Per the product label for On Deck Herbicide: Rainfall or irrigation occurring within 4 hours after postemergence applications may reduce effectiveness
There are certain products that are susceptible to problems with high pH balances in water. If a product label does not specifically address this issue it is because this particular product does not usually have any issues with pH. On Deck Herbicide does not state any such issue, best would be between 5-7.0.
Per the product label for On Deck Herbicide: Rainfall or irrigation occurring within 4 hours after postemergence applications may reduce effectiveness.