▁are ▁found ▁near ▁the ▁lake ▁and ▁ 1 1 ▁ha ▁of ▁the ▁catch ment ▁area ▁is ▁occupied ▁by ▁one - family ▁houses . ▁ ▁While ▁the ▁in flow ▁from ▁the ▁surrounding ▁area ▁contributed ▁some ▁ 2 0 0 , 0 0 0 ▁m ³ ▁ann ually , ▁even ▁small ▁variations ▁in ▁water ▁level ▁in ▁Lake ▁M ä lar en ▁are ▁of ▁greater ▁importance . ▁ ▁Three ▁major ▁ar tery ▁roads ▁and ▁a ▁ground - level ▁met ro ▁track ▁st ret ches ▁through ▁the ▁area ▁of ▁which ▁B rom ma ▁Airport ▁forms ▁a ▁minor ▁part . ▁ ▁Environment al ▁influence ▁ ▁North ▁and ▁east ▁of ▁the ▁lake ▁are ▁a ▁pet rol ▁station ▁and ▁other ▁operations ▁considered ▁as ▁having ▁a ▁negative ▁environmental ▁impact . ▁ ▁Until ▁the ▁mid - 1 9 8 0 s , ▁the ▁lake ▁received ▁considerable ▁amounts ▁of ▁sp ill way ▁overflow ▁which ▁resulted ▁in ▁high ▁levels ▁of ▁col iform ▁b acter ia . ▁ ▁A ▁waste ▁water ▁accident ▁in ▁ 1 9 9 7 ▁caused ▁considerable ▁amounts ▁of ▁poll uted ▁water ▁to ▁leak ▁into ▁the ▁lake , ▁but ▁since ▁ 2 0 0 1 ▁waste ▁water ▁only ▁reach ▁the ▁lake ▁during ▁longer ▁periods ▁with ▁continuous ▁rain . ▁ ▁Low ▁levels ▁of ▁o xygen ▁causes ▁levels ▁of ▁ph osph orus ▁to ▁rise ▁in ▁the ▁bottom ▁of ▁the ▁lake ▁during ▁winter s ▁and ▁during ▁sum mers ▁when ▁the ▁lake ▁is ▁strat ified . ▁ ▁Run - off ▁water ▁from ▁surrounding ▁roads ▁reaches ▁the ▁lake ▁through ▁the ▁d itch ▁d ug ▁when ▁the ▁park