A Better Place by Linda P. Baker. I read it in the anthology Time Twisters, published in 2007, which is apparently the only place it has been published.
The name of the woman is Rosemary, though we don't discover this until the end of the story, and her abusive partner is called Gnash. He is a nasty piece of work. His main weapon is a baseball bat, but he does use concrete in a sock as well:
Gnash went upstairs before we left. He came back carrying a bat. I knew what made the pocket of his coat look so bulky ... ragged pieces of crete, slipped into the toe of a sock. I’d laughed to myself the first time I’d seen him pocket his special weapon, but then I’d seen him use it. I hadn’t laughed afterwards.
The old man is Old Jerry:
Gnash was gone the day I found Old Jerry lying in the street, dying. The old man was in the gutter, so still and blue at first I thought he was dead. Dead ones were nothing unusual, but it was better not to go near them. Don’t go near the dead ones. All kinds of things could come from the dead ones, bugs and disease and worst of all, dreams where the dead one’s face chases you through a fog. But then Old Jerry groaned.
Old Jerry is mostly incoherent but rambles about a treasure leading Gnash to believe Jerry knows the location of something valuable. However Jerry dies without revealing the details.
Gnash searches Jerry's body and discover some nuts that were apparently the treasure. In his anger at this he hits Rosemary but she fights back and drives Gnash away:
Gnash drew back to hit me again with one hand and flung the treasure at my face with the other. Anger, hot as the fire in the picture, raced over me. It was like those little brown balls, hitting my face and my chest, had been sucked through my skin. Like they had lit a fire in my chest, stiffened my backbone. Gnash didn’t see my fist coming, any more than I’d seen his.
Fire exploded over my knuckles as they smashed into his nose. It felt like I’d broken my hand. My eyes and my mouth watered with the pain. He yelped and rocked back, the expression on his face almost funny as blood poured down over his mouth.
...
Gnash stood up slowly. His hand slid toward his pocket.
I steadied the bat. "Go now. Before I call Old Jerry’s friends."
At the end of the story the nuts are revealed to be:
My gram had loved the old oak trees above all other living things on this world, save for me. I’d thought the trees were dying, but somewhere, in one of Old Jerry’s driftings, he’d found an oak tree that wasn’t dying. Maybe that meant there were better places. Places where the earth was starting to live again. My gram had said there would be, if I just had the guts to go looking.
I’d only had pictures. In one of the books Gnash had burned, I’d even marked the page. If he’d ever bothered to look at it, he’d have known what Old Jerry’s treasure was ...
Acorns. Five precious acorns.
The connection to Yellowstone is that the wasteland through which Rosemary and Gnash move was caused by the eruption of Yellowstone. At one point Rosemary and Gansh find a mural depicting the eruption with the text:
Gnash prodded me with his bat. "What does it say?"
I looked away from the picture to where he was standing. At the bottom of the picture, there was writing. I read it, puzzling over a couple of words that didn’t make much sense. "On March 29, 2005, the volcano under Yellowstone National Park, long thought to be dormant, erupted, bleeding a river of red hot lava and spewing clouds of ash and gas into the atmosphere."
