This means that you can use your pressure transducer data directly - provided the probe does not move during the test. If the bailer or slug knocks the probe up or down during the test, you have to redo the test. The difficult part of this test is to produce an "instantaneous" change in water level - especially in highly conductive aquifers. If a slug or a bailer is used the time required is on the order of one or two seconds (depending on the length of the slug or bailer); but if the change of water level is produced by pouring a known volume of water into the piezometer, then the time required will be several seconds, which may be too long. Time permitting, you should repeat your test by removing/inserting the slug. You must check that the yt (or h) has returned to 4 (or H) before repeating the test. Calculations Import your data into a separate Excel sheet, call it "Raw Data". Don't modify the raw data in case you have to return to it later. Copy this sheet to a new sheet, say, "Clean Data", and clean o all unwanted stuff so that you have only time and pressure head. Now copy your clean data to a new sheet, say, "Plots". up Using the Figure 8.20 from Freeze and Cherry's textbook as a guide, plot the data versus time as shown. Determine To as shown (the time corresponding to the Y-axis value of 0.37). Using this value of To, calculate the value of K using the Hvorslev method. This value, To, is related to the dimensions of the piezometer screen (length, radius of the screen and radius of the sand pack) and to the hydraulic conductivity (K) of the aquifer. Having the dimensions of the piezometer screen, one can then calculate K, from the value To as determined from the data, according to: r² In (R) K = 2 L To While the Hvorslev method is best for confined aquifers, it is suitable for the Twin Lakes aquifer because the top of the well screen is below the water table and the screen does not fully penetrate the aquifer (note that there is a distance of 8 m between the bottom of the well screen and the top of the till/bedrock boundary; distance "Li"). Well configuration and aquifer: The data for hydraulic conductivity calculations are from a well in the Twin Lake Aquifer located approximately 5 m from piezometer TL-21. It has the following characteristics: • r = 0.05 m (screen and well casing radius) R = 0.05 m (sand pack radius) L 1.52 m (screen length) Li = 8.00 m (distance between bottom of screen and impermeable layer below; indicates that the screen is not fully penetrating the aquifer) Background When using a pressure transducer, the instrument will give you readings of pressure head, w, (height of water above the probe) and time. To obtain hydraulic head we need to know z, the elevation of the pressure transducer; but knowledge of z is unnecessary if the position of the transducer is not changed during the measurement. In this case, the unrecovered hydraulic head difference will be equal to the unrecovered pressure head difference, because the (constant) elevation head cancels out: H-h (y+z) (y+ + 2) = y - Yr where: h = instantaneous hydraulic head [L] H = static water level (hydraulic head before adding/removing the slug) [L] Ho = initial head immediately after adding/removing the slug [L] Z = elevation head [L] Yt = instantaneous pressure head [L] - straight out of the data logger W logger = static pressure head before adding/removing the slug [L] - initial value from the data Yo = Ho H-h = unrecovered drawdown [L] = y-y (H= wand h = 1 if the pressure transducer is stationary throughout the test) Compare the symbols to those in Figure 8.20 from Freeze and Cherry (1979): 117 to (and t<0) 0.5 0.37 T PA H h H Datum (a) R t=0 H-H 0.2 0.1 2 4 6 8 10 t (hrs) (b) Figure 8.20 Hvorslev piezometer test. (a) Geometry: (b) method of analysis. 1