Biosolids pose an interesting modeling and stakeholder relations problem. They can of course have all sorts of nutrients, chemicals, metals, and of course are chock full of organic matter, but when one is developing a bacteria TMDL, these issues aren't relevant. It seems that there is some disagreement in the scientific field about the actual issues in properly treated biosolids, but that aside, it can be difficult to convince stakeholders to focus solely on the bacteria concerns related to biosolids when developing the bacteria TMDL. Other issues, if they do exist, would be addressed through the normal complaint process or perhaps in a TMDL for another pollutant. And of course if the biosolids AREN'T properly treated and applied, that opens a whole other can of worms I'm not going to consider today...
From a modeling standpoint, biosolids are difficult because their bacteria concentrations can vary quite a bit, but based on the sampling records I've seen, are almost always orders of magnitude below the standard in Virginia - 2,000,000 cfu/gram. In addition, although manure used as a fertilizer may or may not be incorporated into the soil after application, regulations specify that land-applied biosolids MUST be incorporated into the soil within 6 hours of application. This means that whatever bacteria are present in the biosolids are less available to transport by surface runoff.
To top it all off, the fields I've seen in Virginia that receive biosolids do so on a rather sporadic basis - typically the same field will not receive application every year - it will receive it once every 3 years at best. In the area I'm working with, most of the fields are 'backup' fields that are only used if all other fields available to the biosolids company have been exhausted. Additionally the area of cropland that receives biosolids is typically much smaller than the cropland in a given sub-watershed - the finest scale we typically look at in modeling. The 'standard' method of land loading used in the HSPF model is to use a load per acre per day - averaging the once every three years to a small part of the cropland area application to a load/acre/day would be completely unrepresentative of the potential impact of the concentrated application that occurs.
And finally, the biosolids, on their one day of application every three years, typically produce a significantly higher bacteria load than normally experienced by the cropland to which they're applied. Because the die-off on the land surface in HSPF is specified not as a rate but as a limit on the accumulation of the pollutant on the land surface, this raises questions on how to represent the die-off of bacteria in biosolids in HSPF.
The technique I've been using for this TMDL - using HSPF - is to represent the bacteria in biosolids as dry atmospheric deposition of a second quality constituent (where the first quality constituent is the bucket I normally use for bacteria). Representing the bacteria in biosolids as atmospheric deposition addresses the issue of application timing, and representing the bacteria in biosolids as a second quality constituent allows me to uniquely apply the biosolids to a small area of cropland without having to create an additional pervious land segment operation (this could be beneficial if one is approaching the limit on operations enforced by HSPF).
Technical gobbledygook follows... This is done by setting NQUALS to 2 for the relevant cropland PLS, setting QSOFG to 2, setting PQADFG to -1, inputting a MUTSIN timeseries with application on the appropriate dates as PQADFX through the NETWORK block, creating a new, smaller area entry for the relevant cropland PLS in the SCHEMATIC block, and using a new MASS-LINK table with that smaller area entry to that routes ONLY the second QUAL to the general bacteria bucket in the receiving RCHRES (don't want to route the first QUAL or water as they would be double-counted). I assume a 90% reduction in bacteria available to transport in surface runoff before creating the PQADFX timeseries.
To address the die-off problem, and here you really should read the "Accumulate and Remove by a Constant Unit Rate and by Overland Flow" section of the HSPF Manual, it turns out that the limit on surface accumulation (SQOLIM) doesn't actually chop things off - it is used as a ratio with the daily loading (ACQOP) to set a die-off rate. Now, the problem with this of course is that you want ACQOP to be zero - you don't want a daily loading, you only want a loading on the days when biosolids are applied. But if you set the daily loading to something like 1 or even 100 cfu/acre, this is completely insignificant compared even to the wildlife loading. So that makes your denominator in the die-off ratio non-zero. Because SQOLIM is NOT actually the limit on surface accumulation but is instead used as a ratio with ACQOP to set a die-off rate, simply set SQOLIM to be a multiple of your small ACQOP - I used 9*ACQOP to match what we use to calculate die-off for all our other bacteria on the land surface, but you can read up elsewhere (Appendix C) on how you might set an appropriate value. SQOLIM will be much much smaller than the actual load the land is receiving from biosolids, but that is okay. So far this is working well.
The final task will be representing the biosolids during allocation at an appropriate level. To be uniform with our representation of typical permitted operations (e.g., NPDES direct dischargers), we should model the application at the permitted level - the 2,000,000 cfu/gram that is rarely seen in real life. I'm planning to try this out, using the permitted level, in the next week or two as I develop allocation scenario for the TMDL I'm working on. I'll let you know how it goes...
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