Abstract
Wastewater discharges to the Assabet River contribute
substantial amounts of phosphorus, which support accumulations
of nuisance aquatic plants that are most evident in the
river’s impounded reaches during the growing season. To
restore the Assabet River’s water quality and aesthetics, the
U.S. Environmental Protection Agency required the major
wastewater-treatment plants in the drainage basin to reduce the
amount of phosphorus discharged to the river by 2012. From
October 2008 to December 2010, the U.S. Geological Survey,
in cooperation with the Massachusetts Department of
Environmental Protection and in support of the requirements
of the Total Maximum Daily Load for Phosphorus, collected
weekly flow-proportional, composite samples for analysis
of concentrations of total phosphorus and orthophosphorus
upstream and downstream from each of the Assabet River’s
two largest impoundments: Hudson and Ben Smith. The
purpose of this monitoring effort was to evaluate conditions
in the river before enhanced treatment-plant technologies had
effected reductions in phosphorus loads, thereby defining
baseline conditions for comparison with conditions following
the mandated load reductions. The locations of sampling
sites with respect to the impoundments enabled examination
of the impoundments’ effects on phosphorus sequestration
and on the transformation of phosphorus between particulate
and dissolved forms. The study evaluated the differences
between loads upstream and downstream from the impoundments
throughout the sampling period and compared differences
during two seasonal periods of relevance to aquatic
plants: April 1 through October 31, the growing season, and
November 1 through March 31, the nongrowing season, when
existing permit limits allowed average monthly wastewater-treatment-plant-effluent concentrations of 0.75 milligram per
liter (growing season) or 1.0 milligram per liter (nongrowing
season) for total phosphorus. At the four sampling sites during
the growing season, median weekly total phosphorus loads
ranged from 110 to 190 kilograms (kg) and median weekly
orthophosphorus loads ranged from 17 to 41 kg. During the
nongrowing season, median weekly total phosphorus loads
ranged from 240 to 280 kg and median weekly orthophosphorus
loads ranged from 56 to 66 kg.
During periods of low and moderate streamflow,
estimated loads of total phosphorus upstream from the
Hudson impoundment generally exceeded those downstream
during the same sampling periods throughout the study;
orthophosphorus loads downstream from the impoundment
were typically larger than those upstream. When storm
runoff substantially increased the streamflow, loads of total
phosphorus and orthophosphorus both tended to be larger
downstream than upstream.
At the Ben Smith impoundment, both total phosphorus
and orthophosphorus loads were generally larger downstream
than upstream during low and moderate streamflow, but
the differences were not as pronounced as they were at the
Hudson impoundment. High flows were also associated with
substantially larger total phosphorus and orthophosphorus
loads downstream than those entering the impoundment
from upstream.
In comparing periods of growing- and nongrowing-season
loads, the same patterns of loads entering and leaving
were observed at both impoundments. That is, at the Hudson
impoundment, total phosphorus loads entering the impoundment
were greater than those leaving it, and orthophosphorus
loads leaving the impoundment were greater than those entering
it. At the Ben Smith impoundment, both total phosphorus
and orthophosphorus loads leaving the impoundment were
greater than those entering it. However, the loads were greater
during the nongrowing seasons than during the growing seasons,
and the net differences between upstream and downstream
loads were about the same.
The results indicate that some of the particulate fraction
of the total phosphorus loads is sequestered in the Hudson
impoundment, where particulate phosphorus probably undergoes
some physical and biogeochemical transformations to
the dissolved form orthophosphorus. The orthophosphorus
may be taken up by aquatic plants or transported out of the
impoundments. The results for the Ben Smith impoundment
are less clear and suggest net export of total phosphorus and
orthophosphorus. Differences between results from the two
impoundments may be attributable in part to differences in their sizes, morphology, unmonitored tributaries, riparian land
use, and processes within the impoundments that have not
been quantified for this study.