Well test

A well test is conducted to evaluate the amount of water that can be pumped from a particular water well. More specifically, a well test will allow prediction of the maximum rate at which water can be pumped from a well, and the distance that the water level in the well will fall for a given pumping rate and duration of pumping. A well test is conducted to evaluate the amount of water that can be pumped from a particular water well. More specifically, a well test will allow prediction of the maximum rate at which water can be pumped from a well, and the distance that the water level in the well will fall for a given pumping rate and duration of pumping. Well testing differs from aquifer testing in that the behaviour of the well is primarily of concern in the former, while the characteristics of the aquifer (the geological formation or unit that supplies water to the well) are quantified in the latter. When water is pumped from a well the water level in the well falls. This fall is called drawdown. The amount of water that can be pumped is limited by the drawdown produced. Typically, drawdown also increases with the length of time that the pumping continues. The components of observed drawdown in a pumping well were first described by Jacob (1947), and the test was refined independently by Hantush (1964) and Bierschenk (1963) as consisting of two related components, where s is drawdown (units of length e.g., m), Q {displaystyle Q} is the pumping rate (units of volume flowrate e.g., m³/day), B {displaystyle B} is the aquifer loss coefficient (which increases with time — as predicted by the Theis solution) and C {displaystyle C} is the well loss coefficient (which is constant for a given flow rate). The first term of the equation ( B Q {displaystyle BQ} ) describes the linear component of the drawdown; i.e., the part in which doubling the pumping rate doubles the drawdown. The second term ( C Q 2 {displaystyle CQ^{2}} ) describes what is often called the 'well losses'; the non-linear component of the drawdown. To quantify this it is necessary to pump the well at several different flow rates (commonly called steps). Rorabaugh (1953) added to this analysis by making the exponent an arbitrary power (usually between 1.5 and 3.5). To analyze this equation, both sides are divided by the discharge rate ( Q {displaystyle Q} ), leaving s / Q {displaystyle s/Q} on the left side, which is commonly referred to as specific drawdown. The right hand side of the equation becomes that of a straight line. Plotting the specific drawdown after a set amount of time ( Δ t {displaystyle Delta t} ) since the beginning of each step of the test (since drawdown will continue to increase with time) versus pumping rate should produce a straight line.