Inhydrogeology, anaquifer test (or apumping test) is conducted to evaluate anaquifer by "stimulating" the aquifer through constantpumping, and observing the aquifer's "response" (drawdown) in observationwells. Aquifer testing is a common tool that hydrogeologists use to characterize a system of aquifers,aquitards and flow system boundaries.
Aslug test is a variation on the typical aquifer test where an instantaneous change (increase or decrease) is made, and the effects are observed in the same well. This is often used ingeotechnical engineering settings to get a quick estimate (minutes instead of days) of the aquifer properties immediately around the well.
Aquifer tests are typically interpreted by using an analytical model of aquifer flow (the most fundamental being the Theis solution) to match the data observed in the real world, then assuming that the parameters from the idealized model apply to the real-world aquifer. In more complex cases, a numerical model may be used to analyze the results of an aquifer test.
Aquifer testing differs fromwell testing in that the behaviour of the well is primarily of concern in the latter, while the characteristics of the aquifer are quantified in the former. Aquifer testing also often utilizes one or more monitoring wells, or piezometers ("point" observation wells). A monitoring well is simply a well which is not being pumped (but is used to monitor thehydraulic head in the aquifer). Typically monitoring and pumping wells are screened across the same aquifers.
Most commonly anaquifer test is conducted by pumping water from one well at a steady rate and for at least one day, while carefully measuring the water levels in the monitoring wells. When water is pumped from the pumping well the pressure in the aquifer that feeds that well declines. This decline in pressure will show up asdrawdown (change in hydraulic head) in an observation well. Drawdown decreases with radial distance from the pumping well and drawdown increases with the length of time that the pumping continues.
The aquifer characteristics which are evaluated by most aquifer tests are:
Additional aquifer characteristics which are sometimes evaluated, depending on the type of aquifer, include:
An appropriate model or solution to thegroundwater flow equation must be chosen to fit to the observed data. There are many different choices of models, depending on what factors are deemed important including:
Nearly all aquifer test solution methods are based on the Theis solution; it is built upon the most simplifying assumptions. Other methods relax one or more of the assumptions the Theis solution is built on, and therefore they get a more flexible (and more complex) result.
The Theis equation was created byCharles Vernon Theis (working for theUS Geological Survey) in 1935,[1] fromheat transfer literature (with the mathematical help of C.I. Lubin), for two-dimensional radial flow to a pointsink in an infinite, homogeneousaquifer. It is simply
wheres is thedrawdown (change in hydraulic head at a point since the beginning of the test in units of distance),u is a dimensionless parameter,Q is the discharge (pumping) rate of thewell (volume per unit time),T andS are thetransmissivity andstorativity of the aquifer around the well (distance squared per unit time and dimensionless, respectively),r is the distance from the pumping well to the point where the drawdown was observed,t is the time since pumping began, andW(u) is the "Well function" (called theincomplete gamma function,, in non-hydrogeology literature). The well function is given by the infinite series
whereγ is theEuler constant (=0.577216...). Typically this equation is used to find the averageT andS values near a pumpingwell, fromdrawdown data collected during an aquifer test. This is a simple form of inverse modeling, since the result (s) is measured in the well,r,t, andQ are observed, and values ofT andS which best reproduce the measured data are put into the equation until a best fit between the observed data and the analytic solution is found.
The Theis solution is based on the following assumptions:
Even though these assumptions are rarely all met, depending on the degree to which they are violated (e.g., if the boundaries of the aquifer are well beyond the part of the aquifer which will be tested by the pumping test) thesolution may still be useful.
Steady-state radial flow to a pumping well is commonly called the Thiem solution,[2] it comes about from application ofDarcy's law tocylindrical shell control volumes (i.e., a cylinder with a larger radius which has a smaller radius cylinder cut out of it) about the pumping well; it is commonly written as:
In this expressionh0 is the backgroundhydraulic head,h0-h is thedrawdown at the radial distancer from the pumping well,Q is the discharge rate of the pumping well (at the origin),T is thetransmissivity, andR is the radius of influence, or the distance at which the head is stillh0. These conditions (steady-state flow to a pumping well with no nearby boundaries)never truly occur in nature, but it can often be used as an approximation to actual conditions; the solution is derived by assuming there is a circular constant head boundary (e.g., alake orriver in full contact with the aquifer) surrounding the pumping well at a distanceR.
Of critical importance in both aquifer and well testing is the accurate recording of data. Not only must water levels and the time of the measurement be carefully recorded, but the pumping rates must be periodically checked and recorded. An unrecorded change in pumping rate of as little as 2% can be misleading when the data are analysed.[citation needed]
{{cite journal}}:Cite journal requires|journal= (help)TheUS Geological Survey has some very useful free references on pumping test interpretation:
{{cite book}}: CS1 maint: numeric names: authors list (link)Some commercial printed references on aquifer test interpretation:
More book titles can be found in thefurther reading section of the hydrogeology article, most of which contain some material on aquifer test analysis or the theory behind these test methods.
