The 91st's involvement
Company G of the 91st Pennsylvania was assigned to the Long Bridge on 19 March 1862 (Bates, p.186). The regiment was inspected there on 19 April 1862.
Duties
[Here's a general description of the duties of soldiers stationed there:]
"The next reserve to the east was stationed at the Long Bridge at the foot of 14th Street. The river was at this point too wide to enable deserters to swim across, but the locality being directly south of Willard's Hotel on the north and Alexandria road on the south was the central road for passengers and commerce southward. The three officers and one company on duty here were mainly occupied with inspecting stores and passes and patrolling the shore as far as Rock Creek."
[source: William E Doster, Lincoln and episodes of the civil war (New York: G P Putnam's Sons, 1915), pp.69-70]