Purging of Boreholes

Posted on 10 Dec 2021

Generally, the water quality of groundwater sources is considered more stable than surface water sources. For this reason, groundwater sources are usually sampled quarterly which is less frequent than surface water sampling which occurs monthly. To obtain a representative sample, it is often suggested that boreholes be purged prior to doing so.

 

Purging is considered an important part of groundwater monitoring as it removes the stagnant water from borehole casings before a sample is taken. This enables the temperature, pH and Electrical Conductivity of the groundwater to stabilize, ensuring that a true representation of the groundwater is obtained. It is found that many boreholes do not have caps or have caps that are damaged. As a result, it is often found that external factors such as insects, beehives and small reptiles such as Geckos manage to fall into the boreholes and contaminate the water. Therefore, in such instances, purging of the borehole would allow all contaminated water to be pumped out, so that the borehole will be recharged with its true groundwater. The Quality of Domestic Water Supplies Volume 2 recommends that samples of such boreholes be taken 50cm below the water level after the borehole has been purged.

 

According to the Quality of Domestic Water Supplies Volume 2, the following steps need to be taken when purging a borehole:

 

The first step is to measure the static water level which is the distance between the water and the top of the borehole casing. Next, the borehole depth needs to be measured, which is the distance of the entire borehole casing. Following that, the height of the water column needs to be determined which is calculated by subtracting the depth of the borehole with the measured water level. The next step is to calculate the standing volume of water by substituting the following formula:

 

Volume of standing water = πr2 x h x 1000

h = water column height in meters

r = borehole radius in meters

 

The time needed to remove three volumes can then be calculated by making use of the volume determined in the previous step.

 

Purging ensures that the stagnant water of the borehole is removed and replaced with a true representative of the groundwater.

 

ENVASS has an automated purging unit and offers groundwater monitoring as well as the purging of boreholes to ensure that true representative samples are taken on a quarterly basis. For more information about these services contact ENVASS at info@envass.co.za