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Thinking in Terms of a Water Balance
How do you know when water withdrawals or other activities
such as land developement are affecting streamflow and water
quality? One useful tool is a simple "water balance."
Another tool is the more complex groundwater
model.
A water balance is an analysis showing how much
water is entering and leaving a drainage area such as the
Danforth Brook subwatershed. Surprisingly, the Assabet watershed
as a whole is a net importer of water because Hudson, Westborough,
and Maynard withdraw water from wells located in the Sudbury
Watershed, Shrewsbury withdraws water from the Blackstone
Watershed and both Marlborough and Northborough purchase water
from the MWRA. An estimated 9.6 million gallons of water per
day enters the Assabet Watershed from outside the natural
drainage area. But very little of this water recharges aquifers
within the Assabet watershed. Nearly all of it becomes nutrient-rich
wastewater effluent that is discharged directly into the Assabet
River. An estimated 6.4 mgd is withdrawn from groundwater
and surface water sources in the Assabet watershed.
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The consulting firm Earth Tech prepared a water balance
in 2002 for the six Assabet River Consortium communities
(Shrewsbury, Westborough, Marlborough, Northborough,
Hudson, and Maynard) as part of their Comprehensive
Wastewater Facilities Planning work. This analysis shows
that some drainages in the Assabet watershed are hydrologically
stressed, that is, they are net exporters of ground
water. Whether a drainage area is gaining or losing
water may be significant for streams because exporting
drainages, i.e. areas where more groundwater is pumped
out of the ground than is replaced or "recharged,"
are at risk for reduced summer time stream flows because
the aquifers are, in effect, being mined.
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Groundwater well pumphouse, Northborough
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of groundwater could also reduce the amount of water that
can be withdrawn from these aquifers by communities over
the long-term. The Consortium's water balance shows that
in 1999, nearly half of the Assabet subbasins (9 of 22)
within Consortium communities were in "deficit"
because of water withdrawals (see map of the subbasins
in deficit). This means that more water was withdrawn
from these subbasins and discharged to the Assabet River
as wastewater effluent than was returned to the subbasins
as groundwater recharge. Together the communities withdrew
an estimated 6.4 MGD from the Assabet watershed in 1999
and recharged only 2.3 MGD to groundwater. The balance,
4.1 MGD, was discharged to the river as wastewater or
lost through evapotranspiration. |
Surface water withdrawal |
Moreover,
the water balance predicts that in 2025, assuming that the
projected water demands in the Consortium communities are
met, nearly all of the subbasins (18 of 22) would have significant
"deficits" caused by water withdrawals. Under this
scenario, nearly 100% of the water withdrawn from the Assabet
subbasins would be exported from the watershed. These deficits
could reduce the yield of local water sources (particularly
groundwater sources), leading to local water shortages. This
would also further reduce baseflows and dilution of wastewater
effluent in the Assabet River, and decrease minimum flows
in tributary streams.
Additional
information about this water balance may be found in Chapter
7 of the Assabet River Consortium's Phase II CWMP/EIR summary
document dated, May 2002. Summary water balance spreadsheets
prepared by Earth Tech for the Consortium below (in pdf format):
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