Water Quality

Combined Sewer Overflows

In many older cities, a single sewer line is used to convey both sanitary sewage and stormwater. During periods when there is little rain, the sewer and treatment plant have the capacity to handle incoming flows. However, when there is sizable rainfall, the capacity of the sewer and treatment plant is frequently exceeded, and the mixture of sewage and storm runoff is released into nearby waterways to keep the sewer from backing-up and flooding basements or streets. This discharge event is referred to as a combined sewer overflow (CSO).

The degree to which CSOs may increase in frequency in a changing climate is dependent on the rainfall threshold at which a CSO is initiated in a given sewage system. For instance, the City of Rochester reduced CSOs by constructing 34 miles of 12-16 ft diameter tunnels that can store sewage until it can be treated. While these tunnels will store the combined sewage generated by most rainfall events, EPA regulations still permit up to four CSO events per year. An EPA (US EPA 2008) study of such systems concluded that daily CSO discharges could increase from 12% to 50% with climate change. However, as is often not clearly noted, this 12% to 50% increase assumes that only four CSOs currently occur each year.

In contrast, in combined sewage systems that have had no upgrades, a CSO may be initiated with little rainfall and upwards of 50 CSO may already occur per year in certain locations. We assessed two design reports assessing sewer systems in the Harbor Brook watershed in Syracuse and in watershed discharging to the Gowanus Canal in Brooklyn. Gowanus canal. For the Gowanus canal watershed, modeling predicted 50 CSOs each (Montalto et al. 2007). For the Harbor Brook watershed, modeling predicted 58 CSOs each year. Therefore, in these cases, one or two additional large rainfall events each year due to climate change would only slightly increase the total number of CSOs.

CSO Communities

Figure 4. Combined sewer overflow (CSO) communities in NYS

Non-Point Source Pollution

Even without CSOs, precipitation falling onto urbanized and agricultural land uses can pick-up pollutants and transfer them to surface waters. However, with climate change, changes in discharge are not likely to be dramatic enough to greatly alter existing pollutant loads. In watersheds with little impervious surface, potential increases in streamflow due to precipitation will likely be balanced by increases in evaporation (as discussed earlier in terms of water supply). Because pollutant loads are mainly dependent on total runoff volume and not the intensity of runoff (Sartor 1974, Alley 1981, Shaw et al. 2009), the quantity of pollutants entering waterbodies should remain relatively similar to current conditions. In more urbanized areas with more impervious surfaces where there is a greater response in runoff to increases in precipitation, much of the increase in annual precipitation will be in winter and early spring (Hayhoe et al. 2007 Figure 2) when nutrient inputs will not immediately contribute to eutrophication and when there is little recreational activity.

Impacts of Increased Water Temperatures

Increasing air temperatures will result in an increase in the temperature of water in streams and rivers. Up to a water temperature of approximately 77°F, water temperature directly increases with air temperature, albeit with a proportionality constant of 0.6-0.8.

Increasing water temperature can directly stress aquatic biota, in particular cold-water fish species such as trout. Warmer water also holds less dissolved oxygen (DO). Waterbodies that are near the threshold for being DO limited, even for non-trout species, may drop below a critical point more often with climate change.

Increasing temperatures may also have both direct and indirect effects on nutrient export from watersheds. Indirectly, changing water availability in western regions and expanded corn-based ethanol production may lead to increased agricultural land use in NYS. For a study of Michigan Lake basins, Han et al. (2009) predicted that riverine N could increase by up to 24% with climate change and expansion of corn acreage. In terms of direct impacts of temperature increases, Schaefer and Alber (2007) found that 25% of anthropogenic nitrogen inputs in northeastern US watersheds were exported to coastal waters but only 9% of inputs were exported in southeastern watersheds. They hypothesize that higher temperatures encourage gaseous loss of nitrogen, decrease nitrogen loss to waterbodies, and therefore reduce water pollution.

Increased water temperatures are also sometimes associated with greater pathogen survivability in waters. However, there does not appear to be a single general conclusion that can be made about the potential impact of climate change since pathogen viability varies widely among organisms and is also influenced by other environmental conditions.

Finally, increased temperatures may also lead to increased algal growth in waterbodies as well as increased dissolved organic matter exported from soils and wetlands (Futter and Wit 2008). Besides impairing recreational use and normal ecosystem function, this increased organic matter may increase the concentration of disinfection by-products (DBP) in drinking water (potentially harmful chemicals that form when chlorine added to kill pathogens reacts with organic matter). However, DBP formation is dependent on a number of variables (Chowdhury et al. 2009) and there is still limited definitive evidence whether DBP would significantly increase in a changing climate.

Impacts of Decreased Flows

Most waste water treatment plants (WWTPS) in NYS receive a "general" State Pollutant Discharge Elimination System (SPDES) permit that allows them to discharge effluent with a BOD of up to 30 mg/L (background in-stream BOD is around 1 mg/L) The addition of BOD in a waterbody results in a decrease in stream dissolved oxygen near the effluent point and an eventual recovery in stream dissolved oxygen further downstream from the discharge point as reaeration occurs. Increased temperatures could increase the rate at which BOD is consumed and the rate of reaeration (Huber 1993). While these two processes push BOD in opposite directions, BOD consumption is generally assumed to be more sensitive to temperature than rates of reaeration. Thus, with the same amount of BOD released to a river, more rapid consumption could further decrease DO concentrations but limit DO depletion to a shorter section of river.

However, climate change will not only increase stream water temperatures but also potentially result in decreased stream flows, particularly during the summer when stream flow is already at its lowest for the year. At low-flow levels there is less dilution and the pollutant concentration is effectively higher. Thus, decreases in low-flows may require reconsideration of SPDES permit requirements.


There remain gaps in the scientific and regulatory communities' understanding of certain, basic water quality issues related to climate change. Thus, our suggestions for adaptations related to water quality primarily entail additional research and monitoring that will support more directed adaptations in the future.

There is a clear need to better understand the impact of low-flows and higher temperatures on the pollutant assimilative capacity of streams and rivers in the State. This entails better understanding the in-stream chemistry at higher water temperatures (the fundamentals are well established but it should be evaluated on several actual streams) as well as improving means to predict low-flows on streams so that the most vulnerable streams can be identified.

As discussed above, the most likely source of deteriorating water quality in the future will be a shift in land use motivated by climate related factors (i.e. addition of new farm land, unconventional gas well drilling, etc.). Notably, this climate-related land use change may only be a portion of land use change resulting from other causes. Some of this new development will be on marginal land with steep slopes or wet soils, traits which may increase the potential for pollutant generation. Thus, there is a need for additional applied research to identify critical pollutant contributing areas and processes to insure that land likely to cause a disproportionate amount of environmental harm is not brought back into production.

Currently, there is sparse and infrequent pollutant sampling, which limits our ability to separate the impact of the climate and changing land use on water quality. As a starting point, frequent monitoring of primary nutrients, turbidity, and pathogen indicators on major rivers (Chemung, upper Susquehanna, and Delaware) would enable a clearer picture to emerge of the association between climate factors, land use and water quality in New York State at a large spatial scale. This effort could overlap with work to manage nutrient loads to the Chesapeake Bay.


Chowdhury, S., P. Champagne, and P.J. McLellan. 2009. Models for predicting disinfection byproduct formation in drinking waters: A chronological review. Science of the Total Environment. 407: 4189-4206.

Futter, M.N., and H.A. de Wit. 2008. Testing seasonal and long-term controls of streamwater DOC using empirical and process-based models. Science of the Total Environment, 407: 698-707.

Hayhoe, K., C.P. Wake, T.G. Huntington, L. Luo, M.D. Schwartz, J. Sheffield, E. Wood, B. Anderson, J. Bradbury, A. DeGaetano, T.J. Troy, and D. Wolfe. 2007. Past and Future Changes in Climate and Hydrological Indicators in the US Northeast. Climate Dynamics, 28: 381-407.

Huber, W.C. 1993. Contaminant Transport in Surface Water, Chp. 14 in Handbook of Hydrology, ed. D.R. Maidment, McGraw-Hill, NYS.

Montalto, F., C. Behr, K. Alfredo, M. Wolf. M. Arye, and M. Walsh. 2007. Rapid assessment of the cost-effectiveness of low impact development for CSO control. Landscape and Urban Planning. 82, 117-131.

Sartor, J.D., G.B. Boyd, and F.J. Agardy. 1974. Water pollution aspects of street surface contaminants. Journal of the Water Pollution Control Federation. 46: 458-467.

Shaw, S.B., J.R. Stedinger, and M.T. Walter. 2010. Evaluating urban pollutant build-up/wash-off models using a Madison, Wisconsin catchment. Journal of Environmental Engineering, 136: 194-203.

Schaefer, S.C. and M. Alber. 2007. Temperature controls a latitudinal gradient in the proportion of watershed nitrogen exported to coastal systems. Biogeochemistry, 85: 333-346.

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Climate Change Links

Intergovernmental Panel on Climate Change (IPCC) (link)

Northeast Climate Choices (UCS Reports) (link)

Climate Change and Northeast Agriculture (link)

Climate Change and Water Resources (NCAR) (link)

USDA Global Change Program Office (GCPO) (link)