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Impact Study of Grovers Mill Pond and Dam Reconstruction on Big Bear Brook



Grovers Mill Pond

The neighborhood I live in has a special claim to fame as the place in which Martians landed in Orson Welles' famous radio broadcast War of the Worlds. As our local newspaper put it, "Thanks to Orson Welles, Grovers Mill may be the most famous neighborhood in all of central New Jersey" (Trenton Times, May 9, 2004). Grovers Mill Pond and Big Bear Brook make up the watershed that greeted these strange visitors. In the 1800s early settlers to the area built a 400-foot-long earthen dam to trap Big Bear Brook for their grinding mill. Over the years, silt and sediment have spilled into the pond from nearby farms, creating an over-vegetated ecosystem resembling thick green muck (McCrone, 2004). This eutrophication has greatly reduced the amount of dissolved oxygen available to the aquatic organisms in the pond (Greenwood et al, 356). In addition, the dam has become hazardous, with hundreds of trees growing from its earth foundation. This past summer the rehabilitation of Grovers Mill Pond and Dam got underway. Thus far, the pond has been drained but not yet dredged, trees and shrubs have been removed, and efforts are presently underway to reconstruct the old earthen dam.

Insofar as Grovers Mill Pond flows into Big Bear Brook at my study site, I wondered what impact the reconstruction might have on Big Bear Brook.


Big Bear Brook

Walking along Big Bear Brook, one sees an idyllic scene of lush vegetation that includes ferns, vines, and a velvet carpet of green moss. The area is rich in trees, including evergreens, maple, and oak. The brook runs through a forested wetland with random expanses of surface water and mushrooms sprouting in the damp soil. Among the wildlife enjoying the landscape are a resident great blue heron, squirrels, mallards, white-tailed deer, a variety of birds, and dragonflies. Looking into the one-foot-deep brook, one sees leaves, stones, and duckweed.


Litter in Big Bear Brook

Sadly, upon closer inspection there is evidence of pesky "litterbugs." Litter in the form of candy wrappers, empty soda and beer cans, yogurt containers and even a tire, mars the landscape. The general area smells mildly of decay. I believe the odor is caused by decaying plants because I didn't see visible evidence of sewage or waste from agricultural or domestic uses. There are homes near the brook, but no septic tanks.

Along the banks of the brook is a good deal of moss and a small amount of sedge. The vegetation thickens as one walks away from the pond, further along the brook. McCormack, in her interesting book, Vanishing Wetlands, explains that if wetlands are not overburdened with effluence they can act as pollution filter systems for groundwater and streams (McCormack, 1995, p. 46). Lisowski and Williams elaborate: "Wetland plants can absorb and break down impurities, preventing pollutants from contaminating water. In addition, the roots and stems of wetland plants retain silt and sediment, preventing pollutants from traveling farther downstream" (Lisowski and Williams, 1997, pp. 14-15). I wondered what role this wetland plays in the water quality of Big Bear Brook as it travels downstream. I hypothesized that as the water traveled down Big Bear Brook, away from Grovers Mill Pond, the increasing vegetation along the banks would purify the water and indicators of water quality would change positively.


Reconstruction at study site

I decided to collect water samples at five different points along the brook and do a variety of tests to learn more about the quality of the water. As Greenwood, Allan, and Shepard note, "There is no single measure to objectively describe the quality of a stream... Rather, it is defined in terms of various chemical, physical, and biological characteristics. Together, these factors define the 'health' of the aquatic ecosystem..." (Greenwood et al, 2004, p. 358). I tested for benthic macroinvertebrates, total coliform bacteria, dissolved oxygen, nitrates, pH, phosphates, temperature, and turbidity, using the LaMotte Green Water Monitoring Kit. In addition, I contacted Steven E. Yergeau, restoration specialist at the Stony Brook-Millstone Watershed Association, who was kind enough to send me raw data collected since the 1990s on Big Bear Brook. This data would serve as a basis for comparison with my findings to see how the restoration project at Grovers Mill Pond and Dam might be impacting the brook. The raw data from the previous year would be my control. I hypothesized that in the short term the restoration project would negatively impact the quality of water in Big Bear Brook because of the potential stresses that can be experienced during reconstruction. Because the quality of any aquatic habitat is the product of the chemical, biological, and physical conditions of a stream and its watershed, if the hydrology of the watershed is disrupted, it can result in changed concentrations of suspended solids or biostimulatory nutrients such as nitrogen or phosphorus, potentially affecting aquatic life (United States Department of the Interior, Fish and Wildlife Service, 1988 and California Environmental Protection Agency, 2004). Over the long term, I hypothesized that Big Bear Brook would benefit from the rehabilitation of Grovers Mill Pond and Dam because it is expected that the water quality of Grovers Mill Pond will be greatly improved (Snedeker, 2004).


Map of water sampling sites

I gathered the following data from Big Bear Brook in October of this year. I chose to demarcate my study site as the watershed area bounded by Grovers Mill Pond and Mercer County Bridge No. 762.1. I took water samples from the five sites identified in the diagram, as drawn in my field journal. (See the Appendix at the end of the essay for sampling procedures.)

Big Bear Brook Water Sample Results (Click to enlarge)

The data table "Big Bear Brook Water Sample Results" documents my recent findings, as well as those of the Stony Brook-Millstone Watershed Association for October 2003. I chose to compare their raw data from the same month of the previous year because seasonal fluctuation (temperature and precipitation, for example) can affect water samples. Aquatic organisms do not remain stable throughout the year (Lisowski and Williams, 1997). I hoped to lessen the impact of seasonal variation by comparing water samples collected in the early fall. Importantly, I would be able to compare the water quality of Big Bear Brook before the early reconstruction phase of Grovers Mill Pond and Dam with my own more recent findings.

My findings at the various sampling sites were fairly uniform, with the exception of turbidity. The fact that the water quality overall did not change significantly in the farthest samples from the dam disproved my hypothesis that the increasing vegetation along the banks would act to purify the water as it moved downstream. I believe that the increased clarity of the water as it moved downstream was entirely unrelated to the increasing vegetation but rather was a reflection of movement away from the problem of erosion at the dam site. The absence of water-purifying plants such as cattails, reeds, rushes, and water lilies further served to underscore my conclusion.


I tested the water samples for the following indicators of water quality, using the LaMott Green Water Monitoring Kit and Reference Guide (2003):

Nitrates — Nitrogen is a nutrient that acts a fertilizer for aquatic plants. When nutrient levels are high, excessive plant and algae growth creates water-quality problems, as occurred in Grovers Mill Pond. Nitrogen enters the water from human and animal waste, decomposing organic matter, and runoff of fertilizer from lawns and crops. Unpolluted waters usually have a nitrate level below 4 parts per million (ppm). The average nitrate level in my different sampling sites is 2.2 ppm.

Phosphates — Like nitrogen, phosphorus is a nutrient that acts as a fertilizer for aquatic plants. Once again, when levels are high, excessive plant and algae growth creates water-quality problems. Over half of the phosphate found in streams comes from detergents. Fertilizer runoff is another key source. Phosphate levels higher than 0.03 ppm contribute to increased plant growth. The average phosphate level in my different sampling sites is 2 ppm.

pH — The pH scale ranges from 0 to 14 and is a measure of the activity of hydrogen ions. Water samples with a pH below 7 are acidic; those above are basic, with 7 considered neutral. A pH range of 6.5 to 8.2 is optimal for most organisms. Most natural waters have pH values from 5 to 8.5. The average pH level in my different sampling sites is 6.5.

Dissolved oxygen — Fish, invertebrates, plants, and aerobic bacteria all require oxygen for respiration. The amount of oxygen needed varies according to species and stage of life. Dissolved oxygen levels below 3 ppm are stressful to most aquatic organisms. Dissolved oxygen levels below 2 or 1 ppm will not support fish. Levels of 5 to 6 ppm are usually required for growth and activity. The average dissolved oxygen level in my different sampling sites is 5.7 ppm.

Turbidity — Turbid water is caused by suspended matter. It may be the result of soil erosion, urban runoff, algal blooms, and/or bottom sediment disturbances, which can be caused by abundant bottom feeders. The lower the turbidity score, the better. The average turbidity in my different sampling sites is 8 ppm. But in this case, a statistical average can be misleading because the higher turbidity scores are only found at the sampling sites closest to the eroded banks of the dam undergoing reconstruction. The water samples become quite clear as the water moves downstream. It is important to note that turbidity measurements are taken in increments of "20" in my testing kit. Therefore, even the findings at the dam site are not significantly high. According to Richard Snedeker, chairman of the task force on the Grovers Mill Restoration Project, one of the immediate beneficial impacts of the dam reconstruction is going to be "the reduction or elimination of the effects of erosion of the dam's downstream embankment on Big Bear Brook" (Snedeker, 2004).

Temperature — Temperature affects the amount of dissolved oxygen, the rate of photosynthesis, and the sensitivity of organisms to toxic wastes.

Coliform bacteria — Fecal coliform bacteria are rare or absent in unpolluted waters. Its presence may indicate fecal contamination. I found the presence of coliform bacteria in all of my samples. There are mallards, heron, squirrels and white-tail deer in the area.

Benthic macroinvertebrates — Most benthic macroinvertebrates found in streams are the immature aquatic stages of insects. They live on submerged material and are often sensitive to changes in their environment. The presence of these organisms in a stream and the diversity of species present indicate the overall ecological health of the water: "A stream is only as healthy as its bugs" (Why Study the Stream-Bottom Macroinvertebrates? 2004). Water boatmen dominated my samples and are pollution tolerant. However, water pennies were also present, and they are generally sensitive to pollution.

Benthic Macroinvertebrates

Benthic Macroinvertebrates found in Big Bear Brook


Several curious differences emerge when comparing the data collected in October 2003 and October 2004 concerning the benthic macroinvertebrate populations and the phosphate levels, specifically. I did not find evidence of pollution-sensitive mayflies, caddisflies, or stoneflies in my samples, and my phosphate levels were somewhat higher than those of the previous year. I wondered if the differences could be due to the impact of the restoration project, as I predicted in my hypothesis, or were simply a matter of different sampling techniques or sampling locations. I contacted Steve Yergeau, restoration specialist at the Stony-Brook Millstone Watershed Association, to think this through. He agreed that the discrepancies regarding these two indicators were most likely due to differences in sampling sites and equipment, rendering them insignificant.


Reconstruction at study site

Consequently, my data overall is similar to the raw data collected one year ago by the Stony Brook-Millstone Watershed Association. The indicators of water quality, such as nitrate levels, the amount of dissolved oxygen, the pH total, temperature, and turbidity readings, were comparable. Therefore, my hypothesis that the reconstruction would negatively affect the water quality of Big Bear Brook in the short term appears to be invalid. There is no significant difference in the data collected after the reconstruction began and the earlier data. However, insofar as the reconstruction of the Grovers Mill Dam and Pond is only in the early stages, I am unable to say with certainty what the impact of the reconstruction will ultimately have on the brook. I plan to continue sampling periodically until the project is complete, and again one final time after its completion. I hypothesize that once the restoration is complete, the watershed that makes up Grovers Mill will be positively impacted and prove an inviting place for recreational enjoyment—and maybe even another Martian landing.




Sampling Procedures (Using LaMotte Green Water Monitoring Kit):

Nitrate, Phosphate, and pH:

1. Fill test tube with 5 mL of brook water (10mL for pH).
2. Add one Phosporus/Nitrate/pH TesTab.
3. Cap & mix until tablet disintegrates.
3a. (Nitrate only) Add second TesTab and mix.
4. Wait five minutes. (Nitrate and Phosphate only)
5. Compare color of sample to color chart and record results in ppm.

Dissolved Oxygen:

1. Fill small test tube to overflowing with sample water.
2. Add two Dissolved Oxygen TesTabs to the test tube.
3. Cap tube. Be sure no air bubbles are in the sample.
4. Mix by inverting until tablets have disintegrated (about four minutes).
5. Wait five minutes.
6. Compare color of sample to Dissolved Oxygen Color Chart. Record the result as ppm of Dissolved Oxygen.


1. Fill turbidity tube to line.
2. Place base of tube on outline of Turbidity Chart.
3. Use chart to determine turbidity in JTU.


1. Place thermometer four inches below surface for one minute.
2. Remove thermometer, read temperature, and record the results in degrees Celsius.

Coliform Bacteria:

1. Fill the tube to 10mL line.
2. Stand capped tube upright, with test tablet flat on the bottom of the tube.
3. Incubate tube upright, at room temperature, for 48 hours. Store out of direct sunlight.
4. Compare contents to chart and record results as positive or negative.

Benthic Macroinvertebrates:

1. Approach submerged leaves from downstream and scoop up leaves, water, and mud without disturbing macroinvertebrates.
2. Spoon water samples onto Petri dishes and examine with hand lens.
3. Identify macroinvertebrates using pollution-tolerance identification chart.



Fredrickson, Leigh H., and Frederic A. Reid. "Preliminary Considerations for Manipulating Vegetation." Waterfowl Management Handbook. Leaflet 13. Washington D.C.: United States Department of the Interior Fish and Wildlife Service, 1988. Retrieved 13 November 2004 from the World Wide Web.

Global Rivers Environmental Education Network, ed. Green Water Monitoring Kit Guide. 2003.

Greenwood, Tracey, Richard Allan, and Lyn Shepherd. "Water Pollution and Monitoring Water Quality." Senior Biology 1 Student Resource and Activity Manual. 2005 ed. Hamilton, New Zealand: Biozone International, 2004.

Hart, John. "Water Pollution." Microsoft Encarta. 2004 ed. DVD.

Lisowski, Marylin, and Robert A. Williams. Exploring Ecosystems: Wetlands. New York: Franklin Watts (Grolier), 1997.

McCormick, Anita Louise. Vanishing Wetlands. San Diego: Lucent Books, Inc., 1995.

McCrone, Brian X. "Dam, Not Martians, Is Worry at Grovers Mill." Trenton Times 9 May 2004. Retrieved 14 November 2004 from the World Wide Web.

Snedeker, Richard. E-mails to Eitan Paul. 13 and 14 November 2004. The chairman of the task force on Grovers Mill Pond and Dam restoration provided valuable information.

"Water Boatman." Retrieved 4 November 2004 from the World Wide Web.

"Water Penny Beetles." Biological Indicators of Watershed Health. U.S. Environmental Protection Agency. Retrieved 5 November 2004 from the World Wide Web.

"Water Quality/Beneficial Use Stressors Table." California Environmental Protection Agency. Retrieved 14 November 2004 from the World Wide Web.

"Why Study the Stream-Bottom Macroinvertebrates?"  The Stream Study . University of Virginia. Retrieved 5 November 2004 from the World Wide Web.

Yergeau, Steven E. E-mails to Eitan Paul. 2 and 16 November 2004. Mr. Yergeau, restoration specialist at Stony Brook-Millstone Watershed Association, sent raw data collected by the association's biological and chemical analysis teams on Big Bear Brook and answered questions of import to this study.


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