By Sanjay Shukla, Asmita Shukla and Alan W. Hodges
Stormwater detention/retention systems or agricultural ponds can be used to convert the vegetation growing inside to a beneficial commodity while reducing phosphorus discharges. These are the findings of a study conducted by the University of Florida Institute of Food and Agricultural Sciences (UF/IFAS). The study evaluated the environmental and economic feasibility of harvesting vegetation from the ponds, converting the biomass to compost and applying it back to the farm as organic fertilizer.
POND PURPOSES
These ponds are typically found in vegetable, sugarcane and citrus farms in Florida, especially in the southern part of the state. Growers buy and apply compost to their fields every year because of its water and nutrient retention and soil health benefits. Similar to urban areas, ponds in the agricultural landscape exist to satisfy the regulatory requirement of holding a certain amount of stormwater runoff on the property before it can flow out to avoid downstream flooding. The ponds are also considered to be priority conservation practices or best management practices (BMPs) to store and treat farm drainage.
Phosphorus in fertilizer is an essential input for commercial agriculture. However, excess phosphorous in the drainage can lead to ecological degradation of waterbodies such as the Everglades. Numerous BMPs to reduce nutrient losses from farms have been developed by the regulatory agencies and are implemented throughout Florida, including in the greater Everglades region. Regardless of how good of a job farmers do in carefully managing nutrients and water on their farms, there will always be some nutrients lost with the farm drainage. These nutrients will end up in the pond, and part of them will eventually go downstream to rivers and lakes.
Stormwater ponds’ end-of-the-farm location makes them effective for both retaining water and nutrients. Unlike other regions, drainage is pumped into the ponds in South Florida due to flat topography. Pumping mostly occurs after rain events. Once the pond is full, water is discharged through control structures to downstream locations such as canals and rivers.
The farm drainage entering the pond contains dissolved and particulate phosphorous. Once in the pond, the phosphorus can take several pathways. The dissolved phosphorus can be taken up by plants because it is an essential element for their growth, it can be adsorbed on the soil, or it can move out of the pond with surface water or groundwater. Particulate phosphorus mostly remains inside the pond with the sediment. Most of the agricultural ponds in South Florida are over two decades old and have been receiving phosphorus, which is sometimes referred to as legacy phosphorus.
STUDY FINDINGS
A multiyear ongoing study, with the objective of quantifying the phosphorus treatment of ponds and their role as a sustainable BMP, brought to light their ever-decreasing phosphorus retention efficiency due to phosphorus buildup in soil. While a positive phosphorus treatment was observed at the two ponds monitored, a net release of phosphorus was shown to occur during a wet year.
Further analyses showed that ponds often functioned as a source rather than a sink of phosphorus for several rainfall events during the monitoring period. The release of phosphorus occurred because the soil’s capacity to adsorb phosphorus was exhausted (i.e., it was phosphorous-saturated). Sandy soils of Florida have relatively low adsorption capacity. In the future, the ability of these ponds to retain phosphorus will likely be diminished further as they continue to receive drainage.
HARVESTING POND PLANTS FOR COMPOST
In 2014, several short- and long-term management options were explored to renovate the positive phosphorus treatment of these ponds. Vegetation harvesting presented itself as an attractive approach from both economic and environmental standpoints. However, the cost of harvesting plants growing inside had to be considered.
To make it a win-win, the idea of making compost out of the biomass was considered because farmers currently pay to use compost. The technical feasibility analysis of the harvesting-composting showed that the phosphorus retention efficiency could potentially be increased from 50% to 77%. The study also estimated that phosphorus discharge from some ponds could be brought down to a negligible level. These results assumed that 75% of the vegetation would be harvested every year. The vegetation common in the ponds included various kinds of grasses, cattail, water lettuce and willows.
If the aboveground vegetation from the ponds is harvested and turned to compost for use at the same farm, the economic value of compost can help partly pay for the harvesting cost. Harvesting-composting can be incentivized by making it cost-neutral to the producer or paying the producers for the additional phosphorus treated since it is beyond the current set of BMPs they have implemented.
Vegetation harvesting fulfills two needs. It eliminates the plant decay and release of plant phosphorus to water and facilitates its uptake from the saturated soil. As the vegetation is harvested on an annual basis, the excess phosphorus from the soil is mined out of the ponds and soil phosphorus buildup is reduced. If done every year, vegetation harvesting will make the pond go back to adsorbing the phosphorus again and maintaining its treatment capacity in the long term.
ECONOMIC ANALYSIS
Treating phosphorus once it is out of the farms or urban areas is expensive. According to a UF/IFAS study, the cost of phosphorus treatment using the publicly funded stormwater treatment areas in the Everglades basin can range between $158 and $404 per pound. An economic feasibility analysis showed that the harvesting-composting approach could treat phosphorous at 90% less cost of $12 to $19 per pound. The feasibility study included the entire supply chain of compost: harvesting the vegetation, transporting it to a composting facility within a 50-mile radius, composting, and transporting the finished product back to the farm.
The additional phosphorus treatment resulting from this harvesting-composting approach can be sold to the state as a service at a mutually agreeable price, making it a win-win for both farmers and the public. The state would be paying a significantly lower price for treating the phosphorus while the farmers would receive additional income, making it beneficial for all parties involved.
What makes the use of ponds even more attractive is that they already exist. The 50-year net present value of these ponds can typically be up to $1 million. These ponds occupy 10% to 15% of the total farm area, and high land prices drive the said cost. Getting additional services of water treatment extends their beneficial use compared to purchasing land and building new treatment systems such as constructed wetlands.
The benefits of the harvesting-composting approach extend further. Application of compost also increases a farm’s water- and nutrient-holding capacity to reduce fertilization requirements and reduce phosphorus losses. Harvesting biomass on an annual basis may also reduce the carbon footprint, which can become another potential benefit if carbon markets become a reality in Florida. The proposed harvesting-composting concept followed by on-farm application of compost would return the lost phosphorus to the farm in an economic fashion without any additional financial burden to the farmers.
The next step will be to conduct a pilot study to field-verify the concept and demonstrate it to stakeholders.
Sanjay Shuklais a professor at the UF/IFAS Southwest Florida Research and Education Center in Immokalee. Asmita Shuklais a scientific data manager for CSS Inc. in Fairfax, VA. Alan W. Hodgesis a UF/IFAS emeritus professor.