The Sewage Field
We first learn about the sewage field in "Speaking of Courage," when Norman Bowker can't stop driving around a lake while he thinks about what happened to Kiowa. Kiowa, the most moral character in the book and a Native American besides, is a symbol of American decency, drowned in the sewage of the war. The lake in "Speaking of Courage" is a stand-in for the field (as we know from "Notes"), and Norman's driving in circles around the lake shows his inability to talk about any of it. He's not able to cut directly to the heart of the matter – that he feels that decency was destroyed in the war – and so he talks around and around it. Sally and the town, a sort of stand-in for all of small-town America, hear the obscenity he's forced to use instead of listening to the reasons why he's using it. One of the symptoms of PTSD (that's Post-Traumatic Stress Syndrome) is the inability to speak of or remember an event without reliving it. Norman shows this here. Unable to speak about the war, he has to stop driving around and walks into the lake.
Later, in "In the Field," (the field here being both the sewage field that drowned Kiowa and the combat zone) O'Brien discusses the blame for Kiowa's death in the sewage field – or, in keeping with our metaphor, the drowning of American goodness in poop. The soldiers all feel guilty in one way or another – for following orders instead of trusting the Vietnamese, for a moment of stupidity in the field, or for their own brutal and disrespectful natures. Jimmy Cross and Norman Bowker both reflect that the blame is universal. The ignoble death of American decency in war is everybody's fault, in one way or another (which still doesn't mean we should dismiss the idea of personal responsibility).
The next time we see the sewage field is in "Field Trip," when O'Brien goes back to Vietnam to honor Kiowa's memory and to introduce his daughter, Kathleen, to the place that owns such a large part of his soul. Since O'Brien doesn't actually have a daughter named Kathleen and at the time of writing had never been back to Vietnam, we know there's some pretty heavy symbolism going on here.
Kathleen is modern America. She's young, she doesn't really remember the war, and she doesn't understand why her father is so darn obsessed with it, why he's still writing stories. By writing war stories, Tim is metaphorically returning to the sewage field to pay tribute to his dead comrades. He takes us along for the ride to show us what happened. We might not get it – Kathleen certainly doesn't – but that's not what the trip is about. The trip is about honoring the dead and remembering the moral sacrifices that we made in war.
The Dead, Dainty Young Man
The young man represents all of the faceless Vietnamese dead. There were hundreds of thousands of Vietnamese dead, and many went unidentified, burned by napalm or buried by the American soldiers. With the young man on the trail, Tim is trying to give the Vietnamese victims some dignity and an identity.
Mary Anne Bell
Mary Anne is the embodiment of American arrogance in Vietnam. She arrives dressed in her pink sweater and culottes, fresh faced and curious. She wants to know about everything. She treats the deep-rooted conflict as a holiday, blithely treating a Viet Cong stronghold like a tourist town and swimming in a river that's possibly surrounded by snipers. Just like the Americans who thought the war would be easy and over quickly, she thinks she can't be touched.
But she is touched. She's sucked into Vietnam, and she ultimately can't leave. She doesn't want to. Just learning about the country isn't enough. She wants to consume it. And, eventually, she slips away from the soldiers – she's not on their side anymore – and stalks through the night, dealing death.
On the Rainy River
So, O'Brien didn't really work in a meatpacking plant the summer before he went to Vietnam, and he didn't go up to the Canadian border to try to get away from the war and then chicken out and return home. It's a symbol for his mental state at the time. He can't get the nightmarish idea of slaughter out of his head – it's all he can think about – and so he thinks about running away. He's on the edge. Eventually, though, he backs off the edge. He doesn't go to Canada.
Elroy Berdahl is an important symbol in all of this, as O'Brien explicitly states:
He was the true audience. He was a witness, like God, or like the gods, who look on in absolute silence as we live our lives, as we make our choices or fail to make them. (On the Rainy River.74)
If Berdahl is God (or your deity of choice – atheists, feel free to use the universe as a stand-in), then God is ambivalent here. He doesn't push Tim to make one choice or another, and he doesn't judge Tim either way. He's simply there, watching, and his presence is felt.