KEY IDEA 2: Geologically related hazards can result from human activity
WASTE DISPOSAL
One of the most expensive environmental problems
Domestic waste, Hazardous waste, Radioactive waste
DOMESTIC WASTE
18.6 million tonnes domestic solid waste/year in UK
76% disposed of as sanitary landfill
Domestic waste reacts with water to give liquid rich in organic matter, mineral salts, bacteria
Leachate formed when rainfall infiltrates landfill and dissolves soluble fraction of waste
plus soluble products formed by chemical + biochemical processes
conditions usually anaerobic - therefore leachates contain high conc. dissolved organics
Landfill site 1ha - up to 8m3/day
Landfill site 10ha - effluent with same biochemical oxygen demand/year as small rural sewage works
Leachate hazardous - groundwater contamination - any wells at least 1km away
Three fold classification of landfill sites (OHP)
1Unsuitable
2Unsuitable if well nearby
3Suitable - but no guarantee that natural or artificial impermeable linings will
remain impermeable indefinitely
Leachate can be diluted and dispersed or collected by drains and treated
Economic + social factors need to be considered in site selection as well as geology
Need rocks with low permeability - shales, massive igneous, metamorphic
Avoid fissured rocks
Position of water table important- determines whether wet or dry tipping is involved
Avoid wet tipping unless waste is inert
Carbon dioxide + methane generated by domestic waste - amount varies; 2.2 -250l/kg Both gases toxic
Methane highly explosive mixture with air
Many cases of explosions where buildings on top of landfill sites - Avoid
Need to manage gas release by passive or power operated venting
HAZARDOUS WASTE
Largely industrial - toxic, infectious, corrosive, ignitable
Uncontrolled dumping - pollution of soil + groundwater resources
Any well abstracting groundwater at least 2km away
Protect from groundwater contamination by containment
Compacted clay barriers- trench backfilled with compacted clay 150mm thick
- good because of low hydraulic conductivity
+ ability to absorb some wastes
- avoid cracking due to drying during construction
Slurry trench cut off walls - bentonite/soil mix extends down into impermeable
layer
Geomembrane walls - 'U' shaped geomembrane in trench + filed with sand
- monitoring wells in trench to detect and remove any
leakage
- commonly used as composite liner combined with
clay blanket
Sheet piling
Grout curtains - 3 rows of holes - outer rows sealed first, inner rows
last to seal remaining voids
Hydraulic barriers - created by series of extraction wells -used to contain existing waste disposal sites
Liquid hazardous wastes disposed of by injection into deep wells in rocks below aquifers Several hundred or even thousands of metres deep
Limited volume
Potential injection zones often occupied by connate water
Monitoring very important - observation wells in area surrounding disposal wells
RADIOACTIVE WASTE
Low or high level
Low level not hazardous if dealt with by dilute + disperse method
Can be buried in carefully controlled and monitored sites where hydrological and geological conditions severely limit migration of radioactive material
High level waste - needs to be separate from biological systems for hundreds/thousands of years
Disposal very problematic
High level liquid waste - solidified and mixed with inert, non leachable material
- placed in steel + concrete containers - stored underground
- caverns in thick impermeable rock formations: salt, granite, - over 500m deep in geologically stable areas
Salt good- high thermal conductivity - readily dissipates heat from high level waste
- plastic behaviour at depth - fractures will 'self heal'
- gamma ray protection same as concrete
- does not change much when subjected to radioactivity
- impermeable
MINE WASTE
Spoil heaps- disfigure landscape
- costs of restoring sites after extraction need to be included in
resource evaluation
- open cast sites may have recreational use or be used for land fill
Coal spoil heaps - problem of spontaneous combustion - leads to hot spots 600'C
- makes restoration difficult
- emission of gases also a problem ; carbon monoxide, carbon
dioxide, sulphur dioxide
- slope instability - Aberfan 1966 - 144 deaths - mudflow 32km/h
Tailings- fine slurry of waste pumped into lagoons
- need safe dams + embankments
- Buffalo Creek, West Virginia 1972 - dam failure - 118 deaths
Mine spoil heaps- tailings may be serious pollution problem
- common high concentrations of 'heavy metals' - cadmium etc