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Wastewater Treatment Plants and Water Quality

I recently received the following question from Lauren Rosella, a student at College of the Holy Cross. Your question deals with a complicated issue that is also one of the most important challenges that scientists and managers in Long Island Sound and in other estuaries face in trying to improve water quality.

Q:  I was wondering if the upgrades in sewage treatment plants, the addition of the nitrogen credit trading program, or permits have improved the water quality, and if hypoxia has declined in the Long Island Sound due to these mitigation strategies?

Dr. K: Dear Lauren,

The issue that you’re trying to address is very complex, and it’s something that scientists in Long Island Sound and virtually every other coastal water body in the world are dealing with. The relationship between hypoxia and nutrient loading is complicated and non-linear. This management strategy hinges on a long chain of events which starts when nitrogen loadings are reduced, which theoretically causes primary productivity (photosynthesis, which in the ocean is carried out by tiny microscopic plants called phytoplankton) to decrease, which in turn decreases the amount of decaying organic matter which sinks into the benthos. Respiration by bacteria consuming this organic matter is one cause of oxygen drawdown which can lead to hypoxia if the water column is insufficiently ventillated and/or circulated.  However, even in laboratory experiments, the relationship between nutrient loading and primary productivity is non-linear. At high levels of anthropogenic nitrogen loading such as those observed in Western Long Island Sound, it takes a large reduction in loading  to result in even a small reduction in primary productivity. The figure below shows a nutrient enrichment experiment done at the University of Rhode Island Graduate School of Oceanography’s MERL mesocosm lab in the 1980s by Dr. Candace Oviatt.  In the experiment, five-meter-deep mesocosm  tanks designed to replicate the flow and circulation of Narragansett Bay were fertilized with nitrogen at 1,2,4,8,16 and 32X ambient concentrations and the amount of primary productivity was noted. In order to get rapid reductions in primary productivity, it’s necessary to bring nutrient concentrations down into the 5X area or lower, which corresponds with concentrations well below what we presently see in Western Long Island Sound.

At present, Connecticut and New York wastewater treatment facilities have done an excellent job in working to meet or beat their permitted nitrogen discharge targets, and we are on pace to meet, or at least get pretty close to the original goal of a 58 percent reduction in nitrogen loading from this source by 2014. However, sewage is not the only source of nitrogen into Long Island Sound, so this reduction represents a significantly smaller percentage of the overall amount of nitrogen entering the Sound from all sources.

This is further complicated by the wide range of other factors which influence hypoxia (air and water temperature, precipitation, circulation, wind direction and magnitude, etc.), all of which vary year to year, and many of which (like water temperature for example) have a trend of their own that can mask the trend imposed by the reduction (warmer water holds less oxygen than colder water, so as the Sound warms, we should expect more hypoxia). On top of this, nutrient loadings themselves can vary quite a bit from year to year, based on frequency and severity of storms, droughts, etc.

Because there is so much inter-annual variability in the factors which drive hypoxia, even if the reduction in sewage loading had an immediate effect, we wouldn’t expect to be able to detect a clear signal for several years.  As an analogy, think of installing a device on your car that claims to improve your gas mileage by  one mile per gallon.  Even if you did very careful monitoring before you installed the device to record your mileage from each tank for several fill ups (years) It would still take you several fill ups after you installed the device to tell whether the device was really working, because your mileage varies a lot based on how aggressively you drive, whether you’re using the air condition, how long your trips are, and whether you’re driving on the city or highway.

To complicate matters more, many studies from European estuaries (which are way ahead of us on this front) have shown that the ecosystem has a five-to ten-year-lag between when the loadings were reduced and when the primary productivity dropped.  One hypothesis for this is that the sediments have so much nitrogen stored up in them that after the reduction, they release nitrogen back into the water for several years, which masks the impact of the reduction.

In summary, even if the reductions to this point were working perfectly, the year to year variability makes it unlikely that we would be able to detect an immediate reduction in hypoxia associated with a sewage loading reduction of this magnitude if it were occurring (which it may be).  And even if it isn’t occurring yet, it doesn’t mean that these management efforts have failed, because it’s very possible (even likely) that we’ll experience a similar lag in response time of several years. So patience is the watchword here.

In terms of your question about trace metals, in general, trace metals don’t limit primary productivity in nearshore ecosystems.  Trace metals tend to be limiting in areas far from shore where there’s very limited terrestrial input of sediment, which is one problem that LIS does not have at this time.

Jason Krumholz, aka Dr., K, is the NOAA liaison to EPA’s Long Island Sound Office. Dr. Krumholz received his doctorate in oceanography at the University of Rhode Island Graduate School of Oceanography.


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Send an e-mail to Jason Krumholz.. Dr. Krumholz  is a marine scientist working as the NOAA liaison to the EPA Long Island Sound Office. View more of Dr. K’s questions and answers on the Ask Dr. K blog.


Editor’s Note: The opinions expressed in this blog are those of the author. They do not reflect EPA or NOAA policy, endorsement, or action, and EPA or NOAA do not verify the accuracy or science of the contents of the blog.


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