Friday, December 13, 2013
The list of studies investigating what happens to pharmaceutical compounds when they flow from wastewater treatment plants into nearby lakes and rivers is getting longer and longer. But not all wastewater effluent is destined for our waterways. Some of it ends up as irrigation water for farm fields, particularly in areas where rainstorms are few and far between. The treated water seeps into the ground and recharges groundwater supplies. But what happens to the pharmaceuticals it carries?
That is the question that scientists from Hebrew University of Jerusalem and Colorado State University set out to answer with a study recently published in Environmental Pollution. A three-year field study allowed them to compare pharmaceutical concentrations in agricultural soils irrigated with treated wastewater with ones watered with fresh water. Experiments in the field and in the lab also let them examine just how quickly these compounds are degraded by microbial communities in the soil.
What they discovered, unsurprisingly, is that there is no single answer for what happens to pharmaceuticals in soil. How and when the compounds break down appears to depend on two important factors: the physiochemical properties of the pharmaceutical and the time of year. Field and lab samples revealed that compounds like the anti-seizure drug carbamazepine, caffeine, and the blood pressure medication metoprolol break down very slowly, building up over time in soil irrigated with treated wastewater. But anti-inflammatories like ibuprofen breakdown in a few days or weeks. The key is in the chemical bonds. Carbamazepine and other non-ionic compounds – those not held together by the opposite charges of a negative and positive ion – tend to have stronger chemical bonds that are difficult to breakdown. It is easier for bacteria and other microbes to breakdown weaker acidic compounds like ibuprofen.
Regardless of their chemical bond, though, every tested pharmaceutical broke down more quickly before the growing season than after the crop – in this case carrots – had been harvested. The mostly likely reason for this is seasonal differences in the makeup and activity of microbial communities. The warming temperatures and high organic matter left over from the previous year's crop likely makes the time right before farmers start irrigating ideal for building large, diverse communities of bacteria and other microbes. And active communities mean more degradation.
But the study did have one surprising result. Contrary to their hypothesis, researchers discovered that the tested pharmaceuticals weren't degraded any faster in soil that had been irrigated with treated wastewater for multiple seasons. The repeat exposure was expected to build up bacterial communities hungry for pharmaceutical compounds. But it didn't. The exact reason for this is unknown, but it may be because of a phenomenon known as co-metabolism. Soil microbes might not be able to degrade pharmaceuticals directly. It may be instead that bacteria can only breakdown these compounds while they are already metabolizing other organic matter.
A. Grossberger, Y. Hadar, T. Borch, and B. Chefetz. 2014. Biodegradability of pharmaceutical compounds in agricultural soils irregated with treated wastewater. Environmental Pollution 185:168-177.