RAIN WATER ABSORPTION TO REPLENISH BOREHOLES  

 

I’m measuring the static water level in a borehole near Borrowdale Village. It’s dropped from 32 metres in August 2021 to 47 metres at the beginning of January 2022 and has now recovered to 45 metres with absorption from the recent rains. The owner is fortunate as his property lies over 26 metres of topsoil and silt which has the potential to absorb massive volumes of rain water. However the upper levels of the hole are dry for most of the year due to drawdown from boreholes in the neighborhood. Between 26 and 60 metres there is Dolerite with some fracturing and at 60 metres there is a contact zone where water tends to collect. Below 60 metres is solid impervious Dacite rock – and no water – so drilling deeper is not an option.
Due to erratic or nonexistent Municipal water supply most of his neighbours rely on borehole water and many are experiencing drawdown problems or have dry or seasonal boreholes. Some pay for water deliveries. The water table has dropped 15 to 20 metres in the past decade.

The irony is that our owner has 3600 square metres of land and receives (on average) 800mm of rain per year or 2880 cubic metres per year of water over his entire property. That is a lot of water. Most of it will be lost to evaporation and transpiration through trees and other plants ( Evapo/transpiration ).

Because he has a large paved area there will also be loss through Municipal stormwater drains.
Rainwater capture reservoirs tend to be expensive because they have to be large. Rain falls occasionally and seasonally so that storage facilities need to be designed to receive a lot of water in a short time. They also take up a lot of space which may not be available.

So why not enhance your water table by increasing the volume of rain water which goes into the earth and decreasing the volume which is lost to Evapo/transpiration and runoff?

ABSORPTION WELL DESIGN

The primary objective is to rapidly deliver the rainwater below the surface where evaporation takes place and below the topsoil zone where the major rooting systems can be found. Tree roots are highly efficient water extraction machines but they tend to radiate in the shallow soils close to the ground surface. The water should be deposited below the shallow clays which are common in Harare as these clays tend to be semi pervious. A minimum well depth of 10 metres is therefore recommended, though 15 metres would be preferable. The absorption well in the picture is 4 metres deep but a layer of clay is preventing rapid percolation so that the water in the hole is 1 metre below the surface but the resting water level in a borehole nearby is at 31 metres. The water is not going down fast enough. More rain is likely to re

sult in overflow and a rooting system is developing around the top of the well. An absorption well in Granitic sands would not have to be as deep as 10 metres if the clays are absent as water percolates very quickly into Granitic sands.

The distance from the well to the borehole should probably be not less than 10 metres as the water should filter through the sub strata. Both well and borehole should be placed a safe distance from possible sources of pollution such as soak aways.

 

Suggested well design diagram
In the design diagram the top of the well is closed off and covered with soil. In this way evaporation is almost eliminated. If the water source is a clean roof and leaf traps are installed it may never be necessary to remove sediments from the well.

Suitable and unsuitable locations for absorption wells
The ideal location is in the vicinity of borehole which was productive for a number of years but subsequently dried up. This suggests that the historical water has been reduced by excessive pumping but that water will tend to accumulate at that location if the absorption rate can be increased artificially.
Many locations will not be suitable – for example:
• Steep slopes
• Where the bedrock is at the surface or very close to the surface
• In the vicinity of boreholes which were recorded as dry on drilling
• Wetlands where the water table is already very close to the surface.
These places have no absorption capacity.

RICHARD BRIDGES
24/01/22

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