The water quality study measures salinity, oxygen, temperature, suspended sediment, chlorophyll, and the sewage indicating bacterium Enteroccocus.
We use a lab equipped with the IDEXX Enterolert system to test for Enterococcus. Our water quality scoring is based on EPA’s Federal Recreational Water Quality Criteria for safe primary contact (swimming and immersion).
Water from about 30 cm below the surface is continuously pumped over the side and then past an array of sensors on a Hydrolab DS5 data sonde. As the water flows past the sensors, the Hydrolab records temperature, salinity, and turbidity, as well as the oxygen and chlorophyll concentrations. The Hydrolab is interfaced with a GPS unit so that the time and location of each measurement (in latitude and longitude) are also recorded.
Enterococcus is a fecal indicating bacteria that lives in the intestines of humans and other warm-blooded animals. Enterococcus (“Entero”) counts are useful as a water quality indicator due to their abundance in human sewage, correlation with many human pathogens and low abundance in sewage free environments. The United States Environmental Protection Agency (EPA) reports Entero counts as colonies (or cells) per 100 ml of water.
Enterococcus is a genus of Gram positive, facultative anaerobic, a lactic acid bacterium. [Data from 2006-2007 were collected using a plate count described in EPA method 1600. Data from 2008 and later are based on the EPA approved IDEXX Enterolert method.]
There are multiple factors that determine public health risk to people who have primary contact with water, such as swimmers. Exposure to fecal contamination is only one aspect of these risks.
We have based our assessment of acceptable water quality on the 2012 federal Recreational Water Quality Criteria from the US EPA. Unacceptable water is based on an illness rate of 32 per 1,000 swimmers (3.2%).
EPA recommends public notification and possible temporary beach closure for single Entero samples above 60 cells/100 mL. Samples testing above this threshold appear in red on this site, while those below it appear in green.
To avoid exposure to chronic contamination, the geometric mean, a weighted 30-day average, should not exceed 30 cells/100 mL. To avoid exposure to occasional high levels of contamination, no more than 10% of samples should exceed 110 cells/100 mL. Efforts should be made to reduce pollution in recreational waters that exceed these long term measures of water quality.
Water temperature is one of the most basic and most important things we measure during our sampling trips. For example, the activities and metabolic rates of many organisms in the river, from bacteria to fish, are closely connected to water temperature. In general, water temperature in the river changes seasonally, warming through the summer and then cooling as winter approaches. However, surface water temperature can change on small scales in the river. For example, because deeper waters are generally cooler than surface waters, surface temperature can fall if deeper waters are mixed up to the surface. In addition, effluents from industrial facilities, power plants, and sewage treatment plants often are often a different temperature than the rest of the river.
Estuaries like the Hudson are where freshwaters in rivers meet the sea. Salinity tells us about the relative contribution of fresh and salt water within a particular region of the estuary. Salinity generally increases towards the mouth of the estuary, although salinity at any location can change with the tide. Heavy rains will push freshwater further down the estuary, while dry weather will allow salty water to penetrate further north. Many aquatic organisms have a preferred range of salinities where they can best survive.
During our sampling trips, salinity is measured in parts per thousand (ppt). Ocean salinity is about 34 ppt and the freshwaters of the upper Hudson River are close to 0 ppt.
Turbidity is a measure of the amount of fine particles suspended in the water. As turbidity rises, light penetration, a key factor for aquatic plants and algae, diminishes. Particles can be the sites of intense microbial activity and can play a role in the transport of contaminants. In addition, areas of heavy runoff and erosion often have high particle concentration. In general, turbidity in the Hudson River increases in the lower estuary, reaches a maximum around upper Manhattan, and then falls as one goes further south.
During our sampling trips, turbidity is measured as nephelometric turbidity units (NTU). Typical values range from close to zero (not turbid) to several hundred (very turbid).
Oxygen concentrations in the water are closely connected to biological activities. When aquatic plants photosynthesize, they release oxygen into the water. Meanwhile, bacteria and other organisms use up oxygen in their metabolic activities. Higher animals, such as fish and shellfish, need well-oxygenated water to survive. The absolute amount of oxygen that can be dissolved in water is dependent on the temperature and salinity. We therefore record oxygen concentration at different locations as the % concentration relative to full saturation. In aquatic systems in general, surface waters are typically in equilibrium with the atmosphere such that the oxygen concentration near the surface is close to fully saturated (100%).
Because of intense microbial activity, oxygen concentrations in the surface waters of the Hudson are highly variable. We sometimes find areas of the river where the surface oxygen concentrations are too low to support fish or other higher animals (much less than 100%). We also find other locations where the surface waters are supersaturated (more than 100%) because of high photosynthesis from algal blooms.
Chlorophyll is the green pigment that plants use for photosynthesis. Just like plants on land, microscopic algae in the water also contain chlorophyll. Thus, the chlorophyll concentration is a measure of the amount of microscopic algae present in the water. These algae are food for larger organisms and also produce oxygen that many other organisms need to survive.
During our sampling trips, we measure chlorophyll as relative units. Water with no suspended chlorophyll will read zero. Much of the open waters of the river read about 1- 2 units, and in areas with intense algal blooms the reading could exceed 20.
Sewage is just one of the pollutants found in the Hudson River Estuary. Other pollutants include PCBs, radioactive contaminants such as tritium and strontium-90, nutrients such as nitrogen and phosphorus, heavy metals and a variety of toxins.
Some of the toxins in the Hudson come from our wastewater treatment plants, which also treat water from industrial facilities and factories in river communities. Other toxins come from our bodies and homes, via wastewater. These are the byproducts of the medicines, beauty care products, household cleaners, disinfectants, insecticides and other products we use, many of which are not efficiently removed with current wastewater treatment technology and therefore end up in the river.