Chapter 7THE IMPACT OF RICE FIELD PESTICIDES ON THE QUALITY OF FRESHWATER RESOURCESS.I. Bhuiyan and A.R. Castañeda

7.1. Introduction

In recent years, there have been increasing concerns about the possible contamination of freshwater resources by residues of agrochemicals used on agricultural lands. These concerns stem from the awareness that maintaining the quality of the natural resource base is crucial for sustaining agricultural productivity as well as social progress. However, very little is known about how pesticides used in intensive irrigated rice culture systems, which are practiced in over 40 million hectares in Asia, affect the quality of freshwater resources that receive the excess water from these systems.

The need for insecticide application for controlling pests in modem rice culture has remained a debatable issue for a long time. Ecological considerations clearly demand that pesticide use be reduced, or totally eliminated, if possible.

But farmers’ perception of the need for pest control is quite different. Although modem rice varieties have some resistance or tolerance to various insects commonly found in rice fields, pesticide use has become a common feature of rice farming even though farmers do not fully understand the hows and whys of such use (Rola and Pingali, 1993). From an optimistic angle one can expect that in the future, as integrated pest management concepts and procedures become widely used by rice farmers, pesticide use would be reduced. An opposing future scenario is that as rice production is further intensified and higher yields targeted in the presently irrigated areas to meet increasing demands for the cereal, pesticide use in rice fields of Asia may increase. Also, as Rola and Pingali (1993) concluded, there is a concern that where insecticide use is low, poorly implemented integrated pest management programs could increase the amount of insecticide use by farmers.

While there is at least some hope that insecticide use in the future may decline, the prospect for chemical herbicides is different. With the increasing scarcity and cost of rural labor, more and more rice farmers are finding it economically attractive to control weeds by using herbicides. For the same reason, there is a growing popularity of replacing transplanted rice culture with wet-seeded rice culture in which manual or mechanical weed control is very difficult and time-consuming. Furthermore, herbicide availability to farmers has been consistently improving in many countries. All of these developments lead one to conclude that use of herbicides for weed control would continue to increase in many rice-producing countries of Asia.

7.2. Pesticide Movement to Freshwater Bodies and Possible Impact

Only a small fraction of the pesticides applied on rice fields is actually taken up by the plant. The remaining amounts of the applied chemicals are subject to various physicochemical processes for their disposal. Percolation and runoff are solution in rice field water. Percolation, which is the vertical movement of water through the soil profile, allows pesticides in solution and adsorbed by entrained sediment to move downward through the soil profile, which may reach the underground water table. Runoff, on the other hand, allows lateral movement of the chemicals along with water away from the rice field when it is deliberately drained or when excess rainfall or irrigation flow causes overtopping of the field bunds. In a rice irrigation system, all runoff from rice fields are collected in the main drainage system, which ultimately discharges the affluent to a large water body like river, lake, or sea.

While concerns for surface water quality are often separated from those of groundwater quality, the hydrologic cycle provides direct connection between the two in many geologic regions. Depending on hydraulic gradients, surface water may recharge groundwater or be replenished by groundwater (Hicks, Amussen, and Perkins, 1987).

Pesticides resident in drainage water may accumulate in the fresh surface water body to which the drainage system discharges its affluents from the irrigated rice fields, especially if the pesticides degrade

slowly. The impact of such accumulation is far reaching. The water may lose its usefulness as a source of domestic water supply due to the toxicity developed from the pesticides. Aquatic life may be affected. The biggest danger from pesticides in fresh surface water is that they can get into the food chain and affect the health of people and animals consuming the aquatic foods produced in the water body. While acute toxicities cause lethal impact on fish population, sublethal and chronic exposure to pesticides are much more insidious and difficult to identify. Sublethal exposure to pesticides is known to cause suppression in the reproduction success and development of resistant strains in many fish species (Cheng, 1990).

Certain aquatic animals have been found to retain (bioaccumulate) pesticides in concentrations many times more than those found in the water. Metcalf, Sangha, and Kapoor (1971) demonstrated that organochlorines such as DDT would be bioaccumulated 10,000 to 100,000 times the level of the chemical in water. Tejada and Magallona (1986) found from a study of rice-fish culture in the Philippines that fish and snails were able to bioaccumulate carbofuran respectively 110 and 10 times the concentration of residues in water. In Surinam, South America, spraying of PCP and endrin on an 8,000 hectare intensive rice-growing region near the coast caused extensive bird and fish kill. Kites foraging in the rice fields accumulated concentrations of PCP approximately 100 times greater than those from nearby freshwater marshes (Vermeer, Risebrough, Spaans, and Reynolds, 1974). The widespread accumulation of pesticides in fish from all over the United States has been well documented (Henderson, Johnson, and Inglis, 1969; Johnson and Lew, 1970; Stucky, 1970; Hunter, Carroll, and Randolph, 1980; Schmitt, Ludke, and Walsh, 1981).

In Asia, the lethal effects of various rice field pesticides on fish production have been reported by various authors (Lim and Ong, 1977, for Malaysia, and the Punla Foundation, 1981, for the Philippines).

Pesticide contamination of groundwater may render it dangerous for domestic consumption. In rural areas, most rice farmers depend on the abstraction of groundwater from shallow aquifers underneath rice fields for household use. Therefore, pesticides in shallow groundwater are of particular concern. Although much of the pesticides in solution or adsorbed in entrained sediment may be retained in the soil profile as they move downward with percolating water, it is only a matter of time before the adsorption capacity of the soil profile is used up and the pesticides reach the water table and the aquifer. This is especially true for conditions in which intensive rice culture keeps the land saturated for most part of the year. The local hydrology and the type of soil materials that comprise the subsurface profile and the groundwater aquifer mostly determine the extent of pesticides that will leach to the groundwater.

Many studies in the United States (Wartenberg, 1988; Hallberg, 1987; Pionke et al., 1988) and Western Europe have detected the presence of pesticides in groundwater, raising major health concerns. In the United Kingdom, the introduction of highly water soluble herbicides in the 1970s is believed to have caused widespread contamination of groundwater, particularly under the intensive agricultural regions of East Anglia. Some herbicides in groundwater have exceeded the admissible concentrations (Lawrence and Foster, 1987). Localized contaminations have also occurred from the intensive use of insecticides on potatoes and sugar beet grown on permeable soils overlying aquifers. In west European countries, the admissible maximum concentration in groundwater is 0.1 ppb (0.1 µg/L) for a single pesticide and 0.5 ppb (0.5 µg/L) for the sum of multiple pesticides (Leistra and Boesten, 1989). FAO/WHO (1970) set upper limits of concentrations in drinking water for popularly used pesticides based on their toxicity. For example, the acceptable maximum daily intake value for drinking water is 0.0075 mg/kg of body weight (bw) for endosulfan and 0.001 mg/kg-bw for methyl parathion. For butachlor, NRC (1977) of the United States set the upper limit at 0.01 mg/kg-bw for drinking water. The maximum concentration limit for carbofuran, endrin, and lindane are, respectively, 0.04, 0.002 and 0.0002 mg/l (WQA, 1991).

Very few studies have been conducted in the past to investigate the possible contamination of freshwater resources from pesticides used on rice fields. In this paper, two case studies conducted in the Philippines are reported. Case Study 1 addressed the question of pesticide contamination of shallow groundwater resident in aquifers underneath intensively cultivated rice fields. The second study dealt with pesticide concentrations in the drainage affluent from an irrigated area that grows two rice crops in a year.

7.3. Case Study 1: Pesticides in Shallow Groundwater Underneath Irrigated Rice Lands

7.3.1. Study Objectives

The main objectives of this study were to determine (1) whether residues of the commonly used pesticides are present in the shallow groundwater underneath intensively cultivated rice fields and (2) the concentrations of the contaminants present in the water.

7.3.2. Characteristics of the Study Sites

The study was conducted mainly in the service areas of two irrigation systems the Upper Pampanga River Integrated Irrigation System (UPRIIS) in Nueva Ecija province and the Sta. Cruz River Irrigation System (SCRIS) in Laguna province, Philippines. UPRIIS has a total service area of about 90,000 hectares and SCRIS has about 4,000 hectares. A large reservoir supports UPRIIS irrigation. The amount of water stored in the reservoir determines the extent of dry season area that can be irrigated. SCRIS is a run-of-the-river type irrigation system, with no storage facility. Both systems have supported intensive rice culture, with at least two rice crops per year, for over twenty years. Sampling sites were located in three villages within the service area of each irrigation system. For comparison, a 50 to 100 hectare rainfed rice area adjacent to each system was also included in the study. The rainfed area, which did not have access to irrigation supply, grew only one rice crop in the wet season of each year.

7.3.3. Methodology

7.3.3.1. Selection of Tubewells and Water Sampling Procedure.

A large number of manually operated tubewells used for lifting groundwater from shallow aquifers for domestic use were first identified within the service area of each irrigation system. From them, forty-six tubewells were randomly selected for the study. Of these, thirty-two were within the service area of UPRIIS and fourteen within SCRIS. The rainfed area was sparsely populated, hence fewer samples were available. Four tubewells in the rainfed site near UPRIIS, and three in the site near SCRIS were selected. About 90 percent of the selected tubewells were 6 to 12 m deep, the remaining were 13 to 21 m deep. All tubewells had concrete bases that protected the groundwater from surface contaminants. Water samples were taken from each tubewell once every month following a standard practice of purging the well by pumping water for two to three minutes before collecting about a 4 liter sample in a glass container. Water sampling was done for two consecutive rice-growing seasons, wet season (WS) and dry season (DS), during 1989-1991. The samples were analyzed at the Pesticide Residue Laboratory of International Rice Research Institute (IRRI) using gas chromatography in which the minimum detectable concentration was 0.001 ppb or 0.001 µg/L.

7.3.3.2. Farmer Surveys.

During each season, fifty-five to sixty-seven randomly selected rice farmers of the study area in UPRIIS and thirty to thirty-five in SCRIS were interviewed for their pesticide use, rice yields, and major agronomic practices. Seven rainfed farmers from Laguna and five from Nueva Ecija were also interviewed for the same purpose.

7.3.4. Findings and Interpretation

7.3.4.1. Pesticide Use in Rice Culture.

The overwhelming popularity of pesticide use in rice culture was clearly evidenced in the farmer response obtained during the survey. With the exception of one farmer in SCRIS in the WS, all farmers who were interviewed had used pesticides in both seasons (Table 7.1). The farmer who did not use pesticides said that he practiced the integrated pest management (IPM) concept and did not consider it necessary to apply pesticides.

On the whole in the two irrigated rice areas, about 50 percent of annual pesticides used for crop protection was insecticide, 40 percent herbicide, and 10 percent molluscicide.

Compared to UPRIIS, the annual pesticide use in SCRIS was about 25 percent higher due to higher amounts of insecticides and herbicides used in the latter area (Figure 7.1). UPRIIS farmers used more molluscicides because majority of them, 51 percent in the WS and 96 percent in DS, practiced the wet-seeding method of rice crop establishment. At its early stage, wet-seeded rice is more prone to damage by snails, hence the higher molluscicide use in UPRIIS. Higher labor costs and shortage of labor were the main reasons given by farmers for adopting the wet-seeded rice culture in UPRIIS in which land holdings are bigger (Table 7.1).

In the WS, the SCRIS farmers used about 55 percent more pesticides than the adjacent rainfed farmers, higher insecticide use accounting for most of the difference (Figure 7.1). In contrast, UPRIIS farmers used only 15 percent higher amounts of pesticides than the adjacent rainfed farmers and the insecticide uses of the two groups were about the same. Herbicide use in UPRIIS was only 27 percent higher than that in the rainfed area, indicating the growing popularity of herbicide use by farmers even in rainfed, transplanted rice culture.

The total amounts of pesticide use in the two rainfed areas were about the same. But UPRIIS used more insecticides and less herbicides than SCRIS (Figure 7.1). The Nueva Ecija rainfed farms produced higher rice yields than Laguna rainfed farms. The difference could be attributed to the higher nitrogenous fertilizers used by the Nueva Ecija farmers (Table 7.1).

7.3.4.2. Pesticides in Groundwater.

Extent of groundwater contamination. Pesticides found in groundwater are considered in two groups. Group A includes those pesticides that were both reported by farmers to have been used during the year and detected in analysis of water samples. Four pesticides are included in this group: butachlor, carbofuran, endosulfan, and methyl parathion. Of these, butachlor is an herbicide, and the others are insecticides. Carbofuran and methyl parathion are considered extremely hazardous and endosulfan moderately hazardous insecticides (Rola and Pingali, 1993). Group B pesticides are those that were detected in water samples but were not reported by the farmer samples to have been used during the year. These are azinphos, DDT, diazinon, endrin, lindane, malathion, and MIPC. Some popularly used pesticides such as pretilachlor could not be analyzed in the water samples because of limitations in the laboratory equipment.

Both SCRIS and rainfed area of Laguna had much more prevalence of Group A pesticides than the other two sites in Nueva Ecija. On the average, about 47 percent of the water samples from SCRIS and 36 percent of the samples from the rainfed are contained these pesticides. Endosulfan was the most commonly found (88 percent) pesticide in the water samples of SCRIS during the study year, followed by butachlor (51 percent), methyl parathion (33 percent), and carbofuran (15 percent) (Table 7.2). The same order of prevalence was found in the groundwater samples from the adjacent rainfed area. No carbofuran was found in the water samples of the rainfed area. Butachlor and endosulfan were the two most popularly used pesticides in SCRIS in Laguna. Endosulfan was the most popularly used pesticide by the sample farmers of the Laguna rainfed area (Figure 7.2).

In Laguna, Group B pesticides were found in fewer number of water samples than the Group A pesticides. Lindane (66 percent), DDT (44 percent), and diazinon (32 percent) ranked as the top three in order of prevalence in the groundwater samples of SCRIS (Table 7.2). In the rainfed area, the first two were most prevalent (44 percent), which was followed by azinphos (33 percent),and diazinon and MIPC (22 percent).

In Nueva Ecija water samples, the prevalence of Group A pesticides was less compared to Laguna. Methyl parathion was found in 35 percent of the water samples from UPRIIS despite its use in 1990-1991 by only one or two farmers (Figure 7.3). Butachlor was detected in 25 percent of all UPRIIS groundwater samples in the WS but none in the DS or the adjacent rainfed area in the WS. Pretilachlor, a pre-emergence herbicide, was the pesticide most commonly used by the interviewed farmers in both UPRIIS (20 percent) and its adjacent rainfed area (15 percent) (Figure 7.3), but water sample analysis did not include this chemical.

Among the Group B pesticides, diazinon was the most prevalent in the water samples of both UPRIIS (88 percent in WS, 64 percent in DS) and the rainfed area (83 percent). Malathion was the second most prevalent in water samples of both sites (Table 7.3).

Pesticide concentration in groundwater. Among Group A pesticides, the maximum concentration found in the WS water samples of SCRIS was for butachlor (1.14 ppb), with 0.073 ppb as the seasonal mean. Butachlor was found in over three-fourths of the water samples during the season. Although endosulfan was found in nearly all groundwater samples of SCRIS and in 78 percent of the samples from the adjacent rainfed area in the WS (Table 7.2), its mean as well as maximum concentrations in the water were much less than that of butachlor. As in SCRIS, the maximum concentration in Laguna rainfed farms was for butachlor (1.26 ppb), with a seasonal mean of 0.188 ppb (Table 7.4).

In the following DS, the maximum concentration in the SCRIS water samples was for carbofuran (1.15 ppb), with a seasonal mean of 0.095 ppb (Table 7.4). During the season more SCRIS water samples had carbofuran residues than in the previous season, but the prevalence of the other three pesticides decreased. Endosulfan, however, remained by far the most prevalent pesticide during the season (Table 7.2). All seven pesticides of Group B were present in the DS water samples of SCRIS and those of the 1989 WS from the rainfed area, but at much lower concentrations than those of Group A pesticides (Table 7.4).

The maximum concentration of the Group A pesticides in the water samples of UPRIIS in 1990 WS was for carbofuran (1.064 ppb), with a seasonal mean concentration of 0.055 ppb (Table 7.5). In the following DS, the only pesticide that was detected in the UPRIIS water samples was methyl parathion, with a maximum concentration of 0.042 ppb and a seasonal average of 0.007 ppb. Methyl parathion was detected in 42 percent of the groundwater samples during the season (Table 7.3). In general, pesticide concentrations, both maximum and seasonal average, were much less in UPRIIS compared to SCRIS.

The mean concentrations of Group B pesticides did not vary much across sites or seasons; however, individual pesticide concentrations were generally lower than those of Group A pesticides in the water samples of both irrigation systems. Azinphos was found in relatively high maximum concentrations in the water samples of both irrigated sites, especially in UPRIIS in the DS. DDT and endrin were found in water samples of SCRIS and Laguna rainfed areas in both seasons, but not in the samples of UPRIIS (Tables 7.4 and 7.5). As in the case with many other countries in Asia, DDT and endrin are banned for agricultural use in the Philippines (ADB, 1987). Azinphos and endrin are considered highly hazardous pesticides (Rola and Pingali, 1993).

The mean concentrations of Group B pesticides did not vary much across sites or seasons; however, individual pesticide concentrations were generally lower than those of Group A pesticides in the water samples of both irrigation systems. Azinphos was found in relatively high maximum concentrations in the water samples of both irrigated sites, especially in UPRIIS in the DS. DDT and endrin were found in water samples of SCRIS and Laguna rainfed areas in both seasons, but not in the samples of UPRIIS (Tables 7.4 and 7.5). As in the case with many other countries in Asia, DDT and endrin are banned for agricultural use in the Philippines (ADB, 1987). Azinphos and endrin are considered highly hazardous pesticides (Rola and Pingali, 1993).

Most pesticides used for crop protection have half-lives from a few days to a few months in the soil-water system exposed to the normal environmental conditions, but their persistence in the conditions groundwater normally occurs may be a different story. Little is known about the pesticide degradation process in groundwater that is not exposed to the physical, atmospheric, and biological forces that degrade pesticides. Therefore, it is difficult to directly associate the prevalence of pesticides in groundwater samples to their most recent use by farmers of the vicinity because it may take many years for some pesticides to reach groundwater. Similarly, it is not possible to conclude whether the prevalence of Group B pesticides is associated with their most recent use by non-sample farmers, or with previous use by farmers, or both. Although usually at a very slow rate, groundwater is always on the move laterally; therefore one should not also strongly associate the prevalence of pesticides in groundwater at a specific location to their use in that location.

7.3.4.3. The Role of Hydraulic Property of Soil.

Of the different factors that independently and interactively determine the downward movement of chemicals applied on the water above the surface, soil texture is a main factor due to its influence on the hydraulic property of the soil. Because of the presence of more macropores in them, sandy soils allow water infiltration and percolation at faster rates than clay soils. Therefore, pesticides applied on rice field water would move downward more readily in sandy soils. Furthermore, in sandy soils the adsorption of the chemical is less. In the United States, the most susceptible sites for pesticide contamination of groundwater have been identified as those with sandy soils, shallow water tables and high rates of infiltration, together with low soil organic carbon content (Lawrence and Foster, 1987). Some previous studies on the movement of nitrate-nitrogen from fertilizer applied on the rice field water to groundwater table has been explained by the hydraulic property of the soil (Spalding, Junk, and Richard, 1980; Castañeda and Bhuiyan, 1991). SCRIS soils have loamy sand to clay textures, with an average percolation rate of 7.5 mm/day. In contrast, UPRIIS soils have silty clay loam to clay textures and an average percolation rate of 4.9 mm/day. The difference in the hydraulic property of SCRIS and UPRIIS soils may have contributed to the presence of pesticides in a higher proportion of groundwater samples and in higher concentrations in SCRIS.

7.4. Case Study 2: Rice field Pesticides in Runoff Draining into a Freshwater Body

7.4.1. Objective of the Study

This study complements the Sta. Cruz River Irrigation (SCRIS) component of Case Study 1, which has been presented above. The primary objective pursued in this investigation was to determine whether pesticides applied on rice fields are present in the drainage outflows from SCRIS before they leave the system boundary and discharge into the lake called Laguna de Bay.

7.4.2. Characteristics of the Study Site

SCRIS’s irrigation and rice culture background has been discussed in relation to Case Study 1. The system, which has been in operation since 1953, supports two rice crops grown in a year in its service area of about 4,000 hectares. Its DS irrigated area, however, varies from year to year because there is no storage reservoir supporting the system and therefore DS rice culture is largely dependent on the river flow. SCRIS receives a total yearly rainfall of about 2,000 mm, about 20 percent of which is received in the DS. The drainage effluents of the system and the nearby areas discharge into the Laguna de Bay, a very large freshwater lake on the edge of which SCRIS is located.

7.4.3. Methodology

A contiguous, hydrologically bound rice area of about 500 hectares within the service area of SCRIS was selected for this study. The nearly hexagonal area was bounded by roads on five sides and the main irrigation canal on the other. For comparison, rainfed area of about 50 hectares outside of SCRIS service area was chosen. This area grows only one crop of rice in the WS of each year.

Through field survey, three drainage water outflow points and one irrigation water inflow point, which controlled the water inflow-outflow process of the selected area, were identified. At each of these points, water samples were collected in 4-liter glass bottles three times during each of 1989 WS and 1990 DS for pesticide analysis. Drainage outflows from the rainfed area were also collected at the same time for pesticide analysis. The analysis was conducted in the Pesticide Residue Laboratory of IRRI, using the gas chromatography technique. The minimum detection level for the method was 0.001 ppb.

At the end of each season, thirty randomly selected rice farmers from within SCRIS service area were interviewed in 1989 WS and thirty-five in 1990 DS to obtain information on the type and extent of their pesticide use and on rice culture. From the rainfed area, seven randomly selected farmers were interviewed for the same information.

7.4.4 Results and Discussion

7.4.4.1. Agrochemicals Used and Yields Obtained.

In the WS, SCRIS farmers used slightly higher amounts of pesticides (average 1.29 kg ai/ha) compared to the DS (average 1.21 kg ai/ha). The rainfed farmers applied less pesticides in the WS, which averaged 0.82 kg ai/ha (Table 7.6).

Of the pesticides applied during the two seasons, about 58 percent was insecticides, 35 percent herbicides, and 7 percent molluscicides. The average amounts of the three types of pesticides used within the SCRIS area were about the same between the two seasons. All rainfed area farmers used pesticides in their WS rice. In SCRIS, only one farmer in WS and three in DS did not apply pesticides because they practiced the integrated pest management concept and did not consider it necessary to use pesticides.

Rice yields in SCRIS were about 60 percent higher in the DS than in the WS, which could be mostly due to the higher amounts of fertilizer use and solar radiation in the DS. In the WS, rainfed area averaged only 2.35 t/ha yield, which was less than half of the average yield of irrigated rice. Once again, the difference could be attributed to the higher fertilizer use and irrigation water inputs in the SCRIS farms (Table 7.6).

The most commonly used pesticides are listed in Table 7.7. However, due to limitations in available facilities, analysis of irrigation inflow and drainage outflow samples for pesticide residues was conducted for only four commonly used pesticides. These are endosulfan, methyl parathion, carbofuran, and butachlor. The first three are insecticides, and the last one is an herbicide.

7.4.4.2. Pesticides in Irrigation and Drainage Water.

In the WS, the highest pesticide concentration found was 0.54 ppb for carbofuran in irrigation water and 3.46 ppb for methyl parathion in drainage water (Table 7.8). Their average concentrations for the three sampling times in the season were much lower. In the rainfed area, the maximum concentration found for drainage outflow was for butachlor (0.230 ppb). There was no carbofuran detected in drainage outflow. In

the DS, the maximum concentration in irrigation inflow was for endosulfan (0.08 ppb) compared to 1.99 ppb of carbofuran in the drainage water (Table 7.9).

In both seasons, the average pesticide concentration in drainage water was much higher for all pesticides (except butachlor in WS when the concentrations were about equal) than that in irrigation water, which implies that farmer-applied pesticides on the rice fields were transported to drainage water. Although the amounts of pesticide use between the two seasons were about the same, higher runoff of rice field water into the drainage system in the WS, which was caused by higher rainfall, is the likely reason for the higher maximum and average pesticide concentrations in the WS. In the rainfed area, where farmers used much less pesticide, the highest pesticide concentration in drainage water was much lower, 0.23 ppb.

7.5. Conclusions

From Case Study 1, there is clear evidence that in both sites groundwater in the shallow aquifers underneath intensively cultivated irrigated rice fields are receiving a large number of pesticides used by farmers for crop protection. Some of these pesticides are considered extremely hazardous. Although seasonal mean concentrations for individual pesticides in groundwater were much lower than the maximum concentrations detected, a major concern is the multiple toxicity. For example, in SCRIS the WS total of mean toxicity of the eleven pesticides in water samples was 0.283 ppb which is not very far from the maximum concentration of 0.500 ppb that is permissible in drinking water. Furthermore, there are other pesticides that farmers reported using, for which no analysis was conducted. Since in the rural areas water supply for domestic purposes is obtained mostly from shallow groundwater aquifers, the importance of maintaining their quality can hardly be overemphasized.

The process of pesticide movement to groundwater and pesticide persistence in groundwater are not fully understood. In-depth investigations are needed in order to predict the immediate and long-term impact of changes in pesticide use on groundwater quality.

Currently, there is a lack of clarity about the permissible concentrations of different categories of pesticides, especially the new generation pesticides, in drinking water. There is no clear guideline for the multiple-pesticides situation. Strict guidelines from concerned government agencies are needed, along with stronger practical measures to reduce the hazard of pesticide toxicity in natural water bodies. Long-term networks for monitoring water quality impacts of pesticides used for crop protection within major irrigation systems should be established.

Case Study 2 indicates that the drainage outflows of SCRIS and the nearby rainfed areas of Laguna are discharging some of the pesticides used on the rice fields into the freshwater body of Laguna de Bay. Both highly hazardous pesticides such as methyl parathion and carbofuran, and moderately hazardous pesticides such as endosulfan are present in the drainage water that discharges into the lake. The Philippine Department of Environment and Natural Resources (DENR) (1990) allows no toxicity from organophosphates such as methyl parathion in any kind of freshwater body because of health hazards of the chemicals. From this study it is not possible to predict how much of the pesticides in the drainage water would naturally degrade in the process of their transport to the lake or when they are mixed with the lake water, and how much would remain in the freshwater. Therefore, no conclusions can be drawn as to the potential of these residues in drainage water for toxifying the freshwater of the lake.

In the past the Laguna Lake Development Authority (LLDA) found that many pesticides in lake water exceeded the maximum permissible levels for propagation of fish and aquatic resources (NPCC, 1978). LLDA also claimed that there is a reservoir of pesticides contained in the bottom mud of the lake. Edwards (1974) indicated the presence of such an accumulation phenomenon in lakes because pesticides have the tendency to get adsorbed into the bottom sediments on reaching the body of water due to their affinity for organic matter. A cumulative process of pesticide buildup in Laguna de Bay would create an extremely hazardous condition for the aquatic life in the lake and attendant health hazards for humans. Intensive scientific monitoring of the lake water quality as affected by the quality of effluents discharged into the lake should be conducted on a long-term basis.

In the Philippines there is no clear policy at this time for pesticide concentrations in effluents from various sources discharging to freshwater bodies. Without such standards, it will be impossible to regulate the water quality of such water bodies with respect to pesticide toxicity and pollution from various sources.

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