This article describes how to conduct drawdown and buildup tests for the determination of aquifer transmissivity using surface pumps whose design permits the flow rate to be precisely controlled. One surface pump (SHURflo) is recommended where the water level depth is less than 25 feet. Another surface pump (Rotapump) is recommended where the water level depth is greater than 25 feet. By following the procedures outline herein, constant rate aquifer tests can be successfully performed in any diameter well regardless of depth.
â— Install Pumping Tube, Surface Pump, and Speed Controller
For water level depths less than 25 feet, a pumping tube (e.g. 1/2- irrigation tubing) with a check valve (e.g. Brady SLC-75 3/4- with 3/4 x 1/2 bushing and 1/2- NPT x 1/2- hose barb) is installed in the well. At the well head, the pumping tube is attached to the inlet side of a SHURflo 12 VDC suction pump, Model 2088-422-444. The check valve on the end of the pumping tube permits priming of the pump by moving the tube up and down. Once primed, the SHURflo pump lifts water by suction. During pumping, the pumping tube does not move, so no pressure disturbance occurs in the well.
For water level depths greater than from 25 feet, a stilling tube (e.g. 1/2- irrigation tubing) is hung from the well head to the aquifer test depth. This tube may be supported by a hose clamp on the tube bearing on a hole in the well cap. Next, a pumping tube (e.g. 3/8-x1/4- low density polyethylene tubing) with a check valve (e.g. Waterra SS10 offered by Solinst) is placed inside the stilling tube at a minimal distance below the static water level to just accommodate anticipated drawdown due to pumping. At the wellhead, the pumping tube is threaded through the follower on the Rotapump inertial lift pump. As the pumping tube is moved up and down by the Rotapump, water is removed from the top of the stilling tube and relatively unagitated water enters the bottom of the stilling tube. The greater the distance from the end of the pumping tube to the end of the stilling tube, the less the water in the well will be disturbed by the up and down motion of the pumping tube. This distance can be increased by installing a -u-turn- in the stilling tube and extending the tube upwards.
The flow rate of the SHURflo pump or Rotapump is controlled with a speed controller offered by McEdwards Manufacturing / Distribution (www.rotapump.com
â— Pump at a Low Constant Rate and Measure Water Level Changes
To meet as far as practicable the assumption of confined lateral flow when testing unconfined aquifers, the constant flow rate should be as low as possible and but still produce a straight line on a drawdown versus log of pumping time plot (semilog plot). A low and constant flow rate is maintained by the controller. The magnitude of the flow rate is measured by timing filling of a 5 gallon bucket graduated in 1 gallon increments. Water level changes are measured with a pressure transducer.
â— Drawdown Test Analysis
Transmissivity is defined as the product of the hydraulic conductivity and the vertical thickness of the pumped strata. The equation is T = 2.3*Q/(4*Pi*h10). The units of T are ft2/min when Q is in f3/min and h10 is in ft. h10 is the drawdown per log cycle of time since pumping began. The factor 2.3 is the natural log of 10, and appears in the equation to allow the data to be plotted using logarithms of base 10 rather than natural logarithms of base e = 2.72. Once h10 is determined from the semilog plot, the hydraulic conductivity is determined knowing Q and the thickness of the pumped strata.
â— Buildup Test Analysis
If water level data are gathered after pumping is stopped, the Horner Method can be used to determine transmissivity and hydraulic conductivity. This approach eliminates the need to measure water levels during pumping and therefore the need to minimize pressure disturbances during pumping. In practice, application of the Horner Method does not require that a stilling tube be used, only that the flow rate remain constant. Perturbations in water levels during pumping caused by the energetic movement of the inertial lift pumping tube are not important because water levels recorded during pumping are not used in the analysis for transmissivity.
The Horner Method involves plotting water level recovery versus the logarithm of dt/(t+dt) where dt is the variable and is the elapsed time since pumping stopped, and t is the set time the well was pumped at a constant rate. This method of analysis superposes the constant pumping (positive) rate operating for times t + dt with a constant injection (negative) rate of the same magnitude operating for times dt. The change in water level per log cycle of the straight line portion of the Horner plot is taken as h10 in equation for T above. All other factors are the same as for the drawdown test. Recovery water level data are gathered after pumping has stopped and the well is quiescent. The data are extraordinarily noise free as they are recorded as the water level slowly equilibrates to its previous static level. A buildup test is preferred over a drawdown test for water level depths greater than 25 feet where an inertial lift pump yielding a constant flow rate is used.
By Don McEdwards
For more information about using Rotapump in your application, call Don McEdwards of McEdwards Manufacturing & Distribution at 707/354-4618, email firstname.lastname@example.org, or visit www.rotapump.com.