Written by Kes Murray
Many boreholes get over-pumped, leading to pump failure/burnout, borehole clogging, impacts on other groundwater users and spring flows, and even land subsidence. While this is usually only discussed when a neighbouring land owner drills new boreholes or when large infrastructure investments are lost due to yield drops after 2 months or less, a major issue is the lack of proper borehole yield testing.
Why Correctly Testing Borehole Yields Matters
This article discusses the difference between a driller’s airlift yield and a tested yield and also distinguishes between the commonly misused Pump Inlet Test or Farmer’s Test, and the South African National Standards for testing. The recommended yields from these two tests usually differ dramatically and are compared for 25 boreholes in the Western Cape of South Africa, where both tests were performed on each of these boreholes.
Once a new borehole has been drilled, a competent driller will provide the owner with a Drilling Certificate or drilling record, along with the invoice.
This will include information such as:
- What depths were drilled with different diameters,
- How much steel casing was installed,
- If PVC casing and a silica gravel pack were installed,
- Where the perforations are in the PVC casing,
- Simplified geological information with depths,
- What depths was water intersected at and how much at each depth,
- Comments on if the water was clear or turbid and if there was any smell
- Airlift yield/drilling yield.
The airlift yield (also referred to as the driller’s yield or blow yield)
This yield is the measured rate that water comes out the top of the borehole while a large compressor blows air into the bottom of the borehole, lifting the water out. This water is collected by building a small dam wall with the drill chips and channeling it through a V-notch weir, where the flow rate can be measured. Alternatively, it can be channeled through a pipe and caught in a bucket of known volume, which is timed with a stop watch while it fills up to get the flow rate. It will usually be reported in L/hour, m3/hour, L/s, or occasionally gallons/hour.
So, what is this yield?
New borehole owners often make the mistake of assuming that this is the rate that they use to get a pump for their borehole. In fact, in some cases a driller will even tell their client that they can use two-thirds or three-quarters of this rate.
This is NOT the yield of the borehole; it is just a measurement of how much water can be blown out of the borehole over a very short period of time. Typically 30 minutes and almost always less than 2 hours. This airlift yield is useful in identifying at what depth greater or smaller fractures were intersected. But in terms of a borehole’s yield, this is only a rough estimate of which boreholes might be higher yielding than others and what size pump should be used to TEST the borehole’s yield.
From experience, the actual borehole’s yield can range from anything between 10% of the driller’s airlift yield to occasionally even more than the driller’s yield, and everything in between!
Test Pumping Boreholes– What NOT to do
An old and unreliable method of testing boreholes in South Africa is still commonly used today and is frequently responsible for over estimating a borehole’s yield. This is commonly referred to as a pump inlet test, farmer’s test or boeretoets and is mostly done by the company that will be selling and installing the pump, as well as servicing and replacing it with a new one once it has failed from over pumping.
The problem with pump inlet testing
This pump inlet test is usually 6 – 8 hours long, but occasionally 12, 16 or 24 hours, depending on the test contractor’s choice. It involves installing a large pump inside the borehole, either to where the driller recorded the main water strike, or near to the bottom of the hole. The pump is then switched on at maximum capacity to drop the water level all the way down to the pump as quickly as possible, after which the pump inlet flow rate is measured for the next few hours. Sometimes it is measured and recorded each hour, other times it is just measured at the end of the test.
From there, anything between 50 – 100% of that final rate is provided as the borehole’s tested yield, often with the same catch phrase as the drillers use. “You can take out two thirds and leave one third behind for the aquifer”, or three quarters, depending on their favourite fraction. Occasionally a comment about recovery will be made, such as “recovered fully within 5 minutes”. A few different examples from the Western Cape are provided below.
What not to do – example 1
What NOT to do – Example 3
Test Pumping Boreholes– What TO do
About 20 years ago the South African Bureau of Standards (SABS) published the SANS 10299:2003 documents, on the Development, Maintenance and Management of Groundwater Resources.
These cover the acceptable national standard of work in South Africa for:
- Locating and siting boreholes
- Designing, constructing and drilling boreholes
- Test pumping boreholes
- Borehole pump selection
- Borehole pump installation and commissioning
- Rehabilitating boreholes
- Management of boreholes
- Decommissioning of boreholes
The crucial and underlying principle that distinguishes this type of testing from pump inlet testing is that the pumping rate is kept constant while the water level’s drawdown is measured over time (during each step of the Step Test, as well as the CDT). This is completely different from the pump inlet test where the dropping yield is measured over time, while keeping the water level constantly at the pump’s inlet.
The Constant Discharge Test (CDT) or Constant Rate Test (CRT)
The Constant Discharge Test (CDT) or Constant Rate Test (CRT) is in line with international borehole testing standards and is based on aquifer hydraulics theory and practice, which have been used and improved over time, dating back to Theis, 1935. Similarly in line with other international standards is the emphasis on collecting proper recovery test data, with regular measurements of the water level drawdown returning to the starting level over time after the CDT. This often takes between 50 – 200% of the pumping time (so if a 24-hour CDT was done, recovery could even take another 48 hours after that). The minimum standard in South Africa is that this is measured up to 95% or CDT pumping time, whichever comes first.
Summary of Pump Inlet Tests to SANS 10299 Testing Comparison
From a correctly performed test, such as the one shown below, a hydrogeologist can analyse the rate of drawdown in the borehole, compare it to water strike depths and calculate a pumping rate that will keep the water level above these depths over long periods of time, during which rainfall and recharge into the aquifer can replace the abstracted volume.
Example of the water level drawdown and recovery collected during a CDT at 2.1 L/s (+/- 5% after early time)
Why Choosing the Correct Test is Important
Until now this article has mostly discussed differences in the types of yields that might be provided to a new (or old) borehole owner and how to identify whether someone is offering a SANS 10299 test, or something else. The rest of the article will be about why this actually matters. Here are three reasons from different angles:
1. The Return on investment
Drilling, testing and equipping a borehole is an investment in and of itself. One which should bring the owner a reliable water supply to either save municipal tariff costs, provide water when other supplies simply aren’t available, or even to expand current water dependent activities. However, in many cases, most of the financial investment goes into the next step. In domestic and industrial supply, water treatment will often be required, carefully designed and built based on water quality tests and sized specifically to the rate and volume of raw water available. In agriculture, irrigation pipes and infrastructure have to be built and laid out, storage dams may need to be built and fields may need to be prepared and sewn with crop seeds. In both cases, these costs are often 5 – 50 times higher than the cost of the borehole itself and are often rely heavily on a reported borehole yield and water quality. Some small adjustments in pumping rate and quality might be fine, but a 30% drop in yield? 50%? 90%? That could turn a 5 – 10-year return on investment into a loss that does not get returned.
2. Lawful groundwater abstraction
Legally abstracting groundwater for different purposes and of different volumes requires either a registration or license from the Department of Water and Sanitation (DWS). In particular, using groundwater for irrigation, industry, or town supply usually requires a Water Use License. The Water Use License Application (WULA) process requires that the groundwater is correctly tested for yield and quality before it will be approved. For more on WULA processes and finding out whether registration or license is required, see our WULA article at https://geoss.co.za/getting-the-water-act-together/ on the GEOSS Knowledge Hub.
3. Borehole connectivity
Despite frequent comments about “different aquifers” or “’n verskillende aar”, groundwater from different boreholes is often connected. After all, when it starts being pumped out, the new water flowing into the borehole has to come from somewhere (recharge area). If there are 2, 3 or 10 boreholes on a single property or several properties, its quite likely that they all have different “somewhere’s” supplying the water. During a CDT, the influence of the tested borehole (or lack of influence if there really is one) on the other boreholes can be measured and included into the analyses to recommend cumulative pumping rates. Again, this is not something done during a pump inlet type test.
How wrong can the test be?
In order to illustrate the type of error that occurs using pump inlet tests and why just taking a fixed percentage of these won’t work, 25 recommended borehole yields calculated from a pump inlet “Farmer Test” were compared to recommended borehole yields at the same 25 boreholes, calculated from SANS 10299 testing. Test comparisons include boreholes drilled into the Malmesbury Group (fractured bedrock), Table Mountain Group (fractured bedrock) and Quaternary alluvial deposits (porous sands and gravels). The locations include Cape Town, Somerset West, Stellenbosch, Franschhoek, Paarl, Rawsonville, Hermon, Tulbagh, Saron, Citrusdal and Leipoldville, shown below.
Locations of selected boreholes where both a Pump Inlet Farmer’s Test and a correctly performed SANS 10299 pumping test was performed.
The tested yields covered a large range, which will include most boreholes in this area and others, from 0.5 L/s to over 25 L/s (1.8 – 95 m3/hour). The results of the comparison are shown in the graph below, with red bars indicating the recommendation from a pump inlet Farmer Test which can be read off as L/s on the left axis or m3/hour on the right axis. The green bars are the recommendations from a standard SANS 10299 test pumping data analysis and can also be read in L/s or m3/hour.
While some examples are relatively similar (Ex 2, 5, 12, 16 and 19), most are concerningly different! In cases such as crop irrigation or factory use, where the amount of water used can be directly proportional to income and profits; Ex3, 9, 17, 20, 21, 22, 23 and 24 illustrate the potential for business failure. To place the consequences of this outside of the financial realm, if these boreholes were drilled and equipped for emergency town supply in times of drought, the false sense of (water) security could be disastrous for the town in question.
A few summary points from these results:
- The average recommended pumping rate from a Pump Inlet Farmer Test was more than double the rate recommended from SANS 10299 data analysis for the same borehole.
- Half the recommendations made based from Farmer Tests were ~3 times higher than those from SANS 10299 data analysis for the same borehole.
- In one case, a borehole was so over abstracted from pumping at the recommended rate from a Farmer Test, that a 93% reduction in recommended pumping rate was made.
So, what can be done to avoid these types of problems and to plan realistically for a reliable return on investment?
- If the borehole test and quotes are being provided by the same entity that drilled the borehole or that is selling and installing the borehole pump, ask if they are doing a SABS-approved SANS 10299 type test, which has been the industry standard since 2003.
- Check if this includes a Step Test, a Constant Discharge Test and a Recovery Test.
- Make sure that influence measurements are taken in any nearby boreholes.
- Ask a hydrogeologist to analyse the data and recommend a suitable pumping rate, depth to install the pump and the expected dynamic (pumping) water level.
- If you have a production borehole that regularly requires the pump to be pulled out and serviced or replaced, consider having a proper pumping test. Even in cases where this is due to chemical issues like high iron in the water, a correctly determined pumping rate can change the amount of iron clogging the pump.
- Monitor groundwater levels and use!
What is the next step in a long-term reliable groundwater supply borehole?
The next step in a long-term reliable groundwater supply borehole after it has been legally authorised by DWS is to monitor what the water levels inside the borehole do during production. Why is this important? Well, it’s similar to checking the water levels in a storage tank.
Whether that’s having a float in a JoJo tank (or tapping the sides of it and listening to the noise it makes), or taking a look at how full a water supply dam is.
Making the most of your groundwater abstraction:
Adjusting groundwater abstraction and optimising pumping schedules and rates to meet the purpose of the water use is a great way to make sure you get the most benefit out of a borehole.
This could mean:
- The most water possible without running out,
- Saving electrical costs by not switching the pump on and off every few minutes, or
- Pumping lower rates for more hours per day or days per week to reduce pump burn outs.
All of these require groundwater level measurements to inform the adjustments.
These groundwater level measurements get collected with a water level logger installed inside the borehole alongside the pump. The record of these groundwater levels is then compared to the recommendations made from the testing and this is where the optimisation can be made.
Another benefit of groundwater level monitoring
Another huge benefit of groundwater level monitoring is for when droughts occur or when new boreholes are being drilled nearby. A groundwater level record allows the owner or operator of the borehole to see if any effect on their groundwater levels are new, or just the end of a seasonal or long-term trend, and adjust accordingly. An example from our Merweville article is shown below.
In this case, the groundwater level monitoring allowed the operators to increase the abstraction rate substantially and pump fewer hours per day after two major rainfall events recharged the aquifer in October and December 2021. This same data will be used to decrease the abstraction rates and increase pumping hours if similar rains do not fall this year and water levels drop in 2023. Importantly, these levels are compared to the SANS10299 test pumping recommendations, to keep an eye on when these groundwater levels get too close to the fracture in the borehole.
Groundwater level and abstraction rate monitoring