Future Directions


This study was intended to serve as a stepping-stone for future work. Therefore, it is important to consider how the data from this reconnaissance can help inform decisions about sampling design and identify information gaps. This type of information can be combined to more effectively address potential reduction efforts depending on the targeted contaminant class. 


Sampling Design


The samples collected in this study were point or grab samples designed for an initial characterization of the pathways sampled. Through these efforts, it was determined that WWTP effluent contains a wide variety of contaminants from many different compound classes. No clear pattern of detections emerged among the WWTPs based on location, population, treatment type, or size of the plant. This type of ancillary information could not be used to anticipate the type or frequency of detections. Given the various factors influencing the composition of the effluent, it would be difficult to design a study to explain the expected results for WWTP effluent. This pathway can be considered simply as an integrator of human activity and used to evaluate the effects this activity has on the ecosystem. 


At the Portland WWTP, three samples were collected throughout 1 day to examine temporal variability. Given the inherent variability in these types of samples, no obvious differences throughout the day were noted for most compounds. The exception was some of the halogenated compounds (PBDEs, herbicides and insecticides, and triclosan) that exhibited patterns of lower concentrations in the morning with some noon and afternoon concentrations roughly two to four times higher (table 13). This finding illustrates that a single grab sample may not be adequate to characterize the effluent. Time-composited (24-hour) samples may provide a better characterization of the contributions throughout the day. 


Another sampling technique that provides a time-integrated sample is the use of passive samplers (Alvarez, 2010). Passive samplers left in the water for about 30 days integrate the sample by accumulating contaminants, similar to fish or other aquatic biota in the area. In this way, these samples provide a biologically relevant measure of the complex mixtures present. Additionally, because these samplers integrate over time, the contaminants are essentially concentrated into the sampling media, which results in increased sensitivity and lower detection limits than a traditional water sample. This would be important for collecting samples not only from the effluent stream but also from the receiving waters, where dilution makes it difficult to detect these low concentrations using standard methods (table 25). An ideal design for future characterization of not only what is being delivered by the WWTP effluent or stormwater runoff, but also the fate and transport of these contaminants, would combine passive samplers in the waste stream and the receiving water, both in the mixing zone and at some distance downstream and outside the mixing zone. 


Seasonality is a key factor in characterizing stormwater runoff. The length of dry time preceding a runoff event, the magnitude and length of the event, and the time of year (related to pesticide usage, for example) all may have an effect (Sansalone and others, 1998; Flint, 2004). Much less is known about seasonality effects on WWTP effluent. Much will depend on the type of inputs the WWTP receives. For instance, a WWTP receiving wastewater from a cannery or fruit-processing facility will see changes in their influent as the contributing facilities change their operations. How much of an effect this has on the makeup of the effluent could be a study objective. Likewise, seasonal changes in prescription and over-the-counter medicine use also may be noticeable in WWTP effluents. 


The chemical characteristics of the targeted compounds often determine how to focus the sampling media. For hydrophobic compounds, sampling should be targeted at retrieving as much solid material as possible, either through sampling unfiltered water or the solids themselves. The dissolved phase should not be ignored, however, because most compounds are not confined to just one media. The samples measured for estrogenicity in this study indicated that the hormonally active compounds were likely dissolved in the effluent water rather than associated with the solids in the effluent (table 11). 


Information Gaps


The affinity that some contaminants have for the solid phase rather than the aqueous phase raises questions about how many contaminants are sequestered in biosolids during the treatment process and what is their ultimate fate. For most WWTPs, biosolids are transported to a nearby area and spread over the land, many times applied as a fertilizer or nutrient treatment for the land. In 2008, USGS researchers determined that earthworms collected from fields where biosolids and manure had been applied not only picked up drugs and perfumes from the soil but also had bioconcentrated these compounds in their tissues (Kinney and others, 2008). Once these contaminants are spread on the land with the biosolids, little is known about the further transport, degradation, or ultimate effect of these contaminants on the ecosystem. 


This study focused on two known pathways for contaminant transport, but further study could expand this focus to incorporate other potential pathways and sources. These could include but are not limited to other dischargers with an NPDES permit; industries located along a contributing stream; hospitals, nursing homes, hospice and in-home facilities; or manufacturers of emerging contaminants. Further characterization of the land-use types and contributions to stormwater-runoff areas also would help in designing reduction efforts based on these results. The spatial distribution of this study could be expanded to include a further extent of the Columbia River Basin, including pathways to tributaries.


The aquatic communities living in these receiving waters are exposed to complex mixtures of these compounds with unknown toxicity. Further research into sublethal effects, toxicity, bioaccumulation, and mixture effects is needed to further the science of emerging contaminants. 


First posted April 25, 2012