With this approach, a site is monitored at a downstream location before and after the BMP is implemented. The difference in concentration or total loading after implementation can be attributed to the BMP.

Assumptions:

• Conditions (including flow) remain the same over time and therefore all changes are attributable to the BMP implementation.
  

Advantages:

• This approach is most effective if using data from an existing monitoring site with a long record.

Disadvantages:

• It is often difficult to control for other activities upstream of the monitoring site; this approach will not be able to differentiate water quality changes resulting from the BMP from any other changes upstream of the BMP implementation.
• It is also difficult to control for changes that happen over time. For example, if the “before implementation” period happens to be a drought and the “after implementation” monitoring occurs during a high water period, it will be difficult to differentiate drought and climate impacts from changes due to the BMP.

When to use this approach:

• This is a weak monitoring approach and, when possible, should be avoided or used with other approaches.
• This may be a useful approach under very restricted circumstances. For example, it may be used if the BMP is intended to be effective only for a very short time (such as straw bales to capture construction runoff). Even in these cases, however, you must monitor under similar conditions (e.g., a rain event) and natural variability may complicate the results.