Estimation of Hydraulic Conductivity through Slug and Bail Tests

Transcription

1 Estimation of Hydraulic Conductivity through Slug and Bail Tests Xiaoyuan Ren May 5, 2011 Abstract Under the hypothetical situation where the Troll Brothers are going to purchase Vassar Ecological Preserve for housing development, research was carried out to test the availability and accessibility of water resources within this region. The first preliminary research conducted was to test the hydraulic conductivity of potential aquifers through slug and bail test methods. The overall average from these tests suggested a conductivity of 0.9 feet/day, indicating moderately permeable geologic material, which can offer feasible but limited groundwater production. 1

2 1 Introduction Administration of Vassar College has been considering the sale of the Vassar Ecological Preserve. One company, the Troll Brothers, has expressed interest in acquiring the land for housing development. The Troll Brothers need to determine the availability and accessibility of groundwater resources within the area in order to make final purchase decisions. Thus, the company has hired Vassar Hydrology to carry out tests to determine the general groundwater resource characteristics of the Preserve. Several water quality and quantity tests are performed on the Preserve to determine whether the local water resources could support the development, and whether the water is potable. The first test performed is the slug and bail test to estimate hydraulic conductivity of the aquifers in this region. The level of the hydraulic conductivity can largely influence the possibility of houses relying on individual wells. Depending on the results, the Troll Brothers will decide whether or not to move on to subsequent experiments. 2 Background Slug and bail tests are used to determine the hydraulic conductivity of aquifers. In a slug test, a weight is dropped into the well and water is displaced upwards. The increase in water head in the well then drives water outward through the screen and back into the aquifer until water level inside the well matches the water table or the potentiometric surface again. The time that takes for the water head to drop back to 37% of its initial head change is recorded as T 37. T 37 can be determined through a pressure transducer inserted into the well prior to the start of test recordings. This transducer shows changes in water pressure at a certain fixed height in the well. The pressure level corresponds to changes in water height inside the well. A bail test is the opposite of the slug test, during which the slug is rapidly withdrawn, causing decline in the water head. The time for the water head to rise back to 37% of its original displacement is recorded through the same method. The hydraulic conductivity is then obtainable through: K = r2 ln L R 2LT 37 (1) where K is the hydraulic conductivity, r is the radius of well casing, R is the radius of the borehole, and L is the length of well screen and filter packing. 2

3 3 Methods First, records of two consecutive bail-slug tests conducted for the shallow sediment well provided by Senior Hydrogeologist Kirsten Menking were analyzed. The water head change during the four periods, in the order of bail-slug-bail-slug, is shown in Figure 1. Figure 1: Change in water depths for two consecutive bail-slug tests for the shallow well over time of day, where water depth is the water level in reference to a transducer around 35m below the ground level of well opening. The four tests were plotted separately on figures where the water heads are instead displayed in the form of normalized heads. To calculate the normalization, each of the water depth values was represented as a deviation from the regular water level, and this deviation was transformed into a fraction of the largest deviation. This fraction number, a number between 0 and 1, is the normalized head. It was plotted on log10 scale against time of day when data was recorded, as shown in Figure 2. These figures generally produces an overall linear pattern. With the linear pattern, the point where the Y coordinate is 0.37 can be easily identified in all four figures and the corresponding X coordinate was recorded as the recovery time. T 37 is the difference between this time and the beginning time of a slug or bail event. 3

4 Figure 2: The four normalized head versus time plots for the two consecutive bail-slug tests. 4 Results In the experiment, r = 1 inch, L = 84 inches, R = 2.5 inches. With these measurements, the experimental results are shown in the following Table 1. Experiment Bail No.1 Slug No.1 Bail No.2 Slug No.2 T 37 (s) K(feet/day) Table 1: Calculated conductivity for different trials of slug or bail tests. The average conductivity is feet/day, according to Table 1 above. 4

5 The standard deviation is 0.05 feet/day, which gives around 5% range of errors for the final result. Figure 3: Change in water depths over time of day during group bail test, where water depth is water level in reference to a transducer around 32m below the ground level. Figure 4: Normalized head versus time during group bail test. Similarly, a new round of bail test for the same sediment well was conducted by our group. The change of water depth during this group bail test is shown in Figure 3. The figure of change in normalized head over time was shown in Figure 4. 5

6 With the same methods as shown above, it is found found that T 3 7 = 165 s and calculated that K = feet/day, suggesting similar conductivity level as calculated from the previous set of data by Kirsten Menking. 5 Discussion and Conclusions The small deviation of results from four consecutive tests in Kirsten Menking s data set suggested consistency, indicating a relatively reliable result. The calculated conductivity from the group bail test showed consistency with the previous experiment by Kirsten Menking as well, verifying the accuracy of both experiments. The two data sets are collected during different times of the year, suggesting that the aquifer retains a similar hydraulic conductivity throughout the year. The overall average conductivity of both experiments is feet/day, which indicates fine to coarse sand, silt and fractured limestone as possible types of geologic materials in this region. Generally speaking, the geological materials around the Preserve are at most only moderately permeable. These possible sediment materials could still allow for some groundwater production, although it might not be as ideal and effective for a large number of houses each with individual wells. However, the groundwater production is still evaluated to be moderately feasible even though limited. Further actions by the Troll Brothers will require more investigation. 6