GENERAL CONCLUSIONS

The increase in agricultural activity coincides with increased pesticide use and other intensive agriculture activities, especially vegetable cultivation in the upper catchments of river basins in developing tropical countries. High rainfall throughout the year led the soil erosion and increased the amount of suspended sediment in river water. HA is a major component in soils, but the fate of the pesticide in tropical river basins under high sunlight has received little attention.

This study investigated three objectives: 1) to examine the behavior of suspended sediment and loading in tropical rivers under different land use systems, 2) to evaluate farmers’ perceptions and land use pattern impacts to better understand the loading of active ingredients from different pesticides into streams, and 3) to evaluate the effect of HA-bound suspended sediment on the photodegradation of pesticides in tropical rivers. The conclusions are as follows.

In Chapter 2, I examined the suspended sediment results from small-scale river basins and large-scale river basins. According to results, the upper catchments have significant forest cover in both Polwatta and Gin river basins; therefore, the suspended sediment concentration is low. However, in lower parts of the basins, agriculture and other land use patterns increase, except for the forest cover;

therefore, the suspended sediment concentration increases. In addition, higher turbidity in lower basins also signifies the results. The Upper Kotmale basin has a significantly higher percentage of vegetable land use than the other two rivers (Polwatta River and Gin River).

Soil erosion and sedimentation are critical factors associated with suspended sediment in the water, and they are due to land use transformation from forests to intensive agriculture. The conversion of traditional tea lands to annual crop cultivations also significantly contributed to the deterioration of the aquatic environments. This study revealed that higher turbidity, nutrients, and SSC loads are contributed by Nanu Oya to Kotmale Oya proper as compared to the two

88 other subbasins. It is understood that runoff brings agrochemicals from the farmland to river water, which implies that the applied pesticide also drains into streams from farmlands; however, much more attention to this subject is warranted. These pollution loads stemming from the adjacent environment trigger changes to the biological communities of the streams in the Kotmale subcatchment.

As a result of land use changes, the aquatic environment is increasingly vulnerable to the impacts of intensive agriculture practices. Therefore, farmers’

perceptions were evaluated regarding pesticide loading in the river basin and its toxicity to the aquatic environment. The results reveal that most of the farmers are over 30 years old and have high levels of farming experience. In addition, land size and climate were significant influences associated with increased pesticide loading. Forty-six commercial brands and 25 active ingredients were recorded in this basin. The clustering analysis showed four used insecticides (diazinon, deltamethrin, sulfur, and chlorpyrifos), four fungicides (mancozeb, propineb, metiram, and chlorothalonil), and two weedicides (paraquate and metribuzin); and the analysis was based on the usage of the active ingredient per acre. However, previous studies reported significant amounts of pesticide adsorption to farm soil (Halimah et al., 2004; Muhamud et al., 2004; Muhamud et al., 2005; Gerbremarium et al., 2012), and high amounts of applied pesticides are removed from the farmland by volatilization (Leistra et al., 2006). Also, plant uptake, leaching, and photodegradation in farmland also play a key role in the degradation of applied pesticides (EPA, 1986; Gustafson, 1989; Wu et al., 2002; Katagi, 2004; Wang et al., 2007; Rigueira et al., 2013). Remaining pesticides flow through runoff from farmland to river. Some fraction of the farm soil contributes to runoff with adsorbed pesticides; thus, runoff with pesticides is responsible for river toxicity.

According to the toxicity assessment, chlorpyrifos and deltamethrin are toxic (acute and chronic), even if only 1% of the above pesticides (from total applied to farmland) travels from the farmland to stream/rivers (Table 3.4); therefore, these chemicals exceed the toxicity limits (Table 3.3) of aquatic invertebrates at low levels (Table 3.5). The chlorpyrifos active ingredient also showed acute

89 toxicity to Daphnia magna and sediment dwelling aquatic life, and there is the possibility of a potential impact on drinking water. Overall, the October to January season showed the highest toxicity levels (Table 3.4) to aquatic organism as compared to the other two seasons. Runoff with pesticides is responsible for the toxicity levels in river water, and some fraction may end up in rivers or reservoirs with sediment. When assessing the downstream toxicity, the suspended sediment (HABSM) effects of pesticides in tropical regions are unknown. Thus, it is very important to understand the pesticide behavior when water enters a river/stream from farmlands.

Chapter 4 illustrates the findings of the HABSM effect on pesticide photodegradation. Although it is a laboratory experiment in a closed environment, the results imply the vulnerability of the HABSM effect on pesticide photodegradation in tropical rivers. Moreover, it reveals that LHA has the potential to effect chlorpyrifos photodegradation without HA in aqueous media. There is no impact of HA on the adsorption of chlorpyrifos, and the most important finding is that the HABSM effect on chlorpyrifos photodegradation is much higher than the suspended matter (without HA) and LHA. Therefore, these findings suggest that SMBHA has the potential to act as good catalyst for pesticide degradation. This theory is very important when downstream toxicity is examined.

This thesis provides evidence that farmers’ perception and land use patterns significantly impact pesticide loading and aquatic life. Intensive agriculture practices in the upper catchment of river basins are highly susceptible to the deterioration of the aquatic environment based on agrochemical usage, especially regarding pesticides (AI), farmers’ perceptions, and land use changes.

When assessing the downstream toxicity, understanding pesticide behavior in tropical rivers is important, particularly when the pesticides move from farmlands to downstream areas. Also, suspended sediment concentration is highly increased in the rivers because of land use transformation from forest to agriculture. This study reveals that highly concentrated suspended sediment in tropical rivers can potentially catalyze pesticide degradation, and it also plays a significant role in the degradation of pesticides and downstream toxicity in

90 tropical rivers. These results can be used to optimize estimations of downstream toxicity. However, there are certain limitations in this study:

1. Land use percentages in the river basin comparisons may have some effect because the data was collected from different years.

2. The mean value was used to calculate the loading of suspended sediment and nutrients.

3. Pesticide loading was calculated based on three seasons (maximum) per year, and some farmers cultivate four seasons per year.

4. Only four major crops were selected from one farmer, and the mean values were used to calculate pesticide loading.

5. The adsorption experiment was conducted under laboratory conditions (closed environment), and there may be differences when compared with a natural system.