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Raymond M. Wright, Professor Daniel W. Urish, Professor |
The water that flows into Greenwich Bay
comes from a 13,000-acre watershed containing some 19,000 residences.
(See map.) The watershed also contains dairy farms, on-site sewage disposal,
and gas stations. Relatively clean precipitation moves along myriad pathways
to the bay, accumulating anthropogenic contributions, many of which can
be harmful to the bay.
Greenwich Bay is a rich natural resource
with shellfishing, sailing, and swimming. It has a beautiful coastline
and is bordered by a 500-acre state park. During the past two decades,
Greenwich Bay shellfishing areas have been closed due to high levels of
fecal coliform which contaminated the shellfish. There have been beach
closures as well. The loss of a major shellfishing resource was of great
concern to City of Warwick officials. Another contaminant of concern is
nitrogen. When nitrogen is present even in low values in salt water, it
can trigger eutrophication, a condition of overfertilization that results
in oxygen depletion and attendant fish kills. Both fecal coliform and
nitrogen are contained in human and animal waste. The sources of these
contaminants were not known.
A major comprehensive study was funded
in 1994 by the City of Warwick and RI Sea Grant to determine the nature
and origin of pollutants traveling through the watershed into the Bay.
Two principal avenues transport water through
the watershed to the bay---surface stream flow and much slower moving
and unseen groundwater. In the Greenwich Bay watershed, approximately
72 percent of the water enters the Bay as stream flow and 28 percent as
direct groundwater discharge along the coastline.
We began the study by providing an accurate
assessment of the existing water quality conditions in the drainage area
under both dry weather conditions (groundwater) and wet weather conditions
(groundwater, stream sediment resuspension, and storm runoff). Samples
were taken at a series of stations from a stream's headwaters to the confluence
with the bay and at the end of a pipe discharging into the bay.
When it rains, a stream's flow increases
due to surface runoff. At the start of a storm, if there has been no precipitation
for several days, the dry weather stream flow is groundwater. The characteristics
of the storm are represented by a hyetograph which shows precipitation
amounts. The stream flow increases, reaches a maximum, and tapers off
as the storm passes. This response is referred to as a stream's hydrograph
(see Fig. 1).
Three storms were
monitored using the same stations and water quality constituents that
were monitored during the dry-weather program. A prestorm sample defined
the baseline dry-weather loads. Time zero was set at the start of runoff.
Sampling was scheduled at regular intervals and customized to storm and
hydrograph characteristics. Flows were determined for each sample.
Rainfall criteria were critical to the
monitoring program and the interpretation of the data. The goal was to
isolate the effect of a discrete event to permit the characterization
of runoff and the determination of the impact on receiving water quality.
The criteria were designed to sample storms associated with frontal systems
that provided uniform rainfall over the watershed.
In the summer of 1994, the first dry-weather
and wet-weather surveys were completed in Hardig Brook and revealed extraordinarily
high levels of fecal coliform at very different locations. Additional
surveys isolated the sources of the problem to two half-mile stretches
of stream. Subsequent investigations narrowed the search to several hundred
feet and finally to the sources. The dry-weather sources were three raw
sewage discharges discovered under a series of old buildings, and the
wet-weather source of fecal coliform was a dairy farm.
Groundwater discharge is diverse and depends
on the geology and geometry of the shoreline. The sampling process is
a challenge. Thermal infrared aerial imagery was used to identify optimum
locations for groundwater discharge sampling in August 1998. The thermal
infrared image shows the cold groundwater discharge as dark plumes moving
from the shoreline into the warmer water of the bay. One of the regions
of strong groundwater discharge was identified along the eastern shore
of Arnold Neck (see Fig. 2). Sampling of the plume directed by the thermal
infrared image showed major nutrient contamination. Residences are dense
in this area, and the soils are poorly suited to on-site sewage disposal;
this results in little attenuation of pollutants prior to discharge along
the water's edge.
The City of Warwick,
in conjunction with the Rhode Island Department of Environmental Management,
has taken steps to further investigate, correct, or alleviate these contamination
sources. When point sources are discovered, specific remedial action can
be taken by property owners. However, the solutions to widespread non-point
source pollution are larger and more costly. The City of Warwick has embarked
on a $130 million program to sewer the most critical areas. Studies indicate
that sewering the watershed area can reduce harmful nitrogen input from
groundwater sources by 80 percent.
