Lake Mercer Watershed Project
Mentor:  Kathleen Browne

Water is vital to each of our lives and is an invaluable natural resource.  The 1972 Clean Water Act was enacted to restore and maintain the integrity of our nation’s waters.  Its two major goals are to eliminate the discharge of pollutants into water and to achieve water quality levels that are fishable and swimmable.  The CWA provides a framework of standards, technical tools and financial assistance to address the many causes of pollution.  Point source pollution is the easiest to locate and act upon; and has been regulated by the New Jersey Pollutant Discharge Elimination System.  Non-point source pollution is generated in our everyday lives and is carried to streams by precipitation.  It is harder to regulate and has become a bigger problem as agriculture and development increased in the watersheds. 

The state of New Jersey has a watershed-based policy to “restore, maintain and enhance the chemical, physical and biological integrity of its water”. This approach begins with an initial study of the watershed land use, water quality and location of storm water outfalls.  Also, groups and individuals who live and work in the watershed are asked to participate in the watershed management process.  After issues have been identified, a list of potential solutions or Best Management Practices (BMPs) is developed.  These BMPs may be structural in nature, or may simply be a method for educating the public.  The plan is implemented and educational components are presented to schools and to the general public. 

 In the state’s Department of Environmental Protection study of 1998, Assunpink Creek Watershed was determined to be moderately impaired as a result of rapid urbanization of the area.  Studies show that there are numerous  effects of urbanization on run-off water.  These include erosion and flooding, turbidity, temperature change, pollution and increase in nutrients that ultimately may lower the dissolved oxygen available to native species.  According to a 1989 DEP survey, some sources of these problems included loss of natural habitat and its replacement with impenetrable surfaces (pavement, houses, etc.), as well as agricultural land use. 

Our project was part of a larger project underway by Dr. Kathleen Browne, Rider University and Randy Kertes, C.P.G., TRC Omni Environmental Corporation. The project documents the impact of several impoundments on the overall health of Assunpink Watershed.  Their hypothesis is that the installation and configuration of these flood control basins may be contributing to the impairment of the watershed.  Studies have shown that manmade lakes have negative effects on water due mainly to their shallow structure which causes unnatural warming and other problems compared to the stratification of  deep, natural lakes.  Our July, 2001 study involved doing chemical, biological and visual assessments of the water quality and habitats of the Lake Mercer Watershed portion of Assunpink Watershed.  

We chose 11 sites to test along the Assunpink Creek drainage in WMA #11 in New Jersey which makes up the southern boundary of the upper Delaware Water Region.  At the Lake Mercer site, chemical and temperature tests were made at the surface, 50cm, 100cm, 150cm, 200cm, 250cm, and 300cm depths.  We tested for pH, dissolved oxygen, nitrogen-nitrate, phosphate, temperature, zinc, cadmium, and copper.  A hand held multi-parameter  Hach colorimeter was used to measure NO3-N and PO4 from water samples taken at each site. Portable Hach instruments were used to determine the temperature and pH. The dissolved oxygen percent saturation was measured by a YSI 550 DO meter.  A visual assessment of each site was also performed using the EPA guidelines outlined in the USDA Visual Assessment Protocol on water quality assessment with the one exception of the lake site itself. Land use adjacent to each site was determined by using 1995 thermal aerial photographs and USGS topographic maps divided into 7.5 minute quadrangles.

The measurement of  the metals was completed at the Rider University chemistry lab using an ionically-coupled plasma spectrophotometer (ICP). We also performed two benthic assessments; one upstream at site 6 (Mercer Park) and one downstream from Lake Mercer at site 10 (Assunpink Creek at the Quaker Bridge). A kick seine was placed in a riffle to collect the specimens. The specimens were taken back to the biology lab at Rider University and counted and categorized using the Stream Quality Survey for pollution-sensitive species indicator. The macroinvertebrates found were rated by both the "Save Our Streams" BAT (Biological Assessment Team) and Shannon-Wiener Index. Shannon-Wiener Index is a rating scale that compares species biodiversity at two sites by applying the following formula for each site.  Where H is index, pi is the relative abundance of each species, and lnpi is the natural logarithm of the relative abundance. H = - [S(pi)(lnpi)]  

In this study we considered the following factors:

The land use and its effect on water quality and the benthic community; impoundment of Lake Mercer and its effect on water quality and the benthic community; and the BMP’s for maintaining good water quality in the Lake Mercer watershed.

Chemical Assessment:
To assess the chemical condition of the waterways in the Lake Mercer watershed we considered the maximum levels allowable for a freshwater, non-trout category 2 body of water.

For dissolved oxygen, the level should not be below 5.0 ppm at any time, nor should it average less than 6.0 ppm in a period of 24 hours. We had three sites that fell below the standard; the Bridegroom tributary at Highway 130, Bridegroom tributary at Bruntsfield Drive, and the Mercer Park West site. All other dissolved oxygen levels were at or above standards for this type of waterway.

The standard for pH is 6.5-8.5. Again the Bridegroom tributary at both test sites fell below this range. 

The site showing the highest nitrate level was the Assunpink Meadowbrook tributary at 6.3 ppm. The Bridegroom tributary showed a nitrate level of 0.0 ppm at both test sites, which could be due to rapid uptake of nitrogen in the presence of high phosphate concentrations.

The standard for Phosphates is not to exceed 0.05 ppm in any lake, or 0.1 in any stream. The only reading above acceptable levels was the sample from the top of the lake; however, two of the highest stream readings were from the Bridegroom tributary with readings of 0.06 and 0.07 ppm respectively.

Temperatures fluctuations were not unexpected throughout the watershed, given samples were taken on a warm summer day.

In terms of metals, we tested for zinc, copper, and cadmium. Cadmium was found at only two sites, and copper at one. Both readings were just above the detectable level of <3ppb. Zinc was detected at all test sites, and at comparatively high levels at three. Assunpink Meadowbrook tributary had a reading of 34.2 ppb. A highway and a detention basin draining a major subdivision fed into the watershed at this point, therefore these readings would be consistent with one source of zinc contamination.  The bottom of Lake Mercer had a zinc concentration of 18.9 ppb, and the Bridegroom tributary at Meadowbrook had a concentration of 11.8 ppb. Possible sources of these metals include tire wear, moving automobile engine parts, motor oil, and insecticides. Many aquatic species are highly sensitive to increased levels of these metals.

Biological Assessment:
We collected the macro invertebrates at two of our test sites: upstream from Lake Mercer at the Bridegroom tributary on Bruntsfield Drive, and downstream at Assunpink Creek at the Quaker Bridge. From both a BAT assessment and a Shannon- Weiner index, it appears that the biological health of the watershed is impaired following the impoundment. Though there were large numbers of organisms, Caddisflies outnumbered the other species 92 to 1. An overabundance of one species can often indicate an impaired system. However, since the Caddisfly is a pollution sensitive species, without further monitoring, that conclusion cannot be reached for this site.

Visual Assessment:
Using a visual assessment rubric (NWCC) we assessed the area immediately surrounding each test site with the exception of the lake itself. On a scale ranging from <3 being poor to a >9 being excellent, the scores ranged from a 5.1 to a 9.2. The scores varied by land use, and the only correlation is that the lowest score was in an agricultural area (Bridegroom tributary at South Lane) where the stream appeared to have been moved and very little riparian buffer existed. At Assunpink Meadowbrook tributary the low visual assessment of 5.7 can be attributed to human interference. The highest score was at Mercer Park where the Assunpink Creek empties into  Lake Mercer. This is attributed to an obvious effort to keep the area natural.

Lake Mercer

The lake is well mixed as evidenced by the temperature profile graph, which shows the thermocline intersecting the bottom of the lake. The day we tested was windy, perhaps contributing to the very even mixing and solid temperature trend as well as the high and consistent DO levels (10.21-10.66). The pH was much higher in the lake than the either tributary, running from 9.41 at the top to 7.05 at 250 cm. The level of nitrate was 0.0 ppm and the phosphate at the top was 0.06 ppm and the bottom 0.03 ppm. 

In comparing the watersheds of Lake Mercer to Lake Martin, which was a Woodrow Wilson study last year; the Lake Mercer watershed is much less impaired. This may be due to the increased depth of the lake as well as the condition of the riparian buffers along most of its tributaries.

Results indicate that the Lake Mercer impoundment may cause a slight decrease of macroinvertabrate biodiversity.  Lake Mercer seemed to dilute the chemical phosphates, nitrates, and metals.  Lake Mercer replenished dissolved oxygen levels in the lower watershed.

Of the two tributaries that lead into Lake Mercer, the Bridegroom Run seemed to be more impacted by humans due to lack of riparian buffers.  Coupling the lack of buffers with a stream channel that had been diverted seemed to stress the stream water quality in Bridegroom Run.  Assunpink Creek, with strong riparian buffers, appears to be in good health despite the multiple land uses in the area.

Land use alone does not necessarily damage water quality.  A healthy watershed can be maintained by using Best Management Practices.  In this circumstance we recommend a Riparian Forest Buffer be put in place for Bridegroom Run to improve water quality.