A Primer on Water Quality: Impact of Crop Production Practices on Water Quality

Nutrients

• Nitrogen
  Nitrate is a form of nitrogen which can contaminate ground water supplies by moving downward through the soil profile and into ground water. In 1992, the USEPA estimated that 4.5 million people including 66,000 infants under one year of age in the United States were served by community water systems or rural domestic wells that exceeded 10 mg L^-1 of nitrate, the maximum concentration for safe drinking water (Spalding and Exner 1993). For example, ground water wells in California exceeded water quality criteria for human health due to agricultural chemical contamination by nitrate and a few pesticides (Mackay and Smith 1990).

  In most ground water contamination cases, nitrate concentrations are highest at the top of the aquifer. Therefore, the source of nitrate, either point or non-point, is the soil surface. Any plant-available nitrogen within the root zone may be taken up by plants; however, if the amount of plant-available nitrogen is in excess of crop requirements, then the excess may leach below the root zone and into ground water. Point sources include fertilizer spills or leaks at factories and distributorships, livestock manure holding areas, septic systems and farmhouse-barnyard complexes. Non-point sources include field-applied inorganic and organic fertilizers and mineralized nitrogen from crop residues and surface-applied manure. Most nitrate contamination in rural wells is associated with poor siting (close to barnyards and corrals) and substandard construction (poorly sealed wells that channel contaminated surface runoff down to ground water).

  Sandy and gravelly soils, soils with a high water table, and shallow soils over bedrock are vulnerable to contamination by nitrate and some herbicides that can leach through the soil profile (Agriculture Canada 1994). High nitrogen application rates that exceed crop requirements and over-irrigating have been linked to ground water contamination in California. Nitrogen fertilizers and the disposal of wastes from feedlots and poultry ranches were identified as the major sources of nitrate contamination in the United States (Mackay and Smith 1990).

  Farm drinking water well surveys in Montana and Ontario found that more than 15% of the wells exceeded the maximum contaminant level for nitrate. Summerfallow practices and associated mineralization of organic material contribute to regionalized nitrate contamination of shallow ground water in Montana (Bauder et al. 1993). In Ontario, dug/bored wells were contaminated more often than drilled wells (Rudolph and Goss 1993). The frequency of well contamination decreased with depth and increased with age.

  Nitrate contamination of surface water was demonstrated in 11 agricultural watersheds in southern Ontario where 3% of the samples exceeded the 10 mg L^-1 Ontario drinking water standard (Neilsen et al. 1982). Nitrate loads were correlated with total nitrogen application and with percent of cultivated watershed, row crops, corn and tile-drained cropland. About 70% of the nitrate load occurred during spring runoff.

• Phosphorus
  Elevated phosphorus concentrations in surface waters have contributed to the eutrophication of many lakes including Lake Erie. According to Baker (1985), rural non-point sources accounted for 51% of the total phosphorus load to Lake Erie. In two rivers entering Lake Erie, the Maumee and Sandusky Rivers, non-point sources contributed 92% of the total phosphorus load. High phosphorus load occurred even though gross erosion rates were less than the national average for cropland. The high clay content of the soil and the intensive row crop agriculture were identified as the main factors responsible for high phosphorus loading. Phosphorus readily attaches to clay particles and row-crop agriculture promotes soil loss through erosion.

Lake Erie drinking water supplies have been shown to be contaminated by anthropogenic chemicals including highly mobile herbicides (Logan et al. 1987). Anthropogenic chemicals can be easily passed to other species through the food chain; therefore, relatively small loadings of organic chemicals to lakes can have a major impact on higher species. For example, the double-crested cormorant population in the Great Lakes basin decreased by 86% between the 1950's and 1970's from the effects of DDT found in the food chain (Environment Canada 1995).

Both surface and ground water have shown contamination by herbicides throughout North America. For example, a survey of 100 wells in the Delmarva Peninsula, located in Delaware and parts of Maryland and Virginia, detected herbicides more often in shallow than deep ground water supplies (Koterba et al. 1993). Herbicide detections were best related to commonly grown and rotated crops, such as corn, soybean and small grains which tend to be grown on well-drained soils.

Kimball and Goodman (1989) detected pesticides in almost 10% of the samples taken from shallow ground water under dryland farming of row crops, small grains, soybeans and corn in rotation in South Dakota. More than 90% of the detections were herbicides. The two most commonly detected herbicides were alachlor and 2,4-D; however, concentrations of 2,4-D did not exceed the USEPA drinking water standard. Pesticide detections were most frequent during or immediately following their application in ground water.

A comprehensive study on 11 agricultural watersheds in southern Ontario revealed that approximately 60% of the total pesticide (both herbicides and insecticides) contamination of surface water resulted from storm runoff (Frank et al. 1982). Other sources of surface water contamination from both herbicides and insecticides included the following: base flow from internal soil drainage (18%) and carelessness (spills) associated with operating equipment adjacent to streams (22%) (Frank et al. 1982).

Snowmelt and spring rains can move herbicides from fields to surface water. For example, in the midwestern United States, Thurman et al. (1991) found several herbicides to exceed USEPA maximum contaminant levels for drinking water in spring runoff water. Specifically, 52% of the sites sampled exceeded the level for atrazine; 32%, for alachlor; and 7%, for simazine. In addition, Thurman et al. (1991) found that spring runoff contributes to contamination through stream bank infiltration or recharge from flood waters and upland runoff.

Tillage
Conventional and conservation tillage practices affect the movement of water, soil and chemicals within and from a field. Conservation tillage refers to many different types of tillage and planting practices which leave crop residues on the soil surface. Crop residue cushions the erosive impact of raindrops on the soil surface, slows surface water flow, enhances infiltration, reduces wind erosion and conserves soil moisture.

Different tillage practices affect soil properties which subsequently affect contaminant (e.g. agri-chemical and nutrient) movement within and from a field. Tillage reduces the ability for water to infiltrate down through the soil profile by disturbing soil structure. The amount or proportion of water infiltration and runoff can vary greatly among fields, depending on slope, soil texture, structure and internal drainage (Faucett et al. 1994).

Tillage practices can change the volume of runoff and erodible sediment which moves off the field. A review of several paired watershed studies by Faucett et al. (1994) showed that conventional tillage fields tend to have significant water runoff, soil erosion and agri-chemical loss while conservation tillage fields show no seasonal runoff. The authors also noted that conservation tillage systems have often, but not always, increased infiltration and reduced runoff. Although conservation tillage systems have been shown to reduce total water runoff, concentrations of pesticides are sometimes increased by reduced runoff volume. This may be due to an increased reliance on herbicides in some conservation tillage systems.

Irrigation and drainage
Irrigation delivers water to crops to meet demand and improve yield, while drainage systems remove water from poorly drained soils. Since irrigation and drainage systems increase the movement of water, they also affect the movement of contaminants.

Soil and water salinity can potentially occur in arid and semi-arid regions where irrigated agriculture is practiced. In the United States, an estimated 20 to 25% of all irrigated land (about 4 million ha) suffers from salt-caused crop yield reductions (El-Ashry et al. 1985). Salinity is the most serious water quality problem in many arid and semi-arid agricultural areas (El-Ashry et al. 1985).

Increased salt concentrations are due to canal seepage and return flows from irrigated lands and natural sources. High salinity levels can increase water treatment costs, corrode plumbing and reduce crop yields. Polluted ground water from deep percolation of irrigation water and seepage from irrigation conveyance systems can recharge and pollute rivers; for example, the Colorado River is polluted by 500,000 tons (454 000 tonnes) of salt annually (El-Ashry et al. 1985).

Elevated concentrations of selenium in irrigation drain water are linked to waterfowl deaths, deformities and reproductive failures at Kesterton Reservoir in California (Engberg and Sylvester 1993). Median concentrations of selenium exceeded the acute (short term) criterion for protection of freshwater aquatic life in one study location. Three locations exceeded the drinking water standard and two locations exceeded the chronic (long term) criterion for protection of freshwater aquatic life. Additionally, a survey in the western United States found that trace elements including arsenic, boron, mercury and uranium were also of concern (Engberg and Sylvester 1993). In Alberta, irrigation water quality has been assessed in the thirteen irrigation districts from 2006 to 2007 and from 2011 until the present time. For more information, click here for the Water Quality in Alberta’s Irrigation Districts project.

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