Falmouth Academy Student Emily Tietje's Science Fair Project

Changing Salinity at Oyster Pond Weir

Winter/Spring 2001

Introduction

The purpose of this project was to determine how rainfall and inflow of seawater affect salinity measured at the weir in Oyster Pond during the autumn and winter of 2000. Salinity is the amount of dissolved solids in water (Salt, 1965. Encyclopedia Junior Brittanica). A weir is a dam or obstruction built across a river or stream to constrict water flow [See Weir Photograph]. Salinity can be measured with a hydrometer that measures specific gravity that is then converted to salinity in parts per thousand (Tietje, pers. comm., October 10, 2000). Typical ranges of rainwater and freshwater salinity are 0-1 parts per thousand (ppt) and seawater averages 32 ppt (Oyster Pond Remediation Project, August 15, 2000, Internet). Oyster Pond is located on Surf Drive near the ocean. The three sources of water inflow into Oyster Pond are rainfall, surface runoff, and the influx of seawater from tides. A weir was built in 1997 where Oyster Pond and Trunk River meet to lower the level of water in the pond. Prior to 1997, the water level and salinity were alarmingly high, causing the fish to die (Oyster Pond Remediation Project, August 25, 2000. Internet). Since 1997, the weir hs successfully lowered the water level and decreased the salinity to 1ppt or less (Davies, October 10, 2000, email). The water is unable to flow freely into Oyster Pond because Trunk River has gotten clogged with seaweed and other plants (Meyers, pers. comm., October 10, 2000). Construction occurred in late 2000 to build jetties at the mouth of Trunk River to increase the flow of seawater into Oyster Pond and the plants were cut back (Oyster Pond Remediation Project, August 25, 2000. Internet). It was hypothesized that the salinity levels in Oyster Pond woudl remain low.

Materials and Methods

The materials used in this experiment were a hydrometer, 2.4-m long piece of wood, 710-ml soda bottles, a thermometer, charts called "Temperature Correction Table" and "Censity to Salinity Conversion Table," a plastic tube with a diameter of 5.08 cm and 36.83 cm length, a clamp to clip bottle to wood, and duct tape.

Water samples were taken from four locations in Oyster Pond once per week from 10/12/00 to 1/12/01. These locations were as follows: Station T1 was at pond side of the weir, Station T2 was at lagoon in Trunk River, Station T3 was at the dock on Spohr's Garden (Figure 1). Water samples were collected from the surface water at each of the stations every week. These samples were used to measure water salinity.

Samples were collected using a device built with a soda bottle clamped onto a wooden stick. The bottle was manuevered in teh water with the opening facing downwards and then was turned right side up by hand to release air. The bottle was then brought up and back to the work site where salinity tests were immediately conducted.

The water in the bottle was poured into a plastic tube. The thermometer was immediately placed in the water. Once the thermometer equilibrated, the temperature was recorded and the thermometer was removed from the plastic tube. The hydrometer was placed gently in the plastic tube in the water sample, and allowed to float freely in the water. One side of the hydrometer was scaled with markings indicating specific gravity. The mark on the hydrometer even with the water's surface was noted and recorded.

The charts were then used to determine salinity, by determining where the temperature intersected the hydrometer's reading of specific gravity (see Appendix 1 and Appendix 2). This salinity, measured in parts per thousand (ppt), was recorded in a logbook.

Rainfall and snowfall data for November 2000 through January 2001 were acquired from the Falmouth Water Department. These data were collected at nearby Long Pond (in Falmouth), and reported in inches. The data were entered into a MicroSoft Works spreadsheet and plotted.

Construction took place at the mouth of Trunk River, with the goal of causing the salinity in Oyster Pond to increase 4 ppt. Weekly observations were made of the construction process. These observations were recorded in the logbook and pictures were taken to document activities.

Data

Tables 1-4 show salinity data collected at Oyster Pond and Trunk River station locations T1-T4. Table 5 gives average salinity at all stations (T1-T4) by date. Table 6 shows daily snow and rainfall data from November 1, 2000 to January 21, 2001. Table 7 shows total rainfall, in inches, for the 7 days prior to each set of salinity measurements. Graph 1 and Graph 2 illustrate salinity averages by data and location, respectively. Graph 3 displays salinity data for each individual sampling station by date. Graph 4 shows total rain and snowfall, in inches, for the 7 days prior to each set of salinity measurements. The construction at Trunk River started on December 3, 2000, when large boulders and other smaller rocks appeared in the parking lot next to the mouth of Trunk River. About a week later power line protectors were installed and more rocks appeared. On December 24, 2000, a crane and some other CAT equipment moved on site. This equipment was used to remove the eelgrass from Trunk River and to make a rock bottom. Phragmites were also cut back, but grew back within a week during the experiment. During the week of January 12, 2001, the north jetty was nearly completed. By January 28, 2001, the north jetty was finished and the south jetty lacked only a few final rocks. [See Jetty Construction Photo]

Analysis

The purpose of this project was to determine how rainfall and inflow of seawater affect salinity measured at the weir in Oyster Pond during the autumn and winter of 2000/2001. The data in Tables 1-4 and Graph 3 indicate that the salinity in Oyster Pond and Trunk River ranged from 0 parts per thousand (ppt) to 21 ppt over the duration of this project. The salinity at Station T1, located near the weir in Oyster Pond, ranged from 0-14 ppt, averaging 3.4 ppt (Table 1 and Graph 2). The salinity at Station T2, located in a lagoon near the entrance to Oyster Pond, ranged from 0-10 ppt, averaging 3.5 ppt (Table 1 and Graph 2). The salinity at Station T3, located at the entrance to Oyster Pond, ranged from 0-21 ppt, averaging 7.1 ppt (Table 1 and Graph 2). The salinity at Stations T1-T3 rose because of the northeast storm on 12/30/00 that caused the salinity to go from a lowly 1 to 14 ppt., but then gradually lower to 3 ppt. The salinity at Station T4, located at the Spohr's Garden dock, ranged from 0-1 ppt, with an average of 0.3 ppt (Table 1 and Graph 2). The salinity level at T4 stayed low because the ice on the surface of Oyster Pond prevented circulation from distributing influxes of salty water that far back in the pond. Graph 1 and Tables 1-4
  • Table 5 show that the average salinities ranged from 0-17.5 ppt over the sampling period. Table 6 shows daily rainfall and snowfall data from November 1, 2000 to January 21, 2001, which ranged from 0 to 3.14 inches. Table 7 and Graph 4 show rainfall and snowfall data for the preceding 7 days before samples were taken.

    Emery (1997) stated that water flows out of Oyster Pond 98% of the time. This agrees with the results of this study as only one high tide was observed to move water into Oyster Pond during the sampling period. This high tide occurred on 12/30/00. This was supported by the low salinities measured during much of the experiment, except for the dates immediately following the northeast storm that brought large volumes of salt water into Oyster Pond. This storm occurred on 12/30/00. [See Nor'Easter Photo of Weir]

    A limitation in this project was that thick ice at Station T2 prevented samples from being taken there in January, resulting in an incomplete data set. Another limitation was that a fifth station location could not be tested because that station would have needed wading gear. A possible source of error was that the same thermometer was not used throughout the entire project. This could have resulted in inaccuracies in the data.

    An experiment that could be done in the future stemming from this project would be to test how the Oyster Pond water level changes. This would be a good project because it is a concern at Oyster Pond to keep the pond level at a safe height so that the water level does not drop so low that fish can no longer swim in or out.

    The relevance to society for this project is that it adds to the data being collected by the Oyster Pond Environmental Trust. This organization is working towards helping more fish and other animals return to Oyster Pond.

    The purpose of this project was to determine how rainfall and the inflow of seawater affect the salinity measured at the weir in Oyster Pond during the fall and winter of 2000 and 2001. The salt water caused the salinity to go up only when there was a high tide during a northeast storm, but the rain during the storm did not affect the salinity much. The rainfall and snowfall caused the salinity to be very low. The hypothesis of this project was that the salinity levels in Oyster Pond would remain low. The hypothesis was not supported because salinity increased over the duration of the project. This was due to the northeast storm on 12/30/00 and the clearing out of Trunk River.

    Conclusion

    The purpose of this project was to determine how rainfall and the inflow of seawater affect the salinity measured at the weir in Oyster Pond during the fall and winter of 2000 and 2001. The salt water caused the salinity to go up only when there was a high tide during a northeast storm, but the rain during the storm did not affect the salinity much. The rainfall and snowfall caused the salinity to be very low. The hypothesis of this project was that the salinity levels in Oyster Pond would remain low. The hypothesis was not supported because salinity increased over the duration of the project. This was due to the northeast storm on 12/30/00 and the clearing out of Trunk River.

    Acknowledgements

    I would first like to thank the Oyster Pond Environmental Trust for filling me in on what was going on with the construction at the mouth of Trunk River. Second, I would like to thank my wonderful science teacher, Dr. Meyer, for helping me out in figuring out and correcting this project. Thanks to the Falmouth Water Department for supplying me and another student with the rain data. I also would like to thank Ms. Mullen for permitting the use of her salinity measuring equipment and my parents for the constant support and help needed to get this project done. A special thanks to my father who supplied me with all the knowledge I needed.

    Bibliography

    For more information on this project, read OPET Board Member Robert Livingstone's Report on Emily Tietje's Project


    This page updated July 1, 2004