Geochemical characteristics of an urban river: Geochemical contamination and urban stream syndrome

The Anacostia River in Washington, D.C. is among the 10 most contaminated rivers in the USA, containing sewage, metals, PAHs, and PCBs. The biogeochemical characteristics of urban rivers, including the Anacostia, remain largely unstudied. Here we examine the base-flow geochemistry of the tidal freshwater Anacostia over a two-year period (April 2010- April 2012), concentrating on water chemistry (pH, hardness, SAR, alkalinity, Ca, Mg, Na, K, Fe, Mn, Zn, Al, Ba, Ni, P, S, Sr, NO3, NH4, PO4) at 3 locations in the stream. Mean NO3was generally between 1.1 and 1.3 mg/L, although occasionally concentrations increased to 3-4 mg/L at all sites. NH4 was very low generally (0.0 to 0.3 mg/L) with occasional peaks of 1.5-3.9 mg/L downstream. A Principle Components Analysis of stream chemistry showed that the upstream site had two components that explained 34.2 and 29.2% of the data variance; PC1 was most strongly negatively correlated with Ca (-.896), Mg (-.585) and hardness (-.823), and was positively correlated with Ba (which is sometimes associated with disturbance), B, NO3, P, PO4, Sr and Al. PC2 was strongly correlated with Mg, K, S, Ni and NH4. Na was positively and significantly correlated with both components, but more so with PC1. At the middle and downstream sites, two components explained 41 to 44% (PC1) and 22 to 28% (PC2) of the data set variance respectively. The components were essentially the same as the upstream site, with the dominance switched. PC1 was positively and highly correlated with ions associated with bedrock components (Ca, Mg, K, Na, and pH but also S and NH4). PC2 was not positively correlated with any of the dominant geochemical variables, but was negatively correlated with Ca and K and positively correlated with NO3, Ba and Mn. The principle components analysis suggests that there is a strong geochemical component and weaker anion/nitrate component contributing to the ion distribution, and their relative dominance changes moving downstream. Examining ion concentrations, Na and Ca dominate which is a trend observed urban streams. Na is higher than Ca, which is unusual for a non-coastal stream, and points to road chemical treatments as well as Na and Ca dissolution from concrete. Ca is generally in higher concentration than Mg for urban streams, and Na has the highest concentration of all, which is also the case in the Anacostia. Additionally the pH in the Anacostia is between 7.3 and 8, indicating rapid buffering of precipitation (pH 4.9), and suggests the dissolution of concrete. Plotting Na/(Na+Ca) versus total dissolved solids indicates the Anacostia is mostly an urban stream, with some tendency towards "rock dominated" stream water, presumably from groundwater. The Anacostia shows characteristics of "urban stream syndrome" and appears to be so heavily influenced by its altered watershed that, for the most part, it no longer resembles a natural river. The implications for the Anacostia's ecology and future recovery are significant.