A.J. Erickson, P.T. Weiss, J.S. Gulliver, R.M. Hozalski, B.C. Asleson
The assessment of stormwater treatment practices has historically been accomplished with monitoring (e.g., Anderson et al. 1985; Bell et al. Undated; Kovacic et al. 2000; Lin and Terry 2003; Silvan et al. 2004; Winer 2000). Monitoring is the most comprehensive form of assessment and can estimate the multi-objective performance of a stormwater treatment practice within a given watershed. Monitoring programs, however, also require effort for a relatively long period (often 14 or more continuous months) to sample a needed range of storm sizes (U.S. EPA 2002). Unfortunately, the results of monitoring studies are often uncertain because of poor characterization of discharge into and out of the stormwater treatment practice, inaccurate pollutant sampling, or both. There are alternative techniques to monitoring which should be considered before an assessment program is established which may save considerable effort and cost.
Assessment programs and goals
An assessment program is a plan of action for evaluating the functionality and performance of stormwater treatment practices. Before developing an assessment program, it is important to have well-defined goals so that the effort required to develop and implement the program is focused to achieve the desired results. Rather than proposing a series of rigid procedures, several assessment options are provided and can be used in various combinations depending on information needs, budgetary constraints, time frames, and legal requirements. These options are combined to identify specific assessment goals, including uncertainty estimates and information expectations that guide short- and long-term assessment efforts. While most techniques may require technicians with various levels of professional expertise, volunteers may be trained to do some of the visual assessments.
Examples of assessment goals
Some examples of assessment goals include, but are not limited to:
- System-wide visual examination of stormwater treatment practices to determine if they are malfunctioning;
- Annual evaluation of wet pond sediment, bacteria, and phosphorus loading reductions;
- Determination of seasonal performance and function of infiltration basins;
- Identification of maintenance needs and scheduling;
- Determination of sediment particle size removal rates by underground proprietary devices as affected by maintenance;
- Determination of rain garden effects in reducing runoff volumes, phosphorus, and sediment loading rates; and
- Optimization of rain garden life expectancy by scheduling appropriate and timely maintenance procedures.
The details of individual assessment programs will vary depending on the goals of assessment, the stormwater treatment practice(s) to be assessed, and other variables, but the process of developing an assessment program is universal. An assessment program can be outlined by the following five-step process:
- Why assess? What is required by permit, voluntary watershed management goals, TMDL allocation, or protection effort?
- Assessment reconnaissance:
- Identify stormwater treatment practice locations, types, drainage areas, design criteria, and life cycle stage (i.e., age);
- Identify downstream receiving waters by location, proximity to assessment efforts, and designated uses (e.g., cold-water fisheries, etc.);
- Identify how the stormwater treatment practices have been maintained;
- Identify seasonal treatment needs (e.g., winter for chlorides, summer for bacteria); and
- List schedule, budget, and personnel limitations.
- Determine the level of assessment needed to address key needs.
- Revise steps 1, 2, and 3, if necessary, based on the levels of assessment, the budget of the assessment program, and the assessment considerations for the stormwater treatment practice process (e.g., Filtration, Infiltration, Sedimentation, and Biologically Enhanced).
- Fill in the details of the assessment program with dates for visual inspection (level 1 assessment), testing (level 2 and 3 assessment), or monitoring equipment installation (level 4 assessment), where appropriate.
Implementation of an assessment program may begin with assigning tasks or hiring personnel; purchasing, constructing, or acquiring equipment; installing or transporting equipment; acquiring permits or permission; developing and following safety guidelines; developing or revising maintenance programs; or coordinating with municipal, county, or state entities.
Levels of assessment
The answers to the questions listed above will be specific to each assessment goal. Determining how assessment will occur requires an understanding of the four levels of assessment:
- Visual Inspection: A rapid assessment procedure that qualitatively evaluates and documents the functionality of a stormwater treatment practice. The primary purpose of visual inspection is to identify, diagnose, and schedule maintenance for stormwater treatment practices. The results can be used to select and schedule maintenance.
- Capacity Testing: An assessment method that determines the capacity of a stormwater treatment practice through a series of spatially distributed, relatively rapid, and simple point measurements. Specifically, capacity testing can be used to determine the hydraulic conductivity and total infiltration flow rate in volume per time or the sediment removal capacity (remaining sediment storage volume) of a stormwater treatment practice. The results can be used to select and schedule maintenance.
- Synthetic Runoff Testing: An assessment method in which a prescribed amount of synthetic stormwater is applied to a stormwater treatment practice under controlled conditions to assess the effectiveness. Synthetic runoff testing can be used to assess the performance of a stormwater treatment practice for runoff volume reduction (e.g., through infiltration) and pollutant removal efficiency with measurements such as drain time and mass of pollutant capture. Results from synthetic runoff testing can also be used to calibrate watershed models for simulation of performance during natural rainfall events.
- Monitoring: An assessment method which measures performance of a practice during natural rainfall or snowmelt events by measuring influent and effluent flow rates, collecting influent and effluent stormwater samples for analysis, and comparing influent and effluent volume, pollutant concentration, or pollutant load. Monitoring is the most comprehensive form of assessment and can assess multiple aspects of stormwater treatment practice performance (e.g., peak flow reduction and pollutant removal). It also requires a significant amount of data to calculate reliable results because the number and range of variables is large. The results from monitoring can be used to describe the runoff and pollutant load characteristics of a watershed and the associated response of stormwater treatment practice.
Developers of an assessment program should consider each of the four levels of assessment based on effort and uncertainty considerations, and consider the next level only when warranted by the goals of the assessment program. By this process, assessment may include any combination of the four assessment levels but inclusion of all four levels is not mandatory. Each level of assessment will vary in application based on the stormwater treatment practice and the assessment goals, thus detailed descriptions for applying each level of assessment to specific stormwater treatment practices are given in the Filtration, Infiltration, Sedimentation, and Biologically Enhanced Practices sections. A summary of the four levels of assessment including the relative effort, typical elapsed time, advantages, and disadvantages is given in Table 3.1.
Continue to Visual Inspection (level 1).