By Laura Webb
The Water Monitoring Team at EPA Region 7 has been collecting samples in the urban streams of Kansas City since 2006 as shown in the map below. During our non-field sampling season (when the water is generally frozen or near enough to freezing that we don’t want to wade in), we spend a lot of our time evaluating data and trying to figure out what it all means.
One particular contaminant of interest of mine has been PAHs (not to be confused with PAW-PAWs which we also find in streams from time to time). Polynuclear Aromatic Hydrocarbons (PAHs) are organic molecules found in oil and other fossil fuels which are released when fuels are burned. In my previous job as a bench chemist, I once analyzed samples collected on a filter from a charcoal production facility. The black, sooty residue was comprised of many, many PAH molecules. So, when charcoal is used to grill meat, the smoky flavor and blacked exterior of the meat contains, you guessed it, PAHs. Hopefully not a large concentration, though, because oh how I do love grilled steak! PAHs are found in auto exhaust, tire particles, gas residue, and coal-tar based sealant, such as is used on parking lots and road surfaces. In large enough doses, PAHs are toxic to aquatic species that live in streams. Studies have also shown that higher PAH concentration in urban areas can contribute to human health problems, including asthma, anxiety, and lower IQ scores.
In looking at our data, several things are clear. First, that there is not a clear single source of these compounds to our urban waterways in Kansas City. The ratio of PAH compounds to each other and the total can act as a type of chemical fingerprint of the source. Unfortunately for the urban environment, these prints are smeared and difficult to match up to their source.
Compounds can enter the stream water in many ways. For example, there are permitted discharges, storm water runoff, deposition from the air, and even illegal dumping. Some compounds stay dissolved in the water, some combine to form new compounds, some react with sunlight to decompose into other compounds and some, like PAHs, tend to attach to particulates in the water and “sink” down into the sediment (PAHs and many other organic chemicals don’t really like water; they prefer the rich gooey sediments that deposit as water flows around obstacles in the stream). The sediment is a sink, or trap, for these water-phobic compounds. When we collect samples, we obtain a single grab of water at a particular location at a particular time. While it is a single point in time, the flowing water changes every moment, so samples just a few minutes apart could represent completely different pictures of the stream. Sediment samples, on the other hand, are collected throughout the length of the stream site and really represent a depositional history of what was once in the water as it passed a particular spot. That is what causes the smeared chemical fingerprint, and what causes me headaches when trying to figure out where the PAHs in stream sediment come from.
A prime suspect of urban PAH contamination is coal-tar sealant, and it appears to be a heavy contributor in stream sediments especially those related to large areas of impervious surfaces (roads, parking lots, roof tops, surfaces that water cannot penetrate but runs off of). As a matter of fact, the total concentration of PAHs in sediment is closely correlated with both the percentage of impervious surfaces and the percentage of development in the watershed. The fingerprint ratios of sealant versus many urban sediments, especially those with the highest levels of PAHs, match fairly well, although still keeping in mind that sediment is a collection of many sources.
Along with these sealed surfaces are mobile sources in the form of gasoline and diesel burning vehicles. The pollutants deposit from the exhaust, either in the water itself, on the sediment, or surrounding land and are then washed into the streams during runoff events. Some of the sediment fingerprints have traits from petroleum sources but because the sediment is a sink, there is no clear pattern match for this source (but I know it contributes). Along with vehicle exhaust, there is also power plant exhaust, perhaps smoke from nearby restaurants that grill their meat, fireplaces (although we do collect samples in the summer, the deposits occur all year long), waste water treatment discharge, and other industrial processes.There are plenty of sources, and when concentrated together in the urban core, they combine to increase the PAH concentrations in sediment to levels that may be problematic for aquatic life. So that’s a look at one class of compounds we find in the urban environment. For a more complete look, check out our website at www.kcwaters.org.
Laura Webb is a chemist with EPA Region 7’s Water Monitoring Team. She spent her first 16 years with EPA in the regional laboratory, analyzing samples for everything from metals to dioxins. Her current assignment involves ambient water sampling, laboratory analysis, operating the mobile bacteria laboratory, and participating in emergency responses as part of the Response Support Corp.