Isotope Hydrology HW-5, due: Tu 3/11

Radiocarbon Dating of groundwater in Texas

• download the Excel file TexasGwC14.xls which contains the relevant measured data
• the total dissolved inorganic carbon (DIC) content of the water was calculated from pH and alkalinity

1) (5p) Trends

• plot pH, total CO₂, d¹³C, and ¹⁴C vs. distance from the recharge area
• how do you interpret these trends?

2) (10p) Radiocarbon ages

• Apply the following ¹⁴C correction models to the data:
• a) Vogel model (Ao = 85pmc)
• b) a simple chemical dilution model (assuming a fixed value of DIC with 100pmc in the rechange area which is then diluted in the aquifer)
• c) the  d¹³C dilution model (Pearson model)
• Assume that d¹³C of the dissolved limestone and that of the organic matter in the recharge area are 0‰ and -25‰, respectively.
• plot log(DIC) vs. log(d¹³C/(-25 ‰)) and put a line with the slope -1  through the data; what does this plot tell you?

3) (10p) ¹⁴C ages in context

• plot the radiocarbon ages versus distance from the recharge area
• calculate the average standard deviation of the ages derived from the different models to get a feeling for the differences between the models
• calculate the horizontal flow velocity based on the chemical dilution  model
• the pososity of the aquifer is 35%, hydraulic gradient is 1ft/mile,  what is the hydraulic conductivity?
• the hydraulic conducitivity has been measured in the field as:  80 gal/ft^2/day (in the lower part of the aquifer)
• how do the two values compare?