Chwaka Bay is a predominantly rural setting part of Zanzibar Island, far away
from any industrial establishments. Consequently, the area does not directly receive
chemicals from industrial point sources. However, Chwaka Bay is located
relatively close to the rice farms of Cheju, approximately 4 km west of the Bay
(Fig. 1). Chwaka Bay is the drainage basin of the Cheju farms, but there are no
direct surface water pathways between the rice fields and the Bay (Mmochi 2005).
However, during the rain season, when the groundwater table is high, considerable
surface runoff reaches the Bay (Tack 1997). There is also the possibility of
groundwater intrusion due to the coralline structure of the underlying bedrock
(Tack 1997). In Cheju, both fertilizers and pesticides for insect pests and weed
control are used (Mmochi and Mberek 1998), making the Bay potentially vulnerable
to input of agrochemicals.
The southern part of Chwaka Bay is fringed by a large mangrove forest, part of
Jozani Park, which has the potential to trap both pesticides and nutrients from
the Cheju runoff (Mohammed 1998; Kruitwagen et al. 2008). Chwaka Bay may
also receive pesticides from fumigation and malaria control activities in the surrounding
villages and tourist hotels, and nutrients from waste water and domestic
animals. There is no sewerage system in the Chwaka Bay area and most villagers
use pit latrines while some households do not even have toilets (UNEP/FAO/PAP
2000; see chap.1).
Research addressing pollution in Chwaka Bay is scarce. Available documentation
focus on (i) pesticide and nutrient levels in and around the Bay, (ii) nutrient
dynamics in the Bay and exchange with adjacent areas, as well as (iii) the use of
pesticides in agricultural areas in the drainage basin, particularly in Cheju. This
chapter aims at summarizing, discussing and evaluating the available knowledge
on nutrient and pesticide pollution in Chwaka Bay.