Environ. Sci. Technol. 2000, 34, 4490-4495
Persistent Chlorinated Pesticides in Air, Water, and Precipitation from the Lake Malawi Area, Southern Africa H E I D I K A R L S S O N , * ,† D E R E K C . G . M U I R , † CAMILLA F. TEIXIERA,† DEBORAH A. BURNISTON,† WILLIAM M. J. STRACHAN,† ROBERT E. HECKY,† JOSEPH MWITA,‡ HARVEY A. BOOTSMA,‡ NORBERT P. GRIFT,§ KAREN A. KIDD,§ AND BRUNO ROSENBERG§ Environment Canada, National Water Research Institute, 867 Lakeshore Road, Burlington, Ontario, L7R 4A6 Canada, Lake Malawi Nyasa Biodiversity Conservation Project, Salima, Malawi, and Department of Fisheries and Oceans, Freshwater Institute, 501 University Crescent, Winnipeg, Manitoba, R3T 2N6 Canada
Concentrations of chlorinated pesticides were analyzed in air (biweekly 1997-1998), water, and precipitation at Lake Malawi, in southeast Africa. The pesticides in air in Senga Bay on the southwest shore of Lake Malawi were not extensively weathered, implying recent use. Elevated levels of heptachlor, chlorobenzenes, aldrin, and dieldrin were detected periodically, which indicated use on a regular basis. Annual average concentrations for those pesticides ranged from 31 to 257 pg/m3. Levels of HCHs, DDTs, chlordanes, and R-endosufan in air at Senga Bay were comparable to those of the Laurentian Great Lakes, ranging from 24 to 40 pg/m3. Considering air-water gas exchange and wet deposition, the net fluxes of chlorinated pesticides to the lake surface were depositional. Concentrations of chlorinated pesticides in the water from Lake Malawi were relatively low compared to the Laurentian Great Lakes and Lake Baikal. This indicates rapid transformation of chemicals in the water column, which was further supported by high metabolite-to-parent ratios. The results suggests that tropical regions may act as both a global source and sink for chlorinated pesticides, since removal processes may be faster compared to temperate and Arctic regions.
(1). Pesticide import surveys for the country of Malawi in East Africa indicate that chlorinated pesticides such as aldrin and endosulfan continue to be used (2). Others are applied inadvertently since they are impurities in widely used pesticides, e.g. DDT in dicofol, HCB in chlorthaldimethyl, and lindane (3). DDT is also used for malaria vector control upon recommendation by the World Health Organization (WHO). Lake Malawi is an oligotrophic lake situated between Malawi and Tanzania in the rift valley of southeast Africa between 3 and 10 °S (Figure 1). With an area of 28800 km2 and a volume of 8400 km3, Lake Malawi is one of the world’s largest lakes (4). Lake Malawi is also the most species rich lake in the world with an estimated 500-1000 species of cichilids (5). The lake is permanently stratified and anoxic below 200 m and has a water cycling time of 750 years due to low outflow volume (4). On average Lake Malawi receives 39 km3 of precipitation yearly directly on its surface, and it has a single rainy season starting in December and ending in May (4). The climate in Malawi is subtropical and monsoonal. The permanently warm water (22-30 °C in surface water, 22.4 °C in deep water (4)) makes Lake Malawi different from other large, and from a contaminant point of view, well characterized lakes, such as the Laurentian Great Lakes in North America (6, 7) and Lake Baikal in Siberia (8, 9). The slow water cycling time of Lake Malawi makes it particularly vulnerable with regards to chemical pollution since dilution will be ineffective in reducing contaminant concentrations (4, 10). Therefore, it is important to prevent levels of the pollutants such as chlorinated organic pesticides from increasing within the Lake Malawi region as well as in other vulnerable ecosystems (10). While there is some information about use of chlorinated pesticides in East Africa, concentrations in air and water have been characterized only by short-term studies (∼one month) (11) or by longer term studies now more than a decade old (12, 13). In this study we present biweekly air concentration data for chlorinated pesticides at Senga Bay Malawi from the 27th of Feb 1997 to the 2nd of May 1998. Continuous collection and analysis of precipitation during the same time period as well as sampling of lake water allowed estimation of fluxes of chlorinated organic pesticides to/from Lake Malawi. Semivolatile pesticides such as HCHs, chlordanes, and DDTs can be subject to long-range atmospheric transport (14, 15). Therefore, five day air parcel back trajectories were established for Senga Bay for 13 out of 28 air sampling occasions so that possible long-range sources could be identified. This work was part of a larger study on the environmental limnology, biology, and chemistry of Lake Malawi (4, 10, 16).
Experimental Section Introduction Use of persistent chlorinated pesticides continues for agricultural and public health purposes in Africa. The Food and Agriculture Organization (FAO) of the United Nations (UN) has recently estimated that there are 20000 tons of obsolete, toxic, and persistent pesticides and chemicals in Africa (detailed information in Table S1, Supporting Information) * Corresponding author phone: (905)336-4494; fax: (905)336-6430; e-mail:
[email protected]. † Environment Canada, National Water Research Institute. ‡ Lake Malawi Nyasa Biodiversity Conservation Project. § Freshwater Institute. 4490
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 21, 2000
Sampling Methodology. A summary of the sampling procedures is presented here, and full details are available elsewhere (17-20). Large volume water samples were collected at three locations in Lake Malawi (Figure 1) during cruises in April in 1996 and in Feb 1997 and 1999. Lake water was sampled with a submersible pump, pushing water through a filter, and XAD-2 extraction columns at 100 mL/ min. Both surface water and samples from depths of 40 and 80 m were collected. Air samples were collected with a high volume air sampler (General Metalworks PS-1) using two polyurethane foam (PUF) plugs preceded by a GF/A glass fiber filter to trap particulate matter (17, 18). The sampler was located at the Senga Bay research station (approximately 20 m from the lake shore, Figure 1), and samples were 10.1021/es001053j CCC: $19.00
2000 American Chemical Society Published on Web 09/28/2000
TABLE 1. Concentrations of Chlorinated Pesticides in Air at Senga Bay from the Period 27th of Feb 1997 to 2nd of May 1998a concn (pg/m3) av
FIGURE 1. Lake Malawi. Sampling sites for air and precipitation (Senga Bay) as well as water from Lake Malawi. collected biweekly from the 27th of Feb 1997 to the 2nd of May 1998. Approximately, 250 m3 of air was sampled during 24 h every 10 to 14 days. An automated wet-only precipitation sampler (Meteorological Instrument Center, Mississauga, Canada) located at Senga Bay was used to collect rain samples (19). The rainwater was filtered through glass wool at the bottom of the collection funnel, for removal of large particles such as leaves and insects. The wet-only precipitation sampler was equipped with an XAD-2 column that allowed direct extraction of target compounds (19, 20). Rain samples were collected continuously during the single rainy season, every three to four weeks from the 7th of Feb to the 9th of April 1997 and from the 20th of Nov 1997 to the 28th of May 1997. The average rain samples were the accumulated volume of 195 ( 90 mm of precipitation. In summary, 28 air, 10 precipitation, and 11 water samples were collected. Analytical Methodology. Details about the analytical procedures are available elsewhere (17-20), and a brief summary is given here. For air, the PUFs were Soxhlet extracted separately with hexane/dichloromethane (1:1) or hexane. The XAD-2 columns used to collect lake and rainwater were eluted with methanol followed by dichloromethane. For all sample types, combined extracts were then evaporated, the solvent exchanged to isooctane, and further volume reduction was performed with nitrogen evaporation. The samples were fractionated by column chromatography with either Florisil (1.2% deactivated) or neutral silica (activated) eluted sequentially with mixtures of hexane and dichloromethane (18, 20). Fraction A contained PCBs, chlorobenzenes, and p,p′-DDE, and Fraction B included the majority of the chlorinated organic pesticides (18, 20). Samples were evaporated to 1.0 mL with nitrogen. Analysis was performed either with a Varian 3600 gas chromatograph (GC) equipped with one 63Ni electron capture detector (ECD) or a HP5890 GC featuring dual 63Ni ECDs. A DB-5 capillary column (60 m × 0.25 mm × 0.25 µm) was used to separate the target compounds on the Varian 3600
R-hexachlorocyclohexane β-hexachlorocyclohexane γ-hexachlorocyclohexane ∑-hexachlorocyclohexane heptachlor cis-heptachlorepoxide cis-chlordane trans-chlordane trans-nonachlor oxychlordane ∑-chlordane dieldrin aldrin endrin R-endosulfan p,p′-DDT o,p′-DDT p,p′-DDE ∑-DDT hexachlorobenzene pentachlorobenzene 1,2,3,4-tetrachlorobenzene ∑-chlorobenzenes mirex methoxychlor pentachloroanisole trifluralin
max.
min.
N> MDCb MDCc
9.4 ( 9.0
10
1.8
1.3
28 (28)
5.3 ( 8.6
34
