Environmental Engineering Reference
In-Depth Information
Runge, 2002; Goolsby et al., 1999). Corn production is a major contributor to this e
ff
ect
both through direct nitrogen runo
from fertilizer application on farms and through the
use of corn as a feed for livestock whose manure contributes to water pollution.
US corn is also intensive in its use of herbicides and insecticides. Atrazine, the most
common herbicide used in corn production, among other crops - and the most common
pesticide detected in groundwater nationwide - is an endocrine disrupter and possible
human carcinogen (it causes cancer in rats). Exposure to atrazine creates risks for farm
workers, consumers of corn products, and users of groundwater downstream from farm
areas (EPA, 2001a; Repetto and Baliga, 1996; Ribaudo and Bouzaher, 1994; Briggs and
Council, 1992). Metolachlor and S-Metolachlor, both leading herbicides, are possible
human carcinogens (EPA, 2000; Briggs and Council, 1992). Chlorpyrifos, the most
common insecticide used on corn
ff
elds, is a neurotoxin that poses risks for children who
are exposed to it at high levels; it is also used on other foods, and for residential cockroach
and termite control (EPA, 2001b; Briggs and Council, 1992).
Due to important technological improvements, the intensity of herbicide use has
declined in recent years, although atrazine and other chemical herbicides still pollute
drinking water supplies (Benbrook, 2001a).
Pesticide intensity has remained roughly constant, which is disappointing given the
growing use of genetically modi
fi
fi
ed corn. By 2004, just eight years since the introduction
of genetically modi
ed corn varieties in the USA, nearly half of all corn land was planted
in GM crops. More than 30 percent of US corn was planted in varieties engineered with
the Bt (
Bacillus thuringiensis
) endotoxin to
fi
ght some common pests, most notably the
European corn borer. Roughly 15 percent of US corn was genetically engineered for her-
bicide tolerance (USDA-NASS, 2004). While there are widespread concerns about the
risks of such crops to human health and to the environment, their widespread adoption
has not yet produced the environmental bene
fi
t they promised: reduced pesticide appli-
cations (Clark, 1999; Heimlich et al., 2000; Benbrook, 2001b).
In addition, rising exports of unlabeled corn to Mexico, which contain high levels of
GM varieties, have caused signi
fi
cant concern in Mexico since the discovery of traditional
maize varieties contaminated with transgenic corn. This will be discussed later, in the
examination of the environmental impacts of changing USA-Mexico corn trade on
Mexico.
In terms of unsustainable water use, although only about 15 percent of US corn is irri-
gated, the vast majority of irrigated corn land is found in four states: Nebraska, Texas,
Colorado and Oklahoma. These states rely for their irrigation on water from the vast
Ogallala aquifer, an underground reservoir the size of Lake Huron. The Ogallala is being
depleted at unsustainable rates, calling into question the wisdom of expanding corn pro-
duction in areas lacking adequate rainfall (NRC, 1996; Opie, 2000).
Two additional environmental impacts from US corn production are worth examining
brie
fi
y: soil erosion and biodiversity impacts. Conversion to cropland has carried with it
rising problems with soil erosion. Some historical studies suggest that conservation tillage
practices have signi
fl
cantly reduced erosion rates since the 1930s. This would suggest that
expanding corn production may have little impact on soil erosion rates.
Biodiversity impacts are still of concern with expanded corn production. In sharp con-
trast to the situation in Mexico, biodiversity in the corn crop itself is long gone in the USA.
Commercially distributed hybrid varieties have been the norm in US production for
fi
