Last week we have looked at the drilling and construction of a borehole, but the borehole is not yet complete. Certainly one of the most important factors concerning one’s borehole is now on hands. How much water can I pump over the long term without drastically affecting the yield and making sure that the borehole is not pumped dry? It should be stated clearly that a borehole test where only the yield and no water levels has been measured does not mean anything in terms of borehole yield (strength), and is at best a test of how the pump equipment performs.

Important criteria

There are a number of aspects that needs to be taken into consideration when testing a borehole. According to Prof van Tonder at the Institute for Groundwater Studies (Free State University) there are two important rules that needs to be kept in mind when determining the sustainable yield I.e.: The total abstraction from a borehole should be less than the natural groundwater recharge, and secondly, a borehole should be pumped in such a manner that the water level never reaches the position of the main water strike (normally associated with a fracture). Should this happen the yield will inevitably be affected and the borehole would eventually dry up.

Specific information is required to properly test a borehole, these can be listed as follows: what is the rest water level before the start of the test, how does the water level change over time once pumping has started, how long does it take for the water level to recover after the pump has been stopped to recover to the original level – or how far does the water level recover after the same amount of time allowed as for pumping – leaving a residual drawdown.

Why test?

Test pumping of boreholes is normally carried out to meet two main objectives:

1. To establish borehole potential. To estimate the sustainable yield and hydraulic erformance of individual boreholes for water supplies.

2. To establish aquifer potential. To assess the hydraulic characteristics of the aquifer.

How do we test?

Test pumping consists of pumping a borehole at a specified rate and recording the water level (and therefore the drawdown) in the pumping well as well as in nearby observation boreholes at specific time intervals. When these measurements are substituted in appropriate flow equations, certain hydraulic parameters can be calculated. These parameters, together with qualitative assessment of discharge-drawdown characteristics, are then used for the assessment of a recommended yield of the boreholes and or aquifers.

There are three primary types of borehole yield tests commonly used by hydrogeologists I.e. step test, constant rate test and a recovery test). Prior to any test, a calibration test exercise is carried out to adjust and calibrate the pumping equipment at various discharges.

Step test

During this test the pump rate is increased in steps at regular intervals. For example a borehole may be pumped at rate of 1000l/hr for a period of 1 hour and increased thereafter to a rate of say 2000l/hr for the next hour and so on for several more steps. This type of test is particularly useful to determine the effectiveness of the borehole, but not too usefull in determining the long term sustainable yield of a borehole. In this regard the constant rate test is more useful.

Constant Rate Test

In the Constant Rate Test (CRT), the borehole is pumped at a constant discharge rate over a period ranging from 8 to 48 hours (or longer) – the length of the test is normally proportional to the expected yield and importance of the borehole. The discharge is kept constant for the duration of the test, and water levels are recorded in the pumping borehole as well as observation boreholes (if any). The time-drawdown data obtained from the CRT is then analyzed for quantitative (estimation of transmissivity, storativity and hydraulic parameters) and qualitative analysis of borehole and aquifer response to pumping. The analysis provides useful input to assess the sustainable yield of individual boreholes and the potential of aquifers. Hydrogeologists are trained to utilize different mathematical equations to estimate a sustainable yield.

Recovery Test

In this test, recovering water levels are measured in the pumping borehole immediately after the CRT, when the pump is switched off.
This recovery test is very useful in qualitatively assessing the pumping effect and possible dewatering of aquifers that may result due to the limited extent of an aquifer.
Furthermore the recovery test will indicate the level to which the aquifer is actually dewatered by measuring the residual drawdown after the borehole was allowed to recover.
Figure 1 indicates a typical pump test curve obtained from a borehole yield test where the yield has been kept constant.

Water Quality

An important aspect of a borehole yield test is the monitoring of the water quality as pumped from the borehole. A water sample is generally taken at the start of the test as well as at the end of the test for full chemical analysis, while the electrical conductivity and temperature is normally monitored with the water level for the duration of the test.

These information assists in understanding the aquifer behaviour e.g. are we abstracting some of the older water from deeper formations or does the aquifer yield sufficient fresh water quantities? The determination of a sustainable yield will normally take this information into account. The water quality is also of importance to determine whether it is suitable for domestic, irrigation or industrial purposes. An example of this importance for domestic purposes is the nitrate content of water. According to South African Standards the maximum allowable limit is 10mg/l. Too much Nitrate has a major impact on infants less than 6 months old and leads to a condition called methemoglobinemia (blue-baby syndrome) and could be fateful.

Costs

As mentioned in beginning of this series the perception is that it is too expensive to test a borehole. But is this the case? Remember that the pump supplier can only work on the information that the owner of the borehole provides and can thus not be held responsible for pump or borehole failures if the borehole is over pumped. Either of these incidents might lead to a possible over-capitalization on the borehole equipment with a variety of pump protection devices, as well as additional costs for refurbishing and re – installation of a damaged pump.

In many cases the continuous over pumping of a borehole will lead to the eventual complete failure of the borehole resulting in a requirement to have the borehole re – drilled.

Hydrogeologists are specifically trained to apply the science of hydrogeology in determining the sustainable yield of a borehole and could assist in reducing your risk of borehole and pump failures as well as optimizing the operational costs.

A "borehole test" that involves the pumping of a borehole, measuring the yields, but not measuring the water levels, does not mean anything to you as the groundwater user. You are basically testing the yield of the pump. You will get exactly the same results in terms of yield if you put that pump in your swimming pool and pump it!