Regarding Termidor WG, how long does it take for it to bond to the dirt? For example, if you finish a termite job and then it rains heavily for about two or three hours. In this case the house is on level ground and the soil did not wash away.
The active ingredient in Termidor is fipronil, and fipronil is known to have a very LOW water solubility. This means that the active ingredient itself does not readily dissolve in water and therefore is not carried away by water by itself. This is extremely important with respect to ground water and the ability for a pesticide to leach down through the soil and into the underground aquifer. Chemicals that are HIGH in water solubility can therefore dissolve in water in high concentrations and higher amounts of that chemical can then move with water down through the soil profile and into underground water. Fipronil tends not to do this, and very little of it will be carried in water.
We use water only to dilute the insecticide formulation. The fipronil within your spray solution is not dissolved in the water, but only suspended, and with a WG (water dispersible granule) the particles of the dissolving granules themselves, with the fipronil mixed within that particle, also simply suspend in the water mixture. Once we apply this spray solution to soil the fipronil has a strong affinity for the soil particles, and the molecules of fipronil attach readily and stay put very well. Exactly how long this takes would be very hard to say, but the best practice would be to apply your termiticide spray only when it will be allowed to dry in the soil. The Termidor 80 WG label states that we cannot apply the material "while precipitation is occurring", but I believe to be on the safer side we also should not apply it when rain is imminent. If you saturate the soil with your spray application and then heavy rains come on top of this it greatly increases the chances that runoff of the soil could occur. This is the real danger of movement of the fipronil.
In your instance you know that the soil did not wash away from the treated areas, so I believe you are fine. If the soil is still very wet from your application there could be fipronil still in suspension in the water, and then heavy water on top of this could move that fipronil further into the soil, but somehow I doubt that ground water contamination is an issue in this case. What you may have happen is that the fipronil concentration in the soil could have been diluted quite a bit by moving further down with the rain, but hopefully it still exists at a concentration effective on the termites that encounter it later.