Hydrology EESC BC 3025

HYDROLOGY - Homework # 9

1) (10 points) A soil sample is characterized by a volume of 100ml and a weight of 192g. After drying the sample in an oven at 105 oC for a day, the weight was reduced to 182g. The soil was then completely saturated with water resulting in a weight of 212g. Finally, the sample was drained (removal of gravity water) and the weight was determined as 187g. Calculate the (a) porosity, (b) moisture content at the beginning, and (c) the field capacity of the soil in %. Determine also the (d) density of the solid phase (only the soil grains) in g/ml. The density of water is 1g/ml.

2) (10 points) Tensiometers are installed in a soil at 0.4 and 0.3 m above the water table in a uniform sandy soil with the moisture characteristics and hydraulic conductivity curves given in Figures 8.4 and 8.5, respectively. One set of tensiometer readings indicates that the capillary pressure head at the first of these tensiometers in -0.5m and at the second is -0.6m.

a) Make a little diagram of the set-up. Determine the hydraulic head at the two locations. Does the water flow upward or downward?

b) Estimate the hydraulic conductivity in the soil and calculate the specfic discharge using Darcy's law.

3) (10 points) 14C is produced in the higher atmosphere by cosmic rays. Under natural conditions, the 14C activity or concentration in the atmosphere is per definition set to 100 % modern [% mod]. The carbon in the CO2 molecule therefore has a 14C activity of 100 % mod. as well.
14C was measured on two samples taken along a flow line in a confined aquifer as summarized in the following table:
 
Sample
 Distance from the recharge area [km] 14C activity [% mod]
TX-1
10
35
TX-2
30
5

a) Calculate the age of the two samples under the assumption that the initial 14C activity in the recharge area (CO2 in the soil zone) was 85% modern. The halflife of 14C is 5730 years. The age may be calculated in the following way:

b) What is the flow velocity of the water between the two wells? Assume that dispersion and adsorption can be neglected.

c) It turns out that the carbon concentration in the aquifer increases linearly as a function of distance from the recharge area. This is because very old (millions of years old) carbonates are dissolved from the aquifer rocks. Is the velocity calculated in b) under- or overestimating the actual velocity? Please give a brief explanation.

4.) (8 points) Tucson water issues.

Download the TAMA (Tucson Active Management Area) water use data (TAMA_projections.xls) and make (linear?) projections for population and related water demand for the next 50 years. Play with different scenarios, assuming you (a) do and (b) do not have access to CAP water.

What policy recommendations you would make for this region? Use a couple of graphs to support your agrgument.