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.
Conditions (including flow) remain the same over time and therefore all changes are attributable to the BMP implementation.
This approach is most effective if using data from an existing monitoring site with a long record.
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.