I think the concern should be with detecting radium, since all isotopes are radioactive.
The best way to detect radium is with energy dispersive X-Ray fluorescence (EDXRF) spectrometry. They come in two common forms, a desk top and a hand held unit. Desk top units need the samples brought to them and are more expensive. The hand held units are portable. So, the brine on roads could be measured directly.
As far as The spectrometry goes, the spectra are much simpler than something like IR. They are relatively easy to interpret, although there were some issues like escape peaks and artifacts that needed to be dealt with. The newer software maybe able to take care of that now. The area being tested needs to be dry. Detection limits for these elements is usually in the ppm range of lower.
Several companies produce units. They can be optimized for radium (say X-Ray source or americium source and which line ( k,l, or m) is used). Please talk to there tech people. The portable units often use a beryllium window. Beryllium is not good by itself. Then, the window is thin and easily broken. It is expensive to replace.
I haven't gotten a price quote lately, but my guess is these instruments are in the $20 k range.
By the way, you might want to carry Geiger counter, as well. If you see radium, pull out the geiger and make sure it's radioactive.