Borehole Test-Pumping – Why Do It.

Insights / 14 May 2023 by Kobus Prinsloo /
Credit : Umvoto Africa

Groundwater users often neglect test-pumping boreholes once drilling is complete. Test-pumping is crucial towards understanding the yield capabilities of the borehole and greater aquifer into which the borehole is drilled, as well as ensuring the correct borehole pump is selected, before putting the borehole into production. Test-pumping allows the groundwater user to set “safe” or “sustainable” groundwater abstraction yields/rates and operational or pumped groundwater levels in place, which not only protects the groundwater resource, but also improves the life span of the user’s pump and borehole infrastructure.

The National Water Act (No. 36 of 1998) also sets requirements on the groundwater user to legally abstract water from a groundwater resource. To regularise a new groundwater use, the onus sits with the groundwater user to determine whether the new (or existing) groundwater use aligns with catchment management strategies, potentially impacting other registered groundwater users and whether the Resource Quality Objectives (RQOs) are met.

Setup of a large test-pumping rig prior to the commencement of a pumping test.

Test-Pumping Guidelines

In South Africa, there are a couple of guidelines that describe the process of test-pumping a borehole. The Department of Water and Sanitation (DWS), previously known as the Department of Water Affairs and Forestry (DWAF), published the “Minimum Standards and Guidelines for Groundwater Resource Development“ in 1997. This document provides a pragmatic overview of the groundwater resource development process as a whole, including test-pumping.

The most important and commonly used guideline is the “South African National Standard (SANS) 10299-4:2003“ document titled “Test-Pumping of Water Boreholes”, which “covers the requirements for the test-pumping of a water borehole in order to obtain information about its possible long-term pumping rate”. The SANS 10299-4:2003 guidelines provide specifications on the appropriate test-pumping method (including the type and duration of test, and period of recovery) as well as technical aspects thereof, such as test-pumps, test-pump installation depth and test monitoring equipment (dip meters, semi-automated level loggers and flow meters).

Test-pumping of a farm borehole.

Test-Pumping Procedures

Calibration Test

A calibration test is generally performed on existing boreholes where no historic yield or drilling information is available. A calibration test requires the borehole to be pumped at three to four different abstraction rates for short (15 minutes) sequential periods. The rate of groundwater level decline during the calibration test will be used to inform the yields at which further test-pumping procedures such as step drawdown or constant rate discharge tests can be performed.

Step-Drawdown Test

The step-drawdown test is generally performed for two reasons. Firstly, to assess the productivity of a borehole, and secondly to more clearly define the abstraction rate at which a borehole can be subjected to constant rate discharge testing. Step-drawdown tests involve pumping the borehole at three or more sequentially higher abstraction rates or “steps” (typically for 60 minutes or longer). The groundwater level drawdown and abstraction rate are recorded per “step”, with the groundwater level allowed to recover following the step-drawdown test (measured to 95% of the rest or static groundwater level prior to the commencement of the test).

Analysis of step-drawdown test data by an adequately trained professional hydrogeologist can yield valuable borehole performance parameters, such as specific capacity (Sc), well losses (C) and aquifer losses (B).

Constant Rate Discharge Test

A constant rate discharge test is performed to assess the productivity of a borehole (and the immediate surrounding aquifer) according to its response to the abstraction of groundwater over an extended period. The SANS 10299-4:2003 guidelines provide specifications on the duration (12-100 hours or more can be specified) of constant rate discharge tests required, depending on the planned use of the groundwater and the availability of other water resources in the area. Constant rate discharge tests entail pumping the borehole for the duration of the test at a single abstraction rate, with the rate not deviating by more than 5% for the duration of the test. During the test, the groundwater level and abstraction rate must be monitored at pre-determined intervals (as per SANS 10299-4:2003). As with the step-drawdown test, after completion of the constant rate discharge test groundwater level recovery must be measured to 95% of the rest or static groundwater level prior to testing.

Aquifer parameters, such as hydraulic conductivity (K), transmissivity (T) and storage (S) can be determined through analysis of the constant rate discharge test and recovery data. This is done through mathematic curve fitting to areas where derivate test-pumping data indicates periods of radial flow.  

After Test-Pumping

Based on the analysis of the test-pumping data, a professional hydrogeologist will provide borehole operational and groundwater management recommendations, such as production pump installation depth, pump abstraction rate, pump duty cycles (e.g. hours of pumping per day) and operational, dynamic or pumped groundwater levels based on the test-pumping data (as well as any other critical groundwater levels that need to be maintained e.g. to prevent dewatering of specific groundwater-bearing fractured). These recommendations should consider all legislative and resource protection aspects of groundwater abstraction in the project area.

Commonly a groundwater quality sample will also be collected by the hydrogeologist at the end of the constant rate discharge test for hydrochemical analysis at a South African National Standard (SANAS)-accredited laboratory, to determine various groundwater quality parameters and whether groundwater is fit for purpose (with or without treatment) depending on the planned use. The latter is determined by comparing the various analysed hydrochemical parameters with national and international water quality limits, standards and guidelines.

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