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Chapter 10 - Agrochemical Entry into Groundwater

W.D. Reynolds, C.A. Campbell, C. Chang, C.M. Cho, J.H. Ewanek, R.G. Kachanoski, J.A. MacLeod, P.H. Milburn, R.R. Simard, G.R.B. Webster, and B.J. Zebarth

Highlights

• Low-level nonpoint-source entry of agrochemicals into groundwater poses a significant risk because of its extent and the difficulty in controlling it.
• Aquifers in south coastal British Columbia underlie areas of high rainfall and intensive agriculture and often contain nitrate levels above the safe limit.
• The Prairie Provinces are at low risk of agrochemical contamination; point-source contamination of domestic wells occasionally occurs, as does groundwater contamination in areas receiving large amounts of manure or irrigation water.
• Nitrate from fertilizer and bacteria from manure are the main contaminants of groundwater in central Canada. Pesticides, including atrazine (persisting from use in intensive corn-cropping in the past), are often present in groundwater, almost always at levels well below safe limits.
• In the Maritime Provinces, groundwater contamination occurs mainly in areas where potatoes and corn are produced intensively. Leaching of nitrate can be a serious problem, although levels seem to be steady. Pesticides, when detected, are almost always at levels well below the safe limits.
• Entry of agrochemicals into groundwater can be reduced by improved farming methods; these include using catch crops and crop rotations and applying nutrients, pesticides, and irrigation water more efficiently.

Introduction

Groundwater is subsurface water that occurs below the water table in geological materials such as soils, sand and gravel deposits, and bedrock formations. Geological materials that are extensive and permeable enough to yield a steady supply of groundwater to one or more wells are called aquifers. Groundwater pumped from aquifers supplies drinking water to more than six million Canadians (26% of the population). It also contributes substantially to rivers and streams in many parts of Canada. Consequently, groundwater is an important resource that must be protected from the entry of harmful substances including potentially harmful agrochemicals.

The main agrochemicals are fertilizers and pesticides. Crop yields would be unacceptably low in many parts of Canada if fertilizers and pesticides were not used regularly. However, pesticides, some components of fertilizer, such as nitrate, and the bacteria in manure are harmful to humans and animals if present at high enough levels in drinking water.

And our water, the universal solvent, present in the air, in the soil, in plants, animals and man. Without it life could not endure.

J.A. Toodgood
Our Soil and Water

Another important agrochemical is naturally occurring nitrate, produced in the soil by the breakdown of organic matter (decomposed plant and animal residues). Although this nitrate is known to enter groundwater in some situations, our focus is mainly on agrochemicals that are applied to the land. We do not deal with contamination of either surface water or groundwater resulting from salinization, sewage effluent, and the use of sewage water for irrigation.

Agrochemical occurrence in drinking water

Because agriculture is often carried out on large areas of land situated above important aquifers, there is a risk that agrochemicals will enter drinking water. For example, agrochemicals can seep into individual wells because of accidental spills, poor well construction, or improper practices in storing and handling pesticides. This type of high-level point-source entry (Fig. 10-1) is localized and fairly easy to control by education and regulation.

Another way in which agrochemicals enter groundwater is by low-level nonpoint-source entry (Fig. 10-1). This type of entry occurs over large areas, such as agricultural regions and watersheds. It is mainly caused by the slow downward movement of agrochemicals through the soil profile and into the groundwater. (Pesticide use in Canada)

Low-level nonpoint-source leaching of agrochemicals does not always occur in agricultural areas and, when it does occur, often produces levels of agrochemicals in groundwater that are below the current Canadian safe limits for drinking water. However, this type of agrochemical entry into groundwater is usually of greater concern than high-level point-source entry because

• it can be widespread
• it is more difficult to control, because neither the sources nor the leaching behaviour of the agrochemicals are well understood
• it can occur even where agricultural practices are designed to minimize soil and water degradation
• there is evidence that it may be increasing in some areas of intensive agriculture
• the public may have concerns about long-term exposure to even low levels of agrochemicals.

Controlling agrochemical entry into groundwater

Important factors that control the amount and rate of leaching of agrochemicals through the soil profile and into groundwater include:

• type and intensity of agriculture practised
• weather
• land and crop management practices
• type and amount of agrochemicals used
• soil characteristics.

These factors often interact in complex ways. The entry of agrochemicals into groundwater over a particular area depends on which factors are relevant and how they interact at that location.

Type and intensity of agriculture

Agrochemical leaching into groundwater often occurs in regions where intense cropping results in substantial use of chemicals, and where concentrated production of livestock results in high rates of manure being applied to the soil. In both situations, excess agrochemicals can leach through the soil profile. Leaching, especially of nitrate, can also occur in relatively low-intensity dryland farming where frequent summerfallowing is used. Summerfallowing can cause excess water and unused nitrate to build up in the soil profile. The excess water may then percolate downward and carry the unused nitrate into the groundwater zone.

Weather

The amount and timing of precipitation are the main elements of weather that influence agrochemical leaching. Rainfall infiltrating the soil in excess of what crops can use may percolate deep into the soil profile, carrying agrochemicals with it. Most agrochemical leaching takes place during spring and fall, when rainfall is high and use of water by crops is low. The humid regions of Canada are much more prone to agrochemical leaching than the dry regions.

Management practices

Fertilizers, manures, or pesticides may leach into the groundwater when these agrochemicals are applied to the land in excess of crop requirements, and when they are applied at times when the crop cannot use them effectively (early spring or late fall, when crop growth is slow). Excess irrigation water, acting like excess rainfall, can carry agrochemicals deep into the soil profile. The production of crops (such as vegetables) that require high levels of fertilizer, or pesticides, or both, increases the potential for agrochemical leaching into groundwater.

Type and amount of agrochemicals

Some agrochemicals are highly leachable. Because they are slow to break down and are held loosely by the soil, they can move down through the soil profile much faster than a nonleachable substance. A moderately toxic, leachable agrochemical often poses a greater threat to groundwater resources than a highly toxic, nonleachable one. The herbicide atrazine and nitrate fertilizer are only mildly toxic to humans, but both are leachable and have been used extensively for a long time over large areas of land. The potential for widespread leaching of atrazine or nitrate into groundwater is high in some of Canada's prime agricultural regions.

Soil characteristics

Agrochemical leaching tends to increase with increasing soil permeability (a soil's ability to allow the passage of fluids, such as water) and decreasing soil dissipation capacity (a soil's ability to adsorb, or hang on to, and degrade chemical compounds). Coarse sandy soils and fine soils with many cracks, worm holes, and root channels usually have a higher permeability, or lower dissipation capacity, or both, than clay soils and soils with few large pores. Sandy soils therefore usually pose a greater risk for agrochemical entry into groundwater than do clay soils. (Pesticide leaching potential)

Current status and trends

Groundwater monitoring has not been carried out long enough, nor in enough detail, in Canada to give a complete national picture of the current status and trends for agrochemicals entering groundwater resources. However, we have enough information to develop an overall impression of the situation for Canada's main agricultural regions.

British Columbia

Aquifers are an important source of water for municipal, domestic, and agricultural use in the arable valleys of British Columbia. The aquifers in the interior plateau and most of the Peace River area are considered to be at low risk for agrochemical entry because of low rainfall and generally low-intensity agriculture. In southern British Columbia, however, some aquifers are at moderate-to-high risk for agrochemical entry because of

• intensive production of livestock, or crops, or both
• the use of summer irrigation
• the predominance of coarse, sandy soils.

The aquifers in the lower mainland are particularly vulnerable because this area also receives heavy winter rainfall. Significant amounts of agrochemicals (mainly nitrate) have already been detected in the groundwater of several aquifers in the Fraser and Okanagan valleys.

A major concern is the Abbotsford-Sumas aquifer in the lower Fraser Valley, which extends over about 100 square kilometres on both sides of the Canada-United States border. It is one of the most important sources of water in the area for both countries, making agrochemical contamination an international concern. This aquifer is highly susceptible to the entry of agrochemicals because of extensive use of high-nitrogen poultry manure on raspberry and forage crops, combined with climatic and soil conditions that promote leaching.

Nitrate concentrations have increased steadily in large parts of the aquifer over the past 20 years and are often greater than the Canadian safe limit for nitrate-nitrogen of 10 milligrams per litre of water, set out by the 1987 Canadian Water Quality Guidelines. Chlorinated hydrocarbon fumigants are also present in some parts of the aquifer, but at concentrations below provincially established safe limits. Concentrations of these pesticides in the aquifer are expected to decline over time because of their removal from the market.

Significant nitrate leaching into groundwater may also occur in an important aquifer near the town of Osoyoos in the lower Okanagan Valley. About 17% of wells sampled in 1986 and 33% of those sampled in 1987 contained nitrate-nitrogen levels higher than the safe limit. Fertilizer used in intensive, irrigated tree-fruit production is likely the main source of the nitrate. (Nitrate contamination)

The Prairies

Nonpoint-source entry of agrochemicals into groundwater appears to be rare in the dry prairie of southern Alberta and Saskatchewan. In that area agriculture is mainly low intensity (grain farming and low-density grazing of cattle) and the climate is dry--water use by plants usually equals or exceeds the rainfall.

Much of Alberta's groundwater is obtained from deep-bedrock aquifers that are not directly linked to the soil surface. Thus, only about 4-5% of domestic wells have nitrate levels above the safe limit.

In Saskatchewan, 7-17% of domestic wells have nitrate levels above the safe limit. It is now believed, however, that most of this contamination is from point sources (feedlots, spills, and leakage of surface water into shallow wells) and from naturally occurring nitrate.

Although most of the Prairie ecozone is thought to be at low risk of contamination by agrochemicals, their entry into the subsoil and groundwater can be significant under certain conditions. For example, applying feedlot cattle manure on the land at the maximum recommended rates has resulted in substantial soil and groundwater contamination by nitrate at some locations in Alberta. Nitrate-nitrogen levels can be more than 1 tonne per hectare in nonirrigated, continuously cropped soils and as high as 500-600 milligrams per litre in the shallow groundwater under irrigated soils (Fig. 10-2). Pesticides (mainly herbicides) have also been found at concentrations ranging from trace to higher than the safe limit in the groundwater below irrigated soils.

Experiments in Saskatchewan have shown that significant nitrate-nitrogen leaching can occur

• when land is frequently summerfallowed (summerfallowing may allow excess water and nitrate to accumulate in the soil and then percolate below the root zone)
• when crops are overfertilized, leaving excess nitrate in the soil (especially when fertilizer rates are based on general crop and soil characteristics rather than on soil and plant tissue tests)
• in some cases, where crops are under-fertilized (poor crop growth results in low nitrogen use by plants and excess water in the soil profile)
• when frequent irrigation is used (tile-drainage water from some irrigated fields contains nitrate levels above the safe limit, as well as trace amounts of herbicides).

Parkland and boreal transition

In Manitoba, information on the effects of agricultural activities on groundwater quality is limited. Available data suggest that agrochemical entry into groundwater is generally low where low-intensity agriculture is practised and where soils are clayey. In southern Manitoba, however, intensive agriculture is practised. The resulting rapid increase in the use of fertilizer and manure during the last 20 years has caused certain agrochemicals to accumulate under some fields.

A 1992-1993 survey in southern Manitoba found elevated levels of nitrate in the subsoils of fields that were heavily fertilized, or manured, or both, and were cropped to cereal grains or horticultural crops. Although this nitrate does not appear to have entered the groundwater significantly so far, the potential for it to do so in the future is high.

Also of concern in southern Manitoba are aquifers beneath the upper and lower Assiniboine delta. These major sources of drinking and irrigation water for the region underlie sandy and coarse loamy soils that support intensive agriculture, which often uses irrigation. A recent study at one site found nitrate levels above the safe limit in the groundwater below a highly fertilized wheat crop; the extent of this contamination is not known. Analysis of water samples collected in 1991-1993 from a few of the wells used for monitoring groundwater and from wells used to supply irrigation water suggests that pesticides have not contaminated these aquifers.

Central Canada

Many locales in central Canada's prime agricultural regions have diverse combinations of intensive agriculture, wet climate, highly permeable soils, shallow soils, and high water tables. As a result, the risk of agrochemicals leaching into groundwater in those areas is considered to be high.

In southern Ontario, the main agrochemicals entering groundwater are nitrate and the herbicide atrazine, along with bacteria. Nitrate contamination results mainly from a combination of fertilizer use, natural nitrate in the soil, and nitrogen compounds in rainwater. Bacterial contamination results from applying large amounts of liquid manure from large-scale livestock operations (mainly hog, beef, poultry, and dairy facilities). Atrazine occurs mainly as a result of intensive continuous corn cropping in the past; studies in eastern Ontario have shown that atrazine can persist in shallow groundwater for more than 5 years after its use is discontinued.

A 1992 groundwater survey of 900 rural water-supply wells in southern Ontario found that 37% of the wells contained nitrate, bacteria, herbicides, or all three, above safe limits; 15% had nitrate-nitrogen levels above the safe limit; 32% exceeded the safe limit for coliform bacteria; and 12% had detectable levels of pesticides (mainly atrazine), but only two of the wells (0.2%) had pesticide concentrations above safe limits. The incidence of groundwater contamination was widely scattered (Fig. 10-3) and was not related to crop type or cropping practice. However, bacteria contamination occurred more often where manure was applied regularly; nitrate contamination was more frequent under highly permeable soils and in shallow groundwater.

Data for groundwater quality in Quebec is inadequate to give a good picture of the extent and degree of agrochemical contamination in the agricultural areas of this province. Limited surveys in the St. Lawrence lowlands and the Île d'Orléans area found that significant contamination by nitrate and bacteria can occur in areas of intensive potato, vegetable, hog, and poultry production. A 1978-1980 survey of domestic wells in potato-producing areas found that 6% of wells contained nitrate-nitrogen levels above the safe limit. In the Île d'Orléans area, 83% of 35 sampled wells contained bacteria levels above the safe limit for drinking water, probably because of point-source contamination (inadequate waste treatment and surface run-off into improperly sealed wells). The herbicides aldicarb and atrazine were also detected in about 31% of the wells sampled in the ële dÕOrleans area but at concentrations well below the current safe limits. As with Ontario, the incidence of nitrate-contaminated groundwater appears to be widely scattered and more prevalent under highly permeable soils and in shallow groundwater.

While there is a certain public mystique about groundwater and its purity, the farm community understands groundwater better than most; rural residents depend more on groundwater than any other segment of society.

George R. Hallberg
From Hoes to Herbicides:
Agriculture and Groundwater Quality

Atlantic Canada

Agrochemical entry into groundwater in Atlantic Canada occurs mainly in areas producing potatoes and corn intensively, because

• these crops are well fertilized to ensure high yields
• potatoes grow best on coarse, highly permeable soils
• high rainfall in spring and fall causes much leaching.

Average nitrate-nitrogen concentrations in the groundwater in potato-growing areas range from 4 to 10 milligrams per litre. Within this range, average concentrations relate directly to the share of land cropped to potatoes, but evidence suggests that the amount of nonpoint-source nitrate leaching into groundwater is holding steady over time. Certain leachable pesticides used in potato production, such as metribuzin and dinoseb, are also found in the groundwater but usually at concentrations that are only a small fraction of the safe limit values. Nonleachable pesticides, such as chlorothalonil, are detected only occasionally in groundwater, usually at trace levels.

In a recent survey of farm wells in Kings County, Nova Scotia, where most of the province's potatoes and corn are grown, 41% had detectable levels of pesticides (none exceeded safe limits), 9% contained bacteria above the guideline levels, and 13% exceeded the safe limit for nitrate. All pesticides detected were attributed to nonpoint sources, and atrazine occurred most frequently. Elevated concentrations of bacteria were attributed to point sources, and high nitrate levels came from point, nonpoint, and unknown sources.

Production of grass (hay and haylage) is the Atlantic Province's most common agricultural land use. It appears to contribute only low-to-moderate amounts of nitrate to groundwater.

Reducing agrochemical entry into groundwater

As agricultural activity becomes more intense, some degree of agrochemical entry into groundwater is unavoidable. The challenge is to develop ways to reduce agrochemical entry. The methods will vary to suit the wide diversity of climate, soils, and agricultural activity in Canada's agricultural areas. The most promising options for achieving this task are outlined below. (Nitrate leaching in potato production)

Catch crops and crop rotations

The use of catch crops (usually a lower-value crop that is planted in the fall after the main higher-value crop has been harvested) and certain crop rotations can substantially reduce the amount of nitrate leaching into the subsoil, or groundwater, or both. Catch crops use leftover nitrogen and water in the soil, both during the fall after the main crop has been harvested and in the following spring before the main crop is planted. Certain crop rotations reduce nitrate leaching by establishing a situation of mutual benefit. Any leguminous crop in the rotation takes up nitrogen left over from the previous crop and at the same time adds slow-release nitrogen to the soil through its root system. Succeeding crops in the rotation then use the slow-release nitrogen during the growing season, reducing the amount of chemical fertilizer required.

For example, in Atlantic Canada, winter wheat, oilseed radish, and winter rye are effective catch crops for use in potato production. Studies in Prince Edward Island have shown that oilseed radish and winter rye remove unused nitrogen from potato land at about 100 kilograms per hectare. In Saskatchewan, rotating wheat crops with legumes, such as lentil, and fall-seeded crops, such as winter rye, has resulted in less nitrate leaching below the root zone than with the more traditional summerfallow-spring wheat rotation or continuous spring wheat cropping (Fig. 10-4). The lentil crop allows lower rates of chemical fertilization by providing natural, slow-release nitrogen to the soil, which is used by the succeeding wheat crop before leaching occurs. The winter rye crop grows in the fall before freeze-up and starts growing again earlier in the spring than the traditional spring wheat, using the leftover nitrate in the soil before it leaches. (Essential crop nutrients)

Timing, rates, and procedures for applying agrochemicals

Entry of nitrate from commercial fertilizer and manure into surface water and groundwater is the main water-quality problem related to agricultural activity. Although the entry of pesticides into groundwater is currently very low throughout Canada, they still need to be used carefully. Increasing efficiency in the use of nutrients, pesticides, and irrigation water by crops can reduce the potential for nitrate and pesticide leaching.

Nutrients

Ways to improve nutrient use by crops include the following:

• properly accounting for all major sources of nitrogen, including that which occurs naturally in soil and the amounts supplied by plowing under green-manure crops and crop residues and by adding livestock manure
• improving the estimates of each crop's needs and applying agrochemicals only in amounts that the crop can use (for example, nitrogen fertilization rates can be based on soil and plant tissue tests, Fig. 10-5)
• timing agrochemical applications to match times of maximum crop need, while avoiding times of major leaching
• improving ways of applying agrochemicals
• setting goals for crop yield that are both economically and environmentally sustainable
• us ing crops that prevent build-up of unused nitrate after the growing season.

The potential for manure to cause nitrate and bacterial contamination is reduced if dry manure is spread on dry and completely thawed soils and then tilled into the soil soon afterward. In some areas, it is possible to inject liquid manure directly into the root zone, using a tractor-drawn injection implement, rather than spraying it onto the soil surface. This method prevents manure run-off from the soil surface, and at the same time the injector teeth reduce manure leaching below the root zone by breaking up large cracks and worm holes in the soil.

Pesticides

Although it is hard to predict the behaviour and transport of pesticides under field conditions, the following management practices can reduce their entry into the groundwater:

• selecting suitable pesticides and applying them at the recommended rates
• timing pesticide applications and applying them appropriately to reduce drift and evaporation
• reducing pesticide use by adopting integrated pest management strategies (pest control that combines use of crop rotations, cultivation, and biological and chemical pest controls).

Irrigation

Crop use of irrigation water can be made more efficient by

• basing the amount and timing of irrigation on measurements of the amount of soil water still available in the crop-root zone, which prevents excess water being applied
• avoiding soaking by flood or heavy irrigation, which can cause water carrying agrochemicals to leach quickly below the root zone through cracks, worm holes, and root channels.

Improved feed technology and pesticides

Progress is being made in developing animal feed (especially poultry ration) that increases the use of nitrogen by the animal and reduces the nitrogen content of the manure. Many of the persistent leachable pesticides, such as atrazine, are gradually being phased out and replaced by less-persistent nonleachable ones. Also, new pesticide products are being developed that pose less risk to the environment because they are toxic only to the targeted pests and not to other plants and animals.

The impact of modern agricultural practices on water is similar throughout the industrial world.... In all cases the solution lies in the management and control of the inputs used in agricultural production... There is a need to focus on causes rather than symptoms.

Canadian Agricultural Research Council
Agricultural Impacts on Water Quality: Canadian Perspectives

Regulation of agrochemical use

Federal and provincial governments are developing strategies and guidelines for regulating manure handling and pesticide use. For example, British Columbia's Code of Agricultural Practice for Waste Management requires farmers to store and use livestock manure according to environmental guidelines. In many provinces, farmers must pass a course on pesticide safety that teaches proper handling procedures and application rates. Some provinces are developing strategies for reducing pesticide use by 50% or more within the next few years. (Great Lakes Action Plan (GLAP))

Research and monitoring programs

Extensive agrochemical research and monitoring programs are maintained by federal and provincial governments, universities, and the agrochemical industry. The recently completed Soil and Water Environmental Enhancement Program (SWEEP), Great Lakes Action Plan (GLAP), and National Soil Conservation Program (NSCP) are examples of government initiatives that include extensive research on how agrochemicals behave in the environment. Ongoing federal and provincial Green Plan programs also include agrochemical studies. These programs are working to answer the following questions about agrochemical entry into groundwater:

• Where does it occur?
• Are present levels harmful?
• Where is it likely to occur, now and in the future?
• How does it occur? (understanding the leaching process)
• Is it increasing, decreasing, or remaining stable over time?
• How can levels be maintained that are both safe and compatible with sustained high-intensity agricultural production?

Conclusions

Current research and monitoring programs are not sufficient to obtain a comprehensive knowledge of the extent and degree of nonpoint-source agrochemical contamination of Canada's groundwater resources. However, information from various locations in the major areas of intensive agriculture allows an overall assessment.

Nitrate associated with agricultural activities is present in nearly all groundwater underlying the main agricultural regions of Canada. Nitrate levels in groundwater supplies are below the safe limit in most regions of Canada, but in some areas of intensive agriculture they are increasing and have already exceeded the safe limit. The main sources of nitrate are commercial fertilizers, livestock manures, and nitrate occurring naturally in the soil profile. Pesticides are also found in the groundwater of Canada's agricultural regions, but generally at levels that are well below safe limits. Pesticide levels in groundwater generally appear to be decreasing as less persistent and less leachable pesticides are developed and as application methods are improved. Bacterial contamination of groundwater in southern Ontario and Quebec is scattered but widespread, probably resulting from heavy application of liquid manure.

Federal, provincial, university, and private agencies are studying ways to minimize the entry of nonpoint-source agrochemicals into groundwater resources. Important developments include the use of catch crops and crop rotations to reduce the use of pesticides and to intercept unused agrochemicals in the soil; improved technologies for applying agrochemicals and irrigation water; new animal feeds that reduce the nitrogen content of manure; and new pesticide products that are less leachable, less persistent, and more targeted to specific pests. Entry of agrochemicals into groundwater must be regulated so that concentrations do not exceed safe limits and are environmentally acceptable. Continued monitoring, research on agrochemical leaching processes, and development of improved methods for agrochemical use are needed in Canada.

Once polluted, groundwater is difficult, if not impossible, to clean up, since it contains few decomposing microbes and is not exposed to sunlight, strong water flow, or any of the other natural purification processes that cleanse surface water.

Eugene P. Odum
Ecology and Our Endangered Life Support Systems