Pollutant fate and transport in stormwater infiltration systems

As stormwater travels across the land surface into infiltration BMPs, it can pick up various pollutants and deliver them to the subsurface. The fate and transport of these pollutants into soil, the vadose zone and ultimately groundwater depends on the type and amount of pollutant present, the volume of infiltration, the type of infiltration BMP, and subsurface conditions.

Typical stormwater pollutants

Common stormwater pollutants and their most important sources are described in the first table below. The second table provides typical pollutant concentrations in stormwater runoff. The concentrations are based on data from the International Stormwater Database.

Common pollutants of concern and sources in stormwater runoff. Adapted from USGS, 2014.
Link to this table.

Contaminant	Contaminant source1
Nitrogen	Naturally occurring from vegetation decomposition. Anthropogenic sources include fertilizers, farm-animal waste, faulty septic systems
Chloride	Salts applied to roads and parking lots during the winter. Natural sources include mineral dissolution
Copper	Industrial and domestic waste, mining, mineral leaching, automobile parts and fluids
Zinc	Industrial waste; automobile parts and fluids
Manganese	Found naturally in sediment and rocks. Anthropogenic sources include mining waste, industrial waste, automobile parts and fluids
Nickel	Naturally occurring. Anthropogenic sources include stainless steel and alloy products, mining, refining, automobile parts and fluids
Cadmium	Small amounts are naturally occurring. Anthropogenic sources include industrial discharge, mining waste, automobile parts and fluids
Chromium	Old mining operations; fossil-fuel combustion; mineral leaching; automobile parts and fluids
Pesticides	Residential use of lawn care products; commercial landscaping; animal wastes; municipal right-of-ways; agriculture; feedlots
Cyanide	Road salt; fertilizer production
PAHs2	Auto emissions; elicit discharges; asphalt pavement (driveways, roadways and parking lots) with coal tar sealants3
VOCs2	Crude oil; insecticides; varnishes; paints; gasoline products; degreasers; municipal maintenance activities
Oil and grease	Gasoline products; plastics; dyes; rubbers; polishes; solvents; crude oil; insecticides; inks; varnishes; paints; disinfectants; paint removers; degreasers; automobile fluids
Microbes (including fecal coliform, E. coli, and pathogens)	Domestic sewage; animal waste; plant or soil material

¹The list of sources is for stormwater runoff only
²PAHs=polyaromatic hydrocarbons; VOCs=volatile organic compounds
³MPCA, 2014 Source: USGS, 2014, with permission

Concentrations of contaminants found in stormwater. Source: International Stormwater Database⁷. Because the data below are from a single source, values may differ from those contained on this page. We recommend if you are using emcs to quantify pollutant loading, you use this data instead of data from this table. Note that the table does not include information for chloride, a common pollutant in stormwater. Chloride concentrations vary seasonally and would be misrepresented in a single table. For more information on chloride concentrations in stormwater, see here.
Link to this table.

Land use	TSS 1	NO2 + NO3 1	TN 1	TP 1	Cu 2	Zn 2	Ni 2	Cd 2	Cr 2	CN 2,5	Oil and grease 2	VOCs 2,5	Pesticides 2,4,5	FC 3,5	EC 3,5	FS 3,5
Commercial
Number of sites	56	50	13	56	60	62	40	51	38	2	44	4	1	4	--	3
Number of observations	857	786	77	948	785	867	291	543	294	6	394	160	6	19	--	7
% of samples above detection	98.7	98.9	97.4	94.5	85	99.2	51.5	38.1	52.0	0	65.5	65.5	0	73.7	--	100
Minimum	<0.5	<0.1	<1.5	<0.01	<0.2	<0.3	<1	<0.03	<0.7	n/a	<0.5	<0.05	n/a	<200	--	310
Maximum	2385	8.2	18.1	4.27	569.1	3050.5	110	80	100	n/a	359		n/a	28000	--	24000
Median	52	0.6	1.75	0.2	17	110	8	BDL6	4	BDL	5	0.7	n/a	450	--	3100
Industrial
Number of sites	58	51	13	57	65	67	43	60	42	2	48	3	--	6	--	4
Number of observations	619	536	85	638	569	627	300	525	312	9	370	144	--	32	--	12
% samples above detection	99.5	97.0	95.3	95.1	85.1	98.9	58.0	48.6	72.4	0	59.7	10.4	--	90.6	--	91.7
Minimum	<1	<0.02	<1.5	<0.02	<0.2	<0.5	<2	<0.03	<0.7	n/a	<0.5	<0.05	--	<1	--	<1
Maximum	2490	8.4	15.2	7.9	1360	8100	120	334	150	n/a	408		--	3600000	--	48000
Median	75	0.68	1.7	0.23	19	155	10	BDL	10	BDL	5	BDL	--	3950	--	24000
Residential
Number of sites	146	127	20	148	147	151	77	114	72	--	95	7	1	10	3	4
Number of observations	2257	1772	131	2380	1743	2013	418	1123	408	--	694	210	6	94	19	23
% of sample above detection	99.9	99.0	98.5	98.2	86.5	97.0	42.2	40.4	48.8	--	56.8	20.1	0	85.9	100	95.7
Minimum	<0.5	<0.03	<1.5	<0.01	<0.2	<0.5	<0.5	<0.03	<0.7	--	<0.5	<0.05	n/a	<1	10	<1
Maximum	4168	66.4	18.3	19.90	590	14700	100	70	70	--	419	3.42	n/a	5230000	35000	200000
Median	58	0.60	2.24	0.26	11	69.9	5	BDL	3	--	4	BDL	BDL	9400	1000	23500
Open space
Number of sites	15	13	4	15	12	12	9	8	7	3	9	1	--	2	1	--
Number of observations	105	109	13	111	44	49	38	41	36	13	26	5	--	6	5	--
% of samples above detection	97.1	92.7	92.3	93.7	64.4	65.3	23.1	39.0	36.1	15.4	34.6	60.0	--	100	100	--
Minimum	<1	<0.1	<0.5	<0.01	<0.8	<5	<2	<0.04	<0.7	<0.01	<1	<0.2	--	1900	100	--
Maximum	4168	3.4	3.3	0.76	210	390	100	8	120	0.08	11	0.84	--	63000	4700	--
Median	58	0.5	1.1	0.129	6	25	BDL	BDL	BDL	BDL	BDL	0.77	--	2150	1100	--
Rooftop	Water quality from rooftops varies with the type of roof. For more information see the section on Water quality considerations for stormwater and rainwater harvest and use/reuse

TSS=total suspended solids, NO₂=nitrite, NO₃=nitrate, TN=total nitrogen, Cl=chloride, Cu=copper, Zn=zinc, Ni=nickel, Cd=cadmium, Cr=chromium, CN=cyanide, VOC=volatile organic compound, FC=fecal coliform, EC=E. coli, FS=fecal streptococci
¹ Concentrations are in milligrams per liter
² Concentrations are in micrograms per liter
³ Concentrations are in Number per 100 milliliters
⁴Data is for trans-1,3-Dichloropropene and bromomethane
⁵ Data was selected from states with a similar climate to MN. The appropriate states were determined using Figure 1.3 from the Stormwater BMP Design Supplement for Cold Climates document.
⁶BDL = below detection level
⁷The following censoring techniques were used for this data:

• If detection rates were 90% or greater, a value of "0" was substituted for non-detects.
• If detection rates were greater than 50% but 90% or less, a value of 1/2 the detection limit was substituted for non-detects.
• If detection rates were 50% or less, the median was assumed to be below the method detection level.

Nitrogen

Map of nitrate distribution in Minnesota. (Source: “Condition of Minnesota’s Groundwater, 2007-2011, MPCA”, with permission)

Summary of characteristics of nitrate-nitrogen. Sources:Pitt et al., 1994, 1999; Weiss et al., 2008; ATSDR, 2011.
Mobility	Mobile
Solubility	High
Abundance in stormwater	Low/moderate
Toxicity	Low. Primary concern is for infants less than 6 months in age.
Degradation potential	High in anaerobic environments; low in aerobic environments
Adsorption/absorption	Low
Plant uptake	High
Potential risk to groundwater	Low/moderate based on high mobility but relatively low concentrations in urban stormwater.

For an excellent review of nitrogen, link here. The following discussion provides a general overview of nitrogen in stormwater and fate and transport in soil and the vadose zone.

Nitrogen is found in many forms in stormwater runoff, with the most common forms being ammonium+ammonia, organic nitrogen, nitrate, and nitrite. Detectable concentrations of nitrogen occur in more than 95 percent of samples collected from urban runoff (International Stormwater Database). Total nitrogen concentrations in urban stormwater are typically in the 1 to 2 milligram per liter range. Concentrations tend to be somewhat higher in residential areas compared to other land uses (International Stormwater Database).

Ammonium, ammonia, and organic nitrogen comprise the reduced forms of nitrogen and typically account for about two-thirds of total nitrogen in stormwater runoff, although this varies widely with source area (see EPA). Together, these forms are expressed as Total Kjeldahl nitrogen. These forms of nitrogen have low mobility and are attenuated in most stormwater management BMPs through adsorption or oxidation to nitrate.

Nitrate is highly mobile in aerobic environments. It is estimated to be the most common nonpoint-source groundwater contaminant in the world (Gurdak & Qi, 2012). Despite its high solubility, nitrite is detected with much less frequency than nitrate because nitrite oxidizes rapidly to form nitrate. Nitrate concentrations in stormwater are typically 1 milligram per liter or less, well below the drinking water standard of 10 milligrams per liter.

Ammonia is highly toxic to humans and aquatic organisms, but concentrations in stormwater are typically well below levels of concern. Nitrate has relatively low toxicity, although concentrations exceeding 10 milligrams per liter in drinking water can lead to the phenomenon known as “blue baby syndrome” which affects babies less than 6 months old (Prey et al., 2000). Nitrates and nitrites have not been classified as carcinogenic, however a metabolic pathway exists that lead to formation of N-nitroso compounds, some of which are carcinogenic (ATSDR, 2011).

Chloride

Summary of characteristics of chloride. Sources:Pitt et al.; Neiber et al., 2014.
Mobility	Mobile
Solubility	High
Abundance in stormwater	Seasonal high (winter, early spring)
Toxicity	Low
Degradation potential	Low
Adsorption/absorption	Low
Plant uptake	Low
Potential risk to groundwater	High

Chloride in stormwater often comes from the salts used in road surface deicing agents. Only limited data was found in the International Stormwater Database for chloride concentrations in stormwater runoff. Chloride was detected in approximately 38 percent of the 29 samples that were submitted to the database. Only sites in northern climates were included in this analysis.

At elevated concentrations, chloride can become toxic to aquatic life. Elevated levels of chloride can also result in low oxygen conditions, leading to the release of phosphorous and metals sorbed to the solids (Novotny et al., 2008). In addition, high levels of chloride will increase the density of the water, causing the salt containing water to settle to the bottom of the water body. This results in stratification and disrupts lake mixing patterns (New Hampshire Department of Environmental Services).

Cyanide

Summary of characteristics of cyanide. Sources:ATSDR, 2006; EPA Technical Factsheet: Cyanide, N.D.
Mobility	Mobile
Solubility	Depends on form of cyanide. Generally soluble.
Abundance in stormwater	Seasonal (highest in winter, early spring)
Toxicity	High
Degradation potential	Moderate if not at toxic concentrations
Adsorption/absorption	Variable. Nitriles and soluble cyanides (e.g. hydrogen and potassium cyanide) have low absorption potential while insoluble forms may sorb to soil particles.
Plant uptake	Low
Potential risk to groundwater	Low based on low concentrations in stormwater runoff

Cyanide is often found in road salt, where it is used as an anti-caking agent. Another source of cyanide is discharge from industrial facilities. Only limited data was found in the International Stormwater Database for cyanide concentrations in stormwater runoff. Cyanide was detected in approximately 10 percent of the 23 samples submitted to the Database. As with chloride, only sites in northern climates were included.

Mobility in soil depends on the form of cyanide. Nitriles have the potential to leach to ground water as they do not adsorb to soil. They tend to be resistant to hydrolysis in soil or water. Cyanide-containing herbicides have more moderate potential for leaching. Soluble cyanide compounds such as hydrogen and potassium cyanide have low adsorption to soils with high pH, high carbonate and low clay content. At pH less than 9.2, most free cyanide is expected to convert to hydrogen cyanide, which is highly volatile. Soluble cyanides are not expected to bioconcentrate. Insoluble cyanide compounds such as the copper and silver salts may adsorb to soils and sediments EPA.

Cyanide is an extremely toxic pollutant. It prevents the body from using oxygen and at a sufficient concentration it can lead to death. Low exposure can cause headache or dizziness (ATSDR, 2006). Chronic exposure can lead to nerve damage or thyroid problems.