Eutrophic Reservoirs and Chlorophyll a

New York City is experiencing excessive phosphorus contamination in reservoirs supplying its drinking water and has concluded the phosphorus concentration at terminal ( intake ) reservoirs should be 15 ug/ l( micrograms per liter ). 1

The report notes one of its terminal reservoirs, New Croton , ”...has had a mean phosphorus concentration below 20 ug/ l (micrograms per liter) since 1992 and yet it still has a significant use-impairment due to euthrophication.”2

The New Croton reservoir average total phosphorus concentration from 86 samples over the growing season (May-October) 1988-1996 was 20 ug/ l and the average chlorophyll-a value from 84 samples was 8.69 ug/ l.3

It should be noted the North Carolina chlorophyll-a standard, that can not be met in the proposed Randleman project is over 3 times higher than the chlorophyll-a values of the New Croton reservoir, a drinking water supply that , “ frequently taken out of service 2-4 months in the late summer and early fall each year due to eutrophication related water quality problems, such as unacceptable taste and odor, elevated color, and elevated iron and manganese.”4

The following exerts from the report state:

Water bodies are often classified according to their biological productivity
or trophic state. Water bodies with high nutrient levels, high algal
concentration and low transparency are classified as eutrophic.5

While water bodies generally have a natural trophic state, governed by
the physical and chemical characteristics of the watershed and the water
body, many lakes and reservoirs suffer from cultural eutrophication.
Changes to the landscape (e.g. urbanization) or the addition of new
sources of nutrients (e.g. wastewater treatment plants) can dramatically
increase the nutrient load to a water body, resulting in increased productivity
and a shift of the trophic state to a more eutrophic condition. Although
algae need both nitrogen and phosphorus for growth, phosphorus is
often in the least supply and cannot be obtained from atmospheric sources
like nitrogen can. Therefore, the phosphorus concentration is often the
limiting factor in the growth of algae.6

Eutrophic conditions in a reservoir can have significant direct and indirect
negative impacts on water quality. These include production of disin-
fection by-products, unacceptable taste and odors, the release of organic
molecules that can produce gastrointestinal distress, and deposition of organic
matter in deep waters, leading to dissolved oxygen depletions during
microbial respiration. Low dissolved oxygen leads to the appearance of
iron, manganese and hydrogen sulfide in the water column, and the release
of nutrients from sediments.7

More recently, the concern with eutrophic lakes and reservoirs is with
the production of trihalomethane (THMs) and haloacctic acid (HAAs)
precursors since these are a potential health threat. The EPA regulates
the amount of THMs in finished water, and is gradually lowering the federal

As phosphorus loading increases, nitrogen becomes short in supply
by comparison, and this condition favors the growth of nitrogen-fixing,
blue-green algae (cyanophyceae).9

In addition to taste and odor concerns, many species of blue-green algae
produce toxins. Blue-green algae, and their toxins, are on the EPA Drinking
Water Contaminant Candidate List.10

Currently, there are no federal standards for phosphorus, algae or
chlorophyll-a in the source water of a public water supply. EPA is
however, currently pursuing the development of regional nutrient
criteria as part of the Clean Water Action Plan. The individual states
are expected to adopt numeric nutrient criteria not only based on the
type of waterbody and ecoregion, but also the designated use of the
waterbody by 2003. These nutrient criteria will then be the basis
for development of TMDLs or permits for wastewater treatment plants.11

New York State, like many states around the nation, does not have
a phosphorus water quality standard. It does, however, have a total
phosphorus guidance value equal to 20 ug/ l for lakes and reservoirs...
However, this guidance value was based on aesthetic concerns for
primary and secondary recreation, and is meant to avoid turbidity,
low dissolved oxygen and excessive growth of rooted aquatic plants
and surface scum (i.e. algae).12

...operations at a water treatment facility can be negatively impacted
by high levels of organic matter, such as algae, and the effectiveness of
filtration facilities in removing THMs and blue-green algae toxins is
unknown at this time. Notably, the filter plant option only treats the
symptoms of eutrophication, and not the cause.13

The entire report may be accessed on the web at:

1 Development of a water quality guidance value for Phase II Total Maximum Daily Loads (TMDLs) in the New York City Reservoirs, New York City Department of Environmental Protection, March 1999,p.ix.

2 Ibid.p.X

3 Ibid. Table 5.2,p.32.

4 Ibid. p.x.

5 Ibid.p.7

6 Ibid.

7 Ibid.

8 Ibid.p.8.

9 Ibid.

10 Ibid.p.9

11 ibid.

12 Ibid.p.10.

13 ibid.p.17.