This approach is very similar to “monitoring downstream before and after implementation,” but in this case results after implementation “treatment” are compared with data collected at the same location considerably before the implementation.

Advantages:

• More data may be available to use in the “before” period. These can be used to fully establish the “behavior” of the system before implementation of the BMP.

Disadvantages:

• This approach has the same disadvantages as “monitoring downstream before and after implementation.” Changes in weather, hydrology, land use, and other external factors cannot be controlled. The longer the historic data set, the stronger the relationships that can be drawn from the comparison.
• Sampling and analytical techniques may change over time, which can greatly complicate comparisons. Be aware of changes in detection limits, reporting units, or methodology (for example, use of filtered versus non-filtered samples).