Monitoring guidelines: focusing on what's important

Scott de Ridder
Research Manager
Stormwater Management Inc.
Portland, Oregon

Members of the public works community are often responsible for ensuring that stormwater runoff is treated to meet water quality standards before it is discharged to a nearby water body. Monitoring data can be a useful tool for comparing and selecting stormwater treatment technologies based on their ability to meet these standards; yet, individual approaches to monitoring and reporting data vary as much as the technologies themselves.

This article presents some of the challenges of collecting stormwater monitoring data and offers steps to ensure that collected data is comparable and useful when making a decision about a stormwater treatment system.

Develop clear, focused, and realistic monitoring guidelines from the start
The first step toward gathering reliable monitoring data is to develop guidelines. Time spent before monitoring begins, to identify specific goals and processes to reach those goals, will reduce costs and time to treatment. Work should focus on defining the most important water quality goals and developing specific guidelines, such as how storms should be collected and how many storms are required.

While guidelines are invaluable tools, be careful that they are not all-encompassing or open-ended. The more specific the goals, the more efficiently monitoring time will be spent. Temptations to gather as much information about a technology as possible arise because non-point sources are highly variable and there is still much to learn about the effects of non-point source pollution on water bodies. However, this approach can quickly become expensive and time-consuming.

A pole safely elevates a rain gauge and supports a solar panel that keeps sampler batteries charged at a Stormwater Management StormFilter unit that treats runoff from a bridge deck and roadway near Lake Stevens, Washington. The automated samplers are connected to a telephone line that allows the Stormwater Management monitoring team to remotely interface with samplers and download hydraulic data once a storm has been captured. This also helps the team verify that samples were taken before traveling to the site.

Consider that the minimum cost to monitor one system for a single event starts at $3,100. This includes rates for planning, equipment, basic analysis, reporting, and personnel. The cost increases when analysis of additional and/or more specialized pollutants, such as BTEX, is required. And since multiple storm events must be captured in order to reach a level of confidence about a system's performance, the total monitoring cost can run approximately $30,000, assuming there is a dedicated monitoring team and everything goes as planned. Developing a tightly focused plan in advance will help to minimize such costs.

During guideline development, common obstacles to monitoring should also be evaluated and considered. These include: data from some captured storms is not usable—equipment malfunction may cause inconsistencies in the samples; not enough samples may be taken, leading to data that is not a true representation of the entire storm event; or samples from multiple storms may not have been collected consistently, all resulting in non-comparable data.

These obstacles tend to lengthen the monitoring timeline, often requiring that two to three times the amount of storms required in the guidelines be captured in order to collect the minimum needed "usable" data. It could take from eight to 30 months to capture this many events.

Determine what's important to achieving water quality
The next step is to determine what is needed to achieve the water quality desired. Setting standards that are applicable to local conditions is a good place to start. By understanding the pollutants in the area and how they affect local ecosystems, the pollutants with the biggest impact on water quality can be targeted.

For example, when assessing TSS removal, understanding an area's sediment characteristics can help identify which technology will be the most effective at removing TSS in that location. The Washington Department of Ecology (WDOE) has taken this approach, identifying Sil-Co-Sil 106, a commercially-available silica product that represents a silt, as having fundamental particle size distribution characteristics similar to those found in stormwater runoff in the State. By defining the nature of TSS in Washington using this representative sediment texture, WDOE is able to better evaluate TSS removal performance of technologies before approving them for use.

Understanding the sediment texture also allows WDOE to define any additional sample processing methods that are needed to assess removal of TSS, such as requiring pre-screening with a 500 um sieve before analysis. Depending on the sampling methodology used, particle size distributions can range from clay and silt (less than 50 um per USDA definitions) to trash and debris. Thus, use of special pre-analysis processes, such as pre-screening, allow WDOE to evaluate a system's ability to remove those particles it feels are most responsible for the impairment of surface water quality in Washington.

Determine how to collect monitoring data
Another important component to monitoring is the data, in particular how to collect the data. This is critical because any variation in how data is collected, no matter how minor, can affect results, and therefore must be considered.

In order to evaluate a system based on its performance over multiple storm events, it is critical that monitoring samples are consistently collected and analyzed. Detail standardized monitoring procedures before monitoring, including the type of equipment to be used; the types of samples to be taken (composite or discrete); the methods for taking samples (auto sampler, manual grab; flow-based, volume-based); and information on how the samples will be transported and analyzed. This will lead to more consistency in the way sites are monitored and less chance of technical problems occurring in the field.

Once the sampling method is chosen, it is important to consider how monitoring equipment will be installed at the site. For vault-based systems, not only is placement of the samplers and batteries important, but confined space entry is also a factor, requiring two people onsite, special confined space training, and presence of onsite fall-arrest and rescue-retrieval equipment.

For vault-based systems, auto-samplers can be used to collect samples over the course of a storm event. Stormwater Management Inc. (SMI) has found that taking samples at flow-triggered, volume-paced intervals provides a more complete picture of how its systems perform over the entire course of a storm event. For some applications, such as catch basins, auto-samplers may not be practical. In these cases, manual grab samples are a possible alternative, although, through the development of a special insert, SMI has successfully used auto-sampling equipment to collect samples from catch basins. Note that if manual grab samples are taken, this requires the monitoring team to be at the site before, during, and after the storm event.

SMI also chooses to take discrete samples rather than composite samples because each sample can be analyzed separately and made into composites later if needed. This is particularly helpful if there were auto-sampler errors, such as a sampler skipping an interval or taking effluent samples of standing water in a system before the first influent sample of the storm. The ability to evaluate discrete samples means there is less likelihood of having to question and throw out storm event data.

Regardless of the method selected, in order to compare system data, the samplers or the monitoring team must take samples in the same way and the sampling equipment must be similar from site-to-site and test-to-test. This need for consistency applies to both the major and minor details, from the placement of sampling probes in the inlet and outlet pipes to the techniques used to monitor systems with detention, pretreatment, or bypassing.

In the case of systems that have detention or pretreatment, samples should be taken before water enters the detention/pretreatment system, then when it enters the treatment system, and finally when it exits the treatment system. When a high-flow bypass is used, the possibility of bypassed and treated water mixing should be taken into consideration, as should the possibility that high flows may exceed the ability of the sampler to take samples.

Decide whether to do it yourself or enlist help
Rather than establish a monitoring program as described, hiring a third party to do site monitoring is sometimes an option. Before choosing this method, consider the benefits and potential problems. While outsourcing monitoring may free up employee time, there are sometimes training issues. For example, if the third party is not experienced in stormwater sampling, some level of training on how a given system works and on how samples should be taken may be required—a lot of up-front work. And with both experienced and inexperienced teams, there is always the possibility of human error, where samples may not be collected properly or analyzed in the proscribed way.

A set of standardized monitoring procedures created in advance will help in this case too. It will ensure that monitoring is done in such a way that the resulting data meets jurisdictional requirements and is comparable to past and future data collected internally and by other third parties.

Regardless of the method, when evaluating a proprietary technology, include the manufacturer in the process. A manufacturer knows its system better than anyone else and has already invested its own resources into evaluating it. Manufacturers are a great information resource and their input on guidelines will reduce confusion over how samples from their system should be taken.

Hold the bar
Taking time in the beginning to clearly define monitoring guidelines and procedures will make the monitoring and evaluation process more efficient and will provide more complete and comparable data. However, to see the value of all the planning work, these guidelines and procedures need to be followed. Regardless of the technology being evaluated, consistency in process enforcement guarantees that the information collected is usable and comparable. It also ensures an increased understanding of how to treat non-point sources, and makes water quality the top priority.

Several jurisdictions have gone through a thorough process of developing monitoring guidelines. The following web references provide several good examples:

Scott de Ridder can be reached at (503) 240-3393 or at