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Brassica Rice Broccoli Cabbage

Ammonia

Alfalfa

Strontium

Sunflower

Nitrate

Alfalfa

Uranium Zinc

Sunflower Sunflower

Figure 16.61 Representative phytoremediation plant variations.

The majority of phytoremediation plant systems, such as those depicted schematically in Figure 16.61, apply to soil depths extending to the first 2 to 3 m. In turn, most of these plants probably draw their water either from roots closely aligned to the surface or water drawn from vadose-zone pore moisture, as depicted in Figure 16.62. Most poplars, for instance, tend to have shallower root systems, and as a result, these types of plants have an inherent level of reliance on water being precipitated into, and then passed through, the surface soils.

However, there are also deep-rooting plants that maintain a more water-loving lifestyle, in which their root systems extend into the capillary soil region or underlying saturated groundwater zone (see Figure 16.63). These phreatophytic plants tend to have remarkable high summertime water uptake rates, possibly as a competitive means of trying to restrict the growth of their fellow plants. These deep-rooting plants generally have higher levels of plant biomass as well as higher overall growth rates.

With plants such as poplars, although they may not normally pursue this sort of phrea-tophytic growth mode, it is possible to induce these plants to form deep root systems using special drip-irrigation practices that effectively train them to pursue progressively deeper levels of water uptake. Aside from extending the potential reach of these phytoremediat-ing plants to lower levels, this induction of a deep-rooting preference helps the plant with its future water demand. However, at these deeper strata the matter of oxygen availability subsequently becomes a concern, as does the presence of nutrients. Furthermore, deeper soils usually tend to be more tightly packed, such that root impedance may also be an important factor.

When motivated to adopt this phreatophytic mode, though, alders, ash, aspen, river birch, and poplar have proven to be fast growers and rapid water users, with daily uptake rates during peak summertime periods ranging from 100 to as much as 1000L/day per tree. The resulting uptake of water from the deep, saturated soil zone may actually produce a sizable depression of 5 to 10 cm in the water table within the capture zone where water is being used by the trees. Field studies of this sort have been able experimentally to

_Capillary Fringe_

Groundwater Zone

Figure 16.62 Surface and vadose-zone contaminant remediation via phytoremediation.

develop hydraulic barrier strips using deeply rooted trees planted in rows aligned perpendicular to the direction of travel for the contaminated plume (as in Figure 16.64).

Of course, the actual process of evapotranspiration depends not only on the location and depth of the roots, but also on the number of plant and atmospheric parameters. The rate of water use by plants depends on the conductance of water through the plant stoma as well as the cumulative surface area of the leaves through which the water will finally be released into the atmosphere. The air temperature, wind speed, humidity, and radiation intensity will also play a part in the final rate of this release.

For those phytoremediation systems that employ trees, the spacing provided with the original plants can also prove to be an important factor. Of course, trees spaced too far apart will not be able to provide nearly as much of a remediating impact, but plants spaced too tightly together can also experience problems, particularly as they reach maturity. In particular, tightly bunched trees block each other's light, and they will probably end up contending for limiting nutrients. Recommended estimates for seedling plantings are in the neighborhood of several thousand seedlings per hectare, which will drop to a level of a few thousand after natural thinning takes place during the first few years of growth. Clearly, there are instances in which phytoremediation systems can play a significant role in site restoration, but it is not a universal solution for every contaminated soil. In some

Figure 16.63 Groundwater contaminant remediation via phytoremediation.

Groundwater Flow

Drawn Toward —^ Groundwater Table

Phytoremediation Site Depression Induced with"

Evapotranspiration by Deep-Rooted Phytoremediation Plants

Figure 16.63 Groundwater contaminant remediation via phytoremediation.

cases, the contaminants may simply be toxic to the plants, and in others the soils will not be amenable to plant growth. Many strongly sorbed contaminants, such as PCBs or PAHs, may be too tightly adhered onto soils to be amenable to rhizospheric degradation or uptake. There will also be instances involving contaminants that would be bioaccumulated into the plants (either directly or as metabolic intermediates) in forms and at concentrations that are unsafe and unacceptable.

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