( This ( ( may ( mean ( ( the water ) ( has ( ( pathogens or ) ( ( other substances ) ( which ( can ( be harmful ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT This)) (VP (MD may) (VP (VB mean) (SBAR (S (NP (DT the) (NN water)) (VP (VBZ has) (NP (NP (NNS pathogens)) (CC or) (NP (NP (JJ other) (NNS substances)) (SBAR (WHNP (WDT which)) (S (VP (MD can) (VP (VB be) (ADJP (JJ harmful))))))))))))) (. .)))
This may mean the water has pathogens or other substances which can be harmful.
(ROOT (S (NP (NNP Water)) (VP (MD can) (VP (VB be) (VP (VBN boiled) (S (VP (TO to) (VP (VB kill) (NP (NP (NN giardia)) (CC and) (NP (RBS most) (JJ other) (NNS pathogens))))))))) (. .)))