A recent post discussed the role of watershed management in protection vs. restoration of lakes. The reason why watershed management cannot be a mainstay of lake restoration is not obvious to everyone, and here we explore the limits to best management practices in the watershed from the perspective of lake impacts.
Best management practices (BMPs) are procedural or structural techniques used to limit the delivery of contaminants from land to water and eventually the lake. BMPs may restrict what land uses or activities can occur, preventing generation of contaminant loads, or BMPs may focus on trapping contaminants on the way to the lake. “Best” does not necessarily mean adequate or effective, but it is often assumed that if all appropriate practices are applied, the lake will be protected. This is rarely true.
The USEPA has accumulated a huge database on the actual results from various management methods, with a focus on storm water BMPs, as non-point source inputs from developed or agricultural land are by far the biggest input sources these days. The overall average phosphorus reduction that is achieved by individual BMPs is about 50%. This is not the average of all possible projects, but those where the technique was properly applied and monitored; inadequate design, sizing or construction could provide less benefit. A few techniques approach the 90% mark for phosphorus removal, most notably infiltration into appropriate soils and inactivation and filtration, but these are rarely applicable on a watershed-wide basis.
Development and agriculture increase phosphorus loading by an order of magnitude or more in the absence of BMPs, which then reduce those loads by some percentage, averaging 50% on average where properly applied and less in many cases where implementation is incomplete or absent. But if we assume that all developed or agricultural uses are addressed by BMPs that yield a 50% reduction in phosphorus from the tenfold or greater increase expected without BMPs, we have a five-fold increase in loading. Even if we could achieve a 90% reduction, that still represents a doubling of loading from pre-development, pre-agricultural conditions. Human use of land is a losing equation for lakes.
This doesn’t mean that any development or agriculture will doom your lake. Where the human activity occurs and how it is managed matters a lot, and the overall percentage of the watershed in human uses is very important. Most practitioners agree that serious problems can be avoided up to about 25% of the watershed in developed or agricultural uses with judicious BMP application. However, it is simply not reasonable to assume that incoming water quality will be acceptable in urban areas (typically >75% developed) or farm country. The size of the watershed relative to the lake and the depth of the lake will matter too, so simple thresholds will not likely be reliable in all cases. Yet it is clear that where human activities dominate the landscape, application of BMPs will not be able to keep up with the generated loads.