My DS contracted meningitis (meningococcal septicaemia Type B) 11 years ago and passed it on to his teenage (at the time) big sister and because he had been ill for a number of days and had mixed closely (same room, hugs good bye etc) it was thought he could have potentially passed it on to many others.
DS was extremely ill and on a life support machine for many weeks, big DD was taken to hospital a short time after his diagnosis and because they knew what she probably had, although she was seriously ill, her version of the meningitis was treated quickly and within a couple of weeks she was out of hospital and recouperating at home.
Fortunately, DS surprised everyone by pulling through it and 11 years on has only some leg scars (from the septicaemia) to show from those dark days - a small price to pay.
I know things have moved on in the last 11 years (thank goodness) but, as a parent I would be aware of anyone in my vicinity who has it.
Those who have had direct contact with the girl should be cautious especially if they have passed any bodily fluids (cough, cold, shared lollipops/sweets etc) but assuming you have not been this close, I would just observe for any symptons (it would be very unlikely for your child to catch it) and also find out what sort of meningitis the girl in question has contacted and in turn, how she caught it.
As for the contact with lakes and water, that is all new to me and I know that those who are unfortunate enough to catch it, need a lot more than Vitamin C to recover.
I would be cautious for a few weeks and keep in touch with those close to the source to see if it has been passed on, not everyone is capable of getting it thank goodness and there will probably not be a vaccine for those that contract Type B as there are so many different strains of it although they are trying hard to get close.
This is a great link which fills in the gaps and keeps you all up to date www.meningitis.org