Influence of Land Use, Discharge and Impervious Surfaces on the Geochemistry of the Anacostia River, Washington DC.

Challenges facing urban rivers include water stormwater runoff and changing water chemistry, not only from air and water pollution, but also from altered geology with the development of "urban karst" (concrete). The Anacostia River in Washington, D.C. has 75% of its watershed classified as urban or impervious, and is among the 10 most contaminated rivers in the USA. In addition to its relatively well-documented organic contamination problems, we hypothesize that concrete could be substantially altering its geochemistry. Here we report findings from 6 locations along the Anacostia River and its tributaries that indicate both seasonally elevated Na and Cl (becoming brackish, 2000-3000 mg/L Total Dissolved Solids, in a suburban creek), and elevated cations in low flow suburban tributaries. Concentrations of all major cations (Ca, Mg, K, Na) strongly, and positively, covaried (factor scores (FC) >0.88). However Ca/Sr ratios negatively covaried with major cations (FC -0.64). This suggests the weathering of low Sr minerals, such as those in concrete. In urbanized portions of the river, Ca/Sr was >200, which is a concrete weathering indicator in areas with silica mineral bedrock (Anacostia bedrock consists of Precambrian phyllits, sericite, chlorite, quartzite, slate and schist). Mean ± SE Sr was 0.13 ± 0.02 mg/L in the most urban area, but 0.37 ± 0.03 mg/L in the most suburban. This supports the hypothesis that the source of elevated cations in the urban areas is concrete weathering, not bedrock mineral weathering. Inorganic N was not correlated strongly with cations. Mean NO3- was highest at the most suburban site (1.8 ± 0.2 mg/L), but rose above 3 mg/L in some samples at all sites. Elevated NO3- did not appear to vary with season or discharge rate at time of sampling. NH4+ was generally lower than 1 mg/L but spiked to 3.4 mg/L at the most urban site. These data follow patterns expected for "urban stream syndrome". Suburban areas, with their relatively small streams, show greater winter salting effects than more urban areas down stream. Suburban areas also show higher NO3- (and occasionally higher NH4+) than urban areas except in winter. The geochemistry of highly urbanized systems may be significantly altered and understanding this effect will help in the development of plans for more effective watershed rehabilitation.