Attempting to treat sections of a 1-acre pond is akin to letting one rip on one side of the bed and expecting the odor to remain on that side. According to my spouse, it doesn't!! ha!
Seriously, confining the impact of any herbicide based on its placement within a dynamic (constantly transitioning) environment is highly unlikely - even with granular products. Once an active ingredient is applied (whether using a granular or liquid "carrier" to distribute the active), it become mobile within the water-column and is likely to move or diffuse into untreated adjacent areas; although dilution-factors may reduce or impede its activity in said adjacent areas. Regardless, the degree of impact outside of the treated/targeted area will likely be out of one's control - and completely dependent upon several environmental factors that are present.
If you're attempting to selectively take out hydrilla while leaving other plant species intact, you'll need a much more refined tactic - and select an appropriate product. If that's not the case, you can initiate your whole-pond treatment earlier in the season (early spring), before the hydrilla's biomass has grown too large - and while water-temps are still relatively low - and still conduct an aggressive treatment that poses relatively little risk of causing a DO-crash.
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