To begin, mitigation (stream, wetland, buffer, nutrient, carbon, etc.) is an active effort to replace a soon to be lost natural resource. Generally, this is done using either a geographic or tonnage metric. An acre of wetlands will be filled in and two or more acres of wetlands will be constructed to mitigate the loss.
Now this may seem like a laudable effort. However, history has shown that many of the mitigation sites have failed. The benchmark for these projects varies. It is usually some sort of empirical data measurement. Wetlands are usually measured against achieving a certain hydrologic frequency and duration standard (i.e. 5% for the growing season) and a specific planting survivability. For example, if a two-acre mature forested wetland is destroyed a four-acre newly constructed wetland will serve as the mitigation. The wetland will need to remain inundated for approximately 10 days and it will be planted with bare root seedlings at the rate of 320 stems per acre. At the end of 5 years, the site will need to have at least 270 stems per acre and the hydrology will suggest and average of 10 days of contiguous inundation as an annual average.
So if this is accomplished why would it be considered a failure? First, what was lost? The site was a mature system. Bare root seedlings are not going to replace a mature system. Second, what was the site hydrology? The assumption is that all wetlands must be inundated or saturated for at least 5% of the growing season. This is the standard for delineation. However, did anyone actually measure this? Probably not. Third, what functions was this mature system providing? This is almost never addressed. There are some basic percepts that we use, but actual field assessment of the functional loss is not done. Even if it is, how are these wetland functions replaced in the new mitigation site?
Quite frankly, it is not possible to mitigate the functions lost from the mature wetland with an overplanted stormwater management pond. Especially if the functions lost are not even known. The benchmark for success is simply the site holds water and the plants are not dead. Don't ask questions.
Each of the natural systems that we mitigate provides a number of functions. The key is to identify what functions we are trying to replace or restore. This requires that we establish goals based upon an understanding of what was lost. I recently ran across a vernal pool mitigation project in Virginia that was very successful. The benchmark was whether a certain species of concern would naturally colonize the site. This was a specific goal and specific result ensued. We actually saw the species in the site!
Development in and around these unique natural systems are a fact of life. Therefore it is imperative that we strive for success. It is not possible to quantify each and every function. However, it is possible to identify specific goals and develop a means to achieve these goals. Using broad brush generalized concepts as a benchmark will generate ambiguous results with no way to measure uccess. We are left with oversized dry ponds with a bunch of dead sticks stuck in them. The key to success is clear, specific and measurable goals. Sort of like life.