By Dusty Sonnenberg, CCA, Ohio Field Leader
Point source and non-point source phosphorus have become common terminology in the water quality discussion around Ohio. Point source phosphorus is often easy to define as it comes from a specific source and can be traced back to a specific point. Non-point source phosphorus is not as specific, and cannot be traced back to one specific point.
Agriculture is often cited as the entire cause of the non-point source of phosphorus in Ohio’s rivers, however that is not entirely true, according to Elizabeth Toot-Levy, project scientist with Geosyntec Consultants. Geosyntec is contracted by the Ohio Soybean Association to provide water quality support by analyzing the EPA reports.
“Along with agricultural run-off, non-point source phosphorus would also include: streambank erosion, instream processes, assimilation (using nutrients within the stream) and stormwater runoff as just a few examples,” Toot-Levy said.
Every two years, the Ohio EPA is required by legislation (House Bill 64) enacted in 2015 to determine nutrient loads from point and non-point sources for watersheds in Lake Erie and the Ohio River basins. This was done in both 2016 and 2018. It will be done again in 2020. Results are analyzed to determine progress being made in phosphorus load reduction.
Water samples are collected from Heidelberg University tributary monitoring gages at “pour points” in the eight river watersheds that are monitored in Ohio. Those samples are analyzed to determine nutrient content. That information combined with data from the U.S. Geological Survey river gages that monitor the river flow. Those numbers are then combined to determine the total nutrient load. Specific nutrients can be measured in this process and the total phosphorus load can be determined.
The analysis of the total phosphorus load is then a basic algebra equation solving for the variables by isolating the known numbers, and then a process of deduction. The contributors of phosphorus that have relatively easily measured numbers are the point source components. Point source components include: National Pollutant Discharge Elimination System (NPDES) permitted facilities such as municipal waste water treatment facilities and industries; and household sewage treatment systems, such as septic tanks and other home treatment plants, including outhouses. NPDES permitted facilities are limited in the quantities of pollutants they can discharge. Their discharge is monitored and reported to the EPA. Some facilities are metered and some are calculated using modeling.
Determining the phosphorus load from household sewage treatment systems is a process that involves analyzing current census population data, and subtracting the number of citizens residing in municipalities served by permitted municipal waste facilities. The difference in those numbers is the number served by a household sewage treatment system. There are “book values” that estimate the nutrient load produced by an individual person.
“The number used by the EPA is .51 kilograms of phosphorus that is produced per person per year,” Toot-Levy said.
That number is multiplied by the population served by household sewage treatment systems. The EPA does consider the number of home septic systems that are functional and the estimated number of those that are not functional, assigning a separate value to those with functional systems and those with systems that are not functional in their final calculation.
“The EPA calculates the functional home septic treatment systems at 80% efficient. This efficiency number decreases with age. The EPA calculates home septic treatment systems not working as 40% efficient. They also calculate those which they consider ‘direct discharge’ as 6% efficient,” Toot-Levy said.
Once all the point source numbers have been calculated, they are subtracted from the total phosphorus load, and the remaining balance is the non-point source phosphorus load upstream.
There is also a calculation for non-point source phosphorus load downstream of the pour point monitoring station.
“For the non-point source downstream load, the EPA uses the upstream yield (nutrient load per acre) and will look at the land use downstream and compare it to the land use upstream and the EPA makes some assumptions about how similar the land uses are. If the use is similar enough, the EPA can use the yield from upstream to estimate the yield from downstream. The formula the EPA uses is Acres x Yield = Load. For the Maumee River, only 4% of the watershed acreage is downstream of the monitoring point. The non-point source phosphorus load downstream is calculated by multiplying the upstream yield by 4%,” Toot-Levy said. “The EPA also considered that upstream of the gage, the land use is 79% agricultural, and downstream of the gage, the land use is 49% agricultural. There are 6,297 square miles upstream, and 271 square miles downstream in the Maumee River.”
This final number is then added to calculate the total phosphorus load flowing into Lake Erie.
“The non-point source phosphorus load is calculated making some assumptions that no nutrients are removed from the river once they get in the water. The assumption in the calculation is that no phosphorus or other nutrients are used by plants along the way, or get caught-up in wetlands or in sedimentation,” Toot-Levy said.
In addition to agricultural run-off and drainage, the non-point source load also includes: storm water run-off, municipal separated storm water, and any run-off from mining activities.
“All these are non-point sources that there is not good data on to subtract out of the final load number to more accurately quantify agriculture’s contribution,” Toot-Levy said.
In the EPA’s final Nutrient Mass Balance Study for Ohio’s Major Rivers report for 2018, the total phosphorus load calculated 8% of the phosphorus load from NPDES point sources, and 4% from other point sources. The remaining 88% of the load is attributed to non-point sources of phosphorus. That is why the agriculture industry has a target on its back.