TESTING OF BOREHOLES
Drilling of environmental monitoring boreholes, geothermal installation boreholes and water supply boreholes
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TESTING OF BOREHOLES
Testing of Boreholes
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.
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.
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.
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.
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.
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.
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.
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.
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!
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.
4 Groundwater Management/Legalities
We have concluded with a review of different groundwater development phases. All these phases obviously takes place within a certain legal framework and any series on groundwater would not be complete without a look at the legal environment. It is not possible to discuss all aspects of the South African Water legislation in an article such as this, but we are going to look at some fundamental principles, which might affect the local borehole user, as well as other farming activities.
In the preamble to the National Water Act it is recognised that water is a scarce and unevenly distributed national resource, which occurs in many different forms and are all part of a unitary, inter-dependant hydrological cycle, whereas previously groundwater and surface water were treated differently. As such the National Water Act states that there will be no ownership of water (surface and groundwater), only a right to use for basic human needs and the environment. The volume, quality and sustainability of water necessary to sustain human life, and ecological functions on which human life depends, will be reserved in such a manner that the long-term sustainability is not jeopardized.
The water quantity needed to sustain the basic human and environmental needs, will be called and will have absolute priority. The quantity needed to sustain this life will enjoy priority in a catchment and will be calculated and reserved (the reserve) before any additional water rights are allocated for activities such as crop irrigation.
To effectively try to manage the water quantities utilized, the act provides the directive for the registration of water use. All water users, who do not receive their water from a service provider, local authority, water board, irrigation board, government water scheme or other bulk supplier and who are using water for: Irrigation; Mining purposes; Industrial use; Feedlots, or In terms of a General Authorisation; should register their water use. This covers the use of surface and groundwater. Permission to use certain quantities of water will be allocated for a certain time period (maximum 40 years) and should be reviewed at least every 5 years. The National Water Act however, allows a person to use water for reasonable domestic purposes directly from a water resource to which he or she has lawful access.
It also allows a person to take water for small gardening (not for commercial purposes) and the watering of animals (excluding feedlots) on land owned or occupied by that person, from any water resource which is situated on or forms a boundary of that land. This only applies if the use is not excessive relative to the capacity of the water resource and the needs of other users. This means that most users using groundwater in towns and cities and those users with windmills on their own properties need not register.
If you are unsure about your water use registration obligations, contact DWAF or a professional consultant and ask.
Water quality management needs to be implemented in catchment areas. Institutions or individuals that are directly or indirectly responsible for the pollution of both surface and groundwater will be held responsible for the rehabilitation of such resources. This is especially important in areas where major industrial or mining activities takes place and could also have an impact on irrigation where the use of fertilizer could impact on undergroundwater resources.
According to the National Water Act, the Minister has the responsibility to manage and authorise the use of the nations interest toeffectively monitor and manage his water (surface and groundwater) resources.
Through our specialist capabilities we are able to provide aquifer management services to local municipalities and mines. These services include from very basic monitoring and reporting services, to high level pump schedule management, remote controll and monitoring and on site training. Talk to us about your specific needs in order for us to assist in developing tailor made monitoring, managing and training program.