Leaky underground gas tank

Large oil spill from pipeline

The same large oil spill

Pumping out the surface oil

TCE plume

Composting toxic soil

Steam cleaning soil, similar to thermal desorption

Enhanced in situ bioremediation

What happens when we spill?
Q2C: What's the initial effect of soaking the ground with oil?
it becomes anaerobic and many aerobes die off
after entering the ground, the contaminant generally soaks into the soil, eventually finding its way to the groundwater
often the contaminant flows with the groundwater and forms a plume (very much like smoke from a chimney)
sometimes NAPL (non-aqueous phase liquids) like the BTEX solvents will collect in pockets near the groundwater since NAPLs are not water soluble
NAPLs tend to float on top of the water, DNAPLs (dense NAPLs) form their separate phase below the waters

So, what do we do about these spills?
First we identify the risk, we do a risk assessment
wells are bored into the ground to measure the extent and depth of the plume
geological and hydrogeological studies define the site and allow predictions of where a plume may flow

Ex situ remediation
material, like contaminated soil, loaded on trucks and moved somewhere else for treatment
often, this material is brought to hazardous waste incinerator
soil materials are also commonly composted or landfarmed
landfarming is a poor choice since this can contaminate larger areas of land than originally fouled
thermal desorption is good for VOC (volatile organic compounds) - heat is applied and VOCs vaporize and can be captured by condensing (with cold) the gases released
pump and treat - for ground waters - here, water is pumped into one well and then collected at another well downstream. The waters are treated on the surface and pumped back into the ground
all of the ex situ methods are undesirable because of high costs
moving tons of toxic earth by truck can also be hazardous to the communities through which it drives. These risks may be more costly than the actual cleanup
building facilities on-site can lower costs and risks a bit, but lead to other problems
the regulatory aspects can be a nightmare
if the site is in an urban or residential area, one may run into the "not in my backyard" (NIMBY) problem. Who wants a toxic waste incinerator right up the street?

In situ remediation
chemical/physical processes: flushing - pump air or water into the ground and flush out the material
solidification - entomb the waste in concrete - commonly used for heavy metals and radioactive wastes
vitrification - turn it into glass - also for heavy metals and radionuclides
the last two leave unsightly monuments to industrial contamination, and concrete can degrade with time, just releasing the trapped metals
flushing includes a technique called sparging - air is forced into the ground and VOCs are vaporized
a number of collection wells are used to capture the VOC vapors and these are then collected and treated on the surface
in situ treatments are less costly since the soils do not have to be moved and often the equipment costs are lower

What about biological solutions?
in situ biotreatment is popular because it is inexpensive
wells are bored that can deliver nutrients (like N fertilizers) and oxygen to the indigenous microbial population
generally we want an aerobic system with unlimited growth potential for the microbes
the spill site provides large amounts of specific types of carbon sources
certain microbes can eat these compounds
we select for degrading organisms in the environment and enrich the natural population for these degrading bacteria
then we give them plenty of oxygen and N and P and *snap* the bugs eat up our toxic waste
sometimes we can even add some bugs that we selected and enriched for in the laboratory
the soil can also be aerated by churning it up and mixing it
another type of biological solution is to cut off the path of the plume with a wall of bacteria
here bacteria are added to a trench (perhaps with support material) and, as the plume of organics passes through this region, it is degraded
in another technique that is not quite in situ, the soil is excavated and then mixed with water and microbes to make a slurry
this is then placed in a tank, aerated, feed N and P and mixed
this aerobic bioreactor degrades the organic pollutant
the slurry technique is popular because of high reaction rates, but can be too expensive since capital expenditures are needed and there are excavation costs

So what's best?
the hot technique today is "natural attenuation"
leave it where it is and let the natural microbial population in the soil eat it up
if it can be shown that a plume is not getting any larger with time then a site might be a candidate for this program
the site will need to monitored, using wells, perhaps for 30 years or more
but the costs are very low
this technique is very popular with industry because the costs are so low

this marks the end of the section on applied and environmental microbiology