Pollution caused by agricultural runoff is one of the primary causes of water quality issues throughout the US. When polluted runoff, loaded with excessive amounts of nutrients, pesticides, herbicides and other agricultural contaminants, runs into local surface waters, the water becomes polluted and even toxic. Since surface waters such as lakes and rivers supply most of our cities’ drinking water, when these sources are seriously compromised it hits our health and our individual pocketbooks.
The toxins and pollutants that arise from unlimited agricultural runoff are not readily or cheaply removed, and the damage they do to aquatic ecosystems is not easily fixed. The only realistic, sustainable solution to the issue is to manage the problem at the source. Fortunately, there are a broad choice of natural, effective approaches to managing runoff.
Cover Crops
Even in agricultural fields where crops are rotated through the seasons, there may be months where no live plants are growing. In these conditions, precipitation and irrigation water can easily leach nutrients like nitrogen and phosphorus and carry it into local waterways. One of the most effective ways to prevent this kind of pollution is to plant cover crops. Cover crops are fast-growing annual or perennial plants that scavenge and absorb soil nitrogen, keeping it out of waterways and on the land where it’s available to fertilize the next season’s crops. In recent studies, cover crops were found to reduce the amount of nitrogen leaving a field by as much as 89%. While phosphorus hasn’t been as closely studied as nitrogen, water samples have detected phosphorus loads cut by 15% to 92%. In these cases, the mechanism may be as simple as the cover crops’ ability to mitigate erosion by covering the ground and securing soil in place with their roots.
Drainage Water Management
Drainage water management is a practice common in Midwest, agricultural regions where subsurface tile drainage systems are in wide use. Since water draining through agricultural fields can carry soluble forms of both nitrogen and phosphorus, there are significant environmental effects when this drainage enters local groundwater or surface waters. Conservation drainage practice is another term for managing and minimizing the effects of this kind of agricultural runoff on the ecosystem.
At its most basic form, drainage water management helps farmers control the amount of water available to crops by artificially raising and lowering the water table at different periods of the year. Conservation drainage practices integrate conservation benefits and water quality protection with the need for reliable drainage. These practices typically include adding bioreactors and saturated buffers to a drainage system, while modifying the drainage structure itself to include elements like two-stage ditches and other detention or retention structures.
Saturated Buffers and Grassed Waterways
Even in areas that don’t use tile drainage systems, most of these conservation management techniques can be extremely effective. Planting trees, shrubs, and grasses along the borders of agricultural fields can help absorb water and filter nutrients that run off due to over-irrigation or excessive rainfall. Tile drainage systems can simply direct drainage water into these areas. Saturated buffers take very little land out of production, have no impact on crops and provide benefits both in improved water quality and important habitats for beneficial insects and pollinators.
Grassed waterways are an excellent solution in areas where flash flooding is a persistent problem or where seasonal erosion threatens productive fields. These waterways follow the natural route water tends to travel but are typically widened while existing slopes are eased across the course of the waterway. Typically planted with weak-stemmed cool season grasses, the waterways are easy to maintain during dry periods while the roots absorb excess nutrients and keep underlying soil from washing away during wet periods.
Prairie Strips
Placed in between crop rows or along streams and waterbodies, prairie strips are one of the most affordable and environmentally beneficial practices available in agriculture. They help reduce erosion, minimize nutrient runoff, stabilize streambanks, and increase habitat and food sources for wildlife, to an even greater degree than similar plantings with other perennial vegetation types.
In contrast to native tall-grass prairie cultures, grassed waterways are typically planted in monocultures that provide relatively little support for pollinators that need a variety of flowering plants in bloom throughout the growing season. Since grassed waterways are intended to function periodically as a watercourse, weak-stemmed grasses that can bend and move in flowing water without damage are preferred
Native, tall-grass, prairie cultures, however, are characterized by a diverse collection of wildflowers and stiff-stemmed warm season grasses that provide abundant habitat for pollinators, beneficial insects, and other wildlife. As a community, they produce deep and multilayered root systems, and their stiff stems keep them upright even in driving rains.
Bioreactors
Bioreactors are simple edge-of-field structures commonly used in drainage tile systems to remove nitrates leached into tile drains and protect water quality. Tile drainage water flows through an outlet into a trench filled with wood chips where bacteria process nitrates from the water and expel it as gas. In basic function, it’s like a saturated buffer, but without the living plants and habitat opportunities. Bioreactors are a proven low-tech solution that require very little maintenance, have no impact on in-field management, and are effective in decreasing the amount of nitrate-nitrogen that enters local waterways. According to the 2016 Iowa Nutrient Reduction Strategy Annual Progress Report, bioreactors were found to reduce nitrogen loss to surface water by 43 percent on tile-drained land.
Since bioreactors rely on biological processes to break nitrates down into gas, water entering must be retained long enough for the processes to be effective. That’s handled by structures in the tile drainage system that can redirect flow as needed. But nitrate-rich water that’s being treated shouldn’t be allowed to leak out of the trench and enter groundwater, where it can infiltrate drinking water supplies and cause serious health problems. To prevent this, an impermeable geotextile liner is installed across the bottom and up the sides of the trench, separating the bark and any inflowing water from the soil beneath. After an appropriate residence time, water exits from the pit through an outlet at the other end of the trench.
