Investigating the Saw Mill River

New York City is indisputably an urban environment. As such, urban runoff is a leading cause of impairment to its rivers and streams.   Large cities have high percentages of impervious surfaces such as roads and parking lots.  
As rain falls on these surfaces, it does not percolate to the subsurface, and instead runs off the land, carrying contamination along with it (USEPA 2000).  Pollutants usually associated with urban runoff include nutrients (nitrogen and phosphorous), metals (e.g. cadmium, copper, lead and zinc), and coliform bacteria (USEPA 2005).  The Civil and Environmental Engineering Department has been investigating the effect of urban runoff on the water quality of two local water-bodies:  Tibbetts Brook and the Saw Mill River.

For each of the past three years, an assessment of the water quality of Tibbetts Brook has been the final project for the ENGS 115 Environmental Module. Tibbetts Brook is a small freshwater stream that travels from Yonkers into the Bronx through marshlands in the northern end of Van Cortlandt Park.  The brook discharges into Van Cortlandt Lake at the southern end of the park, and the lake outflow is conveyed underground to the Harlem River.   Each year, the ENGS 115 class has designed a water quality sampling plan for the stream, learned proper water quality sampling and laboratory techniques, and analyzed the data to assess whether Tibbetts Brook and Van Cortlandt Lake are impaired due to excessive bacteria, nutrients or metals concentrations.  The project culminates in a final presentation that is made to faculty and students within the CEEN department.   On occasion, members of the NYC Parks Department have also attended planning meetings and the final presentations.  If anyone is interested in hearing about the water quality of Tibbetts Brook or Van Cortlandt Lake, talk to the undergraduate students who have been involved with this project for the past few years.

The Saw Mill River is a tributary to the Hudson River in the Lower Hudson River Drainage Basin.  The River has several designated uses over its entire stretch, indicating diverse surroundings as it travels from Chappaqua to Yonkers, NY where it eventually discharges into the Hudson River.  Additionally, along the Yonkers stretch, the river has been severely altered over the past 30 years by flood control projects and re-routing (Pearce 1999).  The most extreme case of this is in south Yonkers, where the river has a concrete bottom and eventually flows underground for its final 800 feet before reaching the Hudson.   According to USGS estimates (Wall, Riva-Murray et al. 1998), the Saw Mill River watershed of 23.8 mi2 is 63.4% urban, 35.4% forested, and 1.0% agricultural.  This high percentage of urban areas makes the Saw Mill susceptible to contamination from urban runoff.
 
In the summer of 2006, a year-long continuous monitoring program was implemented for the Saw Mill River.  External funding was provided by the New York State Water Resources Institute.  Undergraduates Jason Lumish and Erica Hanley from the CEEN Department took the lead on this work, working full-time for 10 weeks during the summer with additional time spent during the semester.  This work also included partnerships with Groundwork Yonkers and Saunders Trade and Technical High School in Yonkers.  Groundwork Yonkers is a non-profit organization developed in 1999 dedicated to revitalizing, greening, and connecting people to the urban environment in lower Westchester County.  Groundwork Yonkers is the coordinator of the Saw Mill River Coalition, a partnership of non-profit groups, government agencies, and businesses, aimed to revitalize and protect the Saw Mill watershed.   The Coalition played a vital role in disseminating the results of our study to local stakeholders in the Saw Mill River watershed.  Two students from Saunders Trade and Technical High School in Yonkers, Nicole Kerrison and Leslie Guadron, were involved in field sampling and laboratory analysis of water quality parameters through a paid summer internship.   Nicole and Leslie worked in the CEEN laboratories three days a week for eight weeks.   After the conclusion of her work on the project, Nicole continued working in the CEEN labs as part of her senior project.

12 sampling sites along the Saw Mill River (Figure 1) were selected for monitoring.  The following water quality parameters related to urban runoff were measured at each site: nutrients (ammonia, nitrate and total phosphorous) and fecal coliform bacteria.  Temperature, conductivity, pH, total suspended solids and turbidity were also measured.   As an illustration of the findings of this study, the results of fecal coliform bacteria are presented below.  The complete dataset will be made available on the Saw Mill River Coalition website (http://www.sawmillrivercoalition.com).  

Fecal Coliform Bacteria
Total coliform bacteria are a collection of relatively harmless microorganisms that reside in the intestines of warm and cold-blooded animals.  The fecal coliform bacteria are a specific subgroup of the total coliform bacteria, which are associated only with the fecal material of warm-blooded animals.  The presence of fecal coliform bacteria in aquatic environments indicates that the water has been contaminated with fecal material.   Thus, water high fecal coliform bacteria counts may have been contaminated by pathogens which can also exist in fecal material.  Examples of waterborne pathogenic diseases include typhoid fever, viral and bacterial gastroenteritis, and hepatitis A.   The presence of fecal coliform bacteria is an indicator that a potential health risk exists for individuals exposed to this water.   Elevated levels of fecal coliform bacteria occur in surface waters as a result of the overflow of domestic sewage or nonpoint sources of human and animal waste.

Monitoring results for fecal coliform bacteria are presented in Figures 2 and 3.  In Figure 1, a box plot is used to show spatial variability in fecal coliform bacteria concentrations along the entire stretch of the Saw Mill River.   The boundary of the box closest to a value of zero on the y-axis indicates the 25th percentile, a line within the box marks the median, and the boundary of the box farthest from zero indicates the 75th percentile.  The error bars above and below the box indicate the 90th and 10th percentiles, respectively.   Sample sites S11 (Torre Pl., Yonkers, NY) and S12 (Walsh Rd., Yonkers, NY) had the highest median fecal coliform counts.  These two sites are the southern-most sites sampled, and are in a highly urbanized area of downtown Yonkers.  Median fecal coliforms of all other sites were extremely constant.   Sample site S1 (Chappaqua Metro North Station), does not have a significantly different median than sites S2 - S10, however, the highest single fecal coliform measurements were for this site (1.2×105 and 8.4×104 org/100 mL). 

In Figure 3, fecal coliform bacteria is shown as a function of time at sample stations S1, S4, S8 and S12.  Also indicated on the plot is the daily precipitation recorded at Westchester County Airport.  An increase in fecal coliform bacteria seems to be associated with rainfall events.  This can be seen in the plots by examining data points that fall in or slightly to the right of significant rainfall events (more than 1.0 inch/day).  These data points are consistently higher than baseline for all sample sites.
 
All median fecal coliform bacteria values were above 200 org./100 mL.  This is significant because the NYS DEC criteria (1999) for fecal colifoms states that the monthly geometric mean, from a minimum of five examinations, shall not exceed 200 org./100 mL.   Bacterial contamination can originate from point or nonpoint sources.  Point sources may include municipal or Industrial discharges of wastewater.   Nonpoint sources may include storm water runoff, animal waste, application of manure and biosolids to fields, crop irrigation from contaminated storage ponds, failed septic systems, litter or landfill leakage, or direct discharge of marine-craft sewage.   For the Saw Mill River watershed, stormwater runoff, and municipal wastewater discharges are likely causes of the observed high fecal coliform bacteria counts.

Typical pollutant concentrations found in typical urban storm water runoff are on the order of 3600 org./100 mL (MDE 1999).  While fecal coliform bacteria are subject to inactivation upon direct exposure to sunlight, portions of the Saw Mill River are under canopy and rapid die-off is unlikely.   It is therefore possible that the high baseline levels of fecal coliform bacteria are the due directly to urban run-off.

Wastewater generated from much of the area of the Saw Mill River is treated by only the Yonkers Wastewater Treatment plant (Mulligan, Buroughs et al. 2005).   This plant serves a population of half a million people, a total area of 108 mi2 and receives an average daily flow of 96 million gallons per day (Mulligan, Buroughs et al. 2005).  Thus, wastewater generated in the upper Saw Mill River watershed travels south through county trunk lines until it reaches the Yonkers WWTP.  While the recorded values at site S1 may seem extremely high (≈105 org/100mL), they are significant less than levels commonly found in raw sewage and sewer overflows.  Typical fecal coliform bacteria concentrations in raw sewage are on the order of 107 org/100 mL (Thomann and Mueller 1987).   A 1:100 dilution of raw sewage therefore puts it into the range of our observed values.   Coliform bacteria in combined sewer overflows range from 105 to 106 org./100 mL (Thomann and Mueller 1987).   Sewer overflows occurring during wet weather in the upper watershed may therefore be responsible for high fecal coliform bacteria counts at site S1.

For the two sites in Yonkers (S11 and S12), the source of the high coliform levels do not appear to be weather related.  The surrounding area is highly urbanized and has the largest density of impervious services.   Thus, stormwater runoff may be a contributing a larger load in this area than at the more sub-urban upstream sites.  In addition, sewage discharges from homes or businesses tied to storm sewers may be present in this older section of Yonkers.   Further investigation is required if such connections exist.

Works Cited

1. New York State Department of Environmental Conservation (1999). Surface Water and Groundwater Quality Standards and Groundwater Effluent Limitations. 6 NYCRR Part 703.

2. Maryland Department of the Environment (1999). Maryland Stormwater Design Manual. Annapolis, MD, Maryland Department of the Environment.

3. Mulligan, G. E., E. Buroughs, et al. (2005). Databook Westchester County. White Plains, NY, Westchester County Department of Planning.

4. Pearce, W. H. (1999). Saw Mill River Basin, New York.  Reconnaissance study for flood control & ecosystem restoration.  Section 905(b) (WDRA 86) preliminary analysis, US Army Core of Engineers: 27.

5. Thomann, R. V. and J. A. Mueller (1987). Principles of Surface Water Modeling and Control. New York, NY, Harper Collins.

6. USEPA (2000). National Water Quality Inventory, Office of Water, U.S. Environmental Protection Agency.

7. USEPA (2005). National Management Measures to Control Nonpoint Source Pollution from Urban Areas. Washington, D.C., United States Environmental Protection Agency, Office of Water.

8. Wall, G. R., K. Riva-Murray, et al. (1998). Water Quality in the Hudson River Basin, New York and Adjacent States, 1992-1995. U. U.S. Department of the Interior, USGS