Murky, murky waters

Seydou Niang
Anne Gueye-Girardet
Amadou Sall

Dakar’s market gardeners often break into sewage mains to access untreated wastewater to use on their crops. We look at a project employing GIS to analyze the health and environmental risks of this phenomenon.

The processes of economic liberalization have hit the poor of Senegal especially hard. In Dakar, unemployment of 25% has forced many city-dwellers to engage in small-scale farming to feed their families. Some 60% of the city’s vegetable production now comes from urban agriculture.

Sadly, not only work, but water too is scarce. Most urban farmers cannot afford water from the potable distribution network of the water company, Senegalese des Eaux. Worse still, given the proximity of Dakar to the coast, water from wells is highly saline. Farmers thus have no choice but to break into the sewage mains and access untreated domestic wastewater. Just 6% of the city’s wastewater collected is treated before discharge.

Naturally, this raises a host of health and environmental concerns. In areas of Dakar where untreated wastewater constitutes for some farmers an important source of water, many among them are infected with intestinal parasites. Further concern arises regarding consumption of produce cultivated using the wastewater, especially vegetables such as lettuce or tomatoes that tend to be eaten raw.

Since 1998, Canada’s International Development Research Centre (IDRC) has funded the Institut Fondamental d’Afrique Noire, at Cheik Anta Diop University, to lead a group of researchers from the University of Dakar, the Institut Sénégalais de Recherche Agricole and the Senegalese NGO Environnement et développement du tiers-monde (ENDA-TM). The research group has investigated the risks of using this untreated wastewater in urban farming and ameliorate its worst aspects, with the aim of identifying adaptable, long-lasting solutions to enable the healthy use of wastewater. Key to this multidisciplinary investigation has been the use of a geographical information system (GIS) to map the extent of the use of untreated wastewater on urban farms and to cross-reference it with the incidence of negative health and environmental impacts.

User-friendly for market gardeners

Vital to this process was the development of a user-friendly GIS in which the user interface was optimized for the multiple actors involved in urban agriculture, from city planners to the market gardeners themselves. The system had to be durable and applicable to a wide variety of environmental problems. In the long term, the system had to be able to use and simulate multiple forms of spatial knowledge, and to take into account the behaviours of the population, as well as their scientific, social and political understanding.

We have developed a powerful tool that permits researchers not only to define the different components of the practice, to localize and predict pollution risks and the people likely to be affected. It also allows decision-makers to integrate urban agriculture into urban planning, and the producers themselves to become more aware of the risks. Subsequently, the investigation focused on identifying and producing a map showing the typical locations of women vegetable vendors throughout the markets of Dakar. The GIS gave a clear indication of the size of the populations concerned and a representation of the commercialization of vegetable produce from market gardens.

In addition to untreated wastewater, Dakar’s farmers also employ considerable amounts of fertilizer and pesticide. Researchers from the Centre d’Analyse Minérale at the University of Lausanne, Switzerland, thus suspected that the soils would be contaminated with heavy metals, nitrates, organochloride and organophosphorus pesticides, as well as bacteria and pathogenic parasites. The CAM researchers collaborated in the work that was already being done, now with additional funds from the Swiss National Science Foundation.

For the geographical representation of the data on water pollution, the researchers employed software from Delphi and ArcView* to integrate, organize and spatialize the existing data and the new data collected in order to develop scenarios for simulation.

In this way, the GIS permitted the creation of a register of producers by census, then by an attribution of a code related to the plots of land, which were localized by GPS. The plots were digitized from geo-referenced aerial photographs. For each of these plots, the type of water used for irrigation (from shallow wells, deep wells or wastewater) was indexed. Additionally, the agricultural practices, such as the use of different types and quantities of fertilizer and pesticide were indexed based on a representative sample of the plots. We created a soil map layer within the system, as well as a database containing the results of chemical and microbiological analyses of the groundwater and soil samples.

The data collected was then visualized on digitized geo-referenced maps. They showed that the producers didn’t take into account the different types of soil in their dosages of fertilizer, resulting in both economic losses for the farmer and contamination of the groundwater.

It is a complex, costly and time-consuming process, but we hope through these investigations, we can minimize the risks of using untreated wastewater in urban agriculture – a practice that whatever its hazards, greatly improves the food security of the people of Dakar.

Seydou Niang ( seyniang@refer.sn ) is a hydrobiologist and head of the Laboratoire de traitement des eaux usées, Institut Fondamental d'Afrique Noire, Cheikh Anta Diop University, Senegal.

Anne Gueye-Girardet ( anne.gueye@unil.ch ) is a PhD candidate with the Faculty of Geosciences and the Environment, University of Lausanne, Switzerland.

Amadou Sall ( amadou.sall@cse.sn ) is a GIS expert at the Centre de Suivi Ecologique, Senegal.

For more information, visit the UNIL website.

*ArcView GIS software was last covered in ICT Update 27, September 2005.

01 October 2006

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