October 31, 2028: How Virginia MS4 Localities Can Close the Chesapeake Bay Nitrogen Gap
For most Virginia localities with a municipal stormwater permit, the Chesapeake Bay cleanup now has a hard date. Permittees under the 2023 to 2028 Small MS4 General Permit must have 100 percent of their required Bay reductions in place by October 31, 2028. Whatever a locality hasn't built by then, it has to find another way to count.
Where the requirement comes from
The Chesapeake Bay TMDL is carried into local permits through a special condition. Reductions were phased over three five-year permit cycles: 5 percent in the first, 35 percent in the second, and the remaining 60 percent in the third, the current one. The Virginia DEQ MS4 program page lists 100 Phase II operators under the general permit, and each must keep a Bay TMDL action plan showing how it gets to 100 percent. DEQ's review letters to permittees, such as this February 2025 letter, state the October 31, 2028 date directly. Larger Phase I localities hold individual permits with their own schedules; Prince William County's draft action plan, for example, targets June 30, 2028.
The target can also move under you. Several third-cycle action plans, including Albemarle County's, note that the MS4 regulated area is being redrawn using 2020 Census urban areas. More regulated acreage can mean more pounds to reduce.
The bigger picture on the Bay
The region is behind on nitrogen. According to the Chesapeake Bay Program's 2025 assessment, practices in place across the watershed achieve about 57 percent of the nitrogen goal, 90 percent of the phosphorus goal and 100 percent of the sediment goal. In December 2025 the Bay states and EPA approved a revised Chesapeake Bay Watershed Agreement that sets 2040 as the deadline, with targets refined around 2030. Pressure on urban stormwater isn't going away after 2028.
Purchased credits count, one to one
Virginia law gives MS4 localities a direct route. Under subsection I of Va. Code § 62.1-44.15:35, when a locality that operates a regulated MS4 buys nutrient credits from a certified credit-generating bank, independent of what development projects bought for their own compliance, each credit counts one to one toward its Bay TMDL waste load allocation. Credits that redevelopment projects inside the MS4 service area buy also count toward the locality, the same as onsite measures would.
So credits are a compliance tool for localities as well as developers. A locality that is short a few hundred pounds of nitrogen as 2028 approaches can close that gap with credits that are already certified, already in the ground and already monitored, without siting, designing and maintaining a new retrofit.
Credits versus capital projects
The honest comparison isn't price per pound alone. A stream restoration or pond retrofit can be a good investment, and many localities will keep building them. But a capital project carries design, permitting, land or easement costs, construction risk, and a maintenance commitment that never ends, and it has to be finished and credited before the deadline to count. Purchased credits shift those risks to the bank, which is required to protect and maintain the site permanently.
Most localities will use both. The useful exercise is to price the last 10 to 20 percent of your requirement both ways.
A planning sequence for 2027
- Compare reductions achieved to date in your action plan with the 100 percent target, by pollutant.
- Check whether the 2020 Census redraw changes your regulated area.
- Rank remaining capital projects by cost per pound and by their odds of finishing before October 2028.
- Price the gap with nutrient credits, including the geographic and local TMDL rules that apply to your receiving waters.
- Build the purchase into the FY2028 budget and procurement calendar, since council approval and contracting take time.
- Document each purchase in your annual report as a 1:1 reduction.
Working with Potomac
Potomac Mitigation Bank operates DEQ-registered nutrient banks in Loudoun and Westmoreland counties and has supplied credits for municipal facility and infrastructure projects. We can confirm which of our banks can serve your MS4 service area and provide availability and pricing for planning or procurement. Request pricing or call (540) 251-1321.
