Environmental Engineering Reference
In-Depth Information
Table 6 Range of constituents in produced water from shale gas wells
extracting from the Marcellus Shales, Pennsylvania, US, after
Gregory et al.
20
Component
Concentration range (mg/l)
Total dissolved solids
66 000-261 000
Total suspended solids
27-3200
Hardness (as CaCO
3
)
9100-55 000
Alkalinity (as CaCO
3
)
200-1100
Chloride
32 000-148 000
Sulfate
0-500
Sodium
18 000-44 000
Calcium
3000-31 000
Strontium
1400-6800
Barium
2300-4700
Bromide
720-1600
Oil and grease
10-260
present, many of which can be toxic at elevated concentrations. Examples
include arsenic, cadmium and nickel. In addition, shales are hydrocarbon
source rocks and so will also contain a range of hydrocarbons and other
organics such benzene and naphthalene and other oil petroleum-related
compounds. They also may contain relatively high concentrations of uran-
ium and its associated radioactive decay products. This group of materials is
referred to as NORM - Naturally Occurring Radioactive Materials - and can
lead to a radiological hazard if significant concentrations are contained in
the flowback and/or produced waters. Radium-226 and 228 can be present at
up to 1000 pCi g
1
(pico curies per gramme) potentially many times over the
safe disposal limit.
18,19
The safe handling, storage and disposal/recycling of the wastewater are
paramount to avoid risks to humans and the environment. Experience from
the United States has highlighted the challenges that could be faced in the
UK if large-scale shale gas exploitation was to take place. A number of dis-
posal routes for wastewater are used in the United States, some of which are
unlikely to be allowed in the UK due to much stricter regulations. For ex-
ample, direct discharge to surface water would be prohibited.
In the United States a large proportion of the wastewater is disposed of
through deep underground injection. Such an option in the UK is likely to be
limited by the availability of suitable locations and environmental regu-
lation. Other disposal routes will, therefore, need to be considered. A
number of alternatives have been tried in the US, but none appear to offer a
satisfactory solution that could have widespread application in the UK. For
example, municipal wastewater treatment plants would not be able to han-
dle the large volumes of highly saline mineralised water as it would damage
the biological treatment process. Other forms of treatment, such as reverse/
forward osmosis and distillation, are possibilities, but both are energy-in-
tensive processes and the residual waste, although less in volume, would still
