Attempts to quantify impacts of BMPs on water quality often focus directly on the pollutant of concern. Other indirect approaches, however, may be equally or more effective. When developing a monitoring program, be aware of all methods of detecting change, and choose the approach or approaches that work best in a given situation. Different approaches are described below. Refer to Table 2 for a list of common monitored pollutants and a summary of approaches for monitoring and modeling these different pollutants.
Monitoring the pollutant of concern to detect a response to a BMP:
This approach is appropriate when the pollutant or pollutants that are causing the loss of a beneficial use have been clearly identified, a reduction in this pollutant is anticipated as a direct response to a BMP implementation, and there is sufficient time frame to monitor for change. In choosing this approach, make sure to also monitor related parameters that may be critical in interpreting the results. Examples include flow, which is often necessary to help interpret water concentrations, or water temperature and pH, which must be known to determine ammonia toxicity.
Monitoring Surrogates or Response Variables:
In some cases, monitoring the pollutant of concern is expensive or difficult, while monitoring a closely related parameter is relatively straightforward. Surrogates are variables that can be measured more easily and then be correlated with the pollutant of concern. For example, turbidity is easily measured in the field and in many cases can be correlated to suspended sediments in the water column. Response variables are those that change in response to changes in the pollutant of concern and may also be more easily measured than the pollutant of concern. For example, the amount of chlorophyll or plant biomass may be measured in a lake rather than measuring nutrient concentrations directly.
The challenge in these cases is to demonstrate that the variables being measured can be correlated to changes in the pollutant of concern. Note that the relationship between a surrogate or response variable and the pollutant of concern may be different in different watersheds, at different locations within a watershed, or even at different times of the year. The relationship may be sufficiently described and quantified in literature, but in some cases additional monitoring may be necessary to establish the correlation for a particular situation.