Eric V. Novotny

Increase of urban lake salinity by road deicing salt

Over 317,000 tonnes of road salt (NaCl) are applied annually for road deicing in the Twin Cities Metropolitan Area (TCMA) of Minnesota. Although road salt is applied to increase driving safety, this practice influences environmental water quality. Thirteen lakes in the TCMA were studied over 46 months to determine if and how they respond to the seasonal applications of road salt. Sodium and chloride concentrations in these lakes were 10 and 25 times higher, respectively, than in other non-urban lakes in the region. Seasonal salinity/chloride cycles in the lakes were correlated with road salt applications: High concentrations in the winter and spring, especially near the bottom of the lakes, were followed by lower concentrations in the summer and fall due to flushing of the lakes by rainfall runoff. The seasonal salt storage/flushing rates for individual lakes were derived from volume-weighted average chloride concentration time series. The rate ranged from 9 to 55% of a lakes minimum salt content. In some of the lakes studied salt concentrations were high enough to stop spring turnover preventing oxygen from reaching the benthic sediments. Concentrations above the sediments were also high enough to induce convective mixing of the saline water into the sediment pore water. A regional analysis of historical water quality records of 38 lakes in the TCMA showed increases in lake salinity from 1984 to 2005 that were highly correlated with the amount of rock salt purchased by the State of Minnesota. Chloride concentrations in individual lakes were positively correlated with the percent of impervious surfaces in the watershed and inversely with lake volume. Taken together, the results show a continuing degradation of the water quality of urban lakes due to application of NaCl in their watersheds.

National Satellite-Based Land-Use Regression: NO2 in the United States

Land-use regression models (LUR) estimate outdoor air pollution at high spatial resolution. Previous LURs have generally focused on individual cities. Here, we present an LUR for year-2006 ground-level NO(2) concentrations throughout the contiguous United States. Our approach employs ground- and satellite-based NO(2) measurements, and geographic characteristics such as population density, land-use (based on satellite data), and distance to major and minor roads. The results provide reliable estimates of ambient NO(2) air pollution as measured by the U.S. EPA (R(2) = 0.78; bias = 22%) at a spatial resolution (∼ 30 m) that is capable of capturing within-urban and near-roadway gradients in NO(2). We explore several aspects of temporal (time-of-day; day-of-week; season) and spatial (urban versus rural; U.S. region) variability in the model. Results are robust to spatial autocorrelation, to selection of an alternative input data set, and to minor perturbations in input data (using 90% of the data to predict the remaining 10%). The modeled population-weighted (unweighted) mean outdoor concentration in the United States is 10.7 (4.8) ppb. Our approach could be implemented in other areas of the world given sufficient road network and pollutant monitoring data. To facilitate future use and evaluation of the results, concentration estimates for the ∼ 8 million U.S. Census blocks in the contiguous United States are publicly available via the Supporting Information.

Road Salt Impact on Lake Stratification and Water Quality

Runoff from roadways on which road salt (NaCl) has been applied for driving safety in winter can form a saline water layer at the bottom of a lake, pond, reservoir, or river impoundment. Natural vertical mixing of lentic surface water bodies can be hindered by the presence of a benthic saline layer and thereby affect lake water quality and ecology. To study the formation and disappearance of the saline layer, temperature and specific conductance profiles were measured intermittently over two years (2007, 2008) and at high frequency during one year (2009) in an urban lake of the northern temperate region (Tanners Lake, Oakdale, Minnesota). Erosion of the saline layer in the spring occurred only in year 2007. In years 2008 and 2009, the saline layer persisted throughout the summer only to be removed during fall turnover when thermal stratification was at a minimum. In all three years, salinity dominated density stratification after ice-out, but was quickly overtaken by temperature stratification as the epilimnion warmed. The deterministic, unsteady dynamic one-dimensional (1D) lake temperature and dissolved oxygen (DO) model MINLAKE was modified by including vertical salinity gradients, and it was used to simulate summer stratification and mixing dynamics in Tanners Lake. The daily adjustment of the hypolimnetic eddy diffusion as a function of lake number was an important component in the developed model. This addition allowed mixing in the hypolimnion to be stronger in the fall and spring when the lake stratification was weaker than in the summer after thermal stratification formed. Model results of dissolved oxygen in the water column demonstrated that the saline benthic layer can prevent dissolved oxygen from reaching lake sediments. The adverse consequences of dissolved oxygen depletion on phosphorus recycling from the sediments, benthic microbial communities, and fish habitat are well known. Overall, the results show how salinity from road salt applications can influence water quality and natural mixing in urban lakes.