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USGS and DEM
using computer models to evaluate ground-water resources and
management alternatives in the Assabet watershed
The Assabet River Consortium's water balanace
provides a lot of useful information about the hydrologic
health of water resources within the six Consortium communities.
But it is a screening level analysis and does not include
communities outside the Consortium. A more comprehensive and
sophisticated analysis was required.
In response to requests from OAR and the EOEA
SuAsCo Watershed Team (now defunct), the US Geological
Survey (USGS) and the Department of Environmental Management
(MA DEM) agreed to spend $425,000 over three years to evaluate
ground-water management alternatives in the Assabet watershed.
USGS staff in the Massachusetts-Rhode Island office are currently
working on a MODFLOW groundwater model for the watershed to
assess the impact of existing and proposed groundwater pumping
scenarios on streamflows in the Assabet River and its tributaries.
Why
is groundwater managmement so important to the Assabet River?
During
the low flow period of July to September, groundwater or "base
flow" seeps into the river and its tributaries from adjacent
aquifers providing the relatively cool, clean water important
to maintaining good habitat for fish and other aquatic creatures.
In the tributaries baseflow is essential to keep the streams
from drying up in the summer. In the mainstem Assabet it provides
dilution for the nutrients and other pollutants in the wastewater
effluent being discharged to the Assabet. In drier summers
the proportion of effluent to baseflow in the mainstem Assabet
gets very high. For example, during the July 1999 TMDL field
survey, ENSR found that 80% of the flow in the Assabet at
the USGS gage in Maynard could be accounted for by wastewater
effluent.
 Unfortunately,
the Asabet has already lost much of its baseflow because of
existing groundwater withdrawals, sewering, and extensive
paved and otherwise impervious surfaces where water can't
infiltrate back into the aquifers. For this reason, the impacts
of new or increased groundwater withdrawals from the watershed
must be carefully evaluated to ensure that they will not reduce
groundwater or baseflow contributions to the river. Additional
losses of these flows will exacerbate the river's existing
eutrophication problem and undermine existing and future investments
in nutrient control.
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