DRAINAGE AND DISCHARGE OF SALT IN STORMWATERS; A POTENTIAL HAZARD FOR HEAVY METAL POLLUTION OF THE ENVIRONMENT.

Michael J. Watts, Robert J. Hares, Victor H. Zettel, Karen Stead and Neil I. Ward*

ICP-MS Facility, Department of Chemistry, University of Surrey, Guildford, GU2 7XH, UK. n.ward@surrey.ac.uk

ABSTRACT

The heavy metal (Cd, Cr, Cu, Fe, Mn, Ni, Pb, Sb and V) content of stormwater is variable depending roadside management practices, storm events-number, frequency and duration. In general, reported levels for summer and winter periods have typical heavy metal ranges for: Pb: 100-8050; 245-2975 (mg/L); Cd: 0.8-42; 3-105 (mg/L); Cu: 5-178; 7-135 (mg/L) and Zn: 110-3560; 275-4595 (mg/L), respectively. The drainage of these heavy metals in motorway stormwaters is directly into detention ponds, whereby suspended material is separated out to form the sediment layer of the pond. This sediment can be considered to be a long-term reservoir of chemical pollutants. The stormwater discharged into local streams normally contains low levels of heavy metals. However, during winter months the addition of de-icing agents to road surfaces substantially increases the levels of Na⁺, Cl^- and SO₄^2- in stormwater. For example, reported chloride levels for summer periods of 1 - 45 mg/L can increase to 7800 mg/L during winter storms. The addition of such anions / cations to stormwater has a dramatic effect on the heavy metal bioavailability of the detention pond sediments. This paper presents data for two different detention pond systems along a high traffic density motorway (typically 120,000 vehicles per day). In particular, seasonal storm event data confirms how the addition of salt has resulted in the remobilisation of sediment heavy metals into the detention pond water that is discharged into local stream systems.

INTRODUCTION

The constituents of stormwater have been studied in great depth. These constituents include heavy metals (Cd, Cr, Cu, Fe, Mn, Ni, Pb, Sb and V), organometallic species, polyaromatic hydrocarbons (PAH), fuels (petrol, diesel), lubricating and transmission oils, grease, corrosion preventing and anti-freezing agents (Hewitt and Rashed, 1990 Ward, 1995). The concentration and loading rates of these chemicals in stormwater are highly site specific and relate to many factors, such as; the motorway catchment area; local topography and weather conditions; the type and age of vehicles, the average daily traffic density and flow (Ward, 1990). The addition of de-icing salt to the surface of motorways during winter periods has been identified as a major problem. The use of this salt not only increases the Na⁺ and Cl^- content of stormwater runoff, but also enhances the levels of various heavy metals (Cd, Cr, Cu, Mn, Pb and Zn). The impact of salt addition on the chemical quality of stormwater runoff into nearby aquatic or terrestrial environments has attracted a limited number of studies (Stotz and Krauth, 1994; Watts, 1998). One study has shown that the presence of high salt levels in stormwater runoff can alter the bioavailability of certain toxic heavy metals, especially cadmium and zinc, by altering the metal-complex formation in drainage sediments through the production and mobilisation of Cd/Zn-Cl^-/SO₄^2- species (Bingham et al., 1984; Watts, 1998). The environmental fate of heavy metals in freshwater streams and lakes alongside busy motorways may be a potential problem.

MATERIALS AND METHODS

Study sites: Figure 1 shows a schematic of the London Orbital M25 motorway dry detention pond drainage facility near Oxted, Surrey (junctions 5-6). This stretch of motorway has typical daily traffic flows >100,000 vehicles, a rainfall catchment area of 76160 m², from which stormwater is discharged via an oil separator, grit settlement chamber and silt settlement trap into a 2 hectare dry detention/balancing pond. Outflow from this pond is via an interceptor (outflow control device) and an oxygenation cascade before discharge into the Eden river.

Figure 1: Dry detention pond drainage facility, Oxted.

Figure 2 shows a schematic of the London Orbital M25 motorway wet biofiltration detention pond drainage facility near Leatherhead (junction 9). Typical traffic densities at this point are ~140,000 vehicles per day. The facility consists of two basins, a wet biofiltration pond (basin 1) and silt trap (basin 2). Basin 1 is lined with a relatively impermeable clay base and reeds (Typha latifolia). Basin 2 is constructed of concrete and acts as a sedimentation pond, from where stormwater is discharged into the Ryebrook river.

Figure 2: Wet biofiltration detention pond drainage facility, Leatherhead.

All samples were filtered and then analysed for their anion/cation content using a Dionex 2020i ion chromatography system, with an ED40 conductivity detector (Dionex Co., Camberley, UK). Heavy metals (Cd, Cu, Pb, Zn) were also analysed, using a Finnigan MAT Sola Inductively Coupled Plasma Quadrapole Mass Spectrometer (ICP-QMS).

Anion / cation standard solutions were prepared from commercial multi-element solutions (Glen Spectra, Middlesex, UK) and heavy metal standards were prepared from Aristar grade standard solutions (BDH, Poole, UK).

RESULTS

Tables 1 and 2 show that the wet biofiltration and dry detention ponds do not significantly reduce the levels of Na⁺, Cl^- during the treatment process. Low electrolyte levels are achieved only when the stormwater reaches the receiving natural stream (Eden river or Ryebrook).

Table 1: Mean heavy metal and anion / cation content of stormwater throughout the dry detention pond drainage system at Oxted.

	Silt Trap		Detention Pond	Interceptor	Cascade	River Eden	Deer Park
	In	Out
mg/L Pb	50	63	49	51	25	7	6
Cd	2.14	1.94	2.74	2.42	0.41	0.11	0.04
Cu	190	187	195	275	14	11	8
Zn	118	120	124	72	28	18	15
mg/L Na^+	142	141	141	138	127	22	43
Cl^-	190	171	177	173	169	37	68
SO4^2-	77	120	100	105	105	96	56

Table 2: Mean heavy metal and anion / cation content of stormwater runoff throughout the wet biofiltration detention pond drainage system at Leatherhead.

	Basin 1		Interceptor	Basin 2		Interceptor	Dry Pond	Rye-brook
	In	Out		In	Out
mg/L Pb	60	47	50	30	14	12	8	5
Cd	1.76	0.49	0.81	0.62	0.44	0.24	0.21	0.14
Cu	187	87	103	82	16	15	18	19
Zn	238	151	154	194	198	180	45	17
mg/L Na^+	168	160	165	147	137	131	118	54
Cl^-	310	317	336	347	354	252	350	260
SO4^2-	258	252	272	242	226	227	224	188

As a result of the above data an experiment was undertaken to evaluate the mobilisation of heavy metals in sediment taken from the Oxted dry detention pond (silt trap). Material was mixed so as to replicate “idealised” storm conditions ( using batch equilibrium) for 5, 30 and 720 minutes, with solutions of varying salt content (deionised water, 50 and 1000 mg/L). Actual road salt samples were used. The heavy metal chemical composition of the salt samples showed low levels of Zn (6 mg/g) and Cd (< 0.005 mg/g). Figures 3 and 4, report the leaching trends for Zn and Cd after mixing with deionised water (control) and the two salt solutions. The general trend of increased heavy metal release (leaching) from the silt trap sediment with increased heavy salt content confirms the impact of salt addition has on heavy metal mobilisation. Cadmium and zinc form complexes with Cl^- and SO₄^2- more readily than copper and lead. This will have a direct effect on the bioavailability and thereby potential toxicity to the freshwater ecosystems.

Figures 3 and 4: report the leaching trends for zinc and cadmium.

CONCLUSION

The addition of salt to road surfaces needs consideration, although it is an effective de-icing agent it does lead to high levels of anions / cations (Na⁺, Cl^- and SO₄^2- ) in stormwater runoff. Their impact depends upon the treatment of stormwater and the quality of stormwater entering natural watercourses. The ecological impact on aquatic life must also be considered. The passage of stormwater through traditional treatment systems only slightly reduces the levels of anions / cations. The long-term maintenance of stormwater detention ponds is essential as accumulated sediment poses a potential problem via heavy metals being remobilised, and thereby becoming potentially bioavailable to other ecological components, following the addition of de-icing agents every winter season.

REFERENCES

Bingham FT et al. (1984) J. Environmental Quality 13: 71-74.

Hewitt CN, Rashed MB (1990) Science Total Environ. 93: 375-384.

Stotz G, Krauth K (1994) Science Total Environ. 146/147: 465-570.

Ward NI (1990), Science Total Environ. 93: 393-401.

Ward NI (1995), Trace elements, Chapter 15, in Environmental Analytical Chemistry, Fifield F, Haines PJ (eds), Blackie Academic Professional, Glasgow, pp320-351.

Watts MJ (1998) Evaluation of De-icing salt application to British motorways using ion chromatography and ICP-MS, ICP-MS Facility, Dept. of Chemistry, University of Surrey, Guildford, GU2 7XH, UK (MSc Thesis).