pool_mpua: Mass per unit area of pool
In CJBarry/DNAPL: DNAPL Source Term Framework

Mass per unit area of pool

1 2	pool_mpua(hp, phi, rho, Srn, Srw, mode = c("uniform", "linear", "exponential")[1L])

`hp`	numeric `[1]` or `[NLAY]`; pool height, or total height of pools in a layer
`phi`	numeric `[1]` or `[NLAY]`; bulk porosity of each layer
`rho`	numeric `[1]`; solvent density (ensure consistent units, probably kg/m^3)
`Srn`	numeric `[1]`; residual saturations of NAPL and water
`Srw`	numeric `[1]`; residual saturations of NAPL and water
`mode`	character [1]; nature of saturation variation with height above pool base, one of `"uniform"`, `"linear"` or `"exponential"`; for `"uniform"`, it is taken that the pool has saturation `1 - Srw` and for the other two it is taken that the pool has saturation `1 - Srw` at the base and `Srn` at the top