Reference no: EM131496646

Question 1
A lab analysis shows that the alkalinity of a sample is 3.13 meq/L and the pH = 7.7. The sample is groundwater from a limestone aquifer so it is reasonable to assume that alkalinity = mHCO₃ + 2mCO₃ (equation 5.14 from Appelo and Postma). Ignore the difference between activity and concentration and calculate the concentration of HCO3. Provide your answer in units of mmol/L. Hint: Refer to Table 5.3 from Appelo and Postma for the relevant mass action equation(s).

Question 2
Water sampled from a spring is found to be in equilibrium with dolomite. The prevailing CO₂ partial pressure is 10-2.6. Calculate the saturation index of the water sample for calcite (give your answer with two decimal digits).
Hints: Assume that activity equals concentration. Refer to section 5.5.1 in Appelo and Postma to find the concentration of Ca²⁺ as a function of PCO₂. K_dol = 10-16.7 and K_calcite = 10^-8.48.

Use the dissolution reaction of dolomite

CaMg(CO₃)₂ = Ca²⁺ + Mg²⁺ + 2CO₃^2-

to find CO₃^2-. You'll have to make an assumption about the Mg²⁺ concentration.

Question 3
A soil water sample in an agricultural area was found to have the following cation concentrations. Assuming that these are the only cations in the water, and equating activity to concentration, calculate the activity (equivalent fraction) of adsorbed NH₄₊ (ΒNH₄₊). Provide your answer with two decimal digits. Hint: Refer to Table 6.4 in Appelo and Postma for the exchange reactions and constants.

solute	mmol/L
Na	1
K	5
NH4	5.4

Question 4
A water sample has a pH of 7.9 and is in equilibrium with oxygen gas at a partial pressure of PO₂ = 0.209. Calculate the redox potential (Eh) of this water. Express your answer in mV and round to a whole number (no decimal digits).