source of contamination (e.g. latrine) to the water intake (screen) needs to be sufficient so as to pose a “low” to “very low” risk – this translates into a minimum of 25 days of potential travel of pathogens in the ground. Travel time is influenced by porosity, hydraulic conductivity (permeability) and hydraulic gradient. For medium size sand with an average porosity, the distance equivalent to 25 days is around 30 metres, but this can increase to over 100 metres for coarser sediments. However, the distance from contamination to water intake can reduce significantly where the screen intake is at a sufficient depth – this is due to greater variation of aquifer properties in vertical directions than lateral, meaning that a borehole with handpump could be placed very close to a latrine with low risk. However, screen depth must increase with increased extraction rate.
{{procontable | proborder="1" cellpadding="5" cellspacing="0" align="center"|-! width="50%" style="background:#efefef;" | Advantages! style="background:#ffdead;" | Disadvantages|-| valign="top" | - Manual access, that can be deepened or maintained easily at a later date
- Can be privately owned/operated <br>
- Circumstances which make the construction of large diameter wells necessary:
# Where the aquifer (water storage) is of extremely low permeability
# Low cost labour and skills are available
| convalign= "top" | - Greater effort and longer construction time
- Greater safety hazards during and after construction <br>
- Difficulty in preventing contamination <br>
- Can be difficult to sink to required depth especially in areas of flowing sand <br>
- Water levels in the aquifer can drop over time, resulting in possible mining (where aquifer is compressed and cannot hold water any longer afterwards)
|}}
==Resilience to changes in the environment==