CVEN9630 Groundwater Hydrology Assignment

QUESTION 1. SLUGTEST ANALYSIS [20 Marks]

A slug test analysis was performed on a piezometer in a sandy unit. Fig. 1 (see next page) shows the water level response after the pressure in a piezometer is released. Measurements depicted by the dots in Fig. 1 are also recorded in Table 1 (next page). The construction details of the piezometer are as follows: The outside casing and screen diameter (2? ) is 6 cm, while the casing wall thickness is 5 mm. The screen length is 1.5 m. The mean aquifer grain size d50 = 1 mm.

1. Use the data provided in the table to calculate the hydraulic conductivity of the aquifer. Hint: Make sure you normalise your head data.
2. Calculate the maximum velocity at which the water enters the piezometer screen. Hint: What is the maximum velocity at which the water level rises within the piezometer.
3. Does Darcy’s law apply? If not, how else could the slug test be evaluated? Hint: Evaluate the max. Reynolds number. The max. velocity in the aquifer at the screen can be calculated by considering the hydraulics of the slug test

QUESTION 2. NUMERICAL GROUNDWATER MODEL [40 Marks]

The objective is to use an Excel spreadsheet to investigate various boundary conditions for a simple rectangular confined aquifer. The aquifer heads and flow are modeled using a finite difference scheme based on the analysis of Frank and Reilly [1987].

In Excel you need to set up an aquifer model of an area 1100 m wide (in the x-direction) by 1400 m long (in the y-direction). Use a finite difference discrimination where ?x = ?y = 100 m. Your model will then be 12 nodes wide in the x-direction (extending from node 0 to node 11) and 15 nodes long in the y-direction (extending from node 0 to node 14). Assume that the aquifer transmissivity is 300 m2/day. The upper left corner is labelled A, and the other corners are B, C, D (in a clockwise direction).

Model the following three cases:

1. A Dirichlet boundary condition with a linear decrease in head from 7 m to 0 m along BC and AD, a fixed head of 0 m along boundary CD, and a fixed head of 7 m along boundary AB,
2. A Neumann boundary condition with dh/dx=0 across BC and AD, a fixed head of 0 m along boundary CD, and a fixed head of 7 m along boundary AB,
3. A Neumann boundary condition with dh/dx=0 across BC and AD, and an inflow of 2200 m3/day across AB, and a fixed head of 0 m along CD.

Hint: In Excel please make sure that you enable iterations under ”File” then ”Options” then “Formulas” then “Calculation Options”, Activate ”Enable iterative calculation”. Also, set ”Maximum Iterations” to 30,000 and ”Maximum Change” to 0.000001.

Answer the following questions:

1. Explain how you will implement the head calculation (i.e. discretised Laplace equation).
2. Explain how you will implement the boundary conditions (fixed head; no-flow and fixed flow) in a spread sheet.
3. Explain how you will incorporate the groundwater abstraction (pumping) and how you calculate fluxes across the boundary.
4. Run the model for each case with two abstraction bores at nodes (3,5) and (6,7) each pumping at 700 m3/day (e.g. 1400 m3 /day in total) and calculate the drawdowns at each of these nodes. Explain the difference in drawdown between the three boundary condition cases.
5. Plot the flow nets for the 3 different boundary conditions when the pumping is active.
6. Calculate the flows across all boundaries for each of the 3 cases both with and without the abstraction bores pumping. Assume that flow into the model is positive and flow out of the model is negative. Abstraction is therefore negative.

QUESTION 3. AQUIFER TEST ANALYSIS [40 Marks]

A major aquifer test was conducted at site close to Wagga on the Murrumbidgee River (Figure. 2). The borehole locations are shown in Figure 3 and the lithological logs are shown in Figure 4. The pumping test at Bore 30638 started on 26 September 1972 at 15:00 and ran for 3 days. Water levels were recorded inside the bore and in a piezometer installed in the gravel pack between 128.02 m and 128.62 m. This data is presented in Table 2. The pump bore (30638) was screened between 125.58 and 137.76 mAHD. The nominal flow rate was 126.03 l /s. Respective water level data from bores in the vicinity are presented in Tables 2 to 4. The available data sets are presented below.

Complete the following task:

Interpret the data set to derive estimates of transmissivity and storage for this confined aquifer. You should use a combination of log-log and semi-log methods to arrive at your answer. Please clearly state all your assumptions, and importantly when inspecting the results, discuss if any of the assumptions have been violated.

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