A generalised solution for step-drawdown tests including flow dimension and elasticity

Step-drawdown and multi-rate tests present convenient tools for the estimat ion of the long-term yield of boreholes. However, the analytical methods co mmonly employed for the analysis of such tests are all based on the assumpt ion that the drawdown in a borehole is a linear function of the discharge r ate. Numerous constant rate tests, of which a few are discussed in this pap er, has shown that this is not necessarily the case with boreholes drilled in the Karoo formations of South Africa. The drawdowns in these boreholes a re not only influenced by the peculiar geometry of the aquifers, but also t he non-linear deformation of the aquifers during the pumping of a borehole. The two new non-linear models for the analysis of step-drawdown and multi- rate tests introduced here, tries to account for these factors; in particul ar the deformation of the aquifer, flow dimension and dewatering of discret e fractures. Although the model proposed for multi-rate tests is still base d on constant time steps, the one for step-drawdown tests allows the user t o use arbitrary time steps, when performing the test in the field. Non-linearities in drawdown curves should always be treated with caution, e specially when used to assign sustainable yields for boreholes. However, th e example of a step-drawdown test performance at the Campus Test Site of th e University of the Free State, shows that non-linearities can be addressed with an appropriate model.