Effect of rain on water and soil samples contaminated by petroleum
It has much sense since the rain will modify the concentrations of both the heavy metals and the petroleum.
The initial quantities will be redistributed. 
One part will penetrate deeper in the ground, another will be moved due to water runoff and what is left will be measured in your samples. 

If possible, you should take the first samples as soon as possible and the the next ones towards the end of next season. 
It is also important to get the value of the total precipitation during the rainy season. 
This will serve to estimate how long it will take to eliminate the effects of the disaster.