by Don Waye
This simple test operates under the same principle that makes partygoers’ clothing glow brightly under a black light. Special optical brightener dyes that are added to the vast majority of laundry detergents sold in North America (look for the words “brightening agent” or “whitener” in the fine print on your detergent container) are adsorbed onto natural fibres like cotton, causing them to appear whiter undernatural sunlight and to fluoresce under ultraviolet light.
Why OB monitoring?
In most buildings, laundry effluent goes out through the same drains that carry sewage waste. Thus, testing streams or storm drains for optical brighteners is a good way to screen for the presence of wastewater from human sources. The ploy is simple: place OB “traps” containing fabric untainted with optical brighteners in watershed inputs. If the fabric subsequently fluoresces under UV light, it’s a good indication that sewage may be present.
Currently, no method for OB testing is officially approved. Nevertheless, OB monitoring is an effective and inexpensive tool for anyone who would like to determine the extent and locations of illicit discharges.
OB monitoring offers several advantages over alternative methods for detecting sewage contamination, which include bacteria testing, testing for surfactants (another laundry detergent ingredient), and testing for caffeine.
Unlike bacteria testing, OB monitoring is meant to pinpoint human waste. Fecal bacteria from waterfowl produce the same reactions on culture plates as fecal bacteria from human sources, but ducks don’t do laundry.
Most illicit connections are not running 24/7. Because the OB traps are typically left in the field for days at a time, they collect an integrated sample, allowing the detection of intermittent contamination.
And because OB monitoring is the cheapest and simplest of the available methods, it lends itself to wholesale surveys of storm drain outfalls, feeder streams, or rivulets within a given study area.
As useful as OB monitoring is for screening watersheds for sewage inputs, it has its limitations. For instance, it is not likely to detect sewage from most commercial buildings — those that have toilets but no laundry facilities.
OB monitoring is ideally suited for composite sampling during base flow conditions. Wet weather sampling is not usually recommended because the traps can be lost as the storm water pulse rips through the sampling area, and also the sediment load, especially from clay, tends to coat the cotton swatches and mask any OB dye that has been adsorbed onto the fibres.
Although OB monitoring is considered a qualitative measure, some information on the severity of contamination can be gleaned by making note of the relative intensity of each sample’s reaction with UV light. Control samples can be made by diluting laundry detergent in water to various known concentrations. Verified positive results can be obtained by sending samples to a laboratory that can analyze precise intensities of OB dyes by scanning them under a spectrofluorophotometer (the instrument should be set to detect fluorescence in the range of 415.0 to 422.0 nanometres). Some universities have this equipment, usually in their physics departments.
Case study and lessons learned
I used OB monitoring while working for the Northern Virginia Regional Commission (NVRC) in the summers of 1999, 2000, and 2001. For the first summer I hired two college interns to monitor the upper half of the 20-square-mile ulta-urban Four Mile Run watershed at a total cost of $6,800, most of which was the $10/ hour that my agency paid each intern. It took the interns six weeks to survey about 150 sites. The following summer I hired two interns for 120 weeks to monitor every known storm drain outfall in the watershed (299 outfalls in all).
During the second summer, each OB trap was left out in the field between 4 and 72 hours instead of the 7-10 day sampling period used the first summer to minimize interference from wet weather.
We also switched from using hot incandescent UV bulbs (party bulbs) the first summer to a black light fluorescent lamp with its own built-in base housing and cord. The self-contained fluorescent lamp, which we purchased for $24 from Home Depot, yielded more easily interpretable results and was not hot to the touch.
The surveys revealed potential problems with several storm drain outfalls. (For additional information on this project see www.nova region.org/4MileRun/obmslides).
Because we were sampling in a highly urbanized watershed, we placed traps at or near every outfall, whether or not the outfall was flowing. This allowed us to sort out which outfalls experienced intermittent flows, and several sporadic illicit discharges were detected in otherwise dry outfalls.
In the field, we used a Sharpie marker to tag each OB trap with a unique ID and key it to a specific location in the field (e.g., an outfall or rivulet). After recording the ID, location, and pertinent site characteristics (e.g. outfall size) in a field book, we left the trap as long as base flow conditions prevailed. We kept close tabs on the weather and retrieved the traps when rains threatened.
Traps collected from the field were placed in large sturdy plastic baggies and brought back to our agency for black light analysis. All samples flagged as “potential positive” from the quick-and-dirty black light testing were sent to the lab for spectrofluorophotometric analysis, and in every case the presence of OBs was confirmed. For quality control, 5 percent of negative samples were also sent to the lab for confirmation.
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Don Waye is an Environmental Protection Specialist with the US EPA’s Nonpoint Source Control Branch. He may be reached at Waye.Don@epa.gov; 202-566-1170 Reprinted from The Volunteer Monitor, Summer 2003. An earlier article on monitoring optical brighteners, using slightly different methodology, appeared in the Fall 1999 issue of The Volunteer Monitor (pp. 21-23).]
Low-Cost Homemade OB Trap
The key component is the material that will adsorb any OB dye that may be released into a waterway. The trick is not so much in finding the right material (cotton is ideally suited), but in finding a supply that has not already been processed with OB dye. We used 6-inch-square unbleached fabric swatches used by the garment industry to test new dyes, ($1 apiece in bulk order).
To make the traps, we cut 10-foot lengths of 2- inch diameter PVC pipe into 6-inch sections, and drilled two or four holes (for holding wire) in each section. The fabric swatches were stretched over the end of the tubing and held snugly in place with thick, strong rubber bands.
In the field, we placed the traps with the fabric end facing upstream against the direction of flow, and secured them in place by running wire through the holes and wrapping it around nearby tree roots or anything handy at the site. We used 17 gauge spooled aluminium wire, which proved malleable enough to be twisted easily by hand or with needle-nosed pliers.
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