You are correct, leachate results are typically reported against a Soil Sample (Chemistry_Samples.Matrix_Type = Soil), and have a Chemistry_Results.Result_Type = 'Leached_Reg'.

The output units for leached results use the "liquid" output units specified in the zRef_Chemistry_Lookup table (or in SQL Server version zRef_Chemistry_Lookup_Profiles).  This is the same output unit that is used for all liquid outputs (ie Water).

There is no way of setting different units for leached results and water, but it is a good suggestion, and we will look at addressing in a future release.

In the meantime, a possible workaround in the SQL Server version of ESdat is to use the "Chem Profiles" feature, which allows different output units to be used depending on the Chem Profile selected.  Users can switch between chem profles on the fly.

Using this you could generate a Chem Profile specifically for outputting leached soil results (ie Liquid units of mg/L), and then switch to that Profile when outputting leached data.  See http://www.ESdat.net/ESdathelp/index.html?chemistry_profiles.htm 

For Access versions there is no equivalent workaround, other than to change your output units, do the output, then change them back, but that isn't very satisfactory.