported the use of pesticides and herbicides in the removal of weeds in paved areas of the city. This kind of usage is continuous, and poses a major threat to groundwater, as it is continuous and the threat is present in all seasons. Windhoek is further disadvantaged by having a thin soil cover, less than 30 cm in most places before bedrock is reached (Figure 2). Thus contamination via percolation of rainwater into the aquifer is made much easier and faster. In an arid country like Namibia,

pesticides that have been well studied such as DDT (dichloro-diphenyl-trichloroethane), which was used widely across the world in the 1960's and 1970's.

In Karst areas such the Tsumeb-Grootfontein-Otavi (TGO) area of Namibia (Figure 1), that is underlain by limestone and dolomite aquifers and has in excess of 4000 square km where commercial agriculture is practiced, the danger from pesticide poisoning can not be over emphasised. The area receives on average about 760 mm of rain per annum. The area also serves as a major source of groundwater for the capital city Windhoek, which lies some 500 km to the south. Such relationships (groundwater source versus commercial agriculture) are generally present but the effect of pesticide use is not generally given the attention it deserves. No systematic studies have been conducted to especially ascertain attenuation rates of pesticides used on agricultural land.

Fig. 1. Simplified Geological Map of Namibia, showing the crystalline carbonates of the Otavimountainland (Grootfontein-Tsumeb) area shown in black.(Geological Survey Namibia, 2000).

It must be borne in mind that treatment of drinking water only eliminates biological threats, and does not usually address pesticides. Generally water analyses do not target phenyl compounds routinely; such that it is not possible to always know whether a particular area is affected or not. Pesticides may be point sources, i.e., only emanating from one particular location, with one general pesticide; or they could be termed as multiple sources or mixtures

pesticides that have been well studied such as DDT (dichloro-diphenyl-trichloroethane),

In Karst areas such the Tsumeb-Grootfontein-Otavi (TGO) area of Namibia (Figure 1), that is underlain by limestone and dolomite aquifers and has in excess of 4000 square km where commercial agriculture is practiced, the danger from pesticide poisoning can not be over emphasised. The area receives on average about 760 mm of rain per annum. The area also serves as a major source of groundwater for the capital city Windhoek, which lies some 500 km to the south. Such relationships (groundwater source versus commercial agriculture) are generally present but the effect of pesticide use is not generally given the attention it deserves. No systematic studies have been conducted to especially ascertain attenuation

Fig. 1. Simplified Geological Map of Namibia, showing the crystalline carbonates of the Otavimountainland (Grootfontein-Tsumeb) area shown in black.(Geological Survey

It must be borne in mind that treatment of drinking water only eliminates biological threats, and does not usually address pesticides. Generally water analyses do not target phenyl compounds routinely; such that it is not possible to always know whether a particular area is affected or not. Pesticides may be point sources, i.e., only emanating from one particular location, with one general pesticide; or they could be termed as multiple sources or mixtures

which was used widely across the world in the 1960's and 1970's.

rates of pesticides used on agricultural land.

Namibia, 2000).

of point sources. When mixtures of point sources or multiple point sources occur in one particular area, then pesticides may present completely different mechanisms of toxicity. It has also now been known that pesticides can persist in rivers and in groundwater (Chilton, 2000) for long periods of time. This coupled with the fact that these xenobiotics are biocumulative in the ecosystems and in humans (USEPA, 2003), the concern is then raised as to how long their effects take or whether or not the effects will appear as diseases in humans after a critical period of time.

In this paper, the conditions are examined, that should raise concern on the possibility of severe effects appearing among the general population that uses groundwater obtained from areas that have been affected by pesticide exposure. These areas are mainly the Grootfontein-Tsumeb-Otavi (GTO) area in the northern part of the country, including the Tsumeb townlands; the Windhoek area and Okahandja towns, that are sustained from Kombat and Berg Aukas aquifers in the GTO area and the Stampriet area, in the south between Windhoek and Keetmanshoop where the Stampriet aquifer is both used as a water source for domestic use and for farming purposes (Figure 1); the Mariental area, which receives its water from the Hardap dam, whose catchment is a traditionally farming area, situated some 200 km north of Keetmanshoop (Figure 1).
