Tips to improve wet-weather monitoring

Jason Gillespie, Director, Greenville County Soil and Water Conservation District, Greenville, SC
James H. Riddle III, P.E., Project Engineer, Woolpert LLP, Columbia, SC; Presenter, 2004 APWA Congress
Brian Behrens, Environmental Scientist, Woolpert LLP, Greenville, SC

Whether your community has been subject to NPDES monitoring requirements or has been asked to quantify its contribution to a downstream water-quality impairment or proposed TMDL, conducting wet-weather monitoring and obtaining reliable analytical data is not an easy task. Greenville County, South Carolina, is conducting land-use-specific monitoring due to NPDES Phase I requirements, in addition to separate, additional performance monitoring of a local detention pond. Through these monitoring efforts, the community has found that by putting more effort into the "front end" of the monitoring process or planning stages, it could yield far more accurate data, and with fewer problems along the way.

These "front-end" measures have included:

  • Developing a self-imposed Quality Assurance Project Plan—with some aspects of the sampling program that go beyond EPA requirements.
  • Developing and using detailed siting procedures based on a pilot monitoring station project and continual refinement of those procedures to place monitoring equipment in the best possible locations.
  • Selecting the right monitoring equipment and experimenting with different technologies to reduce potential problems and costs.

Here are the details of these three "front-end" measures.

Developing a Quality Assurance Project Plan
Greenville County, a community of about 800 square miles in size and located in the foothills of the Appalachian Mountains, developed a formal Quality Assurance Project Plan before beginning any monitoring work.

"Although such a plan was not an EPA or NPDES requirement, we believe that holding ourselves to a standard streamlines the entire monitoring process," said Jason Gillespie, Director, Greenville County Soil and Water Conservation District.

In coordination with Rogers and Callcott Engineers (the laboratory contracted to produce the analytical data), the county submitted the plan to the South Carolina Department of Health and Environmental Control (SCDHEC), which is the state's NPDES permitting authority.

The plan outlines some aspects of sampling that are not specified by EPA. "We wanted to make sure SCDHEC approved of our methodologies upfront," Gillespie said.

The comprehensive plan includes:

  • A description of sampling requirements and strategies.
  • Worksheets showing the technique used for flow-weighting sample data.
  • Sample handling and chain-of-custody procedures.
  • Preferred methods for sample preservation, preparation, and testing.
  • Operating procedures and proposed training programs for the many potential on-call field personnel.

Among the operating procedures is a detailed checklist that on-call personnel must use to ensure they have all equipment and gear ready to go on a moment's notice.

For additional quality assurance, the county retained an outside consulting firm to perform third-party review of the analytical data. The firm reviews sample results to ensure compliance with the plan as well as provides annual reports outlining any discrepancies noted during the ongoing review and any corrective actions that may have been taken.

A typical Greenville County monitoring station, including a rain gage (not visible in the picture), automatic sampler, flow meter, solar panel, landline interface, and deep-cycle marine battery enclosed in tool box to prevent theft

Selecting an Appropriate Location for Monitoring Equipment
The county began its land-use monitoring program by investigating many potential sites for the location of a pilot monitoring station. Field technicians selected a bridge crossing that provided a uniform cross-sectional area to more easily develop a depth-versus-flow relationship. The contributing watershed, about 1.25 square miles in size, was exclusively single-family residential.

During the pilot year, technicians learned a lot about what worked and didn't work in terms of siting the monitoring station and the equipment itself. "We learned how to operate the equipment, to program it, and to learn its and our own limitations," said Brian Behrens, a Woolpert environmental scientist who monitors the equipment.

Based on this experience and additional experience since the pilot station, the county and its consultant have developed a checklist to evaluate potential future sites. The checklist includes the following primary considerations:

  • Homogenous land use - Finding a watershed that drains from a singular land use (such as an entirely residential or an entirely commercial area) is an important NPDES Phase I permit requirement to meet, even though it can be difficult finding such areas.
  • Drainage area size - A relatively large sub-watershed reduces the chances of constituent variability from small, isolated watersheds; in other words, it's much more likely to produce concentrations similar to runoff from like land use in other parts of the county.
  • Upstream ponds - Watersheds with large ponds upstream should be avoided because some pollutants (such as TSS) tend to collect and settle in ponds as opposed to traveling downstream, which can skew the data for average concentrations of pollutants from runoff.
  • Proximity to office/laboratory - The shorter the distance that field technicians need to travel to get to the sampling site the better, as storms can pop up fast, and sampling of certain pollutants requires a field technician to manually take the sample—often at the beginning of a storm event—as opposed to automated equipment taking the sample.

The checklist includes the following secondary considerations:

  • Uniform openings - Culverts and other structures with uniform openings allow technicians to more easily develop a depth-versus-flow relationship.
  • Headwalls/wingwalls - These structures provide convenient places to mount intake and bubbler tubing.
  • Sedimentation - The presence of significant sediment, which can bury test tubing and area-velocity probes, should be avoided.
  • Laminar (non-turbulent) flow - A smooth, steady flow of water is preferred over areas with a lot of rock or rip rap that create ripple pools and eddies, which can cause intake tubes to pull air.
  • Tree cover - Areas with a lot of trees should be avoided because they can block cell signals used for transferring data from the site to the lab; trees can make it difficult for adequate sunlight to reach solar panels used in charging batteries that keep the equipment operating; and trees can obstruct rain from entering rain gages, thus giving false rain readings.
  • Property owner - If the site is in the public right-of-way, it's important to seek an encroachment permit; if the site is on private property, it's important to seek the permission of the property owner.
  • Equipment accessibility - The site should be accessible and equipment securely installed above potential flood levels.
  • Safety of field personnel - the site should be a location that personnel can safely reach. Personnel should not have to cross highways or traverse down steep embankments—which could be difficult and dangerous to negotiate during wet conditions, particularly when carrying many heavy sample bottles. A road shoulder to pull over on is needed at a minimum, and care should be taken to create footpaths to get to equipment.
  • Potential for vandalism - The site should be in a location where there is likely to be a low occurrence of vandalism.
  • Availability of landline or cellular phone service - The availability of at least one of these services is an important consideration for data transfer.

A field crew member installing sampler intake tubing onto a 60-inch corrugated metal pipe and headwall draining a local residential area

Additional issues to consider include sprinklers—in one testing area, sprinklers were skewing the rain gage data; and, believe it or not, ants—which can get inside the equipment and short-out the circuitry.

Another issue to consider is the importance of having an extra set of equipment should one station need repair. A complete automated monitoring station such as those used for this project typically costs approximately $10,000.

Using Reliable Equipment
Like other stormwater monitoring setups, Greenville County began its monitoring program by installing automated stations that used a stormwater sampler, one of two types of flow meters, a tipping bucket rain gage, and a cellular communications unit. Remote access to real-time data would allow personnel to troubleshoot and download data as needed, saving valuable man-hours required for performing these duties onsite.

"Although some problems were experienced with both the flow meters and samplers, the primary source of equipment issues revolved around communications," Behrens said. "The cellular modems were not as reliable as originally expected, primarily because of typical cellular phone issues. Plus, because of power requirements for the cellular phone, remote access was restricted to only one hour per day to avoid draining the manufacturer-recommended deep-cycle marine battery onsite."

As an alternative, the county obtained a typical landline phone service connection and purchased solar panels as an energy source. The landline proved feasible at remote locations, and new equipment needs for a landline connection were much less expensive than the previously purchased cellular units. Although the solar panels were somewhat costly, eliminating the need to visit sites and continually replace batteries helped the panels to quickly pay for themselves.

Since these modifications, communications and power supplies for each station are much more reliable. Equipment can now be accessed remotely at any time during the day without the previously frequent connection problems and without any risk of draining the power supply. Stations can be accessed just before an approaching storm to ensure the units are functioning properly or for final programming purposes. Stations can also be accessed during and immediately after a storm event to monitor overall rainfall and quickly determine whether rainfall depths were sufficient to perform a full laboratory analysis.

"This time frame is critical due to volatility and subsequent short holding times for many of the tested constituents," Behrens said.

Additional Technical Measures
Space limitations prevent detailed discussion of the more technical aspects of the monitoring program, but a more technical discussion will be presented by representatives of Woolpert and the county at the APWA International Public Works Congress and Exposition in Atlanta, Georgia, on September 12, 2004. However, to summarize, the county concluded that:

  • In-stream sampling of larger watersheds with homogeneous land use provides better averages of runoff pollutant loadings attributed to that type of land use than outfall-specific monitoring for much smaller drainage areas.
  • Analysis of the hydrologic response time of each watershed ensures that sampling sequences begin at appropriate times to pump samples for the duration of the storm event, including during the first flush.
  • Detailed hydraulic analysis used for estimating flow rates provides an accurate data set for flow-weighting composite samples and, in combination with rainfall data, for floodplain model calibration.
  • Development of composite samples in the laboratory is the most viable method for obtaining proper sample volumes, producing the correct flow-weighted composite, and increasing overall quality control.

The monitoring team has currently collected data from 12 different locations in the county over the past three years with plans to continue both the land-use specific monitoring for improvement of the Event Mean Concentrations (EMCs), as well as additional BMP performance monitoring. Greenville County is confident that monitoring data from each station is extremely reliable, representative of conditions throughout a given storm event, and appropriate for use in approximating pollutant loadings to local receiving waters.

"As a result of these 'front-end' planning measures, we now possess a dependable data set for watershed planning and mitigation purposes," said Gillespie. "The highly accurate, localized data set allows Greenville County to gain a much clearer picture of areas that are experiencing or are subject to potential water-quality degradation."

Jason Gillespie can be reached at (864) 467-2756 or at; James H. Riddle III, P.E., can be reached at (803) 731-0261 or at; and Brian Behrens can be reached at (864) 421-9999 or at

James H. Riddle III, P.E., Project Engineer, and Trevor Gauron, Engineering Associate, Woolpert LLP, Columbia, SC, will give a related presentation on September 12 at the APWA Congress in Atlanta, Georgia. The session is entitled "Innovative Tools for NPDES Stormwater Phase I and II Compliance" and begins at 4:00 p.m.

  Typical station storm data showing distribution of samples