Risk-based Prioritization of a New Class of Aquatic Pollutants – Pharmaceuticals and Personal Care Products (PPCPs)
Wastewater is a major conveyor of chemicals from households and commercial establishments. Pharmaceuticals (prescription and over-the-counter) and synthetic food additives enter the sanitary sewer system either unaltered or as their metabolites via urinary and fecal excretion. Cleaning products and personal care products are regularly “rinsed away” as a part of daily maintenance and hygienic activities. In addition, unused products are often disposed of down the drain.
Conventional wastewater treatment plants are ineffective at completely removing many of these compounds before releasing treated wastewater to receiving water bodies. Therefore, after decades of escalating use and unregulated domestic and commercial discharge, these xenobiotic compounds – often referred to as PPCPs (pharmaceuticals and personal care products) – are appearing in measurable concentrations in surface waters and groundwater aquifers ), some of which serve as drinking water sources. PPCPs include antibiotics, detergents, plasticizers, anti-inflammatory drugs, fragrances, hormones and hormone mimics (e.g., estrogen and estradiol), sunscreens, and antidepressants.
The effects of PPCPs once released into the natural environment and drinking water sources are largely unknown. Because many were designed to counteract chemical interactions or to target specific metabolic and biological pathways in humans, there is concern that some PPCPs may disrupt key processes in sensitive non-target organisms, including certain human populations.
Lower production, decreased release, or more efficient removal at wastewater treatment plants are required to curtail the introduction of priority PPCPs into sensitive water bodies. At present, however, it is unclear which of these “new” contaminants should be considered priorities, that is, which PPCPs pose the greatest potential human and ecological health risks. The number of compounds of potential concern is staggering, and thousands of new products are introduced into the market (and the environment) annually. In addition, because of the issue’s recent emergence, published literature describing the occurrence, fate and transport, and effects of these compounds is sparse, incongruent, and scattered relative to more “established” pollutants (e.g., PCBs, PAHs, heavy metals). It is not feasible to approach the issue of PPCPs by conducting detailed field studies of all compounds: PPCPs are too numerous, they behave differently in the environment, and the measurement of PPCPs and their metabolites in environmental samples can be complex and expensive. At the same time, a “scattered” approach – compounds chosen because, for example, they have high name recognition, are produced in large quantities, or are relatively easy to measure – is not prudent.
The overarching goal of this research project is to develop a ranking system for PPCPs that quantifies the potential relative risk that compounds pose to human health and ecological health. We will use this relative ranking process to narrow the list of PPCPs to a more manageable subset of priority PPCPs, those that likely pose the greatest potential risk. These high risk PPCPs will be identified as requiring additional research into their fate and transport, ecotoxicolgy, and human toxicology.
The ranking system involves estimating each PPCP’s relative “toxic load” (TL). TL incorporates a PPCP’s mass loading to the aquatic environment and its toxicity:
TL = ML / TD
where ML = mass loading; and
TD = toxic dose.
We chose to develop TL as a metric because the environmental risk posed by a PPCP is related to both the compound’s mass loading to the environment (which in part controls the concentrations at which the compound will be found in aquatic systems) and the concentration at which the compound is capable of exerting toxic effects. For example, a compound that has a relatively low ML but happens to be highly toxic (i.e., a small TD), may pose a greater risk – and have a greater TL – than a relatively non-toxic compound with higher ML. Therefore, TL has the potential to serve as a better metric for prioritizing PPCPs than methods based solely on mass loading or toxicity alone.
Some of the specific goals of this pilot research are to:
i) Develop a list of PPCPs that may be of human/ecological health significance;
ii) Develop a process for determining mass loading of PPCPs to aquatic systems;
iii) Develop a process for choosing the most relevant measure of PPCP toxicity that allows consistent comparisons across species and across compounds;
iv) Use the above processes to determine mass loadings and toxicities for an abbreviated list of PPCPs (a few hundred);
v) Quantify uncertainty and explore ways to manage/minimize uncertainty related to estimating each of these terms;
vi) Calculate TL for the abbreviated PPCP list, rank compounds based on their TL, and generate a TL distribution;
vii) Using the TL distribution generated above, determine if other PPCPs – those for which ML or TD estimates are unavailable – might be priorities, and therefore warrant more detailed assessment.
In general, our approach to accomplishing these goals is iterative: screen a large number of compounds using the most-easily-obtained estimates of TL (TLscreen); narrow the list to priority compounds by ranking PPCPs using TLscreen; and, for the smaller list of priority PPCPs, recalculate TL (TLrefined) using more accurate (and labor-intensive) measures of ML and TD.