As an industry, we’ve acquired a vast amount of knowledge about stormwater, its adverse impacts, and the best management practices (BMPs) implemented to mitigate them.
However, the spirited debate with regard to whether the field or the laboratory is the best arena for evaluating BMP performance refuses to yield to consensus. Here are some of the arguments for and against field testing.
The Pros: Why Field Testing is Preferred
When executed deliberately the insights field testing provide are invaluable. Field testing provides a “real world” evaluation of BMP performance without oversimplifying pollutant loads and concentrations.
Field testing is also the most viable means of evaluating BMP longevity and overall functionality. Qualitative observations provide insight into the effect of peak flows, washout potential, trash and debris loading, anaerobic conditions, weir heights, etc. on overall performance.
Comparing field monitoring studies is notably challenging but when benchmarking the performance of newer innovative BMPs against traditional land based BMPs like wet ponds and sand filters it is important to keep in mind that most of the performance data for traditional BMPs was collected in the field.
The Cons: Field Monitoring Pit Falls
Anyone who has been involved in field monitoring would likely agree that it’s expensive, time consuming and at times simply frustrating. One of the biggest hurdles for entities without funding is cost. It starts with the monitoring equipment which alone is a sizeable investment. Field monitoring protocols such as the Technology Acceptance Reciprocity Partnership (TARP) Tier II Protocol for Stormwater Best Management Practice Demonstrations and the Technology Acceptance Protocol- Ecology (TAPE) call for a flow paced sampling strategy that requires deploying automated sampling equipment costing thousands of dollars (TARP 2003, TAPE 2008). Sample analysis is another primary contributor to field monitoring costs. Depending on how many different pollutants are being studied and the number of samples submitted for analysis analytical costs can range from hundreds to thousands of dollars per storm.
Since field monitoring is often conducted remotely and/or the entity commissioning the study desires third party involvement, consultants are often relied upon to manage various aspects of the project. Hiring consultants to manage site operations and/or project reporting is another major contributor to project cost. The financial impact is often compounded by the fact that storms rarely seem to occur between 8AM and 5PM Monday through Friday. Dispatching consultants at 3AM on a Sunday morning is not recommended for those on a budget. Often underestimated contributors to field monitoring costs are false starts and failed attempts. This includes dispatching the field crew to find that not enough samples were collected for analysis, the power to the samplers failed, the sample lines have frozen, trash is smothering the flow probe or any number of things that ultimately cost money and do not yield data.
Questions relative to the accuracy of current field monitoring practices, especially those pertaining to the assessment of solids removal, are at the heart of the debate over the value of field monitoring. Most practitioners agree that automated samplers are prone to under sampling coarse solids, but the jury is still out on the extent of the bias on results. Studies suggest that automatic samplers can not representatively sample particles larger than 75-200 micron, but other studies demonstrate that particles larger than 2000 microns are effectively sampled. Questions have also been raised regarding the ability of automatic samplers to obtain representative samples from large diameter drainage pipes with a potentially stratified pollutant load. Sample strainer location becomes critically important to avoid under sampling the solids load, but additional research on this subject is warranted.
Other potential sources of bias include the flow measurements and sample analysis methodology. It is well documented that relying solely on Total Suspended Solids (TSS) analysis often fails to accurately characterize the solids load in stormwater runoff. Different methods of measuring flow including flumes, weirs and area velocity meters are prone to varying degrees error. Flow measurements are particularly troublesome during periods of low flow and when flow fluctuates rapidly. All told, careful planning is required to minimize the introduction of error into field monitoring results.
Another criticism of field monitoring is the fact that it is a long process. Established field monitoring protocols generally mandate sampling 12 or more qualified storm events. What defines qualified varies by protocol, but successfully sampling 12 or more qualified events usually requires 12-24 months in the field. Factor in project planning, data interpretation and reporting and projects routinely span more than two years. For manufacturers introducing a new BMP or academics investigating a particular theory, field monitoring does not offer a short path for doing so.
Part of the challenge is the unpredictable nature of the weather. Prior to each storm event field personnel must carefully track forecasts and make judgment calls regarding sampler programming. If a storm deviates significantly from the forecast the sampler programming may not be adequate, leading to disqualification of the event. Equipment failures, winter conditions and unique monitoring criteria such as capturing infrequent storm events all add to project timelines as well.
In order to make equitable side by side BMP performance comparisons each BMP must be subjected to the same test conditions. Unfortunately, finding two or more field studies where variables such as pollutant concentration, particle size distribution (PSD), inflow rate, etc are the same is essentially impossible. Each field site, as well as each storm, is likely to generate unique pollutant loads and runoff rates. There are so many variables inherent to stormwater runoff that even field data from sites similar in size and in close proximity is rarely ideal for a side by side BMP comparison.
A number of other factors that can hinder the outcome of field monitoring studies are often overlooked until they occur. Power failures routinely disable sampling equipment costing stakeholders both time and money. Local fauna, rodents in particular, have a penchant for munching wires. Food wrappers and other debris obstruct sample strainers and flow probes with regularity. Vandalism can also be a major problem so securing sampling equipment is essential.