Regional Analysis of River Conductivity Maps Salinity Driven Aquatic Habitat Degradation Potential Throughout New England

Abstract:

River salinity is increasing in urban areas throughout the world, and can impact habitat for aquatic organisms. Riverine impairment cascades through the stream network based on the distribution of development and climate conditions that affect dilution capacity. We present a regional river network transport model that assimilates data from the Lotic Volunteer Temperature Electrical Conductivity and Stage (LoVoTECS) network in New Hampshire, USA. LoVoTECS is measuring high temporal resolution stream conductivity at over 100 sites spanning 6 stream orders and ranging from pristine to near complete imperviousness. Total upstream developed land area is a strong predictor of baseflow specific conductance across the region (r²=0.848, p<0.001, n=85). Empirical loading of dissolved solids from head-water specific conductance data, and characteristic storm dilution rates, are incorporated into a network scale transport model using the Framework for Aquatic Modeling in the Earth System (FrAMES). The model predicts specific conductance and derived Cl to develop a continuous spatial mapping of habitat degradation potential. The model performs well against LoVoTECS and USGS station data in high (5 – 7) order rivers with median residual of 7 μS cm^-1 (9.5%), and RMSE of 25 μS cm^-1 (12% of measured range). Summertime exceedances of EPA guidance for chloride (4 days above 230 mg Cl/L) are only predicted in small urban catchments. Potential thresholds for fish community shifts may be as low as 33 to 108 mg Cl/L (Morgan et al 2012), and exceedances of 108 mg Cl/L occur in low (1 – 2) order developed streams. In higher order (3 – 7) streams passing through New England urban centers, dilution from more pristine watershed areas moderates chloride concentration. However, exceedances of 33 mg Cl/L are common and 7^th order reaches exceed this threshold more than lower order reaches due to local urban runoff and the downstream accumulation of dissolved solids. Further simulations will test the influence of changing winter temperatures, summer storm events, and build-out scenarios. Regional maps of predicted salt impairment will identify river reaches at highest risk for reduced aquatic biodiversity and locate communities that would engender greatest habitat improvement through mitigation activities.