Summer/Fall 2001 Table
Balancing the Water Budget
By Tracey Crago, WHOI Sea Grant
Balancing budgets is not exactly what youd
expect to find a marine geochemist doing, but its key to Matt
Charettes current research on coastal ponds.
Charette, an assistant scientist and Coastal Ocean
Institute Fellow at the Woods Hole Oceanographic Institution, is
using a chemical tracer-radium isotopes-to look at sub-surface
groundwater pathways to coastal embayments in southern New England.
These pathways, also known as submarine groundwater discharge (SGWD),
are thought to play a role in delivering nutrients, such as nitrate
and phosphate, to coastal waters.
"Typically," explains Charette, "there
are three ways to measure how much SGWD is coming into an estuary
or coastal ocean. You can create a water budget, manually measure
the groundwater by using seepage meters, or you can use chemical
tracers, which have become popular in the last decade." These
methods are not without limitations, however.
Charettes WHOI Sea Grant-supported
research employs chemical tracers that appear in different concentrations
in groundwater than they do in, say, seawater. "Once the groundwater
concentration is known, you can determine the groundwater flux by
trying to balance the budget," says Charette.
Matt Charette measures nitrate concentration in a Cape Cod
estuary using a nitrate analyzer, which automatically downloads
data into his laptop every five minutes. Photo: Tom Kleindinst,WHOI
As one might expect, each chemical tracer has its
advantages and disadvantages. "Radium is the tool were
using for determining how much submarine groundwater is coming in,"
says Charette, whose study sites for the tracer experiments are
Quonochontaug ("Quonny") Pond in Rhode Island, Sippewissett
Marsh and Pamet River estuary, both on Cape Cod. Both Quonny Pond
and Sippewissett Marsh represent what Charette calls "urbanized
coastal sites," with Pamet serving as his control site.
The field portion of his research involves two separate
sampling protocols: one for groundwater, using residential wells
or piezometers to collect samples; the other for the estuary, using
a multi-probe device that measures salinity, dissolved oxygen, depth,
pH, and tidal height. Groundwater sampling takes place on a different
day than estuary sampling and, says Charette, the rule of thumb
is to collect as many groundwater samples as possible. "Determining
the average concentration of the tracer in groundwater is the main
source of error in the whole approach. The groundwater content of
radium can be variable, so with more samples, the average becomes
more representative of the local conditions."
The radium isotope work takes place back in the
laboratory, using water samples collected in the field. Charette
is assisted in the field and in the lab by Craig Herbold, a research
assistant in the WHOI Marine Chemistry and Geoechemistry department.
In addition to WHOI Sea Grant support, the work is funded by The
Cove Point Foundation.
Sampling in different seasons is important, as is
sampling over the course of a tidal cycle. Charette sets up his
instrumentation at the ocean inlet to "get a good look at nutrient
fluxes" between the water coming into and out of the estuary.
"In March, what came in to Quonny Pond, came out. There isnt
much biological activity in the winter." That was quite different,
says Charette, than sampling in May and August, where he saw negative
values for nutrients, also known as a nutrient "sink."
This came as a surprise. "Quonny Pond is actually
a sink for nutrients, meaning that nutrients are coming into the
pond from Long Island Sound, versus traveling to the sound via the
pond. Those results make it difficult to assess if there is a nitrogen
source to Quonny Pond from groundwater. At this point, if there
is, its minimal," says Charette.
An altogether different kind of surprise presented
itself in Pamet. "Pamet was supposed to be our pristine control
site," begins Charette. "We thought it would be the least
impacted of the three sites. As it turns out, it may be the most
impacted." Charette describes a sampling trip to Pamet in July,
when he saw a rapid increase in nitrate concentration as the tide
went out, over two tidal cycles. "It peaked at 8 ÁM/l (micro-moles
per liter)." As a basis for comparison, the nitrate concentration
in Cape Cod Bay, the body of water that the Pamet estuary empties
into, was less than 0.01 ÁM/liter.
figure illustrates the variation of the salinity, tidal height,
nitrate, and radium-224 during a complete tidal cycle at the
Pamet River Estuary inlet. During the falling tide, radium and
nitrate concentration increased while salinity decreased. Because
the estuary receives minimal surface runoff, these results suggest
that submarine groundwater discharge (SGWD) is an important
source of nutrients to the system.
Charette says that if he backtracks the nitrate
peak to zero salinity, the spike he saw in Pamet corresponds with
the nitrate concentration hes seeing in the groundwater. For
one thing, says Charette, there is not a lot of surface runoff to
Pamet. That, coupled with the results of groundwater samples taken
from wells along the fringe of the estuary, seems to implicate groundwater
as the primary source of nitrate.
As for the surprises, Charette seems more intrigued
than disappointed. "Typically, the higher the housing density
around an estuary, the higher the impact from groundwater."
(Housing density around Pamet, compared to the other sites, is much
lower.) "We definitely didnt expect the nitrate signal
to be as distinct as it was in Pamet." As to why, Charette
cant give a definitive answer, but guesses that older homes,
many with cesspools as opposed to newer, Title V septic systems-and
many located right at the edge of the estuary, have something to
do with the strong nitrate signal. "You need time and distance
for septic systems to dissipate the nitrate signal," he says,
and both are missing in the Pamet equation.
Now that he can determine if groundwater is delivering
nutrients to an estuary, and how much nitrogen, in the form of ammonium
and nitrate, is getting into the estuary through groundwater, Charette
is gearing up to answer perhaps the most important question: what
is the source of the groundwater-borne nitrogen?
To help answer that question, Charette will work
with colleagues at the Boston University Marine Program, professor
Ivan Valiela and graduate student Kevin Kroeger, using stable nitrogen
isotopes. Possible culprits include wastewater and fertilizer, not
surprising in developed areas. However, being able to identify trouble
spots-and sources-will be useful to resource managers
constantly plagued by the effects of too much nitrogen in coastal
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