Land spills, soil and groundwater

A spill on concrete or asphalt stays mostly on the surface, where it can be absorbed and collected. A spill on gravel, soil, grass or snow behaves very differently. The liquid soaks in, and once it is below the surface it is out of sight, hard to recover and able to travel. Fuels and solvents that reach groundwater can contaminate wells and streams for decades. This lesson explains how liquids move in soil, what first responders can do on unpaved ground, and why the later stages of soil and groundwater cleanup need environmental professionals.

How liquids move in soil

Soil is a mixture of mineral grains, organic matter and pore spaces filled with air and water. When a liquid spills on soil, gravity pulls it downward through the pores, while capillary forces spread it sideways. In coarse soils such as sand and gravel, liquid drains down quickly. In fine soils such as clay, it moves slowly and spreads more sideways, but cracks, root holes and old burrows can still carry it deep. Some of the liquid stays behind, clinging to soil grains as it passes, which is why even a small spill leaves a contaminated column of soil.

Below a certain depth, the pores are completely filled with water. The top of this saturated zone is the water table, and the water below it is groundwater. Groundwater is not an underground lake; it is water held in the pores of soil and rock, moving slowly from higher ground toward springs, streams, lakes and wells. Depending on the site, the water table may be a meter below the surface or many meters down, and it rises and falls with the seasons.

Floaters and sinkers

What happens at the water table depends on the liquid. Fuels and most oils are lighter than water and do not mix with it. When they reach the water table, they spread out as a floating layer, sometimes called free product or light non-aqueous phase liquid, or LNAPL. Some components, especially benzene, toluene, ethylbenzene and xylenes, known as BTEX, dissolve slowly into the groundwater and travel with it as a dissolved plume, which can reach far beyond the original spill.

Chlorinated solvents such as trichloroethylene and tetrachloroethylene, once common in degreasing and dry cleaning, are denser than water. They sink below the water table and can collect in pools on clay layers or bedrock, slowly dissolving for decades. These dense non-aqueous phase liquids, or DNAPLs, are among the hardest contaminants to clean up. Water-soluble chemicals such as glycols, some pesticides and road salt mix directly with groundwater and travel with it.

How fast groundwater moves

Groundwater moves much more slowly than surface water, usually centimeters to meters a day. Its speed depends on how easily water passes through the soil, called hydraulic conductivity, how steeply the water table slopes, called the hydraulic gradient, and how much of the soil is open pore space, called porosity. A simple form of Darcy's law estimates the average speed of the water through the pores:

Here \( v \) is the average groundwater velocity, \( K \) is the hydraulic conductivity, \( i \) is the hydraulic gradient and \( n \) is the effective porosity. Dissolved contaminants usually move a little slower than the water because they stick to soil, but the estimate shows the order of magnitude.

# Average groundwater velocity v = K * i / n for different soils (illustrative values).
soils = {"gravel": 1e-2, "clean sand": 1e-4, "silty sand": 1e-6, "clay": 1e-9}   # K in m/s
i, n = 0.01, 0.30                                     # gradient (1 m per 100 m), porosity
for soil, K in soils.items():
    v = K * i / n                                     # m/s
    per_day, per_year = v * 86400, v * 86400 * 365
    print(f"{soil:<11} K={K:.0e} m/s -> {per_day:9.4f} m/day, {per_year:10.2f} m/year")
d = 200
v_sand = 1e-4 * i / n * 86400
print(f"Time for water to travel {d} m through clean sand: about {d / v_sand:.0f} days")

gravel      K=1e-02 m/s ->   28.8000 m/day,   10512.00 m/year
clean sand  K=1e-04 m/s ->    0.2880 m/day,     105.12 m/year
silty sand  K=1e-06 m/s ->    0.0029 m/day,       1.05 m/year
clay        K=1e-09 m/s ->    0.0000 m/day,       0.00 m/year
Time for water to travel 200 m through clean sand: about 694 days

In clean sand, groundwater can travel a hundred meters in a year, so a fuel plume can reach a well across the road within a few years. In clay, the same distance could take thousands of years. This is why site geology matters so much: the same spill can be a minor local problem on clay and a regional water supply problem on sand and gravel.

Why small spills matter to groundwater

Very small amounts of some contaminants make large volumes of water unfit to drink. Benzene, a component of gasoline, is a known human carcinogen. Health Canada's drinking water guideline and the US Environmental Protection Agency's maximum contaminant level for benzene are both 0.005 milligrams per liter, or 5 micrograms per liter. The example below shows how much water the benzene in a single liter of gasoline could contaminate to that level if it all dissolved.

# How much water could the benzene in 1 L of gasoline contaminate? (illustrative)
gasoline_density_kg_per_L = 0.74
benzene_fraction = 0.01          # roughly 1 % by mass in modern North American gasoline
limit_ug_per_L = 5               # drinking water limit for benzene (Canada and US)
benzene_ug = 1 * gasoline_density_kg_per_L * benzene_fraction * 1e9   # micrograms
water_L = benzene_ug / limit_ug_per_L
print(f"Benzene in 1 L of gasoline: about {benzene_ug/1e6:.1f} g")
print(f"Water it could take to the drinking water limit: about {water_L:,.0f} L")
print(f"That is roughly {water_L/2500:,.0f} household water tanks of 2,500 L,")
print(f"or about {water_L/3.785:,.0f} US gallons")

Benzene in 1 L of gasoline: about 7.4 g
Water it could take to the drinking water limit: about 1,480,000 L
That is roughly 592 household water tanks of 2,500 L,
or about 391,017 US gallons

Not all the benzene would dissolve, and much would evaporate or break down, so the real figure is smaller. But the point stands: a few liters of fuel soaking into the ground near a well is a serious matter, and spills to bare ground must be dealt with quickly.

First response on unpaved ground

The goals on soil are the same as on pavement: stop the source, stop the spread and recover as much liquid as possible before it soaks in. Speed matters even more, because every minute more liquid moves below the surface. Stop the source if safe. Contain the spill with socks or by scraping up a small earth berm downhill of it. If liquid is pooling, soak it up with pillows and pads or pump it into a container. Cover the area with plastic sheeting if rain is coming, because rain drives contamination deeper and spreads it. Do not flush the area with water.

Mark the area so that nobody drives over it or digs in it, which can spread contamination. For small spills on soil, the site plan may allow trained workers to dig out the visibly stained soil into lined containers, with care not to spread it. For anything larger, or where the spill may have gone deep or reached groundwater, the plan should call in an environmental consultant or spill contractor. Excavation can disturb buried utilities, so call for a locate before digging, as covered in HSE-217.

Snow, ice and frozen ground

Winter changes how spills behave. Snow absorbs liquid like a sponge, which can help: contaminated snow can be scooped into lined containers before it melts. But snow also hides spills, and when it melts in spring the contamination is released to soil and drains. Frozen ground slows downward movement, so liquid spreads across the surface instead, often farther than expected. Ice on a lake or river can hold a spill on top for collection, but cracks and open water can let it through. Cold temperatures make oils thicker and slower to evaporate. Mark winter spill areas so that they can be checked after melt.

After the first response

Spills that reach soil and groundwater usually need investigation by qualified environmental professionals. They take soil and water samples, compare results with state cleanup standards, and design a cleanup. States set soil and groundwater cleanup levels, often drawing on EPA risk-based screening levels, and many run voluntary cleanup programs. Cleanup may involve excavation and disposal, pumping out free product, treating groundwater, or allowing natural breakdown with monitoring. Leaks from regulated underground storage tanks follow specific federal and state tank rules.

The first responder's job in this phase is to give the professionals good information: what spilled, how much, when, where it went, what was done and where any recovered liquid and contaminated material went. Photos and a sketch of the spill area made at the time are very valuable.