12
1. Selenium Pollution Around the World
is widespread and even pervades university research conducted to develop treatment methods for reducing ecological risks from refinery
effluents (eg, Hawkins et al. 1997). The oil industry transports and
disposes huge volumes of selenium-laden materials on a global scale.
At any point in this process it can become a major contributor to elevated selenium concentrations in aquatic ecosystems.
Agricultural Irrigation
In the early 1980s, a new selenium threat to fish and wildlife emerged:
subsurface irrigation drainage. This drainage water, usually containing elevated concentrations of soil trace elements and other constituents, has poisoned fish and aquatic birds at several locations in the
United States and the Middle East (Table 1.2, Figure 1.1). The mechanism underlying this phenomenon is simple, yet almost insidious. Agricultural irrigation practices in arid and semi-arid regions typically
use water applications in the 60 to 80 cm/yr range. The amount of
water applied is far in excess of what is needed to support crops, but
the excess is used to flush away salts that tend to accumulate in crop
root zones as evaporation occurs and inhibit plant growth. Subsurface
irrigation drainage is produced due to a specific set of soil conditions.
Shallow subsurface (3-10 m) layers of clay impede the vertical movement of irrigation water as it percolates downward. If the irrigation
water is not removed, this results in waterlogging of the crop root zone
and subsequent buildup of salts as excess water evaporates from the
soil surface, exactly the same problem that irrigation is intended to
solve in the first place (Moore et al. 1990).
Several methods of removing excess shallow groundwater can be employed, including the use of wells and surface canals to forcefully pump
and drain the water away. The method of choice in the western United
States is to install rows of permeable clay tile or perforated plastic pipe 3
to 7 m below the surface of agricultural fields (Letey et al. 1986). Once
these drains are installed, irrigation water can be applied liberally, thus
satisfying the water needs of crops while also flushing away excess salts.
The resultant subsurface wastewater is pumped or allowed to drain into
ponds for evaporative disposal, or into creeks and sloughs that are tributaries to major wetlands, streams, and rivers (Moore et al. 1990). Subsurface irrigation drainage is characterized by alkaline pH, elevated
concentrations of salts, trace elements, and nitrogenous compounds, and
low concentrations of pesticides. The natural biological and chemical filter provided by the soil effectively degrades and removes most pesticides
as irrigation water percolates downward to form subsurface drainage. At
the same time, naturally occurring trace elements in the soil, such as
selenium (up to 1400 f..lg/L), are leached out under the alkaline, oxidizing
1. Selenium Pollution Around the World
is widespread and even pervades university research conducted to develop treatment methods for reducing ecological risks from refinery
effluents (eg, Hawkins et al. 1997). The oil industry transports and
disposes huge volumes of selenium-laden materials on a global scale.
At any point in this process it can become a major contributor to elevated selenium concentrations in aquatic ecosystems.
Agricultural Irrigation
In the early 1980s, a new selenium threat to fish and wildlife emerged:
subsurface irrigation drainage. This drainage water, usually containing elevated concentrations of soil trace elements and other constituents, has poisoned fish and aquatic birds at several locations in the
United States and the Middle East (Table 1.2, Figure 1.1). The mechanism underlying this phenomenon is simple, yet almost insidious. Agricultural irrigation practices in arid and semi-arid regions typically
use water applications in the 60 to 80 cm/yr range. The amount of
water applied is far in excess of what is needed to support crops, but
the excess is used to flush away salts that tend to accumulate in crop
root zones as evaporation occurs and inhibit plant growth. Subsurface
irrigation drainage is produced due to a specific set of soil conditions.
Shallow subsurface (3-10 m) layers of clay impede the vertical movement of irrigation water as it percolates downward. If the irrigation
water is not removed, this results in waterlogging of the crop root zone
and subsequent buildup of salts as excess water evaporates from the
soil surface, exactly the same problem that irrigation is intended to
solve in the first place (Moore et al. 1990).
Several methods of removing excess shallow groundwater can be employed, including the use of wells and surface canals to forcefully pump
and drain the water away. The method of choice in the western United
States is to install rows of permeable clay tile or perforated plastic pipe 3
to 7 m below the surface of agricultural fields (Letey et al. 1986). Once
these drains are installed, irrigation water can be applied liberally, thus
satisfying the water needs of crops while also flushing away excess salts.
The resultant subsurface wastewater is pumped or allowed to drain into
ponds for evaporative disposal, or into creeks and sloughs that are tributaries to major wetlands, streams, and rivers (Moore et al. 1990). Subsurface irrigation drainage is characterized by alkaline pH, elevated
concentrations of salts, trace elements, and nitrogenous compounds, and
low concentrations of pesticides. The natural biological and chemical filter provided by the soil effectively degrades and removes most pesticides
as irrigation water percolates downward to form subsurface drainage. At
the same time, naturally occurring trace elements in the soil, such as
selenium (up to 1400 f..lg/L), are leached out under the alkaline, oxidizing