amounts of various nutrients entering the tidally influenced 
Potomac River, during the water years 1979 to 1981 (i.e., Octo¬ 
ber 1978 through September 1981). In this figure, the "River" 
source refers to the watershed upstream of Washington, D.C.? the 
"Non-Point" source refers to all direct runoff and tributary 
inflow below Washington? and "Point" refers to sewage treatment 
plants. 
Restricting this discussion to total phosphorus and total 
nitrogen, approximately one quarter of each comes from non-point 
sources? that is, from the sewage treatment plants. Approxi¬ 
mately half of the supply of each nutrient comes from the 
"River" source and the remaining quarter from local runoff. It 
is important to remember that only 25 percent of total nitrogen 
and total phosphorus come from sewage treatment plants? the 
remainder comes from sources that are much more difficult to 
control. 
Where do nutrients go? Roughly 75 percent of both of these 
nutrients are retained within the system or, in the case of 
nitrogen, denitrified out of the water column. Therefore, be¬ 
tween the head of tide at Chain Bridge and the Chesapeake Bay, 
the Potomac Tidal River and Estuary remove about 75 percent of 
those two nutrients. Most of that removal occurs in the estu¬ 
ary. 
Consider the estuary and recall earlier presentations that 
showed that these nutrients have been collecting in the estu¬ 
arine bottom sediments for quite a number of years. These bot¬ 
tom sediment nutrients are available for recycling during sum¬ 
mertime and produce food for the algae. USGS observations have 
shown that the organic carbon in the bottom sediments changes in 
composition between U.S. Highway 301 Bridge and the mouth of the 
estuary from essentially terrestrial in origin to mostly marine 
in origin. This lends support to the statement made earlier 
that an appreciable fraction of the carbon that produces the 
bottom-water anoxia is grown in place. 
Figure 8 summarizes the results of an in-depth reconnais¬ 
sance conducted by the USGS during the summer of 1984. The 
panel in the lower left shows the proportion of the volume of 
the estuary that was anoxic during the time period from April 
through October. Approximately, 20 percent was anoxic at the 
peak, and this volume covered 30 percent of the bottom area. 
This is obviously an extensive problem. 
Because nutrient removal efforts in the upper part of the 
river have essentially not influenced the nitrogen supply, what 
we've done so far has done nothing and will not do anything to 
significantly alter this situation. I can say this because USGS 
observations from the 1984 summer indicate that if there is a 
limiting nutrient, it is nitrogen. In so far as the estuary 
126 
