Obtain an adequate attenuation of microbiological and chemical contaminant to provide water with a very low risk of contamination.

For microbiological attenuation, it is considered that a low risk is obtained when the shortest pathway for the contaminated water to reach the spring is 25 days of travelling time, whereas 50 days is considered a very low risk¹⁾.

The objective is therefore to guarantee a minimal travel time of 25 days for contaminated water and to promote the extension of protection area to reach 50 days or more.

Ground Water Depth: This is an important component of the attenuation, according to ARGOSS¹, “as little as 2 m of fine unsaturated sand can provide sufficient attenuation of faecal indicator bacteria” and Inter Aide considers that 2 m of well compacted backfill rich in clay (with an attenuation performance much greater than fine sand) must be attained whenever possible.

Inner protection Radius: While this area aims is primarily to protect physically the spring structure (absence of tree and large animals). This area is also considered a critical area, where contaminant must be completely avoided as the risk of penetration is significant due to the limited water depth (especially when less than 2 m), the low lateral attenuation and the possible pathways created by the construction. The area is fenced off to avoid the entrance of human or stray animals. Inter Aide considers that the adequate minimal radius is 5m which must include all areas where water surface ingress is more likely (bare rocks, backfills, etc). If it is impossible to reach this distance an exception can be granted upon justification for distance not under 2m.

Outer Protection Radius: This line of protection is determined by a sufficient lateral attenuation, microbiological contamination that would enter outside this radius would have a sufficient travel time to ensure an adequate die-off rate (25 – 50 days). The distances are calculated taking into account unfavorable soil types likely to be encountered in Ethiopia, Inter Aide thus promote an extremely safe distance of 50 m for this radius and acceptable distance down to 30 m as having a minimal safety margin. Note that the ground water depth is in all likelihood increasing with the distance to the source (ground slope » water table slope) hereby increasing also the vertical attenuation.

Watershed: The watershed area above the outer protection is not considered as area with specific requirement toward protection against contamination. It is however an area where water penetration into the soil must be facilitated (to increase spring recharge) and run-off must be limited. Existing biological coverage (forests, bushes) in that area should be protected for the sake of water quality.

The determination of distance greatly depends of the soil types. We present here a quite unfavorable scenario for the type of soil found in the project area to cover as much as different potential cases in prevention.

Most probable attenuation scenario²⁾

> Conclusion for Sustainability standards: Perimeter radius above and on the side of the spring

The soil found in the project area, are mostly well draining soils quite homogeneous, and where taken as having a permeability always lower than clay loam (at the exclusion of leptosols⁵⁾ where shallow fractured rock might increase significantly permeability rates)⁶⁾