Ashby Pond in the City of Fairfax, Virginia was retrofitted to treat runoff from 54.7 hectares of urban land of mixed use. The pond discharges into Accotink Creek, a highly urbanized tributary of the Potomac River and Chesapeake Bay that is listed on the State of Virginia 303(d) list for multiple impairments. The entire multi-state Chesapeake Bay Watershed is subject to Total Maximum Daily Load (TMDL) restrictions on sediment, phosphorus and nitrogen. Virginia and local municipalities assign pollutant reduction credits to retention ponds that meet certain design requirements. However, to actually meet existing and future water quality goals set by TMDLs, it must be proven that such ponds truly provide the water quality benefits for which they have been credited. The inflow and outflow water quality of Ashby Pond was examined over 7 months from fall 2012 to spring 2013. During that period, the pond provided statistically significant reductions of phosphorus, nitrogen and suspended sediment, but not organic carbon or oxygen demand. Ashby Pond had non-significant export of sodium, chloride and calcium. The pond underperformed when compared to state reduction credits for phosphorus load and concentration, but met and exceeded the credits for nitrogen load and concentration, respectively. The pond was under-sized compared to state design standards, and some underperformance should be expected.