Greg Sandness

North Dakota Department of Health

Sediments, nutrients, and pesticides are the most common nonpoint source (NPS) pollutants associated with crop production in North Dakota. These pollutants can be delivered to waterbodies by way of surface runoff, wind, percolation, or atmospheric deposition. In many watersheds in the State the delivery rate of these pollutants has become excessive. As a result, the water quality of the receiving waterbodies has been graded and beneficial uses; such as recreation, drinking water supply, and fishing, impaired. Indicators of such occurrences include severe algal blooms, accelerated sedimentation, and periodic fish kills.

To effectively reduce or eliminate the transportation of agricultural NPS pollutants to surface and/or groundwater systems, various source control measures have been adopted by the North Dakota NPS Pollution Management Program. In general, these measures include best management practices (BMP) that affect the quantity and/or quality of surface runoff and/or groundwater leachates by;

1) preventing pollutants from leaving agricultural lands;

2) reducing or eliminating the introduction of pollutants;

3) protecting sensitive areas; and/or

4) preventing the interaction between precipitation and pollutants. Examples of cropland source control BMP, which are being utilized within several N.D. NPS Program project areas, include conservation tillage, grassed waterways, nutrient management, and sediment retention dams.

Determining which BMP or combination of BMP's will be most effective for NPS pollution control involves a comparison of existing practices with potential alternative practices. Cost benefits, sustainability of the BMP/farming systems, soil types, slope, land use, and water table depth, are just a few of the variables that need to be considered in this comparison. when establishing these "local" BMP priorities for watersheds, clean tillage is often the main water quality concern associated with cropland management. One method that can be used to determine if conservation tillage would be an effective alternative to clean tillage involves a comparison of the source control attributes of conservation tillage and clean tillage systems. The following provides a summary of such a comparison:

1) PREVENT POLLUTANTS FROM LEAVING CROPLAND ACRES RUNOFF.

• Conservation Tillage Systems: Crop residues retained on the soil surface inhibit the rate of surface runoff, thereby providing more '~exposure time" for water to be absorbed into the soils and allowing more suspended sediments to drop out of the runoff before it exits the field. Wind erosion can also be reduce significantly when adequate residue is left on the soil surface.
• Clean Tillage Systems: Low residue levels on the soil surface offer very little protection from wind and/or water erosion. Surface water flow rates tend to be greater; reducing the opportunity for maximum infiltration and deposition of sediments in the field where the runoff originated. The occurrence of wind blown sediments from the field is also increased as the amount of residue is decreased.

2) Reduce or eliminate the introduction of pollutants.

• Conservation Tillage Systems: Typically conversion to this type of cropping system may require an increased use of chemicals for weed control. However, with adequate levels of residue on the soil surface the threat of pesticides exiting the field in surface runoff can be reduced. In aquifer recharge areas, percolation of nutrients arid pesticides niay also be a concern, depending on soil types, chemical mobility, depth to water table, etc.
• Clean Tillage Systems: Mechanical weed control can reduce the use of pesticides. Under this type of tillage system or a conservation tillage system, all climatic variables and the location of water resources should be taken into consideration prior to any nutrient or pesticide applications.

3) Protect sensitive areas.

• Conservation Tillage Systems; On croplands with highly erodible soils, steep slopes, or adjacent to riparian areas, crop residues can provide the necessary soil protection to effectively reduce wind and water erosion. These areas quite often require the application of several other BMP (e.g. grassed waterways, buffer strips, terraces, etc.) in conjunction with conservation tillage to control erosion. Given the potential increase in pesticide use and infiltration rates associated with conservation tillage systems, application of this type of tillage system over ground water recharge areas iiiay require closer evaluation.
• Clean Tillage Systems; Reduced levels of crop residue on the surface leaves the soils very susceptible to both wind and water erosion. This erosion potential is especially critical if the cropped acres have highly erodible soils, steep slopes, or are adjacent to riparian areas. Without adequate cover, these areas will experience high runoff rates arid lose a considerable amount of soil to both wind arid water erosion.

4) Prevent the interaction between precipitation and pollutants.

• Conservation Tillage Systems; Crop residues protect the soil surface from the impact of rain drops and inhibit runoff rates during spring snow melt or rainfall events. This reduces the amount of water exiting the field as well as the amount of sediments dislodged during rain events.
• Clean Tillage Systems: With low residue cover, the soils can be very susceptible to erosion during rainfall events or spring snow melt. Sediment loadings from low residue fields can be very high during the major rainfall.

Based on this direct comparison, it becomes evident that, under the right conditions, conservation tillage can be an effective solution for local water quality initiatives.

Given the number of cropland acres in North Dakota and increasing public concern regarding water quality, a significant amount of the States future NPS pollution control efforts will obviously be directed toward improving cropland management. One solution to this growing public concern will undoubtedly be conservation tillage Although conservation tillage will to be considered the 'cure all" BMP, it will certainly continue to be promoted as one of the most cost effective and sustainable solutions for NPS pollution control within the many watersheds in North Dakota.