Residue from drugs is increasingly being documented in surface and drinking water, raising questions about endocrine disruption and other adverse health effects in people and the environment. Much remains unknown about which drugs are more likely to leach into the environment, and exactly what the sources are.
The USGS announced on June 4, 2010, that a small study has linked drug manufacturing plants to significant amounts of drugs coming out of nearby wastewater treatment plants (in some cases polluting waters as much as 20 miles downstream), indicating both that the drug plants are a source, and that wastewater treatment plants can't mitigate the problem. USGS says this is the first study identifying drug manufacturing plants as a significant source.
- "Manufacturing Facilities Release Pharmaceuticals to the Environment"  (includes a link to the study in the June 4, 2010, issue of Environmental Science & Technology, and other background information).
Other research has suggested that disposal in landfills or down toilets, and metabolites or unutilized drugs that pass through people who take the drugs, are other contributing sources.
The study results can't be used to identify specific problem plants, since only a few were studied. However, you can begin to get a handle on your geographic beat by identifying drug manufacturing plants, and asking the operators if they have data on drugs they are dumping into area waterways, and if they are pretreating this effluent, or have plans to do so. You can also ask local public health officials if they are concerned about this issue, and are doing any monitoring.
Drug manufacturing plants tend to be identified under the North American Industry Classification System (NAICS, which began replacing the Standard Industrial Classification (SIC) code system in 1997) via the code numbers of 325411 (Medicinal and Botanical Manufacturing) and 325412 (Pharmaceutical Preparation Manufacturing). You may want to search for a few other related manufacturing codes here,  or more broadly here. 
These facilities should be traceable through EPA's National Pollutant Discharge Elimination System,  and identified via the agency's Envirofacts Web site:
- EPA's Envirofacts Warehouse  (some of the options include searching by geographic area under "water" and "permit compliance system" (PCS), or by "facility," or via the EnviroMapper tool).
Another way of approaching the issue is through an industry trade group, which may be able to point you to specific facilities, and to provide the industry's perspective on the issue.