Audi A6 2,5 TDI 1996 loosing water.....

i have just bought this nice car..... with 190.000 km on the clock
i drove i home from Berlin to denmark, with no water loose.
when i drive it normaly, ther is no whater loose......even after 200
km but when i drive hard, for only 20 km the goes to minium and the alarm sounds....
any idees??? what i got be??

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