Plant Interactions with Biogeochemical Environments - Introduction to Phytoremediation of Contaminated Groundwater - page 272

11.4.2 Plants and Biosolids

United States farmland acreage is approved for the applica-
tion of biosolids. Moreover, biosolids destined for use as
fertilizer must conform to chemical limits set in 40 CFR Part
503 Standards for the Use and Disposal of Sewage Sludge
(United States Environmental Protection Agency 1995).
This regulation only applies to the levels of metals in the
biosolids, not organic chemicals. However, the Milwaukee
Metropolitan Sewerage District, a large municipal treatment
plant in Milwaukee, WI, has approval to package its
biosolids as a commercial lawn fertilizer, which can
be purchased in home centers around the United States.
This biosolid purportedly contains dried microbes (fecal
coliforms); trace minerals essential to plant growth, such as
iron and calcium; and other heavy metals, such as cadmium,
lead, and selenium. It meets 40 CFR Part 503 Class A
“Exceptional Quality” requirements. This means that the
biosolids in question have to meet the limits established for
the three criteria for presence of pollutants, pathogens, and
attractiveness to vectors (rodents, mosquitoes; United States
Environmental Protection Agency 1994).
In sum, waste treatment by plants is probably one of the
earliest artificial interactions between plants and man's
wastes. Wastes are applied directly to plants for the benefit
of the plant and to decrease the biosolid volume. In the case
of the phytoremediation of contaminated groundwater, the
plants are directly applied at contaminated site to decrease
the waste concentration.

Much like the hydrologic cycle described in Chap. 2 consists
of a continual, natural cycle of evapotranspiration and pre-
cipitation, man's use of water has added an artificial
subcycle to include consumption, storage, and discharge of
water. For example, water can be taken from a surface-water
source, treated, delivered to homes, used, and then sent to
groundwater through the tile field of a septic system. This
pathway truncates the hydrologic cycle until the groundwa-
ter discharges to a surface-water body and once again can
evaporate.
Any residual material left behind after the physical,
chemical, and biological components of wastewater treat-
ment is called sewage sludge, or biosolid. The term biosolid
has come to replace sewage sludge to reflect more positively
the potential market for its use. This material can be solid,
semisolid, or liquid. This material can be stabilized to reduce
its volume and concentration of pathogenic organisms
through a number of processes. Digestion uses microbes
under aerobic or anaerobic conditions to breakdown the
material into simpler forms. Stabilization also can occur if
the pH of the material is increased by adding chemicals, such
as lime. This also reduces odor-causing compounds and
reduces the number of pathogenic organisms, such as bacte-
ria, viruses, and protozoa. The solids also can be dried by
being spread on paved surfaces or sand beds. Additional
stabilization can occur through composting, heat drying
(pelletization), and chemical fixation. This material has
become a resource for use as fertilizer, after meeting regu-
latory acceptable levels of contaminants and pathogens, that
can be applied to crops. Similar to the natural decomposition
of dead organic matter that makes stored nutrients and
minerals available to living plants, or manure or mulch
applied around plants, the use of biosolids as a way of
releasing nutrients back to plants is a natural part of the
global cycling of nutrients.
One of the concerns with the use of biosolids for
incorporation into the plant base used for consumption is
that the wastewater-treatment plants treat industrial wastes
as well as municipal wastes. Although most wastewater-
treatment plants do not accept industrial wastes unless the
industry performs a pre-treatment, as defined under 40 CFR
Part 403, unpermitted wastes can enter wastewater-treatment
plants and can sometimes lead to disastrous results. An
example is provided by the release of tributyltin (TBT) to a
surface-water system after it was sent (unpermitted) to a
wastewater treatment plant near Columbia, SC (Landmeyer
et al. 2004). This event led to the permanent closure of the
wastewater treatment plant and many private ponds.
The potential for such contamination events to occur and
enter the food chain through biosolid application to farmland
fortunately is small. This is because less than 1% of the

11.4.3 Natural Attenuation

As we saw in the section on cycling of nutrients, it should
become apparent that these systems possess the ability to
deal with permutations to normal conditions. For example, if
an acid is added to a buffered system, the acid will not affect
the pH of the total solution until after the buffer is depleted.
The same process of buffering, or assimilative capacity, in
the environment occurs for a wide range of pollutants. Many
civilizations depend on the assimilative capacity of surface-
water systems to receive untreated effluent.
The process of contaminant reduction as part of the
assimilative capacity is driven by both abiotic and biotic
processes (National Research Council 2000). In groundwa-
ter remediation, intrinsic bioremediation refers to the in situ
capacity for groundwater to clean up contaminants. Because
groundwater is below ground removed from the atmosphere
and direct precipitation, and because of oxygen's low solu-
bility in water, aquifers tend to be oxygen limited. These
limitations can be overcome by engineering ways to
deliver oxygen into the aquifer, which is called engineered
bioremediation.
An additional term regarding the assimilative capacity of
groundwater systems is called natural attenuation, and it is