An Investigation of Water Quality in Mercer County Lake

Part of the Young Naturalist Awards Curriculum Collection.

by Alex, Grade 9, New Jersey - 2007 YNA Winner

Mercer Lake in central New Jersey is a beautiful habitat for thousands of birds, deer, and other wildlife that live near its shores. When people see this large, 300-acre body of water for the first time, most find it hard to imagine that the lake is actually man-made and not a natural feature of the land that has existed for centuries.

Mercer Lake - West Windsor New Jersey
Mercer Lake - West Windsor New Jersey

Mercer Lake is used for boating regattas, including rowing races among area schools. It is also the home of the United States Rowing Association Olympic Training Center. The lake gets a great deal of use from recreational boaters and fishermen, especially during the summer months, but most of the boats are powered by people, not gasoline. A few boats used by the Park Patrol have small engines, and the local schools that use the lake for training their crew teams also have launches powered by small outboard motors.

Given the natural beauty of Mercer Lake and its surroundings, it's easy to forget that it is situated in a densely populated part of the state. Heavily trafficked roads surround it, and two golf courses are in almost year-round operation along its shores. Catered parties at the Mercer County Park Marina may contribute to pollution in and around the lake, along with many thousands of animals that use the lake as a home and as a deposit for their waste.

Mercer Lake Marina
Mercer Lake Marina

On a hot day last summer, I went to a secluded part of Mercer Lake with my dad and our dog Cinnamon. She is a large Labrador retriever who loves the water and feels at home in this remote part of the lake. We wanted her to play in the water and cool off. As I watched her swimming and chasing the birds, she seemed to be having so much fun, I decided to take my shoes and socks off and do a bit of wading myself. Soon after I entered the water, a park ranger came by in a boat to tell us that no wading or swimming was allowed. I quickly walked ashore, but I got to thinking about the reasons why the park ranger wouldn't give me a break on what was one of the hottest days of the year. Was it because there were no lifeguards around? Was the lake clean enough for humans to enter? It certainly didn't seem to bother our dog, but maybe it was polluted and not very safe. I decided I would do my own investigation and find out more about the water in Mercer Lake.


I used the following in my investigation of Mercer County Lake:

  1. phosphorous water test kit
  2. nitrate water test kit
  3. pH test strips
  4. oxygen test kit
  5. pipettes
  6. four plastic bottles
  7. camera

Map of Mercer Lake Collection Areas
Map of Mercer Lake Collection Areas

Key Question to be Answered and Methodology
For the purpose of evaluating the lake's conditions and cleanliness, I decided I would answer a simple question through scientific observation: How safe is the water in Mercer Lake as an occasional play area for local family dogs, including mine? (Safe meaning being secure from a danger or risk of any complications.)

To answer this question, I decided to do an evaluation of water samples over a period of five weeks taken from four different parts of the lake, as follows:

  1. The first area I focused on is near the crew boat house and Olympic Training Center on the north side of the lake, where there are small, gas-powered launches in regular use;
  2. I next focused on the marina area on the south side of the lake, which is in active use for parties and community events;
  3. The third area of focus was in small inlet that lies along Mercer Oaks Golf Course on the north side of the lake; and
  4. A secluded area of the lake on the north side between the Olympic Training Center and the golf courses.

I felt that these four areas should provide a varying range of how the lake is used and would allow me to assess potential reductions in the lake's water quality.


The impacts I expected to find in each region are shown below:

Lake Area Hypothetical Impact(s)
#1 crew boathouse Expected to find high levels of phosphorous and nitrate in the lake from the many small, gas-powered boats used in this area.
#2 marina on south side Expected to find lower oxygen levels in the lake than usual and a pH level that is more acidic (below 7), due to pollution from trash and human activity.
#3 small inlet by golf courses Knowing that the golf courses use large amounts of pesticides and herbicides to keep their fairways green, I expected to find unusual levels of oxygen and nitrate from use of pesticides and herbicides in the lake near Mercer Oaks. Also, I would expect a pH that would indicate that the water has impurities.
#4 secluded area between golf courses and the crew boathouse Knowing that this water is isolated from the rest of the lake and in minimal use by people, I expected to find a pH level of 7 and adequate levels of phosphorous, nitrate, and oxygen. In other words, I expected this part of the lake to have the least amount of impurities.

Test Results Prove Inconclusive
I took water samples from the four different parts of the lake on five successive weekends beginning November 4 and December 2. I evaluated pH, nitrate, phosphorous, and oxygen in each sample (four criteria). The results of my evaluations are summarized on the following data table.

Test Results
South Side of Lake Near Marina

Despite taking 20 water samples from Mercer Lake during the five weeks and evaluating each sample on four criteria, I was unable to determine conclusively whether the lake's water quality is safe enough for my dog to swim in (and occasionally take some drinks of the water).

  • pH Data - The pH evaluation did not provide a clear indication of possible impurities and pollutants in Mercer Lake. The levels of pH among the different samples were mostly 6, with a few outliers of 5. In fact, these samples were not significantly different from the pH of distilled water.
  • Nitrate Data - The nitrate data also did not produce clear results on possible impurities and pollutants in Mercer Lake. Distilled water has a nitrate level of 0.5 mg/l. In the data I collected, some of the nitrate levels actually matched that of distilled water. There were also a number of readings below the 0.5 mg/l level, which tend to suggest limited amounts of gasoline and pesticide pollution in the water.
  • Phosphorous Data - The phosphorous data also did not produce clear results about the possible impurities and pollutants in Mercer Lake. Most of the samples showed that phosphorous levels were depleted or deficient, slightly below the level of distilled water. These levels of phosphorous indicate that there is little gasoline in the water.
  • Oxygen - The oxygen data also did not produce clear results on possible impurities and pollutants in Mercer Lake. Average levels of oxygen in distilled water were 6-8 parts per million. My samples showed that oxygen levels were lowest by the secluded area of the lake (3 ppm), and best at the crew boathouse and marina (6 ppm).

Boats on Mercer Lake
Test results.

Explanation of My Results
Based on the water samples I took over the five weeks in the fall, there is little data to suggest that Mercer Lake has a significant amount of impurities that would make it harmful or unsuitable for my dog to play and swim in. Some possible reasons for my results are as follows:

  1. The fall of 2006 was an extremely rainy period in central New Jersey, which meant Mercer Lake was replenished with large amounts of fresh water over a period of weeks. This could have caused a diluting effect in the water, resulting in a cleaner-than-normal period. However, with more rain there may have been more runoff from the land, and more chemicals may have made it into the water. This could have resulted in a much more polluted lake than normal.
  2. The secluded area of the lake I tested is largely comprised of stagnant (non-moving) water that does not get recycled as rain and is not replenished by streams flowing in and out of the lake. This helps explain the lower oxygen content in the water. Other parts of the lake do benefit from these moving waters, which could help reduce the effects of impurities and pollutants.
  3. Mercer Lake, as already noted, does have a limited number of powerboats. The small engines in use and the number of craft that are gasoline-powered do not produce a high-enough level of pollutants to affect the conditions in this 300-acre lake. It is a credit to the Mercer County Park & Planning Commission that powerboating on the lake is restricted. This clearly seems to be having a positive result, based on my sampling.
  4. The Mercer Oaks golf course has state-of-the-art methods for controlling weeds and insects that do not rely on large amounts of chemicals that could harm the lake. Many golf courses are adopting best practices in their use of pesticides, which can harm neighboring environments and habitats. While I do not know Mercer Oaks' specific policies, I do know that golf courses around New Jersey are focusing on such activities as environmental planning, managing the habitats of wildlife, reducing the use of chemicals, and conserving water. As a county course situated on Mercer Lake, Mercer Oaks appears to be working in conjunction with the community to minimize pollution.
  5. At the marina there are ample trash receptacles, as well as signs encouraging their use and warning people not to pollute the lake. These signs and the management of the marina and the lake are having a positive effect on reducing pollutants and impurities from entering the lake.

Possible Changes to Procedure and Questions for Future Research
A preliminary conclusion based on my results is that the water in Mercer Lake is not harmful to dogs like my Labrador retriever, who has been swimming in the lake a couple of times a month for several years, with no obvious ill effects. This could be due to the factors described above, especially the rainy weather we have been having; a period of dry weather could significantly alter the test results. Also, more gasoline-powered boats and people use the lake, and more pesticide treatments are applied to the golf courses, in the summer rather than the late fall. Therefore, test results in the summer might be different from the ones I found.

South Side of Lake Near Marina
Boats on Mercer Lake

In addition, the lake may still be harmful to dogs or harmful to humans, but the data I was able to produce does not confirm this hypothesis. Additional testing would be needed to confirm the conditions of the lake, such as tests on the microscopic level. I would take the water samples from the lake and put them under a microscope, searching for harmful bacteria. Until these tests are completed, the data at this point are still inconclusive.

As for Mercer County's policy of no swimming in the lake, I recently learned from the county's naturalist that a number of farms and roads drain into the lake, and the runoff from these could impact water quality. He explained that swimming is not allowed because of the varying water quality, the cost of building a bathhouse and having lifeguards on duty, and concerns over potential liability if someone got sick from swimming in the water. This information is consistent with my findings and conclusion that the lake's water quality probably varies at different times of the year.



Hamilton Township, Mercer County, New Jersey. Wikipedia. Retrieved from the World Wide Web on 9 December 2006.,_Mercer_County,_New_Jersey.

Recreational Facilities. Mercer County. Retrieved from the World Wide Web on 9 December 2006.

Rubin, Ken. Reply to Ask-an-Earth-Scientist. Sources of Water Pollution. Retrieved from the World Wide Web on 19 October 2006.