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TECHNICAL REPORTS

Landscape and Watershed Processes

The Impact of Agricultural Runoff on the Quality of Two Streams in Vegetable Farm Areas in Ghana

^a CSIR Water Research Inst., P.O.Box AH 38, Achimota, Ghana
^b West Africa Office, International Water Management Inst., Accra, Ghana
^c Univ. of Ghana, Chemistry Dep., Legon, Ghana
^d UNESCO-IHE Inst. for Water Education, Westvest 7, 2611 AX Delft, The Netherlands
^e UNESCO Jakarta Office, Regional Bureau for Science for Asia and Pacific, JI. Galuh II, Kebayoran Baru, Jakarta 12110, Indonesia

^* Corresponding author (ntow{at}excite.com; w_ntow{at}yahoo.co.uk).

Received for publication March 19, 2007.

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results Discussion Conclusions REFERENCES

 
A study of two small streams at Akumadan and Tono, Ghana, was undertaken during the rain and dry season periods between February 2005 and January 2006 to investigate the impact of vegetable field runoff on their quality. In each stream we compared the concentration of current-use pesticides in one site immediately upstream of a vegetable field with a second site immediately downstream. Only trace concentrations of endosulfan and chlorpyrifos were detected at both sites in both streams in the dry season. In the wet season, rain-induced runoff transported pesticides into downstream stretches of the streams. Average peak levels in the streams themselves were 0.07 µg L^–1 endosulfan, 0.02 µg L^–1 chlorpyrifos (the Akumadan stream); 0.04 µg L^–1 endosulfan, 0.02 µg L^–1 chlorpyrifos (the Tono stream). Respective average pesticide levels associated with streambed sediment were 1.34 and 0.32 µg kg^–1 (the Akumadan stream), and 0.92 and 0.84 µg kg^–1 (the Tono stream). Further investigations are needed to establish the potential endosulfan and chlorpyrifos effects on aquatic invertebrate and fish in these streams. Meanwhile measures should be undertaken to reduce the input of these chemicals via runoff.

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results Discussion Conclusions REFERENCES

 
NONPOINT source agricultural pollution is regarded as the greatest threat to the quality of surface waters in rural areas. One of the most important routes leading to nonpoint source agricultural pollution of surface waters in rural areas is runoff. Runoff from agricultural fields introduces pesticides, soil, organic matter, manure, and fertilizer into small streams, increasing the volume of stream discharge and changing water quality (Neumann and Dudgeon, 2002). The impacts of such runoff are well documented (Cooper, 1993; Castillo et al., 1997; DeLorenzo et al., 2001). Intensive vegetable farm areas of Akumadan and Tono, Ghana (Fig. 1 ) provide an opportunity to study the effect of agriculture runoff on current-use pesticides levels of small streams in the tropics.

View larger version (32K): [in this window] [in a new window]   Fig. 1. Map of Ghana showing the location of the study areas.

Although few studies (Osafo and Frimpong, 1998; Ntow, 2001) have dealt with pesticide residue levels in river ecosystems in farmlands of Ghana, little is known about transport pathways such as rain-induced runoff. Vegetable crops are sprayed with a range of chemical pesticides (Ntow et al., 2006), and fields are cultivated up to the margins of streams where agriculture is practiced. Because of current land tenure systems which support land rotation (Gerken et al., 2001), much agricultural land in Ghana has been abandoned (left to fallow). Active and abandoned agricultural land is generally situated in drainage basins. In this study, we investigated the effects of rain-induced runoff from vegetable fields on water quality by comparing the concentrations of current-use pesticides of paired sites upstream and downstream of vegetable farms along two streams, the Akumadan and the Tono. These sites were sampled at the end of the dry season and again at the start of the wet season when the streams received runoff after rainfall. Our null hypothesis was that the magnitude of the difference in levels of current-use pesticides between the upstream and downstream pairs of sites would remain unchanged between dry and wet season sampling. It was anticipated that any difference arising in the data set would manifest during the wet season, when high levels of current-use pesticides might occur downstream of the vegetable field.

	Materials and Methods

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results Discussion Conclusions REFERENCES

 
Study Area
We selected for study two small streams that flow through areas of intensive cultivation of vegetables. The Akumadan stream is located near the village Akumadan, a prominent tomato-cultivating village in Offinso District in Ashanti Region (Fig. 1). The surrounding areas of the Akumadan stream are characterized by intensive vegetable farming, mostly mixed cropping. Among the major crops cultivated are pepper, eggplants, okra, and tomatoes. Of the vegetables cultivated, tomatoes alone constitute over 90%. The tomato season runs through the whole year in four sub-seasons (Ntow, 2001). The area is subject to rain events of 150 rainy days yr^–1 or more, and an annual precipitation average of 1400 mm (Nurah, 1999). The predominant active ingredients of pesticides used at Akumadan are endosulfan and chlorpyrifos. Applications on vegetable fields occur on a calendar basis (Ntow et al., 2006), at approximately 7-d intervals, and at average rates of 1.0 and 0.04 kg a.i. ha^–1 for endosulfan and chlorpyrifos, respectively.

The Tono stream is located near the Tono Irrigation Project, at Tono near Navrongo in Kassena-Nankana District in Upper East Region (Fig. 1). The Tono Irrigation Project, under the management of Irrigation Company of the Upper Region (ICOUR), was started over a decade ago to promote the production of food crops by small-scale farmers within organized and managed irrigation schemes. The project covering 2490 ha and divided amongst 3000 farmers uses the waters of the Tono stream (Fig. 1) for the purpose. The cropping areas are divided between upland and lowland areas on a ratio of 50:50. Crops grown in upland plots include onions, tomatoes, millet, groundnuts, sorghum and maize. The lowland areas have been developed for rice production. The number of rainy days per year is <70 with an annual precipitation average of about 1000 mm. Predominant active ingredients in use in the area are endosulfan and chlorpyrifos. Applications on vegetable fields occur at average rates of 0.04 and 0.6 kg a.i. ha^–1 for endosulfan and chlorpyrifos, respectively, and when pests appear on crops. The applications are supervised and moderated by ICOUR. The physicochemical properties of endosulfan and chlorpyrifos are given in Table 1 .

Water samples representing the pesticide levels during runoff were collected using a procedure described in Dabrowski et al. (2002b). Briefly, glass-sampling bottles were stored in the stream with the opening approximately 3 cm above the normal water level. During rainfall-induced surface runoff, the rising water level filled the bottles passively. A small glass pipe tied in the opening of the bottle enabled a free flow of water into the bottle while air could flow out via the glass pipe. Retrieval of water samples took place within 24 h of the runoff event. Samples of pesticides associated with streambed sediments were collected manually with a stainless steel spoon from the top 1 cm surface layer into aluminum foils. All the samples were transported to the CSIR Water Research Institute Laboratory within 24 to 48 h on ice in clean ice chests and stored in the laboratory refrigerator at 4°C until time of analyses.

TSS was measured using a turbidity meter (2100P Turbidimeter, Hach Company, Loveland, CO, USA). To calibrate the turbidity measurements as described by Schulz (2001), certain samples were filtered through pre-weighed Whatman GF/F (0.45 µm pore-size) glass microfiber filters and dried at 60°C for 48 h. The filter paper was then re-weighed to determine TSS.

Pesticide Analysis
The extraction and analyses of water were performed following the Association of Official Analytical Chemists 990.06 and 970.52 methods (Rovedatti et al., 2001). Briefly, the 1.0 L unfiltered water samples were extracted sequentially three times with 25 mL n-hexane each time. The extract was dried with anhydrous sodium sulfate and concentrated down to 10 mL by evaporation in a TurboVap (Zymark, Palo Alto, CA, USA). A clean-up system, using a chromatographic column packed with florisil, previously activated for 3 h in an oven at 130°C, and anhydrous sulfate (both rinsed with petroleum ether) was used. The extract was transferred to the column. Three fractions were obtained after elution with 6, 15, and 50% ethyl ether in petroleum ether. Maximal flux rate of elution was 5 mL min^–1. Each eluate was evaporated. The extracts were dissolved in 1.5 mL n-hexane and made up to 2 mL with more n-hexane. The dissolved extracts were injected into a gas chromatographic system for identification and quantification of the pesticides.

Sediment samples were well mixed to obtain a homogeneous sample and then transferred into pans to air-dry at ambient temperature. The air-dried sediment samples were ground in a mortar and sieved (2 mm). The extraction procedures are described by Ntow (2001). Briefly, approx. 5-g representative sieved samples were weighed into extraction thimbles, soxhlet-extracted in methanol, and cleaned up in florisil as described above for water.

Gas chromatography was performed with an Agilent 6890 coupled with Agilent 5973N mass selective detector-electron impact ionization. The capillary column was HP-5MS (length 30 m; I.D 0.25 mm and film thickness 0.25 µm) and packed with 5% phenyl methyl siloxane. The GC–MS was operated in the selected ion-monitoring mode at the following conditions: injection port 250°C (splitless, pressure 22.62 psi; purge flow 50 mL min^–1; purge time 2.0 min; total flow 55.4 mL min^–1). Column oven: initial 70°C, held 2 min, programming rate 25°C min^–1 (70 to 150°C); 10°C min^–1 (150 to 200°C); 8°C min^–1 (200 to 280°C) and held 10 min at 280°C. The carrier gas was nitrogen at 15 psi. The injection volume was 1 µL (Agilent 7683 Series injector).

Analyte recovery experiments were performed with the water and sediment matrices. One-liter samples of distilled tap water were spiked with 0.01 µg of each pesticide standard. The samples were then extracted and analyzed, in accordance with the previously noted procedure. Similarly, uncontaminated sediment (taken from the premises of the CSIR Water Research Institute, Accra, Ghana) was spiked with known quantities of pesticides before extraction (0.1 µg substance^–1 in 5 g of sediment dry mass) and was then processed and analyzed as described previously. Recovery of the different pesticides ranged between 79 and 104% with the variation coefficients not exceeding 13%. The concentration estimates were not corrected for these recoveries. The following quantification limits were obtained for water and sediment: 0.01 µg L^–1 and 0.05 µg kg^–1 dry weight, respectively (calculated from real samples as being 10 times the signal/noise ratio). Three replicates of samples were used. During the sample extraction, blanks were regularly processed (one in ten). The standard error of the pesticides concentrations in three equal samples ranged between 2.9 and 6.1% depending on the sample and on the compound. The screening included the following pesticides: (i) organochlorines: - and β-endosulfan, endosulfan sulfate (ii) organophosphates: chlorpyrifos. Selection of analyzed pesticides was done on the basis of use information gathered during a survey.

Differences in concentration of pesticides between upstream and downstream sites on each sampling date were analyzed by one-way-analysis of variance followed by a Bonferroni test (equal variances assumed) (SPSS, 2004). The data were analyzed for normal distribution (Kolmogorov-Smirnov test) and homogeneity of variance (Levene's Test) before statistical analysis.

	Results

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Pesticides in Water and Sediment
Pesticides measurements undertaken in dry and wet seasons are summarized in Table 2 . In both streams, sites situated upstream and downstream of vegetable fields were free of current-use pesticide contamination in water samples taken in March, during the dry season period of investigation, considering a quantification limit of 0.01 µg L^–1. However, in the wet season in April and May, after runoff events, the pattern in both streams was changed. Water samples taken during the wet season under runoff conditions showed no or, at the most, very low (0.01 µg L^–1) pesticide contamination of upstream sites. Relatively, increased water contaminations by endosulfan and chlorpyrifos, 0.07 and 0.02 µg L^–1, respectively, for the Akumadan stream and 0.04 and 0.02 µg L^–1, respectively, for the Tono stream, were detected at downstream sites in the corresponding samples for the wet season (Table 2). Thus, regarding the downstream sections of the two streams, pesticide concentrations measured in the wet season were increased significantly (p 0.001) above the corresponding concentrations in the dry season in March. Additionally, while there were only minor differences in pesticide levels in water samples collected in dry versus wet season in upstream sites of the streams; the respective differences were very significant (p 0.001) in downstream sites.

To further assess the effect of vegetable field runoff, we calculated the difference between the mean concentration of pesticides at the downstream and upstream sites on each stream on both sampling dates. To account for residues concentrations that were below the limit of quantification, we assumed that actual concentrations for non-detect (nd) samples were equal to the limit of quantification. However, this would underestimate the differences if there would be no contamination of the sampled water and sediment. On the other hand, since the comparison here was relative, not absolute, size of the effect, it was decided to stick to this assumption. The concentration of pesticides was always higher at the downstream sampling points during the wet season (Fig. 2 ). The effect was particularly strong for endosulfan and chlorpyrifos in water and sediment, respectively, for both streams.

View larger version (15K): [in this window] [in a new window]   Fig. 2. Mean (± SE; n = 3) total suspended solids (TSS) levels before and after runoff conditions in Akumadan and Tono streams.

View larger version (12K): [in this window] [in a new window]   Fig. 3. Percentage difference between the mean concentration of pesticides at upstream and downstream sampling points on the Akumadan and the Tono streams. The data are presented separately for water and sediment (see Tables 2 and 3).

	Discussion

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results Discussion Conclusions REFERENCES

There is much evidence to suggest that measured pesticide concentrations were as a result of surface runoff and not via alternate routes such as rapid leaching through the soil substrate (Dabrowski et al., 2002b). Endosulfan and chlorpyrifos were relatively prevalent in downstream water (dissolved + adsorbed) and sediment samples collected during runoff. Both of these pesticides have a high affinity to adsorb to soil particles (Table 1) and are thus relatively immobile in soils (EXTOXNET, 2006). Thus, the most feasible way that these particles could land up in the streams is via surface runoff. The relative increase in turbidity (suspended sediment; Fig. 3) after the storm events is another factor that suggested that runoff had taken place (Dabrowski et al., 2002b). The increases in turbidity must have been as a result of soil and sediment being physically washed into the streams, transporting adsorbed pesticides in the process, and are unlikely to have occurred via subsurface drainage.

The proportion of endosulfan isomers in total endosulfan detected in the study gave further indication that the chemicals detected during runoff in the streams stem from current-use pesticides applied in vegetable production in the catchments. Commercial formulations of endosulfan (e.g., Thionex, Thiodan) used in vegetable production in Ghana (Ntow et al., 2006) contained two isomers, - and β-isomers, with a higher relative amount of -isomer than β-isomer (-isomer/β-isomer = 1.96:1). According to Kimber et al. (1994), Kathpal et al. (1997) and Antonious et al. (1998), although the endosulfan -isomer is about 70% of the active ingredient in commercial formulations, it is found in aquatic environments and solid surfaces at appreciable levels only immediately after spraying, due to its high volatility. Endosulfan -isomer is more volatile (vp = 0.006 mm Hg at 20°C) and less water soluble (2.29 mg L^–1 at 22°C) compared to the β-isomer (vp = 0.003 mm Hg at 20°C and water solubility = 31.1 mg L^–1 at 22°C) (Guerin and Kennedy, 1992; Antonious et al., 1998). Approximately 70% of the total endosulfan detected in the present study was identified as the - and β-isomers, and can therefore be attributed to recently applied chemical instead of residual concentrations.

Pesticide analysis in this study was performed on unfiltered water samples. The relative importance of pesticide transport dissolved in water or adsorbed onto suspended solids has been investigated by several authors on a range of pesticides and reported in Kreuger (1998). These results suggest that the greater part of the pesticide load is carried dissolved in water. Wauchope (1978) summarized several runoff studies and concluded that most of the pesticide contamination is introduced in the water phase, not necessarily because the concentration is higher there but rather because the runoff contains considerably more water than solid components. Nevertheless, the distribution of any substance in the water or solid phase of the runoff depends mainly on its water solubility. According to Wauchope (1978) the importance of particle-bound inputs increases for insecticides with water solubility above 1 mg L^–1. Although the partitioning behavior of endosulfan and chlorpyrifos is in general accordance with their physicochemical properties (Table 1), both chemicals have been found at relatively high concentrations in matrices, water, and suspended particles (Dabrowski et al., 2002b). Therefore, it is reasonable to suppose that, as in the present investigation, the absolute pesticide amounts are generally in both the water and adsorbed phases.

The runoff sampling method used in the present study represents a reasonable technique for sampling in a challenging environment. It is comparatively cheap and easy to install. Reliable qualitative information about the pesticide contamination can be obtained by use of this sampling method. A distinct disadvantage of the method is that it is not suitable for quantitative measurements of chemical transport. In many cases, however, it is more important to know whether any input of pesticides into a stream has occurred, and the method can provide an answer to this question. The equipment for quantitative, timed water/sediment sampling is expensive and usually time-consuming to operate (Schulz et al., 1998). The qualitative measurement offers the opportunity to obtain real contamination data from the field without the necessity of measuring any other parameters, as for a deterministic model (Schulz et al., 1998).

In summary, agricultural field runoff includes pesticides (Wauchope, 1978; Schulz et al., 1998; Neumann and Dudgeon, 2002) and sediments (Schulz, 2001). Both may degrade the water quality, but the contamination is only transient after heavy rainfall. The potential of runoff as a route of entry for pesticides and sediments input from agricultural fields into streams has been illustrated in a number of other studies (e.g., Wauchope, 1978; Readman et al., 1992; DeLorenzo et al., 1999; McDonald et al., 1999; Werner et al., 2000; Schulz, 2001; Schulz et al., 2001; Dabrowski et al., 2002a). Schulz (2001) and Nakano et al. (2004) provide an overview of field studies undertaken in temperate latitudes that establish a relationship between runoff events and increased total suspended sediment and pesticide levels in a river by monitoring their residues in river water or sediments. In this study, we were able to illustrate similar increases in tropical streams by focusing on a comparison of upstream and downstream sites in the streams before and after rainfall events and consequent runoff from vegetable fields. Because the study streams did not receive point-source discharge of pesticides, the only feasible way that the pesticides could have entered the streams was via surface runoff from the vegetable fields. Accordingly, we conclude that it was runoff from the vegetable fields which impacted the downstream sites of the streams. Differences in agricultural activity in the upstream and downstream subcatchments could be invoked to the variation in pesticide contamination between the sampling sites.

	Conclusions

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results Discussion Conclusions REFERENCES

 

1. There is much evidence in the present study to conclude that runoff events led to relative increases in measured levels of current-use pesticides.
   
2. Some general differences in levels of the different pesticides between upstream and downstream sections of the streams were observed.
   
3. The magnitude of the difference between the upstream and downstream pairs of sites varied between dry and wet season sampling.
   

	ACKNOWLEDGMENTS

 
The authors are grateful to the Dutch Government (through the UNESCO-IHE Inst. for Water Education), the International Water Management Inst. (IWMI) and the International Foundation for Science (IFS) for their financial support of this research. The Kinneret Limnological Lab., Migdal, Israel, is acknowledged for technical assistance in the use of GC/MS.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results Discussion Conclusions REFERENCES

 
All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher.

	REFERENCES

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This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Ntow, W. J. Articles by Gijzen, H. J. Search for Related Content PubMed Articles by Ntow, W. J. Articles by Gijzen, H. J. Agricola Articles by Ntow, W. J. Articles by Gijzen, H. J. Related Collections Agricultural Pesticides Runoff Other Environmental Contamination Water Pollution