Biological contamination from ground I took samples of tapwater in order to examine concentrations of Escherichia coli and enterococcus in a rural area. I was told, from an analytical lab, to take samples in four sterilized vessels bought from pharmacy. 

The problem is that at the completion of the last vessel, its cap fell frοm my hand down into the soil and some grains of sand were stuck on the inner surface of the cap. As fast as I could I picked it up and washed it off totally with the water of the sampling tap. After that I closed my last sampling vessel with it and this is the completion of the sampling.

How large is the possibility my sampling has a serious contamination of these twο biological pollutants after this incident?
The point is not how large the possibility is. 
There is a possibility that the flask has been contaminated, so you should have taken a new flask. 
I suppose you did not have a spare one with you. 
Then, the best should have been to sterilize again that flask, but that is not easy in the field. 
I mean, it is much easier to take more sterile flasks with you than to sterilize them in the field. 

Now, if this sample shows that there are no bacteria, you can conclude that there are no bacteria in the water. 
But if there is some concentration of bacteria, you can not know if they come from the water or from the soil or how much come from each part.