How does purchasing nutrient credits help the environment?

Proposed developments are required to provide stormwater absorption on parcels being developed.  Stormwater runoff can sometimes contain high levels of phosphorus and nitrogen and runoff can be detrimental to the local watershed as well as the Chesapeake Bay.  Developers that do not meet the requirements for stormwater runoff on-site, can purchase nutrient credits within the same sub-watershed to “offset” the requirements.