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URI Watershed Watch Program
Block Island Monitoring Results 2001-2006 May 2007
Elizabeth Herron and Linda Green, URI Watershed Watch Summary:
To date both the field monitoring and laboratory analyses indicate
that Block Island’s premier resource, the Great Salt Pond, is in
good health. However, during the six years of monitoring increases
in nutrients have been detected, and a number of tributary streams
have high levels of both nutrients and bacteria, particularly
following rain events. Tributary monitoring has shown a great deal
of variability in water quality conditions, therefore is especially
important to maintain. Continued monitoring of both Great Salt Pond
and its tributary streams is essential in order to determine
whether apparent nutrient increases are an actual trend in water
quality, or due to natural variability in response to weather
patterns and the degree of associated uses of this well-used and
beloved ecosystem. Monitoring enables us to more accurately
pinpoint if and when problems begin to develop, and how the
ecosystem responds to changes in the watershed. Scientists
recommend that ten years of monitoring is needed to fully assess
the health of a water body. During that time there will be drought
and deluge, warm and cool weather, maybe even a hurricane - a full
span of natural conditions. With six years of data available for
the Great Salt Pond and the original three tributary stream sites,
overall water quality status as well as some trends are becoming
discernable.
Table: Block Island Watershed Watch Monitoring Site Information
Great Salt Pond Locations (at 1 meter from the surface and 1.0
meter from the bottom, or mid-depth): GSP #1 mid-harbor @ Green can
#5 GSP #2 Narragansett Inn cove GSP #3 Off Champlin's dock GSP #4
Trim's Pond Tributary Locations: BI Trib #1 Mill Tail Pond, Ocean
Ave. culvert BI Trib #2 Bridgegate Square culvert - "colored side
pipe" BI Trib #3 Cormorant Cove salt marsh culvert BI Trib #4 Beach
Avenue (added in 2003) BI Trib #5 Harris Pt (added in 2003) BI Trib
#6 Scott's (added in 2004)
Introduction: The 2006 monitoring season was the sixth that
dedicated volunteer water quality monitors from the Committee for
the Great Salt Pond (CGSP) monitored four sites on the Great Salt
Pond (GSP) and six tributary stream sites as a part of the URI
Watershed Watch Program (URIWW) (see Table and Figure 1 for site
information). These sites were selected by members of the Committee
for the Great Salt Pond in
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cooperation with URI Watershed Watch staff at a site visit in
2001. Monitoring has been funded through the Block Island and Green
Hill Pond National Decentralized Wastewater Demonstration Project.
Three additional tributary streams were added in 2003 and 2004 in
order to investigate additional potential inland pollution
sources.
Figure 1. Map showing CGSP Monitoring Sites
URI Watershed Watch (URIWW) staff trained CGSP volunteers to
conduct the water quality monitoring, and provided annual
refreshers after the initial training. These water quality monitors
were provided with all the necessary monitoring equipment and
supplies as well as a written monitoring manual that detailed all
the procedures. Their regular monitoring followed a schedule
developed by URIWW designed to cover the period of peak activity on
the island - from mid-June through mid-October each year.
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Unfortunately late season storms that disrupted ferry service
shortened the monitoring season in several years. The monitoring
included biweekly on-site monitoring of water clarity, water
temperature, dissolved oxygen content, salinity, and algae level.
(Please see URIWW monitoring manuals and quality assurance project
plans available on the website at http://www.uri.edu/ce/wq/ww/ for
more specific information.) In 2003 the CGSP purchased a
multi-parameter field meter to measure dissolved oxygen,
temperature and salinity, replacing the URIWW owned and maintained
kits. However the basic field monitoring procedures (monitoring
depth, time of day, etc.) were otherwise kept the same. The on-site
monitoring data was submitted to URIWW on monitoring postcards. In
addition to the bi-weekly on-site monitoring, once a month the
volunteers also collected a suite of samples, packed them on ice in
a cooler and shipped them via ferry to Point Judith. The sample
schedule was set to ensure that samples were collected near low
tide in order to better assess the impact of land based sources
versus in-pond sources, while also making sure that the samples
would reach the URIWW laboratory within acceptable hold times.
URIWW staff picked up the water samples from Point Judith and
delivered them to the URIWW Analytical Laboratory in URI’s College
of the Environment and Life Sciences in Kingston. These samples
were analyzed for bacteria, pH, nitrogen, phosphorus, total
suspended solids and chlorophyll according to standard URIWW
laboratory procedures (please see Quality Assurance Project Plan:
University of Rhode Island Watershed Watch Analytical Laboratory
http://www.uri.edu/ce/wq/ww/Publications/LabSOPs.pdf for additional
information). The analytical laboratory is Rhode Island Department
of Health certified (#LAI00294). Brief summaries and discussion of
the results of the six years of water quality monitoring follow.
Weather summary: Weather can significantly affect water quality,
and can confound the assessment of water quality findings. This
summary (figure 2) is based on weather data from the URI Weather
Station in Kingston, RI. Departures from normal were in relation to
the average temperature and precipitation values over the past
thirty years. Monthly temperature was above normal for the vast
majority of months in the six monitoring seasons, particularly in
August and September. Rainfall was more evenly distributed between
above and below normal in this same time span. The 2001 monitoring
season was generally warmer than average, with about average
rainfall during the monitoring season (that year July - October.)
Droughts during much of the summers of 2002, 2004 and 2005 brought
long periods of sunny days and warmer than average temperatures,
and potentially increased impacts from a better boating season.
Annual precipitation was above average from 2003 through 2006 –
particularly in 2005 and 2006. In 2006, distribution of
precipitation was not as erratic (extended dry periods followed by
unusually wet periods) as in previous years. In 2004 the remnants
of two hurricanes in September produced nearly six inches of rain
over two weekends, ending a summer long drought. In 2005 a wet
winter preceded a record dry summer, which ended with week long
rains including an October record setting rain event. More than 6”
above normal rain in June 2006, and more than 4” above normal in
2006 contributed to the 9+” above normal rain for that year. These
types of extreme weather patterns make assessment of the water
quality data challenging, and argue for continued long-
http://www.uri.edu/ce/wq/ww/http://www.uri.edu/ce/wq/ww/Publications/LabSOPs.pdf
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term monitoring in order to help differentiate between true
water quality trends and weather related impacts.
Figure 2. Weather Summary (Kingston RI Weather Station)
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Degrees F. Temperature, Departure from Normal, 2001-2006
Great Salt Pond field monitoring summaries: Volunteers measured
a number of key water quality indicators using field instruments
and/or kits at four sites on the Great Salt Pond. These field
measurements, which included temperature, dissolved oxygen,
salinity and processing samples for chlorophyll, were typically
done at either mid-depth at sites less than two meters deep, or at
half a meter from the surface and within one meter from the bottom
at sites deeper than two meters. With the exception of the Mid
Harbor site (GSP #1), which was the deepest, water clarity, or
Secchi depth transparency was generally a measurement of bottom
depth, with the Secchi disk usually visible on the bottom. These
water quality indicators pointed to overall excellent water quality
in the Great Salt Pond for all four sites (figures #3 and #4 show
data from the deepest and shallowest sites.) Water clarity at
Mid-Harbor (GSP #1) was excellent, with generally low algae levels.
These both indicate low nutrient, or oligotrophic conditions.
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Figure 3. Great Salt Pond #1 – Mid-harbor Site Field Data
Charts
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SECCHI DEPTH - WATER CLARITY
Depth (m)
Eutrophic - low water clarity
Mesotrophic - Moderate water clarity
Oligotrophic - Excellent water clarityThe Secchi disk has always
been visible on the bottom.
Apparent decline in water clarity was due to a switch to
low-tide monitoring.
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DISSOLVED OXYGEN WITHIN 1 METER OF THE BOTTOMmg/L
Potentially lethal dissolved oxygen levels
Potentially stressful dissolved oxygen levels
Dissolved oxygen at all four sites remained high throughout all
six seasons, an indicator of good tidal flushing as well as good
overall water quality. Salinity measurements showed a slight fresh
water influence at all the pond sites monitored. Salt water
typically has 35 parts per thousand (ppt) salinity, fresh water has
0 ppt. With the exception of 2001, salinity typically ranged from
30 – 33 ppt salinity in the pond sites. The range during 2001 was
broader, with a 35 ppt maximum salinity value, and 25 ppt minimum
value at Mid-Harbor (GSP #1). Salinity values in 2006 and 2005
averaged approximately 29 ppt, reflecting the higher than average
precipitation. This also suggests that ground water contribution to
the pond was fairly stable throughout the monitoring season despite
large fluctuations in rain, and therefore, stormwater runoff.
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Figure 4. Great Salt Pond #4 Trim’s Pond Site Field Data
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CHLOROPHYLL LEVELS AT MID-DEPTHppb
Eutrophic - Elevated algae levels
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DISSOLVED OXYGEN AT MID-DEPTHmg/L
Potentially lethal dissolved oxygen levels
Potentially stressful dissolved oxygen levels
Fecal coliform Bacteria Monitoring: Fecal coliform bacteria are
an indicator of fecal contamination and potentially disease causing
organisms. Both shellfishing and swimming standards for fecal
coliforms in marine waters (14 colony forming units (cfu)/100 mL
and 50 cfu/100 mL respectively) have been established in order to
protect the public from exposure to unsafe levels of pathogens.
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Figure 5. Annual Averages Fecal Coliform Bacteria at Great Salt
Pond Sites
Great Salt Pond Annual Average Fecal Coliform Values (June -
Oct. Geomean)
05
101520253035404550
Great Salt Pond #1(Middle of the harbor)
Great Salt Pond #2(Narragansett Inn Cove)
Great Salt Pond #3(Champlin’s)
Great Salt Pond #4(Trim’s)
2001 2002 20032004 2005 2006
cfu per 100 mL
117 8 4 2
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Shellfish Standard = 14 fecal coliform f /100 l
Note Scale
Swimming Standard = 50 fecal coliform f /100 l
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Average fecal coliform values at the GSP sites typically were
below the stringent 14 cfu/100 ml shellfishing standard the first
three years of monitoring (figure 5). In the past three years
counts above both shellfishing and the 50 cfu/100 ml swimming
standards have become more frequent, at Champlin’s (GSP #3) and
Trim’s Pond (GSP #4) and particularly at Narragansett Inn Cove (GSP
#2). All three sites exceeded both standards in at least four of
the six monitoring seasons, with very high fecal coliform counts
this past summer, 6000 cfu/100ml on July 5, 2006 at Narragansett
Inn Cove (GSP#2), 520 on July 19, and 630 on August 17. The July 5
sample far exceeded the previous maximum of 680 from the 9/21/05
collection. In contrast, the Mid-Harbor site (GSP #1) has never
exceeded the swimming standard in the six years of monitoring. Even
that site had its highest maximum to date in 2006, above the
shellfishing standard but still below the swimming standard.
Tributary stream sites typically had much higher fecal coliform
counts than the Great Salt Pond sites (figure 6a.) While freshwater
fecal coliform standards for recreational contact such as swimming
(200 cfu/100 mL fecal coliform) are much higher than for salt
water, the yearly averages for these sites exceeded even this
higher standard with regularity. Since several of these tributaries
discharge directly into the shellfishing waters of the Great Salt
Pond, identifying the source(s) and taking action to correct
problems in order to attain lower levels is an important goal.
Ocean Ave (#1) and Bridgegate Square (#2) have had the highest
counts since the monitoring began, with values often in excess of
thousands. Figure 6b shows the highest counts for each site for
each monitoring season. The scale of that chart is nearly ten times
higher than for the average counts and clearly illustrates the
contribution of these streams. However, since there were no flow
measurements made, the overall loading or impact from those sources
may be different. It should be noted that during dry periods these
sites have very low bacteria levels, with uniformly low values
during the drought of 2002. With no rain there was no overland flow
or runoff bringing wastes into the streams. Low bacteria counts
during dry weather also suggest that there were no direct
discharges of waste water to the streams, such as from a pipe or a
failed septic system. Overall the tributary site at Harris Point
(#5) showed the least impact from bacterial contamination.
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Figure 6a. Annual Average Fecal Coliform Bacteria – Tributary
Stream Sites
GSP Tributary Streams Annual Average Fecal Coliform (June - Oct.
Geomean)
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Ocean Ave (#1) Bridgegate Sqr(#2)
Cormorant (#3) Beach Ave.(#4)
Harris Pt (#5) Scott’s (#6)
2001 2002 20032004 2005 2006
Note Scale
Freshwater swimming standard (200)
cfu per 100 mL
Figure 6b. Annual Maximum Fecal Coliform Bacteria – Tributary
Stream Sites
GSP Tributary Stream Annual Maximum Fecal Coliform
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Cormorant (#3) Beach Ave.(#4)
Harris Pt (#5) Scott’s (#6)
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Note Scale
Freshwater swimming Standard (200)
cfu per 100 mL
Nutrients: Total, Nitrate-, and Ammonium- Nitrogen. (BayWatchers
II produced by the Coalition for Buzzard’s Bay and available at
http://www.savebuzzardsbay.org/pdf/baywatchers2-pages3-34.pdf,
provided the background information for this section.) Total
nitrogen is widely used by scientists as an indicator of
eutrophication or nutrient enrichment in marine waters. Levels
below 350 ppb are characteristic of low nutrient waters, while
values above 600-700 ppb indicate nitrogen enrichment. Using those
values for comparison, Great Salt Pond water samples were in the
low to moderate range for all years (figure 7.) However, that chart
clearly shows increase in total nitrogen over these years, with the
highest levels at Trim’s Pond (GSP#4). Monitoring the Great Salt
Pond for total nitrogen should definitely be continued. 2006 total
nitrogen levels have nearly tripled since monitoring began in 2001.
Total nitrogen is comprised of nitrate-N, ammonia-N and organic-N.
Nitrate- and ammonia- account for about half the total nitrogen at
each of the sites (data available upon request.)
http://www.savebuzzardsbay.org/pdf/baywatchers2-pages3-34.pdf
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Figure 7. Annual Mean Total Nitrogen – Great Salt Pond Sites
Great Salt Pond Yearly Total Nitrogen Averages (June - Oct.
average)
050
100150200250300350400450500
GSP #1surface
GSP #1deep
GSP #2surface
GSP #2deep
GSP #3surface
GSP #3deep
GSP #4surface
GSP #4deep
2001 2002 2003 2004 2005 2006
Note Scale
ppb or ug/L
Moderate nutrient range
Low nutrient range
Monitoring tributary inlet streams is important for determining
water quality impacts including nutrient loading. Nutrient loading
is defined as the rate at which a substance such as N is added to a
system. The measurement of loading includes multiplying the
concentration of nutrient from a source by the volume of water from
that source (see http://www.dnr.state.md.us/bay/monitoring/water/
for a discussion of the difference between concentration and
loading.) Thus, while a particular tributary may have very high
concentrations, if there is very little flow from that source, its
overall load or contribution, may be very minimal. Since the flow
of these tributary sites has not been measured, it is impossible to
accurately assess the actual nutrient loads to the Great Salt Pond.
However the results to date can help target areas for additional
investigation and monitoring. Total nitrogen values in the inlet
streams were generally at least twice as high as the pond sites
(figure 8). All sites except for Scott’s (#6) have moved from the
low N to the moderate N range. Ocean Ave (#1) and Cormorant (3)
have shown the greatest increase since monitoring began. The
previous 2003 maximum was passed in 2006 at all sites except
Cormorant Cove (#3). It should be noted that total nitrogen levels
in these Block Island tributary streams were within the range of
other tributaries monitored as a part of URIWW. However, Bridgegate
Square (#2), and Beach Ave (#4) have been consistently higher, with
N levels more similar to urban rivers and streams. In light of the
impact that additional nitrogen loads could have on the pond,
continued monitoring of these tributary inlet streams is strongly
recommended.
http://www.dnr.state.md.us/bay/monitoring/water/
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Figure 8. Annual Mean Total Nitrogen – Tributary Sites
Tributary Inlet StreamsYearly Total Nitrogen Averages (June -
Oct. average)
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1400
1600
Ocean Ave (#1) Bridgegate Sqr(#2)
Cormorant (#3) Beach Ave. (#4) Harris Pt (#5) Scott’s (#6)
2001 2002 2003 2004 2005 2006
ppb or ug/LNote Scale
Nutrient enriched range (above 600 - 700 ppb)
Nitrate-nitrogen is a soluble form of nitrogen, readily taken up
and used by algae and submerged vegetation, especially during the
summer growing season. Nitrate-nitrogen levels were very low, below
the detectable range for most of the sampling events in most years
for the pond sites (figure 9.) As with total nitrogen,
nitrate-nitrogen levels in the tributaries inlet streams were also
higher than in the pond at Ocean Ave. (#1) and Bridgegate Sq (#2),
with measurable levels all years (figure 10.) These levels were
within the range of other URIWW monitored tributaries, with
Bridgegate Sq. similar to urban tributary streams. It should be
noted that all levels were well below the 1000 ppb (1 ppm)
nitrate-nitrogen levels that the US Geological Survey considers an
indication of some type of groundwater contamination.
Figure 9. Annual Mean Nitrate-Nitrogen – Great Salt Pond
Sites
Great Salt Pond Yearly Average Nitrate-Nitrogen (June - Oct.
Average)
0
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200
300
GSP #1surface
GSP #1deep
GSP #2surface
GSP #2deep
GSP #3surface
GSP #3deep
GSP #4surface
GSP #4deep
2001 2002 2003 2004 2005 2006
Note Scaleppb or ug/L
analytical limit of detection
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Figure 10. Annual Mean Nitrate-Nitrogen – Tributary Sites
Tributary Stream Yearly Average Nitrate-Nitrogen (June - Oct.
Average)
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400
600
800
1000
Ocean Ave(#1)
BridgegateSqr (#2)
Cormorant(#3)
Beach Ave.(#4)
Harris Pt(#5)
Scott’s (#6)
2001 2002 2003 2004 2005 2006
ppb or ug/LNote Scale
USGS criteria for an indication of groundwater contamination
Ammonium-nitrogen is the other soluble form of nitrogen that the
URI Watershed Watch program analyzes for. It is also readily used
by algae, thereby indirectly contributing to biological oxygen
demand or BOD, which can reduce dissolved oxygen concentration in
water. The acceptable level for ammonia-N depends on pH, salinity
and temperature in salt water, and on temperature, pH and life
stage (of critical organisms) in freshwater, (see
http://www.dem.ri.gov/pubs/regs/regs/water/h20q06.pdf for
specifics.) For the Great Salt Pond it is 411 ppb and for the
tributary inlet streams it is 2100 ppb. Average values for each
year and at each monitoring site in the pond were considerably
below that critical level (Figure 11). While ammonium-nitrogen
levels were low, a very slight trend of increasing values has been
noted, and bears continued watching.
Figure 11. Annual Mean Ammonium-Nitrogen – Great Salt Pond
Sites
Great Salt Pond Annual Ammonium Nitrogen (June - Oct. Mean)
050
100150200250300350400
GSP #1surface
GSP #1deep
GSP #2surface
GSP #2deep
GSP #3surface
GSP #3deep
GSP #4surface
GSP #4deep
2001 2002 2003 2004 2005 2006
Note Scaleppb or ug/L
Critical chronic ammonium-nitrogen level for GSP ≅ 411 ppb
http://www.dem.ri.gov/pubs/regs/regs/water/h20q06.pdf
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Based on the pH and temperature values commonly seen in the GSP
tributaries during the monitoring season, chronic ammonium-nitrogen
values in the range of 2160 to 5030 ppb or higher would be a
concern to salmonid species at various life stages. During the past
six monitoring seasons average values were quite low, and 10% or
less than the values of critical concern (figure 12.) In addition,
ammonium-nitrogen levels in the tributaries have not shown any
clear overall trends, either increasing or decreasing.
Figure 12. Annual Mean Ammonium-Nitrogen – Tributary Sites
Tributary Annual Ammonium Nitrogen (June - Oct. Mean)
0100200300400500600700800900
1000
Ocean Ave(#1)
BridgegateSqr (#2)
Cormorant(#3)
Beach Ave.(#4)
Harris Pt (#5) Scott’s (#6)
2001 2002 2003 2004 2005 2006
ppb or ug/LNote Scale
Critical chronic ammonium-nitrogen level for GSP tributaries
> 2100 ppb
Phosphorus. In most estuaries such as the Great Salt Pond,
nitrogen is the primary nutrient that controls algal & plant
growth. However like nitrogen, phosphorus is also essential for
life, and thus in salt water environments phosphorus levels must be
considered in relationship to nitrogen levels. In estuaries the
recommended level of total phosphorus is 10 to 100 ppb with 100 to
1000 ppb of total nitrogen (a 10:1 ratio of nitrogen to
phosphorus). Under these conditions, most phosphorus-caused algal
blooms may be avoided. (Please see
http://www.water.ncsu.edu/watershedss/info/phos.html for more
information.) Like nitrogen, phosphorus also occurs in the total
form which includes phosphorus bound in particulate (organic and
inorganic) matter and soluble or dissolved forms which are readily
used by algae.
http://www.water.ncsu.edu/watershedss/info/phos.html
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Figure 13. Annual Mean Total Phosphorus – Great Salt Pond
Sites
Great Salt Pond Annual Total Phosphorus (June - Oct.
Averages)
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7080
GSP #1surface
GSP #1deep
GSP #2surface
GSP #2deep
GSP #3surface
GSP #3deep
GSP #4surface
GSP #4deep
2001 2002 2003 2004 2005 2006
ppb or ug/L
Approximate Maximum recommended TP (=10% of TN)
Total phosphorus levels in the Great Salt Pond have been in the
20-40 ppb range (figure 13) and do not show the increases seen with
total nitrogen. They were mostly in the dissolved form (data not
shown), indicating that it was being absorbed and used by
microscopic algae and aquatic plants. Overall total phosphorus is
close to 10% of overall average total nitrogen, indicating that has
the potential to help cause algae blooms.
Figure 14. Annual Mean Total Phosphorus – Tributary Sites
Tributary Streams Annual Total Phosphorus (June - Oct.
Averages)
020406080
100120140160
Ocean Ave(#1)
BridgegateSq (#2)
Cormorant(#3)
Beach Ave.(#4)
Harris Pt (#5) Scott’s (#6)
2001 2002 2003 2004 2005 2006
ppb or ug/L
Recommended Maximum TP for streams
For freshwater streams, a maximum concentration of 50 ppb total
phosphorus is the advised limit for protection of downstream
resources. Concentrations below 50 ppb were generally found in the
tributary sites (figure 14.) In 2006 Cormorant Cove (#3) had an
extraordinarily high total phosphorus value of 724 ppb. This
elevated that year’s average to well above the advisory limit, the
only time this happened during the six
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years of monitoring at that site, although annual values have
been close to the 50 ppb recommended maximum. Bridgegate Square
(#2), Harris Point (#5) and Scott’s (#6), have all had water
samples from multiple dates that exceeded 50 ppb. The regularity
with which these sites have exceeded the recommended total
phosphorus maximum in particular warrant continued monitoring of
phosphorus. However the discussion of concentration versus loading
in the section on total nitrogen also applies here.
Acknowledgements: Volunteer water quality monitoring would not be
possible without the dedication of outstanding volunteers such as
Carl Kaufman, Kyra Riggie, Henry DuPont, Corrie Heinz, Pat and
Shirley Howe. These individuals have devoted a great deal of time
on the water collecting a many, many water samples and in their
kitchens running tests and diligently processing samples for later
URIWW lab analysis. We are indeed indebted to them for providing
the information that has gone into this assessment. We also thank
the Committee for the Great Salt Pond for helping to oversee the BI
monitoring component of the Demonstration Project, and the
technical committee that helped to identify areas of concern. We
are most heartened by the cooperation and collaboration among
various monitoring programs on Block Island that has continued
partially as a result of this project. Finally, we thank Lorraine
Joubert, Galen Howard McGovern and Bill Healy who have managed this
project and enabled us to a get a clearer picture of the status of
Block Island’s greatest resource – the Great Salt Pond and its
watershed.
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