Water quality monitoring
The significant difference in analyzed grab sample chloride concentrations between the “Control” and “Seep” (p < 0.0001) and the “Control” and “Pond Outlet” (p = 0.0001) sites across the two-year study period indicates an apparent impact from the adjacent STF. On average, grab samples from the “Control” and the “Seep” are above US EPA standards for chronic toxicity, with several samples above acutely toxic levels at the “Seep,” a trait never observed at the “Control” site.
Continuously measured conductivity results are consistent with findings from the grab sampling efforts. Chloride concentrations at the “Seep” site exceeded the US EPA acute standard of 860 mg L−1 in the early part of year 1, in addition to exceeding the chronic standard. Concentrations then decreased overall from year 1 to year 2, potentially due to the installation of a roof over the ‘salt offloading’ area located along the STF’s rail spur (in late 2018 or early 2019). Prior to roof installation, salt spilled during unloading could accumulate on the pervious rail bed and infiltrate into native soil during precipitation events. Given the high ratio of salt to water (precipitation), the lack of treatment by any stormwater management features (the area under the rail bed is not hydrologically connected to the STF’s stormwater management features), and the proximity of the “Seep” at 4 vertical and 37 horizontal meters away, high concentrations of chloride appear to daylight at the “Seep” and are readily transported into the LaPlatte River.
The high chloride concentrations at the “Seep” seen in year 2 during spring 2018 that do not track with conductivity measurements may be related to sampling method. Prior to monitoring device installation, samples were collected from surface water at the “Seep” itself where groundwater emerges at the toe of a slope. After the continuous conductivity monitoring device was installed, grab samples were collected from within the vertical chamber (similar to a shallow groundwater well). Grab samples taken prior to the installation of the monitoring device may have been more turbid due to their collection from the surface at the seep itself and without filtration through the monitoring device fabric and stone layer. This may have resulted in the higher Cl− concentration values taken before the monitoring device was installed and may explain the comparatively higher values on the early dates of the study. Therefore, concentrations measured in the sampling device for the majority of the study may be more conservative (lower) than direct surface measurements from the “Seep.”
Where concentrations at the “Seep” varied seasonally and decreased from year 1 to year 2, “Pond Outlet” concentrations remained relatively constant throughout the 2 years and are nearly always above the US EPA chronic standard. The low concentrations observed at the “Pond Outlet” in spring periods are likely due to flooding from Lake Champlain and the LaPlatte River. Floodwaters inundate the “Pond Outlet” monitoring site and dilute the water within. When flooding subsides, chloride concentrations return to above-chronic levels. No significant fluctuation is evident throughout the rest of the study period and chloride concentrations are not notably influenced by precipitation or temperature alone, which differs from patterns observed in other chloride investigations in urban areas draining streets and parking lots where deicing salts are applied following periods of winter precipitation and higher chloride concentrations are subsequently observed during melt periods (temperature-driven) or during and following rain (precipitation-driven) (Torizzo et al. 2016; Mayer et al. 1999). By contrast, the STF receives and stores deicing salts year-round. Salt spillage on impervious surfaces is possible in all seasons and transport to the pond may be occurring consistently, rather than only during winter and early spring periods.
Additionally the conveyance mechanisms for runoff may influence chloride concentrations at the stormwater pond as some of the swales routing runoff are pervious, allowing infiltration of some of the volume and its associated chloride concentration to groundwater. This may indicate that chloride concentrations measured at the “Pond Outlet” are conservative with respect to the total amount of chloride being transported from the STF via surface and sub-surface flows.
Soils
Soils data provide a point of comparison to water quality concentration information. Concentration of ions associated with deicing salts in soils are less ephemeral than concentrations in water. The accumulation and retention of ions in soils can illustrate loading to the environment in a way that measuring concentration in water alone cannot (Ostendorf et al. 2009).
Concentrations in water are greatly influenced by contributing drainage area. In the case of the sentinel monitoring point chosen by the State agency for permitting, a large watershed with relatively uncontaminated runoff seems to easily mask the chloride signal from the STF. The soils data from sites immediately below the “Pond Outlet” and “Seep” show a different story. This indicates that the choice of monitoring location for chloride must take dilution into account as localized impacts to soil chemistry, vegetation, and associated aquatic and riparian species, will not be accounted for.
The elevated levels of Na+ at both test sites (“Pond Outlet” and “Seep”) confirm that the dominant form of deicing salts transported and stored in the adjacent facility are sodium chloride compounds (commonly known as ‘rock salt’). Na+ concentrations are higher at the “Pond Outlet” than the “Seep” or the “Control” likely due to the collection and concentration of runoff from the STF site through the stormwater pond, resulting in higher loading to the wetland where the stormwater pond discharges. While water concentrations of Cl− are not as elevated at the “Pond Outlet” as they are at the “Seep,” much higher volumes of water flow through the “Pond Outlet” point, resulting in lower Cl− concentrations in water but because Na+ will bind to soil particles, higher concentrations of that ion were measured in the wetland soils, indicating that Na+ may be disproportionately loading and displacing native soil cations. This loading impact can’t be observed from measurements of water samples alone as water and its associated chloride concentrations are transient.
No statistically significant difference in Mg2+ in soils among any of the sites was observed which may indicate that MgCl is not the most common salt stored at the site. This is supported by national deicing salt usage which defaults to NaCl due to its lower cost. There is a trend in the Mg2+ data towards significant difference between the “Pond Outlet” and the “Control” (p = 0.07), indicating that over time the Mg2+ ratio in native soils may be similarly changed as a result of deicing salts, but this may take longer than Na+ to appear due to an overall lower volume held on site.
K+ serves as a comparison cation to show likely native conditions of soils on and surrounding the STF. There is no difference for this cation among all monitoring sites, indicating that in natural conditions the soils at the three sites would likely have had very similar soil cation ratios (i.e. for Na+, Mg2+, and K+). Because the soils at each site are mapped as the same type by published USGS Soil Survey maps (Adams and Windsor loamy sands) the relative abundance of each cation would be consistent without influence by the STF. K+ may be in danger of displacement by Na+ and Mg2+ within the native soils over time which could be damaging to soil microbial communities and vegetation.
The lack of soil sampling and analysis in the site’s permit compliance documents (as required by the State stormwater program) eliminates use of a more suitable media for tracking pollutant movement from the site on a time scale that could allow remediation.
Vegetation
Other studies have linked increased chloride concentrations in vegetation to thinning, reduced diversity, and physiological changes resulting in poor performance/ survival (Labashosky 2015; Meter et al. 2011, 2012). Tsuga canadensis are a salt sensitive species and therefore may offer an early indication of impact to the wetland at large (USDA). Additional study focused on vegetation, species diversity, and chloride impacts is needed to conclusively determine the extent of influence from salt loading.
Monitoring program
Despite these clear impacts, the operators of the STF have not indicated to the regulatory authority (VT DEC) that there are any issues resulting from the salt storage facility, attributable to the monitoring point locations and methods. This is in part due to a misapplication of stated water quality management strategy on the part of VT DEC to designate an adequate monitoring point or protocol. As previously noted, the designated sentinel monitoring point is downstream of the STF and subject to considerable dilution from the drainage area. This point forms part of VT DEC’s Integrated Water Information System (IWIS) which is part of the State’s Water Quality Monitoring Program Strategy (VT DEC 2019). The Strategy document discusses the four types of points which are typically designated as monitoring points including ‘targeted’ sites chosen for a specific reason such as stream sections or water body discharge with known problems, ‘probability’ sites which are chosen based on their likelihood to have a water quality issue, ‘river geomorphic assessment’ sites which are targeted for geomorphologic changes, and ‘special or Total Maximum Daily Load (TMDL) studies’ sites which are chosen based on specific changes in pollutant load potential or need to demonstrate compliance with a TMDL mandate. This last site type could apply specifically to the STF site and its concentration of deicing salt storage. However, VT DEC chose not to exercise its stated strategy in designating the sentinel monitoring point but rather chose to use a legacy ‘targeted’ point located near the mouth of the LaPlatte River.
As an EPA designated permit authority, VT DEC also had the option to designate the stormwater BMP ‘outfall’ as a monitoring point in addition to a downstream monitoring point, in accordance with EPA guidelines. However, this was not done, nor was a designation of the Class I wetland as the ‘closest waterway’ exercised as an option as advised under EPA guidelines. Rather, the operator of the STF was allowed to designate monitoring points. This is a known and documented issue with water quality permit compliance monitoring programs as noted by the federal Government Accounting Office (GAO) in 1981 in their finding that better monitoring techniques were needed to assess the quality of rivers and streams. Though improvements have been made, this site illustrates that best practices are not consistently followed and inconsistent oversight leads, as in this case, to pollutant loading.
There are established ‘use cases’ for planning or optimizing an effective Water Quality Monitoring Program (WQMP). While no WQMP will work in all cases, there are common practices that managers can follow. Research indicates that “it is essential that water quality data be relevant, precise and reliable in space and time” and that “watershed managers [have to] adapt their WQMPs to evolving issues of water quality.” Further, “water bodies should not be separated when planning or optimizing and WQMP as rivers feeds lakes and vice versa.” It is logical to extend this connection to other hydrologically connected water bodies such as wetlands and groundwater. A nascent tenet in environmental regulation posits that “the objective is no longer mainly to measure the concentration of chemicals: more and more the objective is shifting towards the evaluation of ecological integrity and the effects of the chemical mixtures” (Behmel et al. 2016).
One such WQMP strategy that could have been employed in this case is outlined in detail by Strobl et al. in a 2006 research paper documenting the use of a ‘Critical Sampling Point (CSP) methodology which takes into account surface and sub-surface conditions and applies ‘fuzzy’ logic to account for real-world lack of distinct boundaries between topographic and land use features which can influence pollutant runoff (Strobl et al. 2006). The model also addresses logistical and economic (cost of monitoring) concerns. The CSP methodology represents just one WQMP strategy that could be employed to designate a more appropriate monitoring point under the VT DEC’s ‘probability’ site strategy protocol.
Stormwater BMPs and chloride reduction performance
Stormwater Best Management Practices (BMPs) aim to reduce the volume of pollutants discharged from a site. The two practices selected for use on this site, a stormwater wet detention pond and a swale with level spreader, were inappropriately employed. The literature has conclusively found these control structures ill suited to reducing salt concentrations due to chloride’s chemical properties.
Barbier et al., in a study of stormwater retention ponds treating deicing salts from roadways found that “almost all of the deicing salt entering the basin was evacuated at the end of March (91%)” and sodium was not retained in pond sediments (Barbier et al. 2018). A similar study from 2006 in Sweden found that “continuous conductivity measurements show that chloride is flushed between [precipitation] events” (Semadeni-Davies 2006). North Carolina’s Department of Transportation specifically reviewed BMPs for salt storage sites in 2006, finding that “runoff from a salt storage area had elevated levels of chloride indicating the need for improved containment of salt” (Line 2006). The same study noted that none of the BMPs used to treat (not contain) salt had any significant effect on conductivity.
Snowmelt storage sites are similar to deicing salt storage facilities in that a relatively small area receives and stores a large amount of deicing salt (in the case of snowmelt storage sites, deicing salts contained within plowed snow and ice transported to the site via truck). A Canadian study from 2019 investigated the effect of a treatment train approach to reducing chloride pollution (Senior et al. 2019). The train, consisting of an impervious melting pad, forebay and grit separator, and extended detention wet pond, served only to dilute, not reduce, chloride concentration through detention of high Cl concentration meltwater and mixing with cleaner precipitation-based runoff from the pad.
Other entities have recognized the inutility of typical stormwater BMPs at salt storage sites. The Virginia Department of Transportation (VDOT) evaluated their treatment approaches which typically consist of wet ponds used to temporarily store high Cl concentration runoff for later treatment either at a public treatment works system or for application on gravel roads for dust suppression (Müller and Gächter 2012). This paper also concluded that their current system is cost-prohibitive and advocated for the development of on-site reverse-osmosis treatment to remove dissolved salts from runoff.