Agriculture's impact on water quality depends on the type of agricultural activity employed. Soil erosion and sedimentation, nutrients, pesticides, and irrigation are the major agricultural concerns to nonpoint source pollution (Academy of Natural Sciences 1996). Further, irrigation practices may have an impact on both water quality and quantity.
Both large- and small-scale studies have been done to investigate this issue. Large-scale approaches often utilize a watershed approach and include the use of GIS to evaluate land use, soil type, elevation, and other mappable characteristics to explain the spatial distribution of nonpoint source contamination in groundwater and surface water. Small-scale approaches include investigation of processes affecting the transport and persistence of nutrients and pesticides to surface water; and in situ chemical processes influencing the transport of nitrate, trace elements, pesticides, and other contaminants to groundwater.
Soil erosion results in nutrient depletion and reduction of soil depth, both of which directly affect plant growth. Soil erosion may also lead to changes in river channels, and to sedimentation in rivers, lakes and reservoirs.
In agriculture, erosion occurs when fields are cleared of vegetation to prepare for crop planting or when vegetation is removed by grazing animals. The physical erosion potential of some soil may be exacerbated by previous agricultural practices which may have reduced the soil’s chemical fertility. The loss in fertility slows vegetative growth and leaves the soil surface exposed to wind and rain.
Where fields are subject to severe erosion, the potential for water quality problems associated with silt and sedimentation is relatively high. Much of the sedimentation can be eliminated by leaving a 35 to 50 foot section of unplowed field next to waterways (Academy of Natural Sciences 1996).
Conservation tillage is a successful method of reducing erosion and runoff from agricultural fields. This practice involves minimizing soil distrubance and water loss by retaining crop residues on the land and leaving the surface rough. Studies have shown that erosion potential is decreased by 30% and runoff by 60% when conservation tillage is used. Sediment loss and phosphorus and pesticide transport can be reduced by as much as 90%. The effectiveness of tillage practices, such as reduced-tillage, ridge-till or no-till varies depending on climate, soil characteristics, and type of crops being grown (Academy of Natural Sciences 1996).
Additional management practices which address agricultural erosion include planting methods such as contouring and strip cropping.
Irrigation practices may affect both water quality and quantity. In some regions of the county, irrigation practices may result in increased salinization of the soil water, due to relatively soluble materials being dissolved. This process may make the soil unusable for plant growth. In other regions, irrigation may compete with other water uses. Potential problems in a stream, such as increased temperatures, decreased dissolved oxygen concentration or concentrated pollutants, may result from decreased streamflow when irrigation water is pumped directly from the stream or from a groundwater source that feeds the stream. Nearby wells may also experience a drop in water pressure. When irrigation is practiced, 80-90% of the water is consumed; it is not returned to flow. (Rogers, 1994).
The following are all solutions which have been used to contend with salty drainage water:
evaporation ponds
intentional
leaching into depleted aquifers
drainage
systems which lead to reservoirs, lakes, wetlands and bays
incorporating
successive cropping patterns, letting drainage water flow from fields planted
with salt-sensitive crops to fields with salt-tolerant crops and then onto
land with salt-resistant plants, to use drainage waters for irrigation
while at the same time removing the salts
Agricultural pollutants may result from leaching, surface runoff, erosion and discharges of animal wastes. The relative importance of these contributions varies considerably and depends upon the type of agricultural operation and management, and site-specific geographical characteristics (Worthington 1986).
Rates of soil erosion are usually much higher on cropland than on grassland or forest because the soil surface is exposed for at least part of the year, during cultivation and the early stages of crop growth. With the rates of soil erosion exceeding the rates of soil formation, there is a net soil loss in cropland area in the U.S. A quantity of soil which takes 10 or more years to form may be lost in a single year of cropping. It has been estimated that several million hectares of cropland have been destroyed by soil erosion in the U.S.
Application
of fertilizers such as nitrogen or phosphorus, may result in pollutants
entering water courses or the groundwater. There is evidence that river
and groundwater nitrate levels have increased as a result of increased
use of nitrogen fertilizers. Fertilizer use may also add phosphorus as
well as nitrogen into surface waters resulting in eutrophication or nutrient
enrichment. Phytoplankton and other aquatic plants become more abundant,
and when the increased mass of organic matter decomposes, the dissolved
oxygen content of the water may be depleted. Under reduced oxygen conditions,
foul odors are generated, fish populations are adversely affected, and
the aesthetic quality and recreational value of the water is reduced. The
Great Lakes as well as many inland lakes have experienced such induced
eutrophication.
Another potential nonpoint source originating from cropped land is pesticides, which include herbicides, insecticides and fungicides. Surface runoff from irrigation or rainfall can wash pesticides from fields into groundwater, streams, and lakes. Some pesticides can also be lost to the atmosphere, either as drift during application or through volatilization from surface of soil or plants. Once airborne, they may become available for redeposition on land or water. They may show up hundreds, or even thousands, of miles away (Academy of Natural Sciences 1996).
The amount of pesticide runoff depends on the grade or slope of an area, the erodibility and texture of the soil, the soil moisture content, the amount and timing of irrigation or rainfall, and properties of the pesticide. Runoff ratings are based on the pesticide's ability to bind to the sediment during a runoff event. The leaching potential depends on whether the pesticide dissolves easily in water, the soil structure and texture, the amount and timing of irrigation or rainfall, the amount of adsoprtion to soil particles, and the persistence of the pesticide (MSU Extension 1990).
While such chemicals may have direct impacts on water quality via aerial drift, runoff and percolation into groundwater, indirect impacts can also result. The use of pesticides to control weeds and pests has made it possible to reduce or eliminate crop rotation. Traditionally, crop rotation was used to control weeds and pests and to maintain the soil fertility. With pesticide usage, the need for crop rotation is decreased, the soil fertility is reduced, and the soil becomes less capable of maintaining a protective vegetative cover.
Another water quality impact from cropland is modification of the drainage network. Drainage and its potential modification can affect the soil structure, soil aeration, hydrology and water chemistry. The history of Michigan's Saginaw Bay Watershed is a prime example of changes in drainage to accomodate agricultural land use, in particular crop production. Drainage has been used as a method of land improvement, particularly during times when wetland sites were considered of little or no value. State and federal government programs added incentives to farmers to drain land to make it viable for agricultural production.
The above figure illustrates the impacts of agricultural drainage to water, plants, wildlife, and other ecosystem properties (derived from Mather 1986).
The result of such land modification has been the loss of wetland habitat, and thus the loss of wetland functions and species diversity. The conterminous United States has lost over 53% of its original wetlands. Within Michigan, approximately 50% of the wetlands have been lost, representing over five million acres (Cwikiel 1992).
Concern about pesticides reaching groundwater and surface water supplies can be addressed through best management practices. The use of buffer zones have been shown to be very beneficial. Research has shown that leaving a band of natural vegetation around a plowed field can reduce the impact of pests such as grasshoppers, potentially reducing the amount of pesticides necessary for control. These buffers also help prevent pesticide drift (Academy of Natural Sciences 1996).
Pesticide reduction may also be achieved through Integrated Pest Management (IPM). In the Great Lakes basin, pesticide reduction has been promoted by Ontario, Canada, with a program that has set an official goal of reducing pesticide use 50% by the year 2002 through IPM. IPM programs emphasize the use of natural predators, special crop combinations and rotations, mechanical control methods, and carefully targeted pesticide applications to reduce pest destruction in agricultural lands (Academy of Natural Sciences 1996).
Other practices which can reduce the potential for surface and groundwater pesticide contamination include:
Using
integrated pest management programs and minimizing pesticide use by combining
with other pest management practices. These may include use of resistant
varieties, crop rotation or biological control agents.
Considering
the geology of the area, noting water table depth and soil permeability
when selecting the pesticide of use.
Considering
soil characteristics and determining the susceptibility of the soil to
leaching or runoff when selecting the pesticide of use.
Selecting
pesticides carefully. Those that are highly soluble, relatively stable,
and not readily adsorbed to soil are the most likely to leach.
Following
label directions, including rates, times, and placement.
Calibrating
and measuring accurately.
Considering
weather and irrigation.
Changing
the location of mixing areas, by mixing and loading pesticides on an impervious
pad, if possible.
Disposing
of wastes properly.
Storing
and mixing pesticides away from water sources such as wells, pond and springs.
Source: (MSU Extension 1990)
Prior to the trend to specialize, farms engaged in both crop and livestock production creating a more balanced system of production. Animal feed was grown on the farm, and the animal manure was returned to the land as fertilizer. With the advent of specialized farming, the intensity of production is increased, making it more difficult for managers to contend with the system's manure. The intensification of animal production has resulted in the production of exorbitant amounts of animal manure.
Number of Hogs in Michigan (from Agricultural Census, 1991/1992)
With increased levels of animal manure, a greater likelihood exists for waste material entering water bodies through leaking slurry tanks, surface runoff of feedlots, or surface runoff from saturated or frozen land. As the organic matter decomposes, oxygen is consumed, leaving less oxygen available for aquatic organisms. Additionally, pollution from animals adds nutrients such as nitrogen and phosphorus to water systems.
A
set of management guidelines for Manure Management and Utilization was
adopted under the Right to Farm Act. These voluntary practices provide
guidance for livestock producers with regards to runoff control and wastewater
management, odor management, design of manure ponds and lagoons, and manure
applications to land.
A nutrient analysis
of the manure should be made prior to application.
Storage
of livestock manure allows farmers to spread it when conditions are most
appropriate for nutrient use by crops and least likely for contamination
to surface water.
Source: (MSU Extension Bulletin MM-2)
