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USE OF GIS TO MAP POLUTION OF NAIROBI RIVER

By Saraflora Sakwa Chachalia

The word Nairobi came from a Masai word 'enairobe' which means "a place of cool waters" The origin of this name was attributed to the three rivers which feed the Nairobi area that include Ngong, Mathare and Nairobi. The cool waters and serene environs made it a suitable place for early settlers to make camp. Trading activities increased due to this and slowly the city became the very heartbeat of Kenya. The saddening part is that the very reason of its existence is at the mercy of water and general environmental pollution causing a huge disruption of the natural ecosystem. The four rivers still flow through Nairobi but due to immense pollution over the past century caused by rapid industrialization and increased human settlement, the rivers are confused to be open sewerage.

A geographic information system(GIS) can be use to address this problem with the aid of a proper application tool whereby various patterns in the pollution cycle can be detected and locations with high pollution rates can be established. This is highly supported by the fact that a GIS is computer based system that supports the study of natural and manmade phenomena. An example of a GIS tool that can be used is arc map. The main methodology that can be employed is collection of water samples at different locations of the rivers say after 5km.as the samples are being collected their respective geographic positions are recorded .This can be done using a geographic positioning system. The collection interval points can be reduced in distance(say 2km) in areas where human population and industrial development is dense for the very reasons that this areas are more prone to high pollution rates. These collected water samples can then be taken to the laboratory and tested for their constituents and recorded.

Assuming that the tests have been carried out different areas of the river samples will have different constituents. Areas of dense industrial will probably reflect chemicals used in those factories in high quantities, petrol effluents from petrol stations or garage. Places with improper drainage facilities or non- existent facilities will reflect presence of raw sewage. These areas may include informal settlements or overloaded sewage drainages. Presence of agro-chemicals reflects possible pollution from farming activities near the river.

The location information will then be uploaded in a geographic information system and a database created for it. This database will include the different data collection point positions and their attributes. The attributes will be the test results from laboratory analysis showing the general water constituents in a particular location. Data on the profile of Nairobi river collected before hand is also incorporated in the data base. This profile may include is length, width, start points(river source) and end points(river drainage ).the profile can be obtained from aerial photography, satellite imaging or ground survey works. Once the data is on arc map or the preferred GIS software, the trends in the data can be analysed and displayed using the tools available in the software. Some of this trends can be;

- Different mineral composition on different sections of the river profile. Areas on the profile with high pollution rates

- The types of pollution and what they could possibly mean E.g. high chemicals will mean industries are draining in the river instead of directing their waste elsewhere.

- Raw sewerage may indicate improper sanitation in surrounding areas possibly in informal settlement. This can also mean strain on current sewerage drains. It can also indicate failure to compliance of the construction contractors who use short cut schemes by directing sewage to the river.

- Plastics and other non-biodegradable may indicate inadequacy in the dumping systems around an area.

- Higher pollution in some areas of the profile may also indicate a trend. For example over population is surrounding areas.

- Areas at river source will tend to have minimal pollution and towards the end maximum pollution.

With such kind of data legislative measures can be applied. A final map reflecting this pollution information can be created to aid general visualization and presentation to the decision makers. Thus GIS will aid in showing possible causes of pollution that may not be so obvious. This data can be integrated with population data of people living along the river to formulate relationships. It can also be integrated with the map data of the drainage system of Nairobi to establish areas of possible direct drainage to the river or areas of possible sewerage overflow due to strain on drainage systems. Analysis can be done on satellite images of areas along the river over the past ten years (or more) as an indicator of the land use changes that could have possibly increased the rate of pollution on the river. The geodatabase can be regularly updated and comparisons made over time to determine the rate of pollution of the river .if legislative measurements have been established this can also serve as an indicator if they are working or not.