Contaminant Sleuthing in the Coastal Environment
Raymond M. Wright, Professor
Department of Civil Engineering
Ray Wright earned a BS in civil engineering from Tufts University.
He has an MS and a PhD, both in civil engineering, from Pennsylvania State University.
He joined the URI faculty in 1981 and specializes in hydrologic research in
lakes, rivers, and estuaries.
Daniel W. Urish, Professor
Department of Civil Engineering
Dan Urish received a BS in civil engineering from the University
of Illinois and an MS in civil and environmental engineering from the University
of Washington. In 1978, after 20 years with the U.S. Navy, he received his PhD
from URI and joined the faculty. His research has focused on island and coastal
hydrogeologic studies.
The water that flows into Greenwich Bay comes
from a 13,000-acre watershed containing some 19,000 residences.
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.
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. 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.