THE PLT RPRTD THAT WHEN HE TOOK OFF FM A NRBY NURSE TRUCK, THE WIND WAS FM THE NE AT 5 TO 7 MPH. HE MADE AN OBSERVATION PASS ACROSS THE AREA TO BE TREATED, THEN BGN A DSCNDG RGT TURN TO MAKE A SWATH RUN IN THE OPPOSITE DRCTN. HE STATED HE WAS UNAWARE OF A WIND-SHIFT TO THE NW. SUBSEQUENTLY, THE HELICOPTER ENCOUNTERED WHAT THE PLT THOUGHT WAS WAKE TURBC FM THE OBSERVATION PASS. THE ACFT BGN TO SETTLE & THE PLT INCREASED THE COLLECTIVE IN AN EFFORT TO STOP THE DSCNT. THE PLT SAID THE 'RECOVERY ENDED IN A DEEP CANYON APPROX 150 TO 200 FT DEEP. EMERGENCY TRIP DOOR SOLENOID TO RELEASE LOAD FAILEDTO WORK. WAS ONLY ABLE TO SUSTAIN HOVER FOR APPROX 2 MIN BEFORE A/C SETTLED ON A 30 TO 40 (DEGREE) EMBANKMENT & SLID DOWN 40 FT TO TREES, TAIL FIRST.' RPRTDLY THE DUMP VLV FAILED TO OPERATE DUE TO CORROSION FM CHEMICALS CARRIED ON THE PREVIOUS FLT.