Road Salt Delivery Mechanisms and Water Quality Impacts in the Hudson River Watershed
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Road Salt Delivery Mechanisms and Water Quality Impacts in the Hudson River Watershed Kate Meierdiercks, Department of Environmental Studies & Sciences, Siena College [email protected] Abstract. Road salt entering surface stream channels can negatively impact water quality and ecosystem and human health. While it is generally understood that salt runs off into surface water with melting snow, much less is known about how and when salt enters surface streams through groundwater baseflow. This project examines how land use and watershed structure control delivery of road salt to the Hudson River. This question is addressed in the Hudson River Watershed using publicly available Hudson River Environmental Conditions Observing System (HRECOS) and US Geological Survey (USGS) data and data from independent researchers participating in The Hudson River Tributary and Subwatershed (THuRST) research network. For the watersheds in this study, results suggest that salt is entering surface streams through groundwater baseflow, but there is less evidence that salt is also delivered by the road network as snowmelt. The one exception is the Patroon Creek, the most urban study watershed, where large fluxes of salt in the winter and spring months are likely the result of road salt runoff. Three Summary Points of Interest Salt concentrations and fluxes (quantified as specific conductance, SpC) are increasing in 86% of the watersheds examined in this study. At most sites, maximum daily discharge is increasing while 7-day minimum discharge is decreasing. All sites exhibit bi-model seasonal SpC response with concentrations peaking around March and again September. However, there is no evidence that the March peak is tied to snow melt. Instead SpC concentrations and fluxes are closely associated with streamflow with high SpC concentrations resulting from low flow rates. Furthermore, SpC concentration is not significantly correlated with road density. Rather, the watershed characteristics that are most strongly and significantly correlated with SpC concentration are imperviousness, soil type, basin slope and outfall density, all characteristics that are known to impact stormwater runoff rate and volume. Results suggest that salt is entering the stream channel through groundwater baseflow and that this may be the more important delivery mechanism. The exception and outlier compared to the other 23 watersheds in this study, is the Patroon Creek Watershed, the most urban of the study watersheds, and one of the watersheds with the highest road density. SpC concentrations in Patroon Creek are an order of magnitude greater than the other study watersheds and SpC fluxes peak in the winter. Results suggest that in the Patroon, salt is delivered to the stream channel both through groundwater baseflow and winter snow melt. Keywords: Road salt, watershed, water quality, Hudson River New York State Water Resources Institute | wri.cals.cornell.edu Road Salt Delivery Mechanisms and Water Quality Impacts in the Hudson River Watershed Introduction Analyze salt concentration (quantified as Road salt is applied to roads during winter specific conductance, SpC) data to identify months to improve driving conditions, but when it runs hotspots and examine spatiotemporal variability. off into surface and drinking water it can lead to Correlate land use and watershed structure negative environmental, ecosystem, and health impacts geospatial characteristics with average annual (Kausal et al., 2018). Increased chloride concentrations and seasonal SpC concentrations. in surface waters can be linked to corrosivity and elevated lead levels in drinking water (Stets et al., 2018), as in the Flint, MI water crisis. There has been an increase in road salt use over the last 50 years in the northeast (Kausal et al., 2005). Kelting et al. (2012) note that New York State is one of the biggest users of road salt. Many studies attribute the rise in salt concentrations in surface water to increased urbanization and road salt applications (see for example, Goodwin et al., 2003). However, some studies have shown that salt concentrations can increase in surface waters without increases in urbanization or salt applications; these studies suggest that road salt is persisting in the environment and entering surface water streams through groundwater baseflow during non- winter months (Kelly et al., 2007; Corsi et al., 2015). However, how and when salt enters surface water bodies through groundwater baseflow is still not well understood (Kelly et al., 2007). Given the spatial variability of salt in groundwater (Kelly et al., 2018) and hydrologic response in urban watersheds (Meierdiercks et al., 2009), it’s unclear how or whether the results from these few watersheds translate to others in the region. This project examined the question, how do land use and watershed structure control delivery of road salt to the Hudson River? Land use and watershed structure characteristics of several Hudson watersheds were Figure 1. Locations of monitoring stations for the watersheds used in this study. computed using GIS data and geoprocessing models. Road salt contamination in these watersheds was Results examined using specific conductance (SpC) water Across 86% (12 out of 14) of this sites with quality data available through the Hudson River long-term data records, regardless of the location within Environmental Conditions Observing System the Hudson River Watershed and decade, SpC (HRECOS), US Geological Survey (USGS), and concentrations and fluxes increase for the period of independent researchers participating in The Hudson record. In many of the sites examined, max daily River Tributary and Subwatershed (THuRST) research discharge is increasing and 7-day minimum discharge is network (Figure 1). Both long-term trend and multi- decreasing. Long-term trends in SpC concentration, SpC watershed comparative analyses were performed on the flux, and discharge are consistent with trends reported water quality data. Relationships between land elsewhere in the literature (Kaushal et al., 2018, for use/watershed structure characteristics and average example). Most sites exhibit a bi-modal seasonal annual and seasonal SpC concentrations were explored response with SpC concentrations peaking around March through correlation analyses. The objectives of this and again around September. The exception is the project are summarized as follows: Patroon Creek Watershed where SpC concentrations and Characterize Hudson subwatershed land use and fluxes peak in the winter months. SpC concentrations in watershed structure through geospatial analyses. This report was prepared for the New York State Water Resources Institute (NYSWRI) and the Hudson River Estuary Program of the New York State Department of Environmental Conservation with support from the NYS Environmental Protection Fund. Road Salt Delivery Mechanisms and Water Quality Impacts in the Hudson River Watershed the Patroon are also an order of magnitude greater than in the environment and entering stream channels through the other study watersheds (Appendix A). groundwater baseflow. There is no significant increase GIS analyses of land surface and drainage in SpC fluxes in the winter and spring in all study network characteristics highlight the diversity of watersheds but one. The exception is the Patroon Creek watershed characteristics and development types Watershed where SpC fluxes increase in the winter and throughout the Hudson River Watershed. The largest spring without a corresponding increase in discharge watershed in this study is 11,900 square miles, while the (Figure 4). These fluxes are likely the result of smallest is less than 1 square mile. The study snowmelt (Erickson, 2004). Further examining watersheds range in percent urban from 96.6% to 0% hydrograph-pollutographs for this station will provide a urban, in road density from 89,000 ft/sq.mi to 0 ft/sq.mi, better understanding of the dynamics of road salt and in slope from 2000 to 151 ft/mi (Appendix A). The delivery mechanisms (Calvi et al., 2018). watershed characteristics with the strongest and most significant correlations with SpC concentrations are percent impervious, percent soil type B, mean basin slope, and outfall density (Figure 2). There is not a strong or significant correlation between SpC concentration and road density (Appendix A) . Figure 3. Relationship between sampled specific conductance values and discharge (left) and specific conductance flux and discharge (right) for USGS 01357500 Mohawk River at Cohoes NY. Discussion Though seasonal SpC concentrations for all sites exhibit a bimodal response with SpC concentrations peaking around March and again around September, these peaks do not appear to be associated with an Figure 2. Specific conductance values (mS/cm) versus GIS increase in salt flux from snowmelt. Furthermore, characteristics for the HRECOS and THuRST stations. although salt concentrations tend to increase with For all study watersheds, SpC concentration is increasing imperviousness, they are not correlated with inversely proportional to discharge: as discharge road density. Instead, the peaks in SpC concentration increases, SpC concentration decreases. Subsequently, are closely tied to seasonal changes in discharge with SpC flux is highly correlated with discharge with SpC peaks in SpC concentration resulting from low discharge flux increasing as discharge increases (Figure 3, for values rather than increases in salt fluxes. Results example; see Appendix