Treating and Recycling Irrigation Runoff
Water Management Guidelines for Nursery/Floral Producers
The potential contamination of surface and groundwater from runoff presents a major challenge for the greenhouse industry. Growers rely heavily on the use of fertilizers and pesticides, as well as water, to produce quality crops. As a result, these operations can pose a threat to our natural water resources. Collecting, treating and recycling greenhouse effluent is one of the best solutions to this environmental problem.
Many states now require a water discharge permit to control irrigation runoff. These permits regulate the level of discharge that flows into surface and groundwater reserves. In many situations quantitative discharge standards are vague and each case is based by the best professional judgment of the regulatory agency.
Although these permits differ somewhat from state-to-state, there are some common features:
Usually 3-5 years in duration
Monitoring is very important in the overall process of treating greenhouse runoff. Knowing what contaminants are present and their relative concentrations, is the basic information required for developing a management plan. Nitrates, salts, pesticides and pathogenic organisms are the principal contami-nants to be on the lookout for. Many of the tests for these materials are quite expensive to run. Also, sampling technique, handling and analytical methodlolgy can impact the results and interpretation. Table 1 provides a list of labs that are currently conducting runoff analysis.
Table 1. Labs currently conducting runoff analysis.
P.O. Box 255
Bedford, Mass 01730
National Testing Laboratories
The best method of managing runoff is to utilize production practices which reduce volume. Obviously, the less runoff you have to deal with, the less of a problem it creates. In addition, these techniques are usually much more economical to implement than large scale treatment and disposal procedures. The following are some basics to consider:
Utilize Efficient Irrigation Systems
A well designed, efficient irrigation system is the foundation of a good water management program. Drip and sub-irrigation do an excellent job of delivering water to the container and are quite efficient. Overhead systems increase the potential for disease and insect problems and create large volumes of runoff. Growers must work towards adapting new irrigation technologies to their production systems to help lower costs and reduce runoff volume.
Watering is perhaps the least precise of all cultural inputs. Growers frequently rely on the "eyeball" method of deter-mining when plants need to be irrigated. As a result, many crops receive excessive amounts of water, creating runoff. Researchers in both Europe and the U.S. are now working towards identifying optimum irrigation regimes based on the water status of the growing medium. These systems will become increasingly important as growers work towards reducing runoff.
Implement Integrated Pest Management
IPM is a common sense approach to controlling pests. This management technique is based on getting the most from chemical pesticides and also incorporates the use of biological controls where feasible. IPM is not only the most economical system for pest control but it also significantly reduces the volume of chemicals that can potentially reach surface and groundwater reserves.
Plants require sufficient nutritional regimes to produce quality greenhouse products. Since fertilizer does not represent a significant production cost, many growers use excessive levels to insure optimum plant growth. This practice largely contributes to the high level of nitrates present in irrigation runoff. Researchers now estimate that most greenhouse crops receive 5 to 10 times more than the amount of fertilizer they require.
Many growers are also evaluating the benefits of slow release fertilizers to help limit nitrate contamination. These materials alone or in combination with soluble fertilizers can be managed effectively to provide optimum nutritional levels, while minimizing the risk to our natural water resources.
Reducing Storm Runoff
Many operations are required to capture storm runoff from production areas. This can represent a significant volume of water to divert and hold. Therefore growers should use space efficiently and reduce as much as wasted surface area as possible. Also, vegetative cover helps limit runoff, so work towards eliminating hard surface areas.
Since up to the first 2 inches of storm runoff must be collected reservoir capacity should be designed on the basis of predict-able storm events. Data on rainfall duration and frequency for your specific location is available from meteorological information sources (i.e. Weather Bureau, Extension Service, etc.). Based on maximum rainfall/hour, annual rainfall and exposed surface area, reservoir size can be calculated. Most designs also allow for enough freeboard to handle unpredictable storm events. These reservoirs should also be designed to prevent seepage (another potential source of groundwater contamination).
Disposing of Greenhouse Runoff
Once you have captured runoff the next question becomes how do you dispose of it? There are numerous commercial systems available for water disposal, but the one best suited for a specific situation may represent a combination of one or all of the following:
Evaporation ponds have not been a viable disposal solution. Although large volumes of water can be lost through evaporation, the quality of water and sediment left behind is extremely poor, creating even more challenging disposal problem.
Land use is one means of disposing of this effluent. Many operations irrigate tracks of land with irrigation runoff. As much as 3-4 inches of water/day can be applied to these areas depending on soil type and vegetation. However, secondary runoff must be avoided. Overhead irrigation is the most feasible system for this application.
Constructed wetlands are now being evaluated for their ability to clean up runoff. These shallow gravel beds, filled with selected vegetation (i.e. cattails), serve as a biological filter for removing chemical pesticides and fertilizers. There is still a great deal of work to be done in this area but it looks promising.
Runoff can be discharged into municipal treatment systems. However, discharge is usually restricted to off-peak hours, the size of existing sewers can be limiting and user fees are typically high. Since most treatment facilities are quickly becoming overloaded, this approach represents a short term solution to runoff disposal.
Treating and Recycling Runoff
Reuse or recycling of runoff is rapidly becoming a common production practice. Careful monitoring of salts, chemicals, nutrients, and pH are extremely critical in managing this water resource. Treating recycled runoff is an important part of this process. A sample treatment system is illustrated in figure 1. Any or all of these procedures can be incorporated into a treatment facility. The order in which water passes through each component can also be rearranged to produce the highest water quality required.
Many growers are now using water treated through a process known as reverse osmosis (RO) to remove potentially harmful salts. These systems are relatively expensive but work well as a source of water for back blending. RO water has virtually no nutrient value and growers have experienced micronutrient deficiencies when plants have been sustained on this water source for extended periods of time.
Like most environmental problems, preventative measures are much more effective than large scale cleanup operations. Common sense is often all that is required to implement cultural inputs that will reduce runoff, as well as improve water quality. However, where more than common sense is required, growers must work towards adapting new technologies for water treatment and recycling.