Black and decker makes a selfpriming washer. If the preasure is basicly gravity or a barrell, and you want it electric, then the self priming feature (regardless of brand) is what you will need.

If you want gas power they can also be made with a waterpump, a hose into a bucket or barrell, and a hose out.

Anothet idea, the next time it is dry, try treating the area with "stuff" (I do not remember the name) to keep the area clear. I am sure someone here knows the name of the chemical.

Personally, I just use bleach and a paint roller. Kills everything about 2 days.

 

can buy a power washer for close on 7k, that is also a mobile unit and pretty light easy to move around more that do the job