Water quality impacts of small-scale hydromodification in an urban stream in Connecticut, USA

Abstract

IntroductionConstruction activities in and along urban streams increase the sediment input into surface waters, causing an overall decline in water quality and aquatic ecosystems. In this case study, we investigate the water quality impacts of local hydromodification in an urban stream (discharge 0.4 m3/s). At the site of interest, workers removed a stream crossing consisting of an embankment with culverts and replaced it with a small bridge (single span of 25 m) in an effort to improve flow capacity.MethodsWater samples were taken at four sites along the North Branch Park River in Connecticut, Northeastern United States. Turbidity and dissolved oxygen (DO) were measured in situ, and nitrate and total phosphorus (TP) were measured in the laboratory. Benthic macroinvertebrate samples were also collected and analyzed for taxon richness and Shannon-Weaver species diversity. Data were compared between upstream and downstream sites and before, during, and after hydromodification. We used one-way ANOVA combined with the post hoc Turkey test to derive statistical significance.ResultsDuring construction, turbidity increased temporarily by 60.9% [from 2.48 Nephelometric Turbidity Units (NTU) over ambient to 4.00 NTU]. Once construction was completed, DO increased locally from 11.0 to 13.0 mg/L. Benthic macroinvertebrate taxon richness and species diversity declined by 61.6 and 32.6% respectively, with no recovery observed in the year following construction. Water quality was only affected within 50 m downstream. Nitrate and TP concentrations were unaffected.ConclusionsSmall-scale hydromodification temporarily increased the turbidity as a result of increased sediment input, approaching the maximum level for clean water (5 NTU). Benthic macroinvertebrate communities declined in the immediate downstream vicinity of construction but are expected to recover soon given that turbidity recovered to pre-construction levels, and DO increased. These outcomes emphasize that environmental assessment is important not only for large-scale hydromodification but also for smaller scale stream modifications.