<I guess, they would have loved to avoid to frac into an aquifer. Do you know what type of methods there are to seal off the water source?>

Depending on their completion, they could activate (close) sliding sleeves in the water producing zones.

http://en.wikipedia.org/wiki/Sliding...

I would imagine that Toreador probably cased to TD and perforated between plugs. If that's the case they likely left each of the "frac" zones isolated from each other, so in most cases an operator will use a sliding sleeve (or something simialr) to open communication between the perf'd casing zone and the production tubing.

Silverhound