You can perform batch leaching tests on the soil/sediment, at natural pH and with varying pH, and with varying solids loading. This will help you understand speciation and solubility, with the aid also of geochemical modeling (e.g. Visual MINTEQ). My attached paper may be of help, but there is a lot of relevant literature available.
Article Comparative study of ageing, heat treatment and accelerated ...
When you study the chromium sources, it is extremely important to characterize the study area based on the anthropogenic sources also. The common sources of chromium can be chromite ore refining, ferrochromium production, refractory production, chromium chemicals production, chromium plating, steel production, leather tanning. I suggest considering these sources as well.