This is definitely one of the more complex topics we'll be discussing on the blog. Physics can be so clean -- bodies of rock moving through space, atmospheric gas escaping -- but biology, especially when we start talking about ecosystems, can be so messy! I mean literally as well as figuratively: people who work on measuring the levels of pharmaceuticals in our waterways have to wade through muck, dig through algae, and get in boats!
The presence of pharmaceuticals and endocrine-disrupting compounds in our waterways is an issue that's only becoming more pressing. I'll concentrate on Minnesota because that's where I live. Remember that flap about water bottles containing bisphenol-A? Maybe you switched your water bottle -- but you can't get away from the fact that small quantities can now be found in over 40 percent of Minnesota lakes . Don't take antidepressants at the moment? Well, maybe you'll get a little when you're swimming: venlafaxine (sold as Effexor) is found in 9.4 percent of stream water samples analyzed by the Minnesota Pollution Control Agency in 2010  and amitriptyline is found in almost 30 percent of lakes randomly sampled in 2012 . Disturbingly, cocaine is found in over 30 percent of lakes, too -- what are Minnesotans doing?!  And you'd think we'd have fewer mosquitos here given that DEET, the insect repellent, is found in over 70 percent of our lakes! 
While the amounts we're talking about are very tiny, they still disrupt fish and mussel life and reproduction. Strangely, fish exposed to some antidepressants are more aggressive predators. Frogs exposed to birth control chemicals can reverse sex. We don't really know what happens to humans exposed to frequent low-level chemical concentrations of this type, although there are disturbing preliminary results relating endocrine disruptors to obesity, diabetes, and endometriosis. We do know that the pharmaceutical industry is hugely important economically, and there is conversation around making the industry greener.
Ok: enough geeking out about the prevalence and importance of drugs in our waterways. Where's the math?
I'm working out a worksheet that will discuss photolysis, the breakdown of chemicals due to light exposure. The rate of photolysis depends on the clearness of the water at a given depth in the photic zone (the zone that light can reach), and this rate constant can be modeled by a
- composition of a rational function and an exponential function.
- This function gives a rate, even though we haven't done any differentiation.
- I'm hoping to work in a page on limits as well.
However, this post is super-long already -- so I'll stop! The worksheet should be up today or tomorrow (Tuesday).
 Pharmaceuticals and Endocrine Active Chemicals in Minnesota Lakes, released May 2013, online at MPCA site on water. Dramatic graph on page 6.
 Pharmaceuticals and Personal Care Products in Minnesota’s Rivers and Streams: 2010, released April 2013, online at MPCA site on water. Pages 2-3 discuss venlafaxine.