Frequently asked questions:
Question 1: Which flow rate do I use?
The flow rate depends on many factors, such as the objectives of the experiment, the hydraulic and chemical properties of the soil in the column, and the time available for completing the experiments.
For saturated columns, the highest flow rate used is often taken to be equal to the saturated hydraulic conductivity of the soil in the column. The lowest flow rate is often chosen equivalent to a flux of about 1 cm/day. Below this flux rate, transport experiments take much time for completion.
For unsaturated experiments, a good initial choice for a flux rate would be 10 cm/day.
To put the flux rate in perspective, note that the average recharge rate in the more humid areas of the United States averages about 30 cm per year, equivalent to a flux rate of 0.082 cm/day. Similarly, many irrigated areas have irrigation efficiencies of around 50% or less, although a few irrigation areas have irrigation efficiencies greater than 80%. If 120 cm of irrigation water is used in a year, and between 20% and 50% recharges to the groundwater, the mean annual recharge rate is 24 to 60 cm per year, equivalent to .0065 and 0.16 cm/day, respectively. Of course during the year the flux rate may vary greatly outside of these averages. However, the point is that average, real, recharge rates are low. In practice, it would take too much time to conduct experiments at these low flux rates, and thus a compromise low flux rate of 1 cm/day may be appropriate.
Question 2: How do I set the syringe pump to get a flow rate of 10 cm/day?
To obtain the low flow rates needed for unsaturated flow experiments, the syringe pump is designed to make one revolution at a time (one pump event). During such a pump event, one or more syringes (up to a maximum of ten syringes) will fill with fluid from a supply bottle and then will discharge the fluid onto the column. The motor, which drives the syringes back and forth, then waits for another signal to repeat the filling and discharging of the syringe(s). One such pump event takes about 9 seconds.
The volume of fluid discharged per pump event depends on the size of the syringes used as well as on the travel distance of the syringe plunger. For example, a standard 3 ml syringe can be set to deliver approximately 0.75 ml per event, 1.5 ml per event, 2.25 ml per event, or 3 ml per event.
A good initial leaching rate for unsaturated columns is 10 cm/day. Assume the inside diameter of the soil column is 7.6 cm and its surface area is 45.6 sq cm. At a leaching rate of 10 cm/day, a total of 456 ml of fluid would move through the column in one day. If the pump is set to deliver 1.5 ml per event, it would have to pump 456/1.5=305 times in a 24 hour day. This amounts to 1440/305=4.72 minutes, or 283 seconds. Thus set the pump at a pump interval of 283 seconds.
If the fraction collector had been set such that effluent is collected for a total of 30 minutes in one test tube (i.e. at 30 minute time intervals), in 24 hours a total of 48 test tubes would be filled. The volume of effluent collected in each of these 48 test tubes is then equal to 456/48= 9.5 ml. However, if each test tube can hold only 10 ml, it may be safer to collect samples at 20 minute intervals (or 72 intervals per day), so that one collects only 456/72= 6.3 ml effluent per test tube. Thus when performing unsaturated column leaching studies, it is important to synchronize the pumping rate with the interval setting on the fraction collector, as well as the size of the fraction collector test tubes.
For column leaching studies, the researcher often has flexibility in setting the leaching rate, which makes it possible to find a practical combination of pumping rate, column size and effluent collection interval. Once a suitable leaching rate is set, it should remain constant for the duration of the experiment. The syringe pump is designed to do this.
Once the pump is set, the actual pumping rate can be determined by pumping fluid into an empty bottle, and then weigh the bottle with fluid over time.
To check the flow rate during an experiment, one should weigh the volume of fluid collected from the column. After removing the test tubes from the fraction collector, one should weigh the test tubes with fluid as well as the empty test tubes. One could also determine the average weight of the empty test tubes before starting the experiment, and subtract this average weight from the test tubes with effluent.
Question 3: What is the level of vacuum I use in the vacuum chamber?
This is best determined with tensiometers in the column. Ideally one should have nearly the same matric potential near the top and bottom of the column. The top matric potential is mostly a function of the column flow rate. Once the flow rate is established, determine the matric potential in the upper tensiometer and then read the matric potential in the lower tensiometer. If the lower matric potential is less negative (soil is wetter) than the upper matric potential, increase the vacuum in the chamber. If the lower matric potential is more negative than the upper matric potential, decrease the vacuum in the chamber. Continue adjusting the vacuum in the vacuum chamber until the upper and lower matric potentials in the column are approximately the same.
Gaber, H.M., S.D. Comfort, W.P. Inskeep, and H.A. El-Attar. 1992. A test of the Local Equilibrium Assumption for Adsorption and Transport of Picloram. Soil Sci. Soc. Am. J. 56:1392-1400.
Risler, P. D., Wraith, J. M., and Gaber, H. M. 1996. Solute Transport under Transient Flow Conditions Estimated Using Time Domain Reflectometry. Soil Sci. Soc. Am. J. 60:1297-1305.
Van Genuchten, M. Th., Wierenga, P.J. 1986. Solute Dispersion Coefficients and Retardation Factors. Klute, A. (ed.) Methods of Soil Analysis, Part I, 2nd ed. Agron. Monogr. 9, ASA and SSSA, Madison, WI p. 1025-1054.
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