Hello Janet,

Unfortunately we can't identify the bee from the photo, as it is too dark, wet (?), and hard to see clearly. If the bees are entering and existing several holes in the ground, then yes, they are a ground-nesting bee species; mining bees are one common group. Maryland is home to hundreds of bee species, many of which will be ground nesters. If they are instead patrolling airspace around holes in wood, then they are likely Carpenter Bees.

Neither group will be motivated to sting, and are best left alone if possible and not treated or removed, especially the ground-nesting species. Habitat is shrinking for many ground-nesting bees, given human development, and they are valued pollinators. How long they will remain active depends greatly on which species they are, but in general, adults are only active for a few weeks (and females stay active longer than males, who only mate before dying off), busily digging and provisioning a nest tunnel for their offspring before dying. Bees collect pollen and sometimes nectar as food for their young, though some bees also chew out cookie-cutter-like sections of plant leaves to wrap around each nest cell. (This does not hurt the plant.) The baby bees spend the rest of the summer in their burrows, slowly feeding on the stored food and maturing, and will emerge the following spring to repeat the cycle.

Miri

Your horse may have tread upon the nest of a social wasp species, such as yellow jackets. Those are not like the docile ground-nesting mining bees which only have one individual in each hole (though there may be lots of holes near each other). 
Social wasps will defend their nest together and will sting when threatened. Our social wasp page was just re-done and is a great reference to learn more: 
https://extension.umd.edu/resource/social-wasps-yellowjackets-hornets-and-paper-wasps/


Christine