How to correctly follow a borehole yield test?
The South African National Standard for the test pumping of water boreholes (SANS 10299-4:2003) clearly prescribes how the borehole yield testing is to be carried out.
Remember the Department of Water and Sanitation (DWS) will only authorise groundwater use if the correct (above) method of borehole yield testing is used.
This spreadsheet provides a template for the data capture.
Here is a summary of the specifications for test-pumping of boreholes.
The yield test would need to include:
4 to 6 x 1 hour steps, each at a different rate – incrementally increasing.
During the last step try to draw the water level down to the pump depth.
The yields to use for the steps are:
- Step1 – One third of the expected yield
- Step2 – two-thirds of the expected yield
- Step3 – equal to the expected yield
- Step4 – one and a half times the expected yield
The planned steps can be adjusted during the test, although the yield during the individual steps must be constant. Yield must be measured at least 3 times during the test to ensure it is constant.
Recovery of the water level after the step test should be monitored till the water level recovers to ~ 1 m hours of the Static water level or for ~ 12 hours.
Constant Discharge Test:
- 24 – 72 hours at a constant rate
- If the water level is drawn down to pump inlet during the test, the pump must be stopped immediately and recovery of the water level monitored.
- The constancy of the yield is very important, otherwise the data cannot be analyzed.
- Send Step Test drawdown and yield information to GEOSS for assessment, and we will provide the recommended yield of the Constant Discharge Test.
- The water level measurements should be taken and recorded according to the attached spreadsheet.
Immediately after the pump is turned off after the pumping test, start measuring water levels.
You need to measure recovery until:
- Water levels recover to less than 5 % of the total drawdown during the constant discharge test
- At least three readings taken in succession are identical
- A time equal to the total time taken for the Constant Discharge Test has elapsed
The data that needs to be collected includes:
- Data and time at commencement of test
- The Static Water level at the start of the test
- The depth of the borehole
- The distance from the borehole to observation boreholes (if applicable)
- Pump installation depth
- Water strike depths (if known from drilling/landowner)
- Borehole diameter
- Rainfall (if it rains during the test)
- Drawdown of the water level
- Rate of discharge (for Steps and constant Tests)
- Attached spreadsheets show measurement intervals and required info.
- Pump must have suitable power drive and have the correct pumping capacity. This needs to be managed properly!
- It is VERY IMPORTANT that the pumping rate is CONSTANT during the individual steps and the constant discharge test. The mathematical equations used for analysing the data are only valid if the flow is constant! If the variation in the pump yield exceeds 5 % the test must be stopped, water levels allowed to recover, and the test restarted using suitable equipment! Valves and flow gauges are needed to monitor and control the flow rates as during pumping the change in head results in the pump yield changing.
- Pump inlet must be at the main water strike. If this is not known, install the pump 3 – 5 m from the bottom of the hole.
- Pump must have a non-return valve to ensure water doesn’t flow back into the borehole
- Flow can be measured using:
- Bucket of known volume and stop watch (most reliable, and should be used to check other methods)
- Flow meter (note that if using a flow meter it will only work when the discharge pipe is full and the flow is not turbulent)
- Orifice weir
- V-notch weir
Boreholes close by should have their water levels monitored during the Test. Boreholes in the area should be rested for at least a day before pumping.
Water from the borehole must be piped 50 – 100 m downgradient from the borehole so that it does not recharge and affect the test.