Water from Stone: The Groundwater Journey From
Bedrock to Coastal Ponds and Beyond
By Monica Allard Cox
If a drop of pond water could be said to have a life history, S. Bradley Moran knows what it is.
For several years, Rhode Island Sea Grant funding has aided Moran, University of Rhode Island oceanography professor,
in his studies of where the water in southern Rhode Island’s coastal ponds—locally called salt ponds—comes from, how
long it circulates around these ponds, and where it goes. Because coastal pond water is a mixture of salt and fresh water
and carries nutrients and contaminants with it, this research has also been of interest to, and funded by, the R.I. Coastal Resources
Management Council. His research has shown that groundwater—as opposed to surface water, such as from rivers
and streams—contributes significantly to these coastal ponds, and that the supply of groundwater changes with the seasons.
These new findings have important implications regarding watershed land use and how contamination sources might affect
coastal water quality.
“There is no question that the problem of understanding and managing the ecological and economic impact of groundwater
and associated chemicals and microbial contaminants on our coastal waters is still a big one,” Moran says. However, he
adds, it is too soon to tell what the longer-term impacts associated with increased building and urbanization on the watershed
will be on groundwater—and thus the coastal ponds and Narragansett Bay.
Moran’s scientific approach involves the use of naturally occurring radium isotopes (223Ra, 224Ra, 226Ra, and 228Ra) as tracers.
As sediment and bedrock weather, they release radium isotopes to groundwater, which retains a radium “fingerprint”
that results from the constant radioactive production of the isotopes. Depending on the type of bedrock—for example,
in the Pettaquamscutt watershed, the bedrock is composed of Rhode Island Formation metasediments, Narragansett Pier
granite, Esmond granite gneiss, and Esmond augen granite gneiss—the characteristics of the radium isotopes differ in known
ways. Therefore, measuring these isotopes can tell scientists where groundwater is coming from as well as the amount of
groundwater flowing into coastal waters.
In addition, because 223Ra and 224Ra have short half-lives, on the order of several days, they can be used to determine
the average length of time that water remains in a coastal pond before circulating out to Narragansett Bay and further offshore.
For example, Moran and his graduate students estimate that surface water in the Pettaquamscutt river estuary spends
an average of about eight days before being flushed out. The waters of Ninigret, Winnapaug, and Point Judith ponds remain
between four and 10 days, while water in Green Hill and Quonochontaug ponds remains between five and six days. Knowing
these time scales is important because the quality and health of a coastal water body is controlled in part by the water’s
residence time.
His group has sampled water at various locations in the water column and in groundwater to determine how radium
isotopes are distributed throughout the ponds. Factored into the analysis are measurements of precipitation and evapotranspiration,
and the results have been compared to results from modeling techniques, in collaboration with John Masterson, Robert Breault, and other colleagues
from the U.S. Geological Survey. For
all their cutting-edge technology and
promising results, however, Moran feels
that “these radium isotope tracers
are not yet a tool in the toolbox” for
coastal management.
“The application of these radium
isotopes to coastal studies has really
taken off in the last 10 years, and these
tracers have certainly provided new
and interesting results,” he adds. “However,
when using these tracers, a key
question is: What are we really measuring?
We still don’t fully understand
what these tracers are telling us; there
is certainly more work that needs to
be done.”
One of the difficulties with conducting
this research is that scientists
are often limited by the number of
samples they can collect. For example,
due to weather conditions, a lack of
equipment, or other such logistical
constraints, some previous investigations
have sampled groundwater only
from shore, or taken water column
samples from a limited number of
locations. Because of the complexity
of groundwater water movement
and the potential for multiple sites of
origin, the accuracy and hence usefulness
of data from such studies could
be compromised. One study Moran
supervised showed a wide variability
not observed in other studies. Moran’s
recent studies have involved a detailed
collection of samples from both
groundwater and the water column;
however, he says that he would prefer
to collect more data.
“We have succeeded in establishing
new limits for the amount of
groundwater and associated nutrients
that flow into these ponds, as well as
the average time water remains in the
ponds,” he says. Observing the variability
in our results has been “interesting
and useful—you appreciate the
complexity of these important ecosystems
—however, for coastal managers,
they want answers.”
Moran notes that using radium
isotopes as tracers in coastal ponds
has uncovered some important information
about how the ponds work.
Future studies of coastal groundwater
will focus both on basic and applied research
questions. In turn, results from
such studies will have direct implications
for the environmental management
of coastal waters.
Moran is proposing to build on
this research with David Smith, URI
oceanography associate professor, using
new techniques to track microbial
source contamination in groundwater
and surface waters of Rhode Island’s
coastal ponds, which has direct, practical
implications. Such studies of the
salt ponds, he says, provide information
pertinent to other water bodies.
“The same concerns regarding the
ecology and health of coastal groundwater
and embayments in Rhode
Island exist for many places around the
world. It turns out that the salt ponds
of southern Rhode Island are useful
natural laboratories for studying these
problems, and the results from this research
have broad implications that are
applicable to other regions.”
For further reading:
Cute, K. 2004. CRMC Funds
Groundwater Research in the Salt
Ponds Region Watershed. Coastal Features
12(1):1–6.
Kelly, R.P. and S.B. Moran. 2002.
Seasonal changes in groundwater input
to a well-mixed estuary estimated using
radium isotopes and implications
for coastal nutrient budgets. Limnol.
Oceanogr. 47(6):1796–1807.
—Monica Allard Cox is a Communicator for Rhode Island Sea Grant.
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