An injection tip assembly 10 and methods for use more reliably provide for delivery of fluid substances, such as materials that promote removal, destruction, or isolation of contaminants, into targeted zones within soil or bedrock. The injection tip assembly 10 permits the application of pressurized fluid 163 so as to erode or cut a desired cavity or eroded volume 164 within the subsurface 14, allows for timely observation, adjustment, and control of pressure within the cavity, and directs the delivery of a second substance or fluid that may incorporate desired materials. The consequence of managed erosion and pressure control is to nucleate and propagate a hydraulic fracture of desirable form that optimally delivers remedial agents throughout the targeted formation.