Mapping of road‐salt‐contaminated groundwater discharge and estimation of chloride load to a small stream in southern New Hampshire, USA

Abstract

Abstract:Concentrations of chloride in excess of State of New Hampshire water-quality standards (230 mg/l) have been measured in watersheds adjacent to an interstate highway (I-93) in southern New Hampshire. A proposed widening plan for I-93 has raised concerns over further increases in chloride. As part of this effort, road-salt-contaminated groundwater discharge was mapped with terrain electrical conductivity (EC) electromagnetic (EM) methods in the fall of 2006 to identify potential sources of chloride during base-flow conditions to a small stream, Policy Brook. Three different EM meters were used to measure different depths below the streambed (ranging from 0 to 3 m). Results from the three meters showed similar patterns and identified several reaches where high EC groundwater may have been discharging. Based on the delineation of high (up to 350 mmhos/m) apparent terrain EC, seven-streambed piezometers were installed to sample shallow groundwater. Locations with high specific conductance in shallow groundwater (up to 2630 mmhos/m) generally matched locations with high streambed (shallow subsurface) terrain EC. A regression equation was used to convert the terrain EC of the streambed to an equivalent chloride concentration in shallow groundwater unique for this site. Utilizing the regression equation and estimates of onedimensional Darcian flow through the streambed, a maximum potential groundwater chloride load was estimated at 188 Mg of chloride per year. Changes in chloride concentration in stream water during streamflow recessions showed a linear response that indicates the dominant process affecting chloride is advective flow of chloride-enriched groundwater discharge. Published in

Impacts of Road Salting on Water Quality in Fractured Crystalline Bedrock

“…This relationship is comparable with those established in Harte and Trowbridge (2010) and Schalk and Stasulis (2012).…”

“…Groundwater contamination by deicing salt primarily occurs near roads, with severity depending on the amount of salt applied (Foos, 2003; Hon et al, 2009). While runoff quickly removes large amounts of road salt (Ostendorf et al, 2009; Corsi et al, 2010; Harte and Trowbridge, 2010), salt‐contaminated groundwater tends to persist long after road salting ceases due to its relatively low velocity (Williams et al, 2000; Foos, 2003; Bester et al, 2006; Ostendorf et al, 2006; Harte and Trowbridge, 2010; Rhodes and Guswa, 2016).…”

Temporal and spatial trends of chloride and sodium in groundwater in New Hampshire, 1960–2011

“…Increases in concentrations of chloride in natural waters from approximately 1945 through 1970 were documented in Hall (1975) where it was noted that increasing concentrations coincided with increased road deicing salt applications. Other studies in New Hampshire have shown that imported road salt may have long-term effects on water quality by raising baseflow concentrations of chloride and sodium due to storage in shallow (Harte and Trowbridge, 2010) and deep (Emery & Garrett Groundwater, Inc., 2007) aquifers. In a regional study of the northern United States, upward trends associated with use of deicing salt for roads and parking areas, number of septic systems, volume of wastewater discharge, and saline groundwater plumes from landfills and salt storage areas were seen in chloride loads (Mullaney and others, 2009).…”

Chloride concentrations, loads, and yields in four watersheds along Interstate 95, southeastern Connecticut, 2008-11: factors that affect peak chloride concentrations during winter storms

“…Attempts to obtain deicer application amounts applied to these impervious areas were unsuccessful, so an estimate of 30 percent of deicers applied to roads was used to represent deicers applied to parking lots, sidewalks, and driveways. Harte and Trowbridge (2010) estimated that parking lots made up about 50 percent of salt inputs in watersheds adjacent to Interstate 93 in southern New Hampshire. …”