Hello delpepo,
could you provide us information on which database (guess ecoinvent, LCI, consequental) and method you used (CML?, our method package or the provide methods)? Moreover, which process you calculated? This would help us.
However, sometimes processes actually remove pollutants from bodies of water as their water output is cleaner than the water input. This can result in negative values. In particular in a system process database. But here it is clearly as Conrad described the consequently system model you are using. Have a look at his answer :)
Best,
Jonas
