Converting methane (CH₄) headspace measurements in ppm to pore water concentrations in mM/L

Conditions:
The sediment samples have been taken as fast as possible from the core catcher
or the cut surface during separation of core liners into sections,
immediately sealed in a headspace vial with a NaCl brine or NaOH receiver,
shaken for 3 min, and either measured within a few hours or stored upside down at -18°C.
For measurement, headspace vials are warmed up to room temperature
and gas samples for analyses are taken from the headspace above the sediment / brine slurry.

Parameters needed:

V_Sed   - amount of sediment sample (e.g  a syringe sample of 2.5 mL) in L
V_vial    - total volume of the headspace vial V_vial (e.g. 20 mL) in L
V_brine  - amount of NaOH or brine receiver in the vial to drive CH₄ to headspace. (e.g. 7.5 mL ) in L
V_Gas   - amount of headspace gas V_Gas (e.g. 10 mL or 0.01 L) in L
             V_Gas   = V_vial - V_sed - V_brine
V_IG     - ideal gas volume at 25°C and 1 bar = 24.789 L/M
C_HS    - methane concentration in headspace gas in ppm
por     - sediment porosity as a fraction of  1

C_pw    - methane concentration in pore water in M/L

           C_pw =  V_Gas / V_IG  * C_HS  x 10^-6 x 1 / (V_sed x por)

If only sediment density ρ_sed is known from e.g. MSCL measurement, porosity may be estimated from
    por = (ρ_rock  - ρ_sed ) / ( ρ_rock - ρ_pw)

Example : Concentration from GC measurement  C_HS = 20000 ppm, porosity  por = 0.8

         C_pw =  0.01 / 24.789  x 20000  x 10^-6 x 1 / (0.0025 x 0.8) = 0.00403 M/L  = 4.03 mM/L

In Excel, the densities from MSCL measurements at the depth of the methane samples
may be extracted from a MSCL dataset  using the VLOOKUP function

assumption:
CH₄  sample depth in column A,
MSCL depth and MSCL densities in columns D and E  e.g. from line 2 to line 100
MSCL depths are increasing, no empty readings in E

densitiy at the sample depth found in cell A2 =VLOOKUP (A2,$D$2:$E$100,2)
if there is no exact match, the VLOOKUP function will use the previous reading
e.g. Methane sample depth at 2.25 m and only MSCL density readings at  2.21 m and 2.26 m exist,
VLOOKUP will return the denstiy from column E at 2.21m.

original time stamp 19.December 2022