Urban streams receive a high pollutant load from residential septic tanks, wastewater effluent, piped storm water drainage, and runoff from parking lots, roads, and fertilized lawns. Urban streams, therefore, have higher concentrations of nutrients and contaminants compared to rural streams. In addition, other activities associated with urbanization, such as stream channel modifications, increased area of impervious surfaces, deforestation of riparian vegetation greatly impacts stream chemistry, morphology, hydrology and ecology. The impact of anthropogenic activities on urban streams calls for a quantification of the stream impairment, in order to monitor and evaluate possible need for restoration. This study strives to quantify the differences between rural and urban stream water biogeochemistry and identify the sources of pollution.
A total of 46 sites from 10 different urban and rural streams were sampled throughout Schenectady County and surrounding areas in New York during the summer of 2016. Water temperature, pH and dissolved oxygen were recorded and ion concentrations and alkalinity were analyzed. Finally, we analyzed the dried filamentous algae for stable nitrogen isotopes, indicative of anthropogenic sources of pollution such as sewage leachate, fertilizer runoff, and nearby wastewater treatment plants.
Our data confirm that urban streams have higher ion concentrations (up to four-fold higher), and that organic pollution is also found at a higher frequency in urban streams. Our data reveal that the largest changes in the urban streams were caused by road salt usage and the high amount of concrete in the urban areas. An extremely strong correlation between sodium and chloride ions indicate the main source for both ions is road salt. This was also confirmed by the ratio of chloride to bromide. Evidence for concrete weathering came from an increased alkalinity as well as increased calcium and magnesium ions in urban waters. The organic pollution, as observed from high d15N values, is probably caused by leaking septic tanks and could be used to pinpoint problem areas if sampling is done at a higher resolution.